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

T₂ MRI oximetry can noninvasively determine oxygen saturation (Y) but requires empirical MR calibration models to convert the measured blood transverse relaxation (T_2b ) into Y. The accuracy of existing T_2b models in the presence of blood disorders such as sickle cell disease (SCD) remains unknown.

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) T_2b 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 T_2b models. Cerebral and forearm oxygen extraction fraction were compared.

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 T_2b model predicted Y_v values with lowest coefficient of variation (compared with existing T_2b models) and the strongest correlation with peripheral venous oximetry (r²  = .29). The HbS T_2b model predicted systematically higher Y_v 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).

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.