Performance Characteristics of BGO Detectors for a Low Cost Preclinical PET Scanner.

PETbox is a low-cost benchtop PET scanner dedicated to high throughput preclinical imaging that is currently under development at our institute. This paper presents the design and characterization of the detectors that are used in the PETbox system. In this work, bismuth germanate scintillator was used for the detector, taking advantage of its high stopping power, high photoelectric event fraction, lack of intrinsic background radiation and low cost. The detector block was segmented into a pixelated array consisting of 20 × 44 elements, with a crystal pitch of 2.2 mm and a crystal cross section of 2 mm × 2 mm. The effective area of the array was 44 mm × 96.8 mm. The array was coupled to two Hamamatsu H8500 position sensitive photomultiplier tubes, forming a flat-panel type detector head with a sensitive area large enough to cover the whole body of a typical laboratory mouse. Two such detector heads were constructed and their performance was characterized. For one detector head, the energy resolution ranged from 16.1% to 38.5% full width at half maximum (FWHM), with a mean of 20.1%; for the other detector head, the energy resolution ranged from 15.5% to 42.7% FWHM, with a mean of 19.6%. The intrinsic spatial resolution was measured to range from 1.55 mm to 2.39 mm FWHM along the detector short axis and from 1.48 mm to 2.33 mm FWHM along the detector long axis, with an average of 1.78 mm. Coincidence timing resolution for the detector pair was measured to be 4.1 ns FWHM. These measurement results show that the detectors are suitable for our specific application.

FLT-PET imaging of radiation responses in murine tumors.

BACKGROUND: 3'-[F-18]fluoro-3'-deoxythymidine (FLT) traces thymidine phosphorylation catalyzed by thymidine kinase during cell proliferation. Knowing the rate of cell proliferation during cancer treatment, such as radiation therapy, would be valuable in assessing whether tumor recurrence is likely and might indicate the need for additional treatments. However, the relationship between FLT kinetics and the effects of radiation is not well-understood. Nor has the method for optimal quantification of FLT uptake within the irradiated tumor microenvironment been extensively examined. MATERIALS AND METHODS: We performed dynamic FLT-positron emission tomography (PET) studies (60 min) on 22 mice implanted subcutaneously with syngeneic mammary MCaK tumors bilaterally in the shoulder area. A day before the FLT-PET imaging, the tumor on the right side was irradiated with a single dose (0, 2.5, 5, 10, or 20 Gy) or with fractionated exposures (4x2.5 Gy given in 12 h intervals). Standardized uptake value (SUVs) of FLT on tumors at 10 and 60 min post injection were calculated; model fitting was used to estimate the kinetic parameters. Significant radiation-induced changes were shown by comparing the irradiated tumor with the control tumor in the same animal and by comparing it to nonirradiated mice. The effect of radiation on MCaK cell cycle parameters and FLT uptake was also examined in vitro. RESULTS: In vivo FLT kinetics were sensitive to radiation doses of 5 Gy and higher (administered 1 day earlier), as judged by SUV semiquantitative measures and by modeling. Single irradiation with 10 Gy had greater impact on SUVs and kinetic parameters than fractionated exposures. Overall, the uptake constant Ki appeared to be the best marker for these radiation effects. FLT uptake by irradiated cells in vitro at various doses gave similar findings, and the in vitro FLT uptake correlated well with Ki. Radiation-induced G2/M arrest appeared to influence FLT uptake, and this was more pronounced after single than fractionated doses. CONCLUSION: The kinetics of FLT uptake into murine mammary tumors was altered 1 day after radiation treatment. The dose-dependent response correlated well with in vitro FLT cellular uptake. Parameters (e.g., Ki) derived from FLT kinetics are expected to be useful for assessing the efficacy of irradiation treatment of tumors.

Distribution volume of radiolabeled large neutral amino acids in brain tissue.

