Value of MRI SEQUENCES in Stroke Imaging

Introductory presentation by radiologist Dr. Sadina Shrestha on basics of MRI Imaging

1. MRI SEQUENCES (BASICS) Presenter: Dr. Sadina Shrestha Department of R adiology, UDMNINAS

2. Hydrogen atom MRI PRINCIPLE Nuclear magnetic resonance (NMR) Hydrogen nuclei is one of them which is not only positively charged, but also has magnetic spin MRI utilizes this magnetic spin property of protons of hydrogen to elicit images

3. Hydrogen ions in body - spinning in a haphazard fashion, and cancel all the magnetism. External magnetic field applied - protons in the body align in one direction. (As the compass aligns in the presence of earth’s magnetic field)

4. NET MAGNETIZATION Half of the protons align along the magnetic field and rest are aligned opposite

5. Basic Sequences: T1 WI T2 WI FLAIR DWI SWI

6. WHICH SCAN BEST DEFINES THE ABNORMALITY T1 W Images: Subacute Hemorrhage Fat-containing structures Anatomical Details T2 W Images: Edema Demyelination Infarction Chronic Hemorrhage FLAIR Images: Edema, Demyelination Infarction esp. in Periventricular location

7. GRE RF pulse is applied that partly flips the NMV into the transverse plane (variable flip angle). Gradients , as opposed to RF pulses, are used to dephase (negative gradient) and rephase (positive gradients) transverse magnetization. Because gradients do not refocus field inhomogeneities , GRE sequences with long TEs are T2* weighted (because of magnetic susceptibility) rather than T2 weighted like SE sequences

8. This feature of GRE sequences is exploited- in detection of hemorrhage, as the iron in Hb becomes magnetized locally (produces its own local magnetic field) and thus dephases the spinning nuclei. Helpful for diagnosing hemorrhagic contusions such as those in the brain and in pigmented villonodular synovitis . SE sequences, on the other hand- relatively immune from magnetic susceptibility artifacts, and also less sensitive in depicting hemorrhage and calcification.

9. Susceptibility weighted imaging (SWI) is an MRI sequence that is particularly sensitive to compounds which distort the local magnetic field and as such make it useful in detecting blood products, calcium, etc.

10. THANK YOU!