Small Animal Positron Emission Tomography Imaging Research Articles

Increased brain metabolism after acute administration of the synthetic cannabinoid HU210: A small animal PET imaging study with 18F-FDG

Cannabis use has been shown to alter brain metabolism in both rat models and humans although the observations between both species are conflicting. In the present study, we examined the short term effects of a single-dose injection of the synthetic cannabinoid agonist HU210 on glucose metabolism in the rat brain using small animal (18)F-2-fluoro-deoxyglucose (FDG) Positron Emission Tomography (PET) 15 min (Day 1) and 24h (Day 2) post-injection of the agonist in the same animal. Young adult male Wistar rats received an intra-peritoneal injection of HU210 (100 μg/kg, n=7) or vehicle (n=5) on Day 1. Approximately 1mCi of (18)F-FDG was injected intravenously into each animal at 15 min (Day 1) and 24h (Day 2) post-injection of HU210. A 5-min Computer Tomography (CT) scan followed by a 20-min PET scan was performed 40 min after each (18)F-FDG injection. Standardised Uptake Values (SUVs) were calculated for 10 brain regions of interest (ROIs). Global increased SUVs in the whole brain, hence global brain metabolism, were observed following HU210 treatment on Day 1 compared to the controls (21%, P<0.0001), but not in individual brain regions. On Day 2, however, no statistically significant differences were observed between the treated and control groups. At the 24h time point (Day 2), SUVs in the HU210 treated group returned to control levels (21-30% decrease compared to Day 1), in all ROIs investigated (P<0.0001). In the control group, SUVs did not differ between the two acquisition days in all brain regions. The present results suggest that high-dose HU210 increases brain glucose metabolism in the rat brain shortly after administration, in line with normalised human in vivo studies, an effect that was no longer apparent 24 h later.

Evaluation of Attenuation and Scatter Correction Requirements as a Function of Object Size in Small Animal PET Imaging

In human emission tomography, an additional transmission scan (x-ray CT or external gamma-source) is often required to obtain accurate attenuation maps for attenuation correction (AC) and scatter correction (SC). These transmission-based correction methods have been translated to small animal imaging although the impact of photon interactions on the mouse/rat-reconstructed images is substantially less than that in human imaging. Considering the additional complexity in design and cost of these systems, the necessity of these correction methods is questionable for small animal emission tomography. In this study, we evaluate the requirement of these corrections for small animal positron emission tomography (PET) through Monte Carlo simulations of the Inveon PET scanner using various sizes of MOBY voxelized phantoms. The 3D sinogram data obtained from simulations were reconstructed in 6 different conditions: Accurate AC+SC, simple (water) AC+SC, accurate AC only, simple AC only, SC only and no correction (NC). Mean error% for 8 different ROIs and 6 different MOBY sizes were obtained against the accurate scatter and attenuation corrections (first on the list). In addition to simulations, real mouse data obtained from an Inveon PET scanner were analyzed using similar methods. Results from both simulation and real mouse data showed that attenuation correction based on solely emission data should be sufficient for imaging animals smaller than 4 cm diameter. For larger sizes, a scatter correction employing an additional transmission scan can also be included depending on the objective of the study.

Design and Characteristics of an Integrated Multichannel Ramp ADC Using Digital DLL Techniques for Small Animal PET Imaging

This paper presents a novel design of an integrated 12-bit multi-channel single-slope ramp analog-to-digital converter (ADC) for a small animal positron emission tomography(PET) imaging system. The proposed ADC is a part of a monolithic front-end readout application-specific integrated circuit(ASIC) which is dedicated to the detector modules consisting of LYSO scintillation crystals read out on both sides by the multi-channel plate (MCP) photodetectors. The function of the ADC is to digitize the voltage signals from a large number of readout channels. Digital delay-locked loop (DLL) techniques are proposed to realize time interpolations in order to reduce the conversion time and to enhance the resolution. Both high precision and low power are obtained. An eight-channel prototype chip is implemented in AMS 0.35 μm CMOS technology. The available resolution of the ADC is 9 ~ 12 bits. The maximum DNL and INL of the fine conversion in the ADC is ±0.75 LSB and ±0.5 LSB, respectively. The static power consumption of the ADC is 3 mW + 0.2 mW/Channel. This ADC architecture provides a possibility to integrate low-noise front-end readout circuits, time-to-digital converters and ADC together into a monolithic ASIC and to output both the energy quantity and the time information with digital representations for PET imaging systems.

