Imaging Renal Urea Handling in Rats at Millimeter Resolution using Hyperpolarized Magnetic Resonance Relaxometry

Abstract

In vivo spin spin relaxation time (T₂) heterogeneity of hyperpolarized [¹³C,¹⁵N₂]urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized ¹³C signal with a macromolecular relaxation agent revealed that a long-T₂ component of the [¹³C,¹⁵N₂]urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the [¹³C,¹⁵N₂]urea to be distinguished via multi-exponential analysis. The T₂ response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized [¹³C,¹⁵N₂]urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-¹³C-cyclopropane-²H₈. Large T₂ increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis. Therefore, [¹³C,¹⁵N₂]urea relaxometry is sensitive to two steps of the renal urea handling process: glomerular filtration and the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Simple motion correction and subspace denoising algorithms are presented to aid in the multi exponential data analysis. Furthermore, a T₂-edited, ultra long echo time sequence was developed for sub-2 mm³ resolution 3D encoding of urea by exploiting relaxation differences in the vascular and filtrate pools.

Figure 1.

Hyperpolarized ¹³C T2 mapping methodology. Hyperpolarized ¹³C-labeled substrates were injected via lateral tail vein catheter inside the magnetic resonance imaging (MRI) scanner. The T2 mapping sequence acquired coronal projection images at 0.9 seconds echo time (TE) intervals while playing 180° pulses for 18 seconds. Each image is then corrected for respiratory motion via rigid translation in the superior/inferior direction. The dynamic images are then denoised using a singular value decomposition (SVD)-based thresholding in the space/time dimensions. The signal component at each T2 is estimated using a regularized version of the T2 nonnegative least squares method, and the long and short T2 signal components are isolated by integrating the T2 distribution.

Figure 3.

[¹³C,¹⁵N₂]urea T2 relaxometry after quenching the vascular signal. Timeline of the substrate injections and imaging (A). The left column shows images from the control experiment, and the center column shows the post-BSA-GdDTPA images. The [¹³C,¹⁵N₂]urea T1/BSA-GdDTPA relaxivity curve is shown on the right. [¹³C,¹⁵N₂]urea signal outside of the kidneys has T2 of <2.5 seconds, which is strongly attenuated by BSA-GdDTPA (B). Pixel distributions from 4 animals are shown on the right. The long T2 urea signal component is confined to the kidneys and is unaffected by the BSA-GdDTPA chaser (C). Pixel distributions from 4 animals are shown on the right. T2 distributions of single pixels selected from the center of the kidneys are showing this short T2 signal attenuation (red arrow) (D). Single-pixel T2 decay curves (corresponding to the distributions in the left and center panels).

Figure 2.

Attenuation of the hyperpolarized [¹³C,¹⁵N₂]urea signal is anatomically consistent with the macromolecular Gd carrier. Large field of view (FOV) images are shown on the top, and the bottom panels are zoomed to the kidney. First time-point ¹³C urea MRI image (A). ¹³C urea MRI acquired 8 seconds after bovine serum albumin-Gd/diethylenetriaminepentaacetic acid (BSA-GdDTPA) infusion shows strong suppression of the vascular signal and interlobular arteries (red arrow) (B). ¹H MRI acquired 5 minutes after BSA-GdDTPA infusion (C). The interlobular arteries show positive contrast in this image (green arrow) demonstrating that BSA-GdDTPA is confined to the vascular space on the imaging time scale. ¹H MRI acquired 5 minutes after GdDTPA infusion (D). In contrast to BSA-GdDTPA (mass ∼85 kDa), GdDTPA (mass ∼938 Da) is freely filtered at the glomerulus (yellow arrow).

Figure 4.

Hyperpolarized [¹³C,¹⁵N₂]urea and HMCP relaxometry during antidiuresis and diuresis. Hyperpolarized [¹³C,¹⁵N₂]urea and HMCP images are shown the left and right columns, respectively. HMCP showed increased medullary T2, but the effect did not vary between antidiuresis (A) and diuresis (B) states. Decay curves are selected from a single pixel in the inner medulla (IM) (C). The red arrow shows the persistence of signal to late TE. Mean renal pixel T2 distributions from 3 animals computed from average values over the cortex, outer stripe of the outer medulla (OSOM), inner stripe of the outer medulla (ISOM), and IM (D). The statistically significant change observed was the [¹³C,¹⁵N₂]urea T2 in the IM consistent with the urea transporter (UT)-A1 and UT-A3 distribution.

Figure 5.

¹³C ureter imaging during diuresis. 10 TE (from 5 to 14 seconds) were averaged after alignment and denoising to improve signal. Ureters could be observed in diuresis with both urea and HMCP (arrows). High urea reabsorption leads to substantial cortical and outer medullary urea signal compared with HMCP.

Figure 6.

T2-edited 3-dimensional (3D) imaging. Sequence design (left) and signal simulations (right 3 panels) of a 1.7 mm³ isotropic resolution image (A). Simulations show the signal response expected from regions with T2 = 1.3 seconds (vascular pool), T2 = 4 seconds (cortex/outer medulla), and T2 = 10 seconds (IM/papilla). Blue arrows indicate sequence parameters used, and thus the signal response expected in the images. The long effective TE (4 seconds) of a 3D acquisition was used for blood pool signal suppression and for encoding the [¹³C,¹⁵N₂]urea at 1.2-mm isotropic resolution (B). ¹³C urea images are acquired at 3 different delay times after 3 different injections. Blood pool suppression is evidenced by the dark interlobular arteries visible on both the [¹³C,¹⁵N₂]urea and ¹H fast spin echo images (magenta arrows).