microPET Data Research Articles

Astrocyte glutamate uptake impacts [18F]FDG‐PET signal

AbstractBackgroundThe role of astrocytes on brain functioning has become an important topic of debate. In this regard, an issue that still remains controversial is the astrocytes contribution to glucose brain metabolism and, therefore, to the uptake of 2‐Deoxy‐2‐[18F]Fluoroglucose([18F]FDG) detected by positron emission tomography(PET). For many years, it was widely assumed that the brain [18F]FDG‐PET signal directly reflects the neuronal use of glucose, overlooking the possible participation of other brain cells. Recently, rodent and human studies have demonstrated that astrocytes may contribute to [18F]FDG‐PET signal. To further explore these early findings, here, we used a pharmacological challenge on rodents. Clozapine was selected because it is known to decrease, on astrocytes, glutamate transport and the glutamate transporter 1 (GLT‐1) levels, an important trigger for glucose uptake in astrocytes.MethodsClozapine was administered for six weeks (25‐35mg/kg) to adult Wistar rats. Glucose brain metabolism was assessed using [18F]FDG‐microPET before and after the treatment. [3H]D‐Aspartate uptake was evaluated on brain slices. [3H]Glutamate release was analyzed on synaptosomal preparations. [3H]D‐Aspartate and [3H]2‐Deoxyglucose uptake were analyzed on primary cortical astrocytic and neuronal cultures treated with clozapine (50uM) for 48h. Immunocontent and expression of GLT‐1 and Glutamate‐Aspartate Transporter(GLAST) were assessed on the cortical tissue and cell cultures.ResultsClozapine treatment significantly reduced [18F]FDG metabolism in the cerebral cortex, hippocampus, and striatum of adult rats. In addition, clozapine decreased cortical glutamate transport in brain slices – indexed by [3H]D‐Aspartate uptake – and reduced the cortical immunocontent and expression of GLT‐1. Astrocytic cultures treated with clozapine presented a decline on GLT‐1 density, [3H]D‐Aspartate uptake and [3H]2‐Deoxyglucose uptake, while the neuronal cultures presented no changes.ConclusionHere, we present microPET data showing that a challenge with clozapine causes a reduction on [18F]FDG signal in brain regions presenting high GLT‐1 density. Our cell culture findings indicate that astrocytes are, at least partially, the cells underlying the brain glucose metabolism response to clozapine. These results corroborate previous studies that highlight the need for a reevaluation in the way that brain [18F]FDG‐PET data is interpreted. Additionally, clozapine is a drug approved for clinical use, therefore, these findings could have high translational implications.

PET imaging of the influence of physiological and pathological α-synuclein on dopaminergic and serotonergic neurotransmission in mouse models.

Alpha-synuclein (α-syn) aggregation is a neuropathological hallmark of neurodegenerative synucleinopathies. This in vivo study explored glucose metabolism and dopaminergic and serotoninergic neurotransmission in KO α-syn, wild-type mice and an accelerated murine model of synucleinopathy (M83). MicroPET acquisitions were performed in all animals aged 5-6months using five radiotracers exploring brain glucose metabolism ([18 F]FDG), dopamine neurotransmission ([11 C]raclopride, [11 C]PE2I) and serotonin neurotransmission ([18 F]MPPF, [11 C]DASB). For all radiotracers, except [18 F]FDG, PET data were analyzed with a MRI-based VOI method and a voxel-based analysis. MicroPET data showed a decrease in [11 C]raclopride uptake in the caudate putamen of KO α-syn mice, in comparison with M83 and WT mice, reflecting a lower concentration of D2 receptors. The increase in [18 F]MPPF uptake in M83 vs WT and KO mice indicates overexpression of 5-HT1A receptors. The lack of change in dopamine and serotonin transporters in all groups suggests unchanged neuronal density. This PET study highlights an effect of α-syn modulation on the expression of the D2 receptor, whereas aggregated α-syn leads to overexpression of 5-HT1A receptor, as a pathophysiological signature.

