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Non-Radiographic Imaging
Published in Eric Ford, Primer on Radiation Oncology Physics, 2020
At some point in time (B in the figure) another pulse is applied. This pulse is a 180-degree pulse which flips the spins to the opposite direction in the x-y plane. Now spins that are precessing faster (green) begin to catch up with spins that are slower (red). At some point in time (C in the figure) the spins have caught up to each other and the net My suddenly increases and there is an abrupt signal (or “echo”) recorded from the large My. The time between the start of the 90-degree pulse and the echo is TE (time to echo), and it is set by the time delay between the 90-degree pulse and the 180-degree pulse. See the videos for further illustration of MRI sequences and the spin-echo process.
Magnetic Resonance Imaging
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
The first step in spin-echo imaging involves using an exciting RF pulse that flips the spins by 90° (Figure 8.18a). After this first pulse has ended, the spins are left precessing in the transverse plane (Figure 8.18a). The spins then rapidly fan out and become decoherent with a relaxation time, T2*, determined by static variations in the magnetic field (Figure 8.18b). (The spins behave in a fashion similar to race cars, which may leave the starting line at the same time, but which spread out as they travel at different speeds.) Thus, while they initially emit a large RF signal, this signal decays rapidly to zero with a relaxation time, T2*.
Biomedical Imaging Magnetic Resonance Imaging
Published in Lawrence S. Chan, William C. Tang, Engineering-Medicine, 2019
Following an excitation RF pulse that produces an FID signal, if a gradient is turned on for a fixed amount of time, then a phase dispersion will be produced across the object due to the spatially dependent frequency variation given by Eq. [8] (Fig. 6a). Such phase dispersion causes the signal to decay rapidly, essentially crushing out the signal when the gradient is sufficiently large and/or the duration sufficiently long. After the initial phase dispersion, if the gradient reverses its polarity, then an opposite phase dispersion is introduced (Fig. 6b). When the reversed gradient causes the same amount of phase “dispersion” as the previous gradient, the net phase dispersion becomes zero, resulting in a strong signal which is known as a gradient echo (Figs. 6c and 6d). Although the gradient echo illustrated in Fig. 6 arises from an FID signal, they can also be formed by utilizing a spin echo. Unlike spin echo which removes dephasing caused by magnetic field inhomogeneities, gradient echo contains the effects of magnetic field inhomogeneities, and thus is sensitive to relaxation.
Imaging-based internal body temperature measurements: The journal Temperature toolbox
Published in Temperature, 2020
Juho Raiko, Kalle Koskensalo, Teija Sainio
In research, magnetic resonance thermometry has usually been performed using customized pulse sequences and the raw data have been transported offline to generate the thermal images [20,43,44]. According to Quesson et al. [9] radio-frequency (RF)-spoiled gradient echo sequences are used for PRFS-based thermometry imaging. Gradient echo is preferred over spin echo because the phase difference induced by the PRFS would be lost in refocusing spin echo sequences. On the other hand, the other MR thermometry methods prefer spin echo sequences over gradient-echo sequences because the phase-difference causes image artifacts as the phase-difference is not the primary interest in those methods [9]. For spectroscopic thermometry, the default pulse sequences of an MRI system can be used as such. The frequency difference between water and a reference metabolite can be determined using an off-line software for magnetic resonance spectroscopy data analysis, such as jMRUI [45] or TARQUIN [46] both of which are free. For the results to be most accurate, calibration measurements are recommended [40].
Frontal aslant tracts as correlates of lexical retrieval in MS
Published in Neurological Research, 2020
Zafer Keser, Argye E. Hillis, Paul E. Schulz, Khader M. Hasan, Flavia M. Nelson
Whole-brain MRI data were acquired on a Philips 3.0 T Intera scanner using a SENSE receive head coil. The MRI protocol included conventional and nonconventional MRI sequences (dual-echo turbo spin echo, fluid attenuation by inversion recovery [FLAIR], and 3-dimensional T1-weighted magnetization prepared rapid acquisition with gradient echo). The T1-weighted sequence spatial resolution was 1 mm × 1 mm × 1 mm, and field-of-view is 256 mm × 256 mm. Diffusion-weighted image (DWI) data were acquired axially from the same graphically prescribed conventional MRI volumes using a single-shot multislice 2-dimensional spin-echo diffusion sensitized, and fat-suppressed echo-planar imaging (EPI) sequence, with the balanced Icosa21 tensor encoding scheme The b-factor = 1000 s/mm2, replication and echo times TR/TE = 7100/65 milliseconds, FOV = 256 mm ×256 mm, and slice thickness/gap/#slices = 3 mm/0 mm/44. The EPI phase encoding used a SENSE k-space undersampling factor of 2, with an effective k-space matrix of 128 × 128, and an image matrix after zero-filling of 256 × 256. The DWI images were converted to nifti format with dcm2niix software (https://www.nitrc.org/plugins/mwiki/index.php/dcm2nii:MainPage) and uploaded to DSIstudio (http://dsistudio.labsolver.org). DWI images were visually inspected for significant motion artifact, and eddy/motion correction was performed. The images then scaled into 1 mm isotropic, and the b-table was checked by an automatic quality control routine to ensure its accuracy [26]. Diffusion tensor imaging was then calculated.
Optic Nerve Traction During Adduction in Open Angle Glaucoma with Normal versus Elevated Intraocular Pressure
Published in Current Eye Research, 2020
Joseph L. Demer, Robert A. Clark, Soh Youn Suh, Joann A. Giaconi, Kouros Nouri-Mahdavi, Simon K. Law, Laura Bonelli, Anne L. Coleman, Joseph Caprioli
The first author performed high resolution MRI with a 1.5T General Electric Signa scanner and surface coils (Medical Advances, Milwaukee, WI) with T2 fast spin echo pulse sequence as57 described.41,57–61 Eye position was controlled by monocular fixation of an illuminated target in central position, or laterally to establish abduction of the fixating eye with simultaneous adduction of its fellow since the surface coil occludes the adducting eye. Axial 2-mm thick images (10–12 cm field of view, 256 × 256 matrix) including both orbits were generally obtained to determine gaze direction and globe axial length (AL). Quasi-coronal sets of 17–20, 2-mm thick planes perpendicular to the long axis of each orbit were obtained separately (field of view 8 × 8 cm, 256 × 256 matrix, resolution 312 microns). Acquisitions were repeated for central gaze, and large (~30°), or both moderate (~25°) and large abduction and adduction of each eye (Figure 1) in 54 control eyes, 34 eyes with OAG-low, and 18 eyes with OAG-high. Imaging was performed with only large ductions in an additional 38 subjects. The data set here required about 1,150 individual MRI acquisitions, for which limited results were included in our earlier publication.41