A feasibility study for quantitative assessment of cerebrovascular malformations using flutriciclamide ([18F]GE-180) PET/MRI

Aim

Neuroinflammation plays a key role in both the pathogenesis and the progression of cerebral cavernous malformations (CCM). Flutriciclamide ([18F]GE-180) is a translocator protein (TSPO) targeting positron emission tomography (PET) tracer, developed for imaging neuroinflammation. The objectives of this study were to describe characteristics of flutriciclamide uptake in different brain tissue regions in CCM patients compared to controls, and to evaluate flutriciclamide uptake and iron deposition within CCM lesions.

Materials and methods

Five patients with CCM and six controls underwent a 60 or 90 min continuous PET/MRI scan following 315 ± 68.9 MBq flutriciclamide administration. Standardized uptake value (SUV) and standardized uptake value ratio (SUVr) were obtained using the striatum as a pseudo-reference. Quantitative susceptibility maps (QSM) were used to define the location of the vascular malformation and calculate the amount of iron deposition in each lesion.

Results

Increased flutriciclamide uptake was observed in all CCM lesions. The temporal pole demonstrated the highest radiotracer uptake; the paracentral lobule, cuneus and hippocampus exhibited moderate uptake; while the striatum had the lowest uptake, with average SUVs of 0.66, 0.55, 0.63, 0.55, and 0.33 for patient with CCM and 0.57, 0.50, 0.48, 0.42, and 0.32 for controls, respectively. Regional SUVr showed similar trends. The average SUV and QSM values in CCM lesions were 0.58 ± 0.23 g/ml and 0.30 ± 0.10 ppm. SUVs and QSM were positively correlated in CCM lesions (r = 0.53, p = 0.03).

Conclusion

The distribution of flutriciclamide ([18F]GE-180) in the human brain and CCM lesions demonstrated the potential of this TSPO PET tracer as a marker of neuroinflammation that may be relevant for characterizing CCM disease progression along with QSM.

A nanoparticle probe for the imaging of autophagic flux in live mice via magnetic resonance and near-infrared fluorescence

Autophagy-the lysosomal degradation of cytoplasmic components via their sequestration into double-membraned autophagosomes-has not been detected non-invasively. Here we show that the flux of autophagosomes can be measured via magnetic resonance imaging or serial near-infrared fluorescence imaging of intravenously injected iron oxide nanoparticles decorated with cathepsin-cleavable arginine-rich peptides functionalized with the near-infrared fluorochrome Cy5.5 (the peptides facilitate the uptake of the nanoparticles by early autophagosomes, and are then cleaved by cathepsins in lysosomes). In the heart tissue of live mice, the nanoparticles enabled quantitative measurements of changes in autophagic flux, upregulated genetically, by ischaemia-reperfusion injury or via starvation, or inhibited via the administration of a chemotherapeutic or the antibiotic bafilomycin. In mice receiving doxorubicin, pre-starvation improved cardiac function and overall survival, suggesting that bursts of increased autophagic flux may have cardioprotective effects during chemotherapy. Autophagy-detecting nanoparticle probes may facilitate the further understanding of the roles of autophagy in disease.

X-ray bi-prism interferometry-A design study of proposed novel hardware

PURPOSE

Advances in X-ray phase-contrast imaging can obtain excellent soft-tissue contrast of phase-shift, attenuation, and small-angle scatter. Here, we present fringe patterns for different design parameters of X-ray bi-prism interferometry imaging systems. Our aim is to develop bi-prism interferometry imaging systems with excellent polychromatic performance that produce high-contrast fringes with spatially incoherent X-ray illumination. We also introduce a novel X-ray tube design that uses temporal multiplexing to provide simultaneous virtual "electronic phase stepping" that replace "mechanical phase stepping" popular with grating-based interferometry setups.

METHODS

In our investigation, we develop expressions for the irradiance distribution pattern of a bi-prism interferometer composed of multiple point sources and multiple bi-prisms. These expressions are used to plot fringe patterns for X-ray design parameters, including size of point source, number of point sources, and point source separation, and bi-prism design parameters including material, angle, number of bi-prisms, period, and bi-prism array to X-ray source and detector distances.