Micro-positron emission tomography for measuring sub-core scale single and multiphase transport parameters in porous media

Accurate descriptions of heterogeneity in porous media are important for understanding and modeling single phase (e.g. contaminant transport, saltwater intrusion) and multiphase (e.g. geologic carbon storage, enhanced oil recovery) transport problems. Application of medical imaging to experimentally quantify these processes has led to significant progress in material characterization and understanding fluid transport behavior at laboratory scales. While widely utilized in cancer diagnosis and management, cardiology, and neurology, positron emission tomography (PET) has had relatively limited applications in earth science. This study utilizes a small-bore micro-PET scanner to image and quantify the transport behavior of pulses of a conservative aqueous radiotracer injected during single and multiphase flow experiments in two heterogeneous Berea sandstone cores. The cores are discretized into axial-parallel streamtubes, and using the reconstructed micro-PET data, expressions are derived from spatial moment analysis for calculating sub-core tracer flux and pore water velocity. Using the flux and velocity measurements, it is possible to calculate porosity and saturation from volumetric flux balance, and calculate permeability and water relative permeability from Darcy’s law. Second spatial moment analysis enables measurement of sub-core solute dispersion during both single phase and multiphase experiments. A numerical simulation model is developed to verify the assumptions of the streamtube dimension reduction technique. A variation of the reactor ratio is presented as a diagnostic metric to efficiently determine the validity of the streamtube approximation in core and column-scale experiments. This study introduces a new method to quantify sub-core permeability, relative permeability, and dispersion. These experimental and analytical methods provide a foundation for future work on experimental measurements of differences in transport behavior across scales.

Upregulated Sphingosine 1-Phosphate Receptor 1 Expression in Human and Murine Atherosclerotic Plaques.

Dysregulation of sphingosine 1-phosphate receptor 1 (S1PR1) signaling contributes to inflammation-related pathophysiological changes in cardiovascular diseases including atherosclerosis (AS). S1PR1-targeting compounds significantly reduce lesion size in murine models of AS. Therefore, characterization of S1PR1 expression in vitro and in vivo in atherosclerotic plaque could enable mechanistic studies and inform S1PR1 targeted therapies. H&E staining and immunostaining studies were performed on variably diseased human femoral endarterectomy plaque specimens, as well as mouse aortic sections from ApoE-/- mice maintained on a high-fat diet (AS mice). In vitro autoradiography study in human femoral plaques was used to confirm the tracer specificity. Micro positron emission tomography (PET) and ex vivo autoradiography studies were conducted in AS mice and their controls using a S1PR1-specific radioligand [11C]TZ3321 for in vivo and ex vivo quantification of S1PR1 expression in mouse aortic plaques. Increased S1PR1 expression was observed in areas of human femoral endarterectomy plaque specimens with foam cell accumulation compared with control tissue; in vitro autoradiography study indicated that SEW2781, a S1PR1 compound was able to reduce the uptake of [11C]TZ3321 by 56%. S1PR1 levels were also upregulated in AS mouse aortic plaques. MicroPET data showed the aorta-to-blood tracer uptake ratio in AS mice was approximately 20% higher than that in controls. Autoradiographic study also revealed elevated tracer accumulation in AS mouse aorta. Upregulated S1PR1 expression in human and mouse atherosclerotic plaques was successfully identified by immunostaining and radioligand-based methods. This data demonstrates that [11C]TZ3321 PET provides great promise in imaging S1PR1 expression in atherosclerotic plaques.

