Motion Dependent and Spatially Variant Resolution Modeling for PET Rigid Motion Correction

Dynamic neuroreceptor positron emission tomography in non-anesthetized rats using point source based motion correction: A feasibility study with [11C]ABP688

To prevent motion artifacts in small animal positron emission tomography (PET), animals are routinely scanned under anesthesia or physical restraint. Both may potentially alter metabolism and neurochemistry. This study investigates the feasibility of fully awake acquisition and subsequent absolute quantification of dynamic brain PET data via pharmacokinetic modelling in moving rats using the glutamate 5 receptor radioligand [11C]ABP688 and point source based motion correction. Five male rats underwent three dynamic [11C]ABP688 PET scans: two test-retest awake PET scans and one scan under anesthesia for comparison. Specific radioligand binding was determined via the simplified reference tissue model (reference: cerebellum) and outcome parameters BPND and R1 were evaluated in terms of stability and reproducibility. Test-retest measurements in awake animals gave reliable results with high correlations of BPND (y = 1.08 × -0.2, r = 0.99, p < 0.01) and an acceptable variability (mean over all investigated regions 15.7 ± 2.4%). Regional [11C]ABP688 BPNDs under awake and anesthetized conditions were comparable although in awake scans, absolute radioactive peak uptakes were lower and relative blood flow in terms of R1 was higher. Awake small animal PET with absolute quantification of neuroreceptor availability is technically feasible and reproducible thereby providing a suitable alternative whenever effects of anesthesia are undesirable, e.g. in sleep research.

Isoflurane and ketamine-xylazine modify pharmacokinetics of [18F]SynVesT-1 in the mouse brain

We investigated the effect of isoflurane and ketamine-xylazine anesthesia on the positron emission tomography (PET) tracer [18F]SynVesT-1 in the mouse brain. [18F]SynVesT-1 PET scans were performed in C57BL/6J mice in five conditions: isoflurane anesthesia (ANISO), ketamine-xylazine (ANKX), awake freely moving (AW), awake followed by isoflurane administration (AW/ANISO) or followed by ketamine-xylazine (AW/ANKX) 20 min post tracer injection. ANISO, ANKX and AW scans were also performed in mice administered with levetiracetam (LEV, 200 mg/kg) to assess non-displaceable binding. Metabolite analysis was performed in ANISO, ANKX and AW mice. Finally, in vivo autoradiography in ANISO, ANKX and AW mice at 30 min post-injection was performed for validation. Kinetic modeling, with a metabolite corrected image derived input function, was performed to calculate total and non-displaceable volume of distribution (VT(IDIF)). VT(IDIF) was higher in ANISO compared to AW (p < 0.0001) while VT(IDIF) in ANKX was lower compared with AW (p < 0.0001). Non-displaceable VT(IDIF) was significantly different between ANISO and AW, but not between ANKX and AW. Change in the TAC washout was observed after administration of either isoflurane or ketamine-xylazine. Observed changes in tracer kinetics and volume of distribution might be explained by physiological changes due to anesthesia, as well as by induced cellular effects.

Validation, kinetic modeling, and test-retest reproducibility of [18F]SynVesT-1 for PET imaging of synaptic vesicle glycoprotein 2A in mice

Alterations in synaptic vesicle glycoprotein 2 A (SV2A) have been associated with several neuropsychiatric and neurodegenerative disorders. Therefore, SV2A positron emission tomography (PET) imaging may provide a unique tool to investigate synaptic density dynamics during disease progression and after therapeutic intervention. This study aims to extensively characterize the novel radioligand [¹⁸F]SynVesT-1 for preclinical applications. In C57Bl/6J mice (n = 39), we assessed the plasma profile of [¹⁸F]SynVesT-1, validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function (AIF), performed a blocking study with levetiracetam (50 and 200 mg/kg, i.p.) to verify the specificity towards SV2A, examined kinetic models for volume of distribution (V_T) quantification, and explored test-retest reproducibility of [¹⁸F]SynVesT-1 in the central nervous system (CNS). Plasma availability of [¹⁸F]SynVesT-1 decreased rapidly (13.4 ± 1.5% at 30 min post-injection). V_T based on AIF and IDIF showed excellent agreement (r² = 0.95, p < 0.0001) and could be reliably estimated with a 60-min acquisition. The blocking study resulted in a complete blockade with no suitable reference region. Test-retest analysis indicated good reproducibility (mean absolute variability <10%). In conclusion, [¹⁸F]SynVesT-1 is selective for SV2A with optimal kinetics representing a candidate tool to quantify CNS synaptic density non-invasively.

Spatiotemporal Kernel Reconstruction for Linear Parametric Neurotransmitter PET Kinetic Modeling in Motion Correction Brain PET of Awake Rats

