This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: Hyperpolarized 13C enables high resolution imaging in which an injected compound and its downstream metabolites can be identified (1) . Rapid imaging is necessary due to T1 signal decay. Although the spectral bandwidth is large, the sparse spectrum can be exploited by spectral undersampling using methods such as spiral CSI (spcsi) (3). Conventional reconstruction involves performing a 1D FFT in kf to separate the metabolites. We propose an alternate reconstruction technique based on a least-squares (LS) algorithm. Methods and Discussion: For a spectrum containing known peaks, spiral CSI in which a single k-space trajectory is repeated for a total of M echoes, the signal for each kx, ky point can be written in matrix form y = Am, where y is a vector corresponding to the M observations, m is a vector describing the spectral components of the point. Information about the peaks, such as chemical shifts and J-couplings are included in A. LS methods can be used to solve for y for each k-space point, and gridding performed to obtain metabolic images. In a technique similar to that developed by Reeder et al for (4), optimal TE values, subject to the FOV and resolution constraints, can be obtained using the condition number of A. Simulations were performed for a set of peaks corresponding to 13C1 lactate, 13C1 alanine, 13C1 pyruvate and 13C bicarbonate. Phantom measurements (three tubes of 1.5M 99% enriched 13C1 ala, lac, and a pyruvate ester) were then performed on a GE 3T MR scanner. The performance of the LS reconstruction exceeded that of conventional reconstruction as seen in lower amounts of ala detected in the lac ROI, lac detected in the ala ROI, and ala detected in the pyr ROI. Additionally, performance of the LS recon with 8 echoes was comparable to performance at 21 echoes. References: (1) Ardenkjaer-Larsen JH et al. PNAS USA 2003;100:10158. (2) Adalsteinsson E et al. MRM 1998;39:889. (4) Reeder SB et al. MRM 2004;51:35. Acknowledgements: Lucas Foundation, NIH RR009784 and CA048269
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