A three-time-point method for assessing kinetic parameters of 64Cu-labeled Ramucirumab trapping in VEGFR-2 positive lung tumors

ObjectiveTo describe a three-time-point method for estimating kinetic parameters involved in 64Cu-labeled Ramucirumab (64Cu-NOTA-RamAb) trapping of VEGFR-2 positive lung tumors. Materials and methodsPositron emission tomography (microPET) data of tumor-bearing mice for 64Cu-NOTA-RamAb trapping in VEGFR-2 positive HCC4006 tumor were used, involving tissue activity measurements acquired at 3, 24 and 48 h post-injection, without and with administration of RamAb blocking dose. A kinetic model provided an analytical formula describing the tissue time-activity-curve, involving 64Cu-NOTA-RamAb uptake (Ki), release rate constant (kR) and fraction of free tracer in blood and interstitial volume (F). ResultsFitting analytical formula outcomes on mean microPET data yielded values of the kinetic parameters: Ki = 0.0314/0.0123 gram of blood per hour per gram of tissue, kR = 0.0387/0.0313 h−1 and F = 0.2075/0.2007 gram of blood per gram of tissue, without/with RamAb blocking dose, respectively (R = 0.99999 for the graph displaying microPET versus theoretical data; P < .01). ConclusionsThree independent kinetic parameters (Ki, kR and F) can be assessed from three data points acquired at early, mid and late imaging, i.e., at 3, 24 and 48 h post-injection, for further characterization of 64Cu-NOTA-RamAb trapping in VEGFR-2 positive lung tumors.

PET imaging of TSPO in a rat model of local neuroinflammation induced by intracerebral injection of lipopolysaccharide

ObjectiveThe goal of this study was to measure functional and structural aspects of local neuroinflammation induced by intracerebral injection of lipopolysaccharide (LPS) in rats using TSPO microPET imaging with [18F]DPA-714, magnetic resonance imaging (MRI), in vitro autoradiography and immunohistochemistry (IHC) in order to characterize a small animal model for screening of new PET tracers targeting neuroinflammation. MethodsRats were injected stereotactically with LPS (50μg) in the right striatum and with saline in the left striatum. [18F]DPA-714 microPET, MRI, in vitro autoradiography and IHC studies were performed at different time points after LPS injection for 1month. ResultsAnalysis of the microPET data demonstrated high uptake of the tracer in the LPS injected site with an affected-to-non-affected side-binding potential ratio (BPright-to-left) of 3.0 at 3days after LPS injection. This BP ratio decreased gradually over time to 0.9 at 30days after LPS injection. In vitro autoradiography ([18F]DPA-714) and IHC (CD68, GFAP and TSPO) confirmed local neuroinflammation in this model. Dynamic contrast enhanced (DCE) MRI demonstrated BBB breakdown near the LPS injection site at day 1, which gradually resolved over time and was absent at 1month after LPS injection. ConclusionThe LPS model is useful for first screening of newly developed tracers because of the easy design and the robust, unilateral inflammatory reaction allowing the use of the contralateral region as control. Additionally, this model can be used to test and follow up the benefits of anti-inflammatory therapies by non-invasive imaging.

PIXSIC, a pixelated β⁺-sensitive probe for radiopharmacological investigations in rat brain: binding studies with [¹⁸F]MPPF.

The aim of this work was to demonstrate the pharmacokinetic potential of a wireless pixelated β(+)-sensitive probe (PIXSIC). The binding of 2'-methoxyphenyl-(N-2'-pyridinyl)-p-[(18)F]fluoro-benzamidoethylpiperazine ([(18)F]MPPF), a 5-HT1A serotonin receptor radiopharmaceutical, was measured in anesthetized rats and compared to microPET data. The effects of a 5-HT1A antagonist injection on in vivo [(18)F]MPPF binding were monitored by PIXSIC. PIXSIC allowed differentiating the radioactive kinetics according to the location of its pixels in the hippocampus, cortex, corpus callosum, and cerebellum. The device accurately detected the changes in [(18)F]MPPF binding, after 5-HT1A antagonist blockade. The time-activity curves were reproducible and consistent with kinetics obtained simultaneously with a microPET camera. These results demonstrate the ability of the PIXSIC device to record reliably the binding of PET ligands, with a high spatiotemporal resolution in anesthetized rodents. These first in vivo results are a key stage on the path to its implementation in awake freely moving animals.

