Year : 2007 | Volume
: 32 | Issue : 2 | Page : 79--80
Review of a Ph.D. thesis entitled 'A study of the assessment of acute ischemic stroke by the use of optimized diffusion-weighted MR imaging'
Department of Nuclear Medicine, All India Institute of Medical Sciences, New Delhi - 110029, India
G S Pant
Department of Nuclear Medicine, All India Institute of Medical Sciences, New Delhi - 110029
|How to cite this article:|
Pant G S. Review of a Ph.D. thesis entitled 'A study of the assessment of acute ischemic stroke by the use of optimized diffusion-weighted MR imaging'.J Med Phys 2007;32:79-80
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Pant G S. Review of a Ph.D. thesis entitled 'A study of the assessment of acute ischemic stroke by the use of optimized diffusion-weighted MR imaging'. J Med Phys [serial online] 2007 [cited 2021 Jan 20 ];32:79-80
Available from: https://www.jmp.org.in/text.asp?2007/32/2/79/33248
Ajai K. Srivastava
Dept. of Radiology, University College of Medical Sciences and GTB Hospital, Dilshad Garden, Delhi - 110 095, India
The author has used diffusion-weighted MR imaging in patients with ischemic stroke. Ischemic stroke is caused by occlusion of a blood vessel that disrupts the flow of blood to the brain and causes sudden onset of focal neurological impairment lasting more than 24 h. Patients are found to be benefited from artery recanalization only if therapy is started within the first 3 h after occlusion. Conventional MR sequences and CT images do not show early changes of ischemic stroke until 6-12 h. However, diffusion-weighted MR imaging (DWI) can detect an infarct within 1-2 h because ischemic tissue exhibits reduced diffusion, which is observed within about half an hour of ischemia. On diffusion-weighted MRI, a proton of water molecule experiences slightly different field strength because of local inhomogeneity in the magnetic field, dependent on its spatial location. Diffusion-weighted images are obtained by adding a series of two gradient pulses, which are applied symmetrically with respect to 180° radio frequency pulse. The first gradient pulse is applied between the 90° pulse and the 180° radio frequency (RF) pulse. Even microscopic motion after this pulse causes molecules to acquire phase shift relative to their transverse magnetization. Both 180° pulse and the second gradient pulse rephase stationary spins.
To evaluate the diagnostic accuracy of diffusion-weighted MR imaging, a study was performed within 12 h of stroke symptoms. To study the time course of ADC , the changes were studied in acute (within 12 h), subacute (at 30 th day) and chronic stages (at 90 th day). The study consisted of 27 patients of acute cerebral ischemic stroke selected from those attending the Medical Emergency/OPD, who were at risk for acute cerebral ischemia. Patients with symptoms suggestive of acute stroke initially underwent a noncontrast CT scan of the head to exclude intracranial hemorrhage. Only those patients who were found negative on CT scans were subjected to diffusion MR imaging in addition to conventional T 1 W, T 2 W and FLAIR . The DWI was performed as early as possible but not later than 12 h (t 1 ). The DWI and conventional MR imaging were also performed at approximately 30 days (t 2 ) and 90 days (t 3 ) for the same patients. The MR studies were performed on a clinical 1.5 Tesla whole body MR system. The standard circularly polarized (CT) phased array head coil was used for excitation and signal reception for the MR studies.
For the ADC estimate, a small region of interest (ROI) was placed at the center of infarct on diffusion isotropic images taken at b = 0 and b = 1000. As a control for signal intensity, a second ROI of size same as the first ROI was placed at the same location on the normal contralateral hemisphere (minor image) for images taken at b = 0, b = 50, b = 500 and b = 1000 in all patients. The study showed that the mean signal intensity at b = 0 s/mm 2 and on diffusion weighted at b = 1000 s/mm 2 were significantly higher than control values for all time periods. The rSI b=0 significantly increased from 1.63 ± 0.20 in the acute stage to 2.19 ± 0.24 in the chronic stage (P DWI decreased from 2.54 ± 0.46 to 1.54 ± 0.22. The mean rADC was found to be 41% lower than mean ADC in the corresponding contralateral hemisphere. Linear regression analysis between rADC and time in log hours showed that pseudo-normalization occurred at 6.61 days (P b=0 ), SI DW and ADC on diffusion-weighted imaging.
This study demonstrates that acute cerebral ischemia can be depicted much earlier by DWI than conventional MR or CT imaging. It has been revealed that the accuracy of T2W and FLAIR scans are only 36 and 55% respectively within 6 h as against 94% with DWI. Apparent diffusion might assist the clinician in selecting patient for salvageable tissue within ischemic penumbra. The mean rADC measurement found in this study was 41% below that of normal tissue in core of the infarct.
The thesis work was conducted under the supervision of Prof. S. K. Bhargava at Guru Teg Bahadur Hospital, Delhi. Dr. R. P. Tripathi, Director, INMAS , Delhi, was one of the co-guides.