U-SPECT-II: An Ultra-High-Resolution Device for Molecular Small-Animal Imaging

Evaluation of a compact, high-resolution SPECT detector based on digital silicon photomultipliers

MicroSPECT is one of the main functional imaging techniques used in the preclinical setting. Even though high-resolution images can be obtained with currently available systems, their sensitivity is often quite low due to the use of multi-pinhole collimation. This results in long acquisition times and hampers dynamic imaging. However, it has already been shown that this limited sensitivity can be overcome using high-resolution detectors. In this article, we therefore investigated the use of a digital photon counter (DPC) in combination with a 2 mm thick monolithic LYSO crystal for SPECT imaging. These light sensors contain arrays of avalanche photodiodes whose signals are directly digitised. The DPCs have the advantage that they are very compact, have a high intrinsic resolution, are MR compatible and allow disabling cells with a high dark count rate. In order to investigate the influence of the temperature dependent dark count rate on the detector performance, we compared it at 3 °C and 18 °C. At 3 °C, we observed an energy resolution of 28.8% and an intrinsic spatial resolution of 0.48 mm. Furthermore, the count rate at 10% loss is 60 kcps. Next, we looked at the event loss at 18 °C caused by the higher dark count rate and found a 5% loss compared to the 3 °C measurements. At this higher temperature the energy resolution becomes 29.2% and the intrinsic spatial resolution decreases to 0.52 mm. Due to the 5% count loss, the count rate at 10% loss increases to 63 kcps. A small degradation of the detector performance is thus observed at 18 °C.These results show the usefulness of this detector for SPECT imaging together with its excellent intrinsic spatial resolution. A drawback of the detector is its low, spatially varying energy resolution. Even though the detection efficiency and intrinsic spatial resolution are better at 3 °C, results are still acceptable at 18 °C.

Multipinhole collimator with 20 apertures for a brain SPECT application

PURPOSE

Several new technologies for single photon emission computed tomography (SPECT) instrumentation with parallel-hole collimation have been proposed to improve detector sensitivity and signal collection efficiency. Benefits from improved signal efficiency include shorter acquisition times and lower dose requirements. In this paper, the authors show a possibility of over an order of magnitude enhancement in photon detection efficiency (from 7.6 × 10(-5) to 1.6 × 10(-3)) for dopamine transporter (DaT) imaging of the striatum over the conventional SPECT parallel-hole collimators by use of custom-designed 20 multipinhole (20-MPH) collimators with apertures of 0.75 cm diameter.

METHODS

Quantifying specific binding ratio (SBR) of (123)I-ioflupane or (123)I-iometopane's signal at the striatal region is a common brain imaging method to confirm the diagnosis of the Parkinson's disease. The authors performed imaging of a striatal phantom filled with aqueous solution of I-123 and compared camera recovery ratios of SBR acquired between low-energy high-resolution (LEHR) parallel-hole collimators and 20-MPH collimators.

RESULTS

With only two-thirds of total acquisition time (20 min against 30 min), a comparable camera recovery ratio of SBR was achieved using 20-MPH collimators in comparison to that from the LEHR collimator study.

CONCLUSIONS

Their systematic analyses showed that the 20-MPH collimator could be a promising alternative for the DaT SPECT imaging for brain over the traditional LEHR collimator, which could give both shorter scan time and improved diagnostic accuracy.

Can 111In-RGD2 monitor response to therapy in head and neck tumor xenografts?

RGD (arginylglycylaspartic acid)-based imaging tracers allow specific imaging of integrin αvβ3 expression, proteins overexpressed during angiogenesis; however, few studies have investigated the potential of these tracers to monitor responses of antiangiogenic or radiation therapy. In the studies presented here, (111)In-RGD2 was assessed for its potential as an imaging tool to monitor such responses to therapies.

METHODS

DOTA-E-[c(RGDfK)]2 was radiolabeled with (111)In ((111)In-RGD2), and biodistribution studies were performed in mice with subcutaneous FaDu or SK-RC-52 xenografts after treatment with either antiangiogenic therapy (bevacizumab or sorafenib) or tumor irradiation (10 Gy). Micro-SPECT imaging studies and subsequent quantitative analysis were also performed. The effect of bevacizumab, sorafenib, or radiation therapy on tumor growth was determined.

RESULTS

The uptake of (111)In-RGD2 in tumors, as determined from biodistribution studies, correlated well with that quantified from micro-SPECT images, and both showed that 15 d after irradiation (111)In-RGD2 uptake was enhanced. Specific or nonspecific uptake of (111)In-RGD2 in FaDu or SK-RC-52 xenografts was not affected after antiangiogenic therapy, except in head and neck squamous cell carcinoma 19 d after the start of sorafenib therapy (P < 0.05). The uptake of (111)In-RGD2 followed tumor volume in studies featuring antiangiogenic therapy. However, the uptake of (111)In-RGD2 in FaDu xenografts was decreased as early as 4 h after tumor irradiation, despite nonspecific uptake remaining unaltered. Tumor growth was inhibited after antiangiogenic or radiation therapy.

CONCLUSION

Here, it is suggested that (111)In-RGD2 could allow in vivo monitoring of angiogenic responses after radiotherapy and may therefore prove a good clinical tool to monitor angiogenic responses early after the start of radiotherapy in patients with head and neck squamous cell carcinoma. Despite clear antitumor efficacy, antiangiogenic therapy did not alter tumor uptake of (111)In-RGD2, indicating that integrin expression was not altered.

Molecular Imaging in the College of Optical Sciences - An Overview of Two Decades of Instrumentation Development

During the past two decades, researchers at the University of Arizona's Center for Gamma-Ray Imaging (CGRI) have explored a variety of approaches to gamma-ray detection, including scintillation cameras, solid-state detectors, and hybrids such as the intensified Quantum Imaging Device (iQID) configuration where a scintillator is followed by optical gain and a fast CCD or CMOS camera. We have combined these detectors with a variety of collimation schemes, including single and multiple pinholes, parallel-hole collimators, synthetic apertures, and anamorphic crossed slits, to build a large number of preclinical molecular-imaging systems that perform Single-Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET), and X-Ray Computed Tomography (CT). In this paper, we discuss the themes and methods we have developed over the years to record and fully use the information content carried by every detected gamma-ray photon.

Cetuximab reduces the accumulation of radiolabeled bevacizumab in cancer xenografts without decreasing VEGF expression

Bevacizumab and cetuximab are approved for the treatment of cancer. However, in advanced colorectal cancer, addition of cetuximab to chemotherapy with bevacizumab did not improve survival. The reason for the lack of activity remains unclear. The aim of this study was to determine the effect of cetuximab on VEGF expression and targeting of bevacizumab to the tumor. Mice with subcutaneous SUM149 or WiDr xenografts were treated with cetuximab, bevacizumab, or a combination of the two. Before the start of cetuximab treatment and after 7 and 21 days of treatment, the uptake of radiolabeled bevacizumab in the tumor was measured by immunoSPECT/CT. Tumor growth of SUM149 xenografts was significantly inhibited by cetuximab, bevacizumab, or their combination, whereas growth of WiDr xenografts was not affected. Cetuximab caused a significant reduction of bevacizumab uptake in SUM149 xenografts, whereas tumor-to-blood ratios in mice with WiDr xenografts did not change. Biodistribution studies with an irrelevant antibody in the SUM149 model also showed significantly reduced tumor-to-blood ratios. Cetuximab treatment did not decrease VEGF expression. Without decreasing VEGF levels, cetuximab reduces tumor targeting of bevacizumab. This could, at least partly, explain why the combination of bevacizumab and cetuximab does not result in improved therapeutic efficacy.

U-SPECT-BioFluo: an integrated radionuclide, bioluminescence, and fluorescence imaging platform

Background

In vivo bioluminescence, fluorescence, and single-photon emission computed tomography (SPECT) imaging provide complementary information about biological processes. However, to date these signatures are evaluated separately on individual preclinical systems. In this paper, we introduce a fully integrated bioluminescence-fluorescence-SPECT platform. Next to an optimization in logistics and image fusion, this integration can help improve understanding of the optical imaging (OI) results.

Methods

An OI module was developed for a preclinical SPECT system (U-SPECT, MILabs, Utrecht, the Netherlands). The applicability of the module for bioluminescence and fluorescence imaging was evaluated in both a phantom and in an in vivo setting using mice implanted with a 4 T1-luc + tumor. A combination of a fluorescent dye and radioactive moiety was used to directly relate the optical images of the module to the SPECT findings. Bioluminescence imaging (BLI) was compared to the localization of the fluorescence signal in the tumors.

