Figure 1.

Representative anatomical T2-weighted (rapid acquisition with relaxation enhancement (RARE)) images in sagittal (A) and axial (B) orientations, with oblique spectroscopy slice geometry shown for a subcutaneous tumor (t) in a rat and for a thermal lactate (lac) phantom for transmit power calibration. To measure within the target tumor, with the scanner reference frequency set to that of [1-¹³C]pyruvate, the slice geometry is prescribed twice: once on the target tumor for [1-¹³C]pyruvate (yellow-and-red outlines) and again offset by the chemical shift displacement of [1-¹³C]lactate (dashed yellow outlines). This produces excitations that, at the location of the tumor, alternate between [1-¹³C]pyruvate and [1-¹³C]lactate frequencies. An additional slice may also be prescribed (dashed blue outlines) to target lactate in the phantom (solid blue outlines). Carbopol^® 980 gel (g, bright contrast) was placed around the tumor to improve its B0 field uniformity. The locations of the ¹³C surface receive coils (c1 and c2, light blue outlines), muscle (m), and a point-resolved spectroscopy (PRESS) voxel (white box surrounding t) for shimming on proton signal within the tumor are also marked. Magnitude spectra (C) from [1-¹³C]lactate in the phantom (blue) and from hyperpolarized [1-¹³C]lactate in the tumor (yellow) after injection, exhibiting a frequency shift between the two sources of lactate signal. In cases where this shift is large, the placement of a slice prescribed to measure within the phantom may benefit from additional spatial offset to compensate for that frequency shift in order to excite the ¹³C in the phantom.