RF coil design for accurate parallel imaging on ¹³C MRSI using ²³Na sensitivity profiles

Abstract

To develop a coil-based method to obtain accurate sensitivity profiles in 13 C MRI at 3T from the endogenous 23 Na. An eight-channel array is designed for 13 C MR acquisitions. As application examples, the array is used for two-fold accelerated acquisitions of both hyperpolarized 13 C metabolic imaging of pig kidneys and the human brain.Methods: A flexible coil array was tuned optimally for 13 C at 3T (32.1 MHz), with the coil coupling coefficients matched to be nearly identical at the resonance frequency of 23 Na (33.8 MHz). This is done by enforcing a high decoupling (obtained through highly mismatched preamplifiers) and adjusting the coupling frequency response. The SNR performance is compared to reference coils. Results:The measured sensitivity profiles on a phantom showed high spatial similarity for 13 C and 23 Na resonances, with average noise correlation of 9 and 11%, respectively.For acceleration factors 2, 3, and 4, the obtained maximum g-factors were 1.0, 1.1, and 2.6, respectively. The 23 Na profiles obtained in vivo could be used successfully to perform two-fold acceleration of hyperpolarized 13 C 3D acquisitions of both pig kidneys and a healthy human brain. Conclusion:A receive array has been developed in such a way that the 13 C sensitivity profiles could be accurately obtained from measurements at the 23 Na frequency. This technique facilitates accelerated acquisitions for hyperpolarized 13 C imaging. The SNR performance obtained at the 13 C frequency, compares well to other state-of-the-art coils for the same purpose, showing slightly better superficial and central SNR.

Considerations for hyperpolarized 13C MR at reduced field: Comparing 1.5T versus 3T

“…While this work is based on large single‐ or dual‐loop coils ($$$ r\approx 10 $$$ cm), often coil arrays of multiple smaller loops are used to improve the detection sensitivity without limiting the field of view, 21 or for making use of parallel imaging 38 . For hyperpolarized $$$ {}^{13} $$$C body imaging, the individual loops of such arrays typically have radii around 5 cm 38,39 (or corresponding rectangular sizes with similar areas 40 ), for which sample noise still slightly exceeds coil noise at 1.5T (Figure 4). However, our Q‐ratio measurements showed that the 16 MHz single‐loop coil with 10 cm radius is already at the limit and hence coil arrays with small loops might be coil noise‐dominated and therefore not beneficial at lower fields.…”

Current methods for hyperpolarized [1‐13C]pyruvate MRI human studies

“…The image reconstruction methods used in human [1- 13 C]pyruvate studies have typically been conventional methods (e.g., FFT, non-uniform FFT, or equivalent). The incorporation of accelerated imaging and advanced reconstruction methods including parallel imaging 10,57,87 and compressed sensing 7 has also been applied in human studies for improved spatial resolution, temporal resolution, and coverage, but have the potential for additional artifacts as well as SNR losses because of ill-conditioning of the reconstruction (e.g., g-factor). The majority of published studies do not use accelerated imaging indicating the resolution and coverage achievable without acceleration is currently adequate for successful data collection.…”

“…For HP 13 C MRI of the human brain, the majority of studies have also used 2D (slice selective) acquisitions, 10–12,14,16,28,33,40–41,44,51,53,60 with a trend toward volumetric coverage using 2D multislice metabolite‐specific imaging. Three dimensional metabolite‐specific imaging of the whole brain, with phase encoding of the slice direction, 34,57 has been shown to provide similar SNR efficiency 88 compared with multislice imaging. A number of studies have used MRSI 5,6,29,31–33,50,55 resulting in a spectrum from each voxel, which has the advantage of not requiring a priori information about which peaks to encode.…”

“…Since then, there have been over 60 papers published with imaging results of human subjects from 13 different sites, with applications including prostate cancer, brain tumors, breast cancer, kidney cancer, pancreatic cancer, metastatic disease, liver disease, ischemic heart disease, diabetes and cardiomyopathies. The vast majority of these studies used [1‐ 13 C]pyruvate, 1–63 where [2‐ 13 C]pyruvate 64 and 13 C‐urea 56 have been demonstrated too.…”

“…This is divided into four major topic areas: (1) HP 13 C‐pyruvate preparation; (2) MRI system setup and calibrations; (3) data acquisition and image reconstruction; and (4) data analysis and quantification (Figure 1). The current successful practices and study methods include a literature review of published peer‐reviewed journal papers showing human HP [1‐ 13 C]pyruvate study data, up to September 2022, 1–63 that cover a variety of anatomical targets (Figure 2), as well as new unpublished information from surveys of HP 13 C study sites. Based on this information, we also highlight the evidence gaps, strengths, and limitations of current practices, which are summarized at the end of each section.…”