Semi-automatic segmentation of subcutaneous tumours from micro-computed tomography images

This paper outlines the first attempt to segment the boundary of preclinical subcutaneous tumours, which are frequently used in cancer research, from micro-computed tomography (microCT) image data. MicroCT images provide low tissue contrast, and the tumour-to-muscle interface is hard to determine, however faint features exist which enable the boundary to be located. These are used as the basis of our semi-automatic segmentation algorithm. Local phase feature detection is used to highlight the faint boundary features, and a level set-based active contour is used to generate smooth contours that fit the sparse boundary features. The algorithm is validated against manually drawn contours and micro-positron emission tomography (microPET) images. When compared against manual expert segmentations, it was consistently able to segment at least 70% of the tumour region (n = 39) in both easy and difficult cases, and over a broad range of tumour volumes. When compared against tumour microPET data, it was able to capture over 80% of the functional microPET volume. Based on these results, we demonstrate the feasibility of subcutaneous tumour segmentation from microCT image data without the assistance of exogenous contrast agents. Our approach is a proof-of-concept that can be used as the foundation for further research, and to facilitate this, the code is open-source and available from www.setuvo.com.

Combined Micro-PET/Micro-CT Imaging of Lung Tumours in SPC-raf and SPC-myc Transgenic Mice

IntroductionSPC-raf and SPC-myc transgenic mice develop disseminated and circumscribed lung adenocarcinoma respectively, allowing for assessment of carcinogenesis and treatment strategies. The purpose of this study was to investigate the technical feasibility, the correlation of initial findings to histology and the administered radiation dose of combined micro-PET/micro-CT in these animal models.Material and Methods14 C57BL/6 mice (4 nontransgenic, 4 SPC-raf transgenic, 6 SPC-myc transgenic) were examined using micro-CT and 18F-Fluoro-deoxyglucose micro-PET in-vivo. Micro-PET data was corrected for random events and scatter prior to reconstruction with a 3D-FORE/2D-OSEM iterative algorithm. Rigid micro-PET/micro-CT registration was performed. Tumour-to-non-tumour ratios were calculated for different lung regions and focal lesions. Diffuse tumour growth was quantified using a semiautomated micro-CT segmentation routine reported earlier. Regional histologic tumour load was assessed using a 4-point rating scale. Gamma radiation dose was determined using thermoluminescence dosimeters.ResultsMicro-CT allowed visualisation of diffuse and circumscribed tumours in SPC-raf and SPC-myc transgenic animals along with morphology, while micro-PET provided information on metabolism, but lacked morphologic detail. Mean tumour-to-non-tumour ratio was 2.47 for circumscribed lesions. No significant correlation could be shown between histological tumour load and tumour-to-nontumour ratio for diffuse tumours in SPC-raf transgenic animals. Calculation of the expected dose based on gamma dosimetry yielded approximately 140 mGy/micro-PET examination additional to approximately 200 mGy due to micro-CT.ConclusionsCombined micro-PET/micro-CT imaging allows for in-vivo assessment of lung tumours in SPC-raf and SPC-myc transgenic mice. The technique has potential for the evaluation of carcinogenesis and treatment strategies in circumscribed lung tumours.

64Cu-Labeled Lissamine Rhodamine B: A Promising PET Radiotracer Targeting Tumor Mitochondria

