Hyperpolarized 13C MRI in hepatocellular carcinoma: Unmet questions during clinical translation

Hyperpolarized (HP) 13C magnetic resonance imaging (MRI) is a powerful molecular imaging technique. With dissolution dynamic nuclear polarization (d-DNP), it can achieve dramatic signal increase and allows for dynamic and quantitative investigation of real-time metabolism in vivo.[1] The typical HP 13C MRI workflow begins with formulating the 13C probe, and then placing it in the hyperpolarization equipment to achieve the desired signal amplification level, which typically amounts to an increase in the MRI signal by more than 10,000 folds. The subsequent data acquisition is supported by a specialized 13C coil and 13C acquisition sequence, and finally quantitative and visualized metabolic information is obtained through a post-processing algorithm. To date, research of HP 13C MRI has focused on diabetes, cardiovascular diseases, and cancers,[2] showing great promises in tumor detection and the evaluation of therapeutic efficacy. Hepatocellular carcinoma (HCC) is the most common liver malignancy and the third leading cause of cancer-related deaths in the world. Given the cirrhotic background of most HCCs, it is challenging to detect the disease at the early stage with regular diagnostic imaging. In addition, the limited information on conventional imaging approaches is not sufficient to comprehensively assess the treatment response of HCC. Since the liver plays a central role in metabolism and is responsible for most chemical synthesis and degradation in the body, it is usually accompanied by the metabolic reprogramming during hepatocarcinogenesis, disease progression, and HCC recovery.[3] Therefore, the valuable metabolic information derived from HP 13C MRI can provide a deeper understanding of HCC [Supplementary Figure 1, https://links.lww.com/CM9/B993] and facilitate its better management, ultimately improving the patient prognosis and alleviating clinical burdens. As a developing technique, however, HP 13C MRI still faces several unsolved barriers in the process of clinical translation, including common translational problems in the field of oncology[2] and special concerns specific to HCC. Herein, we raise these unmet issues and call for international attention and efforts, hoping to promote the clinical application of this promising technology in HCC. How to accelerate the acquisition speed and obtain liver images with high spatial resolution: The hyperpolarization level of 13C probe is transient and non-renewable, requiring fast acquisition sequences to capture the signal and metabolic information. Typically, five dimensions of data are encoded by 13C pulse sequences, including three spatial, one spectral, and one temporal, with spectral encoding for assessing metabolism of substrate and metabolites and temporal encoding for evaluating metabolic kinetics.[4] Currently, spectral-spatial metabolite selective excitation with variable flip angles scheme is popular in research and very likely the choice for clinical use as well.[5] Acceleration techniques, such as compressed sensing or sense reconstruction, can reduce the number of excitations needed to image a given resolution and are thus a very promising addition for fast 13C liver imaging.[6] Moreover, to obtain better liver images with high spatial resolution, advanced shimming techniques and abdominal dual-tuned (1H and 13C) coils are essential, which can help to achieve homogeneous B0 field, increase signal-to-noise ratio (SNR), and optimize clinical workflow. What is the best biomarker for HCC diagnosis and follow-up over time: An increase in aerobic glycolysis and elevated lactate production are often observed in malignant tumors despite the presence of sufficient oxygen availability, also known as the Warburg effect. Hu et al[3] reported a similar phenomenon in the HCC animal model, demonstrating increased [1-13C]pyruvate to [1-13C]lactate conversion as tumors developed. Interestingly, they also found that the conversion of [1-13C]pyruvate to [1-13C]alanine was predominant in the very early stages of hepatocarcinogenesis. Likewise, the increased [1-13C]alanine level and its importance were also noted in other studies,[7,8] which showed the conversion from [1-13C]pyruvate to [1-13C]alanine significantly superseded that of [1-13C]pyruvate to [1-13C]lactate in HCC rats. Based on previous findings, it seems that the relative signal level of [1-13C]alanine may potentially serve as a sensitive biomarker for HCC, but the alterations and diagnostic values of [1-13C]lactate and [1-13C]alanine signal in human HCC remain to be explored. Additionally, other HP 13C agents and their corresponding metabolites, including [5-13C]glutamine and [5-13C]glutamate, [1,4-13C2]fumarate and [1,4-13C2] malate, can be used to assess various histopathological conditions in HCC, and promising results have been achieved in evaluating drug response and monitoring therapeutic efficacy after transcatheter arterial embolization.