Results

Both the phantom and in vivo mouse studies showed that superficial fluorescence signals could be imaged accurately. The SPECT and bioluminescence images could be used to place the fluorescence findings in perspective, e.g. by showing tracer accumulation in non-target organs such as the liver and kidneys (SPECT) and giving a semi-quantitative read-out for tumor spread (bioluminescence).

Conclusions

We developed a fully integrated multimodal platform that provides complementary registered imaging of bioluminescent, fluorescent, and SPECT signatures in a single scanning session with a single dose of anesthesia. In our view, integration of these modalities helps to improve data interpretation of optical findings in relation to radionuclide images.

Design and performance of a compact and stationary microSPECT system

PURPOSE

Over the last ten years, there has been an extensive growth in the development of microSPECT imagers. Most of the systems are based on the combination of conventional, relatively large gamma cameras with poor intrinsic spatial resolution and multipinhole collimators working in large magnification mode. Spatial resolutions range from 0.58 to 0.76 mm while peak sensitivities vary from 0.06% to 0.4%. While pushing the limits of performance is of major importance, the authors believe that there is a need for smaller and less complex systems that bring along a reduced cost. While low footprint and low-cost systems can make microSPECT available to more researchers, the ease of operation and calibration and low maintenance cost are additional factors that can facilitate the use of microSPECT in molecular imaging. In this paper, the authors simulate the performance of a microSPECT imager that combines high space-bandwidth detectors and pinholes with truncated projection, resulting in a small and stationary system.

METHODS

A system optimization algorithm is used to determine the optimal SPECT systems, given our high resolutions detectors and a fixed field-of-view. These optimal system geometries are then used to simulate a Defrise disk phantom and a hot rod phantom. Finally, a MOBY mouse phantom, with realistic concentrations of Tc99m-tetrofosmin is simulated.

RESULTS

Results show that the authors can successfully reconstruct a Defrise disk phantom of 24 mm in diameter without any rotating system components or translation of the object. Reconstructed spatial resolution is approximately 800 μm while the peak sensitivity is 0.23%. Finally, the simulation of the MOBY mouse phantom shows that the authors can accurately reconstruct mouse images.

CONCLUSIONS

These results show that pinholes with truncated projections can be used in small magnification or minification mode to obtain a compact and stationary microSPECT system. The authors showed that they can reach state-of-the-art system performance and can successfully reconstruct images with realistic noise levels in a preclinical context. Such a system can be useful for dynamic SPECT imaging.

Perspectives in molecular imaging through translational research, human medicine, and veterinary medicine

The concept of molecular imaging has taken off over the past 15 years to the point of the renaming of the Society of Nuclear Medicine (Society of Nuclear Medicine and Molecular Imaging) and Journals (European Journal of Nuclear Medicine and Molecular Imaging) and offering of medical fellowships specific to this area of study. Molecular imaging has always been at the core of functional imaging related to nuclear medicine. Even before the phrase molecular imaging came into vogue, radionuclides and radiopharmaceuticals were developed that targeted select physiological processes, proteins, receptor analogs, antibody-antigen interactions, metabolites and specific metabolic pathways. In addition, with the advent of genomic imaging, targeted genomic therapy, and theranostics, a number of novel radiopharmaceuticals for the detection and therapy of specific tumor types based on unique biological and cellular properties of the tumor itself have been realized. However, molecular imaging and therapeutics as well as the concept of theranostics are yet to be fully realized. The purpose of this review article is to present an overview of the translational approaches to targeted molecular imaging with application to some naturally occurring animal models of human disease.

The role of SPECT imaging of the dopaminergic system in translational research on Parkinson's disease

Imaging of the dopaminergic system with single photon emission computed tomography (SPECT), and particularly of the dopamine transporter (DAT) located in the striatum, is a well accepted tool in clinical practice to confirm or exclude loss of nigrostriatal dopamine (DA) neurons in patients suspected to suffer from Parkinson's disease (PD). SPECT techniques were developed successfully to image neurotransmitter systems, including the presynaptic DAT and postsynaptic dopamine D2/3 receptors, in rat and mouse models of PD. Here we review the results of preclinical SPECT studies of the dopaminergic system in rat and mouse models of PD. Initially, SPECT studies in animal models of PD were performed to validate that micro-SPECT is able to accurately assess parts of the dopaminergic system in small animals in-vivo. However, more recently, micro-SPECT DAT is increasingly used as a research tool to support the interpretation of human DAT SPECT studies in PD, including clinical trials examining the effects of potential neuroprotective drugs. Translational research with SPECT is an interesting development which may further increase our understanding of the pathophysiology and treatment of PD.

Performance assessment of a preclinical PET scanner with pinhole collimation by comparison to a coincidence-based small-animal PET scanner

PET imaging of rodents is increasingly used in preclinical research, but its utility is limited by spatial resolution and signal-to-noise ratio of the images. A recently developed preclinical PET system uses a clustered-pinhole collimator, enabling high-resolution, simultaneous imaging of PET and SPECT tracers. Pinhole collimation strongly departs from traditional electronic collimation achieved via coincidence detection in PET. We investigated the potential of such a design by direct comparison to a traditional PET scanner.

METHODS

Two small-animal PET scanners, 1 with electronic collimation and 1 with physical collimation using clustered pinholes, were used to acquire data from Jaszczak (hot rod) and uniform phantoms. Mouse brain imaging using (18)F-FDG PET was performed on each system and compared with quantitative ex vivo autoradiography as a gold standard. Bone imaging using (18)F-NaF allowed comparison of imaging in the mouse body. Images were visually and quantitatively compared using measures of contrast and noise.

RESULTS

Pinhole PET resolved the smallest rods (diameter, 0.85 mm) in the Jaszczak phantom, whereas the coincidence system resolved 1.1-mm-diameter rods. Contrast-to-noise ratios were better for pinhole PET when imaging small rods (<1.1 mm) for a wide range of activity levels, but this reversed for larger rods. Image uniformity on the coincidence system (<3%) was superior to that on the pinhole system (5%). The high (18)F-FDG uptake in the striatum of the mouse brain was fully resolved using the pinhole system, with contrast to nearby regions equaling that from autoradiography; a lower contrast was found using the coincidence PET system. For short-duration images (low-count), the coincidence system was superior.

CONCLUSION

In the cases for which small regions need to be resolved in scans with reasonably high activity or reasonably long scan times, a first-generation clustered-pinhole system can provide image quality in terms of resolution, contrast, and the contrast-to-noise ratio superior to a traditional PET system.

High-Resolution Anamorphic SPECT Imaging

We have developed a gamma-ray imaging system that combines a high-resolution silicon detector with two sets of movable, half-keel-edged copper-tungsten blades configured as crossed slits. These apertures can be positioned independently between the object and detector, producing an anamorphic image in which the axial and transaxial magnifications are not constrained to be equal. The detector is a 60 mm × 60 mm, one-millimeter-thick, one-megapixel silicon double-sided strip detector with a strip pitch of 59 μm. The flexible nature of this system allows the application of adaptive imaging techniques. We present system details; calibration, acquisition, and reconstruction methods; and imaging results.

The role of preclinical SPECT in oncological and neurological research in combination with either CT or MRI

Preclinical imaging with SPECT combined with CT or MRI is used more and more frequently and has proven to be very useful in translational research. In this article, an overview of current preclinical research applications and trends of SPECT combined with CT or MRI, mainly in tumour imaging and neuroscience imaging, is given and the advantages and disadvantages of the different approaches are described. Today SPECT and CT systems are often integrated into a single device (commonly called a SPECT/CT system), whereas at present combined SPECT and MRI is almost always carried out with separate systems and fiducial markers to combine the separately acquired images. While preclinical SPECT/CT is most widely applied in oncology research, SPECT combined with MRI (SPECT/MRI when integrated in one system) offers the potential for both neuroscience applications and oncological applications. Today CT and MRI are still mainly used to localize radiotracer binding and to improve SPECT quantification, although both CT and MRI have additional potential. Future technology developments may include fast sequential or simultaneous acquisition of (dynamic) multimodality data, spectroscopy, fMRI along with high-resolution anatomic MRI, advanced CT procedures, and combinations of more than two modalities such as combinations of SPECT, PET, MRI and CT all together. This will all strongly depend on new technologies. With further advances in biology and chemistry for imaging molecular targets and (patho)physiological processes in vivo, the introduction of new imaging procedures and promising new radiopharmaceuticals in clinical practice may be accelerated.

