Hyperpolarized (HP) 13Cmagnetic resonance spectroscopic imaging (MRSI) is a novel imaging technique that allows rapid and noninvasive monitoring of dynamic pathwayspecific metabolic and physiologic processes [1] with unprecedented gain in sensitivity (10 000–200 000 fold increase) for imaging of 13C-labeled biomolecules that are endogenous, nontoxic, and nonradioactive [2, 3]. We previously reported the first-in-human phase 1 clinical study of HP [13C]-pyruvate MRSI in patients with prostate cancer on active surveillance, and confirmed the feasibility of capturing regions of accelerated HP pyruvateto-lactate flux in high-grade versus low-grade cancer versus benign tissue [4].

Here we describe the first results demonstrating the metabolic response to androgen deprivation therapy (ADT) using HP [13C]-pyruvate MRSI. The patient presented with serum prostate-specific antigen (PSA) of 25.2 ng/ml and Gleason 4+ 5 prostate adenocarcinoma on biopsy. Cross-sectional imaging demonstrated metastases within the pelvic nodes and osseous structures. Baseline multi-parametric (mp) 1H MRI of the prostate (anatomic imaging, diffusion-weighted imaging [DWI], dynamic contrast-enhanced [DCE] imaging, and 3D 1H MRSI) with HP [13C]-pyruvate revealed a bulky tumor involving the left apex, mid gland, and base peripheral and transition zones, and right apex, mid gland, and base peripheral zone, measuring 4.5× 1.5× 5.1 cm3. T2-weighted MRI showed a welldefined focus of low signal intensity (T2 score 5/5; Fig. 1A). The lesion also had marked restricted diffusion (DWI score 5/5; apparent diffusion coefficient [ADC] 930) and was DCE-positive, with increased uptake and washout of contrast agent, and MRSI-positive, with elevated choline and reduced citrate on 1H MRSI. The overall Prostate Imaging-Reporting and Data System v. 2 score was 5.