Renal arterial blood flow measurement by breath-held MRI: Accuracy in phantom scans and reproducibility in healthy subjects

Abstract

This study evaluates reliability of current technology for measurement of renal arterial blood flow by breath-held velocity-encoded MRI. Overall accuracy was determined by comparing MRI measurements with known flow in controlled-flow-loop phantom studies. Measurements using prospective and retrospective gating methods were compared in phantom studies with pulsatile flow, not revealing significant differences. Phantom study results showed good accuracy, with deviations from true flow consistently below 13% for vessel diameters 3mm and above. Reproducibility in human subjects was evaluated by repeated studies in six healthy control subjects, comparing immediate repetition of the scan, repetition of the scan plane scouting, and week-to-week variation in repeated studies. The standard deviation in the 4-week protocol of repeated in vivo measurements of single-kidney renal flow in normal subjects was 59.7 mL/min, corresponding with an average coefficient of variation of 10.55%. Comparison of renal arterial blood flow reproducibility with and without gadolinium contrast showed no significant differences in mean or standard deviation. A breakdown among error components showed corresponding marginal standard deviations (coefficients of variation) 23.8 mL/min (4.21%) for immediate repetition of the breath-held flow scan, 39.13 mL/min (6.90%) for repeated plane scouting, and 40.76 mL/min (7.20%) for weekly fluctuations in renal blood flow.

Figure 1

Breath-held bFFE scout images, used for determination of the renal artery (RA) flow plane; (a) coronal view visualizing the branch of the left RA from the descending aorta, and its direction relative to the axial plane; (b) axial view showing the same RA and its angle with the coronal plane. The flow plane is defined perpendicular to the RA (see arrows) in both views.

Figure 2

PVA Flow phantom (a) and example flow-encoded MRI scans of the flow phantom (b: magnitude, c: velocity map), and of a right renal artery (arrow) of a normal volunteer (d: magnitude, e: velocity map). The vertical (phase encoding) line artifacts in the phantom images are associated with fixed-frequency interference of the flow loop electronics systems scanner's RF chain. The positioning of the phantom within the field of view was adjusted to ensure artifact-free imaging at the flow site.

Figure 3

(a) Renal arterial flow waveform used in pulsatile-flow phantom experiments, and sampling/interpolation of this waveform for prospective gating for a situation with 20 equidistant sampling points spanning 80% of the cardiac cycle; (b) Input renal flow waveform, and sampling points of an actual observed waveform in phantom flow plane (4.76 mm channel) illustrating dampening effects by elastic properties of the flow loop.

Figure 4

Various sources of noise in renal blood flow measurement by MRI, measured and estimated in this study.

Figure 5

(a)Results for phantom experiments with steady flow: Scatter diagram comparing MRI flow measurements with 60-second fluid collection. The reference line shows identity. Data for all flow channel diameters were combined in this plot (N=80). The Concordance Correlation Coefficient (CCC) for these data is 0.992. (b) Results for phantom experiments with steady flow: Bland-Altman plot showing difference vs. average of MRI flow measurements and 60-second fluid collection, with 95% limits of agreement. Data for all flow channels were combined.

Figure 5

(a)Results for phantom experiments with steady flow: Scatter diagram comparing MRI flow measurements with 60-second fluid collection. The reference line shows identity. Data for all flow channel diameters were combined in this plot (N=80). The Concordance Correlation Coefficient (CCC) for these data is 0.992. (b) Results for phantom experiments with steady flow: Bland-Altman plot showing difference vs. average of MRI flow measurements and 60-second fluid collection, with 95% limits of agreement. Data for all flow channels were combined.

Figure 6

(a) Scatter Diagram for flow phantom MRI measurements with pulsatile flow: Programmed flow (true values) and MRI flow. Accumulated results are shown for all vessel diameters, flow rates, and prospective /retrospective gating methods. (b) Bland-Altman diagram showing difference versus average of programmed (“true”) and MRI flow, with 95% limits of agreement (All diameters, prospective /retrospective gating methods, and flow rates combined).

Figure 6

(a) Scatter Diagram for flow phantom MRI measurements with pulsatile flow: Programmed flow (true values) and MRI flow. Accumulated results are shown for all vessel diameters, flow rates, and prospective /retrospective gating methods. (b) Bland-Altman diagram showing difference versus average of programmed (“true”) and MRI flow, with 95% limits of agreement (All diameters, prospective /retrospective gating methods, and flow rates combined).

Figure 7

Variability in RBF measurements for each kidney across the four weekly scans. All measurements in each of four weekly scans are shown together. Data from consecutive weeks are shown left-to-right for one kidney in each of the columns.