Three-dimensional histologic validation of high-resolution SPECT of antibody distributions within xenografts

Longitudinal imaging of intratumoral distributions of antibodies in vivo in mouse cancer models is of great importance for developing cancer therapies. In this study, multipinhole SPECT with sub-half-millimeter resolution was tested for exploring intratumoral distributions of radiolabeled antibodies directed toward the epidermal growth factor receptor (EGFr) and compared with full 3-dimensional target expression assessed by immunohistochemistry.

METHODS

(111)In-labeled zalutumumab, a human monoclonal human EGFr-targeting antibody, was administered at a nonsaturating dose to 3 mice with xenografted A431 tumors exhibiting high EGFr expression. Total-body and focused in vivo tumor SPECT was performed at 0 and 48 h after injection and compared both visually and quantitatively with full 3-dimensional immunohistochemical staining for EGFr target expression.

RESULTS

SPECT at 48 h after injection showed that activity was predominantly concentrated in the tumor (10.5% ± 1.3% of the total-body activity; average concentration, 30.1% ± 4.6% of the injected dose per cubic centimeter). (111)In-labeled EGFr-targeting antibodies were distributed heterogeneously throughout the tumor. Some hot spots were observed near the tumor rim. Immunohistochemistry indicated that the antibody distributions obtained by SPECT were morphologically similar to those obtained for ex vivo EGFr target expression. Regions showing low SPECT activity were necrotic or virtually negative for EGFr target expression. A good correlation (r = 0.86, P < 0.0001) was found between the percentage of regions showing low activity on SPECT and the percentage of necrotic tissue on immunohistochemistry.

CONCLUSION

Multipinhole SPECT enables high-resolution visualization and quantification of the heterogeneity of (111)In-zalutumumab concentrations in vivo.

Performance evaluation of the eXplore speCZT preclinical imaging system

OBJECTIVE

The eXplore speCZT is a recently introduced cadmium zinc telluride-based preclinical SPECT system that has a stationary detector design with interchangeable rotating collimators. Our aim was to evaluate the performance of the eXplore speCZT using 99mTc-sources. In particular, the image quality was assessed using the National Electrical Manufacturers Association NU-4 image quality phantom as well as an in vivo mouse.

METHODS

Energy resolution, sensitivity and spatial resolution were measured using 99mTc sources. Image quality was assessed using NU-4 image quality phantom. The measurements were performed for 4 available collimators: (1) mouse 7-pinhole collimator (mouse PH); (2) mouse 8-slit collimator (mouse SL); (3) rat 5-pinhole collimator (rat PH); and (4) rat 5-slit collimator (rat SL). Furthermore, a mouse bone imaging study was performed using mouse PH and mouse SL.

RESULTS

The system achieved the energy resolution of 5.5% in full-width at half maximum (FWHM) at 140 keV using a 99mTc source. Without resolution recovery function, the system provided a near millimeter transaxial and axial spatial resolution using mouse PH. Mouse SL and rat SL provided reasonably good transaxial (1.79-2.00 mm in FWHM), but much worse axial resolutions (4.55-4.96 mm in FWHM). The use of resolution recovery significantly improved spatial resolution by in average 31±3 or 35±4% in FWHM or full-width at tenth maximum, respectively. In particular, a sub-millimeter resolution of 0.71 mm in FWHM was achieved in either transaxial or axial direction with mouse PH. Using NU-4 phantom, the uniformity of slit collimators as expressed as percentage standard deviation was generally better than that of pinhole collimators. The use of resolution recovery substantially improved uniformity for all the collimators tested, but caused some overestimation in recovery coefficient. Reconstruction settings such as iteration or subset number significantly affected image quality measures. Finally, bone images of acceptable quality were obtained in in vivo mouse using mouse PH with resolution recovery.

CONCLUSIONS

The overall performance shows that the eXplore speCZT system is suitable for preclinical imaging-based research using small-animals.

Imaging integrin αvβ3 on blood vessels with 111In-RGD2 in head and neck tumor xenografts

Arginine-glycine-aspartic acid (RGD)-based imaging tracers allow specific imaging of integrin αvβ3, a protein overexpressed during angiogenesis, leading to the possibility that it might serve as a tool to stratify patients for antiangiogenic treatment. However, these tracers have generally been characterized in xenograft models in which integrin αvβ3 was constitutively expressed by the tumor cells themselves. In the studies presented here, the use of (111)In-RGD2 as a tracer to image only integrin αvβ3 expression on blood vessels in the tumor was determined using tumor xenografts in which tumor cells were integrin αvβ3-negative.

METHODS

DOTA-E-[c(RGDfK)]2 was radiolabeled with (111)In ((111)In-RGD2), and biodistribution studies were performed in squamous cell carcinoma of the head and neck (HNSCC) xenograft mouse models to determine the optimal peptide dose to image angiogenesis. Next, biodistribution and imaging studies were performed at the optimal peptide dose in 3 HNSCC mouse models, FaDu, SCCNij3, and SCCNij202. Immunohistochemical analysis of tumor vascular and cell surface expression of integrin αvβ3 and correlation analysis of vascular integrin αvβ3 and autoradiography were completed.

RESULTS

All 3 HNSCC xenografts expressed integrin αvβ3 on the vessels only. The optimal peptide dose of (111)In-RGD2 was 1 μg or less for specific integrin αvβ3-mediated uptake of the tracer. SPECT/CT imaging showed clear uptake of the tracer in the periphery of the tumors, corresponding with well-vascularized areas of the tumor. Within the tumor, (111)In-RGD2 autoradiography coincided with vascular integrin αvβ3 expression, as determined immunohistochemically. Integrin αvβ3-mediated uptake was also detected in nontumor tissues, which, through immunohistochemical analysis, proved positive for integrin αvβ3.

CONCLUSION

(111)In-RGD2 allows the visualization of integrin αvβ3 in xenograft models in which integrin αvβ3 is expressed only on the neovasculature, such as in the HNSCC tumors. Thus, (111)In-RGD2 allows specific visualization of angiogenesis in tumor models lacking constitutive tumoral integrin αvβ3 expression but may be less useful for this purpose in many tumors in which tumor cells express integrin αvβ3.

A high resolution SPECT detector based on thin continuous LYSO

Single-photon emission computed tomography (SPECT) detectors with improved spatial resolution can be used to build multi-pinhole SPECT systems that have a higher sensitivity or a higher spatial resolution. In order to improve the spatial resolution we investigate the performance of a 2 mm thick continuous Lutetium Yttrium Orthosilicate (LYSO) scintillator and compare it to the performance of a 5 mm thick continuous NaI(Tl) scintillator. The advantages of LYSO are its high stopping power and its non-hygroscopicity. Drawbacks are the lower light output and the intrinsic radioactivity. The hypothesis of this study is that such a thin LYSO scintillator will have a small light spread and, as a consequence, will also have an improved spatial resolution when coupled to a Hamamatsu H8500 position sensitive photomultiplier tube. To optimize the spatial resolution and the useful detector area we used a mean nearest neighbor event-positioning method. Beam source measurements ((99m)Tc, 140 keV) were done to investigate the energy resolution and the spatial resolution of both detectors. The effect of the intrinsic radioactivity of the LYSO scintillator in the energy window was quantified. The mean energy resolution is 9.3% for the NaI(Tl) scintillator and 21.3% for the LYSO scintillator. The LYSO spectrum shows an X-ray escape peak which decreases the detection efficiency with 9.1%. The spatial resolution of the LYSO detector (0.93 mm full width at half maximum (FWHM)) is superior to the spatial resolution of the NaI(Tl) detector (1.37 mm FWHM). The intrinsic radioactivity in the energy window (42% window centered at 140 keV) is low (125.6 cps, 0.024 cps mm(-3)). LYSO is a promising scintillator for small-animal SPECT imaging, where spatial resolution is more important than energy resolution.

ALBIRA: a small animal PET∕SPECT∕CT imaging system

PURPOSE

The authors have developed a trimodal PET∕SPECT∕CT scanner for small animal imaging. The gamma ray subsystems are based on monolithic crystals coupled to multianode photomultiplier tubes (MA-PMTs), while computed tomography (CT) comprises a commercially available microfocus x-ray tube and a CsI scintillator 2D pixelated flat panel x-ray detector. In this study the authors will report on the design and performance evaluation of the multimodal system.

