Moyamoya disease is a progressive steno-occlusive disease causing chronic under-perfusion of brain tissues¹. Transverse MR spin relaxation rates, R₂* (=1/T2*), R₂ (=1/T2), and R₂’ (=R₂*-R₂), have been proposed for brain oxygenation mapping². In this study, we simultaneous mapped these rates and cerebral blood flow (CBF), before and after the injection of acetazolamide (ACZ), to study each parameter’s change and correlation with disease state.

25 pre-operative Moyamoya disease patients (ages 38±12 y, 20 F) were scanned with informed consent and IRB approval, at 3.0T (MR750W, GE Healthcare) with ACZ challenge as part of a clinical diagnostic protocol. Relaxometry was performed using 2D Gradient-Echo Sampling of Free Induction Decay and Echo (GESFIDE: TE_SE/TR 100/2000ms, 40 echoes, TE 5-130ms, resolution 1.9×1.9×1.5mm³ with 34mm-thick coverage)³. CBF was measured using whole-brain 3D multi-delay pseudo-continuous ASL (TE/TR 25.1/6518ms, label time 2000ms, 5 equally spaced PLDs 700-3000ms, resolution 3.4×3.4×4mm³)⁴. Anatomical and angiographic information were acquired with whole-brain T1-weighted 3D IR-FSPGR images and intracranial time of flight (TOF) MRA.

GESFIDE images were analyzed in native space, and other images were coregistered to them. R₂*, R₂, and R₂’ maps were calculated using mono-exponential fitting³. Tissue masks were segmented from anatomical images. Each dataset was analyzed in twelve 3cm annular, mixed-cortical ROIs approximating major arterial territories, with 6 radial segments located at 2 levels (Fig. 1). Each ROI was defined as angiographically “normal” and “abnormal” using blinded inspection of TOF MRA by an experienced neuroradiologist. Statistical significance of ACZ-induced changes and difference between groups was assessed using the 2-tailed t-test and α=0.05.

Results of group-level analysis are shown in Fig. 2-3. In abnormal ROIs, post-ACZ CBF was significantly lower by 12±38%, while pre- and post-ACZ R₂’ were 13±32% and 14±33% higher, respectively. Pre- and post-ACZ R₂ were 2.1±6.8% and 1.6±6.5% lower in abnormal ROIs.

ACZ-induced vasodilation augments CBF, increases tissue oxygenation, and reduces the amount of paramagnetic deoxyhemoglobin per voxel. Indeed, we observed that R₂’, R₂* and R₂ all decreased significantly in normal tissues post-ACZ, by 2.5±13.8%, 1.3±3.1% and 0.2±2.6% (all p<0.05), respectively. However, Moyamoya disease-affected regions experience either submaximal or paradoxical reduction (“steal”) of CBF, which resulted in smaller ΔR₂’, ΔR₂* and ΔR₂ reductions.

Finally, linear regression of each relaxation rate versus CBF (Fig. 4) found R₂’ to be significantly negatively correlated with CBF (R²=0.16, p<0.05). R₂ was uncorrelated (R²<0.01, p>0.05), while R₂* showed very weak correlation (R²=0.05, p<0.05).

We observed R₂’ trends consistent with physiology and a quantitative BOLD (qBOLD) biophysical model⁵ of oxygenation, for both ACZ- and disease-induced differences. qBOLD models tissue R₂’ as proportional to the product of deoxygenated blood volume (DBV) and (1-SO₂), where SO₂ is the tissue oxygen saturation. ACZ-induced CBF augmentation causes competing effects via the two terms, but our observed increase in R₂’ indicated that SO₂ effect dominated, as reported by previous studies⁶. Higher pre-ACZ R₂’ in abnormal ROIs may indicate higher DBV and/or lower SO₂. However, without measuring pre- and post-ACZ blood volume in this cohort, these competing effects cannot be uncoupled. However, our results show that R₂’ is sensitive to oxygenation changes, unlike R₂*, which did not reflect tissue status, consistent with a prior study ⁷.

Modeling R₂ is more complicated. There are two sources of R₂ change: blood compartment and tissue. Like R₂’, blood R₂ is also affected by competing effects of oxygenation⁸ and blood volume⁹. In addition, due to small blood volume in tissues, even large blood R₂ changes can be hard to measure with high SNR. Finally, tissue R₂ can be changed by pathology independently from perfusion and oxygenation changes, e.g. in edema. Therefore, R₂ is suboptimal for measuring tissue oxygenation.

To untangle the competing factors influencing R₂’ and R₂, accurate measurement of DBV is required before and after the ACZ challenge, which is challenging as most methods measure total cerebral blood volume and require contrast, which would not permit subsequent R₂’ measurements. Non-contrast methods such as Vascular-Space-Occupancy (VASO)¹⁰ should be explored for this application.

In this study, we simultaneously characterized transverse spin relaxation rates R₂*, R₂ and R₂’, with perfusion, in an ACZ challenge in Moyamoya disease patients. We observed consistent trends with what we expect from physiological models and prior studies, and physiology. We conclude that R₂’ has the most potential for imaging oxygenation changes in tissue, with superior performance compared to R₂* and R₂.