Whiskers area as extracerebral reference tissue for quantification of rat brain metabolism using (18)F-FDG PET: application to focal cerebral ischemia.

UNLABELLED: Diseases and dysfunction of the central nervous system are often associated with regional changes in cerebral glucose metabolism, which can be measured in vivo by PET using (18)F-FDG as the tracer. For quantification, the arterial tracer input function must be determined. For rodents in particular, direct measurement of blood radioactivity concentration is scarcely feasible for follow-up of individual animals because of the invasiveness of blood sampling. We show that the whiskers area of the rat's muzzle serves as an extracerebral reference region. The derived model also takes into account local variations of the lumped constant, which is crucial in pathologic tissue. METHODS: In 11 rats, the reference tissue kinetic parameters were determined from PET data and measured whole blood radioactivity concentration. Parametric images of cerebral kinetic rate constants were calculated using the directly measured input function, the reference tissue time-activity curve with individually fitted reference kinetic parameters, and the reference time-activity curve with fixed reference kinetic parameters calculated from the fitted parameters averaged over all animals. The need for kinetic modeling in disease models is demonstrated in 5 rats subjected to acute focal cerebral ischemia. (18)F-FDG metabolism and transport rate constants and local cerebral glucose metabolic rates were calculated. RESULTS: Cerebral kinetic constants derived from the 3 methods corresponded closely. The maximum difference in whole-brain kinetic parameters observed between the directly measured input function and the reference tissue time-activity curve with individually fitted reference kinetic parameters was less than 5%. Taking fixed reference parameters (the reference time-activity curve with fixed reference kinetic parameters calculated from the fitted parameters averaged over all animals) still provided whole-brain kinetic parameters with an accuracy of approximately 90%. In the rats subjected to focal cerebral ischemia, (18)F-FDG kinetic parameters in healthy tissue were not significantly different from whole-brain kinetic parameters in naive rats. The ischemic region was characterized by preserved glucose metabolism, although (18)F-FDG uptake was elevated significantly-that is, the lumped constant in the ischemic region was different from that of healthy brain tissue. CONCLUSION: The method presented here allows for the quantitative noninvasive determination of cerebral glucose consumption in rats, takes into account local variations of the lumped constant, and is suitable for follow-up measurements of individuals.

[18F]FLT PET for non-invasive monitoring of early response to gene therapy in experimental gliomas.

The purpose of this study was to investigate the potential of 3'-deoxy-3'-[¹8F]fluorothymidine ([¹8F]FLT) positron emission tomography (PET) to detect early treatment responses in gliomas. Human glioma cells were stably transduced with genes yielding therapeutic activity, sorted for different levels of exogenous gene expression, and implanted subcutaneously into nude mice. Multimodality imaging during prodrug therapy included (a) magnetic resonance imaging, (b) PET with 9-(4-[¹8F]fluoro-3-hydroxymethylbutyl)guanine assessing exogenous gene expression, and (c) repeat [¹8F]FLT PET assessing antiproliferative therapeutic response. All stably transduced gliomas responded to therapy with significant reduction in tumor volume and [¹8F]FLT accumulation within 3 days after initiation of therapy. The change in [¹8F]FLT uptake before and after treatment correlated to volumetrically calculated growth rates. Therapeutic efficacy as monitored by [¹8F]FLT PET correlated to levels of therapeutic gene expression measured in vivo. Thus, [¹8F]FLT PET assesses early antiproliferative effects, making it a promising radiotracer for the development of novel treatments for glioma.

Noninvasive quantification of 18F-FLT human brain PET for the assessment of tumour proliferation in patients with high-grade glioma.

