Rick Kornelsen

Positron emission tomography kinetic modeling algorithms for small animal dopaminergic system imaging

Small animal positron emission tomography (PET) imaging allows in vivo quantification of lesion‐ or treatment‐induced neurochemical changes in animal models of disease. Important for quantification are the kinetic modeling methods used to determine biologically‐relevant parameters of tracer‐tissue interaction. In this work, we evaluate modeling algorithms for the dopaminergic tracers 11C‐dihydrotetrabenazine (DTBZ), 11C‐methylphenidate (MP), and 11C‐raclopride (RAC), used to image the dopaminergic system in the unilateral 6‐hydroxydopamine lesioned rat model of Parkinsons disease. For the presynaptic tracers, PET measures are compared with autoradiographic binding measurements using DTBZ and [3H]WIN 35,428 (WIN). We independently developed a new variant of the tissue‐input Logan graphical modeling method, and compared its performance with the simplified Logan graphical method and the simplified reference tissue with basis functions method (SRTM), for region of interest (ROI) averaged time activity curves (TACs) and parametric imaging. The modified graphical method was found to be effectively unbiased by target tissue noise and has advantages for parametric imaging, while all tested methods were equivalent for ROI‐averaged data. Synapse 64:200–208, 2010.

In-vivo Measurement of LDOPA Uptake, Dopamine Reserve and Turnover in the Rat Brain Using [18F]FDOPA PET

Longitudinal measurements of dopamine (DA) uptake and turnover in transgenic rodents may be critical when developing disease-modifying therapies for Parkinsons disease (PD). We demonstrate methodology for such measurements using [18F]fluoro-3,4-dihydroxyphenyl-L-alanine ([18F]FDOPA) positron emission tomography (PET). The method was applied to 6-hydroxydopamine lesioned rats, providing the first PET-derived estimates of DA turnover for this species. Control (n = 4) and unilaterally lesioned (n = 11) rats were imaged multiple times. Kinetic modeling was performed using extended Patlak, incorporating a kloss term for metabolite washout, and modified Logan methods. Dopaminergic terminal loss was measured via [11C]-(+)-dihydrotetrabenazine (DTBZ) PET. Clear striatal [18F]FDOPA uptake was observed. In the lesioned striatum the effective DA turnover increased, shown by a reduced effective distribution volume ratio (EDVR) for [18F]FDOPA. Effective distribution volume ratio correlated (r > 0.9) with the [11C]DTBZ binding potential (BPND). The uptake and trapping rate (kref) decreased after lesioning, but relatively less so than [11C]DTBZ BPND. For normal controls, striatal estimates were kref = 0.037 ± 0.005 per minute, EDVR = 1.07 ± 0.22 and kloss = 0.024 ± 0.003 per minute (30 minutes turnover half-time), with repeatability (coefficient of variation) ≤11%. [18F]fluoro-3,4-dihydroxyphenyl-L-alanine PET enables measurements of DA turnover in the rat, which is useful for developing novel therapies for PD.

[11C]PBR28 PET Imaging is Sensitive to Neuroinflammation in the Aged Rat:

Neuroinflammation in the aging rat brain was investigated using [11C]PBR28 microPET (positron emission tomography) imaging. Normal rats were studied alongside LRRK2 p.G2019S transgenic rats; this mutation increases the risk of Parkinsons disease in humans. Seventy [11C]PBR28 PET scans were acquired. Arterial blood sampling enabled tracer kinetic modeling and estimation of VT. In vitro autoradiography was also performed. PBR28 uptake increased with age, without differences between nontransgenic and transgenic rats. In 12 months of aging (4 to 16 months), standard uptake value (SUV) increased by 56% from 0.44 to 0.69 g/mL, whereas VT increased by 91% from 30 to 57 mL/cm3. Standard uptake value and VT were strongly correlated (r = 0.52, 95% confidence interval (CI) = 0.31 to 0.69, n = 37). The plasma free fraction, fp, was 0.21 ± 0.03 (mean ± standard deviation, n = 53). In vitro binding increased by 19% in 16 months of aging (4 to 20 months). The SUV was less variable across rats than VT; coefficients of variation were 13% (n = 27) and 29% (n = 12). The intraclass correlation coefficient for SUV was 0.53, but was effectively zero for VT. These data show that [11C]PBR28 brain uptake increases with age, implying increased microglial activation in the aged brain.

Behavioral deficits and striatal DA signaling in LRRK2 p.G2019S transgenic rats: a multimodal investigation including PET neuroimaging.

