Hyperpolarization of spins via dynamic nuclear polarization (DNP) has been explored as a method to non‐invasively study real‐time metabolic rocesses occurring in vivo using ¹³C‐labeled substrates. Recently, hyperpolarized ¹³C pyruvate has been used to characterize in vivo cardiac metabolism in the rat and pig. Conventional 3D spectroscopic imaging methods require in excess of 100 excitations, making it challenging to acquire a full cardiac‐gated, breath‐held, whole‐heart volume. In this article, the development of a rapid multislice cardiac‐gated spiral ¹³C imaging pulse sequence consisting of a large flip‐angle spectral‐spatial excitation RF pulse combined with a single‐shot spiral k‐space trajectory for rapid imaging of cardiac metabolism is described. This sequence permits whole‐heart coverage (6 slices, 8.8‐mm in‐plane resolution) in any plane, allowing imaging of the metabolites of interest, [1‐ ¹³C] pyruvate, [1‐ ¹³C] lactate, and ¹³C bicarbonate, within a single breathhold. Pyruvate and bicarbonate cardiac volumes were acquired, while lactate images were not acquired due to low lactate levels in the animal model studied. The sequence was demonstrated with phantom experiments and in vivo testing in a pig model. Magn Reson Med, 2010.

Survival in glioblastoma might be extended by escalating the radiotherapy dose to treatment-resistant tumour and adapting to tumour changes. Diffusion-weighted imaging (DWI) on MRI-linear accelerators (MR-Linacs) could be used to identify a dose escalation target, but its prognostic value must be demonstrated. The purpose of this study was to determine whether MR-Linac DWI can assess treatment response in glioblastoma and whether changes in DWI show greater prognostic value than changes in the contrast-enhancing gross tumour volume (GTV). Seventy-five patients with glioblastoma were treated with chemoradiotherapy, of which 32 were treated on a 1.5 T MRI-linear accelerator (MR-Linac). Patients were imaged with simulation MRI scanners (MR-sim) at treatment planning and weeks 2, 4, and 10 after treatment start. Twenty-eight patients had additional MR-Linac DWI sequences. Cox modelling was used to evaluate the correlation of overall and progression-free survival (OS and PFS) with clinical variables and volumetric changes in the GTV and low-ADC regions (ADC < 1.25 µm²/ms within GTV). In total, 479 MR-Linac DWI and 289 MR-sim DWI datasets were analyzed. MR-Linac low-ADC changes between weeks 2 and 5 inclusive were prognostic for OS (hazard ratio lower limits ≥ 1.2, p-values ≤ 0.02). MR-sim low-ADC changes showed greater correlation with OS and PFS than GTV changes (e.g., OS hazard ratio at week 2 was 3.4 (p <0.001) for low-ADC versus 2.0 (p = 0.022) for GTV). MR-Linac DWI can measure low-ADC tumour volumes that correlate with OS and PFS better than contrast-enhancing GTV. Low-ADC regions could serve as dose escalation targets.

Hyperpolarization of ¹³C labeled substrates via dynamic nuclear polarization has been used as a method to noninvasively study real‐time metabolic processes occurring in vivo. In these studies, proper calibration of radiofrequency transmit power is required to efficiently observe rapidly decaying magnetization. Conventional transmit radiofrequency field $(B_1^{+})$ mapping methods rely on placing magnetization in a fixed, known state prior to imaging, making them unsuitable for imaging of hyperpolarized magnetization. Recently, a phase‐based B₁ mapping method based on the Bloch‐Siegert shift has been reported. This method uses a B₁‐dependent shift in the resonance frequency of nuclei in the presence of an off‐resonance radiofrequency pulse. In this article, we investigate the feasibility of Bloch‐Siegert B₁ mapping and observation of metabolism of hyperpolarized $[1{-}^{13}{\rm C}]$ pyruvate in vivo, in a single injection. The technique is demonstrated with phantom experiments, and in normal rat and pigs in vivo. This method is anticipated to improve quantitative measurements of hyperpolarized ¹³C metabolism in vivo by enabling accurate flip‐angle corrections. This work demonstrates the use of Bloch‐Siegert B₁ mapping under challenging out‐of‐equilibrium imaging conditions. Magn Reson Med, 2011.

To describe the implementation and initial results of using Chemical Exchange Saturation Transfer (CEST) for monitoring patients with central nervous system (CNS) tumours treated using a 1.5 tesla MR-guided radiotherapy system. CNS patients were treated with up to 30 fractions (total dose up to 60 Gy) using a 1.5 T Elekta Unity MR-Linac. CEST scans were obtained in 54 subjects at one or more time points during treatment. CEST metrics, including the amide magnetization transfer ratio (MTRAmide), nuclear Overhauser effect (NOE) MTR (MTRNOE) and asymmetry, were quantified in phantoms and CNS patients. The signal was investigated between tumour and white matter, across time, and across disease categories including high- and low-grade tumours. The gross tumour volume (GTV) exhibited lower MTRAmide and MTRNOE and higher asymmetry compared to contralateral normal appearing white matter. Signal changes in the GTV during fractionated radiotherapy were observed. There were differences between high- and low-grade tumours, with higher CEST asymmetry associated with higher grade disease. CEST MRI using a 1.5 T MR-Linac was demonstrated to be feasible for in vivo imaging of CNS tumours. CEST images showed tumour/white-matter contrast, temporal CEST signal changes, and associations with tumour grade. These results show promise for the eventual goal of using metabolic imaging to inform the design of adaptive radiotherapy protocols.