PURPOSE: Compartmental modelling of 3'-deoxy-3'-[18F]-fluorothymidine (18F-FLT) PET-derived kinetics provides a method for noninvasive assessment of the proliferation rate of gliomas. Such analyses, however, require an input function generally derived by serial blood sampling and counting. In the current study, 18F-FLT kinetic parameters obtained from image-derived input functions were compared with those from input functions derived from arterialized blood samples. METHODS: Based on the analysis of 11 patients with glioma (WHO grade II-IV) a procedure for the automated extraction of an input function from 18F-FLT brain PET data was derived. The time-activity curve of the volume of interest with the maximum difference in 18F-FLT uptake during the first 5 min after injection and the period from 60 to 90 min was corrected for partial-volume effects and in vivo metabolism of 18F-FLT. For each patient a two-compartment kinetic model was applied to the tumour tissue using the image-derived input function. The resulting kinetic rate constants K1 (transport across the blood-brain barrier) and Ki (metabolic rate constant or net influx constant) were compared with those obtained from the same data using the input function derived from blood samples. Additionally, the metabolic rate constant was correlated with the frequency of tumour cells stained with Ki-67, a widely used immunohistochemical marker of cell proliferation. RESULTS: The rate constants from kinetic modelling were comparable when the blood sample-derived input functions were replaced by the image-derived functions (K1,img and K1,sample, r = 0.95, p < 10(-5); Ki,img and Ki,sample, r = 0.86, p < 0.001). A paired t-test showed no significant differences in the parameters derived with the two methods (K1,img and K1,sample, p = 0.20; Ki,img and Ki,sample, p = 0.92). Furthermore, a significant correlation between Ki,img and the percentage of Ki-67-positive cells was observed (r = 0.73, p = 0.01). CONCLUSION: Kinetic modelling of 18F-FLT brain PET data using image-derived input functions extracted from human brain PET data with the practical procedure described here provides information about the proliferative activity of brain tumours which might have clinical relevance especially for monitoring of therapy response in future clinical trials.

Glioma proliferation as assessed by 3'-fluoro-3'-deoxy-L-thymidine positron emission tomography in patients with newly diagnosed high-grade glioma.

PURPOSE: The aim of this study was to investigate the relationship between the in vivo derived kinetic parameters of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) and the proliferation rate measured in vitro by Ki-67 staining in patients with newly diagnosed high-grade gliomas. EXPERIMENTAL DESIGN: Thirteen patients with newly diagnosed high-grade gliomas were investigated with (18)F-FLT and methyl-(11)C- l-methionine ((11)C-MET) positron emission tomography (PET) and T1-, Gd-T1-, and T2-weighted magnetic resonance imaging on consecutive days. Tracer kinetic parameters of (18)F-FLT as well as the standardized uptake value and the tumor-to-background (T/B) ratio of (18)F-FLT and (11)C-MET were determined. Data of kinetic modeling, standardized uptake value, and T/B values derived from (18)F-FLT-PET were compared with T/B values derived from (11)C-MET-PET and to the in vitro proliferation marker Ki-67. RESULTS: A significant correlation was observed between the metabolic rate constant Ki and the proliferation index as measured by Ki-67 immunostaining [Ki, r=0.79 (P=0.004)]. Also, the phosphorylation rate constant k3 correlated with Ki-67 [k3, r=0.76 (P=0.006)], whereas the rate constant for transport through the blood brain barrier K1 showed a weaker correlation with Ki-67 [K1, r=0.62 (P=0.044)]. No significant correlation between (11)C-MET and (18)F-FLT uptake ratios and Ki-67 was observed. CONCLUSIONS: This study shows that kinetic analysis of (18)F-FLT tracer uptake is essential for the in vivo assessment of tumor proliferation in high-grade gliomas, whereas uptake ratios of (11)C-MET and (18)F-FLT failed to correlate with the in vitro determined proliferation marker. Thus, kinetic analysis of (18)F-FLT might provide an accurate method for the assessment of early response to glioma treatment in the future.

Cortical flattening applied to high-resolution 18F-FDG PET.

UNLABELLED: Group studies using PET and other types of neuroimaging require some means to achieve congruence of brain structures across subjects, such that scans from individuals varying in brain shape and gyral anatomy can be analyzed together. Volume registration methods are the most widely used approach to achieve this congruence. They are fast and typically require little manual interaction, but, unfortunately, it is difficult to achieve a good match between cortical areas in volume space, especially where folding patterns vary across subjects. Cortical flattening is a recent, alternative strategy: Its key features are explicit definition of cortex, such that white matter or cerebrospinal fluid compartments are largely excluded from the analysis volume, and subsequent registration of the cortical sheet in its natural, 2-dimensional topology. This type of registration has been demonstrated to provide better matching of congruent cortical structures than volume methods and, thus, offers a potentially more robust way of analyzing PET data. METHODS: Here, we explore the applicability of cortical flattening of coregistered MRI to (18)F-FDG PET on the HRRT system (high-resolution research tomograph), the highest-resolution whole-head scanner available to date. RESULTS: We report average values and SD of cortical metabolism in a pilot study of the dominant hemisphere in 9 control subjects and provide estimates of group sizes necessary for studies using this technique. CONCLUSION: We conclude that cortical flattening with subsequent surface registration is a feasible and promising strategy for group studies on the HRRT, providing the highest fidelity maps of human cortical glucose consumption to date.

