LAST DIFFUSION perfusion NCI

DIFFUSION & PERFUSION
MRI IMAGING

Dr. Wael Darwish

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DIFFUSION MRI IMAGING

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- History • The feasibility of diffusion images was demonstrated in
the middle 1980s
• Demonstration on clinical studies is more recent ; it
corresponds with the availability of EPI on MR system
• A single shot EPI sequence can freeze the macroscopic
pulsating motion of the brain or motion of the patient’s
head

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Diffusion Weighted Image
• Core of infarct = irreversible damage
• Surrounding ischemic area  may be salvaged
• DWI: open a window of opportunity during which ttt is
beneficial
• DWI: images the random motion of water molecules as
they diffuse through the extra-cellular space

• Regions of high mobility “rapid diffusion”  dark
• Regions of low mobility “slow diffusion”  bright
• Difficulty: DWI is highly sensitive to all of types of
motion (blood flow, pulsatility, bulk patient motion,
……).
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- Diffusion contrast 



Diffusion gradients sensitize MR Image to
motion of water molecules
More motion = Darker image

Freely Diffusing Water = Dark

Restricted Diffusion = Bright


- Principles Velocities and methods of
measurement
Fast Flow

Arteries

~1000mm/ s

“Phase Contrast”

Slow Flow

Veins

~100mm/ s

“Phase Contrast”

Perfusion

Capillaries

~1mm/ s

Contrast Wash out

Diffusion

Intercellular ~ 0,001mm/ s
space

Diffusion weighting

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- Principles About the b factor
• b is a value that include all gradients effect
(imaging gradients + diffusion gradients)

• The b value can be regarded as analogous to
the TE for the T2 weighting

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Low

“b = 5”

Mediu
m

“b = 500”

High

“b = 1000”


- Principles About ADC
The ADC value does not depend on the field strength of
the magnet or on the pulse sequence used (which is
different for T1 or T2)
The ADC obtained at different times in a given patient
or in different patients or in different hospitals can be
compared

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- Principles Isotropic and Anisotropic diffusion
• Diffusion is a three dimensional process, but
molecular mobility may not be the same in all
directions
• In brain white matter, diffusion’s value
depends on the orientation of the myelin fiber
tracts and on the gradient direction*

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Anisotropic diffusion : Individual
direction weighted
X Diffusion - Weighting Y Diffusion - Weighting

Z Diffusion - Weighting


Isotropic diffusion
Isotropic DiffusionIndividual Diffusion
Weighted Image
Directions
Mathematical Combination
(Sorensen et al., MGH)

-+x/


Diffusion weighted image

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Characteristics of diffusion’s contrast
Short TE DWI gives more SNR

TE=100ms
SR 120
b = 1000 s/mm

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TE=75ms
SR150


Characteristics of diffusion’s contrast
Higher b value increases sensitivity
MS

Higher CNR helps
distinguish active lesions

Stroke

Higher CNR

Vasogenic edema

Tumor

Cytotoxic Edema
Vasogenic edema

b = 1000

b= 3000


Mathematical Processing

Diffusion-weighted

ADC
map


Mathematical Processing

Diffusion-weighted

ADC
map

Exponential ADC


Diffusion Imaging Processing

Exponential ADC (ratio of Isotropic DWI/T2)
eliminates T2 shine through artifacts and may
distinguish subacute from acute stroke


Arachnoid Cyst
b=0

b=1000

ADC
eADC


Clinical
Application

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MR Images of 60-Year-Old Man
with Glioblastoma Multiforme
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Figures 1, 2. On (1) T2-weighted fast spin-echo and (2) contrastenhanced T1-weighted spin-echo images, the differential diagnosis
between glioblastoma and abscess is impossible.
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.
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central hypointensity on diffusion-weighted image and
hyperintensity on ADC map, consistent with the diagnosis of
tumor.
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MR Images of 57-Year-Old
Woman with Cerebral
Metastasis
.5

.6

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.7

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central hypointensity on diffusion-weighted image and
hyperintensity on ADC map, consistent with the diagnosis of tumor.

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MR Images of 70-Year-Old Man with History of Recent Vertigo and
Disequilibrium

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.2

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