MR Fat Suppression Techniques - USA

Continue Continue Continue Continue Fat Suppression Techniques OLT Fat Suppression Techniques Online Training This training provides an overview of techniques used to suppress fat in Magnetic Resonance Imaging. Master Template HOOD05162003052540 | Effective Date: 26-Nov-2019 Spectral Fat Saturation 1 STIR 4 Water Excitation 3 ​SPAIR 2 Dixon 5 ? A licensed physician may choose to use FDA-approved contrast agents in conjunction with an MRI exam, based on his/her medical opinion and discretion and in accordance with the instructions for use and indications for use supplied by the pharmaceutical manufacturer for the contrast agents. Welcome [fatsat_olt_en_welcome.mp3] Welcome to the Fat Suppression Techniques Online Training. This training provides an overview of techniques used to suppress fat in Magnetic Resonance Imaging. By the end of this training, you will understand the five fat suppression techniques and be able to summarize their strengths and weaknesses. This allows you to make an educated decision about how to incorporate these techniques into a standard workflow for a patient examination. Let's get started! ? This training is provided to you by our MR Master: Stefan studied Biomedical Engineering and holds a PhD in Medical Physics with the main focus on Magnetic Resonance Imaging. Stefan joined Siemens in 2013 as an MR Application Trainer and is working since 2022 for the Headquarter Support Center as a Problem Manager for MR Applications. Table with 2 columns and 4 rows First Name: Stefan Last Name: Weber Company: Siemens Healthineers Job Title: MR Problem Manager in Headquarter Support Center Introduction of our MR Master ? Navigation Hints Before you start, we would like to give you a few tips on how to navigate: Table with 2 columns and 3 rows Not all pages contain audio. Some pages invite you to read for yourself. All pages show a ? button in the lower-right corner. Select the ? button to get a quick guide through the navigation elements. Some pages show an X button in the upper right corner. Select the X button to return to the overview page and not miss any information provided. Enjoy the course! ? This slide is mandatory for all ED1 trainings. It is displayed directly after the Welcome slide. Please delete what is not needed. Navigation Hints ? Basic knowledge of fat suppression techniques Introduction Introduction [fatsat_olt_en_introduction-intro.mp3] Before we take a closer look at each fat suppression technique, let's briefly talk about existing training material on the underlying MR physics. In addition, you will also learn about the physical phenomenon of chemical shift and challenges for fat suppression techniques. ? Introduction To understand the different fat suppression techniques, a basic understanding of the underlying MR physics is recommended Please check the available online trainings on PEPconnect > Magnetic Resonance Imaging > MR Essentials, e.g., Generating and Acquiring the MR Signal Echoes, Decay, Relaxation, and Contrast Spatial Resolution Pulse Sequences Please note: Some of the trainings are not free of charge and you can get them in the PEPconnect store. Introduction [fatsat_olt_en_intro_physics.mp3] In order to understand the different fat suppression techniques that are covered in this training, a basic understanding of the underlying MR physics is recommended. For this purpose, please check the available online trainings on PEPconnect in the MR Essentials training area. [5 seconds pause] Please note: Some of the trainings are not free of charge and you can get them in the PEPconnect store. ? Chemical Shift Signal water fat f Fat molecule Water molecule 3.5 ppm (chemical shift) 1.5T, ~63 MHz, ~220 Hz 3T, ~123 MHz, ~440Hz Spectral Fat Saturation SPAIR Water Excitation STIR Dixon Chemical Shift [fatsat_olt_en_intro_chemical-shift.mp3] To understand fat suppression techniques we have to learn more about chemical shift which is an important physical phenomenon. Even though fat and water molecules are exposed to the same magnetic field strength within an MR system, they will have slightly different resonance frequencies. The reason for this is the chemical structure of the molecules. The protons of the hydrogen atoms within the water and fat molecules are responsible for the detectable MR signal. Because of the different chemical structure and hence the different environments of their nuclei, and the shielding effects of surrounding electrons, water and fat protons will experience slightly different local magnetic fields. Hence fat will resonate at a slightly lower frequency than water. The chemical shift is a relative offset of 3.5 parts per million to the resonance frequency i.e., at one point five Tesla with the resonance frequency of about 63 Mega Hertz, the chemical shift is a frequency difference of approximately 220 Hertz. At three Tesla with a resonance frequency of about 123 Mega Hertz, the frequency difference of water and fat is approximately 440 Hertz. This characteristic of water and fat molecules can be utilized to suppress fat or to excite only water. The functionality of the different techniques discussed in the following sections include: Spectral Fat Saturation, SPAIR, Water Excitation, STIR and Dixon. ? Challenge of Fat Suppression Techniques Challenge: homogeneous fat suppression over the entire Field-of-View (FoV) Highest homogeneity in the magnetic field is located at isocenter Magnetic field changes with distance to the isocenter at different positions of the human body Different resonance frequencies occur at different locations within the magnetic field Signal f 3.5 ppm (chemical shift) water fat 3.5 ppm Challenge of Fat Suppression Techniques [fatsat_olt_en_intro_challenge.mp3] The challenge of our fat suppression techniques is to provide homogeneous fat suppression over the entire field of view. But why is this so challenging? Let’s have a closer look. In theory, as we have learned on the previous page, water and fat have a slightly different resonance frequency due to what is known as the chemical shift, which is always 3.5 parts per million, e.g., 220 Hertz at one point five Tesla. If we consider just one proton of water and one proton of fat inside one voxel, we could display their frequencies as single lines since they will have one frequency each. An MR system provides within a certain range a uniform strong magnetic field like one point five Tesla or three Tesla. The highest homogeneity in the magnetic field will be in the middle of the magnet, known as the isocenter. The magnetic field changes with distance to isocenter due to the mechanical form of the magnet, magnetic shielding and other elements outside of the magnet. Once the patient is positioned in the magnet, the anatomy and corresponding magnetic properties of the human body will further disturb the magnetic field and hence local magnetic field variations will occur. In other words, we will have a different magnetic field at different positions of the human body. The resonance frequency is proportional to the magnetic field. As a result, we have different resonance frequencies occurring at different locations. Magnetic Field Inhomogeneities FOV 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm frequency FOV 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm 3D shim (patient specific optimization of the magnetic field) 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm ? Reference frequency Reference frequency Reference frequency Reference frequency 3.5 ppm frequency Reference frequency Reference frequency Reference frequency Reference frequency Magnetic Field Inhomogeneities [fatsat_olt_en_intro_inhomogeneities01.mp3] The illustrations on this page demonstrate the consequence of local field variations. First, we will look at four different locations within one large field of view, and for the moment we will consider only one proton of water and one proton of fat at each of the four locations. The frequency difference between water and fat will always be the same, however, since the magnetic field varies, the absolute frequency will be different at the different