Diminished cerebral oxygen extraction and metabolic rate in sickle cell disease using T2 relaxation under spin tagging MRI.

PURPOSE: T2 MRI oximetry can noninvasively determine oxygen saturation (Y) but requires empirical MR calibration models to convert the measured blood transverse relaxation (T2b ) into Y. The accuracy of existing T2b models in the presence of blood disorders such as sickle cell disease (SCD) remains unknown.

METHODS: A Carr Purcell Meiboom Gill T2 preparation sequence was used to make 83 whole blood measurements from 11 subjects with SCD to derive an ex vivo sickle hemoglobin (HbS) T2b model. Forearm venous blood gas, sagittal sinus T2 (T2 Relaxation Under Spin Tagging) and total brain blood flow (phase contrast MRI) were measured in 37 healthy controls and 33 SCD subjects (age 24.6 ± 10.2 years). Cerebral oxygen saturation, extraction fraction, and metabolic rate estimates were calculated using three separate T2b models. Cerebral and forearm oxygen extraction fraction were compared.

RESULTS: Ex vivo, SCD blood had greater saturation dependent relaxivity than control blood, with a weak dependence on HbS and no dependence on hematocrit. In vivo, the HbS T2b model predicted Yv values with lowest coefficient of variation (compared with existing T2b models) and the strongest correlation with peripheral venous oximetry (r2  = .29). The HbS T2b model predicted systematically higher Yv measurements in SCD patients (73 ± 5 and 61 ± 6; P < 0.0001) which was mirrored by peripheral venous measurements (75 ± 20 and 45 ± 20; P < 0.0001).

CONCLUSION: Cerebral and peripheral oxygen extraction are decreased in SCD patients, suggesting either blood flow is increased beyond metabolic demands or the presence of physiological arterial-venous shunting. Magn Reson Med 80:294-303, 2018. © 2017 International Society for Magnetic Resonance in Medicine.