Dynamic contrast‐enhanced MRI (DCE‐MRI)

Separate cohorts of mice, fasted (n = 7) and non‐fasted (n = 7), were used for DCE‐MRI measurements of tumor perfusion, in which the mice were injected with Gd^{3 +}‐DTPA (diethylenetriaminepentaacetic acid; 200 mmol/kg; Magnevist, Bayer Schering Pharma, Leverkusen, Germany) (Table 2). The imaging protocol simulated the protocol used for the ¹³C hyperpolarized measurements. Briefly, for each cohort (seven fasted, seven non‐fasted), four mice were injected with contrast agent (1st scan) and three mice were injected with unlabeled Pyr (82 mm, 10 mL/kg), followed by injection of contrast agent 4 h later (2nd scan) (Fig. 1). Before each injection, animals were submitted to at least 30 min of anesthesia, which is the same time for which they were anesthetized for the hyperpolarized [1‐¹³C]Pyr measurements.

Measurements of tumor perfusion using dynamic contrast agent‐enhanced ¹H MRI

Tumor/muscle (thigh) ratio for the area under the contrast agent concentration curve (AUC) at 45 s after injection of contrast agent for fasted and non‐fasted EL4 tumor‐bearing mice. Acquisitions were performed at the same time points as for the hyperpolarized [1‐¹³C]pyruvate study, with mice from Group 2 having received a dose of unlabeled non‐polarized pyruvate (10 mL/kg, 82 mm) and 30 min of anesthesia 4 h before the contrast agent‐enhanced ¹H images were acquired. Mean ± standard error of the mean.

DCE‐MRI data were acquired using a T ₁‐weighted spin‐echo pulse sequence, as described previously 12. Briefly, first an inversion recovery fast low‐angle shot (FLASH) pulse sequence was used to measure the native spin–lattice relaxation rates (R ₁ = 1/T ₁). These inversion recovery data were fitted, pixel‐by‐pixel, to a mono‐exponential function to obtain a pre‐contrast R ₁ map. A dynamic T ₁‐weighted spin‐echo pulse sequence was then used to collect baseline images (6 or 10) prior to injection of contrast agent. Diluted Gd^{3 +}‐DTPA in sterile saline (0.9% sodium chloride) was then injected as a bolus through a tail vein catheter over 2–3 s and dynamic T ₁‐weighted spin‐echo images were acquired for 10 min post‐injection. These images were then converted to R ₁ relaxation rate maps, as described previously 12. Gd^{3 +}‐DTPA concentration curves were determined for all tumor‐containing slices with a manually delineated region of interest (ROI) in the tumor and also in adjacent thigh muscle. The area under the uptake curve up to 45 s (AUC 45) was calculated for each ROI.