2. WHAT IS DIFFUSION?
random movement of the water protons
Brownian motion
The difference in the mobility of water molecules between tissues gives the
contrast in diffusion weighted imaging
7. BVALUE
magnitude of diffusion weighting provided by the diffusion
gradients
sensitivity of the sequence to the diffusion
sec/mm2
Amplitude, separation and duration of diffusion gradients
increases with diffusion gradient strength, duration of their
application and time between applications of the two gradients
As the b-value increases the signal from water molecules reduces
High b-value: only tissues with very highT2 relaxation time or
those with restricted motion of water molecules will have high
signal.
8. DIFFUSIONTRACE
Removing anisotropy
Higher b-value images need to be taken in three axis- x ,y, and z.
Diffusion changes along all three axes are then averaged to get a ‘trace’ diffusion
image.
9.
10. ADC- APPARENT DIFFUSION COEFFICIENT
measure of the diffusion
Calculated from b-value = 0 and various higher b-
value images
plotted on a graph
‘ADC Map’
The area with reduced ADC (restricted diffusion)
will manifest as a bright area on the diffusion
weighted images (DWI) while the same area will
turn dark on the ADC map.
Can also give quantitative information
11.
12. T2 SHINETHROUGH
The signal intensity of a tissue on DWI depends onT2 relaxation time of the tissue
as well.
Tissues with highT2 can appear bright on DWI even when they are not truly
restricted
.ADC map helps to differentiate
exponential images which are formed by the ratio of DWI images divided byT2-
weighted (b = 0) images in the same series.
18. Can detect early ischemic tissue
as early as minutes to hours
(CBF) below 15–20 ml/100 gm of
brain tissue/minute- bright signal
on DWI
Useful for hypoxic ischemic injury
in newborns
21. EPIDERMOIDVS ARACHNOID CYST
Epidermoid cyst
Composed of keratin, debris and solid
cholesterol
Bright on DWI
Arachnoid cyst
Clear CSF containing cyst
Not bright on DWI (same signal
intensity as CSF.
22. ABSCESSVS SIMPLE CYSTIC LESION
An abscess contains thick fluid with
hindrances to water diffusion.
Cystic lesion with relatively clear fluid
does not show restricted diffusion.
restricted diffusion,
predominantly in the
center
23.
24. BRAINTUMORS
qualitative and quantitative information at cellular level such as cellularity and cell
membrane integrity
tumor with more number of cells and intact cell membranes restricted
diffusion appear bright on DWI
used to detect, characterize and to assess chemotherapy response in tumors.
Bright on DWI- medulloblastoma, ependymoma and lymphoma
25.
26. DWI IN BODY IMAGING
relatively new
Obstacles: motion and shortT2 of various organs
can be performed with breath-hold, with respiratory triggering and even with free
breathing
lower b-values are used in body imaging
difusion-weighted whole-body imaging with background body signal suppression
(DWIBS)- mainly focused in tumor imaging and assessing treatment response
The final DWIBS images show only the diffusion restricted structures and tissues
(normal or abnormal).
Water molecules in the body undergo a random motion called Brownian motion which is result of thermal energy. These constant random motions are uniform in all directions
Diffusion in body is restricted by various organelles, membranes and tissue planes. Intracellular diffusion is more hindered because of cell membrane.
Isotropic- possibility of a water protons moving in any one particular direction is equal to the probability that it will move in any other direction (isotropy = uniformity in all directions)
In Anisotropic di!usion, water di!usion has preferred direction. Water protons move more easily in some direction than other.
water diffusion has preferred direction. Water protons move more easily in some direction than other.
first experimental sequence described for the acquisition of DWI and forms the basis for all DWI performed today. a T2-w spin echo sequence with di!usion gradients applied before and after the 180 degree pulse
Currently diffusion gradients can be applied to various sequences but they are most commonly applied to the echo planar (EPI) sequence with infinite T2.
Isotropic di!usion forms the basis for the routine di!usion imaging. However, there will be some anisotropy to the movement of water molecules in the tissue and especially in the brain from white matter tracts. To reduce this anisotropy, the image with higher b-value like b = 1000 is acquired in three directions along X, Y and Z axes. Di!usion changes along all three axes are then averaged to get a ‘trace’ di!usion image.
Signal attenuation of a tissue with increasing b-value is plotted on a graph with relative signal intensity on y-axis and b-values on x-axis. The slope of the line represents ADC. This is done pixel-by-pixel basis by the computer and is available at a click to the user as images called as ‘ADC Map’.
Drawing region of interest on ADC map gives ADC value of that particular area or tissue expressed in mm2 /sec.
e. (A) DWI shows bright area in the right MCA territory. (B) On the ADC map the area turns dark suggestive of acute infarct. (C) On the Gradient Hemo image there is no evidence of any bleeding, making the infarct a non-hemorrhagic. (D) TOF MRA of the circle of Willis shows absence of the right ICA, MCA and ACA (arrow). (E) TOR MRA of carotid arteries shows complete occlusion of right ICA from its origin (arrow
DWI shows the stroke lesion earliest when all other images including T2-weighted images are normal
vasogenic edema means increased #uid in the extracellular space. Since the extracellular space allows more mobility of water molecules there will be increased di!usion, seen as signal attenuation on DWI and increased signal on the ADC map. A chronic infarct is dark on DWI and bright on ADC map
Chronic infarct. A chronic infarct in the left basal ganglion region (arrows) is hypointense on DWI image (A) and bright on ADC map (B)
Characteristic appearance of an epidermoid cyst in the cerebellopontine angle. The diagnosis is confirmed on DWI which demonstrates very bright signal with intermediate ADC values similar to brain parenchyma.
Abscess. (A) Axial FLAIR image shows a large abscess in the left cerebral hemisphere. (B) DWI image shows central part of the abscess to be bright. (C) On the ADC map the central part of the abscess turns black suggestive of restricted di!usion. Surrounding edema, which is increased #uid in extracellular space, is bright on FLAIR, low on DWI and bright on the ADC map suggestive of increased di!usion
tumor with less cells and broken cell membrane (for example from chemotherapy) will not be restricted on DWI.
there is a sharp increase in the ADC value of some tumors within "rst two weeks of treatment. This increase in ADC value corresponds with volume of the tissue killed and can be used for assessing response to chemotherapy and prognostication.
Ill-de"ned areas of restricted di!usion (arrows) are seen in the left cerebral hemisphere, which are bright on DWI image (A) and dark on the ADC map (B). This turned out to be a primary CNS lymphoma. Lymphoma shows restricted di!usion due to its high cellularity