locations. [fatsat_olt_en_intro_inhomogeneities02.mp3] This is highlighted in the next illustration where we pick the four spectra from our large field of view. What we can see is that the frequencies of water and fat are shifted in relation to a fixed reference frequency. Of course, we will have many more protons that contribute to our MR signal in our imaging volume and they will all experience slightly different local magnetic fields. [fatsat_olt_en_intro_inhomogeneities03.mp3] If we consider all protons together, they will align with a certain distribution around a center frequency of fat and a center frequency of water. The higher the variations of the magnetic field, i.e., the more inhomogeneities, the broader the peaks of the spectrum. The frequency selective Fat suppression and R-F-pulses that we apply in MRI are typically applied over the entire imaging volume. That means they will see all these different frequencies and must be able to excite or suppress the desired water and/or fat molecules. Therefore, the goal in MRI, especially if fat suppression is required, is to keep the variations of the local magnetic field as low as possible. [fatsat_olt_en_intro_inhomogeneities04.mp3] For this purpose, for most of the fat suppression techniques certain adjustments like a 3D shim are automatically applied. The 3D shim tries to improve the magnetic field homogeneity within an adjustment volume which is typically of the same size as the imaging volume or field of view. ? Summary The smaller the adjustment volume, the fewer variations of the magnetic field The better the field homogeneity, the better the fat suppression Highest homogeneity of the magnetic field is located at isocenter Expect increased inhomogeneity with off-center positioning (e.g., elbow or hand/wrist) Inhomogeneities are also induced by the magnetic properties of the human body FOV 3.5 ppm 3.5 ppm 3.5 ppm 3.5 ppm Summary [fatsat_olt_en_intro_summary.mp3] As a general rule: The smaller the adjustment volume, i.e., the smaller the imaging volume, the fewer variations of the magnetic field will occur inside that volume. Or in other words the better the field homogeneity, the better the fat suppression. The highest homogeneity of the magnetic field is located at isocenter. Whenever body parts are acquired off-center, you should expect increased inhomogeneity. For example: elbow, hand, wrist, and hip examinations. Inhomogeneities are also induced by the magnetic properties of the human body, for example in tissue-bone or tissue-air interfaces like in the lungs. Here too we must expect more challenges for our fat suppression techniques. ? Spectral Fat Saturation (FatSat) Chapter 1 Spectral Fat Saturation (FatSat) [fatsat_olt_en_fs-intro.mp3] Now let's take a closer look at each of the fat suppression techniques. We’ll start with "Spectral Fat Saturation" or "Fat-Sat". For this technique Chemical Shift is utilized. Chemical shift makes use of the different resonance frequencies of water and fat molecules. Spectral Fat Saturation (FatSat) ? Resulting transverse magnetization of fat is spoiled (i.e., destroyed), longitudinal magnetization is saturated + Spoiling frequency Fat and water have slightly different resonance frequencies – chemical shift of 3.5 ppm ~220 Hz @1.5T ~440 Hz @3T ~80 Hz @0.55T 3.5 ppm water fat RF saturation pulse is applied only on the fat frequency peak (= spectral / frequency selective), typically, a gaussian shaped pulse RF excitation Spectral Fat Saturation (FatSat) [fatsat_olt_en_fs.mp3] Because of the different chemical structure of water and fat molecules, the resonance frequency for a given magnetic field strength is different between water and fat. Fat has a slightly lower resonance frequency which is around 220 Hertz lower than the resonance frequency of water at one point five Tesla. At three Tesla the frequency difference is around 440 Hertz. At zero point fifty-five Tesla the frequency difference is around 80 Hertz. In order to suppress only fat molecules but not water, a frequency-selective R-F pulse is applied on the resonance frequency of fat molecules only. That means only fat magnetization is excited. This excited magnetization is immediately destroyed after the excitation so that there is no signal from fat. This mechanism is called spoiling. If the pulse sequence is now started it can only excite and collect signal from water molecules since fat no longer has any longitudinal magnetization that could be excited. Consequently, the acquired MR image will only show water signal. The lower the number of lines per shot the better the fat suppression but the longer the scan time 40 - 50 for a VIBE or FLASH 3D sequence with TR 3 ms - 5 ms Spectral Fat Saturation with VIBE ? Spectral Fat Saturation can be combined with most pulse sequences Typically, one fat suppression pulse is applied for each excitation pulse or phase encoding step TR might increase if fat saturation is activated In very fast pulse sequences (e.g., VIBE), multiple excitations and phase encoding steps are acquired after one fat saturation pulse to reduce acquisition time = Fast Fat Saturation Lines per Shot: number of phase encoding steps after one fat saturation pulse Spectral Fat Saturation with VIBE [fatsat_olt_en_fs-with-vibe.mp3] Most of the pulse sequences can be combined with spectral fat saturation. Typically, one fat suppression pulse is applied for each excitation pulse or phase encoding step, respectively. For a Turbo Spin Echo sequence, for example, there is one fat saturation pulse followed by one excitation with multiple spin echoes, i.e., phase encoding steps. The application of a fat saturation pulse, however, requires a little bit of time. Hence depending on the pulse sequence, the T-R might increase if fat saturation is activated. In very fast pulse sequences like the VIBE sequence, multiple excitations and phase encoding steps are acquired after one fat saturation pulse to reduce the acquisition time. This can be done because in the very short repetition times of the sequence, which might be between 3 to 5 milliseconds, almost no T-one relaxation occurs. This technique is called “Fast Fat Saturation” and can be selected in the Contrast parameter card. The number of phase encoding steps after one fat saturation pulse is referred to as “Lines per Shot” which can be seen below the Fast fat saturation option. The number is automatically determined by the system depending on the sequence parameters. Please note that in some situations the parameter “Lines per Shot” may be adjustable by the user. This depends on specific protocol parameters like “Reduced Motion Sensitivity” which is used in breast imaging. The use of Siemens Healthineers default settings is recommended. In general, the lower the number of lines per shot, the better the fat suppression but the longer the scan time. A value of around forty to fifty lines per shot for a VIBE or FLASH three-D sequence with a T-R between three to five milliseconds is reasonable. Spectral Fat Saturation – Joint Mode Spectral Fat Saturation is sensitive to magnetic field inhomogeneities In areas of higher local field inhomogeneities, fat will be suppressed less effectively than in areas of better homogeneity Joint: special fat suppression mode modified RF pulse profile is used which suppresses a larger portion of different fat frequencies optimized for smaller joints If field inhomogeneity becomes stronger (e.g., larger FOVs) the water peak also becomes broader à fat saturation pulse could partly suppress water signal Field Inhomogeneities Default FatSat Joint FatSat ? Spectral Fat Saturation - Joint Mode [fatsat_olt_en_fs-joint-mode.mp3] Because spectral fat saturation uses a frequency-selective R-F pulse, it is sensitive to magnetic field inhomogeneities. Field inhomogeneities will cause the local magnetic field to vary slightly between different locations in the body. Consequently, the resonance frequencies will change slightly and the frequency spectrum considering all protons inside the measurement volume will get broader. The fat saturation pulse, which is centered in the middle of the fat frequency peak, will no longer suppress all fat molecules