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.

Brain glucose metabolism in hypothyroidism: a positron emission tomography study before and after thyroid hormone replacement therapy

CONTEXT

Hypothyroidism is frequently associated with subtle behavioral and psychiatric symptoms. The consequences of inadequate thyroid hormone availability to brain metabolism are poorly understood.

OBJECTIVE

This study assessed the relationships between neuropsychiatric symptoms and changes in relative regional cerebral glucose metabolism in hypothyroid patients undergoing thyroid hormone replacement therapy. DESIGN, SETTING, AND OUTCOME MEASURE: Relative regional cerebral glucose metabolism was compared in 13 previously untreated hypothyroid patients and 10 healthy control participants. Effects of thyroid hormone replacement therapy (levothyroxine, 3 months) were assessed using neuropsychiatric measures and positron emission tomography with [(18)F]fluorodeoxyglucose.

RESULTS

Before treatment, hypothyroid patients exhibited lower regional activity than control subjects in the bilateral amygdala, hippocampus, and perigenual anterior cingulate cortex (ACC), left subgenual ACC, and right posterior cingulate cortex. Severity of depressive symptoms covaried negatively with pretreatment activity in the bilateral middle frontal gyrus and right subgenual and dorsal ACC. Thyroid hormone replacement therapy abolished pretreatment group differences in regional activity, robustly increased activity in the ventral ACC, and significantly reduced both clinician-rated and self-rated behavioral and psychiatric symptoms. Increased activity within the ventral ACC was associated with reduced somatic complaints, whereas increased activity within the dorsal ACC was associated with reduced depressive symptoms.

CONCLUSIONS

Reduction of the behavioral complaints during thyroid hormone therapy is associated with a restoration of metabolic activity in brain areas that are integral to the regulation of affect and cognition. The findings suggest that thyroid hormone modulates regional glucose metabolism and psychiatric symptoms in the mature brain.

PET investigation of post-traumatic cerebral blood volume and blood flow

Hemodynamic changes following traumatic brain injury (TBI) may reflect cellular damage leading to secondary injury. The purpose of this study was to investigate the regional hemodynamic parameters acutely after TBI among regions in and around contusions. Sixteen patients (11 male, 5 female) showing evidence of contusion on CT and 18 normal volunteers (12 male, 6 female) underwent positron emission tomography (PET) with O-15 CO and O-15 H2O to estimate cerebral blood volume (CBV) and cerebral blood flow (CBF), respectively. A flow to volume ratio (FVR = CBF/CBV) was also calculated as an index of vasodilatation. The hemodynamic parameters were compared among contusion, pericontusion, and remote areas. Globally, hemodynamic parameters did not differ between patients and normal volunteers, and did not correlate with intracranial pressure (ICP). Regionally, contusional and pericontusional areas showed significantly lower CBF and FVR compared with normal volunteers, while CBV did not differ significantly. The correlation between CBF and CBV was significant (r = 0.37, p < 0.01). Remote areas did not show a significant difference in any of the PET parameters. In conclusion, regional brain edema is likely to occur in contusion and pericontusion areas, while some of the contusional tissue may show vascular engorgement.

Monitoring adenoviral DNA delivery, using a mutant herpes simplex virus type 1 thymidine kinase gene as a PET reporter gene

Current gene therapy protocols often suffer from an inability to monitor the site, level and persistence of gene expression following somatic DNA delivery. Herpes simplex virus 1 thymidine kinase (HSV1-tk) is currently under intensive investigation as a reporter gene for in vivo imaging of reporter gene expression. The presence of the HSV1-tk reporter gene is repetitively and non-invasively monitored by systemic injection of positron-emitting, radionuclide-labeled thymidine analogues or acycloguanosine HSV1-TK substrates and subsequent detection, by positron emission tomography, of trapped, phosphorylated product. To improve the efficacy of the HSV1-tk PET reporter gene system, both alternative substrates and mutations in the HSV1-tk gene have been described. We used a replication defective adenovirus to deliver the HSV1-sr39tk mutant enzyme and the wild-type HSV1-tk enzyme to mice. HSV1-sr39TK demonstrates greater sensitivity than wild-type HSV1-TK enzyme in vivo, using 9-[(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine as probe, following adenovirus-mediated hepatic expression in mice. Using this adenoviral delivery system, the location, magnitude and duration of HSV1-sr39tk PET reporter gene expression could be non-invasively, quantitatively and repetitively monitored for over 3 months by microPET.

Noninvasive quantitative imaging of protein-protein interactions in living subjects

We are developing methods to image molecular and cellular events in living subjects. In this study, we validate imaging of protein-protein interactions in living mice by using bioluminescent optical imaging. We use the well studied yeast two-hybrid system adapted for mammalian cells and modify it to be inducible. We employ the NF-kappaB promoter to drive expression of two fusion proteins (VP16-MyoD and GAL4-ID). We modulate the NF-kappaB promoter through tumor necrosis factor alpha. Firefly luciferase reporter gene expression is driven by the interaction of MyoD and ID through a transcriptional activation strategy. We demonstrate the ability to detect this induced protein-protein interaction in cell culture and image this induced interaction in living mice by using transiently transfected cells. The current approach will be a valuable and potentially generalizable tool to noninvasively and quantitatively image protein-protein interactions in living subjects. The approaches validated should have important implications for the study of protein-protein interactions in cells maintained in their natural in vivo environment as well as for the in vivo evaluation of new pharmaceuticals targeted to modulate protein-protein interactions.

The usefulness of F-18 deoxyglucose whole-body positron emission tomography (PET) for re-staging of renal cell cancer

PURPOSE

The use of whole-body PET for re-staging of renal cell carcinoma has not been investigated. The aim of the current study was to examine the diagnostic accuracy and clinical usefulness of whole-body PET imaging for re-staging of renal cell cancer.

PATIENTS AND METHODS

Clinical PET was performed for re-staging in 36 patients with advanced renal cell cancer. Written reports of imaging studies (including CT, MRI, US, plain film and bone scan), patient history, and extensive chart notes were used to define the clinical stage before PET (pre-PET stage). The written PET report was used to define the clinical stage after PET (PET stage). Reports were used to determine the accuracy of PET for re-staging renal cell cancer and for defining biopsy proven lesions. Clinical parameters and biopsy proven lesions served as reference for the accuracy of PET for re-staging renal cell cancer.

RESULTS

PET classified the clinical stage correctly in 32/36 patients (89%) and was incorrect in 4/36 (11%) (sensitivity and specificity: 87% and 100%). In 20 patients, 25 suspicious lesions were biopsied within 3.2 +/- 6.7 months of the PET study. Of these, 17 were malignant and 8 were benign. PET correctly classified 21/25 (84%) of the biopsied lesions (sensitivity and specificity: 88% and 75%).

CONCLUSION

PET re-stages renal cell cancer with a diagnostic accuracy of 89%. Its diagnostic accuracy for classifying biopsy proven anatomic lesions as malignant or benign was 84%. These findings suggest that PET is useful in characterizing anatomic lesions of unknown significance in patients with renal cell cancer.

