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Principles behind Magnetic Resonance Imaging (MRI)
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
In order to optimize signal and contrast in MR images, for a given diagnostic purpose, the combined effects of manipulating the magnetization by an RF pulse (Eq. 32.12) and subsequent relaxation processes (Eqs. 32.13–32.14) can be exploited. In its most basic form, a so-called MRI pulse sequence is a scheme of RF pulses and time delays, carefully designed to generate optimal signal and contrast for a given purpose. To introduce this concept, three fundamental MRI pulse sequences are briefly outlined below, at this point primarily described by the effects of RF pulses and relaxation processes.
Magnetic Resonance Imaging
Published in Shoogo Ueno, Bioimaging, 2020
During MRI acquisition, RF fields and gradient fields are applied with precisely designed timing and waveform. Diagramming of these actions results in a pulse sequence. Among many types of pulse sequences developed through previous studies, Figure 4.10 shows a gradient echo pulse sequence, which is one of the most basic type. The horizontal axis is time, and the vertical axis is the intensity of the RF fields and gradient fields.
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 problem in trying to measure relaxation times is that the shortest relaxation time dominates the time behavior of the signal. Unfortunately, this is usually T2*, a quantity that has nothing to do with the body. To measure T1 or T2, a more sophisticated method must be used to separate out these more interesting time behaviors. To accomplish this, most imaging techniques use pulse sequences: many RF pulses emitted and detected in a series. The free induction signals emitted by the body's nuclei after they have been excited by a pulse sequence are measured in order to tease out information about T1 and T2. One commonly used pulse sequence technique is called spin-echo imaging. In understanding this method, you will gain an appreciation of the general ideas behind MRI. (Note that the “echoes” in question only share a name with those used in ultrasound imaging. The underlying physics is very different.)
Using MRI to measure position and anatomy changes and assess their impact on the accuracy of hyperthermia treatment planning for cervical cancer
Published in International Journal of Hyperthermia, 2023
Iva VilasBoas-Ribeiro, Martine Franckena, Gerard C. van Rhoon, Juan A. Hernández-Tamames, Margarethus M. Paulides
The MR imaging protocol was optimized for air pockets visualization, motion compensation, soft tissue contrast, and geometrical precision of the applicator position without significant image wrapping. We used two types of sequences in the protocol. The first scan was to visualize the gastrointestinal air; thus, T1-weighted images were acquired using a 3 D spoiled gradient recalled echo (SPGR) pulse sequence. The acquisition parameters were: TE/TR: 0.62/1.44 ms; slice thickness of 5 mm; flip angle of 2°; the spacing between slices was 2.5 mm; the scan duration was 26 s, and the volume of interest (VOI) was 50 × 50 × 60 cm3. Second, a high resolution anatomic image was taken to capture the volunteer position and the anatomic information. These were T2-weighted MR images acquired using the PROPELLER sequence with the following acquisition parameters: TE/TR: 81/4200 ms; slice thickness of 5.5 mm; flip angle of 125°; there was no spacing between slices; and the total scan duration was 7 min. In order to capture the anatomy inside the applicator, a total of seven scans using the PROPELLER sequence, where a cylindrical VOI of 42.0 × 100.5 cm3 was acquired and reconstructed using the AutoBind tool from GE scanner software. Note that this tool was only used for the PROPELLER sequence since the SPGR sequence takes 3 D volumetric acquisition.
Cerebellar degeneration in primary lateral sclerosis: an under-recognized facet of PLS
Published in Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration, 2022
Eoin Finegan, We Fong Siah, Stacey Li Hi Shing, Rangariroyashe H. Chipika, Orla Hardiman, Peter Bede
T1-weighted images were acquired on a 3 Tesla Philips Achieva system using a 3D Inversion Recovery prepared Spoiled Gradient Recalled echo (IR-SPGR) pulse sequence and an 8-channel receive-only head coil. The following pulse sequence settings were implemented; field-of-view (FOV): 256 × 256 × 160 mm, slice orientation: sagittal, spatial resolution: 1 mm3, TR/TE = 8.5/3.9 ms, TI =1060 ms, flip angle = 8°, SENSE factor = 1.5. 32-direction DTI images were recorded using a spin-echo echo planar imaging (SE-EPI) pulse sequence with the following parameters: TR/TE = 7639/59 ms, slice orientation: transverse (axial), SENSE factor = 2.5, b-values = 0, 1100s/mm2 FOV = 245 × 245 × 150 mm, spatial resolution = 2.5 mm3, 60 slices were acquired with no interslice gaps. Fluid attenuated inversion recovery (FLAIR) images were also acquired for each participant to rule out alternative or comorbid pathologies and assess for vascular white matter lesion burden. FLAIR images were acquired using an Inversion Recovery Turbo Spin Echo (IR-TSE) sequence in axial orientation: TR/TE = 11000/125 ms, TI = 2800 ms, 120° refocusing pulse, FOV = 230 × 183 × 150 mm, spatial resolution = 0.65 × 0.87 × 4 mm, 30 slices with 1 mm gap, with flow compensation and motion smoothing and a saturation slab covering the neck region. Imaging data were evaluated in voxelwise analyses, based on lobular volume and region-of-interest peduncle diffusivity profiles.
Effect of axonal fiber architecture on mechanical heterogeneity of the white matter—a statistical micromechanical model
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2022
Hesam Hoursan, Farzam Farahmand, Mohammad Taghi Ahmadian
The MR images were obtained from a 25-year-old male volunteer. Images were acquired using a Toshiba's Vantage Elan 1.5-Tesla whole body scanner with a 32-channel head coil. In order to extract the anatomical information of the brain structure, an Magnetization Prepared Rapid Gradient Echo (MPRAGE) sequence (TR/TE = 6700/5.5 ms, 1 mm in plane resolution, 1 mm slice thickness, 256 × 256 mm FOV, 176 slices) was applied. Diffusion weighted images (DWI) were acquired using the single shot echo planer imaging (EPI) pulse sequence (b-value = 1000 smm–2. The total acquisition time was approximately 15 min. The FSL software (Jenkinson et al. 2012) was used to extract the brain tissue from the head. Thresholding and segmentation techniques were used to separate white matter, grey matter, and cerebrospinal fluid. Freesurfer (Reuter et al. 2012) was used to verify the segmentation results (Figure 1). The DTI data was registered onto the T1 image and extracted using SPM12 software package in MATLAB in the form of 4D tensors. Explore DTI was used to perform tractography in three regions including the brain stem, corpus callosum, and corona radiata (Leemans et al. 2009) (Figure 2). Following the tractography, the DTI information from the voxels of the brain stem, corpus callosum, and corona radiata were extracted by using a semi-automatic brain atlas (Mazziotta et al. 2001).