Radiopharmaceutical Chemistry with Iodine-124: A Non-Standard Radiohalogen for Positron Emission Tomography

Positron emission tomography (PET) is a powerful molecular imaging technology with the ability to image and monitor molecular events in vivo and in real time. With the increased application of PET radiopharmaceuticals for imaging physiological and pathological processes in vivo, there is a demand for versatile positron emitters with longer physical and biological half-lives. Traditional PET radionuclides, such as carbon-11 ((11)C) and fluorine-18 ((18)F), have relatively short half-lives (20 min and 110 min, respectively). Among the currently available positron emitters, the non-standard radiohalogen iodine-124 ((124)I) has the longest physical half-life at 4.2 d. This, combined with the well characterized radiochemistry of radioiodine, is contributing to the increasing utility of (124)I in investigating slow and complex pharmacokinetic processes in clinical nuclear medicine and small animal PET imaging studies. This review will summarize the progress to date on the potential of (124)I as a positron emitting nuclide for molecular imaging purposes, beginning with the production of (124)I. Particular emphasis will be placed on the basic radiochemistry as it applies to the production of various (124)I-labeled compounds, from small molecules, to biomolecules such as peptides and proteins, and finally to macromolecules like nanoparticles. The review will conclude by highlighting promising future directions in using (124)I as a positron emitter in PET radiochemistry and molecular imaging.

Preclinical Evaluation of 68 Ga-DOTA-Minigastrin for the Detection of Cholecystokinin-2/Gastrin Receptor-Positive Tumors

In comparison to somatostatin receptor scintigraphy, gastrin receptor scintigraphy using 111In-DTPA-minigastrin (MG0) showed added value in diagnosing neuroendocrine tumors. We investigated whether the 68Ga-labeled gastrin analogue DOTA-MG0 is suited for positron emission tomography (PET), which could improve image quality. Targeting of cholecystokinin-2 (CCK2)/gastrin receptor-positive tumor cells with DOTA-MG0 labeled with either 111In or 68Ga in vitro was investigated using the AR42J rat tumor cell line. Biodistribution was examined in BALB/c nude mice with a subcutaneous AR42J tumor. In vivo PET imaging was performed using a preclinical PET-computed tomographic scanner. DOTA-MG0 showed high receptor affinity in vitro. Biodistribution studies revealed high tumor uptake of 68Ga-DOTA-MG0: 4.4 ± 1.3 %ID/g at 1 hour postinjection. Coadministration of an excess unlabeled peptide blocked the tumor uptake (0.7 ± 0.1 %ID/g), indicating CCK2/gastrin receptor-mediated uptake (p = .0005). The biodistribution of 68Ga-DOTA-MG0 was similar to that of 111In-DOTA-MG0. Subcutaneous and intraperitoneal tumors were clearly visualized by small-animal PET imaging with 5 MBq 68Ga-DOTA-MG0. 111In- and 68Ga-labeled DOTA-MG0 specifically accumulate in CCK2/gastrin receptor-positive AR42J tumors with similar biodistribution apart from the kidneys. AR42J tumors were clearly visualized by microPET. Therefore, 68Ga-DOTA-MG0 is a promising tracer for PET imaging of CCK2/gastrin receptor-positive tumors in humans.

Evaluation of 64Cu Labeled GX1: A Phage Display Peptide Probe for PET Imaging of Tumor Vasculature