Positron emission tomography in evaluation of dementia: Regional brain metabolism and long-term outcome

CONTEXT

Deficits in cerebral glucose utilization have been identified in patients with cognitive dysfunction attributed to various disease processes, but their prognostic and diagnostic value remains to be defined.

OBJECTIVE

To assess the sensitivity and specificity with which cerebral metabolic patterns at a single point in time forecast subsequent documentation of progressive dementia.

DESIGN, SETTING, AND PATIENTS

Positron emission tomography (PET) studies of [(18)F]fluorodeoxyglucose in 146 patients undergoing evaluation for dementia with at least 2 years' follow-up for disease progression at the University of California, Los Angeles, from 1991 to 2000, and PET studies in 138 patients undergoing evaluation for dementia at an international consortium of facilities, with histopathological diagnoses an average of 2.9 years later, conducted from 1984 to 2000.

MAIN OUTCOME MEASURES

Regional distribution of [(18)F]fluorodeoxyglucose in each patient, classified by criteria established a priori as positive or negative for presence of a progressive neurodegenerative disease in general and of Alzheimer disease (AD) specifically, compared with results of longitudinal or neuropathologic analyses.

RESULTS

Progressive dementia was detected by PET with a sensitivity of 93% (191/206) and a specificity of 76% (59/78). Among patients with neuropathologically based diagnoses, PET identified patients with AD and patients with any neurodegenerative disease with a sensitivity of 94% and specificities of 73% and 78%, respectively. The negative likelihood ratio of experiencing a progressive vs nonprogressive course over the several years following a single negative brain PET scan was 0.10 (95% confidence interval, 0.06-0.16), and the initial pattern of cerebral metabolism was significantly associated with the subsequent course of progression overall (P<.001).

CONCLUSION

In patients presenting with cognitive symptoms of dementia, regional brain metabolism was a sensitive indicator of AD and of neurodegenerative disease in general. A negative PET scan indicated that pathologic progression of cognitive impairment during the mean 3-year follow-up was unlikely to occur.

Monitoring gene therapy with reporter gene imaging

Rapid advances in imaging technologies and gene transfer strategies offer a great opportunity to optimize clinical trials of human gene therapy. Reporter genes are emerging as very powerful tools to monitor the delivery, magnitude, and time variation of therapeutic gene transfer in vivo. Several reporter genes, such as the herpes simplex virus type 1 thymidine kinase, the dopamine type 2 receptor, and the somatostatin receptor type 2, are currently being successfully used with gamma camera, single photon emission computed tomography, and positron emission tomography imaging. These reporter genes can be coupled with a therapeutic gene of interest to indirectly monitor the expression of the therapeutic gene. Finally, applications of the reporter gene technology to other areas, such as cell trafficking studies and transgenic animal models, are now possible.

Quantitative imaging of gene induction in living animals

Methods to repeatedly, non-invasively, and quantitatively image gene expression in living animals are rapidly emerging and should fundamentally change studies of gene expression in vivo. We previously developed assays utilizing positron emission tomography (PET) to image reporter gene expression. In this paper we: (1) describe a new bi-directional, tetracycline-inducible system that can be used to pharmacologically induce target gene expression and to quantitatively image induced expression by using a PET reporter gene; (2) demonstrate the potential of this system in transient and stable cell transfection assays; and (3) demonstrate the ability to repetitively and quantitatively image tetracycline and tetracycline analog induction of gene expression in living animals. We utilize the dopamine type-2 receptor (D(2)R) and the mutant herpes-simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter genes to validate this system. We utilize microPET technology to show that quantitative tomographic imaging of gene induction is possible. We find a high correlation (r(2) = 0.98) between 'target' and reporter gene expression. This work establishes a new technique for imaging time-dependent variation of gene expression both from vectors with inducible promoters and in transgenic animals in which pharmacologic induction of gene expression must be monitored. These techniques may be applied both in gene therapy and for the study of gene expression in transgenic animals.

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.

Impact of whole-body 18F-FDG PET on staging and managing patients with breast cancer: the referring physician's perspective

FDG PET has emerged as an important clinical imaging modality for diagnosing and staging cancer. However, the impact of FDG PET on staging and managing patients with breast cancer from the referring physician's point of view is unknown.

METHODS

The referring physicians of 160 breast cancer patients received standardized questionnaires inquiring if and how PET findings altered their patient's stage and their clinical management decisions. Management changes were classified as intermodality if the change was from one modality to another (e.g., medical to surgical, surgical to radiation, medical to no treatment, and vice versa) or as intramodality if the change was within the same modality (e.g., altered medical or radiotherapy approach).

RESULTS

Fifty of the 160 surveys were completed (31% response rate). PET changed the clinical stage in 36% of patients (28% upstaged, 8% downstaged) and resulted in intermodality changes in 28% of patients and intramodality changes in 30% of patients.

CONCLUSION

The results of this prospective survey show that FDG PET has a major impact on the management of breast cancer patients, influencing both clinical stage and management in more than 30% of patients.

Effect of whole-body (18)F-FDG PET imaging on clinical staging and management of patients with malignant lymphoma

Correct staging is important in selecting the appropriate treatment for lymphoma patients. PET imaging with (18)F-FDG is useful for staging of lymphoma as well as for monitoring of therapy. However, to our knowledge, the clinical impact of PET on staging and management of lymphoma patients has not been reported.

METHODS

Standardized questionnaires were mailed to referring physicians asking them whether and how the results of PET imaging had influenced clinical staging and management of the disease in their patients. Management changes, when present, were classified as intermodality (e.g., medical to surgical, surgical to radiation, medical to no treatment) or intramodality (e.g., altered medical, surgical, or radiotherapy approach).

RESULTS

The referring physicians returned 52 of 108 questionnaires (48.1%). Physicians indicated that PET led to a change in the clinical stage in 44% of patients: 21% were upstaged and 23% were downstaged. Findings of the PET examination resulted in intermodality changes in management in 42% of patients, in intramodality changes in 10%, and in a combination of the management changes in 10%. Other, not further specified, treatment changes were reported in 6% of patients. PET did not result in any management changes in only 32% of patients.

CONCLUSION

This survey-based study of referring physicians indicates that FDG PET has a major impact on the management of lymphoma patients, contributing to changes in clinical stage in 44% and changes in treatment in >60% of cases.

Human pharmacokinetic and dosimetry studies of [(18)F]FHBG: a reporter probe for imaging herpes simplex virus type-1 thymidine kinase reporter gene expression

9-[4-[(18)F]fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]FHBG) has been used as a reporter probe to image expression of herpes simplex virus type-1 thymidine kinase (HSV1-tk) reporter gene in living animals. Our aim was to study the kinetics, biodistribution, stability, dosimetry, and safety of [(18)F]FHBG in healthy human volunteers, preparatory to imaging patients undergoing HSV1-tk gene therapy.

