mri diffusion .pptx

MRI diffusion

1. Magnetic Resonance Diffusion

2. What is diffusion? random movement of the water protons Brownian motion The difference in the mobility of water molecules between tissues gives the contrast in diffusion weighted imaging

3. types ISOTROPIC DIFFUSION ANISOTROPIC DIFFUSION DWI- Isotropic DTI- Anisotropic

4. Acquisition stejskal-Tanner pulsed gradient spin echo sequence

5. B value magnitude of diffusion weighting provided by the diffusion gradients sensitivity of the sequence to the diffusion sec/mm 2 Amplitude, separation and duration of diffusion gradients increases with diffusion gradient strength, duration of their application and time between applications of the two gradients As the b-value increases the signal from water molecules reduces High b-value: only tissues with very high T2 relaxation time or those with restricted motion of water molecules will have high signal.

6. Diffusion trace Removing anisotropy Higher b-value images need to be taken in three axis- x ,y, and z. Diffusion changes along all three axes are then averaged to get a ‘trace’ diffusion image.

7. ADC- apparent Diffusion Coefficient measure of the diffusion Calculated from b-value = 0 and various higher b-value images plotted on a graph ‘ADC Map’ The area with reduced ADC (restricted diffusion) will manifest as a bright area on the diffusion weighted images (DWI) while the same area will turn dark on the ADC map. Can also give quantitative information

8. T2 Shine through The signal intensity of a tissue on DWI depends on T2 relaxation time of the tissue as well. Tissues with high T2 can appear bright on DWI even when they are not truly restricted .ADC map helps to differentiate exponential images which are formed by the ratio of DWI images divided by T2-weighted (b = 0) images in the same series.

9. Clinical application

10. Neuroimaging Tissue Ischemia Failure of Na-K ATPase pump Influx of extracellular water into cells Reduced diffusion Bright on DWI Dark on ADC CYTOTOXIC EDEMA STROKE

11. Can detect early ischemic tissue as early as minutes to hours (CBF) below 15–20 ml/100 gm of brain tissue/minute- bright signal on DWI Useful for hypoxic ischemic injury in newborns

12. Increased fluid in extracellular space More mobility of water molecules in extracellular space Increased diffusion Signal attenuation on DWI Increased signal on ADC VASOGENIC EDEMA

13. Epidermoid vs Arachnoid cyst Epidermoid cyst Composed of keratin, debris and solid cholesterol Bright on DWI Arachnoid cyst Clear CSF containing cyst Not bright on DWI (same signal intensity as CSF.

14. Abscess vs simple cystic lesion An abscess contains thick fluid with hindrances to water diffusion. Cystic lesion with relatively clear fluid does not show restricted diffusion. restricted diffusion, predominantly in the center

15. brain tumors qualitative and quantitative information at cellular level such as cellularity and cell membrane integrity tumor with more number of cells and intact cell membranes restricted diffusion appear bright on DWI used to detect, characterize and to assess chemotherapy response in tumors. Bright on DWI- medulloblastoma, ependymoma and lymphoma

16. DWI in Body Imaging relatively new Obstacles: motion and short T2 of various organs can be performed with breath-hold, with respiratory triggering and even with free breathing lower b-values are used in body imaging difusion -weighted whole-body imaging with background body signal suppression (DWIBS)- mainly focused in tumor imaging and assessing treatment response The final DWIBS images show only the diffusion restricted structures and tissues (normal or abnormal).

17. Thank you