Variations in the cerebellum to plasma ratio at late times in 6-[18F]fluoro-L-DOPA studies are shown to be consistent with competitive binding of large neutral amino acids for a common transporter in the blood-brain barrier and the stability of brain tissue large neutral amino acid level in the presence of plasma level changes. The distribution volume of an inert large neutral amino acid can be estimated from plasma and tissue large neutral amino acid levels and apparent half-saturation concentrations (Km) of the transporter in the blood-brain barrier. Stability of brain large neutral amino acid levels is supported by literature findings and can be explained by high saturation of the large neutral amino acid transporter at physiologic conditions.

Distribution volume of 3-O-methyl-6.

The distribution volume (DV) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DV as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DV for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DV = 1.5-0.00094*[LNAA], R--2 = 0.79) resulted in an 8% DV decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 +/- 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 +/- 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Effects of large neutral amino acid concentrations on 6-[F-18]Fluoro-L-DOPA kinetics.

6-[F-18]Fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) has been used to measure the central dopaminergic function in many species, including humans and monkeys. For transport across the blood brain barrier (BBB), FDOPA competes with plasma large neutral amino acids (LNAA). In this article we evaluate the effects of normal physiological LNAA concentration variation on BBB transport (K1) and the FDOPA uptake measurement, Ki. We also investigate a method for reducing the dependency of FDOPA quantitation on LNAA. Adult vervet monkeys (Cercopithecus aethiops sabaeus, n = 19) were fasted overnight before FDOPA positron emission tomography scans. Blood samples were drawn for LNAA determination, metabolite analysis, and compartmental modeling. The estimated K1 and Ki were both negatively correlated with LNAA concentrations (r2 = 0.51 and 0.62, respectively). Using an adjustment to K1 and Ki based on these correlations, the LNAA dependency was reduced (SD of the data for K1 was reduced by 33%, for Ki by 40%). Experiments with amino acid loading on an additional six animals indicate that BBB transport can be described using Michaelis-Menten kinetics. Results show a clear dependence of FDOPA uptake on plasma LNAA concentrations, which can be removed to increase the precision of FDOPA quantitation.

Striatal kinetic modeling of FDOPA with a cerebellar-derived constraint on the distribution of volume of 30MFD: a PET investigation using non-human primates.

The peripherally born metabolite of FDOPA, 3-O-Methyl-FDOPA (3OMFD), crosses the blood-brain barrier, thus complicating positron emission tomography-FDOPA (PET-FDOPA) data analysis. In previous reports the distribution volume (DV) of 3OMFD was constrained to unity. We have recently shown that the forward transport rate-constant of FDOPA (K(S1)) and the cerebellum-to-plasma ratio (C(b)/C(p)), a measure for the DV of 3OMFD, are functions of plasma large neutral amino acid (LNAA) concentration. Given large interstudy and intersubject differences in plasma LNAA levels, variations in the DV of 3OMFD are significant. In this report, the authors propose a constraint on the DV of 3OMFD that accounts for these variations. Dynamic PET-FDOPA scans were performed on 12 squirrel monkeys and 12 vervet monkeys. Two sets of constraints were employed on the compartmental model--M1 or M2. In M1, the striatal DV of 3OMFD was constrained to unity; in M2, the striatal DV of 3OMFD was constrained to an estimate derived from the cerebellum. Striatal and cerebellar time-activity curves were fitted using FDOPA and 3OMFD plasma input functions. The estimate of K(S1) and that of the compartmental FDOPA uptake-constant (K(i)), both obtained using M2, were adjusted to values corresponding to average LNAA levels. Finally, K(i) was compared with the graphical uptake-constant (PK(j)). With the use of constraint M2, intersubject variability of squirrel monkey k(S3) and K(i) was reduced by 45% and 53%, respectively; and for vervet monkeys, by 54% and 44%, respectively. Intersubject variability of K(1) and K(i) was further reduced after correction for variations in intersubject plasma LNAA levels (for squirrel monkeys, by 67% and 41%; for vervet monkeys, by 40% and 36%, respectively). K(i) correlation to PK(i) was enhanced to identity. Finally, average cerebellar k(C2) estimates were more than 2.5-fold higher than striatal k(S2) estimates (P < 0.0001). In modeling of PET-FDOPA data, it cannot be assumed that the DV of 3OMFD is unity. The cerebellar-derived constraint furnishes a reliable estimate for the DV of 3OMFD. Invoking the constraint and correcting for variations in plasma LNAA significantly reduced interstudy and intersubject variations in parameter estimates.

