Magnetic Resonance Imaging
Suzanne Amador Kane, Boris A. Gelman in Introduction to Physics in Modern Medicine, 2020
The exact values for TE and TR are adjusted to create images with various types of weighting. For example, for T1-weighting, TE is chosen to be very short to minimize the effects of different T2 values, and several values of TR can be used to better determine T1. The time to take a complete scan also must allow for the system's inherent need to restore its dipoles to their initial state. In practice, for complete relaxation to occur, TR must be equal to several seconds in order to be several times the longest value of T1. This has traditionally placed a constraint on how quickly MRI scans can take place. Some MRI techniques work by exciting only a fraction of the available magnetization, sometimes in combination with flip angles of less than 90°; both maneuvers result in a faster spin recovery, permitting faster scans. Turbo MRI techniques fall into this category. However, by reducing scan times, these methods also lower the signals collected per slice, resulting in noisier, lower-quality images. At the end of this chapter we will discuss a method, echo-planar imaging, that allows reduced scanning times without sacrificing image quality.
Bioengineering Aids to Reproductive Medicine
Sujoy K. Guba in Bioengineering in Reproductive Medicine, 2020
Increasing the slope of the field gradient improves the spatial resolution because then two close by planes parallel to each other will have significantly different resonant frequencies and they may be distinguished readily. Then the number of spin elements in the volume declines and the signal becomes weaker and the signal to noise ratio (S/N) declines thereby adversely affecting the quality of the image. On the other hand having a low slope gradient and thus considering a large bulk of tissue at a time will lead to loss of structural detail and important pathologies may be missed. A compromise suitable for the specific application has to be made. Two additional gradients need to be introduced for defining the y and ζ coordinate positions of the sources. These gradients are taken orthogonal to each other but their directions may be changed to select the imaging plane. In this way the body is viewed from different directions. The next step is to assemble the information to derive a two dimensional image of a plane through the body. A variety of procedures have been proposed. The ones which are more commonly used are Projection reconstructionEcho planar imagingFourier imaging
Diffusion Imaging and Tensor Physics for the Clinician
Andrei I. Holodny in Functional Neuroimaging, 2019
In most practical clinical imaging situations, there are both coherent (blood and CSF flow, patient movement) and incoherent (diffusion) motions present. To measure the small displacements caused by diffusion, the acquisition must be made extremely motion sensitive using strong diffusion-weighting gradients. Fortunately, coherent motion results in phase changes in the resultant images (as in phase-contrast techniques), while incoherent motion results in magnitude changes. Simply ignoring the phase information removes the effects of coherent motion. However, coherent motion is a problem for sequences in which the data are acquired over multiple acquisitions (multishot sequences) because the coherent motion will not be the same for each acquisition. The resultant phase errors cause unacceptable ghosting artifacts. For this reason, single-shot echo-planar imaging (EPI) techniques are usually used for diffusion-weighted imaging, although the resolution that can be attained is quite limited. Recent developments and alternative techniques for measuring diffusion are discussed later.
Multivariate associative patterns between the gut microbiota and large-scale brain network connectivity
Published in Gut Microbes, 2021
N. Kohn, J. Szopinska-Tokov, A. Llera Arenas, C.F. Beckmann, A. Arias-Vasquez, E Aarts
Participants were screened for compatibility with magnetic resonance imaging (MRI). MRI data were acquired using a 3 T MAGNETOM Prisma system, equipped with a 32-channel head coil. After three short task-related fMRI scans (see Papalini et al.), 9 min of resting state fMRI was acquired. 3D echo planar imaging (EPI) scans using a T2*weighted gradient echo multi-echo sequence (Poser, Versluis et al. 2006) were acquired (voxel size 3.5 × 3.5 × 3 mm isotropic, TR = 2070 ms, TE = 9 ms; 19.25 ms; 29.5 ms; 39.75 ms, FoV = 224 mm). The slab positioning and rotation (average angle of 14 degrees to AC axis) optimally covered both prefrontal and deep brain regions. Subjects were instructed to lie still with their eyes open and refrain from directed thought. A whole-brain high-resolution T1-weighted anatomical scan was acquired using a MPRAGE sequence (voxel size 1.0 × 1.0 × 1.0 isotropic, TR = 2300 ms, TE = 3.03 ms, 192 slices).
MRS and DTI evidence of progressive posterior cingulate cortex and corpus callosum injury in the hyper-acute phase after Traumatic Brain Injury
Published in Brain Injury, 2019
Tim P. Lawrence, Adam Steel, Martyn Ezra, Mhairi Speirs, Pieter M. Pretorius, Gwenaelle Douaud, Stamatios Sotiropoulos, Tom Cadoux-Hudson, Uzay E. Emir, Natalie L. Voets
DTI data were pre-processed using FSL tools (49) (https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FDT/UserGuide). Image distortions associated with magnetic field inhomogeneities in echo-planar imaging sequences were corrected using the tool ‘topup’ (50). Data were collected with reversed phase encode blips to estimate the susceptibility induced field based on the five non-diffusion weighted images acquired during the anterior-to-posterior and the posterior-to-anterior phase encode acquisitions. From these pairs of images, with distortions going in opposite directions, the susceptibility induced off-resonance field was estimated (49,51). The resulting fieldmap was fed into the tool ‘eddy’ to simultaneously correct the diffusion-weighted images for susceptibility-induced distortions, eddy current-induced distortions and head movement (50). Finally, diffusion tensors were fit to the data in order to derive fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), lambda 2 and lambda 3 (λ2 and λ3) maps. The mean of λ2 and λ3 was calculated to generate radial diffusivity (RD).
New magnetic resonance imaging sequences for fibrosis assessment in Crohn’s disease: a pilot study
Published in Scandinavian Journal of Gastroenterology, 2022
Bénédicte Caron, Valérie Laurent, Freddy Odille, Silvio Danese, Gabriela Hossu, Laurent Peyrin-Biroulet
All the imaging studies were performed on a 1.5 T MR scanner (MAGNETOM Avantofit, Siemens Healthcare, Erlangen, Germany). The same MR enterography protocol was used in all patients prior MRI examination: patients were prepared and were asked to drink one liter of mannitol preparation 45 min before the MR examination, and 0.5 mg of glucagon was injected just before the contrast agent was injected (Clariscan – GE Healthcare SAS, or Dotarem – Guerbet France). Axial and coronal breath-hold T2-weighted using half-Fourier-acquired single-shot turbo spin Echo (HASTE) and as well as axial and coronal steady state gradient echo (TRUEFISP). Free-breathing axial diffusion weighted imaging was acquired using single-shot echo-planar imaging. Axial and coronal 3D T1-weighted images were acquired using volume interpolated breath-hold technique (VIBE). The VIBE sequence was repeated before and 40 s (arterial), 80 s (portal phase), and the delayed phases at 3 min, and 7 min after contrast agent injection.
Related Knowledge Centers
- Inflammation
- Stroke
- Neoplasm
- Magnetic Resonance Imaging
- Medical Imaging
- Radiology
- Nuclear Medicine
- Contrast Agent
- Intravenous Therapy
- CT Scan