Switching on the lights for gene therapy.

Strategies for non-invasive and quantitative imaging of gene expression in vivo have been developed over the past decade. Non-invasive assessment of the dynamics of gene regulation is of interest for the detection of endogenous disease-specific biological alterations (e.g., signal transduction) and for monitoring the induction and regulation of therapeutic genes (e.g., gene therapy). To demonstrate that non-invasive imaging of regulated expression of any type of gene after in vivo transduction by versatile vectors is feasible, we generated regulatable herpes simplex virus type 1 (HSV-1) amplicon vectors carrying hormone (mifepristone) or antibiotic (tetracycline) regulated promoters driving the proportional co-expression of two marker genes. Regulated gene expression was monitored by fluorescence microscopy in culture and by positron emission tomography (PET) or bioluminescence (BLI) in vivo. The induction levels evaluated in glioma models varied depending on the dose of inductor. With fluorescence microscopy and BLI being the tools for assessing gene expression in culture and animal models, and with PET being the technology for possible application in humans, the generated vectors may serve to non-invasively monitor the dynamics of any gene of interest which is proportionally co-expressed with the respective imaging marker gene in research applications aiming towards translation into clinical application.

Multitracer positron emission tomographic imaging of exogenous gene expression mediated by a universal herpes simplex virus 1 amplicon vector.

To develop efficient and safe gene therapy approaches, the herpes simplex virus type 1 thymidine kinase gene (HSV-1-tk) has been shown to function as a marker gene for the direct noninvasive in vivo localization of thymidine kinase (TK) expression by positron emission tomography (PET) using radiolabeled nucleoside analogues as specific TK substrates. Moreover, the gene encoding dopamine type 2 receptor (d2r) could be used as a PET marker gene using specific radiolabeled receptor binding compounds. Here we describe the quantitative colocalization of d2r and HSV-1-tk gene expression mediated from a universal HSV-1 amplicon vector in a subcutaneous human Gli36dEGFR glioma model by PET. The HSV-1 amplicon vector was constructed using a bicistronic gene cassette to contain (1) the d2r80A mutant, which is able to bind its ligand racloprid but unable to activate downstream signal transduction pathways, and (2) the tk39 mutant with enhanced enzymatic activity toward guanosine analogues fused to the green fluorescent protein gene (tk39gfp) serving as a marker gene in cell culture. After infection of human Gli36dEGFR glioma cells with the HSV-d2r80AIREStk39gfp (HSV-DITG) amplicon vector in cell culture, D2 receptor expression and its targeting to the cell surface were determined by Western blotting and immunolabeling. Vector application in vivo served for quantitative colocalization of d2r80A- and tk39gfp-derived PET signals employing the specific D2 receptor binding compound [(11)C]racloprid and the specific TK39 substrate 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine. Our results demonstrate that for the range of gene expression studied in vivo, both enzymatic and receptor binding assays give comparable quantitative information on the level of vector-mediated gene expression in vivo. The d2r80A in combination with a specific binding compound passing the intact blood-brain barrier might be an alternative marker gene for the noninvasive assessment of vector-mediated gene expression in the brain using PET.

Imaging-guided gene therapy of experimental gliomas.