Ethological and 6-[18F]fluoro-L-DOPA-PET profiles of long-term vulnerability to chronic amphetamine.

A chronic 10-day amphetamine (Amp) protocol was used to induce significant long-term decrements of the striatal [18F]fluoro-L-DOPA influx rate constant (FDOPA Ki) in the vervet monkey. Longitudinal FDOPA-positron emission tomography (PET) assessment in Amp-treated subjects subsequently revealed a gradual recovery of striatal dopamine function: FDOPA Ki values were decreased by approximately 70% at 1 month, approximately 45% at 6 months, approximately 20% at 12 months and were similar to pre-Amp values at 24 months. Motoric and social behavioral measures were obtained on all subjects within a species-typical group setting. Behavioral observations were conducted during both basal and stressor-challenge conditions, the latter being created by placing a potential intruder-animal in an individual cage adjacent to the subject's group enclosure. During basal conditions, post-Amp stereotypies were present at 2 weeks and locomotor behaviors were increased throughout 1 month; both alterations occurred while FDOPA Ki values were significantly decreased. Social behaviors were also significantly affected; affiliative behavior was decreased up to 6 months while aggressive behavior was increased for 12 months. However, a different pattern of behavioral changes emerged under stressor-challenge conditions. Motoric and social changes were of greater magnitude and persisted longer than in basal settings while aggressive behavior remained elevated at 24 months. These results indicate that chronic Amp-induced decreases in FDOPA Ki values and behavioral alterations are reversible. Changes in striatal dopamine function as indexed with FDOPA-PET are not correlated with post-Amp alterations in behaviors and moreover, expression of those behaviors is context-dependent.

Recovery of striatal dopamine function after acute amphetamine- and methamphetamine-induced neurotoxicity in the vervet monkey.

In six vervet monkeys, presynaptic striatal dopamine function was assessed longitudinally by [18F]fluoro-L-DOPA (FDOPA)-positron emission tomography (PET) after administration (2 x 2 mg/kg, i.m., 4 h apart) of either amphetamine (Amp), n = 3, or methamphetamine (MeAmp), n = 3. At 1-2 weeks postdrug, both Amp and MeAmp exposure effected similar decreases (60-70%) in the FDOPA influx rate constant (FDOPA Ki), an index of striatal dopamine synthesis capacity. Subsequent studies in these subjects showed that FDOPA Ki values were decreased by 45-67% at 3-6 weeks, by 25% at 10-12 weeks and by 16% in one Amp-treated subject at 32 weeks. Biochemical analysis showed that striatal dopamine concentrations were decreased by 75% at 3-4 weeks and by 55% at 10-12 weeks. These results indicate that in vervet monkey striatum, an acute Amp or MeAmp drug dosage produces extensive striatal dopamine system neurotoxicity. However, these effects were reversible; observed time-dependent recovery in both FDOPA Ki and dopamine concentrations indicates that neurochemical plasticity remains active in the adult primate striatum. At 3-4 and 10-12 weeks postdrug, the concurrent characterization of the striatal FDOPA Ki and dopamine concentrations for individual subjects showed that Ki decreases between 24 and 67% corresponded to dopamine depletions of 55-85%. These relatively larger postdrug decrements in steady-state striatal dopamine concentrations suggest that compensatory increases in dopamine synthesis capacity develop in the partially lesioned striatum. In contrast to the dopamine depletion in striatum, substantia nigra concentrations remained unchanged from referent values at both 3-4 and 10-12 weeks postdrug. Thus, the integrity of the substantia nigra could not be inferred from decreases in the striatal FDOPA Ki parameter. This disparity between striatum and substantia nigra reactivity to systemic administration of amphetamines suggests that each has unique dopamine system regulatory mechanisms.

