Comparison of pre- and postcontrast 3D time-of-flight MR angiography for the evaluation of distal intracranial branch occlusions in acute ischemic stroke

Abstract

Background and purpose: Three-dimensional time-of-flight (TOF) MR angiography is used routinely in stroke workup to detect arterial occlusions, but a major drawback is its inadequate depiction of vessels with slow or in-plane flow. We hypothesized that the use of contrast-enhanced MR angiography improves delineation of vessels with diminished or absent flow on precontrast MR angiograms.

Methods: Pre- and postcontrast 3D TOF MR angiograms were acquired in 55 consecutive patients with acute stroke. Patency of 480 intracranial vessels was assessed on both the pre- and postcontrast angiograms. Diffusion-weighted (DW) and perfusion-weighted (PW) imaging data were also obtained and results correlated with those of pre- and postcontrast MR angiography.

Results: For 50 abnormal vessel segments seen on precontrast MR angiograms, postcontrast MR angiograms resulted in change in the vascular signal intensity in 70% (35 vessel segments); 94% of these changes showed a greater extent of vessel patency. Venous and soft-tissue contrast enhancement had no effect on assessment in 95% of all 480 vessels examined. Interobserver reliability was moderate, with postcontrast interpretation (kappa = 0.48) showing a slight improvement over precontrast interpretation (kappa = 0.41). Good agreement was found between the TOF results and the pooled DW and PW imaging results.

Conclusions: Compared with precontrast 3D TOF MR angiograms, postcontrast 3D TOF angiograms improve assessment of intracranial vessel patency in acutely ischemic vascular territories. In some patients, an improved understanding of acute ischemic stroke was obtained by viewing the pre- and postcontrast images. Postcontrast MR angiography should be included in the MR evaluation of acute stroke.

Fig 1.

Acquisition geometry for precontrast (A) and postcontrast (B, C) 3D TOF MR angiography. A, Two-slab acquisition used to cover the distal extracranial vessels as well as the intracranial circulation (dashed line indicates slab boundary). B and C, Oblique-axial slabs used to image the intracranial circulation. Slab placement in C was optimal as it excludes the cavernous sinus and thus avoids venous enhancement (dotted line indicates slab position). Effective placement in C can be obtained by projection through only a subset of the sections acquired in B.

Fig 2.

Schematic illustrates labeling of vessel segments and regions report on in this study. ACA indicates anterior cerebral artery region; BA, basilar artery; ICA, internal carotid artery; M1, M1 segment of the MCA; M2, M2 segment of the MCA; P1, P1 segment of the PCA; P2, P2 segment of the PCA; P3, P3 segment of the PCA. For grading purposes, ACA, M2, and P3 were segment regions, and the P1 and P2 segments were scored together and denoted by P1-P2.

Fig 3.

Images in a 78-year-old man with changes in the right anterior circulation. A, Normal DW image. B, Relative mean transit time map shows a region of delayed flow in the right hemisphere (arrows), which was consistent with the findings on the pre- and postconrast MR angiograms (D–F). C, Normal relative cerebral blood volume map. D, Precontrast 3D TOF MR angiogram suggests a diminished right ICA (thick arrow) and M1 and an occluded right M2 branch (thin arrow). E and F, Postcontrast 3D TOF MR angiograms show improved depiction of the right ICA and the M1 and M2 MCA. Some contrast enhancement from the cavernous sinus is evident (arrows in E); however, this problem can be removed by changing the position of the slab. Slab positions for images D–F correspond to Fig 1B and C.

Fig 4.

Images in a 75-year-old woman treated with intravenous tPA 2.7 hours after stroke onset. MR imaging commenced 4.1 hours after onset. A–C, DW image (A) shows diffusion changes (arrow in A) and PW images (B and C) show relative mean-transit time defects (arrow in B) and relative cerebral blood volume defects (arrow in C). D, Precontrast TOF MR angiogram suggests a diminished left MCA, which is consistent with the DW and PW imaging findings. Also, the right M1 and M2 appear diminished. E, Postcontrast TOF MR angiogram confirms the left MCA changes, but the right M1 and M2 appear occluded (arrow). Postcontrast findings suggest that the right MCA territory is also at risk.

Fig 5.

Images in a 91-year-old man treated with intravenous tPA at 2.9 hours after stroke onset. MR imaging commenced 5.5 hours after onset. A, DW image shows an infarct (arrow). B, Precontrast TOF MR angiogram appears to indicate a diminished left M1 and an occluded left M2 MCA (thick arrow). The ACA (thin arrow) appears occluded. C, Postcontrast TOF MR angiogram shows the ACA segment is normal (thin arrow), the left M1 is occluded (thick arrow), and the left M2 is diminished.