# dwi dti.pptx

May. 28, 2023
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### dwi dti.pptx

• 2. What is Diffusion? ⚫Diffusion means random movement of water protons ⚫Brownian motion- water protons diffuse randomly in space ⚫Protons (H2O) diffuse to dissipate their thermal energy ⚫Difference in mobility of H2O molecules b/w tissues gives contrast in DWI ⚫DWI helps to characterize tissues and pathology
• 3. Types of Diffusion ⚫Isotropic Diffusion ⚫Possibility of water protons moving in any one particular direction is equal to the probability that it will move in any other direction ⚫Isotropic means uniformity in all directions ⚫Anisotropic Diffusion ⚫Water diffusion has preferred direction ⚫Water can move easily in one direction than other Isotropic diffusion basis for routine DWI Anisotropic basis for DTI (diffusion tensor imaging) or tractography
• 6. Terms and Concepts ⚫The b-value ⚫It indicates the magnitude of DWI provided by the diffusion gradients ⚫It also indicates sensitivity of the seq; to the diffusion ⚫Expressed in sec/mm2 ⚫Depends on amplitude, separation and duration of DG ⚫The b-value increases with DG strength & Duration of their applications of the two gradients ⚫As b-value increases the signal from water reduces ⚫Highest value of b=1000 only for tissues with very high T2 relaxation time
• 8. Diffusion "trace" ⚫Isotropic diffusion forms the basis for the routine DWI ⚫Also their will be some anisotropic movement of H2O as well ⚫Especially in brain from white matter tracts ⚫To reduce this anisotropy the image with higher b- value like b=1000 is acquired in all three directions X, Y and Z axes ⚫Diffusion changes along all three axes are then averaged to get a ‘trace’ diffusion image
• 9. ADC: Apparent Diffusion Coefficient ⚫ ADC is measure of diffusion ⚫ Calculated mathematically from b-value=0 and higher b-value images ⚫ Signal attenuation of a tissue with increasing value plotted on graph with relative signal intensity on y-axis and b-value of x-axis ⚫ Resultant slope of line is ADC ⚫ To user it is available as ADC map ⚫ Reduced ADC is ‘restricted diffusion’ [bright area] on DWI
• 11. Clinical Applications of DWI DWI in Stroke ⚫Failure of Na-K ATPase pump tissue ischemia ⚫Results in influx of extracellular water into cells ⚫This is called cytotoxic edema ⚫Net shift of water molecules from extracellular into restricted intracellular space ⚫Overall, reduction in diffusion of water molecule in that area ⚫Manifested as bright signal on DWI and dark signal on ADC map ⚫DWI can detect early ischemic tissue (minute to hours) ⚫DWI shows stroke lesion earliest (failure of T2 appears normal)
• 12. DWI and T2-w images in stroke
• 13. Epidermoid versus Arachnoid cyst ⚫Epidermoid composed of keratin, debris and solid cholesterol ⚫Provide hindrance to diffusion of H2O molecules ⚫Epidermoid is seen as bright lesion on DWI ⚫Arachnoid cyst is clear CSF containing cyst, it will not be bright on DWI will be same as CSF in the signal intensity ⚫DWI can detect a residual epidermoid
• 16. Abscess versus simple cystic lesion
• 17. DWI Body Imaging ⚫Use of DWI in body imaging is new ⚫Big obstacles in DWI imaging are motion and short T2 of various organs ⚫Imaging done with breathe-hold & respiratory triggering ⚫DWI mainly used in tumor imaging and in follow- up imaging ⚫For staging tumor and lymphoma whole body imaging with background suppression is used in- replacement to PET ⚫DWIBS (DW whole body imaging with background body signal suppression) ⚫Final DWIBS images shows only diffusion
• 32. Diffusion Tensor Imaging ⚫Routine DTI based on anisotropic diffusion of water molecules ⚫Tensor is mathematical formalism used to model anisotropic dif: Technique ⚫MR scanner X, Y and Z are never perfectly parallel to the WM tracts at every point in the image ⚫In DTI, images are acquired in at least six, usually 12-24 directions instead of three in usual trace diffusion ⚫Pure ADC for each pixel is calculated from these images in multiple directions ⚫This is called ‘principal eigen value’
• 33. ⚫Images formed with principal eigen value is called DTI that gives orientation of fiber tracts Uses: ⚫DTI measure the magnitude of the ADC in the preferred direction of water diffusion and perpendicular to the direction ⚫The resultant image shows WM tracts very well ⚫Hence this technique is called ‘tractography’ ⚫Various maps used to indicate orientation of fiber tracts include FA (fractional anisotropy) ⚫RA (regional anisotropy) and VA(volume ratio) maps ⚫Tractography for the assessment of relationship of tracts with tumor, tumor invasion of tracts and preoperative planning
• 34. Note that the horizontal fibers of the genu and splenium of the corpus callosum at this level are represented in red, whereas the vertically orientated corticospinal tracts in the posterior limb are represented in blue (arrow, B). Anteroposterior fibers such as the SLF are represented in green
• 36. FA and DEC maps obtained from three adults with normal conventional MRIs show age-related white matter degeneration in the corpus callosum (arrows) and the subcortical frontalwhite matter (arrowheads).
• 37. This is a sagittal T2-FLAIR image of the same patient seen in figure 1. The arcuate fasciculus (green fibers) is seen to wrap around the T2-FLAIR hyperintense mass. The arcuate fasciculus connects regions of the brain involved in language. This again indicates that in this patient resection of the mass would likely cause language deficits.
• 38. Axial images obtained from a 35-year-old female with MS. Multiple lesions are revealed in the T2-weighted image (arrows). On the ADC map, lesions appear as hyperintense compared with the surrounding tissue (increased ADC values). On the FA map and DEC images, lesions appear as dark areas within the white matter and have decreased FA values
• 39. MR images from a 39-year-old right-handed male patient with refractory complex partial seizures. FLAIR and T2-weighted images show that the left hippocampus is atrophic with high signal (arrows) compared to the right side, suggesting left mesial temporal sclerosis. Hyperintensity with a significantly increased ADC value of 1.42 10–9 mm2/second and hypointensity with a decreased FA value of 0.147 are demonstrated respectively in the ADC and FA maps.
• 40. An eight-year-old boy with X-linked adrenoleukodystrophy. Hyperintense signal on T2- and FLAIR-weighted images along the splenium of the corpus callosum bilaterally, extends into the peritrigonal white matter of both occipital lobes (arrows). There is markedly increased ADC and decreased FA values. The FA map shows that the splenium of the corpus callosum is dark (arrows) compared with the genus of the corpus callosum (arrowheads). The splenium of the corpus callosum has lost its left to right fiber orientation, which also shown on DEC image.