NEMA NU-4 performance evaluation of PETbox4, a high sensitivity dedicated PET preclinical tomograph.

PETbox4 is a new, fully tomographic bench top PET scanner dedicated to high sensitivity and high resolution imaging of mice. This manuscript characterizes the performance of the prototype system using the National Electrical Manufacturers Association NU 4-2008 standards, including studies of sensitivity, spatial resolution, energy resolution, scatter fraction, count-rate performance and image quality. The PETbox4 performance is also compared with the performance of PETbox, a previous generation limited angle tomography system. PETbox4 consists of four opposing flat-panel type detectors arranged in a box-like geometry. Each panel is made by a 24 × 50 pixelated array of 1.82 × 1.82 × 7 mm bismuth germanate scintillation crystals with a crystal pitch of 1.90 mm. Each of these scintillation arrays is coupled to two Hamamatsu H8500 photomultiplier tubes via a glass light guide. Volumetric images for a 45 × 45 × 95 mm field of view (FOV) are reconstructed with a maximum likelihood expectation maximization algorithm incorporating a system model based on a parameterized detector response. With an energy window of 150-650 keV, the peak absolute sensitivity is approximately 18% at the center of FOV. The measured crystal energy resolution ranges from 13.5% to 48.3% full width at half maximum (FWHM), with a mean of 18.0%. The intrinsic detector spatial resolution is 1.5 mm FWHM in both transverse and axial directions. The reconstructed image spatial resolution for different locations in the FOV ranges from 1.32 to 1.93 mm, with an average of 1.46 mm. The peak noise equivalent count rate for the mouse-sized phantom is 35 kcps for a total activity of 1.5 MBq (40 µCi) and the scatter fraction is 28%. The standard deviation in the uniform region of the image quality phantom is 5.7%. The recovery coefficients range from 0.10 to 0.93. In comparison to the first generation two panel PETbox system, PETbox4 achieves substantial improvements on sensitivity and spatial resolution. The overall performance demonstrates that the PETbox4 scanner is suitable for producing high quality images for molecular imaging based biomedical research.

The use of 18F-fluoride and 18F-FDG PET scans to assess fracture healing in a rat femur model.

PURPOSE: Currently available diagnostic techniques can be unreliable in the diagnosis of delayed fracture healing in certain clinical situations, which can lead to increased complication rates and costs to the health care system. This study sought to determine the utility of positron emission tomography (PET) scanning with (18)F-fluoride ion, which localizes in regions of high osteoblastic activity, and (18)F-fluorodeoxyglucose (FDG), an indicator of cellular glucose metabolism, in assessing bone healing in a rat femur fracture model. METHODS: Fractures were created in the femurs of immunocompetent rats. Animals in group I had a fracture produced via a manual three-point bending technique. Group II animals underwent a femoral osteotomy with placement of a 2-mm silastic spacer at the fracture site. Fracture healing was assessed with plain radiographs, (18)F-fluoride, and (18)F-FDG PET scans at 1, 2, 3, and 4-week time points after surgery. Femoral specimens were harvested for histologic analysis and manual testing of torsional and bending strength 4 weeks after surgery. RESULTS: All fractures in group I revealed abundant callus formation and bone healing, while none of the nonunion femurs were healed via assessment with manual palpation, radiographic, and histologic evaluation at the 4-week time point. (18)F-fluoride PET images of group I femurs at successive 1-week intervals revealed progressively increased signal uptake at the union site during fracture repair. In contrast, minimal tracer uptake was seen at the fracture sites in group II at all time points after surgery. Data analysis revealed statistically significant differences in mean signal intensity between groups I and II at each weekly interval. No significant differences between the two groups were seen using (18)F-FDG PET imaging at any time point. CONCLUSION: This study suggests that (18)F-fluoride PET imaging, which is an indicator of osteoblastic activity in vivo, can identify fracture nonunions at an early time point and may have a role in the assessment of longitudinal fracture healing. PET scans using (18)F-FDG were not helpful in differentiating metabolic activity between successful and delayed bone healing.

