Measurement of renal tissue oxygenation with blood oxygen level-dependent MRI and oxygen transit modeling

Blood oxygen level-dependent (BOLD) MRI data of kidney, while indicative of tissue oxygenation level (PO₂), is in fact influenced by multiple confounding factors, such as R₂, perfusion, oxygen permeability, and hematocrit. We aim to explore the feasibility of extracting tissue PO₂ from renal BOLD data. A method of two steps was proposed: first, a Monte Carlo simulation to estimate blood oxygen saturation (SHb) from BOLD signals, and second, an oxygen transit model to convert SHb to tissue PO₂. The proposed method was calibrated and validated with 20 pigs (12 before and after furosemide injection) in which BOLD-derived tissue PO₂ was compared with microprobe-measured values. The method was then applied to nine healthy human subjects (age: 25.7±3.0 yr) in whom BOLD was performed before and after furosemide. For the 12 pigs before furosemide injection, the proposed model estimated renal tissue PO₂ with errors of 2.3±5.2 mmHg (5.8 ±13.4%) in cortex and -0.1±4.5 mmHg (1.7 ±18.1%) in medulla, compared with microprobe measurements. After injection of furosemide, the estimation errors were 6.9 ± 3.9 mmHg (14.2 ±8.4%) for cortex and 2.6 ±4.0 mmHg (7.7±11.5%) for medulla. In the human subjects, BOLD-derived medullary PO₂ increased from 16.0 ± 4.9 mmHg (SHb: 31 ± 11%) at baseline to 26.2±3.1 mmHg (SHb: 53 ± 6%) at 5 min after furosemide injection, while cortical PO₂ did not change significantly at ~58 mmHg (SHb: 92±1%). Our proposed method, validated with a porcine model, appears promising for estimating tissue PO₂ from renal BOLD MRI data in human subjects.