Quantitative and simultaneous measurement of oxygen consumption rates in rat brain and skeletal muscle using 17 O MRS imaging at 16.4T

Imaging bioluminescence by detecting localized haemodynamic contrast from photosensitized vasculature

Bioluminescent probes are widely used to monitor biomedically relevant processes and cellular targets in living animals. However, the absorption and scattering of visible light by tissue drastically limit the depth and resolution of the detection of luminescence. Here we show that bioluminescent sources can be detected with magnetic resonance imaging by leveraging the light-mediated activation of vascular cells expressing a photosensitive bacterial enzyme that causes the conversion of bioluminescent emission into local changes in haemodynamic contrast. In the brains of rats with photosensitized vasculature, we used magnetic resonance imaging to volumetrically map bioluminescent xenografts and cell populations virally transduced to express luciferase. Detecting bioluminescence-induced haemodynamic signals from photosensitized vasculature will extend the applications of bioluminescent probes.

How quickly does FLASH need to be delivered? A theoretical study of radiolytic oxygen depletion kinetics in tissues

Purpose. Radiation delivered over ultra-short timescales ('FLASH' radiotherapy) leads to a reduction in normal tissue toxicities for a range of tissues in the preclinical setting. Experiments have shown this reduction occurs for total delivery times less than a 'critical' time that varies by two orders of magnitude between brain (∼0.3 s) and skin (⪆10 s), and three orders of magnitude across different bowel experiments, from ∼0.01 to ⪆(1-10) s. Understanding the factors responsible for this broad variation may be important for translation of FLASH into the clinic and understanding the mechanisms behind FLASH.Methods.Assuming radiolytic oxygen depletion (ROD) to be the primary driver of FLASH effects, oxygen diffusion, consumption, and ROD were evaluated numerically for simulated tissues with pseudorandom vasculatures for a range of radiation delivery times, capillary densities, and oxygen consumption rates (OCR's). The resulting time-dependent oxygen partial pressure distribution histograms were used to estimate cell survival in these tissues using the linear quadratic model, modified to incorporate oxygen-enhancement ratio effects.Results. Independent of the capillary density, there was a substantial increase in predicted cell survival when the total delivery time was less than the capillary oxygen tension (mmHg) divided by the OCR (expressed in units of mmHg/s), setting the critical delivery time for FLASH in simulated tissues. Using literature OCR values for different normal tissues, the predicted range of critical delivery times agreed well with experimental values for skin and brain and, modifying our model to allow for fluctuating perfusion, bowel.Conclusions. The broad three-orders-of-magnitude variation in critical irradiation delivery times observed inin vivopreclinical experiments can be accounted for by the ROD hypothesis and differences in the OCR amongst simulated normal tissues. Characterization of these may help guide future experiments and open the door to optimized tissue-specific clinical protocols.

Quantitative 17 O MRSI of myocardial oxygen metabolic rate, blood flow, and oxygen extraction fraction under normal and high workload conditions

PURPOSE

The heart is a highly aerobic organ consuming most of the oxygen the body in supporting heart function. Quantitative imaging of myocardial oxygen metabolism and perfusion is essential for studying cardiac physiopathology in vivo. Here, we report a new imaging method that can simultaneously assess myocardial oxygen metabolism and blood flow in the rat heart.

METHODS

This novel method is based on the 17 O-MRSI combined with brief inhalation of 17 O-isotope labeled oxygen gas for quantitative imaging of myocardial metabolic rate of oxygen consumption (MVO2 ), myocardial blood flow (MBF), and oxygen extraction fraction (OEF). We demonstrate this imaging method under basal and high workload conditions in rat hearts at 9.4 T.

RESULTS

We show that this 17 O MRSI-based approach can directly measure and image MVO2 (1.35-4.06 μmol/g/min), MBF (0.49-1.38 mL/g/min), and OEF (0.33-0.44) in the heart of anesthetized rat under basal and high workload (21.6 × 103 -56.7 × 103 mmHg • bpm) conditions. Under high workload condition, MVO2 and MBF values in healthy rats approximately doubled, whereas OEF remained unchanged, indicating a strong coupling between myocardial oxygen metabolic demand and supply through blood perfusion.

CONCLUSION

The 17 O-MRSI method has been used to simultaneously image the myocardial metabolic rate of oxygen consumption, blood flow, and oxygen extraction fraction in small animal hearts, which are sensitive to the physiological changes induced by high workload. This approach could provide comprehensive measures that are critical for studying myocardial function in normal and diseased states and has a potential for translation.

Dual-frequency resonant coil design for low-γ X-nuclear and proton magnetic resonance imaging at ultrahigh fields

Low-γ X-nuclear MRS and imaging have played a key role in studying metabolism and physiopathology, especially at ultrahigh fields. We design and demonstrate a novel and simple dual-frequency RF resonant coil that can operate at both low-γ X-nuclear and proton frequencies. The dual-frequency resonant coil comprises an LC coil loop and a tuning-matching circuit bridged by two short wires of the desired length to generate two resonant modes: one for proton MRI and the other for low-γ X-nuclear MRS imaging with a large difference in their Larmor frequencies at ultrahigh fields. The coil parameters for the desired coil size and resonant frequencies can be determined via numerical simulations based on LC circuit theory. We designed, constructed, and evaluated several prototype surface coils and quadrature array coils for 1 H and 2 H or 17 O imaging, with small-sized (diameter ≤ 5 cm) coils evaluated using a 16.4 T animal scanner, and a large-sized (15 cm diameter) coil on a 7 T human scanner. All coils could be tuned/matched and driven in the single coil or array coil mode to the resonant frequencies of 1 H (698 and 298 MHz), 2 H (107 and 45.8 MHz), or 17 O (94.7 and 40.4 MHz) for imaging measurements and evaluation at 16.4 and 7 T, respectively. The dual-frequency resonant coil or array provides adequate detection sensitivity for 1 H MRI and excellent performance for low-γ X-nuclear MRS imaging applications, and excellent coil decoupling efficiency between the array coils at both resonant frequencies with an optimal geometric overlap. It provides a simple, cost-effective dual-frequency RF coil solution to perform low-γ X-nuclear MRS imaging for preclinical and human applications, especially at ultrahigh fields.

