Probing Cerebral Metabolism with Hyperpolarized 13C Imaging after Opening the Blood-Brain Barrier with Focused Ultrasound

Focused ultrasound widely broadens AAV-delivered Cas9 distribution and activity

Because children have little temporal exposure to environment and aging, most pediatric neurological diseases are inherent, i.e. genetic. Since postnatal neurons and astrocytes are mostly non-replicating, gene therapy and genome editing present enormous promise in child neurology. Unlike in other organs, which are highly permissive to adeno-associated viruses (AAV), the mature blood-brain barrier (BBB) greatly limits circulating AAV distribution to the brain. Intrathecal administration improves distribution but to no more than 20% of brain cells. Focused ultrasound (FUS) opens the BBB transiently and safely. In the present work we opened the hippocampal BBB and delivered a Cas9 gene via AAV9 intrathecally. This allowed brain first-pass, and subsequent vascular circulation and re-entry through the opened BBB. The mouse model used was of Lafora disease, a neuroinflammatory disease due to accumulations of misshapen overlong-branched glycogen. Cas9 was targeted to the gene of the glycogen branch-elongating enzyme glycogen synthase. We show that FUS dramatically (2000-fold) improved hippocampal Cas9 distribution and greatly reduced the pathogenic glycogen accumulations and hippocampal inflammation. FUS is in regular clinical use for other indications. Our work shows that it has the potential to vastly broaden gene delivery or editing along with clearance of corresponding pathologic basis of brain disease.

State of Practice on Transcranial MR-Guided Focused Ultrasound: A Report from the ASNR Standards and Guidelines Committee and ACR Commission on Neuroradiology Workgroup

Transcranial focused ultrasound (FUS) is a versatile, MR-guided, incisionless intervention with diagnostic and therapeutic applications for neurologic and psychiatric diseases. It is currently FDA-approved as a thermoablative treatment of essential tremor and Parkinson disease. However, other applications of FUS including BBB opening for diagnostic and therapeutic applications, sonodynamic therapy, histotripsy, and low-intensity focused ultrasound neuromodulation are all in clinical trials. While FUS targeting for essential tremor and Parkinson disease has classically relied on an indirect, landmark-based approach, development of novel, advanced MR imaging techniques such as DTI tractography and fast gray matter acquisition T1 inversion recovery has the potential to improve individualized targeting and thus potentially enhance treatment response, decrease treatment times, and avoid adverse effects. As the technology advances and the number of clinical applications increases, the role of the neuroradiologist on a multidisciplinary team will be essential in pairing advanced structural and functional imaging to further this image-guided procedure via a precision medicine approach. This multi-institutional report, written by an experienced team of neuroradiologists, neurosurgeons, and neurologists, summarizes current practices, the use of advanced imaging techniques for transcranial MR-guided high-intensity FUS, recommendations for clinical implementation, and emerging clinical indications.

Functional activation of pyruvate dehydrogenase in human brain using hyperpolarized [1-13 C]pyruvate

PURPOSE

Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [13 C]bicarbonate in the brain from hyperpolarized [1-13 C]pyruvate using carbon-13 (13 C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13 C MRS with hyperpolarized [1-13 C]pyruvate.

METHODS

From seven sedentary adults in good general health, time-resolved [13 C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1-13 C]pyruvate and [1-13 C]lactate, to test the hypothesis that the appearance of [13 C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus-injected [1-13 C]pyruvate being converted to [1-13 C]lactate prior to decarboxylation was investigated by measuring [13 C]bicarbonate production with and without [1-13 C]lactate saturation.

RESULTS

In the course of visual stimulation, the measured [13 C]bicarbonate signal normalized to the total 13 C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1-13 C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate-to-pyruvate ratio by 44.4% ± 9.3% (p < 0.01).

CONCLUSION

We demonstrated the utility of 13 C MRS with hyperpolarized [1-13 C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [13 C]bicarbonate production.

Hyperpolarized 13C metabolic imaging detects long-lasting metabolic alterations following mild repetitive traumatic brain injury

Career athletes, active military, and head trauma victims are at increased risk for mild repetitive traumatic brain injury (rTBI), a condition that contributes to the development of epilepsy and neurodegenerative diseases. Standard clinical imaging fails to identify rTBI-induced lesions, and novel non-invasive methods are needed. Here, we evaluated if hyperpolarized 13C magnetic resonance spectroscopic imaging (HP 13C MRSI) could detect long-lasting changes in brain metabolism 3.5 months post-injury in a rTBI mouse model. Our results show that this metabolic imaging approach can detect changes in cortical metabolism at that timepoint, whereas multimodal MR imaging did not detect any structural or contrast alterations. Using Machine Learning, we further show that HP 13C MRSI parameters can help classify rTBI vs. Sham and predict long-term rTBI-induced behavioral outcomes. Altogether, our study demonstrates the potential of metabolic imaging to improve detection, classification and outcome prediction of previously undetected rTBI.

Imaging immunomodulatory treatment responses in a multiple sclerosis mouse model using hyperpolarized 13C metabolic MRI

BACKGROUND

In recent years, the ability of conventional magnetic resonance imaging (MRI), including T1 contrast-enhanced (CE) MRI, to monitor high-efficacy therapies and predict long-term disability in multiple sclerosis (MS) has been challenged. Therefore, non-invasive methods to improve MS lesions detection and monitor therapy response are needed.

METHODS

We studied the combined cuprizone and experimental autoimmune encephalomyelitis (CPZ-EAE) mouse model of MS, which presents inflammatory-mediated demyelinated lesions in the central nervous system as commonly seen in MS patients. Using hyperpolarized 13C MR spectroscopy (MRS) metabolic imaging, we measured cerebral metabolic fluxes in control, CPZ-EAE and CPZ-EAE mice treated with two clinically-relevant therapies, namely fingolimod and dimethyl fumarate. We also acquired conventional T1 CE MRI to detect active lesions, and performed ex vivo measurements of enzyme activities and immunofluorescence analyses of brain tissue. Last, we evaluated associations between imaging and ex vivo parameters.

RESULTS

We show that hyperpolarized [1-13C]pyruvate conversion to lactate is increased in the brain of untreated CPZ-EAE mice when compared to the control, reflecting immune cell activation. We further demonstrate that this metabolic conversion is significantly decreased in response to the two treatments. This reduction can be explained by increased pyruvate dehydrogenase activity and a decrease in immune cells. Importantly, we show that hyperpolarized 13C MRS detects dimethyl fumarate therapy, whereas conventional T1 CE MRI cannot.

CONCLUSIONS

In conclusion, hyperpolarized MRS metabolic imaging of [1-13C]pyruvate detects immunological responses to disease-modifying therapies in MS. This technique is complementary to conventional MRI and provides unique information on neuroinflammation and its modulation.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans

PURPOSE

This study is to investigate time-resolved ¹³ C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-¹³ C]pyruvate in the human brain.

METHODS

Time-resolved ¹³ C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-¹³ C]pyruvate. ¹³ C MRS with low flip-angle excitations and a multichannel ¹³ C/¹ H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of ¹³ C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise ¹³ C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval.

RESULTS

[1-¹³ C]Lactate relative to the total ¹³ C signal (tC) was 0.21-0.24 in all slices. [¹³ C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- were calculated as 0.014-0.018 s^-1 and 0.0043-0.0056 s^-1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1-¹³ C]pyruvate.

CONCLUSIONS

Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-¹³ C]pyruvate.