Cardiac quantitative susceptibility mapping (QSM) for heart chamber oxygenation

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Mixed‐venous oxygen saturation (SvO2) is an important indicator of cardiopulmonary function that is widely used clinically to assess cardiac function in heart failure patients, and to measure shunt fractions in patients with congenital or acquired heart disease 1. Currently, conventional measurement of SvO2 requires an invasive pulmonary artery catheterization procedure 2 and is therefore both challenging and labor‐intensive for critically ill patients. Cardiac MRI has emerged as a promising non‐invasive method for measuring SvO2. It has been demonstrated that SvO2 can be calculated based on blood T2 relaxation, which requires in vitro calibration of blood samples infused with oxygen at different concentrations 5. This approach is highly dependent on pulse sequence parameters, field strengths, patient blood characteristics 7, and accurate blood calibration curves 14 therefore complicating its clinical applicability 5. In contrast, magnetic susceptibility is a fundamental property of biomagnetic sources, with blood susceptibility being linearly dependent on the oxyheme concentration in blood. Therefore, a magnetic susceptibility‐based oxygenation measurement method would not require calibration 15 or blood withdrawal from patients.
Because of the difference in electron pairing in heme iron caused by oxygen binding, oxyheme is diamagnetic and deoxyheme is paramagnetic 18. Therefore, blood oxygen saturation (SO2) can be measured using quantitative susceptibility mapping (QSM), which is an advanced phase‐based MRI method for measuring the distribution of biomagnetic sources 19. Furthermore, QSM‐based oxygenation quantification has been demonstrated to be feasible in the brain 17, and QSM has been used to study the mouse heart ex vivo 29, but it has not yet been applied to in vivo cardiac MRI because of several technical challenges: cardiac and respiratory motion, chemical shift effects from epicardial fat, and a large range in susceptibilities from air in the lungs and surrounding tissue. As known from coronary MRI, electrocardiogram triggering can be used to acquire data in mid‐diastole to minimize cardiac motion, and breath‐hold or navigator can be used to minimize respiratory motion 30. Recent advancements in QSM precisely account for chemical shift effects using graph cuts 32, and deal with large range in susceptibility using preconditioning 33. By combining the knowledge from these advancements, this study aimed to investigate the feasibility of cardiac QSM for chamber blood SvO2 quantification.

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