The linear parametric neurotransmitter positron emission tomography (lp-ntPET) kinetic model can be used to detect transient changes (activation) in endogenous neurotransmitter levels. Preclinical PET scans in awake animals can be performed to investigate neurotransmitter transient changes. Here we use the spatiotemporal kernel reconstruction (Kernel) for noise reduction in dynamic PET, and lp-ntPET kinetic modeling. Kernel is adapted for motion correction reconstruction, applied in awake rat PET scans. We performed 2D rat brain phantom simulation using the ntPET model at 3 different noise levels. Data was reconstructed with independent frame reconstruction (IFR), IFR with HYPR denoising, and Kernel, and lp-ntPET kinetic parameters (k2a: efflux rate, γ: activation magnitude, td: activation onset time, and tp: activation peak time) were calculated. Additionally, significant activation magnitude (γ) difference with respect to a region with no activation (rest) was calculated. Finally, [11C]raclopride experiments were performed in anesthetized and awake rats, injecting cold raclopride at 20 min after scan start to simulate endogenous neurotransmitter release. For simulated data at the regional level, IFR coefficient of variation (COV) of k2a, γ, td and tp was reduced with HYPR denoising, but Kernel showed the lowest COV (2 fold reduction compared with IFR). At the pixel level the same trend is observed for k2a, γ, td and tp COV, but reduction is larger with Kernel compared with IFR (10–14 fold). Bias in γ with respect with noise-free values was additionally reduced using Kernel (difference of 292, 72.4, and −6.92% for IFR, IFR+KYPR, and Kernel, respectively). Significant difference in activation between the rest and active region could be detected at a simulated activation of 160% for IFR and IFR+HYPR, and of 120% for Kernel. In rat experiments, lp-ntPET parameters have better confidence intervals using Kernel. In the γ, and td parametric maps, the striatum structure can be identified with Kernel but not with IFR. Striatum voxel-wise γ, td and tp values have lower variability using Kernel compared with IFR and IFR+HYPR. The spatiotemporal kernel reconstruction adapted for motion correction reconstruction allows to improve lp-ntPET kinetic modeling noise in awake rat studies, as well as detection of subtle neurotransmitter activations.

Development of a ligand for in vivo imaging of mutant huntingtin in Huntington's disease

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG trinucleotide expansion in the huntingtin (HTT) gene that encodes the pathologic mutant HTT (mHTT) protein with an expanded polyglutamine (polyQ) tract. Whereas several therapeutic programs targeting mHTT expression have advanced to clinical evaluation, methods to visualize mHTT protein species in the living brain are lacking. Here, we demonstrate the development and characterization of a positron emission tomography (PET) imaging radioligand with high affinity and selectivity for mHTT aggregates. This small molecule radiolabeled with ¹¹C ([¹¹C]CHDI-180R) allowed noninvasive monitoring of mHTT pathology in the brain and could track region- and time-dependent suppression of mHTT in response to therapeutic interventions targeting mHTT expression in a rodent model. We further showed that in these animals, therapeutic agents that lowered mHTT in the striatum had a functional restorative effect that could be measured by preservation of striatal imaging markers, enabling a translational path to assess the functional effect of mHTT lowering.

Translation of Preclinical PET Imaging Findings: Challenges and Motion Correction to Overcome the Confounding Effect of Anesthetics

Preclinical brain positron emission tomography (PET) in animals is performed using anesthesia to avoid movement during the PET scan. In contrast, brain PET scans in humans are typically performed in the awake subject. Anesthesia is therefore one of the principal limitations in the translation of preclinical brain PET to the clinic. This review summarizes the available literature supporting the confounding effect of anesthesia on several PET tracers for neuroscience in preclinical small animal scans. In a second part, we present the state-of-the-art methodologies to circumvent this limitation to increase the translational significance of preclinical research, with an emphasis on motion correction methods. Several motion tracking systems compatible with preclinical scanners have been developed, each one with its advantages and limitations. These systems and the novel experimental setups they can bring to preclinical brain PET research are reviewed here. While technical advances have been made in this field, and practical implementations have been demonstrated, the technique should become more readily available to research centers to allow for a wider adoption of the motion correction technique for brain research.

Kinetic Modelling and Test-Retest Reproducibility for the Dopamine D1R Radioligand [11C]SCH23390 in Healthy and Diseased Mice

Purpose

Our aim in this study was to compare different non-invasive pharmacokinetic models and assess test–retest reproducibility of the radioligand [¹¹C]SCH23390 for the quantification of dopamine D₁-like receptor (D₁R) in both wild-type (WT) mice and heterozygous (HET) Q175DN mice as Huntington’s disease (HD) model.

Procedures

Adult WT (n = 9) and HET (n = 14) mice underwent a 90-min [¹¹C]SCH23390 positron emission tomography (PET) scan followed by computed tomography (CT) to evaluate the pharmacokinetic modelling in healthy and diseased conditions. Additionally, 5 WT mice and 7 HET animals received a second [¹¹C]SCH23390 PET scan for test–retest reproducibility. Parallel assessment of the simplified reference tissue model (SRTM), the multilinear reference tissue model (MRTM) and the Logan reference tissue model (Logan Ref) using the striatum as a receptor-rich region and the cerebellum as a receptor-free (reference) region was performed to define the most suitable method for regional- and voxel-based quantification of the binding potential (BP_ND). Finally, standardised uptake value ratio (SUVR-1) was assessed as a potential simplified measurement.

Results

For all models, we measured a significant decline in dopamine D₁R density (e.g. SRTM = − 38.5 ± 5.0 %, p < 0.0001) in HET mice compared to WT littermates. Shortening the 90-min scan duration resulted in large underestimation of striatal BP_ND in both WT mice (SRTM 60 min: − 17.7 ± 2.8 %, p = 0.0078) and diseased HET (SRTM 60 min: − 13.1 ± 4.1 %, p = 0.0001). Striatal BP_ND measurements were very reproducible with an average test–retest variability below 5 % when using both MRTM and SRTM. Parametric BP_ND maps generated with SRTM were highly reliable, showing nearly perfect agreement to the regional analysis (r² = 0.99, p < 0.0001). Finally, SRTM provided the most accurate estimate for relative tracer delivery R₁ with both regional- and voxel-based analyses. SUVR-1 at different time intervals were not sufficiently reliable when compared to BP_ND (r² < 0.66).

Conclusions

Ninety-minute acquisition and the use of SRTM for pharmacokinetic modelling is recommended. [¹¹C]SCH23390 PET imaging demonstrates optimal characteristics for the study of dopamine D₁R density in models of psychiatric and neurological disorders as exemplified in the Q175DN mouse model of HD.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-020-01561-1.