Enhanced mitochondrial potential in carcinoma cells is an important characteristic of cancer. It is of great current interest to develop a radiotracer that is sensitive to mitochondrial potential changes at the early stage of tumor growth. In this report, we present the synthesis and evaluation of (64)Cu-labeled Lissamine rhodamine B (LRB), (64)Cu(DOTA-LRB) (DOTA-LRB = 2-(6-(diethylamino)-3-(diethyliminio)-3H-xanthen-9-yl)-5-(N-(2-(2-(4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclo-dodecan-1-yl)acetamido)ethyl)sulfamoyl)benzenesulfonate) as a new radiotracer for imaging tumors in athymic nude mice bearing U87MG human glioma xenografts by positron emission tomography (PET). We also explored its localization mechanism using Cu(DOTA-LRB) as the fluorescent probe in both the U87MG human glioma cell line and the cultured primary U87MG glioma cells. It was found that (64)Cu(DOTA-LRB) had the highest tumor uptake (6.54 ± 1.50, 6.91 ± 1.26, 5.68 ± 1.13, 7.58 ± 1.96, and 5.14 ± 1.50%ID/g at 0.5, 1, 2, 4, and 24 h postinjection, respectively) among many (64)Cu-labeled organic cations evaluated in the same animal model. The cellular staining study indicated that Cu(DOTA-LRB) was able to localize in mitochondria of U87MG glioma cells due to the enhanced negative mitochondrial potential. This statement is completely supported by the results from decoupling experiment with carbonylcyanide-m-chlorophenylhydrazone (CCCP). MicroPET data showed that the U87MG glioma tumors were clearly visualized as early as 30 min postinjection with (64)Cu(DOTA-LRB). (64)Cu(DOTA-LRB) remained stable during renal excretion, but underwent extensive degradation during hepatobiliary excretion. On the basis of the results from this study, it was concluded that (64)Cu(DOTA-LRB) represents a new class of promising PET radiotracers for noninvasive imaging of the MDR-negative tumors.

In vivo molecular imaging of the GABA/benzodiazepine receptor complex in the aged rat brain

The GABA-ergic system, known to regulate neural tissue genesis during cortical development, has been postulated to play a role in cerebral aging processes. Using in vivo molecular imaging and voxel-wise quantification, we aimed to assess the effects of aging on the benzodiazepine (BDZ) recognition site of the GABAA receptor. To visualize BDZ site availability, [11C]-flumazenil microPET acquisitions were conducted in young and old rats. The data were analyzed and region of interest analyses were applied to validate the voxel-wise approach. We observed decreased [11C]-flumazenil binding in the aged rat brains in comparison with the young control group. More specifically, clusters of reduced radioligand uptake were detected in the bilateral hippocampus, cerebellum, midbrain, and bilateral frontal and parieto-occipital cortex. Our results support the pertinence of voxel-wise quantification in the analysis of microPET data. Moreover, these findings indicate that the aging process involves declines in neural BDZ recognition site availability, proposed to reflect alterations in GABAA receptor subunit polypeptide expression.

MicroPET imaging of noxious thermal stimuli in the conscious rat brain

Small animal positron emission tomography (microPET) has been utilized in the investigation of nociception. However, a possible drawback from previous studies is the reduced activation pattern due to the application of anesthesia. The purpose of the present study was to demonstrate a potential means of avoiding anesthesia during stimulation, as well as minimizing the confounding anesthetic effect. Sodium pentobarbital and ketamine were first evaluated to determine their effect on microPET images in the current study. [18F]-Fluorodeoxyglucose (18F-FDG) was an appropriate radiotracer to reveal activated regions in rat brains. Pentobarbital anesthesia significantly reduced 18F-FDG uptake in neural tissues, blurrier to lower contrast; therefore, ketamine was used to anesthetize animals during microPET. After the rats were anesthetized and secured in a laboratory-made stereotaxic frame, a simple, noninvasive stereotaxic technique was used to position their heads in the microPET scanner and to roughly conform the images in the stereotaxic atlas. For functional imaging, conscious rats were restrained in cages with minimal ambient noise; short repetitive thermal stimuli were applied to each rat's tail subsequently. The rats were adequately anesthetized with ketamine following 30 min of scanning without stimulation. An activation index (AI) was calculated from microPET data to quantify the local metabolic activity changes according to the normalized 18F-FDG dosage. The average AI indicated a side-to-side difference for all innocuous stimulations in the thalamus. However, such side-to-side difference was only observed for noxious heat and cold stimulations in primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and agranular insular cortex (AIC). The present study demonstrated the feasibility of the microPET technique to image metabolic functions of the conscious rat brain, offering better rationale and protocol designs for future pain studies.

