Hyperdense MCA Sign and the "GRE susceptibility vessel sign (GRE SVS)".

Middle cerebral artery dense sign.

Diffusion weighted imaging (DWI) of a patient shows high signal intensity in the right middle cerebral artery (MCA) territory (A). On magnetic resonance angiography (MRA) showing the area, corresponding to the DWI of (A), a right M1 occlusion is present (B). A gradient echo image susceptibility vessel sign (GRE SVS) is shown on a GRE image (C; white arrow). The cross-sectional area of the thrombus was calculated using the Picture Archiving and Communication System (PACS; Maroview version 5.4, MAROTECH Inc., Seoul, Korea). In this case, the calculated cross-sectional area of the thrombus was 25.07 mm2 (D).

Acute ischemic stroke is an event of the highest urgency. Persistent major cerebral arterial occlusion is associated with high mortality and poor neurologic outcome.6),20) Recanalization using intravenous administration of tissue plasminogen activator (IV-tPA) or intra-arterial chemical thrombolysis (IAT) has been shown to improve patient outcome.2),7),22) Endovascular mechanical thrombectomy (MT) has recently been shown to induce high-grade, rapid recanalization, resulting in favorable clinical outcomes.8),9),16)

To date, several studies have reported on the usefulness of assessment of thrombus burden using computed tomography (CT) angiography in prediction of clinical and radiologic outcomes.1),11),21) In addition, a small number of studies using magnetic resonance image (MRI) for estimation of treatment outcomes and thrombus burdens have been reported. Cho et al. suggested that the magnetic susceptibility effect of deoxyhemoglobin in intraluminal clots, which appears as a hypointense signal on T2*-weighted gradient echo (GRE) images, might predict cardioembolic stroke and subsequent recanalization. These authors termed this radiologic finding the "GRE susceptibility vessel sign (GRE SVS)".4) However, Schellinger et al. reported that the GRE SVS can aid in diagnosis of ischemic disease but does not predict recanalization.19) Previous studies have focused solely on the presence of GRE SVS and recanalization after thrombolytic treatment. Correlations between thrombus size according to GRE SVS and recanalization after IV-tPA have not been thoroughly investigated.

The current study was conducted for analysis of the relationships between thrombus size of GRE SVS and recanalization after thrombolytic therapy using IV-tPA.

Image analysis

GRE SVS was defined by hypointense signals in the cerebral artery in axial section T2*-weighted gradient echo images of the territory that corresponded to high signal on the DWIs. MRI examinations were performed on a 1.5 T unit. The common MRI parameters for DWI and GRE were an equivalent slice thickness of 5 mm, an interslice gap of 2 mm, 20 axial slices, and field-of-view of 220 × 220. DWI parameters included a repetition time (TR) of 3000 ms, an echo time (TE) of 73.5 ms, and a matrix number of 160 × 160. GRE parameters were TR of 550 ms, a TE of 20 ms, a matrix number of 256 × 192, and a flip angle of 20°. MRA parameters included a flip angle of 20°, a matrix number of 256 × 192, a field-of-view of 210 × 210, TR of 25 ms, and TE of 6.3 ms.

For estimation of thrombus size, which can represent the actual thrombus volume, the cross-sectional areas of the GRE SVSs on the initial MRI were calculated. The approximate volume of the mass on MRI can be expressed as summation of the products of cross-sectional areas and slice thickness. The average diameter of M1 is 3.35 mm.14) Therefore, GRE SVSs on M1 usually appeared in a slice, and we calculated thrombus cross-sectional area as a representative value of thrombus volume. Radiographic estimations of the cross-sectional areas of the thrombi based on the GRE SVSs were performed using the Picture Archiving and Communication System (PACS; Maroview version 5.4, MAROTECH Inc., Seoul, Korea) and a region of interest (ROI) calculator (Fig. 1). When the GRE SVSs appeared in two slices due to curvature of MCA, the areas were summed.

Recanalization was defined as the reappearance of the distal part of the occluded vessel on the follow-up MRI. In following TFCA, recanalization was graded according to Thrombolysis in Cerebral Infarction (TICI) scores (Grade 0: no perfusion; Grade 1: penetration with minimal perfusion; Grade 2a: partial filling (< 2/3) of the vascular territory; Grade 2b: complete filling of the vascular territory that occurs more slowly than normal; and Grade 3: complete perfusion).10) TICI grades of 2b and 3 were considered to indicate successful recanalization.

CONCLUSION

The authors found a correlation between thrombus size on GRE and radiologic outcome after IV-tPA. Thrombus size on GRE may be a relative predictor of radiologic outcome. M1 occlusions with small thrombi according to GER SVS were more likely to be recanalized following thrombolysis with IV-tPA. Thrombus size on GRE is a simple diagnostic tool that can be easily measured, for quantitative assessment of clot burden in acute M1 occlusion. It can also be helpful in decision making with regard to use of more aggressive recanalization strategies.

Reference