CT Basics - USA

Continue Continue Continue Continue CT Basics Master Template HOOD05162003052540 | Effective Date: 26-Nov-2019 CT Basics Online Training 1 2 3 4 Identify the System Components of a CT Scanner Explain the Functionality of a CT Scanner Describe the Image Reconstruction Parameters Memorize the Radiation Dose Parameters and Definitions This online training will introduce you to the main features and functionalities of CT scanners and scanning, and will cover the following four learning objectives: Welcome [audf_001.mp3] Welcome to the CT Basics Online Training.   This online training will introduce you to the main features and functionalities of CT scanners and scanning, and will cover the following four learning objectives: …    A picture containing indoor, table, sitting, computer Description generated with very high confidence CT Workplace The standard CT workplace is divided into two functional areas:  Scan room Control room The scan room contains the computer tomograph machine. The control room is separated by a radiation protection wall and consists of the radiographer’s workplace (AWP and control box) and may contain optional additional workstations for image post-processing. A desk with a computer on a table Description generated with high confidence Scan room Control room Patient Handling System (PHS) Gantry Acquisition Workplace (AWP) Control box CT Workplace [audf_002.mp3] The standard CT workplace is divided into two functional areas: the scan room and the control room.   The scan room contains the computer tomograph machine.   The control room is separated by a radiation protection wall and consists of the radiographer’s workplace (with the Acquisition Workplace (AWP) and the control box) and optional additional workstations for image post-processing. These add-ons depend on the specific design of the CT facility and might, for example, include a CT workplace or a syngo.via workstation. System Components of a CT Scanner System Components of a CT Scanner Introduction System Components This chapter will introduce you to the system components of a CT scanner with a special focus on: X-ray tube Detector Two scanning advancements made possible by modern detector design will be described: Multislice scanning Acquisition modes Introduction System Components [audf_003.mp3] This chapter will introduce you to the system components of a CT scanner with a special focus on the X-ray tube and the detector.   Two scanning advancements made possible by modern detector design will be described: Multislice scanning and Acquisition modes Overview of System Components Gantry Contains X-ray tube and detector Laser light marker For patient positioning (Isocenter!) Left-side part Contains tilting mechanism (if available) Gantry operator panel For controlling gantry functions and table movement Right-side part Contains power supply, electronics for gantry and table, and cooling connection Patient Handling System For patient positioning Overview of System Components [audf_004.mp3] The computer tomograph consists of a stationary and a rotating part.   The rotating part contains the X-ray tube, detector, several filters and fans, and electronic components (for example those associated with the data transfer).   The stationary part includes further electrics, connections for power supply and cooling, and – for some scanners – the tilting mechanism.   The patient table (also called “Patient Handling System”) is mounted in front of the gantry. X-Ray Tube The following parameters are generated and controlled by the X-ray tube: Tube voltage Tube current Focal spot The tube voltage determines the X-ray spectrum and the maximum energy of the photons. The tube current in combination with the exposure time influences the amount of the photons (mAs product). The focal spot affects the spatial resolution and, in a way, the lifetime of the anode material. Example of a Siemens Vectron X-ray tube X-Ray Tube [1.6 X-Ray Tube_v2.mp3] - By the x-ray tube the following parameters are generated and controlled: > Tube voltage > Tube current > Focal spot - The tube voltage determines the x-ray spectrum and the maximum energy of the photons - The tube current in combination with the exposure time influences the amount of the photons (mAs product) - The focal spot affects the spatial resolution and in a way the lifetime of the anode material Detector Collimator made of tungsten shielding: Suppression of scattered radiation Ultra-Fast Ceramic Detector (UFC): Conversion of X-rays into visible light Photodiode: Conversion of visible light into electrical current A/D (analog/digital) converter: Conversion of electrical signals into digital data Detector [audf_006.mp3] The detector is located on the opposite side of the X-ray tube. It is the other main part of the computer tomograph that is incorporated into the rotating gantry.   The detector converts the X-ray photons into digital data in several steps: - First, the collimator made of tungsten shielding suppresses scattered radiation. - Second, the Ultra-Fast Ceramic Detector (UFC) converts X-rays into visible light. - Third, the photodiode converts visible light into electrical current. and - Fourth, the analog-to-digital converter transforms electrical signals into digital data. Advantage of Multislice CT Scanner The first scanners were equipped with a single-row detector. That means one image is acquired in each rotation. Nowadays, all CT scanners have multi-row detectors. That means multiple images are acquired during a single rotation. This may lead to a reduced scan time and results in the following advantages : Shorter breath hold is required Fewer motion artifacts Multiphase contrast-enhanced scanning is possible einzeiler spiral mehrzeilerspiral Scan with a single-row detector in spiral mode Scan with a multi-row detector in spiral mode Advantage of Multislice CT Scanner [audf_007.mp3] The first scanners were equipped with a single-row detector. That means one image is acquired in each rotation.   