RESULTS

Results show that the fringe patterns for a bi-prism interferometry system are not longitudinally periodic as with grating interferometers that produce a Talbot-Lau carpet. It is also shown that at 59 keV X-rays the bi-prism material should be something comparable to nickel to obtain reasonable fringe visibility.

CONCLUSION

The irradiance distribution pattern demonstrates that bi-prism interferometry may provide comparable or improved fringe visibility to that of gratings. Special care is given to present our findings within the context of previous advancements. A single-shot image acquisition approach using a temporal multiplexed, high-power X-ray source provides virtual electronic phase stepping without focal spot sweeping. This provides multiple images, each at the same exposure and the same projection view, from different fringe locations that allow one to derive the attenuation, phase, and dark-field images of the sample without mechanical phase stepping of a grating.

Tomography of dark-field scatter including single-exposure Moiré fringe analysis with X-ray biprism interferometry-A simulation study

PURPOSE

In this work, we present tomographic simulations of a new hardware concept for X-ray phase-contrast interferometry wherein the phase gratings are replaced with an array of Fresnel biprisms, and Moiré fringe analysis is used instead of "phase stepping" popular with grating-based setups.

METHODS

Projections of a phantom consisting of four layers of parallel carbon microfibers is simulated using wave optics representation of X-ray electromagnetic waves. Simulated projections of a phantom with preferential scatter perpendicular to the direction of the fibers are performed to analyze the extraction of small-angle scatter from dark-field projections for the following: (1) biprism interferometry using Moiré fringe analysis; (2) grating interferometry using phase stepping with eight grating steps; and (3) grating interferometry using Moiré fringe analysis. Dark-field projections are modeled as projections of voxel intensities represented by a fixed finite vector basis set of scattering directions. An iterative MLEM algorithm is used to reconstruct, from simulated projection data, the coefficients of a fixed set of seven basis vectors at each voxel representing the small-angle scatter distribution.

RESULTS

Results of reconstructed vector coefficients are shown comparing the three simulated imaging configurations. The single-exposure Moiré fringe analysis shows not only an increase in noise because of one-eighth the number of projection samples but also is obtained with less dose and faster acquisition times. Furthermore, replacing grating interferometry with biprism interferometry provides better contrast-to-noise.

CONCLUSION

The simulations demonstrate the feasibility of the reconstruction of dark-field data acquired with a biprism interferometry system. With the potential of higher fringe visibility, biprism interferometry with Moiré fringe analysis might provide equal or better image quality to that of phase stepping methods with less imaging time and lower dose.

In vivo quantitative mapping of human mitochondrial cardiac membrane potential: a feasibility study

PURPOSE

Alteration in mitochondrial membrane potential (ΔΨ_m) is an important feature of many pathologic processes, including heart failure, cardiotoxicity, ventricular arrhythmia, and myocardial hypertrophy. We present the first in vivo, non-invasive, assessment of regional ΔΨ_m in the myocardium of normal human subjects.

METHODS

Thirteen healthy subjects were imaged using [¹⁸F]-triphenylphosphonium ([¹⁸F]TPP+) on a PET/MR scanner. The imaging protocol consisted of a bolus injection of 300 MBq followed by a 120-min infusion of 0.6 MBq/min. A 60 min, dynamic PET acquisition was started 1 h after bolus injection. The extracellular space fraction (f_ECS) was simultaneously measured using MR T1-mapping images acquired at baseline and 15 min after gadolinium injection with correction for the subject's hematocrit level. Serial venous blood samples were obtained to calculate the plasma tracer concentration. The tissue membrane potential (ΔΨ_T), a proxy of ΔΨ_m, was calculated from the myocardial tracer concentration at secular equilibrium, blood concentration, and fECS measurements using a model based on the Nernst equation.

RESULTS

In 13 healthy subjects, average tissue membrane potential (ΔΨ_T), representing the sum of cellular membrane potential (ΔΨ_c) and ΔΨ_m, was - 160.7 ± 3.7 mV, in excellent agreement with previous in vitro assessment.

CONCLUSION

In vivo quantification of the mitochondrial function has the potential to provide new diagnostic and prognostic information for several cardiac diseases as well as allowing therapy monitoring. This feasibility study lays the foundation for further investigations to assess these potential roles. Clinical trial identifier: NCT03265431.