The current source of human Alu retroposons is a conserved gene shared with Old World monkey.

A significant fraction of human Alu repeated sequences are members of the precise, recently inserted class. A cloned member of this class has been used as a probe for interspecies hybridization and thermal stability determination. The probe was reassociated with human, mandrill, and spider monkey DNA under conditions such that only almost perfectly matching duplexes could form. Equally precise hybrids were formed with human and mandrill DNA (Old World monkey) but not with spider monkey DNA (New World). These measurements as well as reassociation kinetics show the presence in mandrill DNA of many precise class Alu sequences that are very similar or identical in quantity and sequence to those in human DNA. Human and mandrill are moderately distant species with a single-copy DNA divergence of about 6%. Nevertheless, their recently inserted Alu sequences arise by retroposition of transcripts of source genes with nearly identical sequences. Apparently a gene present in our common ancestor at the time of branching was inherited and highly conserved in sequence in both the lineage of Old World monkeys and the lineage of apes and man.

Sources and evolution of human Alu repeated sequences.

Alu repeated sequences arising in DNA of the human lineage during about the last 30 million years are closely similar to a modern consensus. Alu repeats arising at earlier times share correlated blocks of differences from the current consensus at diagnostic positions in the sequence. Using these 26 positions, we can recognize four subfamilies and the older ones are each successively closer to the 7SL sequence. It appears that there has existed a series of conserved genes that are the primary sources of the Alu repeat family, presumably through retroposition. These genes have probably replaced each other in overlapping relays during the evolution of primates.

6-[18F]fluoro-L-DOPA-PET studies show partial reversibility of long-term effects of chronic amphetamine in monkeys.

The acute and long-term effects of chronic amphetamine administration on the striatal dopamine system in monkeys were assessed with 6-[18F]fluoro-L-DOPA (FDOPA) and positron emission tomography (PET). Vervet monkeys (Cerecopithecus aethiops) were administered amphetamine doses, i.m., that increased from 4 mg/kg/d to 18 mg/kg/d over a 10 day period. Post-amphetamine FDOPA-PET scans at 1-2, 3-4, and 6 week time points in individual subjects showed persistent decrements in dopamine synthesis capacity as reflected by FDOPA influx rate constant (Ki) values being approximately 30% that of pre-drug assessment. In other animals that were administered the same drug regimen, biochemical analysis of striatal regions at 1-2 weeks post-drug indicated that dopamine concentrations were decreased by approximately 95% throughout caudate and putamen regions, while the homovanillic acid/dopamine level ratio was increased 3-10-fold. Post-drug FDOPA-PET Ki values remained consistently low up to 6 weeks; however, at the 5-6 month time point, relative increases in FDOPA-Ki values (approximately 53% of pre-drug values) were observed for all subjects, indicative of partial recovery of striatal dopamine synthesis capacity. These results demonstrate that FDOPA-PET can reveal temporal activity changes within the striatal dopamine system of individual subjects. The apparent, partial reversibility of amphetamine's neurotoxic effects suggests a plasticity of dopaminergic function that may include regeneration of dopaminergic terminals and compensatory increases in residual dopamine synthesis rates. The persistence of the partial decrement in dopamine synthesis capacity, however, may indicate a long term component of amphetamine's toxic effects.

Longitudinal behavioral and 6-[18F]fluoro-L-DOPA-PET assessment in MPTP-hemiparkinsonian monkeys.