Molecular imaging using positron emission tomography (PET) radiotracers targeted to tumor vasculature offers a noninvasive method for early detection of tumor angiogenesis and efficient monitoring of response to anti-tumor vasculature therapy. The previous in vitro results demonstrated that the GX1 peptide, identified by phage display technology, is a tumor vasculature endothelium-specific ligand. In this study, we evaluated a ⁶⁴Cu-labeled GX1 peptide as a potential radiotracer for microPET imaging of tumor vasculature in a U87MG tumor xenografted mouse model. Macrocyclic chelating agent 1,4,7,10-tetraazacyclododecane-N, N', N'', N'''-tetraacetic acid (DOTA)-conjugated GX1 peptide was synthesized and radiolabeled with ⁶⁴Cu (t(1/2) = 12.7 h) in ammonium acetate buffer. The ⁶⁴Cu-labeled GX1 peptide was then subjected to in vitro tumor cell uptake study, small animal PET and direct tissue sampling biodistribution studies in a U87MG tumor xenografted mouse model. The in vitro experiment demonstrated that ⁶⁴Cu-DOTA-GX1 is stable in PBS with more than 91% of ⁶⁴Cu-DOTA-GX1 peptide remaining intact after 24 h of incubation. Cellular uptake and retention studies revealed (64)Cu-DOTA-GX1 binds to U87MG glioma cells and has good tumor cell retention. For small animal PET imaging studies, the U87MG tumors were all clearly visible with high contrast to contralateral background at all measured time points after injection of ⁶⁴Cu-DOTA-GX1 while high accumulation in liver and kidneys were also observed at early time points. The U87MG tumor uptake was determined to be the highest (7.97 ± 0.75%ID/g) at 24 h pi. The blocking experiment was achieved by co-injection of ⁶⁴Cu-DOTA-GX1 with non-radiolabeled GX1 peptide (20 mg/kg) at 24 h pi, suggesting ⁶⁴Cu-DOTA-GX1 is a target-specific tracer. Furthermore, the biodistribution results were consistent with the quantification of microPET imaging, demonstrating the highest ratio (16.09 ± 1.21) of tumor/muscle uptake of ⁶⁴Cu-DOTA-GX1 at 24 h pi for non-blocking group and significant decreased ratio (6.57 ± 0.58) for blocking group. Finally, metabolic studies suggested that ⁶⁴Cu-DOTA-GX1 is stable in mouse blood and urine in vivo at early time point while the metal transchelation may also occur in mouse liver and kidneys. Our studies demonstrate that ⁶⁴Cu-DOTA-GX1 is a promising radiotracer for imaging tumor vasculature.

Radiosynthesis and in vivo evaluation of 1-[ 18F]fluoroelacridar as a positron emission tomography tracer for P-glycoprotein and breast cancer resistance protein

Aim of this study was to label the potent dual P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) inhibitor elacridar ( 1) with 18F to provide a positron emission tomography (PET) radiotracer to visualize Pgp and BCRP. A series of new 1- and 2-halogen- and nitro-substituted derivatives of 1 ( 4a– e) was synthesized as precursor molecules and reference compounds for radiolabelling and shown to display comparable in vitro potency to 1 in increasing rhodamine 123 accumulation in a cell line overexpressing human Pgp (MDCKII-MDR1). 1-[ 18F]fluoroelacridar ([ 18F] 4b) was synthesized in a decay-corrected radiochemical yield of 1.7 ± 0.9% by a 1-step no-carrier added nucleophilic aromatic 18F-substitution of 1-nitro precursor 4c. Small-animal PET imaging of [ 18F] 4b was performed in naïve rats, before and after administration of unlabelled 1 (5 mg/kg, n = 3), as well as in wild-type and Mdr1a/b (−/−) Bcrp1 (−/−) mice ( n = 3). In PET experiments in rats, administration of unlabelled 1 increased brain activity uptake by a factor of 9.5 ( p = 0.0002, 2-tailed Student’s t-test), whereas blood activity levels remained unchanged. In Mdr1a/b (−/−) Bcrp1 (−/−) mice, the mean brain-to-blood ratio of activity at 60 min after tracer injection was 7.6 times higher as compared to wild-type animals ( p = 0.0002). HPLC analysis of rat brain tissue extracts collected at 40 min after injection of [ 18F] 4b revealed that 93 ± 7% of total radioactivity in brain was in the form of unchanged [ 18F] 4b. In conclusion, the in vivo behavior of [ 18F] 4b was found to be similar to previously described [ 11C] 1 suggesting transport of [ 18F] 4b by Pgp and/or BCRP at the rodent BBB. However, low radiochemical yields and a significant degree of in vivo defluorination will limit the utility of [ 18F] 4b as a PET tracer.