METHODS

X-ray transmission measurements are performed based on cone-beam geometry. Individual projections were acquired by rotating the x-ray tube and the 2D flat panel detector, thus making possible a transaxial field of view (FOV) of roughly 80 mm in diameter and an axial FOV of 65 mm for the CT system. The single photon emission computed tomography (SPECT) component has a dual head detector geometry mounted on a rotating gantry. The distance between the SPECT module detectors can be varied in order to optimize specific user requirements, including variable FOV. The positron emission tomography (PET) system is made up of eight compact modules forming an octagon with an axial FOV of 40 mm and a transaxial FOV of 80 mm in diameter. The main CT image quality parameters (spatial resolution and uniformity) have been determined. In the case of the SPECT, the tomographic spatial resolution and system sensitivity have been evaluated with a (99m)Tc solution using single-pinhole and multi-pinhole collimators. PET and SPECT images were reconstructed using three-dimensional (3D) maximum likelihood and ordered subset expectation maximization (MLEM and OSEM) algorithms developed by the authors, whereas the CT images were obtained using a 3D based FBP algorithm.

RESULTS

CT spatial resolution was 85 μm while a uniformity of 2.7% was obtained for a water filled phantom at 45 kV. The SPECT spatial resolution was better than 0.8 mm measured with a Derenzo-like phantom for a FOV of 20 mm using a 1-mm pinhole aperture collimator. The full width at half-maximum PET radial spatial resolution at the center of the field of view was 1.55 mm. The SPECT system sensitivity for a FOV of 20 mm and 15% energy window was 700 cps∕MBq (7.8 × 10(-2)%) using a multi-pinhole equipped with five apertures 1 mm in diameter, whereas the PET absolute sensitivity was 2% for a 350-650 keV energy window and a 5 ns timing window. Several animal images are also presented.

CONCLUSIONS

The new small animal PET∕SPECT∕CT proposed here exhibits high performance, producing high-quality images suitable for studies with small animals. Monolithic design for PET and SPECT scintillator crystals reduces cost and complexity without significant performance degradation.

Fast Count-Regulated OSEM Reconstruction With Adaptive Resolution Recovery

Ordered subsets expectation maximization (OSEM) is widely used to accelerate tomographic reconstruction. Speed-up of OSEM over maximum likelihood expectation maximization (MLEM) is close to the number of subsets (NS). Recently we significantly increased the speed-up achievable with OSEM by specific subset choice (pixel-based OSEM). However, a high NS can cause undesirable noise levels, quantitative inaccuracy or even disappearance of lesions in low-activity image regions, while a low NS leads to prohibitively long reconstructions or unrecovered details in highly active regions. Here, we introduce count-regulated OSEM (CROSEM) which locally adapts the effective NS based on the estimated amount of detected photons originating from individual voxels. CROSEM was tested using multi-pinhole SPECT simulations and in vivo imaging. With the maximum NS set to 128, CROSEM attained acceleration factors close to 128 in high-activity regions and kept quantitative accuracy in low-activity regions close to that of MLEM. At equal cold-lesion contrast in high-activity regions, CROSEM exhibited lower noise than MLEM in low-activity regions. CROSEM is a fast and stable alternative to OSEM, preventing excessive image noise and quantitative errors in low-activity regions while achieving high-resolution recovery in structures with high activity uptake.

On Artifact-Free Projection Overlaps in Multi-Pinhole Tomographic Imaging

In multi-pinhole SPECT, overlapping the projections from different pinholes has been used to increase sensitivity. However, the prevailing view is that the overall quality of the reconstructed image is not improved by the overlaps in the projections. It is often stated in literatures that overlaps introduce ambiguous information, which can lead to nonuniqueness of solution for the inverse problem, and thus artifacts are introduced in the reconstructed image. On the other hand, contrary to the prevailing view, a recent study on slit-slat collimators shows that artifacts can be removed with the "help" of an extra complete nonoverlapped projection data set. In this paper, two types of artifact-free projection overlaps are defined in general, and the criteria for designing artifact-free multi-pinhole systems with overlaps are proposed. It is shown that once the criteria are satisfied, the solution of the inverse problem is unique, and thus no artifact is expected in the reconstructed image. Via extensive simulation study, various artifact-free overlapping multi-pinhole systems are designed and validated. It is shown that overlaps in the artifact-free systems can improve contrast-to-noise ratio (CNR). With a proper design, the CNR for an artifact-free overlapping system can be significantly higher than that for the corresponding nonoverlapping system. The improved image quality is also confirmed with noisy reconstructions.

Performance assessment of the single photon emission microscope: high spatial resolution SPECT imaging of small animal organs

The single photon emission microscope (SPEM) is an instrument developed to obtain high spatial resolution single photon emission computed tomography (SPECT) images of small structures inside the mouse brain. SPEM consists of two independent imaging devices, which combine a multipinhole collimator, a high-resolution, thallium-doped cesium iodide [CsI(Tl)] columnar scintillator, a demagnifying/intensifier tube, and an electron-multiplying charge-coupling device (CCD). Collimators have 300- and 450-µm diameter pinholes on tungsten slabs, in hexagonal arrays of 19 and 7 holes. Projection data are acquired in a photon-counting strategy, where CCD frames are stored at 50 frames per second, with a radius of rotation of 35 mm and magnification factor of one. The image reconstruction software tool is based on the maximum likelihood algorithm. Our aim was to evaluate the spatial resolution and sensitivity attainable with the seven-pinhole imaging device, together with the linearity for quantification on the tomographic images, and to test the instrument in obtaining tomographic images of different mouse organs. A spatial resolution better than 500 µm and a sensitivity of 21.6 counts·s^-1·MBq^-1 were reached, as well as a correlation coefficient between activity and intensity better than 0.99, when imaging ^99mTc sources. Images of the thyroid, heart, lungs, and bones of mice were registered using ^99mTc-labeled radiopharmaceuticals in times appropriate for routine preclinical experimentation of <1 h per projection data set. Detailed experimental protocols and images of the aforementioned organs are shown. We plan to extend the instrument's field of view to fix larger animals and to combine data from both detectors to reduce the acquisition time or applied activity.

Performance evaluation of stationary and semi-stationary acquisition with a non-stationary small animal multi-pinhole SPECT system

PURPOSE

Step-and-shoot mode with many angular steps results in long frame duration limiting the capability of single-photon emission computed tomography (SPECT) for fast dynamic scans. The present study evaluates acquisition with reduced angular sampling for fast imaging in preclinical research with the nanoSPECT/CTplus four-head multi-pinhole system.

PROCEDURES

Measurements with line sources, homogeneity phantoms and a Jaszczak phantom filled with (99m)Tc or (123)I were performed to evaluate the 'stationary' and 'semi-stationary' acquisition mode (one or two detector positions, respectively) with respect to spatial resolution, quantification, noise properties and image artefacts. An in vivo mouse study was performed with (99m)Tc-MAG3.

RESULTS

The fast acquisition modes resulted in only minor degradation of spatial resolution and quantification accuracy. Statistical noise in reconstructed images was significantly reduced compared to conventional SPECT, particularly at low count statistics. Stationary acquisition resulted in streak artefacts and spatial distortion.

CONCLUSIONS

The semi-stationary acquisition mode of the nanoSPECT/CTplus allows fast dynamic SPECT with tolerable loss of image quality.