To further develop gene therapy for patients with glioblastomas, an experimental gene therapy protocol was established comprising a series of imaging parameters for (i) noninvasive assessment of viable target tissue followed by (ii) targeted application of herpes simplex virus type 1 (HSV-1) amplicon vectors and (iii) quantification of treatment effects by imaging. We show that viable target tissue amenable for application of gene therapy vectors can be identified by multitracer positron emission tomography (PET) using 2-(18)F-fluoro-2-deoxy-D-glucose, methyl-(11)C-L-methionine, or 3'-deoxy-3'-(18)F-fluoro-L-thymidine ([(18)F]FLT). Targeted application of HSV-1 amplicon vectors containing two therapeutic genes with synergistic antitumor activity (Escherichia coli cytosine deaminase, cd, and mutated HSV-1 thymidine kinase, tk39, fused to green fluorescent protein gene, gfp) leads to an overall response rate of 68%, with 18% complete responses and 50% partial responses. Most importantly, we show that the "tissue dose" of HSV-1 amplicon vector-mediated gene expression can be noninvasively assessed by 9-[4-(18)F-fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]FHBG) PET. Therapeutic effects could be monitored by PET with significant differences in [(18)F]FLT accumulation in all positive control tumors and 72% in vivo transduced tumors (P = 0.01) as early as 4 days after prodrug therapy. For all stably and in vivo transduced tumors, cdIREStk39gfp gene expression as measured by [(18)F]FHBG-PET correlated with therapeutic efficiency as measured by [(18)F]FLT-PET. These data indicate that imaging-guided vector application with determination of tissue dose of vector-mediated gene expression and correlation to induced therapeutic effect using multimodal imaging is feasible. This strategy will help in the development of safe and efficient gene therapy protocols for clinical application.

Direct demonstration of transcallosal disinhibition in language networks.

Neuroimaging studies in right-handed patients with left hemisphere brain lesions have demonstrated a shift of language activity from left to right inferior frontal gyrus (IFG). This shift may be caused by greater right hemisphere dominance before the injury or by reduced inhibitory activity of the injured left hemisphere. We simulated a brain lesion applying transcranial -magnetic stimulation over left IFG in normal subjects, while simultaneously measuring language activity with positron -emission tomography. Interference with transcranial -magnetic stimulation decreased activity in left and increased it in right IFG in all subjects. We thus demonstrate for the first time that a rightward shift of language activity is caused by the brain lesion and not by greater right-hemisphere dominance, thus supporting the hypothesis of reduced transcallosal inhibition.

18F-fluoro-L-thymidine and 11C-methylmethionine as markers of increased transport and proliferation in brain tumors.

UNLABELLED: Because of the high glucose metabolism in normal brain tissue 18F-FDG is not the ideal tracer for the detection of gliomas. Methyl-11C-l-methionine (11C-MET) is better suited for imaging the extent of gliomas, because it is transported specifically into tumors but only insignificantly into normal brain. 3'-Deoxy-3'-18F-fluorothymidine (18F-FLT) has been introduced as a proliferation marker in a variety of neoplasias and has promising potential for the detection of brain tumors, because its uptake in normal brain is low. Additionally, the longer half-life might permit differentiation between transport and intracellular phosphorylation. METHODS: PET of 18F-FLT and 11C-MET was performed on 23 patients (age range, 20-70 y) with histologically verified gliomas of different grades. On all patients, conventional MRI was performed, and 16 patients additionally underwent contrast-enhanced imaging. Images were coregistered, and the volumes of abnormality were defined for PET and MRI. Uptake ratios and standardized uptake values (SUVs) of various tumors and regions were assessed by region-of-interest analysis. Kinetic modeling was performed on 14 patients for regional time-activity curves of 18F-FLT from tumorous and normal brain tissue. RESULTS: Sensitivity for the detection of tumors was lower for 18F-FLT than for 11C-MET (78.3% vs. 91.3%), especially for low-grade astrocytomas. Tumor volumes detected by 18F-FLT and 11C-MET were larger than tumor regions displaying gadolinium enhancement (P<0.01). Uptake ratios of 18F-FLT were higher than uptake ratios of 11C-MET (P<0.01). Uptake ratios of 18F-FLT were higher in glioblastomas than in astrocytomas (P<0.01). Absolute radiotracer uptake of 18F-FLT was low and significantly lower than that of 11C-MET (SUV, 1.3+/-0.7 vs. 3.1+/-1.0; P<0.01). Some tumor regions were detected only by either 18F-FLT (7 patients) or 11C-MET (13 patients). Kinetic modeling revealed that 18F-FLT uptake in tumor tissue seems to be predominantly due to elevated transport and net influx. However, a moderate correlation was found between uptake ratio and phosphorylation rate k3 (r=0.65 and P=0.01 for grade II-IV gliomas; r=0.76 and P<0.01 for grade III-IV tumors). CONCLUSION: 18F-FLT is a promising tracer for the detection and characterization of primary central nervous system tumors and might help to differentiate between low- and high-grade gliomas. 18F-FLT uptake is mainly due to increased transport, but irreversible incorporation by phosphorylation might also contribute. In some tumors and tumor areas, 18F-FLT uptake is not related to 11C-MET uptake. In view of the high sensitivity and specificity of 11C-MET PET for imaging of gliomas, it cannot be excluded that 18F-FLT PET was false positive in these areas. However, the discrepancies observed for the various imaging modalities (18F-FLT and 11C-MET PET as well as gadolinium-enhanced MRI) yield complementary information on the activity and the extent of gliomas and might improve early evaluation of treatment effects, especially in patients with high-grade gliomas. Further studies are needed, including coregistered histology and kinetic analysis in patients undergoing chemotherapy.