METHODS

[(18)F]FHBG was synthesized with a specific activity of 37,000--444,000 GBq/mmol and a radiochemical purity > 99%. Ten healthy volunteers consented to participate in the study. A transmission scan was obtained before bolus injection of 70.3--229.4 MBq [(18)F]FHBG into a hand vein, followed by dynamic PET imaging with 4 consecutive emission scans. Warmed hand-vein blood was withdrawn at various times after injection for blood time--activity measurements. Electrocardiography, blood pressure, and blood and urine pharmacologic parameters were measured before and after injection of the [(18)F]FHBG tracer (n = 5). The stability of [(18)F]FHBG in the urine was analyzed. Attenuation-corrected images were reconstructed using the ordered-subsets expectation maximization algorithm. Image region-of-interest time-activity data were used with the MIRD program to estimate absorbed radiation dosages.

RESULTS

[(18)F]FHBG had rapid blood clearance; only 8.42% +/- 4.76% (mean +/- SD) of the peak blood activity remained at approximately 30 min. The average ratio of plasma activity to whole-blood activity during the study was 0.91 +/- 0.04. Penetration of [(18)F]FHBG across the blood-brain barrier was not observed. The primary routes of clearance were renal and hepatobiliary. High activities were observed in the bladder, gut, liver, and kidneys, but <0.0002% of the injected dose per gram was observed in other tissues. In the urine, 83% of activity 180 min after injection was stable [(18)F]FHBG. Blood and urine pharmacologic parameters did not change significantly after injection of the [(18)F]FHBG tracer. The bladder absorbed the highest radiation dose.

CONCLUSION

[(18)F]FHBG has the desirable in vivo characteristics of stability, rapid blood clearance, low background signal, biosafety, and acceptable radiation dosimetry in humans. This study forms the foundation for using [(18)F]FHBG in applications to monitor HSV1-tk reporter gene expression.

Estrogen use and brain metabolic change in older adults. A preliminary report

Because estrogen may influence brain blood flow and metabolism in older adults, we used positron emission tomography to evaluate cerebral glucose metabolic change in post-menopausal women and men. Women estrogen users (n=4), women non-users (n=8) and men (n=10) were scanned at baseline and two years later. Analyses focused on glucose metabolism in lateral temporal, inferior parietal and posterior cingulate brain regions, previously reported to decline in non-demented older persons. No metabolic differences in cerebral regions of interest were found among groups at baseline. At follow-up, women estrogen users showed significantly increased glucose metabolism in the lateral temporal region, whereas women non-users and men exhibited no significant metabolic change in this region. These findings suggest that estrogen use may protect against regional cerebral metabolic decline in postmenopausal women.

Regional brain metabolic changes in patients with major depression treated with either paroxetine or interpersonal therapy: preliminary findings

BACKGROUND

In functional brain imaging studies of major depressive disorder (MDD), regional abnormalities have been most commonly found in prefrontal cortex, anterior cingulate gyrus, and temporal lobe. We examined baseline regional metabolic abnormalities and metabolic changes from pretreatment to posttreatment in subjects with MDD. We also performed a preliminary comparison of regional changes with 2 distinct forms of treatment (paroxetine and interpersonal psychotherapy).

METHODS

Twenty-four subjects with unipolar MDD and 16 normal control subjects underwent resting F 18 ((18)F) fluorodeoxyglucose positron emission tomography scanning before and after 12 weeks. Between scans, subjects with MDD were treated with either paroxetine or interpersonal psychotherapy (based on patient preference), while controls underwent no treatment.

RESULTS

At baseline, subjects with MDD had higher normalized metabolism than controls in the prefrontal cortex (and caudate and thalamus), and lower metabolism in the temporal lobe. With treatment, subjects with MDD had metabolic changes in the direction of normalization in these regions. After treatment, paroxetine-treated subjects had a greater mean decrease in Hamilton Depression Rating Scale score (61.4%) than did subjects treated with interpersonal psychotherapy (38.0%), but both subgroups showed decreases in normalized prefrontal cortex (paroxetine-treated bilaterally and interpersonal psychotherapy-treated on the right) and left anterior cingulate gyrus metabolism, and increases in normalized left temporal lobe metabolism.

CONCLUSIONS

Subjects with MDD had regional brain metabolic abnormalities at baseline that tended to normalize with treatment. Regional metabolic changes appeared similar with the 2 forms of treatment. These results should be interpreted with caution because of study limitations (small sample size, lack of random assignment to treatment groups, and differential treatment response between treatment subgroups).

Direct correlation between positron emission tomographic images of two reporter genes delivered by two distinct adenoviral vectors

Biodistribution, magnitude and duration of a therapeutic transgene's expression may be assessed by linking it to the expression of a positron emission tomography (PET) reporter gene (PRG) and then imaging the PRG's expression by a PET reporter probe (PRP) in living animals. We validate the simple approach of co-administering two distinct but otherwise identical adenoviruses, one expressing a therapeutic transgene and the other expressing the PRG, to track the therapeutic gene's expression. Two PET reporter genes, a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) and dopamine-2 receptor (D(2)R), each regulated by the same cytomegalovirus (CMV) promoter, have been inserted into separate adenoviral vectors (Ad). We demonstrate that cells co-infected with equivalent titers of Ad-CMV-HSV1-sr39tk and Ad-CMV-D(2)R express both reporter genes with good correlation (r(2) = 0.93). Similarly, a high correlation (r(2) = 0.97) was observed between the expression of both PRGs in the livers of mice co-infected via tail-vein injection with equivalent titers of these two adenoviruses. Finally, microPET imaging of HSV1-sr39tk and D(2)R expression with 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl) guanine ([(18)F]FHBG) and 3-(2-[(18)F]fluoroethyl)spiperone ([(18)F]FESP), utilizing several adenovirus-mediated delivery routes, illustrates the feasibility of evaluating relative levels of transgene expression in living animals, using this approach.

Impact of 18F-FDG PET on managing patients with colorectal cancer: the referring physician's perspective

Whole-body PET imaging with 18F-FDG has been used successfully to stage colorectal cancer. However, the impact of FDG PET on patient management from the referring physician's point of view has not been determined.

METHODS

A questionnaire was sent to referring physicians to determine whether and how PET altered the management of colorectal cancer patients. Management changes, when present, were classified as intermodality (e.g., medical to surgical, surgical to radiation, medical to no treatment) or intramodality (e.g., altered medical, surgical, or radiotherapy approach).

RESULTS

Of 60 responses from referring physicians, changes in clinical stage were reported for 25 patients (42%). Among these, the disease was upstaged in 20 patients (80%) and downstaged in 5 patients (20%). The PET findings contributed to intermodality management changes in 22 of the 60 patients (37%), intramodality changes in 11 patients (18%), a combination of management changes in 4 patients (7%), and no change in 19 patients (32%). Two of the 60 patients (3%) had other changes, and no response to this question was received for the remaining 2 patients (3%). As a result of PET findings, physicians avoided major surgery in 41% of patients for whom surgery was the intended treatment.

CONCLUSION

This survey-based study of referring physicians shows that FDG PET had a major impact on the management of colorectal cancer patients and contributed to changes in clinical stage and major management decisions in >40% of patients.