Nowadays, all CT scanners have multi-row detectors. That means multiple images are acquired during a single rotation. This may lead to a reduced scan time and results in the following advantages:   A shorter breath hold is required and Fewer motion artifacts occur   Furthermore, multi-row detectors enable multiphase contrast-enhanced scanning. Detector Design and Acquisition Modes Depending on the detector design, different acquisition modes are possible. For most examinations the full detector size is used. But for some special examinations only a part of the detector is used for data acquisition, e.g. in Neuro mode. This avoids geometrical distortion, especially for tiny objects. With the smallest element size (0.6 mm), a thinner reconstruction is possible, e.g. with a slice thickness of 1 mm down to 0.6 mm. Element Design, e.g. of a SOMATOM Edge Plus, SOMATOM X.cite or SOMATOM go.Top Detector Design and Acquisition Modes [audf_008.mp3] Depending on the detector design, different acquisition modes are possible.   For most examinations the full detector size is used. But for some special examinations only a part of the detector is used for data acquisition, for example in “Neuro” mode. This avoids geometrical distortion, especially for tiny objects.   With the smallest element size of 0.6  millimeters, a thinner reconstruction is possible, for example with a slice thickness of 1 millimeter down to 0.6 millimeters. Functionality of a CT Scanner Functionality of a CT Scanner Introduction Scanner Functionality This chapter will address the functionality of a CT scanner by covering the topics: Basic image generation process Sequential versus spiral scanning Automatic scanning parameter adjustments with SureView Introduction Scanner Functionality [audf_009.mp3] This chapter will address the functionality of a CT scanner. After covering the basics of the image generation process, it will discuss sequential versus spiral scanning and the automatic adjustment of the scan parameters using the SureView concept. Image Generation Process The X-ray tube generates the radiation. The rays penetrate the body and are attenuated by the tissue. The attenuated X-ray beams are processed by the detector. The detector converts the radiation into electrical signals. The computer converts this digital data into high-resolution images. Detector X-ray tube Image Generation Process [audf_010.mp3] During the CT examination, the X-ray tube and the detector rotate around the patient in the gantry.   Rotation by rotation, axial slices of the body are captured, one at a time. The sequence of the image generation steps is as follows: - The X-ray tube generates the radiation. - The rays penetrate the body and are attenuated by the tissue. - The attenuated X-ray beams are processed by the detector. - The detector converts the radiation into electrical signals. and - The computer converts this digital data into high-resolution images.   Image Elements Voxel Pixel The image generated by the CT scanner is a digital image and consists of a square matrix of elements (pixels), each of which represents a voxel (3D volume element) of the patient’s tissue. Image Elements [audf_011.mp3] The image generated by the CT scanner is a digital image and consists of a square matrix of elements (called “pixels”), each of which represents a ”voxel” (that is, a 3D volume element) of the patient’s tissue.   Sequential Scanning This is an incremental, slice-by-slice imaging mode without table movement during data acquisition. This scan type results in interscan delays between each acquisition, caused by the movement of the table to the next slice position. With Multislice CT scanners, sequential scanning is performed using different combinations of detector rows and slice collimations. Therefore, one or more slices at different slice widths can be reconstructed from a single multislice sequential acquisition. Scan Scan Scan Scan Scan Move Move Move Move Sequential Scanning [audf_012.mp3] Sequential scanning is an incremental, slice-by-slice imaging mode without table movement during data acquisition. This scan type results in interscan delays between each acquisition, caused by the movement of the table to the next slice position. With Multislice CT scanners, sequential scanning is performed using different combinations of detector rows and slice collimations. Therefore, one or more slices at different slice widths can be reconstructed from a single multislice sequential acquisition. Disadvantage of Sequential Scanning The interscan delays prolong the scan times. Another disadvantage is that sequential scans are prone to slice misregistration. This is caused by scanning at different respiratory levels. Thus, common routine applications of sequential scanning are limited to head and spine imaging because the motion artifacts in these regions are minimal. Most other routine and advanced CT examinations are typically performed using the spiral scan mode. A close up of a logo Description generated with very high confidence Disadvantage of Sequential Scanning [audf_013.mp3] The interscan delays prolong the scan times. Another disadvantage is that sequential scans are prone to slice misregistration. This is caused by scanning at different respiratory levels. Thus, common routine applications of sequential scanning are limited to head and spine imaging because the motion artifacts in these regions are minimal. Most other routine and advanced CT examinations are typically performed using the spiral scan mode. Spiral Scanning This is a continuous volume imaging mode. Data acquisition and table movement are performed simultaneously for the entire scan duration. A typical range can be acquired in a single breath hold. Each acquisition provides a complete volume data set, from which images with overlapping slices can be reconstructed at any slice position. Conventional CT Spiral CT Spiral Scanning [audf_014.mp3] Spiral scanning is a continuous volume imaging mode. Data acquisition and table movement are performed simultaneously for the entire scan duration. A typical range can be acquired in a single breath hold. Each acquisition provides a complete volume data set, from which images with overlapping slices can be reconstructed at any slice position.   