Metabolic rates in small brain nuclei determined by high-resolution PET.

UNLABELLED: Identification of small nuclei in the brain by PET has been limited by the spatial resolution of conventional scanners. The new detector technology and advanced signal analysis of a high-resolution research tomograph (HRRT) has improved 3-dimensional spatial resolution to 2.2 mm at sufficient efficiency and permitted the quantification of tracer concentrations in small volumes. METHODS: In 9 healthy volunteers, cerebral glucose metabolism was investigated after intravenous injection of 370 MBq of (18)F-FDG, and regional cerebral metabolic rates for glucose (rCMRGlc) were determined in various structures of the brain identified on coregistered MR images using stereotactic and topographic anatomic information. RESULTS: rCMRGlc values (in mumol/100 g/min) were higher in the cerebral cortex (33.5 +/- 2.98), the basal ganglia (32.6 +/- 3.04 in the nucleus caudatus and 40.2 +/- 3.50 in the putamen), the thalamus (36.6 +/- 4.72), and the cerebellum (29.8 +/- 2.20) and were lower in the cerebral white matter (12.3 +/- 1.45) than those reported previously with conventional scanners. This resulted in an increased ratio of cortical values to white-matter values. Various nuclei in the basal frontal lobe (21.4 +/- 3.19 in the basal forebrain and 32.3 +/- 2.39 in the nucleus accumbens), the temporal lobe (22.2 +/- 1.74 in the corpus amygdalae), the hippocampus (25.7 +/- 2.11), the diencephalon (23.1 +/- 3.33 in the corpus geniculatum laterale, 20.2 +/- 2.87 in the corpus geniculatum mediale, and 25.2 +/- 3.29 in the nucleus subthalamicus), and the brain stem (24.4 +/- 2.47 in the colliculus superior, 31.4 +/- 3.63 in the colliculus inferior, 31.0 +/- 3.10 in the nucleus ruber, and 22.8 +/- 2.35 in the substantia nigra) could be identified, and the metabolic rate was assessed in these structures. The effect of improved spatial resolution on quantified metabolic rates could directly be demonstrated in a few cases investigated on scanners of different generations. CONCLUSION: The improved spatial resolution of the HRRT decreased partial-volume effects in the quantification of metabolic rates in the brain and increased the accuracy of rCMRGlc values in large structures. For the first time, this scanner has permitted the determination of metabolic rates in small nuclei that are involved in various neurodegenerative disorders.

Hemodynamic changes after occlusion of the posterior superior sagittal sinus: an experimental PET study in cats.

BACKGROUND AND PURPOSE: Occlusion of the anterior third of superior sagittal sinus (SSS) is generally well tolerated because of sufficient collateral venous blood flow. In contrast, the pathophysiologic effects of occlusion of the SSS posterior to the rolandic vein remain controversial. We aimed to identify the specific hemodynamic effects of this subtype of SSS occlusion. METHODS: We ligated the SSS just behind rolandic vein and in the posterior part near the confluens sinus in three anesthetized cats. Regional cerebral blood flow (rCBF) was measured before and at 2 and 24 hours after the SSS occlusion. At around 48 hours, experimental settings were terminated with perfusion fixation with 4% paraformaldehyde solution. Hematoxylin-eosin histologic evaluation was performed. RESULTS: In all three cats with SSS occlusion, rCBF was reduced over the time period of measurement; this finding was observed in areas covering 5-20% of the brain in planes affected by the occlusion. The degree of rCBF reduction and the extension and severity of histologically proved venous infarction were correlated. CONCLUSION: To our knowledge, this is the first demonstration that occlusion of the SSS posterior to the rolandic vein is associated with a significant rCBF reduction to still-viable tissue in the related vascular territory at 24 hours after occlusion. We describe subacute venous infarction in an experimental occlusion of the SSS. Analogous to clinical conditions, occlusion of SSS alone without additional occlusion of bridging veins is adequate for producing a venous circulatory disturbance.

