Microfluidics technology has emerged as a powerful tool for the radiosynthesis of positron emission tomography (PET) and single-photon emission computed tomography radiolabeled compounds. In this work, we have exploited a continuous flow microfluidic system (Advion, Inc., USA) for the [(18) F]-fluorine radiolabeling of the malonic acid derivative, [(18) F] 2-(5-fluoro-pentyl)-2-methyl malonic acid ([(18) F]-FPMA), also known as [(18) F]-ML-10, a radiotracer proposed as a potential apoptosis PET imaging agent. The radiosynthesis was developed using a new tosylated precursor. Radiofluorination was initially optimized by manual synthesis and served as a basis to optimize reaction parameters for the microfluidic radiosynthesis. Under optimized conditions, radio-thin-layer chromatography analysis showed 79% [(18) F]-fluorine incorporation prior to hydrolysis and purification. Following hydrolysis, the [(18) F]-FPMA was purified by C18 Sep-Pak, and the final product was analyzed by radio-HPLC (high-performance liquid chromatography). This resulted in a decay-corrected 60% radiochemical yield and ≥98% radiochemical purity. Biodistribution data demonstrated rapid blood clearance with less than 2% of intact [(18) F]-FPMA radioactivity remaining in the circulation 60 min post-injection. Most organs showed low accumulation of the radiotracer, and radioactivity was predominately cleared through kidneys (95% in 1 h). Radio-HPLC analysis of plasma and urine samples showed a stable radiotracer at least up to 60 min post-injection.

The current report describes the development of a dual modality tomographic agent for both positron emission tomography and magnetic resonance imaging (PET/MRI). The dual-modality agent in this study was based on a 124I (PET) radiolabeled tri-gadolinium endohedral metallofullerene Gd3N@C80 (MRI) nanoprobe platform. The outer surface of the fullerene cage of the Gd3N@C80 metallofullerenes was surface functionalized with carboxyl and hydroxyl groups (f-Gd3N@C80) using previously developed procedures and subsequently iodinated with 124I to produce 124I-f-Gd3N@C80 nanoprobe. Orthotopic tumor-bearing rats were infused intratumorally by convection-enhanced delivery (CED) with the 124I-f-Gd3N@C80 agent and imaged by MRI or micro PET. The anatomical positioning and distribution of the 124I-f-Gd3N@C80 agent were comparable between the MRI and PET scans. The 124I-f-Gd3N@C80­ dual-agent distribution and infusion site within the tumor was clearly evident in both T1- and T2-weighted MR images. The results demonstrate the successful preparation of a dual-modality imaging agent, 124I-f-Gd3N@C80, which could ultimately be used for simultaneous PET/MR imaging.

Background and purposePET imaging with 18F-fluorothymidine (18F-FLT) can potentially be used to identify tumour subvolumes for selective dose escalation in radiation therapy. The purpose of this study is to analyse the co-localization of intratumoural patterns of cell proliferation with 18F-FLT tracer uptake.Materials and methodsMice bearing FaDu or SQ20B xenograft tumours were injected with 18F-FLT, and bromodeoxyuridine (proliferation marker). Ex vivo images of the spatial pattern of intratumoural 18F-FLT uptake and that of bromodeoxyuridine DNA incorporation were obtained from thin tumour tissue sections. These images were segmented by thresholding and Relative Operating Characteristic (ROC) curves and Dice similarity indices were evaluated.ResultsThe thresholds at which maximum overlap occurred between FLT-segmented areas and areas of active cell proliferation were significantly different for the two xenograft tumour models, whereas the median Dice values were not. However, ROC analysis indicated that segmented FLT images were more specific at detecting the proliferation pattern in FaDu tumours than in SQ20B tumours.ConclusionHighly dispersed patterns of cell proliferation observed in certain tumours can affect the perceived spatial concordance between the spatial pattern of 18F-FLT uptake and that of cell proliferation even when high-resolution ex vivo autoradiography imaging is used for 18F-FLT imaging.