Design and Characteristics of a Multichannel Front-End ASIC Using Current-Mode CSA for Small-Animal PET Imaging

This paper presents the design and characteristics of a front-end readout application-specific integrated circuit (ASIC) dedicated to a multichannel-plate photodetector coupled to LYSO scintillating crystals. In our configuration, the crystals are oriented in the axial direction readout on both sides by individual photodetector channels allowing the spatial resolution and the detection efficiency to be independent of each other. Both energy signals and timing triggers from the photodetectors are required to be read out by the front-end ASIC. A current-mode charge-sensitive amplifier is proposed for this application. This paper presents performance characteristics of a 10-channel prototype chip designed and fabricated in a 0.35-μm complementary metal-oxide semiconductor process. The main results of simulations and measurements are presented and discussed. The gain of the chip is 13.1 mV/pC while the peak time of a CR-RC pulse shaper is 280 ns. The signal-to-noise ratio is 39 dB and the rms noise is 300 μV/√(Hz). The nonlinearity is less than 3% and the crosstalk is about 0.2%. The power dissipation is less than 15 mW/channel. This prototype will be extended to a 64-channel circuit with integrated time-to-digital converter and analog-to-digital converter together for a high-sensitive small-animal positron emission tomography imaging system.

Development and Evaluation of Endothelin-A Receptor (Radio)Ligands for Positron Emission Tomography

The expression and function of endothelin (ET) receptors are abnormal in cardiovascular diseases, tumor progression, and tumor metastasis. A previously reported promising radioligand for positron emission tomography (PET) based on the non-peptide ET(A) receptor antagonist PD 156707 showed specific binding to target receptors in the myocardium but high accumulation in bile and intestine, probably because of its high lipophilicity. In this study we describe the synthesis of a series of fluorinated derivatives with hydrophilic building blocks. All compounds were evaluated as high affinity ET(A) receptor ligands (16, 17, 23-26, K(i) = 1.4-7.9 nM) with high subtype selectivity over the ET(B) receptor. [(18)F]3-Benzo[1,3]dioxol-5-yl-4-{3-[1-(2-{2-[2-(2-fluoroethoxy)ethoxy]ethoxy}ethyl)-1H-[1,2,3]triazol-4-ylmethoxy]-4,5-dimethoxybenzyl}-5-hydroxy-5-(4-methoxyphenyl)-5H-furan-2-one ([(18)F]17) was synthesized as one of the radioligands of this series that possesses a higher hydrophilicity and an excellent stability in human serum. Improved clearance properties and specific uptake in target organs have been confirmed by biodistribution studies and small animal PET imaging.

Transforming a Targeted Porphyrin Theranostic Agent into a PET Imaging Probe for Cancer

Porphyrin based photosensitizers are useful agents for photodynamic therapy (PDT) and fluorescence imaging of cancer. Porphyrins are also excellent metal chelators forming highly stable metallo-complexes making them efficient delivery vehicles for radioisotopes. Here we investigated the possibility of incorporating (64)Cu into a porphyrin-peptide-folate (PPF) probe developed previously as folate receptor (FR) targeted fluorescent/PDT agent, and evaluated the potential of turning the resulting (64)Cu-PPF into a positron emission tomography (PET) probe for cancer imaging. Noninvasive PET imaging followed by radioassay evaluated the tumor accumulation, pharmacokinetics and biodistribution of (64)Cu-PPF. (64)Cu-PPF uptake in FR-positive tumors was visible on small-animal PET images with high tumor-to-muscle ratio (8.88 ± 3.60) observed after 24 h. Competitive blocking studies confirmed the FR-mediated tracer uptake by the tumor. The ease of efficient (64)Cu-radiolabeling of PPF while retaining its favorable biodistribution, pharmacokinetics and selective tumor uptake, provides a robust strategy to transform tumor-targeted porphyrin-based photosensitizers into PET imaging probes.

