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1.  A Recommendation for Revised Dose Calibrator Measurement Procedures for 89Zr and 124I 
PLoS ONE  2014;9(9):e106868.
Because of their chemical properties and multiday half lives, iodine-124 and zirconium-89 are being used in a growing number of PET imaging studies. Some aspects of their quantitation, however, still need attention. For 89Zr the PET images should, in principle, be as quantitatively accurate as similarly reconstructed 18F measurements. We found, however, that images of a 20 cm well calibration phantom containing 89Zr underestimated the activity by approximately 10% relative to a dose calibrator measurement (Capintec CRC-15R) using a published calibration setting number of 465. PET images of 124I, in contrast, are complicated by the contribution of decays in cascade that add spurious coincident events to the PET data. When these cascade coincidences are properly accounted for, quantitatively accurate images should be possible. We found, however, that even with this correction we still encountered what appeared to be a large variability in the accuracy of the PET images when compared to dose calibrator measurements made using the calibration setting number, 570, recommended by Capintec. We derive new calibration setting numbers for 89Zr and 124I based on their 511 keV photon peaks as measured on an HPGe detector. The peaks were calibrated relative to an 18F standard, the activity level of which was precisely measured in a dose calibrator under well-defined measurement conditions. When measuring 89Zr on a Capintec CRC-15R we propose the use of calibration setting number 517. And for 124I, we recommend the use of a copper filter surrounding the sample and the use of calibration setting number 494. The new dose calibrator measurement procedures we propose will result in more consistent and accurate radioactivity measurements of 89Zr and 124I. These and other positron emitting radionuclides can be accurately calibrated relative to 18F based on measurements of their 511 keV peaks and knowledge of their relative positron abundances.
PMCID: PMC4159273  PMID: 25202987
2.  Quantitative imaging of disease signatures through radioactive decay signal conversion 
Nature medicine  2013;19(10):1345-1350.
In the era of personalized medicine there is an urgent need for in vivo techniques able to sensitively detect and quantify molecular activities. Sensitive imaging of gamma rays is widely used, but radioactive decay is a physical constant and signal is independent of biological interactions. Here we introduce a framework of novel targeted and activatable probes excited by a nuclear decay-derived signal to identify and measure molecular signatures of disease. This was accomplished utilizing Cerenkov luminescence (CL), the light produced by β-emitting radionuclides such as clinical positron emission tomography (PET) tracers. Disease markers were detected using nanoparticles to produce secondary Cerenkov-induced fluorescence. This approach reduces background signal compared to conventional fluorescence imaging. In addition to information from a PET scan, we demonstrate novel medical utility by quantitatively determining prognostically relevant enzymatic activity. This technique can be applied to monitor other markers and facilitates a shift towards activatable nuclear medicine agents.
PMCID: PMC3795968  PMID: 24013701
Activatable probes; Molecular imaging; Nanoparticles; Cerenkov luminescence
3.  89Zr-Labeled Paramagnetic Octreotide-Liposomes for PET-MR Imaging of Cancer 
Pharmaceutical research  2012;30(3):878-888.
Dual-modality PET/MR platforms add a new dimension to patient diagnosis with high resolution, functional, and anatomical imaging. The full potential of this emerging hybrid modality could be realized by using a corresponding dual-modality probe. Here, we report pegylated liposome (LP) formulations, housing a MR T1 contrast agent (Gd) and the positron-emitting 89Zr (half-life: 3.27 days), for simultaneous PET and MR tumor imaging capabilities.
89Zr oxophilicity was unexpectedly found advantageous for direct radiolabeling of preformed paramagnetic LPs. LPs were conjugated with octreotide to selectively target neuroendocrine tumors via human somatostatin receptor subtype 2 (SSTr2). 89Zr-Gd-LPs and octreotide-conjugated homolog were physically, chemically and biologically characterized.
89Zr-LPs showed reasonable stability over serum proteins and chelator challenges for proof-of-concept in vitro and in vivo investigations. Nuclear and paramagnetic tracking quantified superior SSTr2-recognition of octreotide-LP compared to controls.
This study demonstrated SSTr2-targeting specificity along with direct chelator-free 89Zr-labeling of LPs and dual PET/MR imaging properties.