Advantage of Spiral Scanning Compared to sequential scans, spiral scans offer the following advantages: Examination within one breath hold Shorter scan times facilitate motion artifact reduction and lower the need for contrast medium Better differentiation of contrast-enhanced phases, e.g., arterial and venous phases Better method for 3D reconstructions when scanning restless patients split_second_thorax Advantage of Spiral Scanning [audf_015.mp3] Compared to sequential scans, spiral scans offer the following advantages: - The examination can be completed within one breath hold. - Shorter scan times facilitate the reduction of motion artifacts and lower the need for contrast medium. - There is a better differentiation of contrast-enhanced phases, for instance of arterial and venous phases. - It is a better method for 3D reconstructions when scanning restless patients.   Pitch The pitch (P) is a spiral scan parameter that determines the speed of the table movement and the speed of the acquisition. The higher the pitch, the faster the table feed and the shorter the scan time. Generally, the pitch for single-source scanners is between 0.35 and 1.7 and can be selected in increments of 0.05. Pitch values (approx.): P 0.5 for high detail detectability P 1 for standard examinations P 1.5 for high scan speed (reduction of breathing or motion artifacts) Pitch 0.5 Pitch 1.0 Pitch 1.5 Pitch [audf_016.mp3] The pitch is a spiral scan parameter that determines the speed of the table movement and the speed of the acquisition. The higher the pitch, the faster the table feed and the shorter the scan time.   Generally, the pitch for single-source scanners is between 0.35 and 1.5  and can be selected with an increment of 0.05 . Pitch and SureView Concept When increasing the pitch, the tube current (mA) must also be increased to obtain the same image noise or dose. This is done automatically with the so-called SureView concept. Due to the effective mAs*, the pitch factor no longer has any influence on the image noise because mA is automatically adjusted according to the pitch. The tube current is adjusted to the table feed: Table feed high  higher tube current Table feed low  lower tube current * Effective mAs = mA x Rotation Time Pitch Pitch 1 = no overlap Pitch 0.5 = 50% overlap Z-axis _________________ Pitch and SureView Concept [audf_017.mp3] When increasing the pitch, the tube current (that is, the mA  value) must also be increased to obtain the same image noise or dose. This is done automatically with the so-called SureView concept.   Due to the effective mAs , the pitch factor no longer has any influence on the image noise because mA is automatically adjusted according to the pitch.   The tube current is adjusted to the table feed in the following way: When the table feed is high, higher tube current is applied. When the table feed is low, lower tube current is applied.   When the table feed is low, lower tube current is applied. Image Reconstruction Parameters Image Reconstruction Parameters Introduction Image Reconstruction Parameters This chapter focuses on a set of essential parameters that are involved in the reconstruction of high-resolution images from the detector-generated digital data: Field of View (FoV) (Convolution) Kernel Slice Thickness Increment CT Number CT Window Generated Introduction Image Reconstruction Parameters [audf_018.mp3] This chapter focuses on a set of essential parameters that are involved in the reconstruction of high-resolution images from the detector-generated digital data:   - Field of View - Convolution Kernel (or short: “Kernel”) - Slice Thickness - Increment - CT Number and - CT Window   Field of View (FoV) The anatomical area displayed within the CT image is called Field of View (FoV). The dimension of such a CT image is determined by the matrix size. The spatial resolution can be generally improved by using a smaller FoV because it reduces the pixel size. However, images should always be reconstructed with an appropriate FoV that is adjusted to the anatomical structure of interest. Thus, the visual impression can be improved without over-magnification. Generated (1) FoV = 40cm (2) FoV = 8cm Field of View (FoV) [audf_019.mp3] The anatomical area displayed within the CT image is called Field of View. The dimension of such a CT image is determined by the matrix size. The spatial resolution can be generally improved by using a smaller Field of View because it reduces the pixel size. However, images should always be reconstructed with an appropriate Field of View that is adjusted to the anatomical structure of interest. Thus, the visual impression can be improved without over-magnification.   The Field of View is limited by the maximum size of the scan measurement fields. Generally, you can reconstruct a maximum Field of View of 50 centimeters for the body and of 30 centimeters for the head.   To improve the spatial resolution, the Field of View should always be tailored to the object of interest. That means it should be as small as possible. But take care when defining the Field of View because significant findings may be excluded from the images and missed during data interpretation. Usually the first reconstruction is done with a large Field of View as an overview and additional reconstructions are done with a smaller Field of View according to the organs of interest, for example only one of the lungs or single vertebras.   