PURPOSE:PET imaging allows for the visualization of tumor microenvironment and identification of aggressive or radioresistant tumor subvolumes that can be targeted with an escalated radiation dose. Multiple PET tracers have been developed for visualization of different aspects of tumor microenvironment; however, the spatial distribution of tracers in tumors is equally affected by tumor tissue viability and tracer delivery limitations. Given these issues and the low resolution associated with PET imaging, two different PET tracers can produce very similar images. Therefore, it is important to demonstrate that a novel PET tracer does provide additional useful information to that obtained with other tracers. This study investigates the added value of performing 18F-FLT PET imaging as well as 18F-FDG imaging.METHODS:Head and neck tumor xenografts grown in nude mice were used to study intratumoral tracerdistributions. 18F-FDG and 18F-FLT PET images were obtained on subsequent days using a small animal PET/CT. Pinnacle 9 was used to deformably register the CT image from the FLT PET/CT to the FDG PET/CT image set. The generated deformation was applied to the FLT PET image to achieve an unbiased FLT to FDG PET image registration. The Pearson correlation coefficient between FDG and FLT was calculated voxel- by-voxel within a tumor contour. Overlap analysis of thresholded tracer distributions was carried out by comparing Dice similarity coefficients.RESULTS:Both SQ20B and FaDu tumors showed a moderate voxel-by-voxel correlation between FDG and FLT intratumoral patterns of uptake with an average rho value of .56 and .63 respectively (range .37-.76) despite significant differences in tumor morphology. The average volumes under thedice coefficient surface for SQ20B and FaDu tumors were not significantly different.CONCLUSIONS:Despite being equally affected by the issues of tracer delivery, necrosis and PET resolution, FDG and FLT PET images displayed an observable difference at clinically relevant thresholds.

18F-fluoropaclitaxel is a radiolabeled form of paclitaxel, a widely used chemotherapy agent. Preclinical data suggest that 18F-fluoropaclitaxel may be a reasonable surrogate for measuring the uptake of paclitaxel. As a substrate of P-glycoprotein, a drug efflux pump associated with multidrug resistance, 18F-fluoropaclitaxel may also be useful in identifying multidrug resistance and predicting tumor response for drugs other than paclitaxel. Methods: After informed consent was obtained, 3 healthy volunteers and 3 patients with untreated breast cancer (neoadjuvant chemotherapy candidates, tumor size > 2 cm) received an intravenous infusion of 18F-fluoropaclitaxel and then underwent PET/CT. Healthy volunteers underwent serial whole-body imaging over an approximately 3-h interval, and organ 18F residence times were determined from the time–activity curves uncorrected for decay to determine dosimetry. Radiation dose estimates were calculated using OLINDA/EXM software. For breast cancer patients, dynamic imaging of the primary tumor was performed for 60 min, followed by static whole-body scans at 1 and 2 h after injection. Results: Dosimetry calculations showed that the gallbladder received the highest dose (229.50 μGy/MBq [0.849 rad/mCi]), followed by the small and large intestines (161.26 μGy/MBq [0.597 rad/mCi] and 184.59 μGy/MBq [0.683 rad/mCi]). The resultant effective dose was 28.79 μGy/MBq (0.107 rem/mCi). At approximately 1 h after injection, an average of 42% of the decay-corrected activity was in the gastrointestinal system, with a mean of 0.01% in the tumor. All 3 breast cancer patients showed retention of 18F-fluoropaclitaxel and ultimately demonstrated a complete pathologic response (no invasive cancer in the breast or axillary nodes) to chemotherapy that included a taxane (either paclitaxel or docetaxel) at surgical resection. The tumor-to-background ratio increased with time to a maximum of 7.7 at 20 min. Conclusion: This study demonstrates the feasibility of using 18F-fluoropaclitaxel PET/CT tumor imaging and provides radiation dosimetry measurements in humans. Although further study is needed, it is hoped that the measured intratumoral 18F-fluoropaclitaxel distribution can serve as a surrogate for paclitaxel, and potentially other chemotherapeutic agent retention, in solid tumors.