and an incomplete or inhomogeneous fat suppression results. In areas of higher local field inhomogeneities, the fat will be suppressed less effectively than in areas with better homogeneity. A typical example of where this type of incomplete fat suppression may occur is shown here in the foot. To improve the fat saturation in more inhomogeneous areas such as this, a special fat suppression mode called “Joint” is available. It can be accessed via the options button behind Fat Saturation. If the “Joint” region is selected, a modified R-F pulse profile is used which suppresses a larger portion of different fat frequencies. As the name indicates, this is optimized for smaller joints such as hand/wrist, elbow, foot/ankle or the knee. However, the “Joint” mode could also be used for the shoulder or hip. The image example provided here shows the improved fat suppression in the foot using the “Joint” mode. Care must be taken if the field inhomogeneity becomes stronger, for example in larger fields of view. In this case the water peak also becomes broader, and the fat saturation pulse could partly suppress the water signal. Spectral Fat Saturation – Summary ? Field Inhomogeneities Disadvantages Increases scan time Sensitive to B0 variations Sensitive to B1 variations Advantages Can be combined with most pulse sequences and contrasts Can be used post contrast agent injection Weak or strong fat saturation possible Water signal is not affected unsuppressed fat, partial water saturation Fat FatSat Water Fat FatSat Water Spectral Fat Saturation - Summary Image source – Name: Vida_Shoulder/ID:Siemens_MAC/study date:9.21.2021 [fatsat_olt_en_fs-summary.mp3] Spectral fat saturation is widely used in M-R imaging in all kinds of applications where fat suppression is needed. Particularly for orthopedic imaging, spectral fat saturation is often the preferred fat suppression technique since it works well and does not need much additional scan time. It can be combined with most of the different pulse sequences and contrasts we want to achieve. Spectral fat suppression can also be used after contrast agent injections. Like with other fat suppression modes, weak fat suppression can be selected in order to retain some of the fat signal to get a better anatomical overview. Also the water signal is not affected. However, spectral fat saturation will usually increase the scan time compared to a non-fat saturated sequence. How much depends on the specific protocol parameters like the T-R of the sequence. Spectral fat saturation is sensitive to the magnetic field inhomogeneity, i.e., if the local magnetic fields varies, the fat suppression may become worse. This typically happens for example in off-center imaging or in the periphery of larger fields of view, especially at higher field strengths like three Tesla. Spectral fat saturation also depends on the transmission field, known as the B-one field. That means if the transmission field varies the fat saturation pulse will become less effective. B-one field variations are a typical phenomenon at field strengths of three Tesla or higher due to the physical nature. For this purpose, other techniques like SPAIR fat suppression may be the proper choice in certain situations. ? SPAIR Spectrally Adiabatic Inversion Recovery Chapter 2 SPAIR - Spectrally Adiabatic Inversion Recovery [fatsat_olt_en_spair-intro.mp3] SPAIR, which stands for “Spectrally Adiabatic Inversion Recovery”, is another fat suppression technique based on the chemical shift between fat and water protons. It combines the advantages of STIR and spectral fat suppression. SPAIR – Spectrally Adiabatic Inversion Recovery excitation MZ TISPAIR time fat water Adiabatic inversion of fat 180o ? Uses an adiabatic pulse Less sensitive to inhomogeneities in the excitation B1 field higher energy deposition, needs noticeably longer time SPAIR - Spectrally Adiabatic Inversion Recovery (1) [fatsat_olt_en_spair01.mp3] Like spectral fat saturation, SPAIR uses a frequency-selective RF pulse which is applied on the fat frequency. In common with STIR it inverts the longitudinal magnetization, but unlike STIR it only inverts the fat magnetization because of the fat frequency-selective RF pulse. That means that the water signal is not affected. After the inversion, there is a wait of a certain time (the T-I time) until the longitudinal fat magnetization is zero. At this time point the excitation of the pulse sequence starts so that the full water signal but no fat signal is acquired. SPAIR uses a special type of RF pulse, known as an adiabatic pulse. Adiabatic pulses are more complex pulses that are less sensitive to inhomogeneities in the excitation B-one field, in other words they widely guarantee that the desired flip angle is achieved homogeneously in the entire imaging volume. This is especially important with higher field strengths like 3 Tesla where the transmission field has larger variations due to the physical nature. The disadvantages of adiabatic pulses are a higher energy deposition, i.e., higher SAR values and they need a noticeably longer time. This is why adiabatic pulses are not used for conventional imaging, but they are perfectly suited for preparation pulses like fat suppression pulses. ? VIBE with SPAIR VIBE with FatSat SPAIR – Spectrally Adiabatic Inversion Recovery SPAIR improves fat suppression quality and eliminates shading! SPAIR - Spectrally Adiabatic Inversion Recovery [fatsat_olt_en_spair02.mp3] These 3-Tesla Breast image examples show the advantages of SPAIR in contrast to spectral fat saturation. With SPAIR a homogenous fat suppression in both breasts is achieved. With spectral fat saturation, however, the fat suppression is more inhomogeneous between both breasts due to variations of the transmission field which are naturally at 3 Tesla or higher field strengths. The improved fat suppression with SPAIR is mostly achieved due to the adiabatic preparation pulse which compensates the variational transmission field. SPAIR improves fat suppression quality and eliminates shading! ? SPAIR – Spectrally Adiabatic Inversion Recovery HASTE with SPAIR HASTE with FatSat SPAIR - Spectrally Adiabatic Inversion Recovery [fatsat_olt_en_spair03.mp3] Another example demonstrating the improved fat suppression characteristics of SPAIR versus spectral fat saturation is shown here. SPAIR is not only less sensitive to B-one variations but is also slightly less sensitive to B-zero variations due to a sharper pulse profile of the SPAIR pulse in comparison to the default fat saturation pulse. This is beneficial in larger fields of view, therefore SPAIR is usually preferred for example in abdominal imaging. SPAIR – Special Options at 3T Field Inhomogeneities ? Fat SPAIR Water Fat Special SPAIR modes Water Improved fat suppression unsuppressed fat Fat SPAIR Water 3T systems higher signal-to-noise ratio more inhomogeneous magnetic fields Typical challenges large field-of-views in the Abdomen or Thorax region special SPAIR options available at 3T SPAIR - Special Options at 3T [fatsat_olt_en_spair-special-options01.mp3] Three-Tesla systems offer a lot of advantages compared to one point five Tesla systems. The most prominent feature is the higher signal-to-noise ratio. However, Three-Tesla systems also bring some more challenges, i.e.: stronger magnetic susceptibility effects resulting in a more inhomogeneous magnetic field. Consequently, this causes more challenges for fat suppression. If the magnetic field becomes more inhomogeneous, different more resonant frequencies occur within the imaging volume. The larger the imaging volume and the higher the susceptibility differences between the tissues, the stronger the variation of the frequencies. As a result, frequency spectra of water and fat will widen, and the applied spectral RF pulse won’t be able to suppress all fat molecules at the same time. Insufficient fat suppression may occur. Typical challenges are large fields of view in the abdomen or thorax region, but these may also be found in other body regions such as the pelvis or breast. Therefore, special SPAIR options are available at 3 Tesla focusing on specific body regions by using