The recently introduced OxFlow sequence for rapid measurement of the cerebral metabolic rate of oxygen (CMRO₂) is based on Cartesian k-space coverage¹. OxFlow simultaneously yields superior sagittal sinus (SSS) flow via phase-contrast and intravascular venous oxygen saturation (SvO₂) via magnetic susceptometry². Temporal resolution is enhanced by employing a BRISK view-sharing scheme, yielding a maximum temporal resolution of 3 seconds^1,3. However, this approach is not ideal as it provides limited flexibility in that the temporal resolution has to be chosen a priori. In contrast, a golden-angle radial sampling strategy allows for a variety of reconstruction options tailored to the specific application’s needs.

Purpose: To design and validate a method for rapid quantification of whole-brain CMRO₂ via simultaneous measurement of SvO₂ and SSS blood flow using a velocity-encoded, multi-echo, golden-angle radial imaging sequence.

A radial OxFlow (rOxFlow) sequence (Figure 1) was designed in SequenceTree with sequence parameters closely matching the standard Cartesian version (cOxFlow)¹. Sequence parameters: TR/TE=19.2 ms/6.752 ms , flip angle=15°, slice thickness=4 mm. Acquisition matrix size was 208×208 for cOxFlow (80% phase-encoding coverage, 176×176 FOV) and 240×240 for radial (15790 radial lines). Temporal resolution was 3s for cOxFlow and 1.29s for rOxFlow. Radial images were reconstructed every 34 views. The sequences were optimized for a protocol involving continuous scanning for 10 minutes.

All experiments were conducted at 1.5T (Siemens AVANTO). Results from rOxFlow were compared to cOxFlow in six volunteers during a volitional apnea paradigm. All subjects were instructed to perform five successive breath-holds, each 30s in duration, separated by 90s. For both rOxFlow and cOxFlow, SvO₂ was calculated from 1st and 3rd echoes and phase-contrast velocity was computed from the first echo collected with different first gradient moments (Figure 1). Total CBF (tCBF) was estimated by upscaling SSS velocity by a factor acquired from a preceding calibration sequence¹. Bland-Altman analysis⁴ was conducted to investigate inter-sequence bias. Means of baseline and peak SvO₂ and velocity were averaged over 5 breath-hold and rest periods, respectively, from both Cartesian and radial data, and compared via paired t-tests.

Figure 2 shows a sagittal scout angiogram and magnitude axial image indicating measurement locations. Sample velocity and phase cOxflow and rOxFlow images and response time courses are displayed in Figure 3 highlighting the similarity of the two sequences in both the parametric images and over the entire time course of the breath-hold response. A Bland-Altman plot comparing cOxFlow to rOxFlow in one subject during the time-course of the experiment is given in Figure 4. The data suggests minimal bias given that >95% of the differences were within the ±1.96 SD limit. Table 1 lists SvO₂ and tCBF data in all six subjects as well as means and standard deviations. There was no significant difference between the two methods from the paired t-tests involving all six subjects for any of the parameters measured (p>0.20 in all cases).In general, the results from the two sequences were found to be in good agreement with each other. While the data from the six subjects did not suggest a bias between sequences, comparisons involving a larger number of subjects would be required to corroborate this result. In addition to providing superior effective temporal resolution, rOxFlow was found to have better SNR than its Cartesian counterpart.