Imaging integrin alpha-v-beta-3 expression in tumors with an 18F-labeled dimeric RGD peptide

Integrin αv β3 receptors are expressed on activated endothelial cells during neovascularization to maintain tumor growth. Many radiolabeled probes utilize the tight and specific association between the arginine-glycine-aspartatic acid (RGD) peptide and integrin αv β3 , but one main obstacle for any clinical application of these probes is the laborious multistep radiosynthesis of (18)F. In this study, the dimeric RGD peptide, E-[c(RGDfK)]2, was conjugated with NODAGA and radiolabeled with (18)F in a simple one-pot process with a radiolabeling yield of 20%, the whole process lasting only 45 min. NODAGA-E-[c(RGDfK)]2 labeled with (18)F at a specific activity of 1.8 MBq nmol(-1) and a radiochemical purity of 100% could be achieved. The logP value of (18)F-labeled NODAGA-E-[c(RGDfK)]2 was -4.26 ± 0.02. In biodistribution studies, (18)F-NODAGA-E-[c(RGDfK)]2 cleared rapidly from the blood with 0.03 ± 0.01 percentage injected dose per gram (%ID g(-1)) in the blood at 2 h p.i., mainly via the kidneys, and showed good in vivo stability. Tumor uptake of (18)F-NODAGA-E-[c(RGDfK)]2 (3.44 ± 0.20 %ID g(-1), 2 h p.i.) was significantly lower than that of reference compounds (68) Ga-labeled NODAGA-E-[c(RGDfK)]2 (6.26 ± 0.76 %ID g(-1) ; p <0.001) and (111) In-labeled NODAGA-E-[c(RGDfK)]2 (4.99 ± 0.64 %ID g(-1) ; p < 0.01). Co-injection of an excess of unlabeled NODAGA-E-[c(RGDfK)]2 along with (18)F-NODAGA-E-[c(RGDfK)]2 resulted in significantly reduced radioactivity concentrations in the tumor (0.85 ± 0.13 %ID g(-1)). The αv β3 integrin-expressing SK-RC-52 tumor could be successfully visualized by microPET with (18)F-labeled NODAGA-E-[c(RGDfK)]2 . In conclusion, NODAGA-E-[c(RGDfK)]2 could be labeled rapidly with (18)F using a direct aqueous, one-pot method and it accumulated specifically in αv β3 integrin-expressing SK-RC-52 tumors, allowing for visualization by microPET.

Design and performance evaluation of a 20-aperture multipinhole collimator for myocardial perfusion imaging applications

Single photon emission computed tomography (SPECT) myocardial perfusion imaging remains a critical tool in the diagnosis of coronary artery disease. However, after more than three decades of use, photon detection efficiency remains poor and unchanged. This is due to the continued reliance on parallel-hole collimators first introduced in 1964. These collimators possess poor geometric efficiency. Here we present the performance evaluation results of a newly designed multipinhole collimator with 20 pinhole apertures (PH20) for commercial SPECT systems. Computer simulations and numerical observer studies were used to assess the noise, bias and diagnostic imaging performance of a PH20 collimator in comparison with those of a low energy high resolution (LEHR) parallel-hole collimator. Ray-driven projector/backprojector pairs were used to model SPECT imaging acquisitions, including simulation of noiseless projection data and performing MLEM/OSEM image reconstructions. Poisson noise was added to noiseless projections for realistic projection data. Noise and bias performance were investigated for five mathematical cardiac and torso (MCAT) phantom anatomies imaged at two gantry orbit positions (19.5 and 25.0 cm). PH20 and LEHR images were reconstructed with 300 MLEM iterations and 30 OSEM iterations (ten subsets), respectively. Diagnostic imaging performance was assessed by a receiver operating characteristic (ROC) analysis performed on a single MCAT phantom; however, in this case PH20 images were reconstructed with 75 pixel-based OSEM iterations (four subsets). Four PH20 projection views from two positions of a dual-head camera acquisition and 60 LEHR projections were simulated for all studies. At uniformly-imposed resolution of 12.5 mm, significant improvements in SNR and diagnostic sensitivity (represented by the area under the ROC curve, or AUC) were realized when PH20 collimators are substituted for LEHR parallel-hole collimators. SNR improves by factors of 1.94-2.34 for the five patient anatomies and two orbital positions studied. For the ROC analysis the PH20 AUC is larger than the LEHR AUC with a p-value of 0.0067. Bias performance, however, decreases with the use of PH20 collimators. Systematic analyses showed PH20 collimators present improved diagnostic imaging performance over LEHR collimators, requiring only collimator exchange on existing SPECT cameras for their use.

Imaging of epidermal growth factor receptor expression in head and neck cancer with SPECT/CT and 111In-labeled cetuximab-F(ab')2

Combined treatment of advanced head and neck squamous cell carcinomas (HNSCC) with radiotherapy and the epidermal growth factor receptor (EGFR) inhibitor cetuximab improves clinical outcome in comparison to radiotherapy alone but is effective only in a few cases. To select those patients most likely to benefit from EGFR inhibition, it can be advantageous to quantify the tumor EGFR status before and possibly during therapy. The aim of this study was to develop and characterize the (111)In-cetuximab-F(ab')2 tracer to image EGFR targeting in vivo.

METHODS

The affinity and internalization kinetics of (111)In-cetuximab-F(ab')2 were determined in vitro. The optimal protein-fragment dose for imaging was determined in nude mice with a subcutaneous head and neck carcinoma model (FaDu). Mice with FaDu tumors were imaged using ultra-high-resolution SPECT with (111)In-cetuximab-F(ab')2 or (111)In-cetuximab IgG at 4, 24, 48, and 168 h after injection. Tumor tracer uptake was determined on micro-SPECT and autoradiography images of tumor sections. Immunohistochemical staining was used to analyze EGFR expression in the tumor.

RESULTS

In vitro, more than 50% of (111)In-cetuximab-F(ab')2 was internalized into FaDu cells within 24 h. The half maximal inhibitory concentration (IC50) of (111)In-cetuximab-F(ab')2 and (111)In-cetuximab was similar: 0.42 ± 0.16 nM versus 0.28 ± 0.14 nM, respectively. The protein dose-escalation study showed that the highest uptake of (111)In-cetuximab-F(ab')2 in tumors was obtained at doses of 10 μg/mouse or less (13.5 ± 5.2 percentage injected dose per gram [%ID/g]). Tumor uptake of (111)In-cetuximab was significantly higher (26.9 ± 3.3 %ID/g, P < 0.01). However, because of rapid blood clearance, tumor-to-blood ratios at 24 h after injection were significantly higher for (111)In-cetuximab-F(ab')2 (31.4 ± 3.8 vs. 1.7 ± 0.2, respectively; P < 0.001). The intratumoral distribution of (111)In-cetuximab-F(ab')2 correlated well with the immunohistochemical distribution of EGFR (r = 0.64 ± 0.06, P < 0.0001). micro-SPECT images of (111)In-cetuximab-F(ab')2 clearly showed preferential uptake in the tumor from 4 h onward, with superior tumor-to-background contrast at 24 h, compared with (111)In-cetuximab (107.0 ± 17.0 vs. 69.7 ± 3.9, respectively; P < 0.05).

CONCLUSION

(111)In-cetuximab-F(ab')2 displays higher tumor-to-blood ratios early after injection than (111)In-cetuximab in an HNSCC model, making it more suitable for EGFR visualization and potentially for selecting patients for treatment with EGFR inhibitors.

The feasibility study on 3-dimensional fluorescent x-ray computed tomography using the pinhole effect for biomedical applications

We propose a 3-dimensional fluorescent x-ray computed tomography (CT) pinhole collimator, aimed at providing molecular imaging with quantifiable measures and sub-millimeter spatial resolution. In this study, we demonstrate the feasibility of this concept and investigate imaging properties such as spatial resolution, contrast resolution and quantifiable measures, by imaging physical phantoms using a preliminary imaging system developed with monochromatic synchrotron x rays constructed at the BLNE-7A experimental line at KEK, Japan.

Methods for in vivo molecular imaging

Visualization of single molecules and specific subsets of cells is widely used for studies of biological processes and particularly in immunological research. Recent technological advances have provided a qualitative change in biological visualization from studying of "snapshot" pictures to real-time continuous observation of cellular dynamics in vivo. Contemporary methods of in vivo imaging make it possible to localize specific cells within organs and tissues, to study their differentiation, migration, and cell-to-cell interactions, and to follow some intracellular events. Fluorescence intravital microscopy plays an especially important role in high resolution molecular imaging. The methods of intravital microscopy are quickly advancing thanks to improvements in molecular sensors, labeling strategies, and detection approaches. Novel techniques allow simultaneous detection of various probes with better resolution and depth of imaging. In this review, we describe current methods for in vivo imaging, with special accent on fluorescence approaches, and discuss their applications for medical and biological studies.

Rapid additive manufacturing of MR compatible multipinhole collimators with selective laser melting of tungsten powder

PURPOSE

The construction of complex collimators with a high number of oblique pinholes is very labor intensive, expensive or is sometimes impossible with the current available techniques (drilling, milling or electric discharge machining). All these techniques are subtractive: one starts from solid plates and the material at the position of the pinholes is removed. The authors used a novel technique for collimator construction, called metal additive manufacturing. This process starts with a solid piece of tungsten on which a first layer of tungsten powder is melted. Each subsequent layer is then melted on the previous layer. This melting is done by selective laser melting at the locations where the CAD design file defines solid material.