Metabolic recovery following human traumatic brain injury based on FDG-PET: time course and relationship to neurological disability

OBJECTIVE

Utilizing [(18)F]fluorodeoxyglucose positron emission tomography (FDG-PET), we assessed the temporal pattern and the correlation of functional and metabolic recovery following human traumatic brain injury.

DESIGN AND SUBJECTS

Fifty-four patients with injury severity ranging from mild to severe were studied. Thirteen of these patients underwent both an acute and delayed FDG-PET study.

RESULTS

Analysis of the pooled global cerebral metabolic rate of glucose (CMRglc) values revealed that the intermediate metabolic reduction phase begins to resolve approximately one month following injury, regardless of injury severity. The correlation, in the 13 patients studied twice, between the extent of change in neurologic disability, assessed by the Disability Rating Scale (DRS), and the change in CMRglc from the early to late period was modest (r = -0.42). Potential explanations for this rather poor correlation are discussed. A review of the pertinent literature regarding the use of PET and related imaging modalities, including single photon emission tomography (SPECT) for the assessment of patients following traumatic brain injury is given.

CONCLUSION

The dynamic profile of CMRglc that changes following traumatic brain injury is seemingly stereotypic across a broad range and severity of injury types. Quantitative FDG-PET cannot be used as a surrogate technique for estimating degree of global functional recovery following traumatic brain injury.

Decision analysis for the cost-effective management of recurrent colorectal cancer

OBJECTIVE

To determine whether the use of [(18)F]2-fluoro-2-deoxyglucose positron emission tomography (FDG PET) in addition to computed axial tomography (CT) is helpful in managing recurrent colorectal cancer (CRC).

SUMMARY BACKGROUND DATA

There is no consensus on a management algorithm for CRC. However, when recurrence is suspected, CT is generally used for further evaluation and staging of disease.

METHODS

The authors used decision trees based on theoretical models to assess the cost-effectiveness of a CT + FDG PET strategy for the diagnosis and management of recurrent CRC compared with a CT-alone strategy. These theoretical models focus on patients with hepatic recurrence who are potentially curable through surgical hepatic resection. The population entering the decision trees consisted of patients with CRC who had undergone surgical resection of their primary CRC and who were suspected of having recurrence based on elevated levels of carcinoembryonic antigen.

RESULTS

The CT + FDG PET strategy was found to be cost-effective for managing patients with elevated carcinoembryonic antigen levels who were candidates for hepatic resection. The CT + FDG PET strategy was higher in mean cost by $429 per patient but resulted in an increase in the mean life expectancy of 9.527 days per patient.

CONCLUSIONS

These results show, through rigorous decision tree analysis, the potential cost-effectiveness of FDG PET in the management of recurrent CRC. The decision trees can be used to model various features of the management of recurrent CRC, including the cost-effectiveness of other newly emerging technologies.

8-[18F]Fluoropenciclovir: an improved reporter probe for imaging HSV1-tk reporter gene expression in vivo using PET

We have synthesized and evaluated 8-[18F]fluoropenciclovir (FPCV) and compared it with 8-[18F]fluoroganciclovir (FGCV) for monitoring the expression of herpes simplex virus type 1 thymidine kinase (HSV1 -tk) reporter gene in cell culture and in vivo.

METHODS

C6 rat glioma cells stably transfected with HSV1-tk (C6-stb-tk+) and control C6 cells were evaluated for their ability to accumulate FGCV versus FPCV. For in vivo studies, 15 mice were injected by tail vein with increasing levels of an adenoviral vector carrying HSV1-tk. Forty-eight hours later the mice were injected with FPCV and killed 3 h later. The percentage injected dose per gram (%ID/g) liver was then determined. Two additional mice were studied by microPET and autoradiography using FPCV to image adenoviral-mediated hepatic HSV1-tk reporter gene expression. A tumor-bearing mouse (C6 control and C6-stb-tk+) was imaged with FDG, FGCV, and FPCV. Two mice carrying tumors expressing two different reporter genes, HSV1-tk and dopamine type 2 receptor (D2R), were also imaged by microPET using FPCV (day 1) and 3-(2'-[18F]fluoroethyl)spiperone (FESP) (day 2).

RESULTS

FPCV shows a significantly greater accumulation in C6-stb-tk+ cells than does FGCV (P < 0.05). Over identical ranges of adenoviral administration, mouse liver shows a higher %ID/g liver for FPCV (0%-9%) compared with our previously reported results with FGCV (0%-3%). In C6 control and C6-stb-tk+ tumor-bearing mice, FPCV has a greater accumulation than does FGCV for equal levels of HSV1-tk gene expression. In mice carrying tumors expressing either HSV1-tk or D2R reporter genes, there is a corresponding retention of FPCV and FESP, respectively.

CONCLUSION

These results indicate that FPCV is a better reporter probe than is FGCV for imaging lower levels of HSV1 -tk gene expression in vivo. The results also reveal the ability to monitor the expression of two distinct reporter genes in the same animal using reporter probes specific for each gene.

Cognitive and metabolic responses to metrifonate therapy in Alzheimer disease

OBJECTIVE

The objective of this study was to identify the relation between the cognitive benefit seen with the cholinesterase inhibitor metrifonate and changes in brain metabolism as visualized with [18F]-fluorodeoxyglucose positron emission tomography (FDG-PET).

BACKGROUND

The regional metabolic correlates of treatment with cholinesterase inhibitors are poorly understood.

METHODS

Six patients with mild to moderate Alzheimer disease (AD) were evaluated before and after treatment with the long-lasting cholinesterase inhibitor metrifonate. Patients were given 60 or 80 mg of metrifonate per day (based on weight) for 6 to 12 weeks. Clinical evaluations included the cognitive portion of the Alzheimer's Disease Assessment Scale (ADAS-cog), the Mini-Mental State Examination (MMSE), and the Neuropsychiatric Inventory. Imaging was carried out using FDG-PET. The PET studies, registered to a probabilistic anatomic atlas, were normalized across the group's mean intensity levels and subjected to voxel-by-voxel subtraction of the posttreatment minus pretreatment studies. Subvolume thresholding corrected random lobar noise to produce a three-dimensional functional significance map.

RESULTS

The criteria for cognitive improvement with treatment were met for the MMSE (>2 points improvement from baseline), and the drawing subscale of the ADAS-cog was significantly improved with treatment. The three-dimensional significance map revealed a significant metabolic increase of the dorsolateral frontoparietal network on the left and bilateral temporal cortex with metrifonate treatment.

CONCLUSION

The clinical benefits observed in AD with cholinesterase inhibitor therapy are associated with a metabolic increase of heteromodal cognitive and medial temporal networks.

