Three‐frequency RF coil designed for optimized imaging of hyperpolarized, 13C‐labeled compounds

Imaging exams involving hyperpolarized, ¹³C‐labeled compounds require novel RF coils for efficient signal utilization. While ¹³C coils are required for mapping the spatial distribution of the hyperpolarized compounds, imaging/pulsing at different frequencies is also needed for scan setup steps prior to the image acquisition. Imaging/pulsing at the ¹H frequency is typically used for anatomical localization and shimming. Flip angle (FA) calibration, which is difficult or impossible to achieve at the ¹³C frequency, can be accurately performed at the ²³Na frequency using the natural abundance signal that exists in any living tissue. We demonstrate here a single RF resonant structure that is capable of operating linearly at the ¹H and ²³Na frequencies for scan setup steps, and in quadrature at the ¹³C frequency for imaging. Images at the three resonant frequencies of this coil are presented from an exam involving hyperpolarized ¹³C compounds in vivo. Magn Reson Med 60:928–933, 2008. © 2008 Wiley‐Liss, Inc.     

Accurate flip-angle calibration for 13C MRI.

(13)C imaging and spectroscopy in the presence of injected labeled compounds can vastly extend the capability of MRI to perform metabolic imaging. The details of imaging in the presence of injected compounds, however, pose new technological challenges. Pulse sequences, in general, rely on precise flip-angle (FA) calibration to create high signal-to-noise ratio (SNR), artifact-free images. Signal quantification also requires precise knowledge of the excitation FA. In MRI scans that rely on signal acquisitions from injected compounds, however, such FA calibration is challenged by low natural-abundance (13)C signal levels before injection, and by time-varying signal following injection. A method to precisely set the FA at the (13)C frequency based on FA calibration at the (23)Na frequency is presented here. A practical implementation of a coil (a dual-tuned, (23)Na/(13)C low-pass birdcage coil) suitable for this calibration in vivo is also documented. Accurate FA calibration is demonstrated at the (13)C frequency for in vivo rat experiments using this approach.     

Magnetization transfer measurements of exchange between hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate in a murine lymphoma

Measurements of the conversion of hyperpolarized [1‐¹³C]pyruvate into lactate, in the reaction catalyzed by lactate dehydrogenase, have shown promise as a metabolic marker for the presence of disease and response to treatment. However, it is unclear whether this represents net flux of label from pyruvate to lactate or exchange of isotope between metabolites that are close to chemical equilibrium. Using saturation and inversion transfer experiments, we show that there is significant exchange of label between lactate and pyruvate in a murine lymphoma in vivo. The rate constants estimated from the magnetization transfer experiments, at specific points during the time course of label exchange, were similar to those obtained by fitting the changes in peak intensities during the entire exchange time course to a kinetic model for two‐site exchange. These magnetization transfer experiments may therefore provide an alternative and more rapid way of estimating flux between pyruvate and lactate to serial measurements of pyruvate and lactate ¹³C peak intensities following injection of hyperpolarized [1‐¹³C]pyruvate. Magn Reson Med 63:872–880, 2010. © 2010 Wiley‐Liss, Inc.     

Computational and experimental optimization of a double‐tuned 1H/31P four‐ring birdcage head coil for MRS at 3T

Purpose

To optimize the homogeneity and efficiency of the B₁ magnetic field of a four‐ring birdcage head coil that is double‐tuned at the Larmor frequencies of both ³¹P and ¹H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants.

Materials and Methods

We developed a finite difference time domain (FDTD) tool in‐house to iteratively compute and seek the range of geometric and electromagnetic parameters of a dual‐tuned, four‐ring birdcage coil that would produce the desired resonance patterns, optimize homogeneity of the B₁‐field, and maximize efficiency of the coil. To demonstrate the validity of our computational results, we constructed three RF coils: one dual‐tuned coil that was based on the calculated optimized parameters and two single‐tuned coils that had dimensions similar to those of the dual‐tuned coil, but tuned at the Larmor frequencies of both ³¹P and ¹H, respectively. We then tested and compared the performances of the dual‐tuned coil and single‐tuned coils at both of these frequencies.

Results

We found that a dual‐tuned, four‐ring birdcage coil with a diameter of 180 mm, an inner birdcage length of 100–300 mm, and an outer birdcage length of 25–100 mm produces the desired resonance patterns. For the use of this coil with human infants, optimization of the homogeneity of the B₁ field, combined with improved coil efficiency, yielded a dual‐tuned birdcage coil with diameter of 180mm, an inner birdcage length of 150 mm, an outer birdcage length of 25 mm, and corresponding inner and outer capacitances of 17.2 pF and 7.6 pF, respectively. The experimental results from a constructed coil having the sameparameters with the modeled coil agreed well with the computational results from the modeled coil. This optimized design overcame the deficiencies of existing dual‐tuned, four‐ring birdcage coils.

Conclusion

The homogeneity and efficiency of the B₁ field for ³¹P/¹H dual‐tuned, four‐ring birdcage coils can be optimized well using our FDTD tool, especially at high static magnetic fields (B₀). J. Magn. Reson. Imaging 2009;29:13–22. © 2008 Wiley‐Liss, Inc.     

Saturation‐recovery metabolic‐exchange rate imaging with hyperpolarized [1‐13C] pyruvate using spectral‐spatial excitation

Within the last decade hyperpolarized [1‐¹³C] pyruvate chemical‐shift imaging has demonstrated impressive potential for metabolic MR imaging for a wide range of applications in oncology, cardiology, and neurology. In this work, a highly efficient pulse sequence is described for time‐resolved, multislice chemical shift imaging of the injected substrate and obtained downstream metabolites. Using spectral‐spatial excitation in combination with single‐shot spiral data acquisition, the overall encoding is evenly distributed between excitation and signal reception, allowing the encoding of one full two‐dimensional metabolite image per excitation. The signal‐to‐noise ratio can be flexibly adjusted and optimized using lower flip angles for the pyruvate substrate and larger ones for the downstream metabolites. Selectively adjusting the excitation of the down‐stream metabolites to 90° leads to a so‐called “saturation‐recovery” scheme with the detected signal content being determined by forward conversion of the available pyruvate. In case of repetitive excitations, the polarization is preserved using smaller flip angles for pyruvate. Metabolic exchange rates are determined spatially resolved from the metabolite images using a simplified two‐site exchange model. This novel contrast is an important step toward more quantitative metabolic imaging. Goal of this work was to derive, analyze, and implement this “saturation‐recovery metabolic exchange rate imaging” and demonstrate its capabilities in four rats bearing subcutaneous tumors. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.