Physiological system analysis of the kidney by high‐temporal‐resolution monitoring of an oxygenation step response

Abstract

Purpose: Examine the feasibility of characterizing the regulation of renal oxygenation using high-temporal-resolution monitoring of the T * 2 response to a step-like oxygenation stimulus. Methods: For T * 2 mapping, multi-echo gradient-echo imaging was used (temporal resolution = 9 seconds). A step-like renal oxygenation challenge was applied involving sequential exposure to hyperoxia (100% O 2), hypoxia (10% O 2 + 90% N 2), and hyperoxia (100% O 2). In vivo experiments were performed in healthy rats (N = 10) and in rats with bilateral ischemia-reperfusion injury (N = 4). To assess the step response of renal oxygenation, a second-order exponential model was used (model parameters: amplitude [A], time delay [Δt], damping constant [D], and period of the oscillation [T]) for renal cortex, outer stripe of the outer medulla, inner stripe of the outer medulla, and inner medulla. Results: The second-order exponential model permitted us to model the exponential T * 2 recovery and the superimposed T * 2 oscillation following renal oxygenation stimulus. The in vivo experiments revealed a difference in D outer medulla between healthy controls (D < 1, indicating oscillatory recovery) and ischemia-reperfusion injury (D > 1, reflecting aperiodic recovery). The increase in D outer medulla by a factor of 3.7 (outer stripe of the outer medulla) and 10.0 (inner stripe of the outer medulla) suggests that this parameter might be rather sensitive to (patho)physiological oxygenation changes. Conclusion: This study demonstrates the feasibility of monitoring the dynamic oxygenation response of renal tissues to a step-like oxygenation challenge using | 335 ZHAO et Al.

Cerebral perfusion metrics calculated directly, model-free, from a hypoxia-induced step change in deoxyhemoglobin

“…The use of rapid conversion of dOHb to oxyhemoglobin to generate a step function has been previously reported in mechanically ventilated rats by Zhao et al (2021) 19 for studying renal hemodynamics. Unfortunately, due to a large apparatus dead space, their reoxygenation took 30 s compared to the 2-3 s achieved here (Figure 7C and Figure 2 in 1 ).…”

“…The abrupt increased SaO 2 is conducted into the pulmonary vein, left atrium and ventricle, enters the arterial tree with the same abrupt leading edge of hemoglobin saturation at every branching. This rapid transition from dOHb to oxyhemoglobin describes a step susceptibility input function 19 . We propose that the resulting increase in the T2*-weighted signal can be used to directly calculate model-free resting perfusion metrics.…”

Noninvasive Assessment of Diabetic Kidney Disease With MRI: Hype or Hope?

“…Furthermore, the sensitivity of T 2 * scales supra-linearly with the magnetic field strength (B 0 ), which might be a recognized detection level limitation of studies performed at B 0 ≤ 1.5 T. The move to higher magnetic field strengths B 0 ≥ 3.0 T in basic research and clinical application lowers the detection level for renal T 2 * changes and benefits explorations into the role of intrarenal oxygenation in DKD. 90 Recently, Vinovskis et al proposed to evaluate the relationship between renal oxygen availability and expenditure by the ratio of T 2 * to eGFR. Their results demonstrated relative hypoxia in early DKD in T1DM.…”

“…This is most likely one reason behind some of the inconsistent results obtained in such studies. Furthermore, the sensitivity of T 2 * scales supra‐linearly with the magnetic field strength ( B 0 ), which might be a recognized detection level limitation of studies performed at B 0 ≤ 1.5 T. The move to higher magnetic field strengths B 0 ≥ 3.0 T in basic research and clinical application lowers the detection level for renal T 2 * changes and benefits explorations into the role of intrarenal oxygenation in DKD 90 …”

Evaluation of renal cold ischemia–reperfusion injury with intravoxel incoherent motion diffusion-weighted imaging and blood oxygenation level-dependent MRI in a rat model

“…( Le Bihan D et al, 1986 ). Blood oxygenation level-dependent (BOLD) utilized deoxyhemoglobin to evaluate the blood oxygenation level in tissue by magnetic susceptibility alternations and the provided R2* measurement was sensitive to blood oxygenation, blood flow, and blood volume ( Zhao et al, 2021 ). Although BOLD are currently used to assess renal function in various kidney diseases ( Pedersen et al, 2010 ; Hu et al, 2014 ), evaluation of renal CIRI has been limited.…”