METHODS

A complex collimator with 20 loftholes with 500 μm diameter pinhole opening was designed and produced (16 mm thick and 70 × 52 mm(2) transverse size). The density was determined, the production accuracy was measured (GOM ATOS II Triple Scan, Nikon AZ100M microscope, Olympus IMT200 microscope). Point source measurements were done by mounting the collimator on a SPECT detector. Because there is increasing interest in dual-modality SPECT-MR imaging, the collimator was also positioned in a 7T MRI scanner (Bruker Pharmascan). A uniform phantom was acquired using T1, T2, and T2* sequences to check for artifacts or distortion of the phantom images due to the collimator presence. Additionally, three tungsten sample pieces (250, 500, and 750 μm thick) were produced. The density, attenuation (140 keV beam), and uniformity (GE eXplore Locus SP micro-CT) of these samples were measured.

RESULTS

The density of the collimator was equal to 17.31 ± 0.10 g∕cm(3) (89.92% of pure tungsten). The production accuracy ranges from -260 to +650 μm. The aperture positions have a mean deviation of 5 μm, the maximum deviation was 174 μm and the minimum deviation was -122 μm. The mean aperture diameter is 464 ± 19 μm. The calculated and measured sensitivity and resolution of point sources at different positions in the field-of-view agree well. The measured and expected attenuation of the three sample pieces are in a good agreement. There was no influence of the 7T magnetic field on the collimator (which is paramagnetic) and minimal distortion was noticed on the MR scan of the uniform phantom.

CONCLUSIONS

Additive manufacturing is a very promising technique for the production of complex multipinhole collimators and may also be used for producing other complex collimators. The cost of this technique is only related to the amount of powder needed and the time it takes to have the collimator built. The timeframe from design to collimator production is significantly reduced.

Oral contrast enhances the resolution of in-life NIS reporter gene imaging

NIS reporter gene imaging is an excellent technology for noninvasive cell fate determination in living animals unless the NIS-transduced cells reside in perigastric organs such as spleen, liver, diaphragm, omentum, pancreas, perigastric lymph nodes or perigastric tumor deposits. Here we report that orally administered barium sulfate enhances CT definition of the stomach, masks background gamma ray emissions from the stomach, and enhances signal detection from radiotracer uptake in NIS-transduced organs.

Imaging capabilities of the Inveon SPECT system using single-and multipinhole collimators

The Inveon small-animal SPECT system comes with several types of multipinhole collimator plates. We evaluate here the performance measurements of the Inveon SPECT system using 6 different collimators: 3 dedicated for mouse imaging and 3 for rat imaging.

METHODS

The measured performance parameters include the sensitivity, the spatial resolution using line sources, the ultra-micro Derenzo phantom, the recovery coefficient and the noise measurements using the National Electrical Manufacturers Association NU-4 image quality phantom, obtained with the 2 reconstruction algorithms available with the Inveon Acquisition Workplace, version 1.5-the 3-dimensional ordered-subset expectation maximization (3DOSEM) and the 3-dimensional maximum a posteriori (3DMAP). Further, the overall performance of the system is illustrated by an animal experiment.

RESULTS

The results show that the Inveon SPECT scanner offers a spatial resolution, measured at the center of the field of view, ranging from 0.6 to 1 mm with the collimator plates dedicated to mouse imaging and from 1.2 to less than 2 mm with rat collimator plates. The system sensitivity varies from 29 to 404 cps/MBq for mouse collimators and from 53 to 175 cps/MBq for rat collimators. The image quality study showed that 3DMAP allows better noise reduction while preserving the recovery coefficient, compared with other regularization strategies such as the premature termination of the 3DOSEM reconstruction or 3DOSEM followed by gaussian filtering.

CONCLUSION

The acquisition parameters, such as the collimator set and the radius of rotation, offer a wide range of possibilities to apply to a large number of biologic studies. However, special care must be taken because this increase in sensitivity can be offset by image degradation, such as image artifacts caused by projection overlap and statistical noise due to a higher number of iterations required for convergence. 3DMAP allowed better noise reduction while maintaining relatively constant recovery coefficients, as compared with other reconstruction strategies.

Development and characterization of [123I]iodotiagabine for in-vivo GABA-transporter imaging

The increased knowledge of molecular changes associated with different neurological disorders calls for the development of novel radioligands. Tiagabine (Gabitril) is an anticonvulsive drug that binds selectively to GABA transporter-1 and thereby inhibits GABA uptake. As radioligands for in-vivo imaging of the GABA transporter are not yet available, we radiolabelled tiagabine and assessed its efficacy for in-vivo imaging of these transporters. Tiagabine was first brominated at its vinylic part, which was then exchanged with I. Next, anaesthetized rats received a bolus injection of [I]iodotiagabine in their tail vein, which was immediately followed by acquisition of planar and high-resolution micro-single-photon emission computed tomography (SPECT) images of the total body with special focus on the brain. Uptake in anatomical regions was assessed by coregistration of micro-SPECT with micro-CT images. Tiagabine labelling with I resulted in 50% yield and 99.7% radiochemical purity. Within 3 h after injection, SPECT demonstrated an increased signal-to-background ratio in the nasal mucosa and/or the Harderian glands but not in the brain. In addition we observed an increased signal-to-background ratio in organs such as the thyroid, heart, liver, kidney and bladder. More than 99% pure I-labelled tiagabine can be obtained and applied in animal micro-SPECT studies. However, this new radioligand is not taken up sufficiently by the brain and therefore cannot be used to successfully detect cerebral GABA transporters.

Bevacizumab reduces tumor targeting of antiepidermal growth factor and anti-insulin-like growth factor 1 receptor antibodies

Bevacizumab (antivascular endothelial growth factor [anti-VEGF]) and cetuximab (antiepidermal growth factor receptor [anti-EGFR]) are approved antibodies for treatment of cancer. However, in advanced colorectal cancer, the combination fails to improve survival. As the reason for the lack of activity is unknown, our study aims to determine the effect of bevacizumab on targeting of anti-EGFR and insulin-like growth factor 1 receptor (IGF-1R) antibodies in tumors with single-photon emission computed tomography (SPECT)/CT imaging. Mice with subcutaneous EGFR and IGF-1R-expressing SUM149 xenografts received a single dose of bevacizumab (10 mg/kg) or saline. After 4 days, mice were injected with radiolabeled cetuximab or R1507, an anti-IGF-1R antibody. A control group received a radiolabeled irrelevant IgG (hLL2). Three days later, SPECT/CT images were acquired and mice were dissected to determine the concentration of antibodies in the tissues. Tumors were analyzed immunohistochemically to determine vascular density (CD34), VEGF, EGFR and IGF-1R expression. SPECT/CT imaging revealed that bevacizumab treatment significantly reduced tumor targeting of radiolabeled cetuximab by 40% from 33.1 ± 1.1 %ID/g to 19.8 ± 5.7 %ID/g (p = 0.009) for untreated and bevacizumab-treated tumors, respectively. A similar effect was found for (111) In-R1507: tumor targeting of R1507 decreased by 35%. No significant differences in tumor uptake were observed in mice that received an irrelevant IgG. Uptake in normal organs was not altered by bevacizumab. Immunohistochemical analysis showed that vascular density decreased with 43%, whereas EGFR and IGF-1R expression was unaltered. In conclusion, bevacizumab treatment significantly reduces tumor targeting of anti-EGFR and anti-IGF-1R antibodies. This emphasizes the importance of timing and sequencing of bevacizumab in combination with other antibodies.