tailored RF pulse profiles for each individual body part that is supported. The pulses use typically broader profiles, but are optimized for the expected frequency distributions in the different body regions. The special options for SPAIR at 3 Tesla can be accessed via the options menu by clicking on the button with the three dots behind SPAIR. The following body regions are supported: Abdomen and Pelvis, Breast and Thorax. The following example will demonstrate the use cases of the special SPAIR regions. ? SPAIR – Abdomen & Pelvis Region at 3T TSE with Default TSE with Abdomen & Pelvis HASTE with Default HASTE with Abdomen & Pelvis SPAIR - Abdomen & Pelvis Region at 3T [fatsat_olt_en_spair-abdomen-pelvis.mp3] These images demonstrate the effects of the special SPAIR region "Abdomen and Pelvis" used for a HASTE transverse and coronal examination on the MAGNETOM M-R three-T Skyra system. With the default SPAIR, certain areas with fat are not sufficiently suppressed, … whereas with the optimized “Abdomen & Pelvis” option the fat suppression within the volume of interest can be clearly improved. ? Abdomen & Pelvis Default Thorax SPAIR – Thorax Region at 3T The Abdomen & Pelvis mode can lead to water saturation if used in other body regions (especially in Thorax). SPAIR - Thorax Region at 3T [fatsat_olt_en_spair-thorax.mp3] These image examples illustrate the various SPAIR modes used in a thorax examination. The thorax region has even higher susceptibility effects than the liver region because of its anatomy and the high amount of air in the lungs. This also causes the water peak within the frequency spectrum to get broader… … and the SPAIR mode “Abdomen & Pelvis” can suppress not only the fat but also parts of the water signal. Therefore, the “Thorax” region was introduced. It yields better fat suppression than the “Default” mode since it is optimized for the expected frequency spectrum in the thorax, but has a greater distance to the water peak in order not to suppress the water signal. ? SPAIR – Breast Region at 3T VIBE with Default SPAIR VIBE with SPAIR Breast SPAIR - Breast Region at 3T [fatsat_olt_en_spair-breast.mp3] For breast imaging the SPAIR “Breast” region was introduced. This mode is optimized not only to improve the fat suppression in the breast but also to homogeneously suppress silicone signal if silicone is present. The illustration shows how fat suppression is improved compared to the "Default" SPAIR mode and at the same time suppresses the silicone implants. In this example the silicone implants were placed on top of the breasts. SPAIR with VIBE ? Multiple phase encoding steps after one fat saturation pulse = Fast Fat Saturation Option for VIBE or 3D FLASH Same principle is applied for SPAIR Lines per Shot: number of phase encoding steps after one SPAIR pulse The lower the number of lines per shot the better the fat suppression but the longer the scan time 40 - 50 for a VIBE or FLASH 3D sequence with TR 4 ms - 5 ms SPAIR with VIBE [fatsat_olt_en_spair-with-vibe.mp3] As already learned in the spectral fat saturation chapter, in certain situations time can be saved by using multiple phase encoding steps after one fat saturation pulse. This was called “Fast Fat Saturation” and is an option for the VIBE or 3D FLASH sequence. The same principle is applied for SPAIR, for example if a VIBE protocol is used with SPAIR, the system will automatically acquire multiple phase encoding steps after one SPAIR pulse. The number of phase encoding steps after one SPAIR pulse is displayed as “Lines per Shot”. The same characteristics apply for SPAIR as with the “Fast Fat Saturation”. The system will automatically select the correct lines per shot. In some specific situations such as that shown here for a breast protocol, the number of “Lines per Shot” is adjustable. This depends on specific protocol parameters like “Reduced Motion Sensitivity” which is used in breast imaging. The use of Siemens Healthineers default settings is recommended. In general, the lower the number of lines per shot, the better the fat suppression but the longer the scan time. A value of around forty to fifty lines per shot for a VIBE or FLASH three-D sequence with a T-R between four to five milliseconds is reasonable. SPAIR – Summary Advantages Insensitive to B1 variations Water signal is not affected Can be used post contrast agent injection Can be combined with most pulse sequences and contrasts Special optimizations for specific body parts at 3T Disadvantages Sensitive to B0 variations Increases scan time Increases SAR ? SPAIR - Summary Image source – Name: Sola_Abdomen/ID:Siemens_MAC/study date:5.7.2021 [fatsat_olt_en_spair-summary.mp3] In summary, SPAIR offers improved fat suppression especially at 3 Tesla. This is because the adiabatic R-F pulses used are less sensitive to the variations of the transmission field. As with spectral fat saturation, the water signal is not affected, offering full water signal which is not the case for STIR fat suppression. Therefore, like spectral fat saturation and in contrast to STIR, SPAIR can be used after a contrast agent injection and SPAIR can be combined with most of the available pulse sequences and contrasts. At 3 Tesla special tailored SPAIR options are available for various body regions. However, like spectral fat saturation SPAIR is also sensitive to variations of the magnetic field. Furthermore, SPAIR increases the scan time of the pulse sequence used, and increases SAR. Because SPAIR uses an inversion recovery technique with a waiting time to null the longitudinal magnetization of fat, the scan time with SPAIR typically takes longer than with spectral fat saturation. ? Water Excitation Chapter 3 Water Excitation [fatsat_olt_en_water-excitation-intro.mp3] Another method to suppress fat is “Water Excitation”. Actually, water excitation is not a fat suppression technique but rather a way of NOT exciting fat at all. Let's take a closer look at how the principle behind this works. ? Water Excitation 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 Water Excitation uses binominal pulses different resonance frequencies of water and fat different dephasing effects of water and fat (opposed- and in-phase effects) Select the numbered steps below to learn more. water fat Water Excitation [fatsat_olt_en_water-excitation00.mp3] Water Excitation uses what are known as “binominal pulses” and makes use of the different resonance frequencies of water and fat. However, it uses the different dephasing effects of water and fat, known as opposed- and in-phase effects of water and fat. The following illustrations demonstrate the functionality of water excitation. In our example, we want to achieve a 90° excitation pulse for water but no excitation of fat. Select the numbered steps below to learn more. [fatsat_olt_en_water-excitation01.mp3] Initially, a 22.5 degree pulse will be applied on both the water and fat frequency. By doing so, both water and fat are excited, and their longitudinal magnetization vectors are tilted by 22.5 degrees towards the transverse orientation. [fatsat_olt_en_water-excitation02.mp3] After excitation, fat and water protons are in phase, i.e., they point in the same direction. However, because of their spin property and the fact that fat and water have slightly different resonance frequencies, they will run out of phase. At a given time point which depends on the field strength, fat and water protons will look in exactly the opposite directions. This is called the opposed phase status. That means in our example the water magnetization is flipped by 22.5 degrees but the fat magnetization by negative 22.5 degrees. [fatsat_olt_en_water-excitation03.mp3] If we now apply a 45 degree pulse,… …water will be flipped from the 22.5 degree angle to 67.5 degrees whereas fat will be flipped from negative 22.5 degrees to plus 22.5 degrees. [fatsat_olt_en_water-excitation04.mp3] Here too, after the pulse fat and water are in phase, but will run out of phase again. In the opposed phase state, the magnetization of fat will again be at negative 22.5 degrees. [fatsat_olt_en_water-excitation05.mp3] Once the opposed phase status is reached a last 