PET imaging of transgene expression

A vital step in transgenic animal study and gene therapy is the ability to assay the extent of transgene expression. Unfortunately, classic methods of assaying transgene expression require biopsies or death of the subject. We are developing techniques to noninvasively and repetitively determine the location, duration, and magnitude of transgene expression in living animals. This will allow investigators and clinicians to assay the effectiveness of their particular experimental and therapeutic paradigms. Of radionuclide (single photon emission computed tomography, positron emission tomography [PET]), optical (green fluorescent protein, luciferase), and magnetic (magnetic resonance imaging) approaches, only the radionuclide approach has sufficient sensitivity and quantitation to measure the expression of genes in vivo. We describe the instrumentation involved in high resolution PET scanning. We also describe the principles of PET reporter gene/reporter probe in vivo imaging, the development of two in vivo reporter gene imaging systems, and the validation of our ability to noninvasively, quantitatively, and repetitively image gene expression in murine viral gene transfer and transgenic models. We compare the two reporter gene systems and discuss their utility for the study of transgenic animals and gene therapies. Finally, we mention alternative approaches to image gene expression by using radiolabeled antibody fragments to image specific proteins and radiolabeled oligonucleotides to image RNA messages directly.

Oral 18F-fluoro-2-deoxyglucose for primate PET studies without behavioral restraint: demonstration of principle

We describe a method of orally administering 18F-fluoro-2-deoxyglucose (FDG) for positron emission tomography (PET) scans to determine local cerebral metabolic rates for glucose (LCMRGlc), normalized to that of whole brain, in fully conscious, non-restrained primates. Oral FDG-PET studies were performed in both non-restrained and chaired monkeys, and in one human where results could be compared with traditional intravenous FDG administration. The oral route of FDG administration gave images and whole brain-normalized PET LCMRGlc results comparable to those obtained by the intravenous route. This oral FDG-PET method may provide a useful means by which to obtain measures of LCMRGlcs for brain structures, relative to each other, in non-restrained, non-drugged primates in field and laboratory studies. This method might also have clinical applications for PET studies of children.

Positron emission tomography provides molecular imaging of biological processes

Diseases are biological processes, and molecular imaging with positron emission tomography (PET) is sensitive to and informative of these processes. This is illustrated by detection of biological abnormalities in neurological disorders with no computed tomography or MRI anatomic changes, as well as even before symptoms are expressed. PET whole body imaging in cancer provides the means to (i) identify early disease, (ii) differentiate benign from malignant lesions, (iii) examine all organs for metastases, and (iv) determine therapeutic effectiveness. Diagnostic accuracy of PET is 8-43% higher than conventional procedures and changes treatment in 20-40% of the patients, depending on the clinical question, in lung and colorectal cancers, melanoma, and lymphoma, with similar findings in breast, ovarian, head and neck, and renal cancers. A microPET scanner for mice, in concert with human PET systems, provides a novel technology for molecular imaging assays of metabolism and signal transduction to gene expression, from mice to patients: e.g., PET reporter gene assays are used to trace the location and temporal level of expression of therapeutic and endogenous genes. PET probes and drugs are being developed together-in low mass amounts, as molecular imaging probes to image the function of targets without disturbing them, and in mass amounts to modify the target's function as a drug. Molecular imaging by PET, optical technologies, magnetic resonance imaging, single photon emission tomography, and other technologies are assisting in moving research findings from in vitro biology to in vivo integrative mammalian biology of disease.

High-resolution microPET imaging of carcinoembryonic antigen-positive xenografts by using a copper-64-labeled engineered antibody fragment

Rapid imaging by antitumor antibodies has been limited by the prolonged targeting kinetics and clearance of labeled whole antibodies. Genetically engineered fragments with rapid access and high retention in tumor tissue combined with rapid blood clearance are suitable for labeling with short-lived radionuclides, including positron-emitting isotopes for positron-emission tomography (PET). An engineered fragment was developed from the high-affinity anticarcinoembryonic antigen (CEA) monoclonal antibody T84.66. This single-chain variable fragment (Fv)-C(H)3, or minibody, was produced as a bivalent 80 kDa dimer. The macrocyclic chelating agent 1,4,7, 10-tetraazacyclododecane-N,N',N", N"'-tetraacetic acid (DOTA) was conjugated to the anti-CEA minibody for labeling with copper-64, a positron-emitting radionuclide (t(1/2) = 12.7 h). In vivo distribution was evaluated in athymic mice bearing paired LS174T human colon carcinoma (CEA positive) and C6 rat glioma (CEA negative) xenografts. Five hours after injection with (64)Cu-DOTA-minibody, microPET imaging showed high uptake in CEA-positive tumor (17.9% injected dose per gram +/- 3.79) compared with control tumor (6.0% injected dose per gram +/- 1.0). In addition, significant uptake was seen in liver, with low uptake in other tissues. Average target/background ratios relative to neighboring tissue were 3-4:1. Engineered antibody fragments labeled with positron-emitting isotopes such as copper-64 provide a new class of agents for PET imaging of tumors.

A meta-analysis of the literature for whole-body FDG PET detection of recurrent colorectal cancer

A meta-analysis of the literature for the use of FDG PET in the detection of recurrent colorectal cancer (CRC) was conducted to evaluate the quality of the reported studies. Overall values for the sensitivity and specificity of whole-body FDG PET and an overall FDG PET-directed percentage change in management were also determined through this analysis.

METHODS

Guidelines to evaluate the articles were formulated on the basis of the U.S. medical payer source criteria for assessing studies that report information on usage of new medical technology. A metaanalysis was conducted using methodology described in the peer-reviewed literature.

RESULTS

On the basis of the guidelines established for our review, the availability of necessary information for assessing the reliability of the FDG PET data for diagnosing recurrent CRC was less than ideal. Through a meta-analysis of 11 articles, we determined, within a 95% confidence level, an overall sensitivity of 97% (95% confidence level, 95%-99%) and an overall specificity of 76% (95% confidence level, 64%-88%) for FDG PET detecting recurrent CRC throughout the whole body. Furthermore, through pooling of the change-in-management data, an overall FDG PET-directed change in management was calculated to be 29% (95% confidence level, 25%-34%).

CONCLUSION

Our review suggests that improvements can be made to more effectively report the results of these FDG PET studies. The overall values determined through the meta-analysis indicate the potential benefits of using FDG PET as a diagnostic or management tool. Furthermore, these values should prove to be useful to assess the cost-effectiveness of using FDG PET in the management of patients with recurrent CRC.

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.

A review of the literature for whole-body FDG PET in the management of patients with melanoma

BACKGROUND

A review and meta-analysis of the literature on the use of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) in the detection of recurrent melanoma was conducted. The goals were to evaluate the quality of data reporting and to determine the overall values for the sensitivity and specificity of whole body FDG PET and management changes.

METHODS

Guidelines to evaluate reporting within articles were formulated based on the United States medical payer source criteria for assessing studies reporting information on the utilization of new medical technology. A meta-analysis was conducted using methodology described in the peer reviewed literature.

RESULTS

Our MEDLINE PLUS search resulted in a universe of 89 total articles. Within these 89, 19 were categorized in our targeted content area of which 13 were selected for analysis in our targeted subset, with the remaining 70 covering 24 different related content areas. Five of 13 (38%) articles in the target subset reported data which was adequate for incorporation into modeling objectives based on PET sensitivity and specificity values, with 1 of 13 (8%) in the same target subset reporting data adequate for modeling based on change-in-management data. Through a meta-analysis of the 13 target articles we determined, within a 95% confidence level, an overall sensitivity of 92% (95% confidence level 88.41%-95.82%) and an overall specificity of 90% (95% confidence level 83.26%-96.05%) as calculated by number of lesions, for FDG PET detecting recurrent melanoma throughout the whole body. Furthermore, limited data available for change-in-management suggests an overall FDG PET directed change-in-management value of 22%.