Radiofluorinated Rhenium Cyclized α-MSH Analogues for PET Imaging of Melanocortin Receptor 1

In order to accomplish in vivo molecular imaging of melanoma biomarker melanocortin 1 receptor (MC1R), several α-melanocyte-stimulating hormone (α-MSH) analogues have been labeled with N-succinimidyl-4-¹⁸F-fluorobenzoate (¹⁸)F-SFB) and studied as positron emission tomography (PET) probes in our recent studies. To further pursue a radiofluorinated α-MSH peptide with high clinical translation potential, we utilized 4-nitrophenyl 2-¹⁸F-fluoropropionate (¹⁸F-NFP) to radiofluorinate the transition metal rhenium cyclized α-MSH metallopeptides for PET imaging of MC1R positive malignant melanoma. Metallopeptides Ac-d,Lys-ReCCMSH(Arg¹¹) (two isomers, namely RMSH-1 and RMSH-2) were synthesized using conventional solid phase peptide synthesis chemistry and rhenium cyclization reaction. The two isomers were then conjugated with ¹⁹F-NFP or ¹⁸F-NFP. The resulting cold or radiofluorinated metallopeptides, (¹⁸/¹⁹)F-FP-RMSH-1 and (¹⁸/¹⁹)F-FP-RMSH-2, were further evaluated for their in vitro receptor binding affinities, in vivo biodistribution, and small-animal PET imaging properties. The binding affinities of ¹⁹F-FP-RMSH-1 and ¹⁹F-FP-RMSH-2 were determined to be within low nanomolar range. In vivo studies revealed that both F-labeled metallopeptides possessed good tumor uptake in the B16F10 murine model with high MC1R expression, while possessing much lower uptake in A375M human melanoma xenografts. Moreover, ¹⁸F-FP-RMSH-1 displayed more favorable in vivo performance in terms of higher tumor uptake and much lower accumulation in the kidney and liver, when compared to that of ¹⁸F-FP-RMSH-2 at 2 h postinjection (p.i.). ¹⁸F-FP-RMSH-1 also displayed lower liver and lung uptake when compared with that of the same peptide labeled with ¹⁸F-SFB (named as ¹⁸F-FB-RMSH-1). Small animal PET imaging of ¹⁸F-FP-RMSH-1 in mice bearing B16F10 tumors at 1 and 2 h showed good tumor imaging quality. As expected, much lower tumor uptake and poorer tumor/normal organ contrast were observed for A375M model compared to those of the B16F10 model. ¹⁸F-FP-RMSH-1 also exhibited higher tumor uptake and better tumor retention when compared with ¹⁸F-FB-RMSH-1. ¹⁸F-FP-RMSH-1 demonstrates significant advantages over ¹⁸F-FB-RMSH-1 and ¹⁸F-FP-RMSH-2. It is a promising PET probe for imaging MC1R positive melanoma and MC1R expression in vivo.

Synthesis of novel 68Ga-labeled amino acid derivatives for positron emission tomography of cancer cells

We developed amino acid derivatives of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A) and 1,4,7,10-tetraazacyclododecane-1,4,7,-triacetic acid (DO3A) that can be labeled with (68)Ga, and we investigated their basic biological properties. Alanine derivatives of DO2A and DO3A were synthesized by regiospecific nucleophilic attack of DO2tBu and DO3tBu on the β-position of Boc-l-serine-β-lactone, followed by acid hydrolysis. Also, homoalanine derivatives were synthesized by reacting with the protected bromo derivative of homoalanine, which was synthesized from N-Cbz-l-homoserine lactone. Further catalytic reduction and acid cleavage of protected groups resulted in the required products. All derivatives were labeled with (68)Ga. Cell uptake assays were carried out in Hep3B (human hepatoma) and U87MG (human glioma) cell lines at 37°C. Positron emission tomography (PET) imaging studies were performed using balb/c mice xenografted with CT-26 (mouse colon cancer). All compounds were labeled with >97% efficiency. According to in vitro studies, the labeled amino acid derivatives showed significantly greater uptakes than the control ((68)Ga 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) in cancer cells. Small animal PET images for labeled compounds showed high tumor uptake, as well as kidney and bladder uptakes, at 30 min postinjection. (68)Ga-DO3A-homoalanine showed the highest standardized uptake value ratio (3.9 ± 0.3), followed by (68)Ga-DO2A-alanine (3.1 ± 0.2), (68)Ga-DO3A-alanine (2.8 ± 0.2) and (68)Ga-DO2A-homoalanine (2.3 ± 0.2). These derivatives were found to have high labeling efficiencies, high stabilities, high tumor cell uptakes, high tumor/nontumor xenograft uptakes and low nonspecific uptake in normal organs, except for the kidneys. However, the uptake mechanism of these derivatives remains unclear, and uptake via specific amino acid transporters needs to be demonstrated.