22.5 degree pulse is sent out… …that flips water from 67.5 to 90 degrees, fat will be flipped from negative 22.5 degrees plus 22.5 degrees to zero, … … hence water was fully excited, and fat has not been excited at all. [fatsat_olt_en_water-excitation06.mp3] This illustration is called 1-2-1 water excitation, i.e., the excitation of water is achieved by three R-F pulses in total. Water Excitation – Summary 6 22.5° 45° 22.5° in-phase opp-phase opp-phase in-phase dephasing dephasing water fat 1 2 1 Water Excitation – Step 5 5 water fat 22.5° pulse is applied on water and fat frequency Water is flipped from 67.5° to 90°, fat is flipped from -22.5° to 0° 22.5° Water Excitation – Step 4 4 in-phase opp-phase dephasing water fat In-phase: fat and water protons point in the same direction after excitation Dephasing: fat and water protons run out of phase Opp-phase: magnetization of fat is again at -22.5° Water Excitation – Step 3 3 45° pulse is applied on water and fat frequency Water is flipped from 22.5° to 67.5°, fat is flipped from -22.5° to +22.5° 45° water fat Water Excitation – Step 2 2 22.5° in-phase water fat In-phase: fat and water protons point in the same direction after excitation Dephasing: fat and water protons run out of phase because of their spin property and slightly different resonance frequencies of water and fat Opp-phase: fat and water protons look in exactly the opposite directions In this example: water 22.5°, fat -22.5° opp-phase in-phase dephasing water fat Water Excitation – Step 1 1 22.5° water fat 22.5° pulse is applied on water and fat frequency Water and fat are excited Longitudinal magnetization vectors are tilted by 22.5° towards the transverse orientation Water Excitation ? … 1-3-3-1: 11.25º-33.75º-33.75º-11.25º 1-1: 45º-45º 1-2-1: 22.5º-45º-22.5º Contrast parameter card > Fat-Water Contrast > Water Excitation Fast Water Excitation Images taken from MAGNETOM Flash 1/2011 maximum excitation at water frequency minimum excitation at fat frequency frequency RF exitation 1-1 1-2-1 1-3-3-1 1-4-6-4-1 fat water Water Excitation [fatsat_olt_en_water-excitation002.mp3] Water Excitation can be selected in some of the pulse sequences in the “Contrast” parameter card under “Fat-Water Contrast” within a drop-down menu. Usually two options are available: “Water Excitation” and “Fast Water Excitation”. “Water Excitation” uses a scheme of 3 RF pulses as shown on the previous page, … … whereas “Fast Water Excitation” uses only two pulses, for example two 45° pulses to achieve a 90° excitation. The more pulses are used the better the excitation profile, i.e., the less fat will be excited. However, the more pulses are used the longer the excitation. Water Excitation – Summary Advantages Reduced sensitivity to B1 inhomogeneity Less SAR warnings Applicable post contrast agent injection Disadvantages Sensitive to B0 variations Increases scan time ? Image of water excitation (e.g. in orthopedic imaging) available? (matching to the protocol shown below) Water Excitation - Summary Image source – Name: Vida_knee/ID:Siemens_MAC/study date:7.1.2021 [fatsat_olt_en_water-excitation-summary.mp3] Water Excitation is used by certain pulse sequences, mainly in orthopedic imaging. Sequences using water excitation typically have “W-E” written in their protocol name as shown here for a VIBE protocol from the knee body region. Water excitation has the advantage that fat suppression is not dependent on the transmission homogeneity so it might be an important feature at 3Tesla. If the flip angle differs from the desired flip angle, then an inhomogeneous water signal may result, but fat won’t be excited. Water excitation has lower SAR because the flip angle is split into several smaller flip angles. Water excitation is also applicable after contrast agent injection. One of the disadvantages of Water Excitation is that it is sensitive to the magnetic field homogeneity like spectral fat suppression techniques (i.e., Fat Saturation and SPAIR). Furthermore, Water Excitation needs a certain time for the excitation scheme since several pulses are used instead of one. This means it is not applicable for Turbo Spin Echo sequences where multiple spin echoes are generated in a short time. Beside the knee exam shown in the example we might consider that on the other hand water excitation is not used in fast sequences like VIBE in the Abdomen because the repetition and hence the breath-hold time would be longer. Knowledge Check You have just learned about the fat suppression techniques Spectral Fat Saturation, SPAIR and Water Excitation. Select Start to test your knowledge of the presented content. Start ? Knowledge Check Select three (3) answers. ? Question 1 of 3 Which statements about magnetic field homogeneity are correct? The smaller the adjustment volume, the fewer variations of the magnetic field The better the field homogeneity, the better the fat suppression Highest homogeneity of the magnetic field is located at isocenter Inhomogeneities are not induced by the magnetic properties of the human body Inhomogeneities are not induced by the magnetic properties of the human body Inhomogeneities are not induced by the magnetic properties of the human body Question 1 One or more answers are incorrect. Select three (3) answers. ? Question 2 of 3 Which SPAIR body regions are available at 3T? Abdomen & Pelvis Thorax Breast Foot/Ankle Question 2 One or more answers are incorrect. Select two (2) answers. ? Question 3 of 3 Which of the following fat suppression techniques are less sensitive to transmit field variations (B1)? SPAIR Water Excitation Spectral Fat Saturation Question 3 One or more answers are incorrect. Knowledge Check Completion Review Retry Retry Continue Continue ? You have just completed the Knowledge Check. Select Review to assess how your responses compare to the correct answers. Select Retry to test your knowledge again or select Continue to advance through the course. Knowledge Check Completion ? STIR – Short Tau Inversion Recovery Chapter 4 STIR - Short Tau Inversion Recovery [fatsat_olt_en_stir-intro.mp3] Another fat suppression technique used to suppress fat signal is known as STIR, which stands for "Short Tau Inversion Recovery" or "Short T-I Inversion Recovery". STIR – Short Tau Inversion Recovery TI: 150ms – 180ms @1.5T 200ms – 240ms @3T TI time fat water MZ Inversion of fat and water 180o Excitation (start of the sequence, e.g. TSE) ? T1 relaxation-dependent technique Fat has much shorter T1 relaxation time than other water-based tissues It can be separated by using a magnetization preparation technique STIR - Short Tau Inversion Recovery [fatsat_olt_en_stir.mp3] STIR is a T-one relaxation-dependent technique based on the different T-one relaxation behavior of water and fat. Since fat has a much shorter T-one relaxation time than other water-based tissues it can be separated by using a magnetization preparation technique. Prior to the excitation pulse, an inversion pulse is applied which inverts the longitudinal magnetization for all tissues. Then the longitudinal magnetization for all tissues is negative and T-one relaxation occurs to return longitudinal magnetization to its initial positive state. Because fat has a much faster T-one relaxation, the longitudinal magnetization recovers faster for fat than for other, water-based tissues. At a given time which depends on the exact T-one relaxation time of fat, the longitudinal magnetization of fat will be zero. This time is called the “Inversion time”, “T-I” for short. If the acquisition of the pulse sequence is triggered at this time point, there won’t be any signal from fat because of the zero longitudinal magnetization. However, since the longitudinal magnetization of water is not zero, water tissues will have a signal. That signal is negative, but it will be recognized as a positive MR signal in the MR image which is finally reconstructed, since typically the magnitude of the signal is used in MRI. The T-one time of fat is well known, therefore we can set the T-I in such a way that fat magnetization will be nulled. The T-I time is typically set between 150 milliseconds and 180 milliseconds at one point five Tesla, depending on the organ or body part under investigation. If some residual signal of fat is wanted, a shorter T-I time can also be set, for example 140 milliseconds. At 3 Tesla the T-I is at around 200 to 240 milliseconds since the T-one relaxation time is longer at 3 Tesla and hence the recovery of the magnetization takes longer. ? 