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DIFFUSION & PERFUSION MRI IMAGING

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

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DIFFUSION & PERFUSION MRI IMAGING

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  1. DIFFUSION & PERFUSIONMRI IMAGING Dr. Wael Darwish

  2. DIFFUSION MRI IMAGING

  3. - 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

  4. 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,……).

  5. - Diffusion contrast - • Diffusion gradients sensitize MR Image to motion of water molecules • More motion = Darker image Freely Diffusing Water = Dark Restricted Diffusion = Bright

  6. - Principles -Velocities and methods of measurement

  7. - 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

  8. Medium High Low “b = 500” “b = 1000” “b = 5”

  9. - 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

  10. - 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*

  11. Anisotropic diffusion : Individual direction weighted X Diffusion - Weighting Y Diffusion - Weighting Z Diffusion - Weighting

  12. Isotropic diffusion Isotropic Diffusion- Weighted Image Individual Diffusion Directions Mathematical Combination (Sorensen et al., MGH) - + x /

  13. Diffusion weighted image

  14. Characteristics of diffusion’s contrast 2 b = 1000 s/mm Short TE DWI gives more SNR TE=100ms SR 120 TE=75ms SR150

  15. Characteristics of diffusion’s 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

  16. Mathematical Processing Diffusion-weighted ADC map

  17. Mathematical Processing ADC map Diffusion-weighted Exponential ADC

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

  19. Arachnoid Cyst b=0 b=1000 ADC eADC

  20. Clinical Application

  21. . 2. 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.

  22. . 3. 4. central hypointensity on diffusion-weighted image and hyperintensity on ADC map, consistent with the diagnosis of tumor.

  23. 5. 6. MR Images of 57-Year-Old Woman with Cerebral Metastasis

  24. 7. 8. central hypointensity on diffusion-weighted image and hyperintensity on ADC map, consistent with the diagnosis of tumor.

  25. 1. 2. MR Images of 70-Year-Old Man with History of Recent Vertigo and Disequilibrium

  26. 3. 4. A brain abscess with Streptococcus anginosus was found at surgery.

  27. 5. 6. MR Images of 57-Year-Old Woman with Cerebral Metastasis the differential diagnosis between metastasis and abscess is impossible.

  28. 7. 8. Central hypointensity is seen on the diffusion-weighted image and hyperintensity on the ADC map, consistent with the diagnosis of tumor.

  29. APPLICATIONS SPINE

  30. BENIGN VERSUS MALIGNANT FRACTURE

  31. 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

  32. 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.

  33. T2-weighted MR image shows ovoid hypointense mass in spinal canal.

  34. T1-weighted sagittal MR image after infusion of gadolinium contrast material shows diffuse signal enhancement of mass.

  35. T1-weighted transverse MR image after infusion of contrast material shows extent of tumor in spinal canal and C4-C5 neural foramen

  36. 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).

  37. ADC map shows mass (arrows) as structure of intermediate intensity. MENINGIOMA

  38. 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.

  39. Perfusion imaging • Definitions • Principles • Some more definitions • Perfusion technique • Applications • Future

  40. 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

  41. 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

  42. 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

  43. 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.

  44. At each voxel we observe : slice n ~ ‘mean transit time’ time Integral:= cerebral blood volume intensity time

  45. 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

  46. Principles • Dynamic Susceptibility Contrast Imaging Extract time-intensity curves Perform mathematical manipulation Generate functional maps + + NEI - + x / MTE Negative Enhancement Integral Map(NEI) Qualitative rCBV map Mean Time to Enhance (MTE)Map Ischaemic Penumbra First Pass Contrast bolus

  47. Hemodynamics Bl. volume Bl. flow Aim 1. Diagnosis 2. Monitoring management 3. Understanding intracranial lesions Dynamic MR perfusion

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

  49. 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)

  50. 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 1:34 min *Push Gadolinium with 20 cc of saline flush

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