This study focused on the chronic phase after combined cerebral revascularization for MMD. The sum of the donor vessel cross-sectional areas of the STA, MMA, and DTA was evaluated as TVA using TOF-MRA, and RCBF was evaluated by SPECT. In addition, the preoperative and postoperative change ratios were defined as ΔTVA and ΔRCBF, respectively. The TVA and RCBF significantly increased postoperatively, but no significant correlation was found between ΔTVA and ΔRCBF. In addition, when divided into adult and pediatric groups, ΔRCBF tended to be significantly higher in the adult group, whereas ΔTVA tended to be higher in the pediatric group.
Evaluation of donor vessel cross-sectional area by TOF-MRA
TOF-MRA has long been the focus of attention as an alternative to digital subtraction angiography (DSA) for assessing donor vessel development . In recent years, Uchino et al. reported that postoperative STA and DTA dilation correlates with DSA gradual revascularization assessment . In addition, one report suggested that if MMA is preserved intraoperatively without sacrifice, the resulting increase in MMA diameter after surgery may reinforce the development of indirect pathways . The significant development of donor vessels after surgery in these reports is consistent with our results. Therefore, the development of donor vessels observed using TOF-MRA is considered a beneficial finding that predicts the enhancement of revascularization.
However, it should be noted that previous reports evaluated donor vessel development by vessel diameter. This is because the blood flow through the blood vessels depends on the blood flow velocity and cross-sectional area of the blood vessel. We adopted the vessel cross-sectional area rather than the vessel diameter to estimate the blood flow through the vessel more accurately. In fact, an algorithm for evaluating CBF using the cross-sectional area of blood vessels has been proposed , and a report that used the algorithm with magnetic resonance imaging suggested that CBF decreases with age .
Meanwhile, FWHM has the advantages of minimizing interobserver variation caused by window-level adjustments and improving measurement reproducibility. FWHM has been previously reported as an effective tool for assessing the residual components of aneurysms after coiling treatment . In addition, if the vessel cross-section is oval during the vessel diameter measurement, the minor diameter is measured to avoid overestimating the actual radius.
Evaluation of CBF by SPECT
The CBF quantitative value by SPECT is an established evaluation method of cerebral circulation metabolism, but the fluctuation between patients is large . Since RCBF calculates relative values based on the cerebellum of the same patient, it is expected to reduce evaluation errors. In previous reports, RCBF has been adopted as an effective semiquantitative parameter for understanding postoperative cerebral hemodynamics [12, 19].
Donor vessel development and increased CBF are not correlated
We hypothesized that TVA and RCBF reflect revascularization effects and correlate with their tendency to strengthen. However, contrary to expectations, there was no correlation between ΔTVA and ΔRCBF. To explain this result, we present three possible factors.
First, microvessels that were not evaluated as donor vessels may have enhanced CBF in the postoperative craniotomy area. The unique microvascular hyperplasia of the cerebral cortex of patients with MMD has been implicated in the development of collateral circulation that compensates for ischemia . Furthermore, angiogenic activity in the craniotomy area has been reported to be activated by the intervention of revascularization . These reports reinforce the first hypothesis. Second, the development of the donor vessels may have the effect of strengthening the external carotid (EC) blood flow system, which, in contrast, reduces the internal carotid (IC) blood flow system. This phenomenon is a dynamic change in the blood flow system characteristic of MMD and is called IC-EC conversion . Similar to the first hypothesis, interventions in revascularization surgery have been reported to promote IC-EC conversion [12, 23]. In this study, ΔTVA increase was detected as strengthening the EC blood flow system. However, ΔRCBF may decrease with the weakening of the IC blood flow system under the influence of IC-EC conversion. Therefore, IC-EC conversion may have disrupted the correlation between ΔTVA and ΔRCBF. Finally, RCBF may reflect diminished cerebellar blood flow opposite the supratentorial lesion, called crossed cerebellar diaschisis (CCD) . Of the 24 patients (31 hemispheres) included in this study, 18 (25 hemispheres) had contralateral lesions. Therefore, if CCD lowers the cerebellar blood flow on the subject side, it will affect the calculation of RCBF.
These hypotheses warn of the difficulty of postoperative cerebral circulation metabolism assessment in patients with MMD. Furthermore, it may be important to use a combination of multiple parameters to verify cerebral circulation metabolism more accurately. According to a report in China, CBV in the MCA region decreased significantly 3–14 months after surgery .
This study has some limitations. First, this was a retrospective cohort study conducted in a single center and with small sample size. Therefore, the results should be carefully interpreted. Second, the study included cases where a decrease in preoperative RCBF was not apparent. A correlation may be confirmed between ΔTVA and ΔRCBF by extracting and verifying cases with a marked decrease in RCBF. Third, in TOF-MRA images, the brightness of the vessel lumen depends on blood flow velocity, but the brightness has not been evaluated. Therefore, to determine donor blood flow more accurately, the brightness of the vessel lumen should be investigated using TOF-MRA. Finally, the postoperative chronic phase was defined as after 6 months without an upper limit. However, there is little evidence that donor vessel development and cerebral blood flow remain stable after 6 months. Thus, individual differences in strengthening TVA and RCBF that occur after 6 months were not considered.