Frequency correction method for improved spatial correlation of hyperpolarized 13C 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 ¹³C metabolic imaging in vivo. 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 ¹³C 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.