Biochemical and behavioral criteria were established to determine the long-term stability of a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced unilateral striatal dopamine deficiency in the vervet monkey. At time points over a 12-month period, post-MPTP striatal dopamine synthesis capacity was indexed with 6-[18F]fluoro-L-DOPA (FDOPA)-positron emission tomography. For the MPTP-treated subjects (n = 4), an intrasubject FDOPA influx rate constant (Ki) ratio method of right (lesioned) striatum/left (unlesioned) striatum values was used to assess changes in striatal activity. Striatal FDOPA Ki ratios differed less than 5% between studies conducted at 1-2, 5-7, and 9-11 months post-MPTP; these results indicated a stable MPTP-induced striatal lesion over this time period. At the 5-7 and 9-11 month time points, behavioral indices of the MPTP-induced deficits were obtained within a species-typical group setting. For three of the four subjects, persistent decrements in motoric, affiliative, and vigilance behavior were observed while the frequency of aggression toward group members was increased. At the 9-11 month time point, one subject showed a 30% improvement in the social measures, indicative of a partial recovery from the MPTP-induced behavioral decrements although its striatal FDOPAKi ratio remained unchanged. Thus, behavioral and noninvasive biochemical methods can provide complementary indices to assess individual differences in sensitivity to MPTP-induced deficits. Both types of data are required to determine lesion stability and, subsequently, the efficacy of interventions designed to restore normal function in this primate Parkinsonian model.

Nigrostriatal reduction of aromatic L-amino acid decarboxylase activity in MPTP-treated squirrel monkeys: in vivo and in vitro investigations.

Aromatic L-amino acid decarboxylase (AAAD) activity was examined in vivo with positron emission tomography (PET) using 6-[18F]fluoro-L-DOPA (FDOPA) in squirrel monkeys lesioned with graded doses of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In vitro biochemical determinations of AAAD activity in caudate, putamen, substantia nigra, and nucleus accumbens were performed in the same animals to establish a direct comparison of in vivo and in vitro measurements. In vivo and in vitro AAAD activities in caudate/ putamen were substantially reduced in animals treated with the highest dose of MPTP (2.0 mg/kg). The percent change in the striatal FDOPA uptake (K(i)) and decarboxylation rate constant (k3) values resulting from MPTP treatment showed highly significant correlations with in vitro-determined AAAD activities. However, decarboxylase rates within individual animals presented as approximately 10-fold difference between in vivo and in vitro values. Lower in vivo k3 measurements may be attributed to several possibilities, including transport restrictions limiting substrate availability to AAAD within the neuron. In addition, reductions in AAAD activity in the substantia nigra did not parallel reductions in AAAD activity within the striatum, supporting the notion of a nonlinear relationship between nigrostriatal cell degeneration and terminal losses. This work further explores the role of AAAD in Parkinson's disease, a more important factor than previously thought.

Novel observations with FDOPA-PET imaging after early nigrostriatal damage.

Striatal 6-[18F]fluoro-L-DOPA (FDOPA) kinetic rate constants were measured by positron emission tomography (PET) in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated squirrel monkeys. After scanning, stereological counts of dopaminergic neurons were done in substantia nigra, and dopamine (DA) and metabolite concentrations were determined in the caudate, putamen, and substantia nigra. Graded doses of MPTP produced animals with mild to moderate reductions (10-35%) in dopaminergic neurons, where the percent of cell loss was proportional to the amount of MPTP given. Striatal DA and metabolite concentrations were relatively unchanged in animals given 1.0 and 1.5 mg/kg of MPTP, but were significantly reduced after 2.0 mg/kg of MPTP. All animals injected with a single dose of MPTP showed no overt signs of parkinsonism. In contrast, DA and metabolite concentrations in the substantia nigra were significantly reduced for all MPTP-treated animals. Reduction of dopaminergic indices in the substantia nigra did not parallel reductions in the striatum, indicating differential sensitivity of the nigrostriatal pathway to the neurotoxic effects of MPTP. The percent change in FDOPA uptake (Ki) and decarboyxlation (k3) after MPTP showed significant positive correlations to striatal DA levels, but not to the number of dopaminergic neurons. This suggests that FDOPA is a good index of striatal DA levels.