Magnetic Resonance Diffusion

Diffusion weighted imaging basic principles
Different DWI sequences

1. Rahman Ud Din Lecturer Medical Imaging NWIHS Magentic Resonance Diffusion

2. Learning Outcomes What is Diffusion? How DWI is acquired? Clinical applications of DWI Diffusion Tensor Imaging?

3. Introduction Diffusion Weighted Imaging (DWI) Effective for diagnosis of various diseases Vital technique Brain imaging protocols (integral part) It’s role is expanding to other body imaging

4. What is Diffusion? Diffusion means random movement of water protons Brownian motion- water protons diffuse randomly in space Protons (H2O) diffuse to dissipate their thermal energy Difference in mobility of H2O molecules b/w tissues gives contrast in DWI DWI helps to characterize tissues and pathology

5. Types of Diffusion Isotropic Diffusion Possibility of water protons moving in any one particular direction is equal to the probability that it will move in any other direction Isotropic means uniformity in all directions Anisotropic Diffusion Water diffusion has preferred direction Water can move easily in one direction than other Isotropic diffusion basis for routine DWI Anisotropic basis for DTI (diffusion tensor imaging) or tractography

6. H ow do we acquire DWI? “ Stejskal -Tanner pulsed gradient spin echo seq ;” First experimental sequence described for the acquisition of DWI Forms the basis for all DWI performed today It is a T2-w SE sequence with diffusion gradients applied before and after the 180 degree pulse Now, diffusion gradients can be applied to various seq ; Mostly applied to EPI seq ; with infinite T2

7. Stejskal-T a nner's Sequence

8. T erms and Concepts The b-value It indicates the magnitude of DWI provided by the diffusion gradients It also indicates sensitivity of the seq ; to the diffusion Expressed in sec/mm2 Depends on amplitude, separation and duration of DG The b-value increases with DG strength & Duration of their applications of the two gradients As b-value increases the signal from water reduces Highest value of b=1000 only for tissues with very high T2 relaxation time

9. Diffusion "trace" Isotropic diffusion forms the basis for the routine DWI Also their will be some anisotropic movement of H2O as well Especially in brain from white matter tracts To reduce this anisotropy the image with higher b-value like b=1000 is acquired in all three directions X, Y and Z axes Diffusion changes along all three axes are then averaged to get a ‘trace’ diffusion image

10. ADC: Apparent Diffusion Coefficient ADC is measure of diffusion Calculated mathematically from b-value=0 and higher b-value images Signal attenuation of a tissue with increasing value plotted on graph with relative signal intensity on y-axis and b-value of x-axis Resultant slope of line is ADC Done on pixel by pixel basis by computer To user it is available as ADC map ADC is independent of ‘B’ Reduced ADC is ‘restricted diffusion’ [bright area] on DWI While on same area will be dark on ADC map ADC value from map for AOI measured in mm2/sec

11. ADC plot

12. T2 Shine Through Signal intensity on DWI (higher b-value images) not only depends on ADC but also on T2 relaxation time of tissue High T2 tissue appears bright on DWI (even not restricted) ADC map helps to differentiate T2 shine through from actual Or T2 shine through are exponential images formed by ratio of DWI images divided by T2-w (b=0) images in same series These images are called eADC by some vendors (Philips) Truly restricted area is bright on eADC eADC

13. Images available to view DWI routinely performed with EPI sequence Acquisition time is typically less than a minute With preset post-processing few sets of images are available for viewing immediately after acquisition Depending in the number of b-values used these images typically include b=0, higher b-values images and ADC map Higher b-value images considered as DWI

14. Clinical Applications of DWI DWI in Stroke Failure of Na-K ATPase pump tissue ischemia Results in influx of extracellular water into cells This is called cytotoxic edema Net shift of water molecules from extracellular into restricted intracellular space Overall, reduction in diffusion of water molecule in that area Manifested as bright signal on DWI and dark signal on ADC map DWI can detect early ischemic tissue (minute to hours) DWI shows stroke lesion earliest (failure of T2 appears normal) Vasogenic edema= increased fluid in extracellular space A chronic infract is dark on DWI and bright on ADC map

15. DWI and T2-w images in stroke

16. Epidermoid versus Arachnoid cyst Epidermoid composed of keratin, debris and solid cholesterol Provide hindrance to diffusion of H2O molecules Epidermoid is seen as bright lesion on DWI Arachnoid cyst is clear CSF containing cyst, it will not be bright on DWI will be same as CSF in the signal intensity DWI can detect a residual epidermoid

17. Abscess versus simple cystic lesion

18. DWI in brain tumors Abscess on DWI Lymphoma on DWI Moreover, following imaging are done on DWI as well Medulloblastoma Ependymoma Abdominal Imaging

19. DWI Body Imaging Use of DWI in body imaging is new Big obstacles in DWI imaging are motion and short T2 of various organs Imaging done with breathe-hold & respiratory triggering DWI mainly used in tumor imaging and in follow-up imaging For staging tumor and lymphoma whole body imaging with background suppression is used in-replacement to PET DWIBS (DW whole body imaging with background body signal suppression) Final DWIBS images shows only diffusion restricted structures and tissues (normal and abnormal)

20. DWI whole body

21. Diffusion Tensor Imaging Routine DTI based on anisotropic diffusion of water molecules Tensor is mathematical formalism used to model anisotropic dif: Technique MR scanner X, Y and Z are never perfectly parallel to the WM tracts at every point in the image In DTI, images are acquired in at least six, usually 12-24 directions instead of three in usual trace diffusion Pure ADC for each pixel is calculated from these images in multiple directions This is called ‘principal eigen value’ Principal eigen value is calculated true axis of diffusion called ‘ eigen vector’.

22. Images formed with principal eigen value is called DTI that gives orientation of fiber tracts Uses: DTI measure the magnitude of the ADC in the preferred direction of water diffusion and perpendicular to the direction The resultant image shows WM tracts very well Hence this technique is called ‘ tractography ’ Various maps used to indicate orientation of fiber tracts include FA (fractional anisotropy) RA (regional anisotropy) and VA (volume ratio) maps Tractography for the assessment of relationship of tracts with tumor, tumor invasion of tracts and preoperative planning Used to evaluate WM tracts in various congenital anomalies and dysplasias