CONCLUSIONS

Our review suggests that improvements can be made to more effectively report the results of these FDG PET studies. The overall values determined through the meta-analysis indicate the potential benefits of using FDG PET as a diagnostic/management tool. Furthermore, these values should prove useful to assessing the cost effectiveness of utilizing FDG PET in the management of recurrent melanoma.

In vivo imaging of neuronal activation and plasticity in the rat brain by high resolution positron emission tomography (microPET)

The study of neural repair and neuroplasticity in rodents would be enhanced by the ability to assess neuronal function in vivo. Positron emission tomography (PET) is used to study brain plasticity in humans, but the limited resolution and sensitivity of conventional scanners have generally precluded the use of PET to study neuroplasticity in rodents. We now demonstrate that microPET, a PET scanner developed for use with small animals, can be used to assess metabolic activity in different regions of the conscious rodent brain using [18F]fluorodeoxyglucose (FDG) as the tracer, and to monitor changes in neuronal activity. Limbic seizures result in dramatically elevated metabolic activity in the hippocampus, whereas vibrissal stimulation results in more modest increases in FDG uptake in the contralateral neocortex. We also show that microPET can be used to study lesion-induced plasticity of the brain. Cerebral hemidecortication resulted in diminished relative glucose metabolism in the neostriatum and thalamus ipsilateral to the lesion, with subsequent, significant recovery of metabolic function. These studies demonstrate that microPET can be used for serial assessment of metabolic function of individual, awake rats with a minimal degree of invasiveness, and therefore, has the potential for use in the study of brain disorders and repair.

Cerebral metabolic and cognitive decline in persons at genetic risk for Alzheimer's disease

The major known genetic risk for Alzheimer's disease (AD), apolipoprotein E-4 (APOE-4), is associated with lowered parietal, temporal, and posterior cingulate cerebral glucose metabolism in patients with a clinical diagnosis of AD. To determine cognitive and metabolic decline patterns according to genetic risk, we investigated cerebral metabolic rates by using positron emission tomography in middle-aged and older nondemented persons with normal memory performance. A single copy of the APOE-4 allele was associated with lowered inferior parietal, lateral temporal, and posterior cingulate metabolism, which predicted cognitive decline after 2 years of longitudinal follow-up. For the 20 nondemented subjects followed longitudinally, memory performance scores did not decline significantly, but cortical metabolic rates did. In APOE-4 carriers, a 4% left posterior cingulate metabolic decline was observed, and inferior parietal and lateral temporal regions demonstrated the greatest magnitude (5%) of metabolic decline after 2 years. These results indicate that the combination of cerebral metabolic rates and genetic risk factors provides a means for preclinical AD detection that will assist in response monitoring during experimental treatments.

Dissociation of cerebral glucose metabolism and level of consciousness during the period of metabolic depression following human traumatic brain injury

Utilizing [18F]fluorodeoxyglucose positron emission tomography (FDG-PET), we studied the correlation between CMRglc and the level of consciousness within the first month following human traumatic brain injury. Forty-three FDG-PET scans obtained on 42 mild to severely head-injured patients were quantitatively analyzed for the determination of regional cerebral metabolic rate of glucose (CMRglc). Reduction of cerebral glucose utilization, defined as a CMRglc of < or =4.9 mg/100 g/min, was present regionally in 88% of the studies. The prevalence of global cortical CMRglc reduction was higher in severely head-injured patients (86% versus 67% mild-moderate), although the absolute magnitude was similar across the injury severity spectrum (mean CMRglc 3.9 +/- 0.6 mg/100 g/min). The level of consciousness, as measured by the Glasgow Coma Scale, correlated poorly with the global cortical CMRglc value (r = 0.08; p = 0.63). With regards to severity of head injury, this correlation was worst for the severely injured (r = -0.11; p = 0.58) and better for the mildly injured patients (r = 0.50; p = 0.07). In most cases, intraparenchymal hemorrhagic lesions were associated with either focal CMRglc reduction or elevation. It is concluded that the etiologies of CMRglc reduction are likely multifactorial given the complex nature of traumatic brain injury and that the reduction of CMRglc represents a fundamental pathobiologic state following head injury that is not tightly coupled to level of consciousness.

Dynamic changes in cerebral glucose metabolism in conscious infant monkeys during the first year of life as measured by positron emission tomography

Recently, advances in spatial resolution have provided the opportunity to utilize positron emission tomography (PET) to examine local cerebral metabolic rates for glucose (lCMR(glc)) in large animals noninvasively, thereby allowing repeated lCMR(glc) measurements in the same animal. Previous studies have attempted to describe the ontogeny of cerebral glucose metabolism in anesthetized nonhuman primates using [18F]fluorodeoxyglucose (FDG) and PET. However, the use of sedation during the tracer uptake period may influence lCMR(glc). This study was conducted to describe lCMR(glc) in conscious infant vervet monkeys (Cercopithecus aethiops sabaeus) during the first year of life utilizing FDG-PET. Cross-sectional studies (n=23) displayed lowest and highest lCMR(glc) in all structures at the 2-3 and 8-9 month age groups, respectively. The metabolic pattern suggested an increase in lCMR(glc) values between 2 and 8 months of age with decreased metabolism observed at 10-12 months of age in all regions. Peak lCMR(glc) values at 8 months were an average of 84+/-24% higher than values seen at the youngest age examined quantitatively (2-3 months). The regions of greatest and smallest increases in lCMR(glc) at 8 months were the cerebellar hemispheres (90%) and the thalamus (39%), respectively. Longitudinal analysis in 4 animals supported this developmental pattern, demonstrating the ability to detect changes in cerebral glucose metabolism within animals and the potential for FDG-PET in nonhuman primate models of brain maturation. By determining the normative profile of lCMR(glc) during development in monkeys, future application of FDG-PET will provide the opportunity to longitudinally assess the effects of environmental or pharmacological intervention on the immature brain.

