Frezghi Habte

Investigation of position sensitive avalanche photodiodes for a new high-resolution PET detector design

We are developing a high-resolution PET detector design with a goal of nearly complete scintillation light collection in /spl les/1 mm width, /spl ges/20 mm effective thickness LSO crystals. The design uses position sensitive avalanche photodiodes in novel layered configurations that significantly improve the light collection aspect ratio. To reduce design complexity and dead area we are investigating the use of 1 mm thick sheets of LSO in addition to discrete crystal rods, and the use of PSAPDs which require only four readout channels per device. The raw spatial response of a 1 mm thick crystal sheet coupled to a PSAPD exhibits a compressed dynamic range compared to that observed with discrete crystals. Measurements with the proposed configurations using /sup 22/Na irradiation achieved 10%-13% FWHM energy resolution at 511 keV and 2 ns coincidence time resolution. 1 mm width crystals with a saw cut surface finish an no inter-crystal reflector were well resolved in flood images.

Impact of high energy resolution detectors on the performance of a PET system dedicated to breast cancer imaging.

We are developing a high resolution, high sensitivity PET camera dedicated to breast cancer imaging. We are studying two novel detector technologies for this imaging system: a scintillation detector comprising layers of small lutetium oxyorthosilicate (LSO) crystals coupled to new position sensitive avalanche photodiodes (PSAPDs), and a pure semiconductor detector comprising cadmium zinc telluride (CZT) crystal slabs with thin anode and cathode strips deposited in orthogonal directions on either side of each slab. Both detectors achieve 1 mm spatial resolution with 3-5 mm directly measured photon interaction depth resolution, which promotes uniform reconstructed spatial resolution throughout a compact, breast-size field of view. Both detector types also achieve outstanding energy resolution (<3% and <12%, respectively for LSO-PSAPD and CZT at 511 keV). This paper studies the effects that this excellent energy resolution has on the expected system performance. Results indicate the importance that high energy resolution and narrow energy window settings have in reducing background random as well as scatter coincidences without compromising statistical quality of the dedicated breast PET data. Simulations predict that using either detector type the excellent performance and novel arrangement of these detectors proposed for the system facilitate approximately 20% instrument sensitivity at the system center and a peak noise-equivalent count rate of >4 kcps for 200 microCi in a simulated breast phantom.

Reporter gene imaging of targeted T cell immunotherapy in recurrent glioma

PET gene reporter imaging can be used to monitor the trafficking of therapeutic cytotoxic T cells in glioma patients. T cells reporting for duty Cytotoxic T cells engineered to kill tumor cells are becoming a mainstay of cancer immunotherapy. However, no matter how precisely they are engineered, once they are injected into a patient, they are no longer directly monitored or controlled by the researchers. As a result, if the treatment fails to work or causes toxicity, it is not clear whether the therapeutic cells are ineffective or whether they scattered through normal tissues and never reached the tumor. Keu et al. have designed a method to engineer these T cells with a reporter gene such that they can be tracked in people by positron emission tomography. The authors present a clinical trial demonstrating the feasibility and safety of this approach in glioma patients. High-grade gliomas are aggressive cancers that often become rapidly fatal. Immunotherapy using CD8+ cytotoxic T lymphocytes (CTLs), engineered to express both herpes simplex virus type 1 thymidine kinase (HSV1-TK) and interleukin-13 (IL-13) zetakine chimeric antigen receptor (CAR), is a treatment strategy with considerable potential. To optimize this and related immunotherapies, it would be helpful to monitor CTL viability and trafficking to glioma cells. We show that noninvasive positron emission tomography (PET) imaging with 9-[4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine ([18F]FHBG) can track HSV1-tk reporter gene expression present in CAR-engineered CTLs. [18F]FHBG imaging was safe and enabled the longitudinal imaging of T cells stably transfected with a PET reporter gene in patients. Further optimization of this imaging approach for monitoring in vivo cell trafficking should greatly benefit various cell-based therapies for cancer.

Optical coherence contrast imaging using gold nanorods in living mice eyes

Optical coherence tomography (OCT) is a powerful imaging modality to visualize tissue structures, with axial image pixel resolution as high as 1.6 μm in tissue. However, OCT is intrinsically limited to providing structural information as the OCT contrast is produced by optically scattering tissues.

