Abstract
In recent years, interest has grown in the potential for magnetic resonance imaging (MRI) measures of venous oxygen saturation (Y(v)) to improve neurological risk prediction. T(2)-relaxation-under-spin-tagging (TRUST) is an MRI technique which has revealed changes in Y(v) in patients with sickle cell anemia (SCA). However, prior studies comparing Y(v) in patients with SCA relative to healthy controls have reported opposing results depending on whether the calibration model, developed to convert blood T(2) to Y(v), is based on healthy human hemoglobin (HbA), bovine hemoglobin (HbBV) or sickle hemoglobin (HbS). MRI Quantitative Susceptibility Mapping (QSM) is an alternative technique that may hold promise for estimating Y(v) in SCA as blood magnetic susceptibility is linearly dependent upon Y(v), and no significant difference has been found between the magnetic susceptibility of HbA and HbS. Therefore, the aim of this study was to compare estimates of Y(v) using QSM and TRUST with five published calibration models in healthy controls and patients with SCA. 17 patients with SCA and 13 healthy controls underwent MRI. Susceptibility maps were calculated from a multi-parametric mapping acquisition and Y(v) was calculated from the mean susceptibility in a region of interest in the superior sagittal sinus. TRUST estimates of T(2,) within a similar but much smaller region, were converted to Y(v) using five different calibration models. Correlation and Bland-Altman analyses were performed to compare estimates of Y(v) between TRUST and QSM methods. For each method, t-tests were also used to explore group-wise differences between patients with SCA and healthy controls. In healthy controls, significant correlations were observed between QSM and TRUST measures of Y(v,) while in SCA, there were no such correlations. The magnitude and direction of group-wise differences in Y(v) varied with method. The TRUST-HbBV and QSM methods suggested decreased Y(v) in SCA relative to healthy controls, while the TRUST-HbS (p < 0.01) and TRUST-HbA models suggested increased Y(v) in SCA as in previous studies. Further validation of all MRI measures of Y(v), relative to ground truth measures such as O(15) PET and jugular vein catheterization, is required in SCA before QSM or TRUST methods can be considered for neurological risk prediction.