[9–10] In terms of clinical application, the best biomarker for diagnosing and following up in human HCC should be identified with further efforts. What are the effects of the dual blood flow in observed metabolism: The normal liver has a dual blood supply with two inputs, mostly from the portal veins and partly from the hepatic arteries. According to these unique characteristics, we should expect another small signal peaks after the main peaks,[11] or even a sustained peak of certain metabolite,[12] since the remaining HP 13C substrate and its metabolites produced by the intestines can be brought back to liver through the portal vein. However, the results of previous studies are inconclusive, and the contributions of flowback substances and their signals on the overall observed metabolism remain unknown. Notably, due to the abnormal neovascularization during tumorigenesis, HCCs typically exhibit altered patterns of blood flow, which are dominated by the hepatic arteries. Compared with adjacent liver parenchyma, the blood supply from the portal vein is largely reduced in HCC, resulting in altered imaging appearance and metabolism. It is of great importance to know the impact of vascular malformations on metabolic activities and the sources of measured metabolite signals. A preliminary study from Menzel et al[13] may give some clues, who established a preclinical HCC model and analyzed the 13C metabolite dynamics in vena cava, tumor, and gastrointestinal tract (GIT). In specific, they found a significant smaller amount of [1-13C]pyruvate reached the tumors than the GIT, but the metabolic conversion of [1-13C]pyruvate to [1-13C]alanine or [1-13C]lactate was higher in the tumors compared to GIT. Yet, studies in real patient scenario are missing, and further investigations are required to explore the influence of dual blood supply and cancerous blood flow changes on hepatic metabolism. What is the best injection timing for HP13C probes: In view of the importance of hemodynamic changes during tumor development, MRI examination with gadolinium-based contrast agent is necessary to identify the enhancement pattern and to assist the diagnosis of HCC in a clinical setting. However, it is currently unclear whether the HP 13C probes should be injected before or after the administration of gadolinium, especially when using the liver-specific contrast agent. In principle, gadolinium will change the MRI properties and corresponding signals of the tissues and may also affect the hepatic metabolism. Therefore, Perkons et al[14] suggested to perform the HP 13C MRI scanning before contrast enhancement. On the other hand, given that liver-specific contrast agent (gadoxetic acid) can be taken up by normal hepatocytes, previous studies found that the HP 13C signals from normal liver could be selectively suppressed by injecting gadolinium first.[15] These findings were significant as they could be applied to separate the HP 13C signals arising from small tumors and surrounding healthy hepatocytes with high metabolic activity, which might eventually improve the assessment of HCC metabolism. Future researches are needed to determine the best injection timing of HP 13C probes relative to gadolinium, and further explore the confounding effects of gadolinium contrast materials on HP signals. What is the optimal acquisition timing for HP13C imaging: Appropriate acquisition timing for HP 13C imaging is essential to achieve sufficient SNR of substrate and metabolites. To maximize the observed signals of [1-13C] lactate, a recent preclinical HCC study initiated the image acquisition 25s following the commencement of HP [1-13C]pyruvate injection.[14] Nevertheless, the delay time between HP 13C probe injection and image acquisition was significantly varied in previous liver studies. Most researchers started acquisition simultaneously with intravenous injection to capture the full dynamics of HP 13C substrate and metabolites, while others acquired the imaging data with a delay of 5–30s after the end of injection.[3,8,11] These empirically determined delay times can be problematic because the inherent physiological variability between patients may affect the robustness of interpreted results.[5] Hence, there is a need to explore the optimal acquisition timing and reach an international consensus on the delay time for HP 13C liver imaging before this technique can be finally integrated into the clinical workflow of HCC. Other challenges: HP 13C MRI remains technically demanding and costly, despite its potential advantages in terms of safety and patient acceptability. The complexity of the current hyperpolarization process, requirements for special facility setup, and relatively high costs limit the widespread use of this technique; and these issues need to be addressed before it can be truly translated into clinical practice. Future improved and simplified HP 13C MRI workflow is warranted with the help of continuing research and experience from multicenter studies. HP 13C MRI is a novel molecular imaging technique with great potential for clinical translation in HCC diagnosis and management. However, current unmet technical issues should be addressed before its final application in the clinic. Funding This work was supported by a grant from the National Natural Science Foundation of China (No. 82302161) and the China Postdoctoral Science Foundation (No. 2023M732464). Conflicts of interest None.