Remodeling of Endogenous Mammary Epithelium by Breast Cancer Stem Cells

Poorly regulated tissue remodeling results in increased breast cancer risk, yet how breast cancer stem cells (CSC) participate in remodeling is unknown. We performed in vivo imaging of changes in fluorescent, endogenous duct architecture as a metric for remodeling. First, we quantitatively imaged physiologic remodeling of primary branches of the developing and regenerating mammary tree. To assess CSC‐specific remodeling events, we isolated CSC from MMTV‐Wnt1 (mouse mammary tumor virus long‐term repeat enhancer driving Wnt1 oncogene) breast tumors, a well studied model in which tissue remodeling affects tumorigenesis. We confirm that CSC drive tumorigenesis, suggesting a link between CSC and remodeling. We find that normal, regenerating, and developing gland maintain a specific branching pattern. In contrast, transplantation of CSC results in changes in the branching patterns of endogenous ducts while non‐CSC do not. Specifically, in the presence of CSC, we identified an increased number of branches, branch points, ducts which have greater than 40 branches (5/33 for CSC and 0/39 for non‐CSC), and histological evidence of increased branching. Moreover, we demonstrate that only CSC implants invade into surrounding stroma with structures similar to developing mammary ducts (nine for CSC and one for non‐CSC). Overall, we demonstrate a novel approach for imaging physiologic and pathological remodeling. Furthermore, we identify unique, CSC‐specific, remodeling events. Our data suggest that CSC interact with the microenvironment differently than non‐CSC, and that this could eventually be a therapeutic approach for targeting CSC. STEM Cells2012;30:2114–2127

Development of A Novel Long-Lived ImmunoPET Tracer for Monitoring Lymphoma Therapy in a Humanized Transgenic Mouse Model

Positron emission tomography (PET) is an attractive imaging tool to localize and quantify tracer biodistribution. ImmunoPET with an intact mAb typically requires two to four days to achieve optimized tumor-to-normal ratios. Thus, a positron emitter with a half-life of two to four days such as zirconium-89 [(89)Zr] (t1/2: 78.4 h) is ideal. We have developed an antibody-based, long-lived immunoPET tracer (89)Zr-Desferrioxamine-p-SCN (Df-Bz-NCS)-rituximab (Zr-iPET) to image tumor for longer durations in a humanized CD20-expressing transgenic mouse model. To optimize the radiolabeling efficiency of (89)Zr with Df-Bz-rituximab, multiple radiolabelings were performed. Radiochemical yield, purity, immunoreactivity, and stability assays were carried out to characterize the Zr-iPET for chemical and biological integrity. This tracer was used to image transgenic mice that express the human CD20 on their B cells (huCD20TM). Each huCD20TM mouse received a 7.4 MBq/dose. One group (n = 3) received a 2 mg/kg predose (blocking) of cold rituximab 2 h prior to (89)Zr-iPET; the other group (n = 3) had no predose (nonblocking). Small animal PET/CT was used to image mice at 1, 4, 24, 48, 72, and 120 h. Quality assurance of the (89)Zr-iPET demonstrated NCS-Bz-Df: antibody ratio (c/a: 1.5 ± 0.31), specific activity (0.44-1.64 TBq/mol), radiochemical yield (>70%), and purity (>98%). The Zr-iPET immunoreactivity was >80%. At 120 h, Zr-iPET uptake (% ID/g) as mean ± STD for blocking and nonblocking groups in spleen was 3.2 ± 0.1% and 83.3 ± 2.0% (p value <0.0013.). Liver uptake was 1.32 ± 0.05% and 0.61 ± 0.001% (p value <0.0128) for blocking and nonblocking, respectively. The small animal PET/CT image shows the spleen specific uptake of Zr-iPET in mice at 120 h after tracer injection. Compared to the liver, the spleen specific uptake of Zr-iPET is very high due to the expression of huCD20. We optimized the radiolabeling efficiency of (89)Zr with Df-Bz-rituximab. These radioimmunoconjugate lots were stable up to 5 days in serum in vitro. The present study showed that (89)Zr is well-suited for mAbs to image cancer over an extended period of time (up to 5 days).

A fieldable-prototype, large-area, gamma-ray imager for orphan source search

We have constructed a unique instrument for use in the search for orphan sources. The system uses gamma-ray imaging to ldquosee throughrdquo the natural background variations that effectively limit the sensitivity of current devices. The imager is mounted in a 4.9-m-long trailer and can be towed by a large personal vehicle. Source locations are determined both in range and along the direction of travel as the vehicle moves. A fully inertial platform coupled to a Global Positioning System receiver is used to map the gamma-ray images onto overhead geospatial imagery. The resulting images provide precise source locations, allowing rapid follow-up work. The instrument simultaneously searches both sides of the street to a distance of 50 m (100-m swath) for millicurie-class sources with excellent performance as determined using source injection studies and receiver-operator-characteristic techniques.