PMCID: PMC3578092  PMID: 23224977
nanoparticles; octreotide; PET-MRI; targeted molecular imaging; 89Zr
4.  Pharmacokinetic Assessment of the Uptake of 16β-[18F]-Fluoro-5α Dihydrotestosterone (FDHT) in Prostate Tumors as Measured by PET 
The aim of this study was to develop a clinically applicable non-invasive method to quantify changes in androgen receptor (AR) levels based on 18F-FDHT PET in prostate cancer patients undergoing therapy.
Thirteen patients underwent dynamic 18F-FDHT PET scans over a selected tumor. Concurrent venous blood samples were acquired for blood metabolite analysis. A second cohort of 25 patients, injected with 18F-FDHT underwent dynamic PET imaging of the heart. These data were used to generate a population-based input function, essential for pharmacokinetic modeling. Linear compartmental pharmacokinetic models of increasing complexity were tested on the tumor tissue data. Four suitable models were applied and compared using the Bayesian Information Criterion (BIC). Model 1 consisted of an instantaneously equilibrating space followed by a unidirectional trap. Models 2a and 2b contained a reversible space between the instantaneously equilibrating space and the trap, into which metabolites were excluded (2a) or allowed (2b). Model 3 built upon Model 2b with the addition of a second reversible space preceding the unidirectional trap and from which metabolites were excluded.
The half-life of the 18F-FDHT in blood was determined to be between 6-7 minutes. As a consequence, the uptake of 18F-FDHT in prostate cancer lesions reached a plateau within 20-minutes as the blood-borne activity was consumed. Radiolabeled metabolites were shown not to bind to AR in in-vitro studies with CWR22 cells. Model 1 produced reasonable and robust fits for all datasets and was judged best by the BIC for 16 out of 26 tumor scans. Models 2a, 2b and 3 were judged best in seven, two and one case, respectively.
Our study explores the clinical potential of using 18F-FDHT PET to make estimates of free AR concentration. This process involved the estimation of a net-uptake parameter such as Model 1’s ktrap that could serve as a surrogate measure of AR expression in metastatic prostate cancer. Our initial studies suggest a simple body-mass normalized standard uptake value (SUV) is reasonably well correlated to model based ktrap estimates, which we surmise may be proportional to AR expression. Validation studies to test this hypothesis are underway.
PMCID: PMC2866076  PMID: 20080885
5.  Image-Guided PO2 Probe Measurements Correlated with Parametric Images Derived from 18F-Fluoromisonidazole Small-Animal PET Data in Rats 
18F-fluoromisonidazole PET, a noninvasive means of identifying hypoxia in tumors, has been widely applied but with mixed results, raising concerns about its accuracy. The objective of this study was to determine whether kinetic analysis of dynamic 18F-fluoromisonidazole data provides better discrimination of tumor hypoxia than methods based on a simple tissue-to-plasma ratio.
Eleven Dunning R3327-AT prostate tumor-bearing nude rats were immobilized in custom-fabricated whole-body molds, injected intravenously with 18F-fluoromisonidazole, and imaged dynamically for 105 min. They were then transferred to a robotic system for image-guided measurement of intratumoral partial pressure of oxygen (PO2). The dynamic 18F-fluoromisonidazole uptake data were fitted with 2 variants of a 2-compartment, 3-rate-constant model, one constrained to have K1 equal to k2 and the other unconstrained. Parametric images of the rate constants were generated. The PO2 measurements were compared with spatially registered maps of kinetic rate constants and tumor-to-plasma ratios.
The constrained pharmacokinetic model variant was shown to provide fits similar to that of the unconstrained model and did not introduce significant bias in the results. The trapping rate constant, k3, of the constrained model provided a better discrimination of low PO2 than the tissue-to-plasma ratio or the k3 of the unconstrained model.
The use of kinetic modeling on a voxelwise basis can identify tumor hypoxia with improved accuracy over simple tumor-to-plasma ratios. An effective means of controlling noise in the trapping rate constant, k3, without introducing significant bias, is to constrain K1 equal to k2 during the fitting process.
