Diffusion-weighted imaging of hyperpolarized [13 C]urea in mouse liver

Abstract

Purpose: To compare the apparent diffusion coefficient (ADC) of hyperpolarized (HP) [¹³ C,¹⁵ N]urea to the ADC of endogenous water in healthy and fibrotic mouse liver.

Materials and methods: ADC measurements for water and [¹³ C]urea were made in agarose phantoms at 14.1T. Next, the ADC of water and injected HP [¹³ C,¹⁵ N]urea were measured in eight CD1 mouse livers before and after induction of liver fibrosis using CCl₄ . Liver fibrosis was quantified pathologically using the modified Brunt fibrosis score and compared to the measured ADC of water and urea.

Results: In cell-free phantoms with 12.5% agarose, water ADC was nearly twice the ADC of urea (1.93 × 10^-3 mm² /s vs. 1.00 × 10^-3 mm² /s). The mean ADC values of water and [¹³ C,¹⁵ N]urea in healthy mouse liver (±SD) were nearly identical [(0.75 ± 0.11) × 10^-3 mm² /s and (0.75 ± 0.22) × 10^-3 mm² /s, respectively]. Mean water and [¹³ C,¹⁵ N]urea ADC values in fibrotic liver (±SD) were (0.84 ± 0.22) × 10^-3 mm² /s and (0.75 ± 0.15) × 10^-3 mm² /s, respectively. Neither water nor urea ADCs were statistically different in the fibrotic livers compared to baseline (P = 0.14 and P = 0.99, respectively). Water and urea ADCs were positively correlated at baseline (R² = 0.52 and P = 0.045) but not in fibrotic livers (R² = 0.23 and P = 0.23).

Conclusion: ADC of injected hyperpolarized urea in healthy liver reflects a smaller change as compared to free solution than ADC of water. This may reflect differences in cellular compartmentalization of the two compounds. No significant change in ADC of either water or urea were observed in relatively mild stages of liver fibrosis.

Level of evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:141-151.

Keywords: MRI; apparent diffusion coefficient; carbon tetrachloride; dynamic nuclear polarization; hyperpolarization; mouse liver.

FIGURE 1

Diagram of the liver spaces: intracellular space (IC), extracellular extravascular space (EC-EV), and extracellular intravascular space (EC-IV). Endogenous water is found both in the intra- and extracellular space, while exogenously injected hyperpolarized urea is mainly extracellular. Fibrosis is characterized by collagen deposition in the EC-EV space.

FIGURE 2

DWI and ADC measured in cell-free agarose phantoms. Representative dataset (0% agarose) for (a) water ¹H-DWI and (b) urea ¹³C-DWI with b-value factors displayed on each image in units of s/mm². (c) ADC maps and fit of the mean of the signal intensity for the dataset in (a,b). (d) The ADC decreased with increased agarose concentration. Error bars for the first three water experiments and the fourth urea experiment are smaller than the marker.

FIGURE 3

(a) Representative T₂-weighted abdominal anatomic axial and coronal images of a mouse with the slab boundaries for the DWI experiments shown in red for ¹H-DWI and green for ¹³C-DWI. (b) Timeline for liver fibrosis studies. MR images were acquired prior to the first CCl₄ treatment and 1 week following the final CCl₄ treatment. Mice were sacrificed on the same day of the final imaging session.

FIGURE 4

DWI acquisition scheme and representative ¹H- and ¹³C-DWI dataset. (a) Schematic representation of the trigger time-points for acquisition and ¹H-DWI. (b) ¹³C-DWI and acquisition scheme: two images were acquired during each respiratory cycle. (c) ¹H and ¹³C ADC maps with a typical ROI drawn on them (middle) and fits of the data excluding images corrupted by motion (sides).

FIGURE 5

(a) Correlation of ¹³C and ¹H ADC measured in each mouse at baseline and (b) after 7 or 11 weeks of treatment with CCl₄. See also Table 2.

FIGURE 6

Trichrome (left) and H&E (right) images of the livers of a representative mouse from (a) group 1 (7 weeks of CCl₄ treatment) and (b) group 2 (11 weeks of CCl₄ treatment). Based on a modified Brunt staging system, (a) developed fibro-sis stage 2 with centrizonal pericellular fibrosis, and (b) developed fibrosis stage 2–3 with centrizonal early septal fibrosis.

FIGURE 7

Comparison of baseline and post-CCl₄ treatment ADCs of the right lobe of the liver for water (proton) and [¹³C, ¹⁵N]urea (carbon). The mean (±SD) of each group is displayed under each dataset.

FIGURE 8

Post-CCl₄ treatment ADCs of the right lobe of the liver for different stages of fibrosis (a) for water (proton) and (b) [¹³C, ¹⁵N]urea (carbon). These data are also shown in Fig. 7. The mean ADC across all fibrosis stages is shown with a dashed line, and the baseline ADC is shown with a solid black line.