A new scaleable modular data acquisition system for SPECT (PET)

Describes a modular data acquisition system that continuously samples shaped PMT pulses from a SPECT detector. The pulse waveform data are used by signal processors to accurately reconstruct amplitude and time for each scintillation event. Data acquisition for a PMT channel is triggered in two alternative ways, either when its own signal exceeds a selected digital threshold, or when it receives a trigger pulse from one of its neighboring PMTs. The triggered region is restricted to seven, thirteen or nineteen neighboring PMT channels. Each acquisition module supports three PMT channels and connects to all other modules and a reconstruction computer via Firewire-a high speed serial bus. 24 modules are needed to cover the 72 channels in the Stockholm University/Karolinska Hospital cylindrical SPECT camera.

A Fieldable-Prototype, Large-Area, Gamma-Ray Imager for Orphan Source Search

We have constructed a unique instrument for use in the search for orphan sources. The system uses gamma-ray imaging to ldquosee throughrdquo the natural background variations that effectively limit the sensitivity of current devices. The imager is mounted in a 4.9-m-long trailer and can be towed by a large personal vehicle. Source locations are determined both in range and along the direction of travel as the vehicle moves. A fully inertial platform coupled to a Global Positioning System receiver is used to map the gamma-ray images onto overhead geospatial imagery. The resulting images provide precise source locations, allowing rapid follow-up work. The instrument simultaneously searches both sides of the street to a distance of 50 m (100-m swath) for millicurie-class sources with excellent performance as determined using source injection studies and receiver-operator-characteristic techniques.

64Cu-Labeled Divalent Cystine Knot Peptide for Imaging Carotid Atherosclerotic Plaques

The rupture of vulnerable atherosclerotic plaques that lead to stroke and myocardial infarction may be induced by macrophage infiltration and augmented by the expression of integrin αvβ3. Indeed, atherosclerotic angiogenesis may be a promising marker of inflammation. In this study, an engineered integrin αvβ3–targeting PET probe, 64Cu-NOTA-3-4A, derived from a divalent knottin miniprotein was evaluated in a mouse model for carotid atherosclerotic plaques. Methods: Atherosclerotic plaques in BALB/C mice, maintained on a high-fat diet, were induced with streptozotocin injection and carotid artery ligation and verified by MR imaging. Knottin 3-4A was synthesized by solid-phase peptide synthesis chemistry and coupled to 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) before radiolabeling with 64Cu. PET probe stability in mouse serum was evaluated. Mice with carotid atherosclerotic plaques were injected via the tail vein with 64Cu-NOTA-3-4A or 18F-FDG, followed by small-animal PET/CT imaging at different time points. Receptor targeting specificity of the probe was verified by coinjection of c(RGDyK) administered in molar excess. Subsequently, carotid artery dissection and immunofluorescence staining were performed to evaluate target expression. Results: 64Cu-NOTA-3-4A was synthesized in high radiochemical purity and yield and demonstrated molecular stability in both phosphate-buffered saline and mouse serum at 4 h. Small-animal PET/CT showed that 64Cu-NOTA-3-4A accumulated at significantly higher levels in the neovasculature of carotid atherosclerotic plaques (7.41 ± 1.44 vs. 0.67 ± 0.23 percentage injected dose/gram, P < 0.05) than healthy or normal vessels at 1 h after injection. 18F-FDG also accumulated in atherosclerotic lesions at 0.5 and 1 h after injection but at lower plaque–to–normal tissue ratios than 64Cu-NOTA-3-4A. For example, plaque–to–normal carotid artery ratios for 18F-FDG and 64Cu-NOTA-3-4A at 1 h after injection were 3.75 and 14.71 (P < 0.05), respectively. Furthermore, uptake of 64Cu-NOTA-3-4A in atherosclerotic plaques was effectively blocked (∼90% at 1 h after injection) by coinjection of c(RGDyK). Immunostaining confirmed integrin αvβ3 expression in both the infiltrating macrophages and the neovasculature of atherosclerotic plaques. Conclusion: 64Cu-NOTA-3-4A demonstrates specific accumulation in carotid atherosclerotic plaques in which macrophage infiltration and angiogenesis are responsible for elevated integrin αvβ3 levels. Therefore, 64Cu-NOTA-3-4A may demonstrate clinical utility as a PET probe for atherosclerosis imaging or for the evaluation of therapies used to treat atherosclerosis.