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Diffusion Diffusion Presentation Transcript

  • DIFFUSION & PERFUSION MRI IMAGING Dr. Wael Darwish
  • DIFFUSION MRI IMAGING
  • - 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
  • 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,……).
  • - 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
  • - 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 = 5” “ b = 500” “ 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
  • - 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*
  • X Diffusion - Weighting Y Diffusion - Weighting Z Diffusion - Weighting Anisotropic diffusion : Individual direction weighted
  • Individual Diffusion Directions Mathematical Combination (Sorensen et al., MGH) Isotropic Diffusion- Weighted Image - + x / Isotropic diffusion
  • Diffusion weighted image
  • TE=100ms SR 120 TE=75ms SR150 Short TE DWI gives more SNR Characteristics of diffusion’s contrast b = 1000 s/mm 2
  • Higher b value increases sensitivity b = 1000 b= 3000 MS Stroke Tumor Vasogenic edema Cytotoxic Edema Higher CNR helps distinguish active lesions Higher CNR Vasogenic edema Characteristics of diffusion’s contrast
  • Diffusion-weighted ADC map Mathematical Processing
  • Diffusion-weighted ADC map Exponential ADC Mathematical Processing
  • Exponential ADC (ratio of Isotropic DWI/T2) eliminates T2 shine through artifacts and may distinguish subacute from acute stroke Diffusion Imaging Processing
  • b=0 eADC ADC b=1000 Arachnoid Cyst
  • Clinical Application
  • 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. . 2.
  • central hypointensity on diffusion-weighted image and hyperintensity on ADC map, consistent with the diagnosis of tumor. . 3. 4.
  • MR Images of 57-Year-Old Woman with Cerebral Metastasis 5. 6.
  • central hypointensity on diffusion-weighted image and hyperintensity on ADC map, consistent with the diagnosis of tumor. 7. 8.
  • MR Images of 70-Year-Old Man with History of Recent Vertigo and Disequilibrium 1. 2.
  • A brain abscess with Streptococcus anginosus was found at surgery. 3. 4.
  • the differential diagnosis between metastasis and abscess is impossible. MR Images of 57-Year-Old Woman with Cerebral Metastasis 5. 6.
  • Central hypointensity is seen on the diffusion-weighted image and hyperintensity on the ADC map, consistent with the diagnosis of tumor. 7. 8.
  • APPLICATIONS SPINE
  • BENIGN VERSUS MALIGNANT FRACTURE
    • 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
    • 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.
  • T2-weighted MR image shows ovoid hypointense mass in spinal canal.
  • T1-weighted sagittal MR image after infusion of gadolinium contrast material shows diffuse signal enhancement of mass.
  • T1-weighted transverse MR image after infusion of contrast material shows extent of tumor in spinal canal and C4-C5 neural foramen
  • 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 ).
  • ADC map shows mass ( arrows ) as structure of intermediate intensity. MENINGIOMA
    • 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.
  • Perfusion imaging
    • Definitions
    • Principles
    • Some more definitions
    • Perfusion technique
    • Applications
    • Future
  • 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
  • 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
  • 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
    • 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.
  • slice n time intensity ~ ‘mean transit time’ Integral:= cerebral blood volume At each voxel we observe : time
  • 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
  • Principles Extract time-intensity curves Perform mathematical manipulation Generate functional maps + + - + x / Negative Enhancement Integral Map(NEI) Qualitative rCBV map First Pass Contrast bolus Mean Time to Enhance (MTE)Map Ischaemic Penumbra MTE NEI
    • Dynamic Susceptibility Contrast Imaging
    • Hemodynamics Bl. volume
    • Bl. flow
    • Aim 1. Diagnosis
    • 2. Monitoring management
    • 3. Understanding intracranial lesions
    Dynamic MR perfusion
  • rCBV rCBV, processed with “Negative Enhancement Integral”(NEI) is related to area under curve
  • 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)
  • Cerebral blood perfusion by bolus tracking power injector - Gadolium 5ml/sec Procedure : 1 - Start Imaging 2 - Inject Contrast* 3 - Continue Imaging Requires very high speed imaging 10 slices - 50 images of each slice - TOTAL time 1:34 min * Push Gadolinium with 20 cc of saline flush
  • Applications of Perfusion MRI
    • Neurology
    • Gerontology
    • Neuro-oncology
    • Neurophysiology
    • Neuropharmacology
  • Perfusion Imaging: Findings in Infarction
    • CBV
      • regional perfusion deficit
      • compensatory increased volume
    • MTT
      • regional prolongation of transit time
    Stroke
  • Head Trauma T2 image showing bifrontal volume loss FLAIR image showing bifrontal gliosis and encephalomalacia
  • Head trauma:Hypo-perfusion rCBV MAP Tc-HMPAO SPECT Hypo-perfusion
  • E.g. 1 : Left hemisphere stroke, 4.5 hrs after onset of symptoms 3D-TOF Vascular FSE-T2W FSE-FLAIR
  • Same patient with DWI and FLAIR 4.5 hrs EPI FLAIR 4.5 hrs 24 hrs Diffusion imaging shows lesion early. b=0 b=800 FLAIR shows enhanced changes after 24 hrs.
  • Isotropic diffusion image b=800 ADC map Apparent diffusion coefficient ADC
  • Contrast enhanced perfusion imaging 24 slices 3 seconds/acquisition Time/intensity graph
  • Mean Time To Enhance delayed compensatory hyper perfusion delayed hypo perfusion
  • EPI Perfusion EPI Diffusion and Perfusion mapping EPI Diffusion MTT Mean Time To Enhance CBV Negative Enhancement Integral ADC Diffusion Coefficient *
  • 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
  • Alzheimer’s disease
    • FDG PET
      • marked temporo-parietal hypometabolism
    • Tc-HMPAO SPECT
      • marked temporo-parietal hypoperfusion
    • DSC MRI
      • correlates well with SPECT
    Findings with Perfusion imaging for Gerontology
  • 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
  • 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
    • 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
    Dynamic MR perfusion
  • 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
  • 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
  • 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
    • low rCBV in tumour infers low grade glioma
    rCBV maps
  • Eg2 Diffused tumor: Abnormal 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
  • Recurrent Tumor Eg3 tumor vs.radiation necrosis Conventional T2 CBV Non specific changes