In-vivo Imaging of Mitochondrial Depolarization of Myocardium With Positron Emission Tomography and a Proton Gradient Uncoupler

BACKGROUND

We recently reported a method using positron emission tomography (PET) and the tracer 18F-labeled tetraphenylphosphonium (18F-TPP+) for mapping the tissue (i.e., cellular plus mitochondrial) membrane potential (ΔΨT) in the myocardium. The purpose of this work is to provide additional experimental evidence that our methods can be used to observe transient changes in the volume of distribution for 18F-TPP+ and mitochondrial membrane potential (ΔΨm).

METHODS

We tested these hypotheses by measuring decreases of 18F-TPP+ concentration elicited when a proton gradient uncoupler, BAM15, is administered by intracoronary infusion during PET scanning. BAM15 is the first proton gradient uncoupler shown to affect the mitochondrial membrane without affecting the cellular membrane potential. Preliminary dose response experiments were conducted in two pigs to determine the concentration of BAM15 infusate necessary to perturb the 18F-TPP+ concentration. More definitive experiments were performed in two additional pigs, in which we administered an intravenous bolus plus infusion of 18F-TPP+ to reach secular equilibrium followed by an intracoronary infusion of BAM15.

RESULTS

Intracoronary BAM15 infusion led to a clear decrease in 18F-TPP+ concentration, falling to a lower level, and then recovering. A second BAM15 infusion reduced the 18F-TPP+ level in a similar fashion. We observed a maximum depolarization of 10 mV as a result of the BAM15 infusion.

SUMMARY

This work provides evidence that the total membrane potential measured with 18F-TPP+ PET is sensitive to temporal changes in mitochondrial membrane potential.

Lessons learned about [F-18]-AV-1451 off-target binding from an autopsy-confirmed Parkinson's case

[F-18]-AV-1451 is a novel positron emission tomography (PET) tracer with high affinity to neurofibrillary tau pathology in Alzheimer’s disease (AD). PET studies have shown increased tracer retention in patients clinically diagnosed with dementia of AD type and mild cognitive impairment in regions that are known to contain tau lesions. In vivo uptake has also consistently been observed in midbrain, basal ganglia and choroid plexus in elderly individuals regardless of their clinical diagnosis, including clinically normal whose brains are not expected to harbor tau pathology in those areas. We and others have shown that [F-18]-AV-1451 exhibits off-target binding to neuromelanin, melanin and blood products on postmortem material; and this is important for the correct interpretation of PET images. In the present study, we further investigated [F-18]-AV-1451 off-target binding in the first autopsy-confirmed Parkinson’s disease (PD) subject who underwent antemortem PET imaging. The PET scan showed elevated [F-18]-AV-1451 retention predominantly in inferior temporal cortex, basal ganglia, midbrain and choroid plexus. Neuropathologic examination confirmed the PD diagnosis. Phosphor screen and high resolution autoradiography failed to show detectable [F-18]-AV-1451 binding in multiple brain regions examined with the exception of neuromelanin-containing neurons in the substantia nigra, leptomeningeal melanocytes adjacent to ventricles and midbrain, and microhemorrhages in the occipital cortex (all reflecting off-target binding), in addition to incidental age-related neurofibrillary tangles in the entorhinal cortex. Additional legacy postmortem brain samples containing basal ganglia, choroid plexus, and parenchymal hemorrhages from 20 subjects with various neuropathologic diagnoses were also included in the autoradiography experiments to better understand what [F-18]-AV-1451 in vivo positivity in those regions means. No detectable [F-18]-AV-1451 autoradiographic binding was present in the basal ganglia of the PD case or any of the other subjects. Off-target binding in postmortem choroid plexus samples was only observed in subjects harboring leptomeningeal melanocytes within the choroidal stroma. Off-target binding to parenchymal hemorrhages was noticed in postmortem material from subjects with cerebral amyloid angiopathy. The imaging-postmortem correlation analysis in this PD case reinforces the notion that [F-18]-AV-1451 has strong affinity for neurofibrillary tau pathology but also exhibits off-target binding to neuromelanin, melanin and blood components. The robust off-target in vivo retention in basal ganglia and choroid plexus, in the absence of tau deposits, meningeal melanocytes or any other identifiable binding substrate by autoradiography in the PD case reported here, also suggests that the PET signal in those regions may be influenced, at least in part, by biological or technical factors that occur in vivo and are not captured by autoradiography.