05zp_C28_Abdomen_T2_TIRM_tra_384_G2 STIR – Summary STIR uses non-frequency pulses and utilizes the different T1 times of fat and water to suppress fat. Advantages Difficult regions with increased magnetic field inhomogeneities like neck, c-spine, breast, abdomen, whole body Disadvantages Water signal is reduced compared to other fat suppression techniques Not applicable post contrast agent injection Additional SAR and time required for inversion T2 TSE with STIR (TIRM) TIRM = Turbo Inversion Recovery Magnitude STIR - Summary [fatsat_olt_en_stir-summary.mp3] STIR uses non-frequency pulses and utilizes the different T-one times of fat and water to suppress fat. The STIR technique is considered to be one of the most robust fat suppression techniques. This is because it is not dependent on frequency differences between fat and water molecules that could be disturbed by magnetic field inhomogeneities. The inversion pulse used has a broad frequency spectrum to invert water and fat molecules at their different resonance frequencies. Therefore, STIR is very useful in body regions with increased magnetic field inhomogeneities, like for example the neck region or in the abdomen and for whole body imaging. In combination with a Turbo Spin Echo sequence, STIR is sometimes called also “TIRM”, which stands for Turbo Inversion Recovery Magnitude. However, compared to other fat suppression techniques, STIR has the disadvantage that the water signal will also be affected by the inversion pulse. Consequently, because the magnetization of water will also relax during the chosen T-I time, the water signal is reduced. This also means that STIR cannot be used after contrast agent injection, because water tissue with contrast media will have a shortened T-one time which might be similar to fat, i.e. this water signal affected by the contrast media may be suppressed as well. Furthermore, additional SAR and time are required for inversion. ? Dixon Chapter 5 Dixon [fatsat_olt_en_dixon-intro.mp3] The last method that we present to you is Dixon. In contrast to spectral fat suppression techniques, Dixon is actually not a fat suppression but rather a fat-water separation technique. ? Dixon Utilizes different resonance frequencies of water and fat corresponding different dephasing effects similar to water excitation Requires measurement of two contrasts at two different echo times one echo at the opposed-phase state another echo in the in-phase state of water and fat Ink artifact: Same amount of water and fat protons within a voxel à signal will become zero TE = 0.0 ms TE=1.3 ms TE=2.6 ms W F W F W F Images taken from MAGNETOM Flash 1/2011 in-phase image opposed-phase image Dixon [fatsat_olt_en_dixon.mp3] Dixon is a technique that utilizes the different resonance frequencies of water and fat and the corresponding different dephasing effects similar to water excitation. The Dixon method requires us to measure two contrasts at two different echo times, where one echo is acquired in the opposed-phase state and the other echo in the in-phase state of water and fat. Shown on this page is an example for a VIBE Dixon sequence at three Tesla. Immediately after excitation, water and fat will have the same phase. With time they will run out of phase since fat has a lower resonance frequency than water. At three Tesla, at approximately 1.3 milliseconds fat and water will be completely in opposed phases and we will acquire the first echo which is the “opposed-phase echo”. In the resulting image, the signals of water and fat will cancel out each other, i.e., if a voxel contains both water and fat protons the signal will be reduced. If the same amount of water and fat protons are within a voxel, the signal will become zero, a phenomenon also referred to as “ink artifact” since it looks like the organs have a black contour. The reason for this is that organs are typically surrounded by a fat layer and because of partial volume effects the voxels at the border of the organs contain a similar amount of water and fat protons. The second echo is acquired when water and fat are aligned again which is the “in-phase echo”. In the resulting image, water and fat signals add up if a voxel contains both fat and water protons. Dixon – Principle From measured opposed-phase and in-phase images we can calculate "pure" fat and water images. Simplified equation: SIWater = (SIin-phase + SIopp-phase) ÷ 2 SIFat = (SIin-phase – SIopp-phase) ÷ 2 Example: Unknown: Water = 100, Fat = 20 Measured: In-phase → 120 Opp-phase → 80 Calculated: W = (120 + 80) ÷ 2 = 100 F = (120 - 80) ÷ 2 = 20 Method only works for perfect magnetic field homogeneity, otherwise water and fat cannot be separated correctly Advanced fat water separation algorithms are used opposed-phase in-phase fat image water image ? Dixon - Principle [fatsat_olt_en_dixon-principle.mp3] From the measured opposed-phase and in-phase images we can calculate "pure" fat and water images. In simple terms, we can assume that the signal of water is the sum of the in-phase and opposed-phase signals divided by 2. Fat can be calculated by subtracting the opposed-phase signal from the in-phase signal and dividing it by 2. Let’s take a simple example. Let us assume that we have a voxel that contains a water signal of 100 and a fat signal of 20. This might be for example one voxel within the liver. In our MR images that we acquire we don’t yet know the quantity of water and fat signals we have within our voxel. With Dixon we are now able to calculate the amount of water and fat in this voxel: What we will measure is an in-phase echo where water and fat will be added together and an opposed-phase echo where the signal from water and fat cancel out each other. Hence, the signals that we will measure in the voxel for our example are in the in-phase 120 (i.e., 100 plus 20) and in the opposed-phase 80 (i.e., 100 minus 20). From those two measured signals we now try to calculate the amount of water and fat within our voxel. If we add the in- and opposed phase the result is 120 + 80 which is 200, this divided by 2 results in 100. That is the amount of water signal within our voxel. Let’s now calculate the fat signal. If we subtract the opposed phase from the in-phase we will have 120 – 80 which is 40. Divided by 2 this results in 20 which is the fat signal within our voxel. This describes the method behind the Dixon technique. However, this only works for perfect magnetic field homogeneity, otherwise water and fat cannot be separated correctly. For this purpose, advanced fat water separation algorithms are used. Dixon – Swap Artifacts ? D:\TrainingCentre\Courses\3T\Data\JPGs\Dixon\COR_DIXON_W_1.MR.0023.0053.2014.04.04.08.32.00.625984.16930289.png Local water-fat swap within the liver (result from older Dixon algorithm) Global water-fat swap, i.e., the water image is labeled as fat and the fat image is labeled as water (result from older Dixon algorithm) Local magnetic field inhomogeneities are always present in reality Any kind of motion causes local variations of magnetic field Additional variations of resonance frequency, additional dephasing of spins Advanced Dixon algorithm determine additional field variations distinguish water and fat correctly Swaps: water and fat signals are swapped if separation is not successful Always reconstruct water and fat images to identify local swaps and to avoid loss of information in the event of a global swap Dixon - Swap Artifacts [fatsat_olt_en_dixon-swap-artifacts.mp3] Local magnetic field inhomogeneities are induced by the magnetic properties of the magnet and the human body and are always present. Moreover, any kind of motion like gross patient motion or flow within the blood vessels will cause local variations of the magnetic field. These inhomogeneities will cause additional variations of the resonance