Histopathologic validation of a PET tracer requires assessment of colocalization of the tracer with its intended biologic target. Using thin tissue section autoradiography, it is possible to visualize the spatial distribution of the PET tracer uptake and compare it with the distribution of the intended biologic target (as visualized with immunohistochemistry). The purpose of this study was to develop and evaluate an objective methodology for deformable coregistration of autoradiography and microscopy images acquired from a set of sequential tissue sections. Methods: Tumor-bearing animals were injected with 3′-deoxy-3′-18F-fluorothymidine (18F-FLT), 14C-FDG, and other markers of tumor microenvironment including Hoechst 33342 (blood-flow surrogate). After sacrifice, tumors were excised, frozen, and sectioned. Multiple stacks of sequential 8 μm sections were collected from each tumor. From each stack, the middle (reference) sections were used to obtain images of 18F-FLT and 14C-FDG uptake distributions using dual-tracer autoradiography. Sections adjacent to the reference were used to acquire all histopathologic data (e.g., images of cell proliferation, hematoxylin and eosin). Hoechst images were acquired from all sections. To correct for deformations and misalignments induced by tissue processing and image acquisition, the Hoechst image of each nonreference section was deformably registered to the reference Hoechst image. This transformation was then applied to all images acquired from the same tissue section. In this way, all microscopy images were registered to the reference Hoechst image. The Hoechst-to-autoradiography image registration was done using rigid point-set registration based on external markers visible in both images. Results: The mean error of Hoechst to 18F-FLT autoradiography registration (both images acquired from the same section) was 30.8 ± 20.1 μm. The error of Hoechst-based deformable registration of histopathologic images (acquired from sequential tissue sections) was 23.1 ± 17.9 μm. Total error of registration of autoradiography images to the histopathologic images acquired from adjacent sections was evaluated at 44.9 μm. This coregistration precision supersedes current rigid registration methods with reported errors of 100–200 μm. Conclusion: Deformable registration of autoradiography and histopathology images acquired from sequential sections is feasible and accurate when performed using corresponding Hoechst images.

Background: Glioblastoma multiforme is the most common and most lethal primary brain tumor in humans, with median survival of approximately 1 year. Owing to the ability of glioma cells to aggressively infiltrate normal brain tissue and survive exposure to current adjuvant therapies, there is a great need for specific targeted nanoplatforms capable of delivering both therapeutic and imaging agents directly to invasive tumor cells. Method: Gadolinium-containing endohedral fullerenes, highly efficient contrast agents for MRI, were functionalized and conjugated with a tumor-specific peptide and assessed for their ability to bind to glioma cells in vitro. Results: We report the successful conjugation of the carboxyl functionalized metallofullerene Gd3N@C-80(OH)(similar to 26)(CH2CH2COOH)(similar to 16) to IL-13 peptides and the successful targeting ability towards brain tumor cells that overexpress the IL-13 receptor (IL-13R alpha 2). Conclusion: These studies demonstrate that IL-13 peptide-conjugated gadolinium metallofullerenes could serve as a platform to deliver imaging and therapeutic agents to tumor cells.

