title: Frequency correction method for improved spatial correlation of hyperpolarized 13C metabolites and anatomy.
abstract: 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 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 (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.
authors.affiliation: Department of Medical Biophysics and Sunnybrook Health Sciences Centre, Toronto, Canada; Imaging Research, Sunnybrook Health Sciences Centre, Toronto, Canada.
authors.forename: Charles H
authors.initials: CH
authors.lastname: Cunningham
authors.forename: William
authors.initials: W
authors.lastname: Dominguez Viqueira
authors.forename: Ralph E
authors.initials: RE
authors.lastname: Hurd
authors.forename: Albert P
authors.initials: AP
authors.lastname: Chen
mesh_terms: D000465:Algorithms; D000818:Animals; D016477:Artifacts; D002247:Carbon Isotopes / Q000493:pharmacokinetics; D017352:Echo-Planar Imaging / Q000379:methods*; D007668:Kidney / Q000033:anatomy & histology* / Q000378:metabolism*; D019344:Lactic Acid / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D057054:Molecular Imaging / Q000379:methods; D019289:Pyruvic Acid / Q000378:metabolism*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D011923:Rats, Nude; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013223:Statistics as Topic; D014018:Tissue Distribution; D014508:Urea / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D019275:Radiopharmaceuticals; D019344:Lactic Acid; D019289:Pyruvic Acid; D014508:Urea
keywords: echo-planar imaging; hyperpolarzied; imaging; lactate; metabolic imaging; mutual information; pyruvate; spectral-spatial excitation