Vascular space occupancy asymmetric spin echo (VASO-ASE) for non-invasive quantification of cerebral oxygen extraction fraction.

Abstract

Asymmetric spin echo (ASE) MRI is a method for measuring regional oxygen extraction fraction (OEF); however, extravascular tissue models have been shown to under-estimate OEF. The hypothesis investigated here is that the addition of a vascular-space-occupancy (VASO) pre-pulse will more fully suppress blood water signal and provide global OEF values more consistent with physiological expectation and 15 O positron emission tomography (PET)-validated T2 -relaxation-under-spin-tagging (TRUST) OEF measures. Healthy adults (n= 14; age=27.7 ± 5.2y; sex=7/7 male/female) were scanned at 3.0T. Multi-echo ASE without inter-readout refocusing (ASERF- ), multi-echo ASE with inter-readout refocusing (ASERF+ ), and single-echo VASO-ASE were acquired twice each with common spatial resolution=3.44 × 3.44 × 3.0 mm and τ= 0-20 ms (interval=0.5ms). TRUST was acquired twice sequentially for independent global OEF assessment (τCPMG = 10ms; effective TEs=0, 40, 80, and 160 ms; spatial resolution=3.4 × 3.4 × 5 mm). OEF intraclass-correlation-coefficients (ICC), summary statistics, and group-wise differences were assessed (Wilcoxon rank-sum; significance: two-sided p< 0.05). ASERF+ (OEF=36.8 ± 1.9%) and VASO-ASE (OEF=34.4 ± 2.3%) produced OEF values similar to TRUST (OEF=36.5 ± 4.6%, human calibration model; OEF=32.7 ± 4.9%, bovine calibration model); however, ASERF- yielded lower OEF (OEF=26.1 ± 1.0%; p< 0.01) relative to TRUST. VASO-ASE (ICC=0.61) yielded lower ICC compared to other ASE variants (ICC >0.89). VASO-ASE and TRUST provide similar OEF values; however, VASO-ASE spatial coverage and repeatability improvements are required.