Hemodynamic and metabolic effects of decompressive hemicraniectomy in normal brain. An experimental PET-study in cats.

Hemicraniectomy is increasingly used as treatment option in stroke and in head trauma, but little is known on the (patho)physiological regional effects of hemicraniectomy in the normal brain. A standard left-sided craniectomy was performed in three cats. Regional cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO(2)) and cerebral metabolic rate of glucose (CMR(glc)) were measured from the brain tissue underneath the craniectomy at 2, 20 and 28 h after hemicraniectomy. CBF significantly decreased (P<0.01) and oxygen extraction fraction (OEF) (P<0.05) significantly increased. CMRO(2) and CMR(glc) decreased only in regions with most severe CBF reduction. These effects remained for at least a day irrespective of corrective sustaining cranioplasty. The authors demonstrated for the first time that decompressive hemicraniectomy in the cat decreases CBF, and to a lesser extent CMR02 and CMR(glc) 2 h after hemicraniectomy in normal brain tissue that last for at least 1 day. Even though the underlying basis of these phenomena are not fully understood, this finding implies that persisting pathophysiological processes are induced by hemicraniectomy and should be taken into consideration for surgical indications.

Improved herpes simplex virus type 1 amplicon vectors for proportional coexpression of positron emission tomography marker and therapeutic genes.

For the development of efficient and safe gene therapy protocols for clinical application it is desirable to determine the tissue dose of vector-mediated therapeutic gene expression noninvasively in vivo. The herpes simplex virus type 1 thymidine kinase gene (HSV-1-tk) has been shown to function as a marker gene for the direct noninvasive in vivo localization of thymidine kinase (TK) expression by positron emission tomography (PET). Using bicistronic or multicistronic gene-expressing cassettes with tk as the PET marker gene, the quantitative analysis of tk gene expression may indirectly indicate the distribution and the level of expression of linked and proportionally coexpressed genes. Here, we describe the construction and functional evaluation of HSV-1 amplicon vectors mediating proportional coexpression of HSV-1-tk as PET marker gene and the enhanced green fluorescent protein gene (gfp) as proof of principle and cell culture marker gene and the Escherichia coli cytosine deaminase (cd) as therapeutic gene. Several double-/triple-gene constructs expressing HSV-1-tk, gfp, and E. coli cd were engineered based on gene fusion or the use of an internal ribosome entry site (IRES). Functional analysis in cell culture (green fluorescent protein [GFP] fluorescence and sensitivity to the prodrugs ganciclovir [GCV] and 5-fluorocytosine [5-FC]) and Western blots were carried out after infection of proliferating rat 9L gliosarcoma and human Gli36 glioma cells with helper virus-free packaged HSV-1 amplicon vectors. To study the ability of PET to differentiate various levels of tk expression noninvasively in vivo, retrovirally transduced and selected populations of rat F98 and human Gli36dEGFR glioma cells with defined levels of proportionally coexpressed tk and gfp genes were grown as subcutaneous tumors in nude rats and nude mice, and tk imaging by PET was performed. To study HSV-1 amplicon vector-mediated gene coexpression in vivo, HSV-1 amplicon vectors bearing coexpression constructs were injected (4 x 10(7) to 1 x 10(8) transducing units) into subcutaneously growing Gli36dEGFR gliomas in nude animals, and tk imaging was performed 24 hr later. All vector constructs mediated GFP expression and sensitized 9L and Gli36 cells toward GCV- and 5-FC-mediated cell killing in a drug dose-dependent manner, respectively. The levels of gene expression varied depending on the location of the genes within the constructs indicating the influence of the IRES on the level of expression of the second gene. Moreover, functional proportional coexpression of the PET marker gene HSV-1-tk and the linked therapeutic E. coli cd gene was observed. In selected tumor cell populations, subtle IRES-dependent differences of tk gene expression could be noninvasively distinguished by PET with good correlation between quantitative assays for IRES-dependent attenuated GFP and TK expression in culture and in vivo. After infection of subcutaneously growing gliomas with HSV-1 amplicon vectors, various levels of TK expression were found ranging from 0.011-0.062 percentage injected dose per gram (%ID/g). These values were 4.0- to 5.7-fold lower than positive control tumor cells. TK expression could be imaged by PET in vivo even with the tk gene located at the weak position downstream from the IRES. In conclusion, these HSV-1 amplicon vectors carrying HSV-1-tk as PET marker gene and any linked therapeutic gene will serve an indirect noninvasive assessment of the distribution of therapeutic gene expression by PET. Monitoring the correlation between primary transduction and therapeutic efficiency of a given vector is highly desirable for the development of safe and efficient gene therapy and vector application protocols in clinical applications.