PMCID: PMC3784982  PMID: 22933821
hypoxia; [18F]-FMISO; [18F]-fluoromisonidazole; kinetic modeling; parametric images; OxyLite probe
6.  Positron Lymphography: Multimodal, High-Resolution, Dynamic Mapping and Resection of Lymph Nodes After Intradermal Injection of 18F-FDG 
The lymphatic system plays a critical role in the maintenance of healthy tissues. Its function is an important indicator of the presence and extent of disease. In oncology, metastatic spread to local lymph nodes (LNs) is a strong predictor of poor outcome. Clinical methods for the visualization of LNs involve regional injection and tracking of 99mTc-sulfur colloid (99mTc-SC) along with absorbent dyes. Intraoperatively, these techniques suffer from the requirement of administration of multiple contrast media (99mTc-SC and isosulfan blue), unwieldy γ-probes, and a short effective surgical window for dyes. Preclinically, imaging of transport through the lymphatics is further hindered by the resolution of lymphoscintigraphy and SPECT. We investigated multimodal imaging in animal models using intradermal administration of 18F-FDG for combined diagnostic and intraoperative use. PET visualizes LNs with high sensitivity and resolution and low background. Cerenkov radiation (CR) from 18F-FDG was evaluated to optically guide surgical resection of LNs.
Imaging of 18F-FDG uptake used PET and sensitive luminescent imaging equipment (for CR). Dynamic PET was performed in both sexes and multiple strains (NCr Nude, C57BL/6, and Nu/Nu) of mice. Biodistribution confirmed the uptake of 18F-FDG and was compared with that of 99mTc-SC. Verification of uptake and the ability to use 18F-FDG CR to guide nodal removal were confirmed histologically.
Intradermal injection of 18F-FDG clearly revealed lymphatic vessels and LNs by PET. Dynamic imaging revealed rapid and sustained labeling of these structures. Biodistribution of the radiotracer confirmed the active transport of radioglucose in the lymphatics to the local LNs and over time into the general circulation. 18F-FDG also enabled visualization of LNs through CR, even before surgically revealing the site, and guided LN resection.
Intradermal 18F-FDG can enhance the preclinical investigation of the lymphatics through dynamic, high-resolution, and quantitative tomographic imaging. Clinically, combined PET/Cerenkov imaging has significant potential as a single-dose, dual-modality tracer for diagnostics (PET/CT) and guided resection of LNs (Cerenkov optical).
PMCID: PMC3537831  PMID: 22872741
lymph node mapping; PET/CT; Cerenkov; intraoperative
7.  Cerenkov imaging - a new modality for molecular imaging 
Cerenkov luminescence imaging (CLI) is an emerging hybrid modality that utilizes the light emission from many commonly used medical isotopes. Cerenkov radiation (CR) is produced when charged particles travel through a dielectric medium faster than the speed of light in that medium. First described in detail nearly 100 years ago, CR has only recently applied for biomedical imaging purposes. The modality is of considerable interest as it enables the use of widespread luminescence imaging equipment to visualize clinical diagnostic (all PET radioisotopes) and many therapeutic radionuclides. The amount of light detected in CLI applications is significantly lower than other that in other optical imaging techniques such as bioluminescence and fluorescence. However, significant advantages include the use of approved radiotracers and lack of an incident light source, resulting in high signal to background ratios. As well, multiple subjects may be imaged concurrently (up to 5 in common bioluminescent equipment), conferring both cost and time benefits. This review summarizes the field of Cerenkov luminescence imaging to date. Applications of CLI discussed include intraoperative radionuclide-guided surgery, monitoring of therapeutic efficacy, tomographic optical imaging capabilities, and the ability to perform multiplexed imaging using fluorophores excited by the Cerenkov radiation. While technical challenges still exist, Cerenkov imaging has materialized as an important molecular imaging modality.
PMCID: PMC3477724  PMID: 23133811
Cerenkov radiation; PET; optical imaging; fluorescence
8.  Quantitative Modeling of Cerenkov Light Production Efficiency from Medical Radionuclides 
PLoS ONE  2012;7(2):e31402.
There has been recent and growing interest in applying Cerenkov radiation (CR) for biological applications. Knowledge of the production efficiency and other characteristics of the CR produced by various radionuclides would help in accessing the feasibility of proposed applications and guide the choice of radionuclides. To generate this information we developed models of CR production efficiency based on the Frank-Tamm equation and models of CR distribution based on Monte-Carlo simulations of photon and β particle transport. All models were validated against direct measurements using multiple radionuclides and then applied to a number of radionuclides commonly used in biomedical applications. We show that two radionuclides, Ac-225 and In-111, which have been reported to produce CR in water, do not in fact produce CR directly. We also propose a simple means of using this information to calibrate high sensitivity luminescence imaging systems and show evidence suggesting that this calibration may be more accurate than methods in routine current use.
