Matthew D. Walker

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.

Characterization of a New MR Compatible Small Animal PET Scanner Using Monte-Carlo Simulations

We are currently designing a small animal PET insert for use in an MRI with a bore size that constrains the insert inner diameter to be no larger than 66 mm while leaving 25 mm for ring thickness. The insert will be made from 10 mm thick DOI-capable Dual Layer Offset LYSO blocks coupled to MR-compatible SiPMs. The block is made from a 9 × 9 array of 1.345 × 4 mm3 crystals in the front layer and a 10 × 10 array of 1.345 × 1.345 × 6 mm3 crystals in the back layer (crystal pitch = 1.422 mm). A ring of blocks is made by repeating a block around a ring with inner diameter of 64.776 mm 16 times. Here, GATE simulations have been made to estimate mousenoise-equivalent count rate (NECR), mouse-scatter fraction (SF), peak sensitivity (Sp) resolution, and resolution uniformity to evaluate the design of our PET insert. These simulations make use of hardware performance estimates measured from a prototype block. For the one, three, and six ring tomographs, NECR curves, SF, and fígures were produced for the best and worst expected hardware performance. Simulations of a point source in a one-ring tomograph were made to estimate resolution across the field of view (FOV). For a six-ring tomograph with a 250-750 keV energy window and best expected hardware performance, the peak NECR, peak NECR activity, and Sp were 1273 kcps, 96 MBq and 10.0%. With three rings, these figures were 389 kcps at 95 MBq, and 5.9%. And with one-ring, they were 43 kcps, 85 MBq, and 2.0%. SF was ~ 16% in these three cases. Spatial resolution in the radial direction was found to change from 1.0 to 1.9 mm FWHM moving from the center of the FOV to a 15 mm offset. These results indicate that our scanner design is highly suited for high-resolution preclinical mouse imaging.

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.

Commentary: An Eye on PET Quantification

Positron emission tomography (PET) is generally considered to be a quantitative imaging modality, allowing assessment of regional differences in radiotracer accumulation and the derivation of quantitative physiological information. Due to the increasing complexity of PET technology, the quantitative accuracy of PET images has to be continually reassessed if PET is to maintain its quantitative reputation. In this commentary, we discuss the results from a recent inter-scanner study in which the quantitative outcome measures from human studies were compared for three different radiotracers. The approach is a useful complement to standard phantom tests such as those prescribed by NEMA, but the resulting data are more difficult to interpret.

Characterization of a Small Animal PET Detector Block Incorporating a Digital Photon Counter Array

A Small Animal PET detector block made with a Dual Layer Offset crystal array with 1.27 mm wide LYSO crystals on a Philips PDC3200-22-44 Digital Photon Counter (DPC) array was characterized while operating near room temperature. Crystal map peak to valley ratio, energy resolution, and timing resolution were characterized as a function of various device settings of the DPC and temperature. In addition, rates of count loss due to the phenomena of incomplete neighbor logic and dark-readout deadtime were measured. Device settings of interest were: Trigger scheme-defining the threshold of when a DPC will generate a timestamp and enter a readout cycle, inhibit fraction-the fraction of noisy cells which are disabled, and RTL-refresh-a setting which reduces the probability of the DPC being triggered from dark noise. At 15°C, peak to valley ratios were measured to be around 11, and energy resolution around 11.5% regardless of device settings. Timing resolution ranged from near 300 ps to 1.5 ns. Count loss from dark readout deadtime was insignificant compared to incomplete neighbor logic, which ranged from as high as 95% to 5% of coincidences. It was found that trade-offs had to be made between timing resolution and count loss. With the most optimal device settings for small animal PET, a timing resolution of 1.4 ns and coincidence losses of 5% were achieved. At these settings, the detector block had little sensitivity to a 5°C temperature fluctuation.

Estimation of NECR, scatter fraction, and sensitivity of a new MR compatible small animal PET insert based on Monte-Carlo simulations

We are currently designing an LYSO-coupled to SiPM PET insert for use in small animal PET/MR. The inner diameter of the insert is constrained to be no larger than 6.4 cm due to the allowable space in the 7T magnet that the insert is being design for. The design utilizes a dual layer offset crystal design to minimize the depth of interaction effect. Each detector block is simulated to be composed of a 10 × 10/9 × 9 array of 1.345 × 1.345 × 6/4 mm3 crystals (pitch = 1.422 mm) in the front/back layer. The tomograph is modeled as one, three, or six rings (ring pitch = 16 mm), leading to an axial extent of 14, 30, or 46 mm. GATE simulations were performed to estimate some performance characteristics of the tomograph NEC, sensitivity, and scatter fraction. Based on measured data from a single detector block, the energy resolution was estimated to be between 15 and 17%, the timing between 3 and 4 ns, and dead-time between 500 and 600 ns. A set of NEC curves was produced from the simulated data with different permutations of the above hardware characteristics. The simulations were made with a mouse-sized phantom as outlined in the NEMA NU4-2008 protocol. Simulations of a low activity point source in the centre of the tomograph were also made to estimate peak sensitivity. For a six-ring tomograph with the 250-750 keY window applied, it was found that peak NEC, peak NEC activity, and peak sensitivity would be 754 kcps at 56 MBq and 8.3% under the most ideal hardware conditions. With only three rings, these figures would be 245 kcps at 53 MBq, and 5.0%. When reduced to one ring, these figures would be 29 kcps, 54 MBq, and 1.9%. Regardless of the number of rings, scatter fraction was always -16%. Results are also reported for the worst hardware configuration, and also with an energy window of 350 750 keV.

Comparison of two small animal PET scanners: Pinhole collimation vs. electronic collimation

PET imaging of rodents is increasingly used in preclinical research, but its utility is limited by the spatial resolution and statistical quality of the images. In a new approach, a specially designed pinhole collimator enables high-resolution, simultaneous imaging of PET and SPECT tracers. Such a physical collimation technique strongly departs from traditional electronic collimation achieved via coincidence detection in PET. This work compares two small animal PET scanners, one with electronic collimation (Focus120) and one with physical collimation using clustered pinholes (VECTor). Data were compared from Jaszczak (hot-rod) and uniform phantoms and point source measurements. Mouse brain images from [18F]FDG PET were acquired on both systems, and compared with quantitative ex-vivo autoradiography as a gold standard. Using typical reconstruction settings, the pinhole system resolved the smallest rods (0.85 mm diameter) in the Jaszczak phantom while the coincidence system resolved 1.3 mm diameter rods. This was in agreement with the recovered contrast. The contrast-to-noise ratio was better for the pinhole system when imaging small rods (<;1.1 mm) for a wide range of activity levels, but this reversed for larger rods. Image uniformity on the coincidence system (<;3%) was superior to that on the pinhole system (10%), as was the peak sensitivity (4% compared to 0.4%). The high [18F]FDG uptake in the striatum of the mouse brain was fully resolved using the pinhole system with contrast to nearby regions equaling that from autoradiography; a lower contrast was found using the coincidence PET system. To conclude, in cases where small regions need to be resolved in scans with reasonably high activity or reasonably long scan times, a lower sensitivity clustered pinhole system with higher reconstructed spatial resolution (in this case the first generation VECTor with sub-optimal reconstruction software) can provide superior image quality in terms of contrast and the contrast-to-noise ratio as compared to a traditional system.