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Anatomical and Biological Imaging of Pediatric Brain Tumor
Published in David A. Walker, Giorgio Perilongo, Roger E. Taylor, Ian F. Pollack, Brain and Spinal Tumors of Childhood, 2020
Rob A. Dineen, Shivaram Avula, Andrew C. Peet, Giovanni Morana, Monika Warmuth-Metz
Although tractography can play an important role in neurosurgical planning, it is prudent to be aware of the variation in construction of the tracts by the different post-processing tools available for clinical use.27,28 The use of DTI in conjunction with functional MRI (fMRI) has been proposed to increase the reliability of DTI in presurgical planning.29 Future directions in the field of tractography include the use of newer advanced diffusion tensor techniques such as high-definition fiber tractography (HDFT) and high angular resolution diffusion imaging (HARDI).30,31
Acquisition Strategies
Published in Luisa Ciobanu, Microscopic Magnetic Resonance Imaging, 2017
Besides the three basic types of contrast discussed in the previous section, MR images can be sensitized to other physical processes such as molecular diffusion, perfusion, oxygenation level, etc. Among these, diffusion imaging is often used in magnetic resonance microscopy. The evolution of magnetization in a magnetic field in the presence of molecular diffusion is described by the Bloch‐Torrey equation, which is obtained by including an additional diffusion term to the Bloch equation:
Diffusion-Weighted Imaging in Stroke
Published in Andrei I. Holodny, Functional Neuroimaging, 2019
Pamela W. Schaefer, William A. Copen
Chapter 11 outlines the physical principles of diffusion imaging. When evaluating a patient for acute stroke, DWI images and exponential images or ADC maps should be available for review (Fig. 1, Table 2). It is important to understand that the DWI image has T2 contrast as well as contrast due to differences in diffusion. To remove the T2 contrast, the DWI can be divided by the echo planar spin-echo (SE) T2 image (or b = 0 image), which does not have a diffusion component, to give an exponential image whose signal intensity is exponentially related to the ADC. Alternatively, an ADC map, whose signal intensity is linearly related to the ADC, can be created by taking a logarithm of the exponential image. On DWI images, regions with decreased diffusion are relatively hyperintense. Regions with elevated diffusion [such as the cerebrospinal fluid (CSF) spaces] are usually hypointense but may be isointense, or slightly hyperintense, depending on the strength of the diffusion and T2 components. On ADC maps, regions with decreased diffusion are relatively hypointense, while regions with elevated diffusion are relatively hyperintense. On exponential images, regions with decreased diffusion are relatively hyperintense while lesions with elevated diffusion are relatively hypointense. For lesions with decreased diffusion, the DWI images have superior lesion contrast. However, since hyperintense signal abnormality on DWI images could result from the T2 component rather than from abnormal diffusion, review of the ADC maps or the exponential images is important. The exponential image and ADC map are also useful for detecting areas of increased diffusion that may be masked by T2 effects on the DWI images.
Diffusion tensor imaging findings and neuropsychological performance in adults with TBI across the spectrum of severity in the chronic-phase
Published in Brain Injury, 2021
Katherine L. Zane, Jeffrey D. Gfeller, P. Tyler Roskos, Jeff Stout, Tony W. Buchanan, Thomas M. Malone, Richard Bucholz
Another limitation pertains to the limitation in our collection of diffusion imaging parameters (15 directions), which reduces the sensitivity and specificity of tract selection and raises concerns about crossing fibers. This data was collected at a time when fewer directions were standard protocol (2009–2013), while more current data collection favors a higher number of directions. Thus, our use of 15 directions may have influenced our data acquisition and the conclusions we reached. In addition, the present study relied solely on mean FA values to detect white matter abnormalities. As summarized by Alexander, et al. (62), other common DTI measures include mean diffusivity, radial diffusivity, and axial diffusivity. In the absence of other information, FA is highly sensitive to microstructural changes, but perhaps nonspecific to certain types of changes (e.g., radial or axial). This imposes a challenge on the interpretation of DTI measurements for diagnostic and therapeutic applications. To maximize the utility of DTI findings, future studies should consider using multiple diffusion tensor indicators to better characterize the tissue microstructure (62). In addition, more recent studies have used a cluster approach for their DTI methods, which may be more sensitive to within and between-group differences among individuals with TBI (63).
Dynamic changes in white matter following traumatic brain injury and how diffuse axonal injury relates to cognitive domain
Published in Brain Injury, 2021
Daphine Centola Grassi, Ana Luiza Zaninotto, Fabrício Stewan Feltrin, Fabíola Bezerra Carvalho Macruz, Maria Concepción García Otaduy, Claudia Costa Leite, Vinícius Monteiro Paula Guirado, Wellingson Silva Paiva, Celi Santos Andrade
Still, some caveats of the deterministic streamline tractography approach should be mentioned. This technique indirectly estimates fibre tract anatomy based on the main direction of water molecules diffusion in each voxel (in the order of millimetres), by far much bigger than the axonal diameter (in the order of microns). This assumption of homogeneous unidirectional vectors is unrealistic and gives erroneous estimations of fibre pathways in areas of crossing fibres. Furthermore, longer acquisition times and motion artefacts limit increases in spatial resolution. Other robust diffusion analysis techniques that soothe some of these limitations are evolving steadfastly, such as global probabilistic tractography, high angular resolution diffusion imaging (HARDI), q-ball imaging and diffusion kurtosis analysis (DKI) (15,18,75–77,78). So far, however, these approaches require more sophisticated processing algorithms and are less feasible for implementation in clinical settings to evaluate individual patients with TBI.
Comparison between CT and MRI in detection of metastasis of the retroperitoneum in testicular germ cell tumors: a prospective trial
Published in Acta Oncologica, 2020
Marjut Laukka, Susanna Mannisto, Annette Beule, Mauri Kouri, Carl Blomqvist
MRI of the abdomen from the level of thoracic vertebra 10 to the groins without contrast medium was performed using a 1.5 T GE machine, Optima MR450w (2011). To improve the detection of small lymph nodes, we used DWI, but no quantitative ADC measurements were carried out. The machine was updated in 2014, and at that time, three b-value DWI was introduced in addition into the protocol. Because of this technical development, 21 of patients examined in 2015–2017 underwent both diffusion imaging protocols. Imaging of the abdomen: coronal and axial T1-weighted images, 4.4- to 5-mm slice, overlap 2.2–2.5 mm, FOV 42 × 33.6–42 (LAVA flex); axial T2-weighted images, 5-mm slice, gap 1 mm, FOV 42 × 29.4 respiratory triggering. Axial diffusion-weighted images were with two p values 0 and 600 (all patients). Due to system update at the end of 2014, three p value diffusion-weighted images were added to the protocol using p values 50, 200 and 800. Imaging of the pelvis: axial T1-weigted images 5-mm slice, 0 gap, FOV 42 × 42 and axial T2-weighted images 5-mm slice, 0 gap, FOV 42 × 42. Axial diffusion-weighted images with two p values 0 and 600 and with three p values 50, 200 and 800 were performed in the same way as in the abdomen.