frequency and an additional dephasing of the spins. The task of our advanced Dixon algorithm is to determine these additional field variations in order to distinguish correctly between water and fat. If this separation is not successful, water protons may be wrongly identified as fat, and fat protons identified as water. This effect is referred to as “swaps”, i.e., the water and fat signals are swapped. There are two kind of swaps that may occur in very rare cases: A global swap occurs where the entire water image is swapped, i.e., we only see fat in the water-labeled image and only water in the fat-labeled image. This may happen if the total amount of fat in the imaging volume is very low. A local swap occurs where locally parts of the water signals are wrongly identified as fat. This may happen in areas of high flow, such as close to larger arteries. Therefore, it is strongly recommended to always reconstruct both the water and fat images in order to identify local swaps and to avoid loss of information in the event of a global swap. Nevertheless, over the past decade the Dixon algorithm has been remarkably improved, including the use of artificial intelligence in order to avoid local and global swapping artifacts. Dixon – Parameter Card ? Contrast parameter card > Common > Fat-Water Contrast > Dixon Dixon cannot be selected in a conventional TSE sequence TSE Dixon protocol must be selected within Siemens library Only VIBE and Turbo Spin Echo support Dixon Dixon - Parameter Card [fatsat_olt_en_dixon-parameter-card.mp3] Dixon is accessed in the Contrast parameter card with the “Fat-Water Contrast” parameter. Via the options menu which is accessed by the button with the three dots behind the parameter, we can select the contrasts we want to create. It is strongly recommended to always reconstruct water and fat images as described earlier. In addition to the “Water” and “Fat” series, the original “In-Phase” and “Opposed-Phase” series can be selected and will be reconstructed as additional separate series. “Original Echoes” reconstructs the “In-phase” and “Opposed-Phase” series as well, so you can select either this or the individual in- and opposed-phase series. In some of the older software versions only “Original Echoes” may be visible and must be selected if in- and opposed phase series are desired. Furthermore, since Dixon needs certain specific requirements, Dixon cannot be selected in a conventional T-S-E sequence. Instead, the T-S-E Dixon protocol must be selected within the Siemens library. The Dixon method is not possible with every pulse sequence. Only the VIBE and Turbo Spin Echo sequences support the Dixon water-fat separation method. ? Dixon – Turbo Spin Echo Trick: Two TSE series “Normal” TSE sequence Same sequence parameters, echo readout is shifted for each individual spin echo within the echo train by the opposed phase time In- and opposed phase echoes are generated Dixon can be used with TSE sequences Only gradient echoes have the effect of in-phase and opposed-phase echoes For spin-echo based sequences like the TSE, fat and water will always be in-phase Dixon - Turbo Spin Echo [fatsat_olt_en_dixon-tse01.mp3] As described, Dixon can be used with T-S-E sequences. But actually, only the gradient echo sequences will have the effect of in-phase and opposed-phase echoes. For spin-echo based sequences like the T-S-E, fat and water will always be in-phase. However, with a special trick that we apply we are also able to get the Dixon water-fat separation for a T-S-E sequence. This requires the acquisition of two T-S-E series. In one acquisition the in-phase spin echoes are acquired, so this behaves as a “normal” T-S-E sequence. In the second acquisition the same sequence parameters are used but the echo readout is shifted for each individual spin echo within the echo train by the opposed phase time. With this trick both in- and opposed phase echoes are generated and the Dixon algorithm to separate water and fat can be applied. ? Dixon – Turbo Spin Echo TSE Dixon: scan time is doubled compared to other protocols lower resolution and/or use of parallel acquisition techniques or fewer averages to realize a reasonable scan time Dixon FatSat more robust and homogeneous fat suppression with Dixon Dixon - Turbo Spin Echo [fatsat_olt_en_dixon-tse02.mp3] Because T-S-E Dixon requires two acquisitions, the scan time is doubled compared to a T-S-E sequence without Dixon or a T-S-E sequence with spectral fat saturation or SPAIR. Therefore, T-S-E Dixon protocols may have a lower resolution and/or use parallel acquisition techniques or use fewer averages in order to realize a reasonable scan time. The example here shows a comparison of a T-S-E protocol with spectral fat saturation and a T-S-E protocol with Dixon for a coronal proton density contrast in the knee at 3 Tesla. As can be seen in this particular example, the T-S-E sequence parameters slightly differ in order to achieve similar scan times, i.e., the matrix was reduced from 512 to 448 and the number of averages was changed from two to one. The benefit with Dixon, however, is a more robust and homogeneous fat suppression compared to spectral fat saturation or SPAIR, especially at 3 Tesla and larger fields of view or off-center imaging like hand, wrist or elbow where more magnetic field inhomogeneities will occur. Dixon – Turbo Spin Echo with “Fast Dixon” ? Fast Dixon Acquire in- and opposed phase echoes within a single Turbo Spin Echo acquisition Both echoes are acquired within each echo spacing of the multi echo train Contrast parameter card > Common > Fat-Water Contrast > Fast Dixon To get two echoes consecutively, bandwidth must be increased, and base matrix must be reduced Other changes possible (e.g., increased Field-of-View) Mainly an option for larger Fields-of-View like in the Abdomen Dixon - Turbo Spin Echo with Fast Dixon [fatsat_olt_en_dixon-tse-fast-dixon01.mp3] There is also a way to acquire the two desired in- and opposed phase echoes within a single Turbo Spin Echo acquisition. In this case, both echoes are acquired within each echo spacing of the multi echo train. This option is called “Fast Dixon” and can be selected in the drop-down menu from the “Fat-Water Contrast” parameter. Since two echoes are now acquired instead of one, this has great demands on the gradient system and is only possible under certain conditions. In order to be able to get the two echoes consecutively, the bandwidth must be increased, and the base matrix must be reduced. Other changes may also be possible like an increased field of view. If the current protocol parameters are too demanding the option may not be visible at all. If “Fast Dixon” is selectable and is activated, the system will automatically suggest protocol adaptations. Depending on the protocol but also on the MR system type and gradient system, these changes might be too large for a desired protocol. “Fast Dixon” is mainly an option for larger fields of view like in the abdomen but may not be suited for high resolution small field of view imaging like in smaller joints. ? Dixon – Turbo Spin Echo with “Fast Dixon” Dixon (1 average, TA 2:21) Fast Dixon (2 averages, TA 2:21) Dixon - Turbo Spin Echo with Fast Dixon [fatsat_olt_en_dixon-tse-fast-dixon02.mp3] This is an example where “Fast Dixon” is very useful. Because of the large field of view in this abdominal examination, “Fast Dixon” can be selected with reasonable limitations. It can be used to reduce the acquisition time or, as is the case here, to acquire two averages at the same time as one average with conventional Dixon. With the averaging effect potential motion artifacts can be reduced. Dixon – Summary Advantages: Insensitive to B0 variation Difficult areas (e.g., neck / abdomen) Large FOVs (spine) 4 contrasts available (water, fat, in-phase, opposed phase) Can be used post Gadolinium Preferred fat suppression in abdominal imaging, cervical spine or neck soft tissue Disadvantages: Increased scan time Water-Fat swaps possible opposed-phase in-phase fat image water image ? Images taken from MAGNETOM Flash 1/2011 Dixon - Summary [fatsat_olt_en_dixon-summary.mp3] To summarize, Dixon is one of