Synopsis

We assessed microbleed burden and cerebral oxygenation in veins and tissue in stroke patients by means of Quantitative Susceptibility Mapping (QSM). Results showed significant susceptibility difference for ipsi- and contralateral veins, and smaller area measurements for cerebral microbleeds compared to SWI and magnitude images due to elimination of blooming effects. Strong susceptibility difference of microbleeds compared to other brain tissue suggest the possibility of quantifying microbleeds by thresholding the images. QSM may be employed in stroke studies to study cerebral oxygenation in veins and microbleed assessment.

Introduction

Stroke is the second leading cause of death in the world¹. Ischemic stroke accounts for about 80% of all cases¹. Recently, Quantitative Susceptibility Mapping (QSM) has been used for examination of cerebral tissue and venous oxygenation² and to study major veins in traumatic brain injury patients³ and patients with acute middle cerebral artery occlusion (a type of ischemic stroke)⁴. Although QSM measurements of severe ischemic stroke have been performed, a wider range of stroke severity has not been studied. QSM can give a quantitative measure of brain oxygenation and unlike other MR phase based methods⁵, is not dependent on the orientation of the blood vessels⁶. Similarly, cerebral microbleeds (CMBs), prevalent in cerebrovascular diseases, have been examined using QSM, and may enable better quantification of microbleed size and intensity than standard MRI methods such as T2*-weighted and Susceptibility Weighted Imaging (SWI)^7-9.

Our purpose was to assess the full value of QSM in ischemic stroke patients, by evaluating tissue and draining veins from stroke and control hemispheres and assessing microbleeds in comparison to standard SWI.

Materials and methods

Patient study: MRI was performed on 45 ischemic stroke patients 30 days after symptom onset from June 2016-June 2017 including the following sequences: DWI, SWI and time-of-flight MR angiography. Seven subjects were chosen for this study based on the inclusion criteria: 1) unilateral stroke; 2) evident hyper/hypo intensities on DWI/ADC images; 3) lesion volume measured on DWI, greater than 8mL. In addition, 10 of the total patients had CMBs, which were also assessed with QSM.

Experiment: All images were acquired on a 3T Prisma Siemens system using the following protocol for three-dimensional SWI: TR/TE=28ms/20ms, flip angle=15°, voxel size=0.68×0.68×2 mm3, number of slices=72, bandwidth=100 Hz/pixel, matrix size=290 × 320.

Data processing and analysis: QSM images were obtained from the raw phase images, using PRELUDE for unwrapping, RESHARP for background field removal (kernel radius = 3mm, Tikhonov regularization parameter=0.001) followed by dipole inversion with Total Variation (TV) regularization (TV parameter =0.0005).

The vein susceptibility measurements were taken from the Maximum Intensity Projection (MIP) QSM over the slices that contained the infarct. For tissue, regions-of-interest (ROIs) were drawn on DWI images in the hemisphere of the infarction and in the contralateral hemisphere and were checked on minimum Intensity Projection (mIP)SWI images to ensure veins did not contribute to the ROI signal. The ROI for major draining veins and local veins within the infarct territory were drawn on SWI images in both hemispheres. Figure 1 shows the infarct on DWI and selected ROIs for tissue, local veins and major veins on QSM.

CMBs were defined to be small, oval, hypo-intense/hyper-intense lesions on SWI/QSM images (Figure 2) and their areas were segmented by computing the width at half maximum height along their greatest diameter on twice interpolated images.

Results

Tissue and veins: Results from all seven subjects for susceptibility measurements in tissue and vein, summarized in Table 1, show that the susceptibility and therefore blood deoxygenation was significantly higher in major veins draining from the contralateral side than that from the lesion territory. This difference was significant for local veins within the infarcted region compared to its contralateral region while there was no significant difference in tissue susceptibility between two hemispheres.

Microbleeds: As results for CMB assessment in Table 2 show, on QSM the size of CMBs were significantly smaller (Figure 3) in comparison to SWI (p < 0.001759) and magnitude images (p < 0.00640) and the range for mean QSM value for CMBs was found to be 313.9 ± 126 ppb.

Discussion

Lower relative susceptibility in local and major veins in the stroke hemisphere suggests decreased cerebral metabolism and this effect is more visible in veins than tissue.

QSM can differentiate between calcium and microbleeds¹⁰, and removes the blooming effect which is present on magnitude and SWI that might lead to inaccurate CMB size measurements. Susceptibility values suggest that QSM could be used to automatically distinguish CMBs from small veins (susceptibility in range of 108±56ppb¹¹) by thresholding, and it is easy to differentiate between large veins and CMBs due to vein continuity.

Conclusion

Acknowledgements

References

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