Synthesis and in vivo evaluation of [11C]tariquidar, a positron emission tomography radiotracer based on a third-generation P-glycoprotein inhibitor

The aim of this study was to develop a positron emission tomography (PET) tracer based on the dual P-glycoprotein (P-gp) breast cancer resistance protein (BCRP) inhibitor tariquidar (1) to study the interaction of 1 with P-gp and BCRP in the blood–brain barrier (BBB) in vivo. O-Desmethyl-1 was synthesized and reacted with [11C]methyl triflate to afford [11C]-1. Small-animal PET imaging of [11C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15mg/kg, n=3) or the dual P-gp/BCRP inhibitor elacridar (5mg/kg, n=2), as well as in wild-type, Mdr1a/b(−/−), Bcrp1(−/−) and Mdr1a/b(−/−)Bcrp1(−/−) mice (n=3). In vitro autoradiography was performed with [11C]-1 using brain sections of all four mouse types, with and without co-incubation with unlabeled 1 or elacridar (1μM). In PET experiments in rats, administration of unlabeled 1 or elacridar increased brain activity uptake by a factor of 3–4, whereas blood activity levels remained unchanged. In Mdr1a/b(−/−), Bcrp1(−/−) and Mdr1a/b(−/−)Bcrp1(−/−) mice, brain-to-blood ratios of activity at 25min after tracer injection were 3.4, 1.8 and 14.5 times higher, respectively, as compared to wild-type animals. Autoradiography showed approximately 50% less [11C]-1 binding in transporter knockout mice compared to wild-type mice and significant displacement by unlabeled elacridar in wild-type and Mdr1a/b(−/−) mouse brains. Our data suggest that [11C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [11C]-1 behaves in vivo as a transported or a non-transported inhibitor.

Small-Animal PET Imaging of Human Epidermal Growth Factor Receptor Positive Tumor with a 64Cu Labeled Affibody Protein

Epidermal growth factor receptor (EGFR) has become an attractive target for cancer molecular imaging and therapy. Affibody proteins against EGFR have been reported, and thus, we were interested in evaluating their potential for positron emission tomography (PET) imaging of EGFR positive cancer. An Affibody analogue (Ac-Cys-Z(EGFR:1907)) binding to EGFR was made through conventional solid phase peptide synthesis. The purified protein was site-specifically coupled with the 1,4,7,10-tetraazacyclododecane-1,4,7-tris-aceticacid-10-maleimidethylacetamide (maleimido-mono-amide-DOTA) to produce the bioconjugate, DOTA-Z(EGFR:1907). (64)Cu labeled probe (64)Cu-DOTA-Z(EGFR:1907) displayed a moderate specific activity (5-8 MBq/nmol, 22-35 microCi/microg). Cell uptake assays by pre-incubating without or with 300 times excess unlabeled Ac-Cys-Z(EGFR:1907) showed high EGFR-specific uptake (20% applied activity at 0.5 h) in A431 epidermoid carcinoma cancer cells. The affinity (K(D)) of (64)Cu-DOTA-Z(EGFR:1907) as tested by cell saturation analysis was 20 nM. The serum stability test showed excellent stability of the probe with >95% intact after 4 h of incubation in mouse serum. In vivo small-animal PET imaging showed fast tumor targeting, high tumor accumulation (approximately 10% ID/g at 1 h p.i.), and good tumor-to-normal tissue contrast of (64)Cu-DOTA-Z(EGFR:1907) spiked with a wide dose range of Ac-Cys-Z(EGFR:1907). Bio-distribution studies further demonstrated that the probe had high tumor, blood, liver, and kidney uptakes, while blood radioactivity concentration dropped dramatically at increased spiking doses. Co-injection of the probe with 500 microg of Ac-Cys-Z(EGFR:1907) for blocking significantly reduced the tumor uptake. Thus, (64)Cu-DOTA-Z(EGFR:1907) showed potential as a high tumor contrast EGFR PET imaging reagent. The probe spiked with 50 microg of Ac-Cys-Z(EGFR:1907) improved tumor imaging contrast which may have important clinical applications.

