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 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 - 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 “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 - 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* 10 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 Findings with Perfusion imaging for Gerontology 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 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 Dynamic MR perfusion Clinical applications:- • Intracranial neoplasm N.B angiogenesis usually = aggressiveness Exceptions:- Meningioma 2.Choroid plexus papilloma 1.Glioma Grading Biopsy D.D recurrence from radiation necrosis 65 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 66 is much Meningioma Hypervascular Extra axial Has leaky and permeable capillaries causing no recovery of T2* signal to basline Tumor mimicking lesions e.g cerebral infections lesions tumefactive demyelinating less commonly infarcts 67 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 68 69 70 71 72 Neuro-oncology rCBV maps • low rCBV in tumour infers low grade glioma 73 Eg2 Diffused tumor: Abnormal capillary density Glioblastoma multiform Hyper perfusion Excised region Before surgery MTSE shows blood brain / After surgery rCBV map shows diffuse disease in right frontal lobe barrier breakdown (bbbb) Eg3 tumor vs.radiation necrosis Conventional T2 Non specific changes CBV Recurrent Tumor ... 10 Anisotropic diffusion : Individual direction weighted X Diffusion - Weighting Y Diffusion - Weighting Z Diffusion - Weighting Isotropic diffusion Isotropic DiffusionIndividual Diffusion Weighted... 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... Flow Veins ~100mm/ s “Phase Contrast” Perfusion Capillaries ~1mm/ s Contrast Wash out Diffusion Intercellular ~ 0,001mm/ s space Diffusion weighting - Principles About the b factor • b is a value