Kernel The (convolution) kernel is a reconstruction parameter affecting image sharpness and noise. The kernel applies a specific mathematical algorithm that digitally filters the raw data during image reconstruction. The image sharpness is defined by the numeric values: the higher the kernel number, the sharper the image but the greater the image noise. The letter after the number indicates the scan mode: "s“ stands for standard mode, "f" for fast mode, and "h" for high-resolution mode. Table with 3 columns and 5 rows Image Appearance Typical Kernel Numbers Visualization/Purpose Smooth 10 - 20 3D post-processing, noise reduction with thin slices Medium 30 - 50 General soft tissue display Sharp 60 - 70 Lung or bone visualization with edge enhancement High Resolution 80 - 90 High spatial resolution assessment of minute structure, e.g., inner ear Raw data Image data Kernel (e.g. B31s) Kernel [audf_020.mp3] The convolution kernel (or short: “kernel”) is a reconstruction parameter affecting image sharpness and noise. The kernel applies a specific mathematical algorithm that digitally filters the raw data during image reconstruction. The image sharpness is defined by the numeric values in the kernel name: the higher the kernel number, the sharper the image but the greater the image noise. The letter after the number indicates the scan mode: "s" stands for standard mode, "f" for fast mode, and "h" for high-resolution mode. Updated Kernel Concept (since 2013) The kernel concept changed in 2013 for Siemens Healthineers CT scanners in the Somaris 5 and 7 platforms, and on all systems going forward, an updated kernel labeling system will be used. The first upper-case letter stands for the kernel family, the second lower-case letter stands for the kernel group. The resolution, which refers to the image sharpness, is defined by the subsequent number. The overall principle is the same as before: The higher the number, the sharper the image impression. *Somaris 7 platform (including SOMATOM Definition AS, SOMATOM Confidence, SOMATOM Definition AS+, SOMATOM Definition Edge, SOMATOM Edge Plus, SOMATOM Definition Flash, SOMATOM Drive, SOMATOM Force) Position 1 : Kernel Family Pos 1 B Pos 2 r Pos 3 32 S = Special Kernel H = Head B = Body Q = Quantitative Sb = Special Body 49 Hc = Head Crisp 40 - 44 Qr = Quantitative Regular 32 - 69 Sh = Special Head 49 Bf = Body Fine Noise 32 - 44 Bl = Body Lung Special 57 - 64 Br = Body Regular 32 - 69 Bv = Body Vascular 36 - 59 Hf = Head Fine 38 - 40 Hp = Head Pediatric 38 Hr = Head Regular 32 - 69 Ub = UHR Body 36 - 59 U = UHR Uh = UHR Head 36 - 59 Ul = UHR Lung Special 57 - 64 Ur = UHR Regular 69 - 89 Position 2 : Kernel Group, Position 3 : Resolution Updated Kernel Concept (since 2013) [1.24 Updated Kernel Concept.mp3] The kernel concept changed in 2013 for Siemens Healthineers CT scanners in the Somaris 5 and 7 platforms, and on all systems going forward, an updated kernel labeling system will be used.   The first upper-case letter stands for the kernel family, the second lower-case letter stands for the kernel group. The resolution, which refers to the image sharpness, is defined by the subsequent number. The overall principle is the same as before: The higher the number, the sharper the image impression.  Slice Thickness (Width) This parameter describes the thickness of the reconstructed images. Slice width should not be confused with collimation. However, for single-slice CT imaging these terms are regarded as synonyms because in this case only images with a width equal to scan collimation can be reconstructed. With MSCT imaging, the possible slice widths depend on the chosen collimation and scan mode. Several slice widths can be selected post-acquisition by combining data from the various detector rows to reconstruct the axial images. Generated Generated (1) 0.75mm slice width, more noise, but more details in the image (2) 5mm slice width, smooth image with excellent soft tissue contrast (1) 0.6mm slice, excellent image details (2) 3.0 mm slice, poor image details Slice Thickness (Width) [audf_022.mp3] The parameter slice thickness (also known as slice width) describes the thickness of the reconstructed images.   Slice width should not be confused with collimation. However, for single-slice CT imaging these terms are regarded as synonyms because in this case only images with a width equal to scan collimation can be reconstructed.   With multi-slice CT imaging, the possible slice widths depend on the chosen collimation and scan mode. Several slice widths can be selected post-acquisition by combining data from the various detector rows to reconstruct the axial images. A narrower reconstructed slice width results in improved axial resolution but also increased noise. You can see the effects in the images on the right side. Increment Increment is the distance (in mm) between the reconstructed images in z-direction. It can be freely adapted from 0.1 mm upwards for spiral or 3D sequential scanning. If the increment is smaller than the slice width, the images are created with overlap. This provides better detail of the anatomy as well as high quality 2D and 3D post-processing. For routine 2D viewing, image series with contiguous increment are typically reconstructed. This reduces the total number of images for documentation and archiving. Two adjacent slices: CT value underrated Object overlooked Smaller objects are overlayed (1) (2) (2) Three slices with 50% overlap: Full density at least in one slice  correct CT value Real size of the object More details are visible Increment [audf_023.mp3] Increment is the distance (in millimeters) between the reconstructed images in z -direction. It can be freely adapted from 0.1  millimeters upwards for spiral or 3D sequential scanning.   If the increment is smaller than the slice width, the images are created with overlap. This provides better detail of the anatomy as well as high quality 2D and 3D post-processing.   For routine 2D viewing, image series with contiguous increment are typically reconstructed. This reduces the total number of images for documentation and archiving. CT Number (Hounsfield Units) The CT value is used to represent the mean X-ray attenuation value of the corresponding voxel within the slice. CT numbers are normally expressed in terms of Hounsfield units (HU). The CT number scale is defined so that water has a value of 0 HU; denser substances are given positive values and less dense materials negative numbers. Normally, CT values are measured in the range from -1024 to +3071, with values around -1000 HU representing air, and the highest numbers corresponding to very dense materials such as bones. Air 1000 Water Fat HU -1000 -500 Bone 0 500 Soft tissue CT Number (Hounsfield Units) [audf_024.mp3] The CT value is used to represent the mean X-ray attenuation value of the corresponding volume element (or: voxel) within the slice. CT numbers are normally expressed in terms of Hounsfield units. The CT number scale is defined so that water has a value of 0 Hounsfield units; denser substances are given positive values and less dense materials negative numbers. Normally, CT values are measured in the range from minus 1024 to plus 3071, with values around minus 1000 Hounsfield units representing air, and the highest numbers corresponding to very dense materials such as bones.   