Protective effects of repetitive transcranial magnetic stimulation in a rat model of transient cerebral ischaemia: a microPET study

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive method to excite neurons in the brain. However, the underlying mechanism of its therapeutic effects in stroke remains unclear. The aim of this study was to investigate the neuroprotective effect of high-frequency rTMS in a rat model of transient cerebral ischaemia using positron emission tomography (PET). Sprague-Dawley rats (n=30) were anaesthetized with chloral hydrate and subjected to 90 min of intraluminal middle cerebral artery occlusion (MCAO) with subsequent reperfusion in three groups: control (n=10), rTMS (n=10), or sham-rTMS groups (n=10). In the rTMS group, rTMS was given 1 h after ischaemia and every 24 h for 7 days after MCAO. In all three groups, small-animal PET (microPET) imaging with (18)F-FDG was used to evaluate brain glucose metabolism. Apoptotic molecules were measured in the infarct margin using immunohistochemical staining. The neurological scores of the rats in the rTMS group were higher than in those of the control group over the whole 7-day observation period. The total, cortical and striatal infarct volumes were significantly less in the rTMS group than in the control group, as measured by 2,3,5-triphenyltetrazolium chloride staining. (18)F-FDG microPET images showed significantly higher standardized uptake values in the cortex and striatum in the rTMS group than in the control group in the affected hemisphere. The number of cells positive for caspase-3 was significantly lower in the rTMS group than in the control group, while the Bcl-2/Bax ratio was significantly higher in the rTMS group than in the control group. rTMS therapy increased glucose metabolism and inhibited apoptosis in the ischaemic hemisphere. (18)F-FDG PET could be used to monitor rTMS therapy in transient cerebral ischaemia in animal studies and in future clinical trials.

Single-Input–Dual-Output Modeling of Image-Based Input Function Estimation

Quantification of small-animal positron emission tomography (PET) images necessitates knowledge of the plasma input function (PIF). We propose and validate a simplified hybrid single-input-dual-output (HSIDO) algorithm to estimate the PIF. The HSIDO algorithm integrates the peak of the input function from two region-of-interest time-activity curves with a tail segment expressed by a sum of two exponentials. Partial volume parameters are optimized simultaneously. The algorithm is validated using both simulated and real small-animal PET images. In addition, the algorithm is compared to existing techniques in terms of area under curve (AUC) error, bias, and precision of compartmental model micro-parameters. In general, the HSIDO method generated PIF with significantly (P < 0.05) less AUC error, lower bias, and improved precision of kinetic estimates in comparison to the reference method. HSIDO is an improved modeling-based PIF estimation method. This method can be applied for quantitative analysis of small-animal dynamic PET studies.

Positron Emission Tomography Imaging of Neuroinflammation

Injury to the central nervous system is characterized by localization of activated microglia at the site of injury. The peripheral benzodiazepine receptor expressed on the outer mitochondrial membrane of the activated microglia is a sensitive biomarker for the detection of this neuroinflammatory response to an insult. PK11195, an isoquinoline ligand that specifically binds peripheral benzodiazepine receptor, can be tagged with a positron emitter and used as a tracer for molecular imaging of this receptor in vivo by positron emission tomography (PET). [(11)C](R)PK11195 has been used in the imaging of various neuroinflammatory disorders, such as Alzheimer disease and multiple sclerosis. On the basis of our small-animal PET imaging studies using a neonatal rabbit model of maternal inflammation-induced cerebral palsy, we propose that PET imaging using [(11)C](R)PK11195 may be a valuable tool for detecting neuroinflammation in the brain of newborns born to mothers with chorioamnionitis.