TH‐C‐204B‐04: Assessing the Potential of Cerenkov Radiation for in Vivo Imaging of Tumor Hypoxia

Purpose: To assess whether in vivo optical imaging using Cerenkov radiation can (a) produce results that are comparable to those from microPET tumor imaging and (b) provide information that facilitates future dual‐tracer imaging experiments. Method and Materials: (a) Subcutaneous tumors were established by injecting HT‐29 human colon adenocarcinoma cells into the shoulders of 4 nude mice and were grown until they reached a volume of 1cm3. 200microCi of 18F‐EF5 was injected via intraperitoneal injection and 3 mice were imaged using microPET whilst the other was imaged in an IVIS Spectrum. Images were acquired immediately and 1 hour following injection on both machines using acquisition times of 10mins (PET) and 2mins (IVIS). (b) A mathematical model of the Cerenkov emission spectrum was developed and used to predict the ability of Cerenkov imaging to resolve signals from different tracers. Results: (a) In the initial PET and IVIS images the tracer signal was confined to the region of the gastrointestinal region and there was no accumulation in the tumor. After 1 hour both sets of images showed similar biodistribution throughout the body along with uptake in the tumor. (b) The model predicted that different radionuclides can be distinguished through an energy‐dependent red‐shift in their emission spectra leading to higher intensity signals in the optical spectrum. Conclusion: We have shown that 18F‐EF5 Cerenkov imaging of subcutaneous tumors can be performed optically. More specifically our results show that the biodistribution and tumor uptake observed using this technique is in agreement with the microPET data. Our modeling suggests that the spectral differences between radionuclides coupled with the differences in decay rates between tracers labeled with different radioisotopes could be used to develop techniques for separating signals from two tracers for surface‐level tumors thus potentially allowing multiple aspects of tumor biology to be imaged simultaneously.

Small animal tumour imaging with MRI and the ECAT EXACT scanner: application of partial volume correction and comparison with microPET data

Partial volume effects caused by limited spatial resolution of conventional positron emission tomography (PET) scanners result in an underestimation of the activity concentration in small tumours. The aim of the study was to evaluate the feasibility of small animal tumour imaging with the clinical PET scanner ECAT EXACT after partial volume correction based on MRI calculations. The same tumour model was examined additionally with the small animal PET system, microPET focus 120. Before the ECAT EXACT studies recovery coefficients for different sphere volumes were generated with phantom experiments. For the following in-vivo study DS-sarcoma cells were implanted on both hind foot dorsum of male Sprague-Dawley rats. In-vivo tumour volume calculations were done with the high-resolution MRI system, Magnetom Vision Experimental. Dynamic F-fluorodeoxyglucose (FDG) PET was performed with the scanner ECAT EXACT (5 MBq intravenous, two-dimensional mode, n = 16 tumours) or with the microPET focus 120 (20 MBq intravenous, two-dimensional mode, n = 10 tumours). The animals were then killed, the tumours rapidly explanted, weighed and homogenized. The concentration of F-FDG was measured with a gamma counter and decay corrected; the ex-vivo F-FDG concentration was compared with the mean tumour activity concentration of the PET data. Using the ECAT EXACT mean underestimation of actual tumour F-FDG concentration was 35.4%, for partial volume-corrected data this error decreased to 1.7%. In addition, after partial volume correction congruence and linear correlation between the regions of interest-based activity concentration and ex-vivo measurements were excellent (r = 0.98). These results were quite similar to the microPET experiments without partial volume correction: r = 0.99. These data indicate that partial volume correction might allow use of the clinical PET system, ECAT EXACT, for the metabolic assessment of small animal tumours >/=10 mm with sufficient accuracy if no dedicated animal PET is available.

