Accurate flip‐angle calibration for ¹³C MRI

Abstract

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 flipangle (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 Given the ubiquitous presence of carbon atoms in most metabolic processes, and the large chemical shift range of the 13 C nuclei, 13 C spectroscopy is very promising for following metabolism in vivo. Static or dynamic 13 C images or spectra from animals and humans have been reported using both natural abundance and infusion of 13 C labeled compounds (1-6). Given the low gyromagnetic ratio of the 13 C nuclei, however, even images/spectra aided by the infusion of labeled compounds suffer from limited spatial resolution and low signal-to-noise ratio (SNR) at infusion doses and imaging times compatible with a routine clinical exam.The recent development of hyperpolarization techniques (7,8) can dramatically change the impact that 13 C MRI and MRS can have in managing a variety of pathological conditions. Real-time imaging of metabolism has been reported in a variety of animal models of disease using MRI of 13 C labeled, hyperpolarized compounds (9 -12). In order to successfully translate the results of research performed in animals to humans, care must be taken when imaging such compounds to ensure that the maximum information is extracted using the minimum relevant agent dose. The hyperpolarized signals are large, nonrenewable, and fast decaying, and require special attention when imaged to extract the maximum image SNR per injected agent dose.Many pulse sequences require precise flip-angle (FA) calibration to produce high-SNR, artifact-free images. Moreover, quantification of compound concentration relies on precise knowledge of the excitation FA. For most MRI or MRS scans, FA calibration is performed in a prescan step, at the same frequency as the one used for imaging. For infusion or injection of 13 C labeled (or labeled and hyperpolarized) compounds, such a calibration step is challenged by the low availability of natural-abundance signal prior to injection, and by the variable nature of the signal following injection. One approach to overcome these difficulties is to use a phantom that loads the coil in a manner similar to that in which a patient would load it, and perform a FA calibration on that phantom prior to any patient scan. The same transmit gain (TG) setting would then be used for all of the in vivo studies. However, in 23 Na scans of human brains, for example, the transmit power c...

Integrated Bloch‐Siegert B1 mapping and multislice imaging of hyperpolarized 13C pyruvate and bicarbonate in the heart

“…Recently, the development of a triple-tuned 1 H/ 13 C/ 23 Na coil has been reported as a method to indirectly tune the 13 C transmit B 1 field (26,27). This method relies on the conversion of a quadrature 13 C transmit coil into a linear 23 Na transmit coil.…”

Transmit gain calibration for nonproton MR using the Bloch–Siegert shift

“…TG calibration becomes somewhat more involved when imaging nonproton nuclei, such as 13 C, 3 He or 129 Xe 1–7. The signal from natural‐abundance, thermally polarised spins is typically not detectable.…”

A fast and simple method for calibrating the flip angle in hyperpolarized 13C MRS experiments

“…The flip angle calibration can be performed with a process called transmit gain calibration: at first, the 1808 flip angle is determined by manually increasing the transmit gain until the signal vanishes, successively 6 dB of power are subtracted to the obtained value to provide the 908 flip angle. Alternative methods have also been published, particularly addressed to studies with hyperpolarized 13 C, such as the Bloch-Siegert shift (18) or indirectly through flip angle calibration at the natural abundance 23 Na frequency (very close to that of 13 C) (10).…”

“…Starting from Eqs. [9] and [10], the following ratio can be defined, B 1peak being fixed: [11] which can be also expressed as:…”