Rapid T2- and susceptometry-based CMRO2 quantification with interleaved TRUST (iTRUST).

Susceptometry-based oximetry (SBO) and T2-relaxation-under-spin-tagging (TRUST) are two promising methods for quantifying the cerebral metabolic rate of oxygen (CMRO2), a critical parameter of brain function. We present a combined method, interleaved TRUST (iTRUST), which achieves rapid, simultaneous quantification of both susceptometry- and T2-based CMRO2 via insertion of a flow-encoded, dual-echo gradient-recalled echo (OxFlow) module within the T1 recovery portion of the TRUST sequence. In addition to allowing direct comparison between SBO- and TRUST-derived venous oxygen saturation (Yv) values, iTRUST substantially improves TRUST temporal resolution for CMRO2 quantification and obviates the need for a separate blood flow measurement following TRUST acquisition. iTRUST was compared directly to TRUST and OxFlow alone in three resting subjects at baseline, exhibiting close agreement with the separate techniques and comparable precision. These baseline data as well as simulation results support the use of two instead of the traditional four T2 preparation times for T2 fitting, allowing simultaneous quantification of susceptometry- and T2-based Yv (and CMRO2) with three- and six-second temporal resolution, respectively. In 10 young healthy subjects, iTRUST was applied during a 5% CO2 gas mixture-breathing paradigm. T2-based Yv values were lower at baseline relative to susceptometry (62.3 ± 3.1 vs. 66.7 ± 5.1 %HbO2, P<0.05), but increased more in response to hypercapnia. As a result, T2-based CMRO2 decreased from 140.4 ± 9.7 to 120.0 ± 9.5 μMol/100g/min, a significant -14.6 ± 3.6% response (P < 0.0001), whereas susceptometry-based CMRO2 changed insignificantly from 123.4 ± 18.7 to 127.9 ± 25.7, a 3.3 ± 9.7% response (P = 0.31). These differing results are in accord with previous studies applying the parent OxFlow or TRUST sequences individually, thus supporting the reliability of iTRUST but also strongly suggesting that a systematic bias exists between the susceptometry- and T2-based Yv quantification techniques.