PET: the merging of biology and imaging into molecular imaging

PET and SPECT are molecular imaging techniques that use radiolabeled molecules to image molecular interactions of biological processes in vivo. PET imaging technologies have been developed to provide a pathway to the patient from the experimental paradigms of biological and pharmaceutical sciences in genetically engineered and tissue transplanted mouse models of disease. PET provides a novel way for molecular therapies and molecular diagnostics to come together in the discovery of molecules that can be used in low mass amounts to image the function of a target and, by elevating the mass, to pharmacologically modify the function of the target. In both cases, the molecules are the same or analogs of each other. PET can be used to titrate drugs to their sites of action within organ systems in vivo and to assay biological outcomes of the processes being modified in the mouse and the patient. The goal is to provide a novel way to improve the rates of discovery and approval of radiopharmaceuticals and pharmaceuticals. Extending this relationship into clinical practice can improve drug use by providing molecular diagnostics in concert with molecular therapeutics. Diseases are biological processes, and molecular imaging with PET is sensitive and informative to these processes. This sensitivity is exemplified by the detection of disease with PET without evidence of anatomic changes on CT and MRI. These biological changes are seen early in the course of disease, even in asymptomatic stages, as illustrated by the metabolic abnormalities detected with PET and FDG in Huntington's and familial Alzheimer's diseases 7 and 5 y, respectively, before symptoms appear. Differentiation of viable from nonviable tissue is fundamentally a metabolic question, as shown by the use of PET to differentiate patients with coronary artery disease who will benefit from revascularization from those who will not. Although beginning within a specific organ, cancer is a systemic disease the most devastating consequences of which result from metastases. Whole-body PET imaging with FDG enables inspection of glucose metabolism in all organ systems in a single examination to improve the detection and staging of cancer, selection of therapy, and assessment of therapeutic response. In lung and colorectal cancers, melanoma, and lymphoma, PET FDG improves the accuracy of detection and staging from 8% to 43% over conventional work-ups and results in treatment changes in 20%-40% of the patients, depending on the clinical question. Approximately 65% are upstaged because unsuspected metastases are detected, and 35% are downstaged because a structural diagnosis of lesions is changed from malignant to benign. Similar results are now being shown for other cancers. The main difference between CT, sonography, MRI, and PET or SPECT is not technologic but, rather, a difference between detecting and characterizing a disease by its anatomic features as opposed to its biology. The importance and success of developing new molecular imaging probes is increasing as PET becomes integral to the study of the integrative mammalian biology of disease and as molecular therapies targeting the biological processes of disease are developed.

Long-term methamphetamine-induced decreases of [(11)C]WIN 35,428 binding in striatum are reduced by GDNF: PET studies in the vervet monkey

The effects of glial cell line-derived neurotrophic factor (GDNF) pretreatment on methamphetamine (METH)-induced striatal dopamine system deficits in the vervet monkey were characterized with [(11)C]WIN 35,428 (WIN)-positron emission tomography (PET). WIN, a cocaine analog that binds to the dopamine transporter (DAT), was used to provide an index of striatal dopamine terminal integrity. In two subjects, GDNF (200 microg/40 microl) was injected into the caudate and putamen unilaterally vs. saline contralaterally. After 1-2 weeks, + and -GDNF striatal WIN-PET binding values were equivalent as calculated by multiple time graphic analysis, suggestive of an absence of unilateral DAT up-regulation. Three other subjects (n = 3) received GDNF injections into the caudate and putamen unilaterally and one week later, were administered METH HCl (2 x 2 mg/kg; i.m., 24 hours apart; a neurotoxic dosage for this species). At 1 week post-METH, WIN-PET studies showed that mean WIN binding was decreased by 72% in the +GDNF and by 92% in the -GDNF striatum relative to pre-drug assessment values. Thus, GDNF pretreatment reduced the extent of METH-induced decreases in WIN binding. Subsequent WIN-PET studies (1.5-9-month range) showed a protracted recovery of WIN binding in each striatum, indicative of long-term but partially reversible METH neurotoxicity. Further, at each time point, WIN binding remained relatively higher in the +GDNF vs. -GDNF striatum. These results provide further evidence that the adult non-human primate brain remains responsive to exogenously administered GDNF and that this pharmacotherapy approach can counteract aspects of neurotoxic actions associated with methamphetamine.

Seeing is believing: non-invasive, quantitative and repetitive imaging of reporter gene expression in living animals, using positron emission tomography

The ability to monitor reporter gene expression in living animals and in patients will permit longitudinal examinations both of somatically transferred DNA in experimental animals and patients and of transgenic constructs expressed in experimental animals. If investigators can non-invasively monitor the organ and tissue specificity, the magnitude and the duration of gene expression from somatically transferred DNA and from transgenes, conceptually new experimental paradigms will be possible. If clinicians can non-invasively monitor the location, extent and duration of somatically transferred genes, they will be better able to determine the correlations between expression of therapeutic genes and clinical outcomes. We have developed two reporter gene systems for in vivo reporter gene imaging in which the protein products of the reporter genes sequester positron-emitting reporter probes. The "PET reporter gene" dependent sequestration of the "PET reporter probes" is subsequently measured in living animals by Positron Emission Tomography (PET). We describe here the principles of PET reporter gene/PET reporter probe in vivo imaging, the development of two imaging systems, and the validation of their ability to non-invasively, quantitatively and repetitively image reporter gene expression in murine viral gene transfer and transgenic models.

A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography

We are developing assays for noninvasive, quantitative imaging of reporter genes with positron emission tomography (PET), for application both in animal models and in human gene therapy. We report here a method to improve the detection of lower levels of PET reporter gene expression by utilizing a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) as a PET reporter gene. The HSV1-sr39tk mutant was identified from a library of site-directed mutants. Accumulation (net uptake) of the radioactively labeled substrates [8-(3)H]penciclovir ([8-(3)H]PCV), and 8-[(18)F]fluoropenciclovir (FPCV) in C6 rat glioma cells expressing HSV1-sr39tk is increased by a factor of approximately 2.0 when compared with C6 cells expressing wild-type HSV1-tk. The increased imaging sensitivity of HSV1-sr39tk when FPCV is used is also demonstrated in vivo both with tumor cells stably transfected with either HSV1-tk or HSV1-sr39tk, and after hepatic delivery of HSV1-tk or HSV1-sr39tk by using adenoviral vectors. The use of HSV1-sr39tk as a PET reporter gene and FPCV as a PET reporter probe results in significantly enhanced sensitivity for imaging reporter gene expression in vivo.

Long-term methamphetamine-induced decreases of [(11)C]WIN 35,428 binding in striatum are reduced by GDNF: PET studies in the vervet monkey

The effects of glial cell line-derived neurotrophic factor (GDNF) pretreatment on methamphetamine (METH)-induced striatal dopamine system deficits in the vervet monkey were characterized with [(11)C]WIN 35,428 (WIN)-positron emission tomography (PET). WIN, a cocaine analog that binds to the dopamine transporter (DAT), was used to provide an index of striatal dopamine terminal integrity. In two subjects, GDNF (200 microg/40 microl) was injected into the caudate and putamen unilaterally vs. saline contralaterally. After 1-2 weeks, + and -GDNF striatal WIN-PET binding values were equivalent as calculated by multiple time graphic analysis, suggestive of an absence of unilateral DAT up-regulation. Three other subjects (n = 3) received GDNF injections into the caudate and putamen unilaterally and one week later, were administered METH HCl (2 x 2 mg/kg; i.m., 24 hours apart; a neurotoxic dosage for this species). At 1 week post-METH, WIN-PET studies showed that mean WIN binding was decreased by 72% in the +GDNF and by 92% in the -GDNF striatum relative to pre-drug assessment values. Thus, GDNF pretreatment reduced the extent of METH-induced decreases in WIN binding. Subsequent WIN-PET studies (1.5-9-month range) showed a protracted recovery of WIN binding in each striatum, indicative of long-term but partially reversible METH neurotoxicity. Further, at each time point, WIN binding remained relatively higher in the +GDNF vs. -GDNF striatum. These results provide further evidence that the adult non-human primate brain remains responsive to exogenously administered GDNF and that this pharmacotherapy approach can counteract aspects of neurotoxic actions associated with methamphetamine.