the most robust fat suppression techniques that exist. Compared to spectral fat saturation or SPAIR it is much less sensitive to the magnetic field inhomogeneities… … and can be used for reliable and homogeneous fat suppression in difficult body regions like the neck or abdomen. It automatically generates up to four different contrasts within one acquisition… … and can be used post contrast agent injections. Because of the benefits, Dixon is the preferred fat suppression in abdominal imaging, especially at 3 Tesla. In other body regions like the cervical spine or neck soft tissue, Dixon is also the preferred fat suppression choice. Compared to STIR, which is also considered a very robust fat suppression technique that is widely independent from the magnetic field homogeneity, Dixon can be used after contrast agent injection which is not suitable for STIR. On the downside Dixon requires prolonged scan times. For example, in a VIBE protocol with Dixon the repetition time must be longer compared to spectral fat saturation or SPAIR in order to acquire two echoes instead of one. Hence the breath-hold time may be increased. However, the longer T-R will increase the Signal-to-Noise ratio of the protocol. In the T-S-E sequence, two acquisitions are required for Dixon, making the scan time longer compared to a spectral fat saturation or a SPAIR protocol if Fast Dixon cannot be applied. In rare cases the water-fat separation can fail causing either a local or global swap. Therefore, water and fat images must be acquired in order to identify these effects. ? Course Review Congratulations. You have completed the Fat Suppression Techniques Online Training. Select the numbered buttons below to review the material before proceeding to the final assessment. If you are interested to learn more about this topic, please have a look at the MAGNETOM Flash Issue 74 (3/2019). Dixon STIR SPAIR Spectral Fat Saturation 1 1 2 2 2 4 4 5 5 Water Excitation 3 3 3 Course Review Dixon Advantages: Insensitive to B0 variation Difficult areas (e.g., neck / abdomen) Large FOVs (spine) 4 contrasts available (water, fat, in-phase, opposed phase) Can be used post Gadolinium Disadvantages: Increased scan time Water-Fat swaps possible opposed-phase in-phase fat image water image Images taken from MAGNETOM Flash 1/2011 STIR 05zp_C28_Abdomen_T2_TIRM_tra_384_G2 T2 TIRM STIR uses non-frequency pulses and utilizes the different T1 times of fat and water to suppress fat. Advantages Difficult regions with increased magnetic field inhomogeneities like neck, c-spine, breast, abdomen, whole body Disadvantages Water signal is reduced compared to other fat suppression techniques Not applicable post contrast agent injection Additional SAR and time required for inversion Water Excitation Advantages Reduced sensitivity to B1 inhomogeneity Less SAR warnings Applicable post contrast agent injection Disadvantages Sensitive to B0 variations Increases scan time SPAIR Advantages Insensitive to B1 variations Water signal is not affected Can be used post contrast agent injection Can be combined with most pulse sequences and contrasts Special optimizations for specific body parts at 3T Disadvantages Sensitive to B0 variations Increases scan time Increases SAR Spectral Fat Saturation (FS) Fat FatSat Water unsuppressed fat, partial water saturation Fat FatSat Water Field Inhomogeneities Disadvantages Increases scan time Sensitive to B0 variations Sensitive to B1 variations Advantages Can be combined with most pulse sequences and contrasts Can be used post contrast agent injection Weak or strong fat saturation possible Water signal is not affected Disclaimer ? Please note that the learning material is for training purposes only. For the proper use of the software or hardware, please always use the Operator Manual or Instructions for Use (hereinafter collectively “Operator Manual”) issued by Siemens Healthineers. This material is to be used as training material only and shall by no means substitute the Operator Manual. Any material used in this training will not be updated on a regular basis and does not necessarily reflect the latest version of the software and hardware available at the time of the training. The Operator Manual shall be used as your main reference, in particular for relevant safety information like warnings and cautions. Please note: Some functions shown in this material are optional and might not be part of your system. Certain products, product related claims or functionalities (hereinafter collectively “Functionality”) may not (yet) be commercially available in your country. 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You will have 3 attempts to take this assessment and to successfully pass this course. You must receive a score of 80% or higher. You will receive your score once you complete the assessment. Start Assessment Select three (3) answers. ? Question 1 of 10 Which of the following methods are suitable for suppressing fat signal in MRI? STIR SPAIR SPACE Dixon Multiple Answer Select the best answer. ? Question 2 of 10 Which method uses the different T1 times of water and fat to suppress the fat signal? STIR STIR STIR Dixon Dixon Dixon Spectral Fat Suppression Water Excitation Multiple Choice Select the best answer. ? Question 3 of 10 _____ can be used to reduce the acquisition time or acquire two averages at the same time. DIxon Dixon SPAIR STIR Fast Dixon Multiple Choice Select the best answer. ? Question 4 of 10 What is the frequency difference of fat and water at 1.5T? 220 Hz 220 Hz 220 Hz 440 Hz 440 Hz 440 Hz 660 Hz 660 Hz 660 Hz 110 Hz 110 Hz 110 Hz Multiple Choice Select the best answer. ? Question 5 of 10 Which method requires you to measure two contrasts at two different echo times, where one echo is acquired in the opposed-phase state and the other in the in-phase state of water and fat? SPACE SPACE STIR Dixon SPAIR Multiple Choice Select the best answer. ? Question 6 of 10 How many RF pulses are used for the regular Water Excitation (not Fast Water Excitation)? 3 3 3 1 1 1 2 2 2 4 4 4 Multiple Choice Select the best answer. ? Question 7 of 10 With _____, homogeneous fat suppression in both breasts can be achieved 3 3 SPAIR STIR Dixon SPACE Multiple Choice Select the best answer. ? Question 8 of 10 Which sequences use “Fast Fat Saturation”, where several lines per shot are acquired? VIBE VIBE VIBE TSE TSE TSE SE SE SE SPACE SPACE SPACE Multiple Choice Select the best answer. True Question 9 of 10 Inhomogeneities are also induced by the magnetic properties of the human body such as tissue-bone or tissue-air interfaces. False Multiple Choice Select the best answer. STIR Stack Question 10 of 10 A special fat suppression mode called ____ is available to improve the fat saturation in more inhomogeneous areas, such as the foot. Joint Ortho Multiple Choice Retry Assessment Results %Quiz1.ScorePercent%% %Quiz1.PassPercent%% Continue YOUR SCORE: PASSING SCORE: Assessment Results You have exceeded your number of assessment attempts. Exit You did not pass the course. Select Retry to continue. Congratulations. You passed the course. Exit To access your Certificate of Completion, select the Launch button drop down on the course overview page. You can also access the certificate from your PEPconnect transcript. You have completed the Fat Suppression Techniques Online Training. Completion Navigation Help Select the icon above to open the table of contents. Click Next to continue. Next Welcome Slide The timeline displays the slide progression. Slide the orange bar backwards to rewind the timeline. Click Next to continue. Next Tmeline Some images may have a magnifier icon. Select the image to see an enlarged view. Select it again to return to the normal view. Click Next to continue. Next Zoom Slide Some images have a magnifier icon in the bottom-left corner. Select these image to see an enlarged view of the image. Select the image again to return to the normal view. Select Submit to record your response. Click the X in the upper right corner to exit the navigation help. Assessment Slide Question Bank 1 HOOD05162003328000 | Effective Date: 31 Jan 2023 Here are some useful links and documents: