2.3. In vivo experiments

All animal experiments were carried out under a protocol approved by the institutional animal care and use committee. Animal preparation and handling procedures have been previously described.24 For this feasibility study, a Yorkshire pig was scanned (weight = 25 kg, heart rate = 60 bpm). An intravenous catheter was placed in the ear vein for injection of hyperpolarized [1‐¹³C]pyruvate. An oral carbohydrate load (25 g glucose in 1 L, Life Brand, Toronto) was administered approximately 1 hour before the pyruvate injection, to raise cardiac pyruvate dehydrogenase activity. The animal was positioned supine and feet first within the volume transmit coil system.

A total of 500 mg of [1‐¹³C]pyruvic acid containing 15‐mM AH111501 sodium salt was polarized using a GE SpinLAB polarizer for approximately 3 hours. The acid was neutralized with a stoichiometric quantity of a base solution (720‐mM NaOH, 400‐mM Trizma pH 7.6, 100 mg/L ethylenediaminetetraacetic acid) and diluted to a concentration of 250 mM. After dissolution, a volume of 20 mL of the 250‐mM hyperpolarized [1‐13C]pyruvate solution was injected at 4 mL/s, followed by a 25‐mL saline flush at 5 mL/s.

The single‐shot spiral sequence was used in a spectrally selective manner to sequentially excite the resonances arising from [1‐¹³C]lactate, ¹³C‐bicarbonate, and [1‐¹³C]pyruvate (parameters: TR = 1 R‐R interval, FOV = 480 mm, readout duration = 32 ms, nominal in‐plane resolution = 7.5 × 7.5 mm², 24 total slices, 3 excitations per heartbeat, slice thickness = 10 mm, gap = 0 mm, flip angle = 60 ° for all metabolites). The full 3D FOV was 48 × 48 × 24 cm³. To test the effects of SMS excitation, 2 injections were performed, using either the single‐slice pulse or the multiband pulse for RF excitation. The unaccelerated scan time to image the 3 resonances was 24 heartbeats. The 2‐fold accelerated scan time was 12 heartbeats, and this was repeated 2 times to maintain a total breath‐held scan time of 24 heartbeats. For a heart rate of 60 bpm, this corresponded to a total scan time of 24 seconds. The receive coils were placed on the chest wall, with the linear row of elements oriented parallel to the axial imaging slices. Fiducial markers visible on MRI (MR‐SPOTS, Beekley Medical, Bristol, CT) were placed on the edges of the coil housing for each paddle. The scan started 20 seconds after the start of the injection to maximize the ¹³C‐bicarbonate signal in the myocardium.24 Axial, cardiac‐gated breath‐held SSFP cine ¹H images were acquired for anatomical reference (TR = 4.2 ms, TE = 1.8 ms, FOV = 240 mm, slice thickness = 5 mm, matrix size = 224 × 224).