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.

Regulation of myocardial blood flow response to mental stress in healthy individuals

Mental stress testing has been proposed as a noninvasive tool to evaluate endothelium-dependent coronary vasomotion. In patients with coronary artery disease, mental stress can induce myocardial ischemia. However, even the determinants of the physiological myocardial blood flow (MBF) response to mental stress are poorly understood. Twenty-four individuals (12 males/12 females, mean age 49 +/- 13 yr, range 31-74 yr) with a low likelihood for coronary artery disease were studied. Serum catecholamines, cardiac work, and MBF (measured quantitatively with N-13 ammonia and positron emission tomography) were assessed. During mental stress (arithmetic calculation) MBF increased significantly from 0.70 +/- 0.14 to 0.92 +/- 0.21 ml x min(-1) x g(-1) (P < 0.01). Mental stress caused significant increases (P < 0.01) in serum epinephrine (26 +/- 16 vs. 42 +/- 17 pg/ml), norepinephrine (272 +/- 139 vs. 322 +/- 136 pg/ml), and cardiac work [rate-pressure product (RPP) 8,011 +/- 1,884 vs. 10,416 +/- 2,711]. Stress-induced changes in cardiac work were correlated with changes in MBF (r = 0.72; P < 0.01). Multiple-regression analysis revealed stress-induced changes in the RPP as the only significant (P = 0.0001) predictor for the magnitude of mental stress-induced increases in MBF in healthy individuals. Data from this group of healthy individuals should prove useful to investigate coronary vasomotion in individuals at risk for or with documented coronary artery disease.

Synthesis of 8-[(18)F]fluoroguanine derivatives: in vivo probes for imaging gene expression with positron emission tomography

A new method for the preparation of 8-[(18)F]fluoroguanine derivatives based on a direct radiofluorination reaction has been developed. The radiofluorination of ganciclovir (1a) with [(18)F]F(2) was carried out in absolute ethanol in the presence of tetraethylammonium hydroxide at room temperature to give 8-[(18)F]fluoroganciclovir (3a) in an approximately 1% radiochemical yield. Similarly, 8-[(18)F]fluoropenciclovir (3b), 8-[(18)F]fluoroacyclovir (3c), and 8-[(18)F]fluoroguanosine (3d) were synthesized from penciclovir (1b), acyclovir (1c), and guanosine (1d), respectively, using [(18)F]F(2). The structural analyses of the final products (3a, 3b, 3c, and 3d) were carried out after (18)F decay by (1)H, (13)C, and (19)F nuclear magnetic resonance and high resolution mass spectroscopy.

Imaging transgene expression with radionuclide imaging technologies

A variety of imaging technologies are being investigated as tools for studying gene expression in living subjects. Noninvasive, repetitive and quantitative imaging of gene expression will help both to facilitate human gene therapy trials and to allow for the study of animal models of molecular and cellular therapy. Radionuclide approaches using single photon emission computed tomography (SPECT) and positron emission tomography (PET) are the most mature of the current imaging technologies and offer many advantages for imaging gene expression compared to optical and magnetic resonance imaging (MRI)-based approaches. These advantages include relatively high sensitivity, full quantitative capability (for PET), and the ability to extend small animal assays directly into clinical human applications. We describe a PET scanner (microPET) designed specifically for studies of small animals. We review "marker/reporter gene" imaging approaches using the herpes simplex type 1 virus thymidine kinase (HSV1-tk) and the dopamine type 2 receptor (D2R) genes. We describe and contrast several radiolabeled probes that can be used with the HSV1-tk reporter gene both for SPECT and for PET imaging. We also describe the advantages/disadvantages of each of the assays developed and discuss future animal and human applications.

Effect of mental stress on myocardial blood flow and vasomotion in patients with coronary artery disease

In patients with coronary artery disease (CAD), mental stress may provoke ischemic electrocardiograph changes and abnormalities in regional and global left ventricular function. However, little is known about the underlying myocardial blood flow response (MBF) in these patients.

METHODS

We investigated the hemodynamic, neurohumoral, and myocardial blood flow responses to mental stress in 17 patients with CAD and 17 healthy volunteers of similar age. Mental stress was induced by asking individuals to solve mathematic subtractions in a progressively challenging sequence; MBF was quantified at rest and during mental stress using 13N ammonia PET.

RESULTS

Mental stress induced significant (P < 0.01) and comparable increases in rate-pressure product, measured in beats per minute x mm Hg, in both patients (from 7826 +/- 2006 to 10586 +/- 2800) and healthy volunteers (from 8227 +/- 1272 to 10618 +/- 2468). Comparable increases also occurred in serum epinephrine (58% in patients versus 52% in healthy volunteers) and norepinephrine (22% in patients versus 27% in healthy volunteers). Although MBF increased in patients (from 0.67 +/- 0.15 to 0.77 +/- 0.18 mL/min/g, P < 0.05) and healthy volunteers (from 0.73 +/- 0.13 to 0.95 +/- 0.22 mL/min/g, P < 0.001), the magnitude of flow increase was smaller in patients (14% +/- 17%) than in healthy volunteers (29% +/- 14%) (P = 0.01). The increase in MBF during mental stress correlated significantly with changes in cardiac work in healthy volunteers (r = 0.77; P < 0.001) but not in patients.

CONCLUSION

Despite similar increases in cardiac work and comparable sympathetic stimulation in CAD patients and healthy volunteers, CAD patients exhibit an attenuated blood flow response to mental stress that may contribute to mental stress-induced ischemic episodes in daily life.

Localized orbitofrontal and subcortical metabolic changes and predictors of response to paroxetine treatment in obsessive-compulsive disorder

Previous positron emission tomography (PET) studies of patients with obsessive-compulsive disorder (OCD) have found elevated glucose metabolic rates in the orbitofrontal cortex (OFC) and caudate nuclei that normalize with response to treatment. Furthermore, OCD symptom provocation differentially activates specific subregions of the OFC, which have distinct patterns of connectivity and serve different functions. Therefore, we sought to determine the role of specific subregions of the OFC and associated subcortical structures in mediating OCD symptoms, by determining how glucose metabolism in these structures changed with paroxetine treatment of OCD patients. We also sought to determine whether pretreatment OFC metabolism would predict response to paroxetine, as it has for other OCD treatments. Twenty subjects with OCD received [18F]-fluorodeoxyglucose (FDG)-PET scans before and after 8 to 12 weeks of treatment with paroxetine, 40 mg/day. In patients who responded to paroxetine, glucose metabolism decreased significantly in right anterolateral OFC and right caudate nucleus. Lower pretreatment metabolism in both left and right OFC predicted greater improvement in OCD severity with treatment. These results add to evidence indicating that orbitofrontal-subcortical circuit function mediates the symptomatic expression of OCD. Specific subregions of the OFC may be differentially involved in the pathophysiology of OCD and/or its response to pharmacotherapy.