Correction for collimator-detector response in SPECT using point spread function template

Compensating for the collimator-detector response (CDR) in SPECT is important for accurate quantification. The CDR consists of both a geometric response and a septal penetration and collimator scatter response. The geometric response can be modeled analytically and is often used for modeling the whole CDR if the geometric response dominates. However, for radionuclides that emit medium or high-energy photons such as I-131, the septal penetration and collimator scatter response is significant and its modeling in the CDR correction is important for accurate quantification. There are two main methods for modeling the depth-dependent CDR so as to include both the geometric response and the septal penetration and collimator scatter response. One is to fit a Gaussian plus exponential function that is rotationally invariant to the measured point source response at several source-detector distances. However, a rotationally-invariant exponential function cannot represent the star-shaped septal penetration tails in detail. Another is to perform Monte-Carlo (MC) simulations to generate the depth-dependent point spread functions (PSFs) for all necessary distances. However, MC simulations, which require careful modeling of the SPECT detector components, can be challenging and accurate results may not be available for all of the different SPECT scanners in clinics. In this paper, we propose an alternative approach to CDR modeling. We use a Gaussian function plus a 2-D B-spline PSF template and fit the model to measurements of an I-131 point source at several distances. The proposed PSF-template-based approach is nearly non-parametric, captures the characteristics of the septal penetration tails, and minimizes the difference between the fitted and measured CDR at the distances of interest. The new model is applied to I-131 SPECT reconstructions of experimental phantom measurements, a patient study, and a MC patient simulation study employing the XCAT phantom. The proposed model yields up to a 16.5 and 10.8% higher recovery coefficient compared to the results with the conventional Gaussian model and the Gaussian plus exponential model, respectively.

Effects of attenuation map accuracy on attenuation-corrected micro-SPECT images

Background

In single-photon emission computed tomography (SPECT), attenuation of photon flux in tissue affects quantitative accuracy of reconstructed images. Attenuation maps derived from X-ray computed tomography (CT) can be employed for attenuation correction. The attenuation coefficients as well as registration accuracy between SPECT and CT can be influenced by several factors. Here we investigate how such inaccuracies influence micro-SPECT quantification.

Methods

Effects of (1) misalignments between micro-SPECT and micro-CT through shifts and rotation, (2) globally altered attenuation coefficients and (3) combinations of these were evaluated. Tests were performed with a NEMA NU 4–2008 phantom and with rat cadavers containing sources with known activity.

Results

Changes in measured activities within volumes of interest in phantom images ranged from <1.5% (¹²⁵I) and <0.6% (²⁰¹Tl, ^99mTc and ¹¹¹In) for 1-mm shifts to <4.5% (¹²⁵I) and <1.7% (²⁰¹Tl, ^99mTc and ¹¹¹In) with large misregistration (3 mm). Changes induced by 15° rotation were smaller than those by 3-mm shifts. By significantly altering attenuation coefficients (±10%), activity changes of <5.2% for ¹²⁵I and <2.7% for ²⁰¹Tl, ^99mTc and ¹¹¹In were induced. Similar trends were seen in rat studies.

Conclusions

While getting sufficient accuracy of attenuation maps in clinical imaging is highly challenging, our results indicate that micro-SPECT quantification is quite robust to various imperfections of attenuation maps.

Performance evaluation of small-animal multipinhole μSPECT scanners for mouse imaging

PURPOSE

We compared the performance of three commercial small-animal μSPECT scanners equipped with multipinhole general purpose (GP) and multipinhole high-resolution (HR) collimators designed for imaging mice.

METHODS

Spatial resolution, image uniformity, point source sensitivity and contrast recovery were determined for the U-SPECT-II (MILabs), the NanoSPECT-NSO (BioScan) and the X-SPECT (GE) scanners. The pinhole diameters of the HR collimator were 0.35 mm, 0.6 mm and 0.5 mm for these three systems respectively. A pinhole diameter of 1 mm was used for the GP collimator. To cover a broad field of imaging applications three isotopes were used with various photon energies: (99m)Tc (140 keV), (111)In (171 and 245 keV) and (125)I (27 keV). Spatial resolution and reconstructed image uniformity were evaluated in both HR and a GP mode with hot rod phantoms, line sources and a uniform phantom. Point source sensitivity and contrast recovery measures were additionally obtained in the GP mode with a novel contrast recovery phantom developed in-house containing hot and cold submillimetre capillaries on a warm background.

RESULTS

In hot rod phantom images, capillaries as small as 0.4 mm with the U-SPECT-II, 0.75 mm with the X-SPECT and 0.6 mm with the NanoSPECT-NSO could be resolved with the HR collimators for (99m)Tc. The NanoSPECT-NSO achieved this resolution in a smaller field-of-view (FOV) and line source measurements showed that this device had a lower axial than transaxial resolution. For all systems, the degradation in image resolution was only minor when acquiring the more challenging isotopes (111)In and (125)I. The point source sensitivity with (99m)Tc and GP collimators was 3,984 cps/MBq for the U-SPECT-II, 620 cps/MBq for the X-SPECT and 751 cps/MBq for the NanoSPECT-NSO. The effects of volume sensitivity over a larger object were evaluated by measuring the contrast recovery phantom in a realistic FOV and acquisition time. For 1.5-mm rods at a noise level of 8 %, the contrast recovery coefficient (CRC) was 42 %, 37 % and 34 % for the U-SPECT-II, X-SPECT and NanoSPECT-NSO, respectively. At maximal noise levels of 10 %, a CRCcold of 70 %, 52 % and 42 % were obtained for the U-SPECT-II, X-SPECT and NanoSPECT-NSO, respectively. When acquiring (99m)Tc with the GP collimators, the integral/differential uniformity values were 30 %/14 % for the U-SPECT-II, 50 %/30 % for the X-SPECT and 38 %/25 % for the NanoSPECT-NSO. When using the HR collimators, these uniformity values remained similar for U-SPECT-II and X-SPECT, but not for the NanoSPECT-NSO for which the uniformity deteriorated with larger volumes.

CONCLUSION

We compared three μSPECT systems by acquiring and analysing mouse-sized phantoms including a contrast recovery phantom built in-house offering the ability to measure the hot contrast on a warm background in the submillimetre resolution range. We believe our evaluation addressed the differences in imaging potential for each system to realistically image tracer distributions in mouse-sized objects.

Intratumoral administration of holmium-166 acetylacetonate microspheres: antitumor efficacy and feasibility of multimodality imaging in renal cancer

Purpose

The increasing incidence of small renal tumors in an aging population with comorbidities has stimulated the development of minimally invasive treatments. This study aimed to assess the efficacy and demonstrate feasibility of multimodality imaging of intratumoral administration of holmium-166 microspheres (¹⁶⁶HoAcAcMS). This new technique locally ablates renal tumors through high-energy beta particles, while the gamma rays allow for nuclear imaging and the paramagnetism of holmium allows for MRI.

Methods

¹⁶⁶HoAcAcMS were administered intratumorally in orthotopic renal tumors (Balb/C mice). Post administration CT, SPECT and MRI was performed. At several time points (2 h, 1, 2, 3, 7 and 14 days) after MS administration, tumors were measured and histologically analyzed. Holmium accumulation in organs was measured using inductively coupled plasma mass spectrometry.

Results

¹⁶⁶HoAcAcMS were successfully administered to tumor bearing mice. A striking near-complete tumor-control was observed in ¹⁶⁶HoAcAcMS treated mice (0.10±0.01 cm³ vs. 4.15±0.3 cm³ for control tumors). Focal necrosis and inflammation was present from 24 h following treatment. Renal parenchyma outside the radiated region showed no histological alterations. Post administration CT, MRI and SPECT imaging revealed clear deposits of ¹⁶⁶HoAcAcMS in the kidney.

Conclusions

Intratumorally administered ¹⁶⁶HoAcAcMS has great potential as a new local treatment of renal tumors for surgically unfit patients. In addition to strong cancer control, it provides powerful multimodality imaging opportunities.

Preclinical SPECT and SPECT/CT

The molecular processes underlying carcinogenesis and malignant spread are the foundation of future drug development for the treatment of cancer. Understanding these processes requires study of the interaction of complex biologic systems in a way that spatially and temporally recapitulates that seen in humans. Likewise, once an anticancer agent is developed, its intended antitumor action and its unintended side-effects must be studied in a rigorous and reproducible manner prior to its introduction into the clinic, a process that can benefit from methods that elucidate specific molecular processes and that can be performed serially. Recent advances in small-animal models of cancer, radiochemistry of single photon emitting radionuclides, single photon emission tomography systems, and image reconstruction techniques have set the stage for an ever-increasing use of SPECT and SPECT/CT in preclinical oncology-related applications. Several of these advances as well as several specific applications in oncology are highlighted and areas needing further improvement are identified.

Quantifying the effect of repetitive transcranial magnetic stimulation in the rat brain by μSPECT CBF scans

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is used to treat neurological and psychiatric disorders such as depression and addiction amongst others. Neuro-imaging by means of SPECT is a non-invasive manner of evaluating regional cerebral blood flow (rCBF) changes, which are assumed to reflect changes in neural activity.