CT Window Since the human eye is only able to distinguish approximately twenty gray tones, it is not practical to always represent all possible HU in a single comprehensive grayscale. Therefore, CT Window limits the image display to tissue of interest. This results in a better differentiation of tissue with similar densities. All pixels with values above this range are represented in white, and those below this range in black. The Window Center (also called Window Level) and the Window Width define this range of CT numbers displayed in grayscale. Generated Typical window settings for visualization of the soft tissue Mediastinum and lung parenchyma: (1) Soft tissue window (W= 350, C= 40) reveals structures within the Mediastinum but the lung parenchyma cannot be seen (2) Lung window (W= 1500, C= -600) depicts details of the lung parenchyma, but poor visualization of the Mediastinum (1) (2) CT Window [audf_025.mp3] Since the human eye is only able to distinguish approximately twenty gray tones, it is not practical to always represent all possible Hounsfield units in a single comprehensive grayscale. Therefore, the parameter CT Window limits the image display to tissue of interest. This results in a better differentiation of tissue with similar densities.   All pixels with values above this range are represented in white, and those below this range in black.   The Window Center (also called “Window Level”) and the Window Width define this range of CT numbers displayed in grayscale. Radiation Dose Parameters and Definitions Radiation Dose Parameters and Definitions Introduction Radiation Dose The last chapter of this training is dedicated to the important subject of radiation dose and covers the topics: Background radiation Radiation dose in CT Patient dose parameters Radiation dose definitions (absorbed, equivalent, effective dose) Introduction Radiation Dose [audf_026.mp3] The last chapter of this training is dedicated to the important subject of radiation dose.   It gives you an overview of the exposure to background radiation and the dose considerations applying to CT procedures. Furthermore, it explains patient dose parameters and provides dose definitions for the absorbed, equivalent, and effective dose. Background Radiation Humans are constantly exposed to natural radiation (cosmic and terrestrial). This dose varies, depending on the location, between 1 mSv and 6 mSv per year (average 2.4 mSv per year). If the artificial radiation by medical imaging procedures and human nuclear activities is added, the average radiation dose received is about 4 mSv per year for German and 6.2 mSv per year for US citizens. Effective Dose in mSv Average Annual Natural Background Radiation 2.4 0.13 X-Ray Chest Standard CT Chest 0.09 0.13 CT Chest Low Dose CT Cardiac Background Radiation [audf_027.mp3] Humans are constantly exposed to natural radiation (cosmic and terrestrial). This dose varies, depending on the location, between 1 and 6 milli sieverts per year (with an average of 2.4 milli sieverts per year).   If the artificial radiation by medical imaging procedures and human nuclear activities is added, the average radiation dose received is about 4 milli sieverts per year for German and about 6.2 milli sieverts per year for US citizens. Radiation Dose in CT Computed Tomography is a very valuable diagnostic tool, and it provides much more information than a conventional radiograph. However, the dose in CT can be much higher compared to conventional examinations. Therefore, it is important to limit the patient dose by using dose reduction techniques and applications. Patients should not be exposed to excessive irradiation, and the clinical indications must be verified before performing a CT exam. The common practice is to apply the ALARA principle. Radiation Dose in CT [audf_028.mp3] Computed Tomography is a very valuable diagnostic tool and it provides much more information than a conventional radiograph. However, the dose in CT can be much higher compared to conventional examinations.   Therefore, it is important to limit the patient dose by using dose reduction techniques and applications (for example CareDose 4D). Patients should not be exposed to excessive irradiation, and the clinical indications must be verified before performing a CT exam.   The common practice is to apply the ALARA principle (where ALARA stands for: “As Low As Reasonably Achievable”). The aim of ALARA is to minimize the risk of radioactive exposure to a level that is necessary to obtain the desired diagnostic information. Dose Overview and Absorbed Dose We all have an intuitive understanding of what “dose” is, but a radiation dose that reflects the potential damage to organic tissue cannot be defined simply as a certain amount of radiation energy per kg or sq. cm of body surface. That is why three different definitions are used: absorbed, equivalent, and effective dose. The energy dose or absorbed dose (D) characterizes the amount of energy deposited in matter after being exposed to a certain amount of radiation. What is Dose? Equivalent Dose H Absorbed Dose D Effective Dose E Definition D = Absorbed Radiation Energy/kg of matter; measured in 1 Gy = 1 J/kg Physical Definition: When we irradiate 1 kg of water with 1 Gy, the water stores 1 J and ist temperature increases by only 0.00024°C. Dose Overview and Absorbed Dose [audf_029.mp3] We all have an intuitive understanding of what “dose” is, but a radiation dose that reflects the potential damage to organic tissue cannot be defined simply as a certain amount of radiation energy per kilogram or square centimeter of body surface. That is why three different definitions are used: absorbed, equivalent, and effective dose.   The energy dose or absorbed dose (D) characterizes the amount of energy deposited in matter after being exposed to a certain amount of radiation.   