Purpose: To utilize serial PET imaging it is important to minimize the effect of repositioning errors and anatomical changes. Compensating for these errors requires an objective and reliable method of deformable image registration. Here we report on the methodology used to deformably co‐register serial PET images using CT anatomy and compare intratumoral distributions of FDG and FLT as imaged in the same animal with PET/CT on two consecutive days. Methods: Nude mice bearing FaDu (human H&N) tumor xenografts were imaged with 18F‐FDG and 18F‐FLT on two consecutive days using a dedicated small animal PET/CT scanner (Siemens Inveon). Both data sets were reconstructed and loaded into Pinnacle 9.1. Despite careful repositioning of the animal using an animal‐specific pad with recorded landmarks, misalignment of FDG and FLT PET images hindered direct voxel‐by‐voxel analysis. To perform an objective co‐registration of the PET images, we relied on associated CT images. Animal bodies and tumors were contoured on both CT scans. A mesh was generated from the contours for visualization purposes. The CT images were deformably registered with a demons algorithm, and the resulting displacement vector field was applied to the FLT image, allowing for voxel‐by‐voxel analysis of co‐registered FLT and FDG PET images in Matlab. Results: Based off of visual inspection, the deformable image registration tools available in Pinnacle are adequate for co‐registration of the animal PET/CT images despite the deformations caused by repositioning. Voxel‐by‐voxel analysis of co‐registered FLT and FDG PET/CT images produced correlation coefficients ranging from .45 to .55, p<10–4. Conclusions: Using corresponding CT anatomy with the tools in Pinnacle 9.1 to generate deformation matrices is a viable approach to deformable PET image registration. The images produced facilitated a voxel‐by‐voxel comparison of FLT vs. FDG.

R. Ribes and J.C. Vilanova, eds. New York, NY: Springer, 2010, 280 pages, $59.95 This brief text on musculoskeletal imaging is a good, concise handbook of interesting cases compiled by many authors from Spain and showcasing a diverse spectrum of musculoskeletal diseases. The book is easy to read

Purpose: To propose a new objective method for deformable coregistration of multimodality images acquired with digital autoradiography (DAR) and microscopy in the context of PET tracer histopathological validation. To analyze the spatial concordance between the uptake pattern of 18F-fluorothymidine (FLT) as imaged with DAR and the distribution of cell proliferation as revealed by immunofluorescence microscopy imaging.Methods: Tumor-bearing mice were injected with FLT and other markers including bromodeoxyuridine (cell proliferation). After sacrifice, tumors were excised, frozen and sectioned. Multiple stacks of sequential 8μm sections were collected from each tumor. Selected sections were used for DAR to image FLT uptake distribution. Adjacent sections were used to acquire histopathological data. To correct for imperfections of the tissue cutting and collection, all images were deformably coregistered to the FLT DAR image based on biological images of tumor blood flow (Hoechst) that was acquired from each tissue section used. For each FLT DAR — cell proliferation microscopy image pair, object-based analysis was conducted, including overlap and relative operating characteristics (ROC) analysis. Results: Total registration error of proposed coregistration method was 44.86μm. This supersedes current rigid registration methods with reported errors of 100–200μm. In tumors with well-compartmentalized functional aspects, area under the ROC curve (AUCroc= 0.7) indicated FLT DAR image thresholding as an accurate method of detecting cell proliferation. For these tumors, Dice overlap index indicated maximum detection rates at thresholds between 20% and 40% of the maximum DAR intensity. For the tumors characterized by more heterogeneous distribution of cell proliferation across the tumor section, FLT DAR image thresholding could not predict cell proliferation beyond random chance. Conclusions: We developed a comprehensive method of obtaining and analyzing coregistered images of cell proliferation markers and intratumoral uptake of FLT. Tumor microenvironment heterogeneity is a significant factor affecting the utility of FLT for imaging cell proliferation.

In this communication, we describe the successful encapsulation of Lu-177 into the endohedral metallofullerene (LuxLu3-xN)-Lu-177@C-80 (x = 1-3) starting with (LuCl3)-Lu-177 in a modified quartz Kraschmer-Huffman electric generator. We demonstrate that the Lu-177 (beta-emitter) in this fullerene cage is not significantly released for a period of up to at least one-half-life (6.7 days). We also demonstrate that this agent can be conjugated with an interleukin-13 peptide that is designed to target an overexpressed receptor in glioblastoma multiforme tumors. This nanoparticle delivery platform provides flexibility for a wide range of radiotherapeutic and radiodiagnostic multimodal applications.