OBJECTIVE

rCBF changes induced by rTMS are evaluated by comparing stimulation on/off in different stimulation paradigms using microSPECT of the rat brain.

METHODS

Rats (n = 6) were injected with 10 mCi of (99m)Tc-HMPAO during application of two rTMS paradigms (1 Hz and 10 Hz, 1430 A at each wing of a 20 mm figure-of-eight coil) and sham. SPM- and VOI-based analysis was performed.

RESULTS

rTMS caused widespread significant hypoperfusion throughout the entire rat brain. Differences in spatial extent and intensity of hypoperfusion were observed between both stimulation paradigms: 1 Hz caused significant hypoperfusion (P < 0.05) in 11.9% of rat brain volume while 10 Hz caused this in 23.5%; the minimal t-value induced by 1 Hz was -24.77 while this was -17.98 due to 10 Hz. Maximal percentage of hypoperfused volume due to 1 Hz and 10 Hz was reached at tissue experiencing 0.03-0.15 V/m.

CONCLUSION

High-frequency (10 Hz) stimulation causes more widespread hypoperfusion, while 1 Hz induces more pronounced hypoperfusion. The effect of rTMS is highly dependent on the electric field strength in the brain tissue induced by the TMS coil. This innovative imaging approach can be used as a fast screening tool in quantifying and evaluating the effect of various stimulation paradigms and coil designs for TMS and offers a means for research and development.

SPEM: a state-of-the-art instrument for high resolution molecular imaging of small animal organs

OBJECTIVE

To describe the Single Photon Emission Microscope (SPEM), a state-of-the-art instrument for small animal SPECT imaging, and characterize its performance presenting typical images of different animal organs.

METHODS

SPEM consists of two independent imaging devices based on high resolution scintillators, high sensitivity and resolution Electron-Multiplying CCDs and multi-pinhole collimators. During image acquisition, the mouse is placed in a rotational vertical holder between the imaging devices. Subsequently, an appropriate software tool based on the Maximum Likelihood algorithm iteratively produces the volumetric image. Radiopharmaceuticals for imaging kidneys, heart, thyroid and brain were used. The mice were injected with 74 to 148 MBq/0,3mL and scanned for 40 to 80 minutes, 30 to 60 minutes afterwards. During this procedure, the animals remained under ketamine/xilazine anesthesia.

RESULTS

SPEM images of different mouse organs are presented, attesting the imaging capabilities of the instrument.

CONCLUSION

SPEM is an innovative technology for small animal SPECT imaging providing high resolution images with appropriate sensitivity for pre-clinical research. Its use with appropriate radiotracers will allow translational investigation of several animal models of human diseases, their pharmacological treatment and the development of potential new therapeutic agents.

The sodium iodide symporter (NIS) as an imaging reporter for gene, viral, and cell-based therapies

Preclinical and clinical tomographic imaging systems increasingly are being utilized for non-invasive imaging of reporter gene products to reveal the distribution of molecular therapeutics within living subjects. Reporter gene and probe combinations can be employed to monitor vectors for gene, viral, and cell-based therapies. There are several reporter systems available; however, those employing radionuclides for positron emission tomography (PET) or singlephoton emission computed tomography (SPECT) offer the highest sensitivity and the greatest promise for deep tissue imaging in humans. Within the category of radionuclide reporters, the thyroidal sodium iodide symporter (NIS) has emerged as one of the most promising for preclinical and translational research. NIS has been incorporated into a remarkable variety of viral and non-viral vectors in which its functionality is conveniently determined by in vitro iodide uptake assays prior to live animal imaging. This review on the NIS reporter will focus on 1) differences between endogenous NIS and heterologously-expressed NIS, 2) qualitative or comparative use of NIS as an imaging reporter in preclinical and translational gene therapy, oncolytic viral therapy, and cell trafficking research, and 3) use of NIS as an absolute quantitative reporter.

Improved quantification in pinhole gated myocardial perfusion SPECT using micro-CT and ultrasound information

Absolute quantification using single photon emission computed tomography (SPECT) was demonstrated in vitro and in large immobile organs in vivo. To determine the feasibility of in vivo quantification of myocardial perfusion in pinhole gated SPECT, we added an ultrasound derived partial volume correction factor to attenuation and scatter corrections, in combination with gated acquisitions. In nine male Wistar rats, cardiac ultrasound was performed prior to SPECT/CT scans to determine the myocardial wall thickness. SPECT/CT scans were then performed 30 min after injection of (99m) Tc Tetrofosmin. Animals were killed and six midventricular segments of the left ventricle were excised and counted in a γ-well counter. Using AMIDE, regional myocardial activity was measured after combined scatter correction (SC) and attenuation correction (AC). These image derived activities were compared with the ex vivo counted activity. To correct for the partial volume effect, a recovery coefficient was determined from a phantom study, to determine the thickness specific partial volume effect. Combined AC and SC led to a significant underestimation of activity compared with ex vivo data (root mean squared error = 0.145 mCi g(-1)). The recovery coefficient calculated from the phantom study showed a linear relationship with object size from 1 to 6 mm, positioned in the vicinity of the center of the field of view (R(2)  = 0.98). Correction of nongated SPECT images with a recovery coefficient derived from the diastolic phase results in a global overestimation with root mean squared error = 0.04 mCi g(-1). Nongated SPECT images corrected with a recovery coefficient with a weighted average ratio diastolic and systolic phase led to an improved root mean squared error of 0.03 mCi g(-1). Combining attenuation correction with scatter correction and a gated partial volume correction yields the best correlation with ex vivo counting (root mean squared error = 0.021 mCi g(-1) (systolic) and 0.025 mCi g(-1) (diastolic). This study demonstrates a method for improved segmental myocardial perfusion quantification in pinhole gated SPECT, using combined attenuation-, scatter- and ultrasound-derived partial volume effect corrections.

Optimization of IGF-1R SPECT/CT imaging using 111In-labeled F(ab')2 and Fab fragments of the monoclonal antibody R1507

The insulin-like growth factor 1 receptor (IGF-1R) is a potential new target for the treatment of breast cancer. Patients with breast cancer lesions that express IGF-1R may benefit from treatment with anti-IGF-1R antibodies. IGF-1R expression can be visualized using radiolabeled R1507, a monoclonal antibody directed against IGF-1R. However, antibodies clear slowly from the circulation, resulting in low tumor-to-background ratios early after injection. Therefore, we aimed to accelerate targeting of IGF-1R using radiolabeled R1507 F(ab')2 and Fab fragments. In vitro, immunoreactivity, binding affinity and internalization of R1507 IgG, F(ab')2 and Fab were determined using the triple negative IGF-1R-expressing breast cancer cell line SUM149. In vivo, pharmacokinetics of (111)In-labeled R1507 IgG, F(ab')2 and Fab were studied in mice bearing subcutaneous SUM149 xenografts. SPECT/CT images were acquired and the biodistribution was measured ex vivo. The in vitro binding characteristics of radiolabeled R1507 IgG and F(ab')2 were comparable, whereas the affinity of Fab fragments was significantly lower (Kd: 0.6 nM, 0.7 nM and 3.0 nM for R1507 IgG, F(ab')2 and Fab, respectively). Biodistribution studies showed that the maximum tumor uptake of (111)In-R1507 IgG, F(ab')2 and Fab was 31.8% ID/g (72 h p.i.), 10.0% ID/g (6 h p.i.), and 1.8% ID/g (1 h p.i.), respectively. However, maximal tumor-to-blood ratios for F(ab')2 (24 h p.i.: 7.5) were more than twice as high as those obtained with R1507 (72 h p.i.: 2.8) and Fab (6 h p.i.: 2.8). Injection of an excess of unlabeled R1507 significantly reduced tumor uptake, suggesting that the uptake of R1507 IgG and F(ab')2 was specific for IGF-1R, while the major fraction of the tumor uptake of Fab was nonspecific. IGF-1R-expressing xenografts were visualized with (111)In-F(ab')2 SPECT/CT as early as 6 h p.i., while with R1507 IgG, the tumor could be visualized after 24 h. No specific targeting was observed with (111)In-Fab. (111)In-F(ab')2 fragments showed improved targeting of IGF-1R expressing tumors. Tumor-to-blood ratios were twice as high as those obtained with (111)In-R1507, and adequate tumor targeting on SPECT/CT images was observed as early as 6 h p.i. For individualization and optimization of IGF-1R targeted therapy, (111)In-F(ab')2 may be the tracer of choice.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.