Phase I Randomized Trial of 17 O-Labeled Water: Safety and Feasibility Study of Indirect Proton MRI for the Evaluation of Cerebral Water Dynamics

BACKGROUND

¹⁷ O-labeled water (PSO17) is a contrast agent developed to measure brain water dynamics and cerebral blood flow.

PURPOSE

To evaluate the safety and feasibility of PSO17.

STUDY TYPE

Prospective study.

SUBJECTS

A total of 12 male healthy volunteers (23.1 ± 1.9 years) were assigned to three groups of four subjects: placebo (normal saline), PSO17 10%, and PSO17 20%.

FIELD STRENGTH/SEQUENCE

Dynamic 3D fluid attenuated inversion recovery (FLAIR, fast spin echo with variable refocusing flip angle) scans of the brain were performed with 3-T MRI.

ASSESSMENT

Contrast agents were injected 5 minutes after the start of a 10-minute scan. Any symptoms, vital signs, and blood and urine tests were evaluated at five timepoints from preinjection to 4 days after. Blood samples for pharmacokinetic analysis, including half-life (T1/2), maximum fraction (C_max ), time-to-maximum fraction (T_max ), and area under the curve (AUC), were collected at 13 timepoints from preinjection to 168 hours after. Regions of interest were set in the cerebral cortex (CC), basal ganglia/thalamus (BG/TM), and white matter (WM), and ¹⁷ O concentrations were calculated from signal changes and evaluated using C_max .

STATISTICAL TESTS

All items were compared among the three groups using Tukey-Kramer's honestly significant difference test. Statistical significance was defined as P < 0.5.

RESULTS

No safety issues were noted with the intravenous administration of PSO17. The T1/2 was approximately 160 hours, and the AUCs were 1.77 ± 0.10 and 3.75 ± 0.36 in the PSO17 10% and 20% groups, respectively. ¹⁷ O fractions calculated from MRI signals were higher in the PSO17 20% group than in the 10% and placebo groups. Significant differences were noted between all pairs of groups in the CC and BG/TM, and between PSO17 20% and both placebo and 10% groups in the WM.

DATA CONCLUSION

PSO17 might be considered safe as a contrast medium. Dynamic 3D-FLAIR might detect dose-dependent signal changes and estimate ¹⁷ O.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 1.

Noninvasive assessment of myocardial energy metabolism and dynamics using in vivo deuterium MRS imaging

PURPOSE

The assessment of cellular energy metabolism is crucial for understanding myocardial physiopathology. Here, we conducted a pilot study to develop an alternative imaging approach for the assessment of myocardial energy metabolism.

METHODS

We developed a deuterium MRSI method to noninvasively monitor the accumulation of deuterated downstream metabolites and deuterated water in rat hearts infused with deuterated glucose or acetate substrate on a 16.4 Tesla animal scanner.

RESULTS

We found that the deuterated water accumulation rate and isotopic turnover rate of deuterated glutamate/glutamine via the tricarboxylic acid cycle and exchange in rat hearts were much higher when infused with acetate compared to that with glucose, demonstrating the myocardium substrate preference for acetate over glucose.

CONCLUSION

We demonstrated the feasibility of deuterium MRSI for noninvasive imaging and assessment of myocardial energy metabolism in vivo. Although the strong signal and large dynamics of myocardial deuterated water may provide a sensitive imaging biomarker, quantifying the metabolic rates still poses a challenge due to the confounding effects of blood recirculation, perfusion, and multiple deuterated water production pathways. In contrast, the deuterated glutamate/glutamine signal and change should directly reflect the metabolic activity of the myocardial tricarboxylic acid cycle, which can be used to study the metabolic shift in substance preference between acetate and glucose in the diseased state. Deuterium MRSI is noninvasive and robust and may have the potential to assess myocardial energy metabolism in human patients.

Zero Echo Time 17O-MRI Reveals Decreased Cerebral Metabolic Rate of Oxygen Consumption in a Murine Model of Amyloidosis

The cerebral metabolic rate of oxygen consumption (CMRO₂) is a key metric to investigate the mechanisms involved in neurodegeneration in animal models and evaluate potential new therapies. CMRO₂ can be measured by direct ¹⁷O magnetic resonance imaging (¹⁷O-MRI) of H₂¹⁷O signal changes during inhalation of ¹⁷O-labeled oxygen gas. In this study, we built a simple gas distribution system and used 3D zero echo time (ZTE-)MRI at 11.7 T to measure CMRO₂ in the APP_swe/PS1_dE9 mouse model of amyloidosis. We found that CMRO₂ was significantly lower in the APP_swe/PS1_dE9 brain than in wild-type at 12-14 months. We also estimated cerebral blood flow (CBF) from the post-inhalation washout curve and found no difference between groups. These results suggest that the lower CMRO₂ observed in APP_swe/PS1_dE9 is likely due to metabolism impairment rather than to reduced blood flow. Analysis of the ¹⁷O-MRI data using different quantification models (linear and 3-phase model) showed that the choice of the model does not affect group comparison results. However, the simplified linear model significantly underestimated the absolute CMRO₂ values compared to a 3-phase model. This may become of importance when combining several metabolic fluxes measurements to study neuro-metabolic coupling.