Dysregulation of pH in solid tumors is a hallmark of cancer. In recent years, the role of altered pH heterogeneity in space, between benign and aggressive tissues, between individual cancer cells, and between subcellular compartments, has been steadily elucidated. Changes in temporal pH-related processes on both fast and slow time scales, including altered kinetics of bicarbonate-CO₂ exchange and its effects on pH buffering and gradual, progressive changes driven by changes in metabolism, are further implicated in phenotypic changes observed in cancers. These discoveries have been driven by advances in imaging technologies. This review provides an overview of intra- and extracellular pH alterations in time and space reflected in cancer cells, as well as the available technology to study pH spatiotemporal heterogeneity.

Hyperpolarization (HP) of a carbon-13 molecule via dynamic nuclear polarization (DNP) involves polarization at low temperature, followed by a dissolution procedure producing a solution with highly polarized spins at room temperature. This dissolution DNP method significantly increases the signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) over 10,000-fold and facilitates the use of magnetic resonance spectroscopy (MRS) to image not only metabolism but also the extracellular microenvironment. The extracellular tumor microenvironment (TME) closely interacts with tumor cells and stimulates their growth and metastasis. Thus, the ability to detect pathological changes in the TME is pivotal for the detection and study of cancers. This review highlights the potential use of MRS to study features of the TME-elevated export of lactate, reduced interstitial pH, imbalanced redox equilibrium, and altered metal homeostasis. The promising outcomes of both in vitro and in vivo assays suggest that DNP-MRS may be a useful technique to study aspects of the TME. With continued improvements, this tool has the potential to study the TME and provide guidance for accurate patient stratification and precise personal therapy. Graphical Abstract.

Purpose

To assess the utility of furosemide diuresis and the role of an improved scatter correction algorithm in reducing scatter artifact severity on Ga-68- Prostate-specific membrane antigen (PSMA)-11 positron emission tomography (PET).

Materials and methods

A total of 139 patients underwent Ga-68-PSMA-11 PET imaging for prostate cancer: 47 non-time-of-flight (non-TOF) PET/computed tomography, 51 PET/magnetic resonance imaging (MRI) using the standard TOF scatter correction algorithm, and 41 PET/MRI using an improved TOF scatter correction algorithm. Whole-body PET acquisitions were subdivided into 3 regions: around kidneys; between kidneys and bladder; and around bladder. The images were reviewed, and scatter artifact severity was rated using a Likert-type scale.

Results

The worst scatter occurred when using non-TOF scatter correction without furosemide, where 42.1% of patients demonstrated severe scatter artifacts in 1 or more regions. Improved TOF scatter correction resulted in the smallest percentage of studies with severe scatter (6.5%). Scatter ratings by region were lowest using improved TOF scatter correction. Furosemide reduced mean scatter severity when using non-TOF and standard TOF.

Conclusions

Both furosemide and scatter correction algorithm play a role in reducing scatter in PSMA PET. Improved TOF scatter correction resulted in the lowest scatter severity.

There is an unmet clinical need for new and robust imaging biomarkers to distinguish indolent from aggressive prostate cancer. Hallmarks of aggressive tumors such as a decrease in extracellular pH (pH_e) can potentially be used to identify aggressive phenotypes. In this study, we employ an optimized, high signal-to-noise ratio hyperpolarized (HP) ¹³C pH_e imaging method to discriminate between indolent and aggressive disease in a murine model of prostate cancer. Transgenic adenocarcinoma of the mouse prostate (TRAMP) mice underwent a multiparametric MR imaging exam, including HP [¹³C] bicarbonate MRI for pH_e, with ¹H apparent diffusion coefficient (ADC) mapping and HP [1-¹³C] pyruvate MRI to study lactate metabolism. Tumor tissue was excised for histological staining and qRT-PCR to quantify mRNA expression for relevant glycolytic enzymes and transporters. We observed good separation in pH_e between low- and high-grade tumor regions, with high-grade tumors demonstrating a lower pH_e. The pH_e also correlated strongly with monocarboxylate transporter Mct4 gene expression across all tumors, suggesting that lactate export via MCT4 is associated with acidification in this model. Our results implicate extracellular acidification as an indicator of indolent-to-aggressive transition in prostate cancer and suggest feasibility of HP pH_e imaging to detect high-grade, clinically significant disease in men as part of a multiparametric MRI examination.