Insulin resistance as a mechanism of preserved myocardial function in heart failure animals fed a high fat diet

Heart failure (HF) rats fed a high saturated fat diet (SAT) have improved left ventricular contractile function compared to HF animals fed normal chow (NC) diets. Both HF and SAT are individually associated with the development of insulin resistance (IR), but the role of IR in HF+SAT is unknown. We hypothesize that the cardioprotective effect seen in HF+SAT results from defects in insulin signaling that decrease glucose uptake in IR myocardium. Rats underwent coronary ligation or sham (SH) surgery &amp; were fed NC or SAT for 8 weeks. Cardiac contractility (peak positive dP/dt) was preserved in HF+SAT animals versus HF+NC. SH+SAT, HF+NC &amp; HF+SAT animals have abnormal glucose tolerance tests &amp; elevated insulin levels compared to SH+NC, indicating peripheral IR. MicroPET data shows HF+SAT animals have myocardial IR relative to all other groups. Protein targets of insulin signaling (IRS‐1, Akt, pAkt, GLUT4) are currently being analyzed. HF+SAT animals are IR in both peripheral tissue &amp; the myocardium, while SH+SAT &amp; HF+NC animals are IR in the periphery only. This correlates to preserved contractile function in HF+SAT versus HF+NC. We predict that insulin signaling targets such as pAkt and GLUT4 are downregulated, which may be the mechanism by which myocardial insulin resistance preserves contractile function in HF+SAT animals. (Grant # NHLBI HL‐081857)

1127 IS THERE NEED TO A NEW HEPATITIS B VACCINE SCHEDULE IN CHILDREN WITH CELIAC DISEASE?

Material and Methods: We complexed Cu (MS Nordion) to L-histidine (Sigma) and verified the extent of complexing with chromatography. 3.7–5.5 MBq of complexed Cu-histidine was injected into healthy Long-Evans Agouti (LEA) rats, as well as mutant Long-Evans Cinnamon (LEC) rats lacking Atp7b. Radioactivity was measured in bile, blood and organ tissues after times ranging from 1h to 24h. Furthermore, rats were subjected to dynamic microPET imaging for up to 24h. The effect of acute or chronic liver injury on handling of Cu-histidine complex was determined in LEA rats treated once or repeatedly with CCl4. To identify further mechanisms in targeting of Cu, we studied Cu complexed to hepatocyte-specific ligands. Finally, dynamic microPET imaging was performed in LEC rats after transplantation of healthy hepatocytes isolated from LEA rats. Results: Cu was efficiently complexed to histidine. The complex was incorporated largely in the liver and kidney of animals followed by rapid excretion of radiocopper in bile of healthy LEA rats and not in diseased LEC rats. MicroPET data showed a mean time to peak liver activity of 13±2 min in LEA rats, but in LEC rats hepatic SUV (standard uptake value) increased over the entire period of investigation. Dynamic microPET imaging for 30–60 min was sufficient to show divergent patterns of Cu accumulation in WD (+ 35%) and loss of Cu in the healthy state (−10%). In CCl4-treated LEA rats, the kinetics or magnitude of hepatic copper excretion were unchanged or significantly different from LEC rats, respectively (p < 0.05). Enhanced hepatic targeting of Cu complexed with ligands to asialoglycoprotein receptor showed greater fractional biliary copper clearances from the liver (p< 0.05). Moreover, the assay demonstrated correction of Atp7b deficiency in LEC rats repopulated with healthy hepatocytes from LEA rats. Conclusion: Hepatic targeting of Cu will be effective for identifying abnormality in biliary copper excretion. This assay will facilitate management of Wilson’s disease.

MicroPET study of brain neuronal metabolism under electrical and mechanical stimulation of the rat tail

Small-animal positron emission tomography (microPET) has been widely used for measuring various molecular processes in the rodent brain. The somatotopic projection, however, has not been identified earlier using microPET under electrical stimulation (ES) and mechanical stimulation (MS). This study aimed to utilize microPET to investigate the glucose metabolism of cortical and thalamic responses to ES and MS of the rat tail. The rats were anesthetized by ketamine and a custom-built stereotaxic frame was used to fix the rat head to ensure that the scanned images were concordant with an atlas. [F]-fluorodeoxyglucose (FDG) was used as a radiotracer to reveal the brain metabolic changes. An activation index (AI) was calculated from microPET data o quantify the changes in local metabolic activities normalized to variations in FDG dosage between animals. The results showed that ES increased FDG uptake in both the contralateral thalamus (AI=18) and cortex (AI=12.5), with significant side-to-side differences (P<0.05, paired t-test). MS also significantly increased FDG uptake in both cortical and thalamic regions, although lateralization was absent in the thalamus. This study indicated that microPET can be used to elucidate the functional and quantitative neuronal activities of brain structures of rodents under peripheral stimulation, and could be applied in investigations of brain sensory functions.

