MR Essentials - Spatial Resolution Online Training

Welcome to the MR Essentials - Spatial Resolution Online Training. This course will discuss how an image with spatial structures can be generated from an MR signal that is packaged as different gray values.   Upon successful completion of this course, you will be able to: Explain how images with spatial structures are generated from MR signals Describe how magnetic field gradients are applied to alter the magnetic field strength Summarize how the signal information is obtained from frequencies Identify the key components in a pulse sequence Congratulations. You have completed the MR Essentials - Spatial Resolution Online Training. Listed below are the learning objectives presented in the course. Take time to review the material before proceeding to the final quiz. Describe how magnetic field gradients are applied to alter the magnetic field strength Gradients are a controlled change in the magnetic field strength in a certain direction, either a linear increase or decrease Applying gradients disrupts the homogeneity of the magnetic field and cause the spins at different locations to precess at different frequencies Explain how images with spatial structures are generated from MR signals By switching gradients, a mixture of signals for a slice image can be obtained Slice selection involves only selecting spin within a certain slice A 2D scan matrix is acquired via frequency and phase encoding in the slice The MR system reconstructs the MR image from raw data measured using a 2D Fourier Transform   Summarize how the signal information is obtained from frequencies k-space is the raw data matrix. The axes of the raw-data matrix (kx and ky) designate the “spatial frequencies” Raw data determines whether and how intensively a certain stripe pattern contributes to the image Low spatial frequencies have a wide stripe pattern and are close to the center. They contribute primarily contrast to the image High spatial frequencies have a fine stripe pattern and is far from the center. They contribute primarily edge definition to the image Fourier Transform allows us to determine the signal contribution of each frequency component and reallocates individual frequencies to their location along the x-axis. The individual signal strength obtained determines the gray value of the allocated pixel. The Fourier Transform relies on raw data values in the k-space to calculate the gray value distribution in the image   Identify the key components in a pulse sequence 90o pulse Slice-selection Gradient (Gs) Frequency-encoding Gradient (Gf) Phase-encoding Gradient (Gp) 180o pulse Spin Echo Echo Time (TE) Repetition Time (TR) Select the link to view/print your review material before proceeding to the final quiz.   What is a Gradient? A slope and a vector - defined by amplitude and direction Units mT/m Consists of an amplifier and a coil When applying the gradient, it will be ramped up to the desired amplitude. After the gradient has been applied long enough, it will return to 0 or be ramped up to another amplitude, depending on the sequence.   Gradient Coils Learn about how gradient coils work. Element HTMLThe MR system has 3 gradient coil arrangements for all three spatial directions (x, y, and z). The gradient coils are operated in pairs in a specific direction with the same current strength but opposite polarity. One coil increases the static magnetic field and the opposing coil reduces it.B B0 Z Sound File Audio ScriptThe MR system has three gradient coil arrangements for all three spatial directions (x, y, and z) positioned around the magnet bore. The gradient coils do not create a permanent magnetic field; instead, they are switched on briefly and multiple times during the examination. The gradient coils are operated by special power supplies called gradient amplifiers. The gradient coils are operated in pairs in a specific direction with the same current strength but opposite polarity. One coil increases the static magnetic field and the opposing coil reduces it. The gradient is centered at the center of the magnet. As a result, the magnetic field with original strength B0 (pronounced "bee-naught") changes like the incline of a road. Slice-selection gradient (Gs) is switched in the z-direction simultaneously with the RF pulse A slice is a defined resonance area of the proton spins Defined by the application of a gradient simultaneous with a RF pulse broadcasting at the matching Larmor Frequency (ω) Slice position is the resonance location z0 Original field strength B0 occurs at one location only, z0 Slice Thickness (Δzo) is a tissue volume in the z-direction Determined by the bandwidth of neighboring frequencies about the center frequency (Δωo)   Slice Position Learn about the slice positioning. Element HTMLxyztransversecoronalsagittaldouble-obliqueobliqueXYZ Sound File Audio ScriptA big advantage of gradients in MRI is that they allow the positioning of arbitrary slice planes. Recall that for transverse slices, the z-gradient is switched on when the RF Pulse is applied. For sagittal slice positioning, the x-gradient is switched on. For coronal slice positioning, the y-gradient is switched on. To obtain oblique slices, two or three gradients must be switched on simultaneously with the RF pulse and their effect is superimposed. A single oblique slice is generated by two gradients, for example, in the z- and y-direction. The angle between the two directions determines the amplitude for each gradient. For a double-oblique slice, the system switches all three gradients simultaneously.   