This study applied a dual-agent, ¹³C-pyruvate and ¹³C-urea, hyperpolarized ¹³C magnetic resonance spectroscopic imaging (MRSI) and multi-parametric (mp) ¹H magnetic resonance imaging (MRI) approach in the transgenic adenocarcinoma of mouse prostate (TRAMP) model to investigate changes in tumor perfusion and lactate metabolism during prostate cancer development, progression and metastases, and after lactate dehydrogenase-A (LDHA) knock-out. An increased Warburg effect, as measured by an elevated hyperpolarized (HP) Lactate/Pyruvate (Lac/Pyr) ratio, and associated Ldha expression and LDH activity were significantly higher in high- versus low-grade TRAMP tumors and normal prostates. The hypoxic tumor microenvironment in high-grade tumors, as measured by significantly decreased HP ¹³C-urea perfusion and increased PIM staining, played a key role in increasing lactate production through increased Hif1α and then Ldha expression. Increased lactate induced Mct4 expression and an acidic tumor microenvironment that provided a potential mechanism for the observed high rate of lymph node (86%) and liver (33%) metastases. The Ldha knockdown in the triple-transgenic mouse model of prostate cancer resulted in a significant reduction in HP Lac/Pyr, which preceded a reduction in tumor volume or apparent water diffusion coefficient (ADC). The Ldha gene knockdown significantly reduced primary tumor growth and reduced lymph node and visceral metastases. These data suggested a metabolic transformation from low- to high-grade prostate cancer including an increased Warburg effect, decreased perfusion, and increased metastatic potential. Moreover, these data suggested that LDH activity and lactate are required for tumor progression. The lactate metabolism changes during prostate cancer provided the motivation for applying hyperpolarized ¹³C MRSI to detect aggressive disease at diagnosis and predict early therapeutic response.

The differentiation of bacterial infection from other causes of inflammation is difficult in clinical practice and is critical where patient outcomes rely heavily on early interventions. In addition to physical exam and laboratory markers, several imaging modalities are frequently employed, but these techniques generally target the host immune response, rather than the living microorganisms themselves. Here, we describe a method to detect bacteria-specific metabolism using hyperpolarized (HP) ¹³C magnetic resonance spectroscopy. This technology allows visualization of the real-time conversion of enriched ¹³C substrates to their metabolic products, identified by their distinct chemical shifts. We have identified the rapid metabolism of HP [2-¹³C]pyruvate to [1-¹³C]acetate as a metabolic signature of common bacterial pathogens. We demonstrate this conversion in representative Gram-negative and Gram-positive bacteria, namely, Escherichia coli and Staphylococcus aureus, and its absence in key mammalian cell types. Furthermore, this conversion was successfully modulated in three mutant strains, corresponding to deletions of relevant enzymes.

Purpose

Rapid chemical exchange can affect SNR and pH measurement accuracy for hyperpolarized pH imaging with [¹³ C]bicarbonate. The purpose of this work was to investigate chemical exchange effects on hyperpolarized imaging sequences to identify optimal sequence parameters for high SNR and pH accuracy.

Methods

Simulations were performed under varying rates of bicarbonate-CO₂ chemical exchange to analyze exchange effects on pH quantification accuracy and SNR under different sampling schemes. Four pulse sequences, including 1 new technique, a multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms using hyperpolarized [¹³ C]bicarbonate, varying parameters such as tip angles, repetition time, order of metabolite excitation, and refocusing pulse design. In vivo hyperpolarized bicarbonate-CO₂ exchange measurements were made in transgenic murine prostate tumors to select in vivo imaging parameters.

Results

Modeling of bicarbonate-CO₂ exchange identified a multiple-excitation scheme for increasing CO₂ SNR by up to a factor of 2.7. When implemented in phantom imaging experiments, these sampling schemes were confirmed to yield high pH accuracy and SNR gains. Based on measured bicarbonate-CO₂ exchange in vivo, a 47% CO₂ SNR gain is predicted.

Conclusion

The novel multi-EPI pulse sequence can boost CO₂ imaging signal in hyperpolarized ¹³ C bicarbonate imaging while introducing minimal pH bias, helping to surmount a major hurdle in hyperpolarized pH imaging.

A robust and selective late-stage deuteration methodology was applied to 13C-enriched amino and alpha hydroxy acids to increase spin-lattice relaxation constant T1 for hyperpolarized 13C magnetic resonance imaging. For the five substrates with 13C-labeling on the C1-position ([1-13C]alanine, [1-13C]serine, [1-13C]lactate, [1-13C]glycine, and [1-13C]valine), significant increase of their T1 was observed at 3 T with deuterium labeling (+26%, 22%, +16%, +25% and +29%, respectively). Remarkably, in the case of [2-13C]alanine, [2-13C]serine and [2-13C]lactate, deuterium labeling led to a greater than four fold increase in T1. [1-13C,2-2H]alanine, produced using this method, was applied to in vitro enzyme assays with alanine aminotransferase, demonstrating a kinetic isotope effect.

Alterations in Zn²⁺ concentration are seen in normal tissues and in disease states, and for this reason imaging of Zn²⁺ is an area of active investigation. Herein, enriched [1-¹³ C]cysteine and [1-¹³ C₂ ]iminodiacetic acid were developed as Zn²⁺ -specific imaging probes using hyperpolarized ¹³ C magnetic resonance spectroscopy. [1-¹³ C]cysteine was used to accurately quantify Zn²⁺ in complex biological mixtures. These sensors can be employed to detect Zn²⁺ via imaging mechanisms including changes in ¹³ C chemical shift, resonance linewidth, or T₁ .