However, this physical definition is not suited for living organisms since it does not reflect the biological effects. The type of radiation and the damage it might cause in different tissues have to be taken into account. Equivalent Dose The biological damages caused by different types of radiation are not the same. Therefore, even if an absorbed dose of X-rays or α-rays is similar, the damage can be dramatically different. The equivalent dose (H) for any type of radiation is defined as the absorbed dose (D) multiplied by a factor (wr) that weights the damage caused to biological tissue by a particular type of radiation. In the case of X-rays used in CT, the weighting factor is 1. Therefore, the equivalent dose is the same as the absorbed dose. What is Dose? Equivalent Dose H Absorbed Dose D Effective Dose E The equivalent dose H is: H = D x wr where wr is an estimate of the amount of biological damage caused by 1 Gy of the corresponding type of radiation (for X-rays: wr = 1; for α-rays: wr = 20). The unit used to measure the equivalent dose is the sievert (Sv). Equivalent Dose [audf_030.mp3] The biological damages caused by different types of radiation are not the same. Therefore, even if an absorbed dose of X-rays or α-rays  is similar, the damage can be dramatically different. The equivalent dose (H) for any type of radiation is defined as the absorbed dose (D) multiplied by a factor (wr ) that weights  the damage caused to biological tissue by a particular type of radiation. In the case of X-rays used in CT, the weighting factor is 1. Therefore, the equivalent dose is the same as the absorbed dose. In the case of α–rays (that occur naturally and are emitted, for example, by some types of uranium isotopes) the absorbed dose has to be multiplied by a factor of 20. This indicates that α-rays cause much more damage to biological tissue. Please mind the „t“ in this word. It is the verb “to weight” and not the verb “to weigh”. wr and f later on are weighting factors. Patient Dose Parameters Three main quantities that describe the patient dose in CT: • CTDIvol (Volume CT Dose Index) • DLP (Dose-Length Product) • Effective Dose (E) The CTDIvol describes a local dose, or "dose density": energy accumulated per mass. Contrary to common thinking, the CTDIvol does not change when the scan length (coverage) is increased. However, the DLP and the effective dose values take the scan length into consideration. They are more consistent with the general understanding of the patient dose. Table with 4 columns and 4 rows Quantity Description Meaning Unit CTDIvol Average dose over the total volume scanned for the selected CT conditions of operation Basic dose parameter in CT mGy DLP Product of the CTDIvol and the scan range Main descriptor of the total energy deposited in the body mGy*cm Effective Dose Average dose to the whole body, which is the weighted average of all affected organs Describes the radiation risk mSv Patient Dose Parameters [audf_031.mp3] In radiology, the term "dose" or "patient dose" is always related to the absorbed dose: a measure of the energy deposited in a patient‘s body by ionizing radiation. There are three main quantities that describe the patient dose in CT: • CTDIvol  (the Volume CT Dose Index) • DLP (the Dose-Length Product) and • Effective Dose (E) The CTDIvol describes a local dose, or "dose density": energy accumulated per mass. Contrary to common thinking, the CTDIvol does not change when the scan length (the coverage) is increased. However, the DLP and the effective dose values actually do take the scan length into consideration. They are more consistent with the general understanding of the patient dose. Volume CT Dose Index (CTDIvol) This is a measure of the dose absorbed during a CT examination. It is a local quantity describing the energy deposited in a unit of mass. The CTDIvol is calculated from the integral of the dose profile produced in a single axial scan corrected for the weight of the patient and the pitch factor. The CTDIvol gives a good estimation of the average dose applied in the scanned volume, as long as the patient size (cross section) is similar to the size (diameter) of the respective dose phantoms. Since the body size can be smaller or larger than the 32-cm plexiglass phantom, the CTDIvol value displayed can deviate from the dose in the scanned volume. Volume CT Dose Index (CTDIvol) [audf_032.mp3] The CTDIvol is a measure of the dose absorbed during a CT examination. It is a local quantity describing the energy deposited in a unit of mass. The CTDIvol is calculated from the integral of the dose profile produced in a single axial scan corrected for the weight of the patient and the pitch factor. The CTDIvol gives a good estimation of the average dose applied in the scanned volume, as long as the patient size (that is: the cross section) is similar to the size (that is: the diameter) of the respective dose phantoms. Since the body size can be smaller or larger than the 32-centimeter plexiglass phantom, the CTDIvol value displayed can deviate from the dose in the scanned volume. Dose-Length Product (DLP) Generated DLP = CTDIvol x L * * L = Length of scan range in cm The X-ray tube and the detector scan the patient along L (examination range) on the z-axis. The product of CTDIvol and the length L (cm) of the scan range is called Dose-Length Product (DLP). In order to calculate the total absorbed dose for a complete CT examination, the scan range that is being examined must be taken into account. Because DLP takes into consideration the geometrical extent of irradiation, it is considered to be a better indicator of the patient dose than CTDIvol. It describes the total energy deposited in the body. Therefore, it is often kept with the patient record. With the use of specific organ-related factors, DLP can be used to calculate the effective dose. Dose-Length Product (DLP) [audf_033.mp3] The product of CTDIvol and the length L (in centimeters) of the scan range is called Dose-Length Product (DLP).   