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DIFFUSION & PERFUSIONDIFFUSION & PERFUSIONMRI IMAGINGMRI IMAGING

Dr. Mohamed El Safwany, MD.Dr. Mohamed El Safwany, MD.

Intended learning outcome

• The student should learn at the end of this lecture principles of Diffusion and Perfusion MRI studies.

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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

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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

treatment 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

Diffusion gradients sensitize MR Image to Diffusion gradients sensitize MR Image to motion of water moleculesmotion of water molecules

More motion = Darker imageMore motion = Darker image

Freely Diffusing Water = DarkFreely Diffusing Water = Dark Restricted Diffusion = BrightRestricted Diffusion = Bright

- Diffusion contrast -

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- Principles -Velocities and methods of

measurementFast Flow Arteries ~1000mm/ s “Phase Contrast”

Slow Flow Veins ~100mm/ s “Phase Contrast”

Perfusion Capillaries ~1mm/ s Contrast Wash out

Diffusion Intercellular space

~ 0,001mm/ s 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

Low Medium

High

““b = 5b = 5”” ““b = 500b = 500”” ““b = 1000b = 1000””

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- 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

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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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Diffusion weighted image

TE=100msTE=100msSR 120SR 120

TE=75msTE=75msSR150SR150

b = 1000 s/mmb = 1000 s/mm22

Short TE DWI gives more SNRShort TE DWI gives more SNR

Characteristics of diffusion’s contrastCharacteristics of diffusion’s contrast

Higher b value increases sensitivityHigher b value increases sensitivity

b = 1000 b= 3000

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Stroke

TumorVasogenic edema

Cytotoxic Edema

Higher CNR helpsdistinguish active lesions

Higher CNR

Vasogenic edema

Characteristics of diffusion’s contrastCharacteristics of diffusion’s contrast

Diffusion-weighted Diffusion-weighted ADCADCmapmap

Mathematical ProcessingMathematical Processing

Diffusion-weightedDiffusion-weighted ADCADCmapmap

Mathematical ProcessingMathematical Processing

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

Diffusion Imaging ProcessingDiffusion Imaging Processing

b=0b=0

eADCeADCADCADC

b=1000b=1000

Arachnoid CystArachnoid Cyst

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Clinical Clinical Application Application

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MR Images of 60-Year-Old Man with Glioblastoma

Multiforme.

2. Figures 1, 2. On (1) T2-weighted fast spin-echo and (2) contrast-enhanced T1-weighted spin-echo images, the differential diagnosis between glioblastoma and abscess is impossible.

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3. 4. 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

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

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

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3. 4. A brain abscess with Streptococcus anginosus was found at surgery.

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6. the differential diagnosis between metastasis and abscess is impossible.

MR Images of 57-Year-Old Woman with Cerebral Metastasis

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8 . Central hypointensity is seen on the diffusion-weighted image and hyperintensity on the ADC map, consistent with the diagnosis of tumor.

Text Book

• David Sutton’s Radiology

• Clark’s Radiographic positioning and techniques

Assignment

• Two students will be selected for assignment.

Question

• Describe the effect of b value upon diffusion sensitivity?