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Editorial for “Whole‐Abdomen Metabolic Imaging of Healthy Volunteers Using Hyperpolarized [1‐13C]pyruvate MRI”

Editorial for “Whole-Abdomen Metabolic Imaging of Healthy Volunteers Using Hyperpolarized [1-C]pyruvate MRI” Contrast agents based on hyperpolarized (HP) carbon-13 have the potential to significantly advance MRI as a molecular imaging paradigm. Development of HP pyruvate as a method for detection and staging of human disease is accelerating, with important advances in brain and heart imaging demonstrating the immense potential of the technique. Each of these studies have been posited in an organspecific manner, where the detection coils needed for observing C frequencies can be purpose built for the specific application. However, imaging of primary cancers and metastatic lesions remains a high priority target for HP imaging. Masses in the abdominal cavity are commonly discovered incidentally by MRI, leading to significant stress for the patient and triggering a costly cascade of medical tests in an attempt to determine whether the mass is benign or malignant. In the case of renal clear cell carcinomas (RCCs) occurring at 36,000 cases/year in the United States, total nephrectomy is not sufficient to prevent the disease recurrence. The 5-year survival rate of RCC is such that nephrectomy is the best option of care in many circumstances where the lesions cannot be properly biopsied for diagnosis. Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are liver diseases that are producing an incredible impact on healthcare, rapidly becoming the number one etiology responsible for liver transplant. NAFLD/NASH is a metabolic disease associated with dysregulated energy metabolism. Metabolically sensitive MRI of the whole abdomen is therefore a rational and important technical goal in and of itself, as a means of assessing metabolism in abdominal organs and across its entire volume simultaneously would significantly enhance our diagnostic capabilities. In this case, the field-ofview (FOV) must be large enough to accommodate the entire abdomen. Surface coils for excitation/detection do not lend themselves to this application and are likely to be incapable of imaging lesions in regions that are not amenable to traditional biopsy due to their limited active volumes. Volumetric C transmit coils are the appropriate hardware for implementing imaging of the whole abdomen, with a paired C array to maximize detected signal throughout the volume. This hardware configuration is executed in the work of Lee et al., but this is only the first step in producing images that can be registered with H scans to produce a diagnostic tool. HP C imaging places constraints on acquisition schemes that are not limiting with thermally polarized scans. In common with fast H imaging, echo planar readouts of the magnetization are widely used for C detection. In contrast to H MRI, the signals of the substrate and metabolites are selectively excited with different flip angles, as depolarization of the initial agent costs signal in the downstream products. Therefore, B0 homogeneity becomes even more important than for H scans, as the selective excitation schemes depend upon using the correct frequency for all the targeted metabolic signals. In the work of Lee, a paired set of flexible coils were developed to fit around the subject, with larger, outer excite coils and a closer fitting set of receive coils providing the FOV needed for the studies. Tests with phantoms using standard Bloch–Siegert calibrations demonstrated suitable B1 homogeneity. Drawing upon previous work, a ΔB0 field map was constructed using H imaging and generalized for use in assigning the exact frequencies of the C excitation pulses. The H-derived B0 map was proven to be of sufficient accuracy to remove the need for spectroscopic imaging, significantly simplifying the overall workflow. Using a set of 13 patients, HP imaging was executed in a reproducible manner, enabling a first of its kind examination of abdominal pyruvate utilization. The production of kpl and kpa maps describing rates of lactate and alanine production across the abdomen provides important, organ specific insights into pyruvate metabolism. While the inputless model for estimating kpl and kpa neglects pyruvate uptake and extravasation as controlling parameters, it has the favorable quality of simplicity and ease of implementation, allowing robust estimates of pyruvate kinetics across patients. As the authors point out, the data can serve as the first inter-organ atlas of pyruvate kinetics and provides some background for what can be considered as a normal physiological response. The findings are compelling, as the individual estimated values range considerably across the organs, and the kpl/kpa