Slice Thickness Learn about selecting slice thickness. Element HTMLω ω0B B0z0Zω ΔωoΔz0                    Z  ω Δωoa bΔza Δzb   Sound File Audio ScriptThe RF pulse has a certain bandwidth of neighboring frequencies about its center frequency (pronounced "omega-naught"). As a result, the RF pulse can excite a desired spatial area of slice thickness (pronounced "delta-zee-naught") in the presence of the slice gradient. The slice thickness can be changed by changing the RF pulse bandwidth in the presence of a constant gradient amplitude. More commonly, the slice thickness can be modified by keeping the RF pulse bandwidth constant while changing the gradient amplitude. A steeper gradient ramp will excite a thinner slice, while a shallower gradient excites a thicker slice. This relationship between the gradient ramp and slice thickness is illustrated in the graph below. The Fourier Transform is applied to raw data to make image data. Each raw pixel contains information for the entire image and each image pixel contains information from all the raw data. Center raw data: primarily structure and contrast Outer raw data: primarily spatial resolution   Phase Frequency 256 times RF GS GF GP Phase-Encoding Gradient (GP) - a gradient in the vertical, y-direction during the time between the RF pulse and the echo Used for the definition of multiple lines of voxels The amount of measurement required depends on the matrix size in the phase encoding direction (i.e. 128, 256, 512) Phase-Encoding - the spins precess at different speeds for a short time After the gradient is switched off, the spins along the y-axis show different phase shifts directly proportional to their locations.  Frequency-Encoding Gradient (GF) is the gradient applied in the horizontal or x-direction The gradient is turned on as data is being received Frequency Encoding is the spin ensembles of the individual voxels that precess along the x-axis at an increasing frequency The resonance frequency is linearly changing along this gradient direction The Fourier Transform enables the signal contribution of each frequency component to be determined Individual frequencies are reallocated to their location of origin along the x-axis RF GS GF ky kx k-space Image space       256           Fourier Transform           256 ky kx k-space is the raw data matrix. The axes (kx and ky) designate the spatial frequencies The raw data determines whether and how intensely a certain strip pattern contributes to the image. Rough stripe pattern (close to the center) shows low spatial frequency Fine stripe pattern (far from the center) shows a high spatial frequency RF GS GF GP FID Spin echo TE TR 90o 180o 256 times Pulse Sequence - a spin echo is created by a 90o pulse, which generates the FID, followed by a 180o pulse which generates the spin echo in the echo time TE Repetition Time (TR) - the pulse sequence is repeated with this time interval until the raw-data matrix is filled with echoes Scan Time = Np x TR Np = number of phase encoding steps Slice-Selection Gradient (Gs) - switched simultaneously with the 90o pulse Followed by a rephasing gradient of opposite polarity and half the duration  Phase-Encoding Gradient (GP) - switched on briefly between the slice selection and spin echo Frequency-Encoding Gradient (GF) - also known as the readout gradient   ω ω0   B B0 weaker field as before stronger field RF GF - + FID T2* TE Gradient echo Gradient Applying a gradient pulse directly after the RF pulse (-) artificially dephases the spin frequencies Dephasing - spins precess at different speeds and lose their phase more quickly Reversing the polarity of the gradient (+), rephases the spins Rephasing - causes the spins to become "in-phase" again We measure an echo during the rephasing of the FID. Since this echo is generated by a gradient, it is called the gradient echo. 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 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. Please contact your local Siemens sales representative for the most current information. Functionalities described in the material or parts of this functionality may not yet be released for customers and not yet be commercially available in every country. Due to regulatory requirements, the future availability of said functionalities or parts thereof in any specific country is not guaranteed. The Operator Manual shall be used as your main reference, in particular for relevant safety information like warnings and cautions. 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 rights, including rights created by patent grant or registration of a utility model or design, are reserved. Copyright © Siemens Healthcare GmbH, 2018 Fourier Transform Frequency encoding "Zip Code" by frequencies The MR signal is a mixture of the signals of all excited spins along the x-axis. At a resolution of 256 voxels, an echo includes 256 "notes" of different frequencies.