Frequency correction method for improved spatial correlation of hyperpolarized 13 C metabolites and anatomy

Blip‐reversed echo‐planar imaging (EPI) is investigated as a method for measuring and correcting the spatial shifts that occur due to bulk frequency offsets in 13 C metabolic imaging in viv o . By reversing the k ‐space trajectory for every other time point, the direction of the spatial shift for a given frequency is reversed. Here, mutual information is used to find the ‘best’ alignment between images and thereby measure the frequency offset. Time‐resolved 3D images of pyruvate/lactate/urea were acquired with 5 s temporal resolution over a 1 min duration in rats ( N  = 6). For each rat, a second injection was performed with the demodulation frequency purposely mis‐set by +35 Hz, to test the correction for erroneous shifts in the images. Overall, the shift induced by the 35 Hz frequency offset was 5.9 ± 0.6 mm (mean ± standard deviation). This agrees well with the expected 5.7 mm shift based on the 2.02 ms delay between k ‐space lines (giving 30.9 Hz per pixel). The 0.6 mm standard deviation in the correction corresponds to a frequency‐detection accuracy of 4 Hz. A method was presented for ensuring the spatial registration between 13 C metabolic images and conventional anatomical images when long echo‐planar readouts are used. The frequency correction method was shown to have an accuracy of 4 Hz. Summing the spatially corrected frames gave a signal‐to‐noise ratio (SNR) improvement factor of 2 or greater, compared with the highest single frame. Copyright © 2013 John Wiley & Sons, Ltd.