DWI DTI MRI

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.