DIFFUSION & PERFUSION MRI IMAGING

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DIFFUSION PERFUSIONMRI 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 patients 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
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- Principles -Velocities and methods of
measurement
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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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Medium
High
Low
b 500
b 1000
b 5
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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 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, diffusions 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
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Isotropic diffusion
Isotropic Diffusion- Weighted Image
Individual Diffusion Directions
Mathematical Combination (Sorensen et al., MGH)
- x /
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Diffusion weighted image
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Characteristics of diffusions contrast
Short TE DWI gives more SNR
TE100ms SR 120
TE75ms SR150
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Characteristics of diffusions contrast
Higher b value increases sensitivity
MS
Higher CNR helps distinguish active lesions
Stroke
Higher CNR
Vasogenic edema Cytotoxic Edema
Tumor
Vasogenic edema
b 1000 b 3000
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Mathematical Processing
Diffusion-weighted
ADC map
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Mathematical Processing
ADC map
Diffusion-weighted
Exponential ADC
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Diffusion Imaging Processing
Exponential ADC (ratio of Isotropic
DWI/T2) eliminates T2 shine through artifacts and
may distinguish subacute from acute stroke
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Arachnoid Cyst
b0
b1000
eADC
ADC
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Clinical Application
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MR Images of 60-Year-Old Man with Glioblastoma
Multiforme
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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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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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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A brain abscess with Streptococcus anginosus was
found at surgery.
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MR Images of 57-Year-Old Woman with Cerebral
Metastasis
the differential diagnosis between metastasis and
abscess is impossible.
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Central hypointensity is seen on the
diffusion-weighted image and hyperintensity on
the ADC map, consistent with the diagnosis of
tumor.
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APPLICATIONS

• SPINE

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BENIGN VERSUS MALIGNANT FRACTURE
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• This finding indicates that the lack of signal
  reduction in malignant vertebral fractures is
  caused by tumor cell infiltration
• Different diffusion effect is caused by more
  restriction or hindrance in densely packed tumor
  cells compared with more mobile water in
  extracellular volume fractions in fractures

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• diffusion-weighted spin-echo sequences could
  differentiate benign fracture edemas and
  fractures caused by tumor infiltration due to
  higher restriction of water mobility in tumor
  cells.

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T2-weighted MR image shows ovoid hypointense mass
in spinal canal.
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T1-weighted sagittal MR image after infusion of
gadolinium contrast material shows diffuse signal
enhancement of mass.
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T1-weighted transverse MR image after infusion of
contrast material shows extent of tumor in spinal
canal and C4-C5 neural foramen
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Diffusion-weighted sagittal MR image using
peripheral pulse gating and navigator correction
shows signal intensity of mass (open arrows) to
be intermediate, less than that of brainstem
(large solid arrow) and greater than that of
vertebral bodies (small solid arrows).
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ADC map shows mass (arrows) as structure of
intermediate intensity.
MENINGIOMA
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• In that study, tumors with high cellularity had
  low mean ADC values, and tumors with low
  cellularity had high mean ADC values.
• In addition, the relatively high ADC value seen
  in our patient corresponded to a low degree of
  cellularity, such as has been reported in
  cerebral gliomas.

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Perfusion imaging

• Definitions
• Principles
• Some more definitions
• Perfusion technique
• Applications
• Future

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Definitions

• Perfusion is refer to the delivery of oxygen
  and nutrients to the cells via capillaries
• Perfusion is identified with blood flow
  which is measured in milliliters per minute
  per 100 g of tissue

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Principles
After injection of a contrast agent

• In normal brain, the paramagnetic contrast agent
  remains enclosed within the cerebral
  vasculature because of the blood brain barrier
• The difference in magnetic susceptibility
  between the tissue and the blood results in
  local magnetic field ?finally to large signal loss

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Some more Definitions

• rCBF the rate of supply of Gd chelate to a
  specified mass ( ml / 100g / min)
• rCBV - the volume of distribution of the Gd
  chelate during its first passage through the
  brain ( or ml / 100g )
• MTT - the average time required for any given
  particle to pass through the tissue, following an
  idealised input function (min or s)
  MTT rCBV / rCBF

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• Passage of Gd. can be followed by the changes in
  the relaxation rates concentration of
  local contrast.
  
• Linear relation bet. concentration and rates of
  signal changes can be expressed as curve.
• Tissue contrast concentration time curve can be
  used to determine tissue micro vascularity,
  volume and flow.