Synthesis and Small-Animal Positron Emission Tomography Evaluation of [11C]-Elacridar As a Radiotracer to Assess the Distribution of P-Glycoprotein at the Blood−Brain Barrier

With the aim to develop a positron emission tomography (PET) tracer to assess the distribution of P-glycoprotein (P-gp) at the blood-brain barrier (BBB) in vivo, the potent third-generation P-gp inhibitor elacridar (1) was labeled with (11)C by reaction of O-desmethyl 1 with [(11)C]-methyl triflate. In vitro autoradiography and small-animal PET imaging of [(11)C]-1 was performed in rats (n = 3), before and after administration of unlabeled 1, as well as in wild-type, Mdr1a/b((-/-)) and Bcrp1((-/-)) mice (n = 3). In PET experiments in rats, administration of unlabeled 1 increased brain activity uptake 5.4-fold, whereas blood activity levels remained unchanged. In Mdr1a/b((-/-)) mice, brain activity uptake was 2.5-fold higher compared to wild-type animals, whereas in Bcrp1((-/-)) mice, brain activity uptake was only 1.3-fold higher. In vitro autoradiography showed that 63% of [(11)C]-1 binding was displaceable by an excess of unlabeled 1. As the signal obtained with [(11)C]-1 appeared to be specific for P-gp at the BBB, its utility for the visualization of cerebral P-gp merits further investigation.

Development of a 64-channel APD detector module with individual pixel readout for submillimetre spatial resolution in PET

Visualization and quantification of biological processes by molecular imaging in small animals such as rats and especially mice require the best possible spatial resolution in positron emission tomography (PET). A new avalanche photodiode (APD) detector module, the LabPET II, was developed to achieve submillimetre spatial resolution for this purpose. The module consists of two monolithic APD arrays of 4×8 pixels, each with an active area of 1.1×1.1 mm 2 at a 1.2 mm pitch. The two APD arrays mounted in a custom ceramic PGA holder are coupled to an 8×8 tapered LYSO scintillator array designed to accommodate one-to-one coupling between individual APDs and crystal pixels. To investigate the module performance, an analog test board with four 16-channel preamplifier ASICs was designed to be interfaced with the LabPET™ digital processing electronics. At a standard APD operating bias, a mean energy resolution of 27.5±2.1% was typically obtained with a relative standard deviation of 13.8% in signal amplitude for the 64 individual pixels when irradiated with 511 keV photons. With two modules in coincidence, a mean timing resolution of 5.0±0.2 ns FWHM was measured. Finally, an intrinsic spatial resolution of 0.82 mm FWHM was measured by sweeping a 22Na point source between the two detector arrays. The LabPET II detector module demonstrates promising characteristics for dedicated small animal PET imaging at submillimetre resolution and, with some further optimization, would be suitable as the building block for a dual-modality combined PET/CT system.

64Cu-Labeled PEGylated Polyethylenimine for Cell Trafficking and Tumor Imaging

In this study, we exploited the potential of (64)Cu-labeled polyethylenimine (PEI) for cell trafficking and tumor imaging as compared to copper-64-pyruvaldehyde-bis(N (4)-methylthiosemicarbazone) ((64)Cu-PTSM). U87MG cells were labeled with both (64)Cu-PEI and (64)Cu-PTSM, and their in vivo distributions in mice were tracked by positron emission tomography (PET). The tumor imaging ability of (64)Cu-PTSM and (64)Cu-PEI was investigated in U87MG human glioblastoma xenograft model. (64)Cu-PEI-polyethylene glycol (PEG) was also synthesized, and the cell uptake, efflux, cytotoxicity, and the biodistribution were carried out and compared with (64)Cu-PEI. Both (64)Cu-PEI and (64)Cu-PEI-PEG were obtained in high labeling yield without the need of macrocyclic chelating agents. (64)Cu-PEI showed lower cell labeling efficiency than (64)Cu-PTSM. Small-animal PET images of living mice indicate that tail-vein-injected U87MG cells labeled with (64)Cu-PTSM or (64)Cu-PEI traffic to the lungs and liver. In a subcutaneous U87MG xenograft model, (64)Cu-PEI had higher tumor uptake (18.7 +/- 2.2 %ID/g at 24 h) than (64)Cu-PTSM (12.4 +/- 1.7 %ID/g at 24 h). In comparison with (64)Cu-PEI, (64)Cu-PEI-PEG had decreased toxicity and increased cell uptake in cell culture, as well as higher tumor uptake and better tumor-to-background contrast in U87MG xenograft model. (64)Cu-labeled polyethylenimine can be used for both cell trafficking and tumor imaging. PEGylation reduces the toxicity of (64)Cu-PEI and improves the tumor imaging ability.