In order to calculate the total absorbed dose for a complete CT examination, the scan range that is being examined must be taken into account. The Dose-Length Product is measured in milli gray centimeters. Because DLP takes into consideration the geometrical extent of irradiation, it is considered to be a better indicator of the patient dose than CTDIvol.   It describes the total energy deposited in the body. Therefore, it is often kept with the patient record. When multiple series are performed, the DLP value for all series should be added to obtain the total DLP, which reflects the total dose for the entire examination. With the use of specific organ-related factors, DLP can be used to calculate the effective dose.   Effective Dose The weighted average of Organ Dose values for the irradiated organs is defined as the effective dose (E) expressed in mSv. It describes the stochastic radiation risk. The Organ Dose is the product of the average absorbed dose in an organ, multiplied with a weighting factor. This factor indicates the effects of the different types of radiation, such as X-rays or ionizing neutrons. The effective dose can also be estimated by multiplying the DLP with a conversion factor determined from measurements or computer simulations. The conversion factors (f) are reported by the European Commission. E = DLP x f Region of body Head Head & Neck Neck Chest Abdomen & Pelvis Trunk Conversion factor, f (mSv/(mGy cm) 0.0021 0.0031 0.0059 0.014 0.015 0.015 Region of body Head Neck Thorax Abdomen Pelvis Cardio Calculated effective organ dose (mSv) Dual Source CT 0.1 - 1.8 1.2 0.7 - 4.0 3.8 - 4.5 2.6 - 5.3 1.0 - 9.2 Single Source CT 0.2 - 1.7 1.9 - 2.7 0.4 - 3.5 3.8 - 4.8 1.5 - 4.6 0.9 - 4.2 Effective Dose [audf_034.mp3] The weighted average of Organ Dose values for the irradiated organs is defined as the effective dose (E) expressed in milli sieverts. It describes the stochastic radiation risk. The Organ Dose is the product of the average absorbed dose in an organ, multiplied with a weighting factor. This factor indicates the effects of the different types of radiation, such as X-rays or ionizing neutrons. The effective dose can also be estimated by multiplying the DLP with a conversion factor determined from measurements or computer simulations. The conversion factors (f) are reported by the European Commission. The calculation of E using Siemens default protocols depends on the CT source.   Describe the Image Reconstruction Parameters Explain the Functionality of a CT Scanner Identify the System Components of a CT Scanner Course Review Congratulations. You have completed the CT Basics course. Select the objectives listed below to review the material before proceeding to the final assessment. 1 1 1 2 2 2 3 3 3 Memorize the Radiation Dose Parameters and Definitions 4 4 Course Review Memorize the Radiation Dose Parameters and Definitions Exposure to background radiation Radiation dose in CT: Important to limit the patient dose by using dose reduction techniques and applications. Patients should not be exposed to excessive irradiation. Common practice: ALARA principle (As Low As Reasonably Achievable = Minimize risk of radioactive exposure to a level that is necessary to obtain the desired diagnostic information). Absorbed dose (D, energy dose): Amount of energy deposited in matter after being exposed to a certain amount of radiation. Equivalent dose (H): Defined as the absorbed dose (D) multiplied by a factor (wr) that weights the damage caused to biological tissue by a particular type of radiation. For X-rays used in CT, the weighting factor is 1  Equivalent dose = Absorbed dose. Effective dose (E in mSv): Defines the weighted average of Organ Dose values for the irradiated organs. Describes the stochastic radiation risk. Patient dose parameters: Three main quantities that describe the patient dose in CT CTDIvol (Volume CT Dose Index in mGy): Average dose over the total volume scanned for the selected CT conditions of operation, basic dose parameter in CT DLP (Dose-Length Product in mGy*cm): Product of CTDIvol and scan range, main descriptor of total energy deposited in body Effective Dose (E in mSv): See above Table with 2 columns and 2 rows Natural radiation (cosmic and terrestrial) Artificial radiation by medical imaging procedures and human nuclear activities Between 1 mSv and 6 mSv per year depending on location (average 2.4 mSv per year) Average radiation dose received is about 4 mSv per year for German and 6.2 mSv per year for US citizens Describe the Image Reconstruction Parameters Field of View (FoV): Anatomical area displayed within the CT image. Please note: Always reconstruct with an appropriate FoV that is adjusted to the anatomical structure of interest. (Convolution) Kernel and updated kernel concept: Reconstruction parameter affecting image sharpness and noise (indicated by kernel number). The kernel applies a specific mathematical algorithm that digitally filters the raw data during image reconstruction. Slice thickness (width): Describes the thickness of the reconstructed images. Unlike for single-slice CT imaging, for MSCT imaging the possible slice widths depend on the chosen collimation and scan mode. Increment: Distance (in mm) between the reconstructed images in z-direction, freely adaptable from 0.1 mm upwards for spiral or 3D sequential scanning. If the increment is smaller than the slice width, the images are created with overlap. They provide better detail of the anatomy as well as high quality 2D and 3D post-processing. CT number (Hounsfield Unit, HU): Used to represent the mean X-ray attenuation value of the corresponding voxel within the slice. CT number scale: Water has a value of 0 HU; denser substances are given positive values and less dense materials negative numbers (e.g. around -1,000 HU for air). CT window: Limits the image display - the range of CT numbers (HUs) displayed in grayscale - to tissue of interest. This results in a better differentiation of tissue with similar densities. Explain the Functionality of a CT Scanner Basic image generation process: X-ray tube generates radiation → X-rays penetrate body → tissue attenuates X-rays → detector processes attenuated X-ray beams → detector converts radiation into electrical signals → computer converts these digital data into high-resolution images (square matrix of