Performance evaluation of the microPET R4 PET scanner for rodents.

The microPET R4 scanner is a dedicated positron emission tomograph (PET) for studies of rodents. A number of scanner parameters such as spatial resolution, sensitivity, scatter, and count rate performance were determined in this work, which showed that the microPET R4 is a suitable PET scanner for small animals like mice and rats. In the center of the field of view (FOV) a maximal sensitivity of 43.66 cps/kBq for a centered point source was calculated from a measurement with a germanium-68 line source within an energy widow of 250-750 keV. A spatial resolution of 1.85 mm full-width at half-maximum (FWHM) in the axial direction and 1.66 mm FWHM in the transaxial direction was measured in the center with a 1-mm-diameter sodium-22 point source. Within the inner 20 mm of the FOV the volumetric resolution is better than 15.6 micro l, corresponding to a linear resolution of less than 2.5 mm in all three dimensions. Images of a high-resolution phantom and from mice and rat studies illustrate the good performance of the scanner. A maximal noise equivalent count rate (NECR) was reached at 174 kcps for a mouse phantom and at 93 kcps for a rat phantom (energy window 250-750 keV). Scatter fractions were measured between 0.30 and 0.42 for an energy window of 250-750 keV and phantom diameters similar to mice and rats. A comparison with the microPET P4 model for primates illustrates the gain in sensitivity due to a smaller detector ring diameter but also the changes in NECR.

Deep brain stimulation of the subthalamic nucleus does not increase the striatal dopamine concentration in parkinsonian humans.

Deep brain stimulation of the subthalamic nucleus (STN-DBS) has become an effective treatment option in advanced Parkinson's disease (PD). Recent animal studies showed an increase of neuronal firing in dopaminergic neurons under effective STN-DBS. Increased striatal dopamine levels may also contribute to the stimulation's mechanism of action in humans. We investigated the striatal dopamine release in 6 patients with advanced PD under effective bilateral STN-DBS with positron emission tomography (PET) of the reversible dopamine-D2/3-receptor ligand [(11)C]raclopride (RACLO). Although STN-DBS proved to be a highly effective treatment in these subjects, we found no significant difference of the striatal RACLO binding between the STN-DBS-on and -off condition. The changes of radioligand binding did not correlate with the patients' improvement in clinical rating scales or with the stimulation amplitudes. Therefore, our PET data in living parkinsonian humans do not provide evidence for an increased striatal dopamine concentration under effective STN-DBS. We conclude that the modulation of dopaminergic activity does not seem to play a crucial role for the stimulation's mechanisms of action in parkinsonian humans.

Measurement of glucose consumption using [(18)F]fluorodeoxyglucose.

The [(18)F]fluorodeoxyglucose (FDG) method to measure glucose metabolism quantitatively in humans is reviewed. The assumptions and the mathematical formulation of the underlying autoradiographic Sokoloff model and its adaptation to positron emission tomography (PET) are described. Various implementations to estimate glucose consumption from measured tissue activity with PET are presented. The dependence on the "lumped constant" and on the accuracy of the input function is discussed. Recommendations for the practical application of different procedures for performing FDG studies are given.