Susceptibility weighted imaging: does it give information similar to perfusion weighted imaging in acute stroke?

Abstract

In a patient presenting with acute stroke it is essential to make a quick decision on thrombolysis based on history, clinical findings and the information obtained on an urgent CT or MRI. Though CT is considered the imaging technique of choice to rule out intracerebral haemorrhage, MRI using a gradient T2*/SWI sequence has been found to be equally good [1]. If the presentation is beyond the first 3 h, there is a need to image the salvageable penumbra. MRI gives information about the penumbra by using the diffusion–perfusion imaging. The role of susceptibility weighted imaging (SWI) in patients with stroke has already been documented [2]. We report two interesting cases of acute stroke where the findings in SWI helped in early diagnosis and management. A 43-year-old male presented with wake up stroke and was rushed to our hospital. The patient could not move the right sided limbs and had aphasia. Magnetic resonance imaging done at 7.30 a.m, soon after his arrival in the hospital, revealed a left middle cerebral territory infarct involving the frontal operculum, insular and lateral frontal cortex. The diffusion restriction was minimal. Prominent veins were noted in the SWI posterior to the area of infarct. The same area showed reduced regional cerebral blood flow and increased mean transit time (Fig. 1). The diffusion restricted areas showed normal cerebral blood flow and mean transmit time. The second patient was a 44-year-old male who presented with sudden onset of weakness of right sided limbs and complete inability to speak at 8.30 a.m. The patient was immediately taken to a local hospital from where a CT scan was done and this was reported to be showing a doubtable dense MCA sign. At around 10.30 a.m his weakness started improving, and by 11 a.m his speaking difficulty also improved. In view of complete improvement, thrombolysis was deferred. He was referred to our hospital for further evaluation. An MRI done on the same day in the afternoon revealed no areas of restricted diffusion. The SWI showed MCA susceptibility vessel sign [3] and prominent veins in the left MCA territory. Hyperintense vessel sign [3] was noted in the FLAIR sequence. The MR angiography showed thrombus close to the MCA bifurcation (Fig. 2a–j). Since the patient improved and thrombolysis was not being planned, a perfusion MRI was not done. As part of workup for the transient ischemic attack, an MR angiography of neck vessels, FLAIR and diffusion weighted imaging of brain was repeated after 3 days. This revealed diffusion restriction in the left caudoputaminal region (Fig. 2k, l). In the first patient, the findings in SWI were corresponding to the areas showing change in the cerebral blood flow and mean transit time. Prominence of veins following stroke has been described earlier [4–6]. The prominence is thought to be due to increased deoxyhemoglobin in the veins. This may be secondary to increased oxygen extraction fraction due to reduced blood flow [7]. The findings in the second patient show that the SWI changes can be seen even without changes in diffusion imaging. In this patient, the clinical picture was that of transient ischemic attack. A reduction in cerebral blood flow due to C. Kesavadas (&) B. Thomas H. Pendharakar Department of Imaging Sciences and Interventional Radiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, India e-mail: chandkesav@yahoo.com