PMCID: PMC3282695  PMID: 22363636
9.  Imaging of Alkaline Phosphatase Activity in Bone Tissue 
PLoS ONE  2011;6(7):e22608.
The purpose of this study was to develop a paradigm for quantitative molecular imaging of bone cell activity. We hypothesized the feasibility of non-invasive imaging of the osteoblast enzyme alkaline phosphatase (ALP) using a small imaging molecule in combination with 19Flourine magnetic resonance spectroscopic imaging (19FMRSI). 6, 8-difluoro-4-methylumbelliferyl phosphate (DiFMUP), a fluorinated ALP substrate that is activatable to a fluorescent hydrolysis product was utilized as a prototype small imaging molecule. The molecular structure of DiFMUP includes two Fluorine atoms adjacent to a phosphate group allowing it and its hydrolysis product to be distinguished using 19Fluorine magnetic resonance spectroscopy (19FMRS) and 19FMRSI. ALP-mediated hydrolysis of DiFMUP was tested on osteoblastic cells and bone tissue, using serial measurements of fluorescence activity. Extracellular activation of DiFMUP on ALP-positive mouse bone precursor cells was observed. Concurringly, DiFMUP was also activated on bone derived from rat tibia. Marked inhibition of the cell and tissue activation of DiFMUP was detected after the addition of the ALP inhibitor levamisole. 19FMRS and 19FMRSI were applied for the non-invasive measurement of DiFMUP hydrolysis. 19FMRS revealed a two-peak spectrum representing DiFMUP with an associated chemical shift for the hydrolysis product. Activation of DiFMUP by ALP yielded a characteristic pharmacokinetic profile, which was quantifiable using non-localized 19FMRS and enabled the development of a pharmacokinetic model of ALP activity. Application of 19FMRSI facilitated anatomically accurate, non-invasive imaging of ALP concentration and activity in rat bone. Thus, 19FMRSI represents a promising approach for the quantitative imaging of bone cell activity during bone formation with potential for both preclinical and clinical applications.
PMCID: PMC3143164  PMID: 21799916
10.  Imaging Transgene Activity 
Cancer research  2008;68(8):2878-2884.
The successful translation of gene therapy for clinical application will require the assessment of transgene activity as a measure of the biological function of a therapeutic transgene. While current imaging permits the non-invasive detection of transgene expression, the critical need for quantitative imaging of the action of the expressed transgene has not been met. Magnetic resonance spectroscopic imaging (MRSI) was applied to quantitatively delineate both the concentration and activity of a cytosine deaminase-uracil phosphoribosyltransferase (CD-UPRT) fusion enzyme expressed from a transgene. MRSI enabled the generation of anatomically accurate maps of the intratumoral heterogeneity in fusion enzyme activity. We observed an excellent association between the CD-UPRT concentration and activity and the percentage of CD-UPRT+ cells. Moreover, the regional levels of UPRT activity, as measured by imaging, correlated well with the biological affect of this enzyme. This study presents a translational imaging strategy for precise, in vivo measurements of transgene activity with potential applications in both pre-clinical and clinical settings.
PMCID: PMC3043616  PMID: 18413756
11.  Possible axonal regrowth in late recovery from the minimally conscious state 
Journal of Clinical Investigation  2006;116(7):2005-2011.
We used diffusion tensor imaging (DTI) to study 2 patients with traumatic brain injury. The first patient recovered reliable expressive language after 19 years in a minimally conscious state (MCS); the second had remained in MCS for 6 years. Comparison of white matter integrity in the patients and 20 normal subjects using histograms of apparent diffusion constants and diffusion anisotropy identified widespread altered diffusivity and decreased anisotropy in the damaged white matter. These findings remained unchanged over an 18-month interval between 2 studies in the first patient. In addition, in this patient, we identified large, bilateral regions of posterior white matter with significantly increased anisotropy that reduced over 18 months. In contrast, notable increases in anisotropy within the midline cerebellar white matter in the second study correlated with marked clinical improvements in motor functions. This finding was further correlated with an increase in resting metabolism measured by PET in this subregion. Aberrant white matter structures were evident in the second patient’s DTI images but were not clinically correlated. We propose that axonal regrowth may underlie these findings and provide a biological mechanism for late recovery. Our results are discussed in the context of recent experimental studies that support this inference.
PMCID: PMC1483160  PMID: 16823492

Results 1-11 (11)