BACKGROUND A major risk-factor for developing Parkinsons disease (PD) is genetic variability in leucine-rich repeat kinase 2 (LRRK2), most notably the p.G2019S mutation. Examination of the effects of this mutation is necessary to determine the etiology of PD and to guide therapeutic development. OBJECTIVE Assess the behavioral consequences of LRRK2 p.G2019S overexpression in transgenic rats as they age and test the functional integrity of the nigro-striatal dopamine system. Conduct positron emission tomography (PET) neuroimaging to compare transgenic rats with previous data from human LRRK2 mutation carriers. METHODS Rats overexpressing human LRRK2 p.G2019S were generated by BAC transgenesis and compared to non-transgenic (NT) littermates. Motor skill tests were performed at 3, 6 and 12 months-of-age. PET, performed at 12 months, assessed the density of dopamine and vesicular monoamine transporters (DAT and VMAT2, respectively) and measured dopamine synthesis, storage and availability. Brain tissue was assayed for D2, DAT, dopamine and cAMP-regulated phosphoprotein (DARPP32) and tyrosine hydroxylase (TH) expression by Western blot, and TH by immunohistochemistry. RESULTS Transgenic rats had no abnormalities in measures of striatal dopamine function at 12 months. A behavioral phenotype was present, with LRRK2 p.G2019S rats performing significantly worse on the rotarod than non-transgenic littermates (26% reduction in average running duration at 6 months), but with normal performance in other motor tests. CONCLUSIONS Neuroimaging using dopaminergic PET did not recapitulate prior studies in human LRRK2 mutation carriers. Consistently, LRRK2 p.G2019S rats do not develop overt neurodegeneration; however, they do exhibit behavioral abnormalities.

Measuring dopaminergic function in the 6-OHDA-lesioned rat: a comparison of PET and microdialysis

Background[18 F]fluorodopa (FDOPA) positron emission tomography (PET) allows assessment of levodopa (LDOPA) metabolism and is widely used to study Parkinsons disease. We examined how [18 F]FDOPA PET-derived kinetic parameters relate the dopamine (DA) and DA metabolite content of extracellular fluid measured by microdialysis to aid in the interpretation of data from both techniques.Methods[18 F]FDOPA PET imaging and microdialysis measurements were performed in unilaterally 6-hydroxydopamine-lesioned rats (n = 8) and normal control rats (n = 3). Microdialysis testing included baseline measurements and measurements following acute administration of LDOPA. PET imaging was also performed using [11C]dihydrotetrabenazine (DTBZ), which is a ligand for the vesicular monoamine transporter marker and allowed assessment of denervation severity.ResultsThe different methods provided highly correlated data. Lesioned rats had reduced DA metabolite concentrations ipsilateral to the lesion (p < 0.05 compared to controls), with the concentration being correlated with FDOPAs effective distribution volume ratio (EDVR; r = 0.86, p < 0.01) and DTBZs binding potential (BPND; r = 0.89, p < 0.01). The DA metabolite concentration in the contralateral striatum of severely (>80%) lesioned rats was lower (p < 0.05) than that of less severely lesioned rats (<80%) and was correlated with the ipsilateral PET measures (r = 0.89, p < 0.01 for BPND) but not with the contralateral PET measures. EDVR and BPND in the contralateral striatum were not different from controls and were not correlated with the denervation severity.ConclusionsThe demonstrated strong correlations between the PET and microdialysis measures can aid in the interpretation of [18 F]FDOPA-derived kinetic parameters and help compare results from different studies. The contralateral striatum was affected by the lesioning and so cannot always serve as an unaffected control.

Methods for Parkinson’s rat model PET image analysis with regions of interest

Accurate methods are required for analysis of microPET dopamine (DA) receptor or transporter images of unilaterally 6-hydroxydopamine-lesioned rat models of Parkinsons disease. Heavily lesioned striata and the cerebellum do not appear distinctly in PET images when presynaptic tracers such as [11C]-(+)-dihydrotetrabenazine (DTBZ) are used, and are difficult targets on which to place reliably regions of interest (ROIs) without additional guidance. Registration of a brain atlas to DA receptor/transporter images significantly improves reproducibility and reliability of ROI-based analyses, as measured by discrepancy between calculated binding potentials (BP) of repeated scans of the same animal, and correlation with autoradiographic binding measurements with the same tracer (DTBZ). Averaging over 3 or 5 axial planes to generate time activity curves gives equivalent reproducibility and reliability. Scan-to-scan coregistration with automated image registration (AIR) can be successful with appropriate masking. Coregistered image analysis produces statistically equivalent results to separately placing ROIs on images that are being directly compared, though coregistered images require only one set of ROIs to be placed, reducing analysis effort.