Measuring Regional Changes in the Diastolic Deformation of the Left Ventricle of SHR Rats Using microPET Technology and Hyperelastic Warping

The objective of this research was to assess applicability of a technique known as hyperelastic warping for the measurement of local strains in the left ventricle (LV) directly from microPET image data sets. The technique uses differences in image intensities between template (reference) and target (loaded) image data sets to generate a body force that deforms a finite element (FE) representation of the template so that it registers with the target images. For validation, the template image was defined as the end-systolic microPET image data set from a Wistar Kyoto (WKY) rat. The target image was created by mapping the template image using the deformation results obtained from a FE model of diastolic filling. Regression analysis revealed highly significant correlations between the simulated forward FE solution and image derived warping predictions for fiber stretch (R (2) = 0.96), circumferential strain (R (2) = 0.96), radial strain (R (2) = 0.93), and longitudinal strain (R (2) = 0.76) (p < 0.001 for all cases). The technology was applied to microPET image data of two spontaneously hypertensive rats (SHR) and a WKY control. Regional analysis revealed that, the lateral freewall in the SHR subjects showed the greatest deformation compared with the other wall segments. This work indicates that warping can accurately predict the strain distributions during diastole from the analysis of microPET data sets.

MicroPET Detection of Regional Brain Activation Induced by Colonic Distention in a Rat Model of Visceral Hypersensitivity

Using microPET and (18)F-fluorodeoxyglucose ((18)F-FDG) as a tracer, we investigated regional brain activation in a rat model of visceral hypersensitivity induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS). TNBS injection into the proximal colon through laparotomy resulted in a significant, sustained decrease in the pain threshold to mechanical distention of the distal colon, indicating a phenomenon referred to as visceral hypersensitivity. When TNBS-induced colonic hypersensitivity was fully developed, all the TNBS-treated rats presented characteristic pain behaviors in response to colonic distention at previously innocuous pressure (0-35 mmHg) that produced no abdominal pain in sham-operated control animals. In microPET study, colonic distention at the normally non-painful pressure produced significant increases in (18)F-FDG uptake in the thalamus and sensory cortex I of TNBS-treated rats. Since the increases in (18)F-FDG uptake in the brain regions were completely abolished by an analgesic dose of morphine (375 microg/kg, s.c.), it is most likely that the regional brain activation detected by microPET is a pain-related central event. The pharmacological and microPET data indicate that colonic distention at the normally non-painful pressure activates specific brain regions in rats with TNBS-induced visceral hypersensitivity, and the microPET protocol described here could provide an objective measure to test visceral analgesic compounds.

The application of maximum likelihood factor analysis (MLFA) with uniqueness constraints on dynamic cardiac microPET data

A maximum likelihood framework for factor analysis (MLFA) of dynamic images was developed. This framework allows for the introduction of weighting factors to account for the differences in signal-to-noise ratio and frame durations in the dynamic images. An efficient iterative algorithm was developed to solve the maximum likelihood problem with non-negativity constraint embedded inside the iteration loop. To address the non-uniqueness issue, blood samples as well as two different constraint strategies were employed to resolve ambiguities in the time activity curves (TAC) and factor images. Four weighting mechanisms were implemented and the algorithms were thoroughly tested using simulation studies at different signal-to-noise ratios and with different kinetic parameter realizations. In addition, rodent dynamic cardiac FDG microPET datasets were used for further validation of our algorithms. It was demonstrated that with appropriate constraints, our MLFA approach is capable of generating accurate blood input function and pure tissue TACs. The MLFA algorithm with an accompanying graphical user interface (GUI) is available at http://www.chempet.wustl.edu/faculty/shoghik.