Sylvana García-Rodríguez1, Alejandro Roldán-Alzate1,2, Camilo A. Campo1, Scott B. Reeder1, Oliver Wieben1,3, and Christopher J. François1
1Radiology, University of Wisconsin-Madison, Madison, WI, United States, 2Mechanical Engineering, University of Wisconsin-Madison, Madison, WI, United States, 3Medical Physics, Univerisity of Wisconsin-Madison, Madison, WI, United States
Synopsis
This study investigated the diurnal changes in renal blood flow in healthy volunteers using 4D flow MRI, to determine the optimal time of day to perform renal blood flow measurements. Five 4D flow MRI acquisitions were performed throughout the day in seven healthy subjects to mimic potential imaging scheduling time points. Significant differences in renal blood flow were observed depending upon time of day and prandial status. This study confirms the importance of timing of renal MRI studies assessing kidney function.Introduction
Magnetic resonance imaging (MRI) is increasingly being used to non-invasively quantify renal function, including renal blood flow.1-3 Although normal renal blood flow response to meal challenges have been presented,2 no studies have reported diurnal variations in renal blood flow.Purpose
To evaluate diurnal changes in renal blood flow in normal subjects using four-dimensional flow-sensitive magnetic resonance imaging (4D flow) MRI.4,5Methods
In this IRB-approved and HIPAA-compliant study, 7 healthy volunteers with no history of renal disease (36 ± 9 years, 88 ± 8 kg) were imaged after written informed consent was obtained.
Diurnal protocol: The first MRI scan (pre-breakfast, 8 am) was performed following ≥ 5 hours of fasting. Subsequently, subjects ingested 574 mL EnSure PlusÒ (Abbot Laboratories, Columbus, OH; 700 cal, 28% from fat, 57% from carbohydrates) orally. A second acquisition (post-breakfast, 9 am) was started 20 min after the meal challenge. A third scan was performed before lunch (pre-lunch, noon). Subjects were asked to have a normal lunch at the cafeteria, 20 minutes after which a fourth acquisition took place (post-lunch, 1:30 pm). Finally, a fifth scan was performed (afternoon, 4 pm).
MRI: Studies were conducted on a clinical 3T scanner (Discovery MR 750, GE Healthcare, Waukesha, WI) with a 32-channel body coil (NeoCoil, Pewaukee, WI). 4D flow MRI was conducted using a time resolved 3D radially undersampled phase contrast acquisition (5-point PC-VIPR)6,7 and comprehensive coverage of the upper abdomen. 4D flow MRI parameters were: 32 x 32 x 24 cm imaging volume, 1.25 mm acquired isotropic spatial resolution, TR/TE = 6.4/2.2 ms, retrospective ECG gating. All subjects received 0.03 mmol/kg of gadofosveset trisodium (Lantheus, N. Billerica, MA), an intravascular gadolinium-based contrast agent used to maximize SNR performance and injected prior to the first scan. The velocity encoding (Venc) was adjusted for pre- and post-meal acquisitions to provide optimal velocity encoding with expected increases in flow velocities post-meal.
Analysis: Vessel segmentation was performed in Mimics (Materialise, Leuven, Belgium) from PC angiograms (Figure 1). Flow visualization and quantification was performed in EnSight (CEI, Apex, NC). Flow was measured at the supraceliac Aorta (SCAo), right renal artery (RRA) and left renal artery (LRA). In the presence of two renal arteries on the same side, the data was recorded as the sum of the two.
Statistics: For each vessel, flow values were compared before and after each meal using a paired Student t-test (p = 0.05). Similarly, the same analysis was performed between pre-breakfast and afternoon measurements. Renal blood flow was also analyzed in terms of percentage of SCAo blood flow, for which the same comparisons were made.
Results and Discussion
Mean volumetric blood flow values measured throughout the day for the SCAo and renal arteries are presented in Table 1. Pre-breakfast blood flow values were the lowest for all vessels; maximum renal blood flow was present pre-lunch while post-breakfast was the highest in the SCAo. Statistically significant (p < 0.05) increases in blood flow were present in the SCAo (Figure 2) in response to breakfast (30%, increase) and lunch (21% increase). Renal blood flow increased after breakfast, 12% for LRA and 14% for RRA (Figure 3). Renal blood flow then increased further pre-lunch and in the afternoon. No significant changes were seen in response to lunch, after which blood flow slightly decreased. By the afternoon, blood flow was slightly increased to levels near pre-lunch. Significant differences in renal flow were present between (a) pre-breakfast and afternoon bilateral renal blood flow and (b) pre-breakfast and pre-lunch left renal blood flow. As expected, relative renal blood flow decreased after each meal (Figure 4).Summary
4D flow MRI confirmed the presence of diurnal and post-prandial changes in renal blood flow healthy volunteers. The greatest change in renal blood flow occurred four hours after breakfast, when renal blood flow significantly increased. The findings of this study support the importance of taking the time of day and prandial status into consideration when performing renal functional studies.Acknowledgements
We acknowledge support from the NIH (R01 DK096169 and R01HL072260). We also thank GE Healthcare for their support.References
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