pixels, each of which represents a voxel - a 3D volume element - of the patient’s tissue). Pitch (P): Spiral scan parameter that determines the speed of the table movement and the speed of the acquisition. SureView concept: Automatic adjustment of spiral scanning parameters pitch (P) and tube current (mA) → increasing the pitch → tube current must also be increased to obtain the same image noise or dose Table with 2 columns and 3 rows Sequential Scanning Spiral Scanning Incremental, slice-by-slice imaging mode without table movement during data acquisition → Interscan delays caused by the movement of the table to the next slice position Continuous volume imaging mode with data acquisition and table movement performed simultaneously for the entire scan duration Disadvantages: Longer scan times, prone to slice misregistration caused by scanning at different respiratory levels → Sequential scanning limited to head and spine imaging because of minimal motion artifacts in these regions Advantages: Examination within one breath hold, shorter scan times → facilitate motion artifacts reduction and lower need for contrast medium, better differentiation of contrast-enhanced phases, e.g. arterial and venous phases, better method for 3D reconstructions when scanning restless patients Identify the System Components of a CT Scanner X-ray tube: At constant tube current (mA), an increased voltage (kV) leading to increased penetration capacity of the X-ray photons, increased dose, reduced image noise, reduced low-resolution contrast (e.g. in soft tissue) Detector components: Collimator suppressing scattered radiation Ultra-Fast Ceramic Detector (UFC) converting X-rays into visible light Photodiode converting visible light into electrical current A/D (analog/digital) converter converting electrical signals into digital data Multislice scanning possible due to multi-row detectors. Advantages: Shorter breath holding required, fewer motion artifacts, multiphase contrast-enhanced scanning possible Acquisition modes: Depending on detector design (full detector size vs. part of the detector, e.g. in Neuro mode), smaller detector element sizes enable thinner image reconstructions (of 1 mm down to 0.6 mm) 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. Due to regulatory requirements, the future availability of said Functionalities in any specific country is not guaranteed. Please contact your local Siemens Healthineers sales representative for the most current information. The reproduction, transmission or distribution of this training or its contents is not permitted without express written authority. Offenders will be liable for damages. All names and data of patients, parameters and configuration dependent designations are fictional and examples only. All rights, including rights created by patent grant or registration of a utility model or design, are reserved. © Siemens Healthcare GmbH 2023 Siemens Healthineers Headquarters\Siemens Healthcare GmbH\Henkestr. 127\ 91052 Erlangen, Germany\Telephone: +49 9131 84-0\siemens-healthineers.com Disclaimer Assessment Welcome to the assessment. For each question, select your answer and then select Submit. 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 when you have completed the assessment. Start Assessment … reduces the dose for the patient. … reduces the image noise. … increases the soft tissue contrast. … increases the scan time. Increasing the kV value … Question 1 of 5 Select the correct answer. Multiple Choice Question Sequential scanning is a slice-by-slice imaging mode with table movement during data acquisition. Sequential scans are very well suited for 2D and 3D reconstructions. Sequential scans are prone to slice misregistration. Sequence scans are the typically performed scan mode for CT. Which statement is correct? Question 2 of 5 Select the correct answer. Multiple Choice Question … should be as low as possible to reduce scan time. … is another term for the CTDIvol. … determines the increment of the acquired data. … doesn‘t influence the total scan time. The pitch …? Question 3 of 5 Select the correct answer. Multiple Choice Question … applies a specific mathematical algorithm that digitally filters image data. … is just important for bony scans. … is a reconstruction parameter to determine the image sharpness. … subsequent number shows: The lower the number, the sharper the image impression. The kernel … ? Question 4 of 5 Select the correct answer. Multiple Choice Question It is the lead glass window to observe the patient during the scan. It is the gantry bore hole through which the patient moves during the scan. It is a range that limits the image display to the tissue of interest. It is a display method that can‘t be adjusted. What is the CT Window? Question 5 of 5 Select the correct answer. Multiple Choice Question Retry Assessment Results %Quiz2.ScorePercent%% %Quiz2.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 CT Basics Online Training. Completion Question Bank 1 ct_basics_usa_ 1 CT Basics Welcome to CT Basics 1.1 Welcome 1.2 CT Workplace 1.3 System Components of a CT Scanner 1.4 Introduction System Components 1.5 Overview of System Components 1.6 X-Ray Tube 1.7 Detector 1.8 Advantage of Multislice CT Scanner 1.9 Detector Design and Acquisition Modes 1.10 Functionality of a CT Scanner 1.11 Introduction Scanner Functionality 1.12 Image Generation Process 1.13 Image Elements 1.14 Sequential Scanning 1.15 Disadvantage of Sequential Scanning 1.16 Spiral Scanning 1.17 Advantage of Spiral Scanning 1.18 Pitch 1.19 Pitch and SureView Concept 1.20 Image Reconstruction Parameters 1.21 Introduction Image Reconstruction Parameters 1.22 Field of View (FoV) 1.23 Kernel 1.24 Updated Kernel Concept (since 2013) 1.25 Slice Thickness (Width) 1.26 Increment 1.27 CT Number (Hounsfield Units) 1.28 CT Window 1.29 Radiation Dose Parameters and Definitions 1.30 Introduction Radiation Dose 1.31 Background Radiation 1.32 Radiation Dose in CT 1.33 Dose Overview and Absorbed Dose 1.34 Equivalent Dose 1.35 Patient Dose Parameters 1.36 Volume CT Dose Index (CTDIvol) 1.37 Dose-Length Product (DLP) 1.38 Effective Dose 1.39 Course Review 1.40 Disclaimer 1.41 Assessment