Manganese concentration mapping in the rat brain with MRI, PET, and autoradiography

Purpose: Mn2+ is used as a contrast agent and marker for neuronal activity with magnetic resonance imaging (MRI) in rats and mice, but its accumulation is generally not assessed quantitatively. In this work, nonradioactive Mn and 52Mn are injected simultaneously in rats, and imaged with MRI, positron emission tomography (PET) and autoradiography (AR). Mn distributions are compared between modalities, to assess the potential and limitations on quantification of Mn with MRI, and to investigate the potential of multimodal measurement of Mn accumulation. Methods: MRI (in vivo), PET (in vivo and post mortem), and AR (ex vivo) were acquired of rat brains, for which animals received simultaneous intraperitoneal (IP) or intracerebrovertricular (ICV)‐targeted injections containing the positron‐emitting radionuclide 52Mn and additional nonradioactive MnCl2, which acts as an MRI contrast agent. Pre and postinjection MR images were fit for the longitudinal relaxation rate (R1), coregistered, and subtracted to generate R1 difference maps, which are expected to be proportional to change in Mn concentration in tissue. AR and PET images were coregistered to smoothed R1 difference maps. Results: Similar spatial distributions were seen across modalities, with Mn accumulation in the colliculus, near the injection site, and in the 4th ventricle. There was no 52Mn accumulation measurable with PET in the brain after IP injection. In areas of very highly localized and concentrated 52Mn accumulation in PET or AR, consistent increases of R1 were not seen with MRI. Scatter plots of corresponding voxel R1 difference and PET or AR signal intensity were generated and fit with least squares linear models within anatomical regions. Linear correlations were observed, particularly in regions away from very highly localized and concentrated Mn accumulation at the injection site and the 4th ventricle. Accounting for radioactive decay of 52Mn, the MnCl2 longitudinal relaxivity was between 4.0 and 5.1 s−1/mM, which is within 22% of the in vitro relaxivity. Conclusions: This proof‐of‐concept study demonstrates that MR has strong potential for quantitative assessment of Mn accumulation in the brain, although local discrepancies from linear correlation suggest limitations to this use of MR in areas of inflammation or very high concentrations of Mn. These discrepancies also suggest that a combination of modalities may have additional utility for discriminating between different pools of Mn accumulation in tissue.

In-vivo DAT occupancy studies with 11C-metylphenidate in unilaterally 6-OHDA lesioned rodents

Introduction: Dopamine transporter (DAT) function is altered by many neurodegenerative diseases. In addition, in Parkinsons disease (PD), DAT was found to play a role in treatment related motor complications in humans and in rodent models of disease. Positron emission tomography with the DAT marker C-methylphenidate (MP) is often used to asses DAT function. In light of the biological relevance of DAT and increasing efforts in developing suitable animal disease models it is important to (i) evaluate the transporter occupancy as a function of administered tracer mass (or specific activity (SA)) to determine the useful SA range for imaging (negligible mass effect) and (ii) determine the impact of the disease model, here 6-OHDA lesioning, on DAT by evaluating separately Bmax and Kd, here done with an in-vivo Scatchard method (1).

MicroPET/sup /spl reg// R4 neuro-imaging in rats and post-mortem measures

Small animal studies performed on dedicated small animal scanners may be technically very demanding and, especially in the case of brain receptor studies, susceptible to confounds such as anesthesia and tracer mass effect. In addition, studies might require a large number of animals to achieve statistically meaningful results. Here we investigated accuracy and limitations of PET data compared to post-mortem studies and techniques to reduce scanning times in the investigations of the dopaminergic system in a rat model of Parkinsons disease (PD) using a microPET/sup /spl reg// R4. We found that (i) different anesthetics influence tracer kinetics in a tracer specific manner and thus must be taken into account, especially when an intertracer comparison is performed, (ii) for selected tracers it might be possible to reduce scanning times and still obtain reliable and biologically meaningful information from the data, (iii) mass effect studies indicate that with a shorter scanning protocol it is indeed possible to perform at least two scans with the same tracer delivery thus significantly reducing the cost of scanning without affecting the accuracy of the results. Taken together these results provide some guidance in achieving highly time and cost efficient scanning protocols leading to biologically accurate and meaningful results.