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At each voxel we observe
slice n
mean transit time
time
Integral cerebral blood volume
intensity
time
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Principles

• Each one of these effects is linearly
  proportional to the concentration of the
  paramagnetic agent
• To date, this technique results in
  non-quantitative perfusion parameters (like
  rCBV,rCBF or MTT) because of the ignorance of
  the arterial input function

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Principles

• Dynamic Susceptibility Contrast Imaging

Extract time-intensity curves
Perform mathematical manipulation
Generate functional maps


NEI
- x /
MTE
Negative Enhancement Integral Map(NEI) Qualitativ
e rCBV map
Mean Time to Enhance (MTE)Map Ischaemic Penumbra
First Pass Contrast bolus
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Dynamic MR perfusion

• Hemodynamics Bl. volume
• Bl. flow
• Aim 1. Diagnosis
• 2. Monitoring management
• 3. Understanding intracranial
  lesions

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rCBV
rCBV, processed with Negative Enhancement
Integral(NEI) is related to area under curve

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MTT
MTT is related to the time to peak and to the
width of the peak it is processed with Mean
Time to Enhance(MTE)

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Cerebral blood perfusion by bolus tracking
Requires very high speed imaging
power injector - Gadolium 5ml/sec
Procedure 1 - Start Imaging 2 - Inject
Contrast 3 - Continue Imaging
10 slices - 50 images of each slice - TOTAL
time 134 min
Push Gadolinium with 20 cc of saline flush
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Applications of Perfusion MRI

• Neurology
• Gerontology
• Neuro-oncology
• Neurophysiology
• Neuropharmacology

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Perfusion Imaging Findings in Infarction
Stroke

• CBV
• regional perfusion deficit
• compensatory increased volume
• MTT
• regional prolongation of transit time

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Head Trauma
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Head traumaHypo-perfusion
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E.g. 1 Left hemisphere stroke, 4.5 hrs after
onset of symptoms
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Same patient with DWI and FLAIR
4.5 hrs
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Apparent diffusion coefficient ADC
ADC map
Isotropic diffusion image b800
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Contrast enhanced perfusion imaging
24 slices 3 seconds/acquisition
Time/intensity graph
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Mean Time To Enhance
delayed compensatory hyperperfusion
delayed hypoperfusion
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EPI Diffusion and Perfusion mapping
EPI Diffusion
EPI Perfusion
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Findings with Perfusion Imaging for Infarction

• Changes seen almost immediately after the
  induction of ischemia
• more sensitive than conventional MRI
• Perfusion findings often more extensive than
  those on DW-EPI in early stroke
• more accurately reflects the amount of tissue
  under ischemic conditions in the hyperacute
  period than DW EPI
• Abnormal results correlate with an increased risk
  of stroke
• PerfEPI - DWEPI tissue at risk

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Alzheimers disease
Findings with Perfusion imaging for Gerontology

• FDG PET
• marked temporo-parietal hypometabolism
• Tc-HMPAO SPECT
• marked temporo-parietal hypoperfusion
• DSC MRI
• correlates well with SPECT

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Findings with Perfusion imaging for
Neurophysiology and pharmacology

• Traumatic brain injury
• focal rCBV deficits that correlate with cognitive
  impairment
• Schizophrenia
• decreased frontal lobe rCBV
• HIV/ AIDS
• multiple discrete foci of decreased CBV
• Polysubstance abuse
• multiple discrete foci of decreased CBV

New Jersey Neuroscience Institute
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Findings with Perfusion imaging for Neuro-oncology

• Critical imaging to BBBB imaging of neoplasm
• many tumors have high rCBV
• regions of increased rCBV correlate with areas of
  active tumor.
• heterogeneous patterns of perfusion suggest high
  grade
• radiation necrosis typically demonstrates low
  rCBV
• Lesion characterization may be possible
• meningiomas have very high CBV in contrast to
  schwannomas

New Jersey Neuroscience Institute
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Dynamic MR perfusion

• Clinical applications-
• Intracranial neoplasm
• N.B angiogenesis usually aggressiveness
• Exceptions- 1. Meningioma
• 2.Choroid plexus papilloma
• 1.Glioma Grading
• Biopsy
• D.D recurrence from radiation
  necrosis

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2.Metastasis Can differentiate solitary
metastasis from 1ry brain neoplasm (glioma) by
measuring the peritumoral relative blood
volume. 3.1ry cerebral lymphoma Can help in
differentiating lymphoma from glioma as lymphoma
is much less vascular
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4. Meningioma Hypervascular
Extra axial Has leaky and permeable capillaries
causing no recovery of T2 signal to basline. 5.
Tumor mimicking lesions e.g.
cerebral infections tumefactive
demyelinating lesions less
commonly infarcts
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6.Tumefactive demyelinating lesions No
neo-vascularization in demyelinating lesions To
conclude MR perfusion should be included in
routine evaluation of brain tumor as it improve
diagnostic accuracy.
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Neuro-oncology
rCBV maps

• low rCBV in tumour infers low grade glioma

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Eg2 Diffused tumorAbnormal capillary density
Glioblastoma multiform
Hyper perfusion
Excised region
Before surgery MTSE shows blood brain / barrier
breakdown (bbbb)
After surgery rCBV map shows diffuse disease in
right frontal lobe
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Eg3 tumor vs.radiation necrosis
CBV
Conventional T2
Recurrent Tumor
Non specific changes