Po-topic I-02: Bold MRI applied to renal medullary hypoxia

Abstract

Renal medulla operates in a hypoxic environment (1) and results in hypoxic injury when oxygenation or blood flow is further impaired. Acute renal failure (ARF) involves damage to the medulla (1) probably initiated by a reduction in medullary blood flow or increase in oxygen demand. Hence, an understanding of mechanisms that influence medullary hypoxia is important in the study of the pathophysiology of ARF. The conventional measurements of intrarenal oxygen tension depend on implanted oxygen-sensitive microelectrodes into the renal parenchyma of animals, a delicate and tedious procedure and impractical for human studies. The Blood Oxygenation Level Dependent (BOLD) magnetic resonance imaging (MRI) depends on the principle that variations in the oxygen saturation of hemoglobin result in changes in local magnetic susceptibility and hence in the signal intensity of T2∗-weighted MR images. Because the ratio of oxyhemoglobin to deoxyhemoglobin is related to the pO2 of blood, and since the pO2 of capillary blood is thought to be in equilibrium with the surrounding tissue, changes in BOLD MRI signals can be interpreted as changes in tissue pO2(2). The sigmoid relationship of blood pO2 to the hemoglobin oxygen saturation makes the BOLD MRI technique especially suited for oxygenation measurements in the hypoxic renal medulla because it falls on the linear regime.