A prototype instrument for single pinhole small animal adaptive SPECT imaging

The authors have designed and constructed a small-animal adaptive SPECT imaging system as a prototype for quantifying the potential benefit of adaptive SPECT imaging over the traditional fixed geometry approach. The optical design of the system is based on filling the detector with the region of interest for each viewing angle, maximizing the sensitivity, and optimizing the resolution in the projection images. Additional feedback rules for determining the optimal geometry of the system can be easily added to the existing control software. Preliminary data have been taken of a phantom with a small, hot, offset lesion in a flat background in both adaptive and fixed geometry modes. Comparison of the predicted system behavior with the actual system behavior is presented, along with recommendations for system improvements.     

Attenuation correction for small animal SPECT imaging using x-ray CT data

Photon attenuation in small animal nuclear medicine scans can be significant when using isotopes that emit lower energy photons such as iodine-125. We have developed a method to use microCT data to perform attenuation corrected small animal single-photon emission computed tomography (SPECT). A microCT calibration phantom was first imaged, and the resulting calibration curve was used to convert microCT image values to linear attenuation coefficient values that were then used in an iterative SPECT reconstruction algorithm. This method was applied to reconstruct a SPECT image of a uniform phantom filled with 125I-NaI. Without attenuation correction, the image suffered a 30% decrease in intensity in the center of the image, which was removed with the addition of attenuation correction. This reduced the relative standard deviation in the region of interest from 10% to 6%.     

Design and performance of a multi-pinhole collimation device for small animal imaging with clinical SPECT and SPECT-CT scanners

A multi-pinhole collimation device is developed that uses the gamma camera detectors of a clinical SPECT or SPECT-CT scanner to produce high-resolution SPECT images. The device consists of a rotating cylindrical collimator having 22 tungsten pinholes with 0.9 mm diameter apertures and an animal bed inside the collimator that moves linearly to provide helical or ordered-subsets axial sampling. CT images also may be acquired on a SPECT-CT scanner for purposes of image co-registration and SPECT attenuation correction. The device is placed on the patient table of the scanner without attaching to the detectors or scanner gantry. The system geometry is calibrated in-place from point source data and is then used during image reconstruction. The SPECT imaging performance of the device is evaluated with test phantom scans. Spatial resolution from reconstructed point source images is measured to be 0.6 mm full width at half maximum or better. Micro-Derenzo phantom images demonstrate the ability to resolve 0.7 mm diameter rod patterns. The axial slabs of a Micro-Defrise phantom are visualized well. Collimator efficiency exceeds 0.05% at the center of the field of view, and images of a uniform phantom show acceptable uniformity and minimal artifact. The overall simplicity and relatively good imaging performance of the device make it an interesting low-cost alternative to dedicated small animal scanners.     

Radiation dose estimate in small animal SPECT and PET

Calculations of radiation dose are important in assessing the medical and biological implications of ionizing radiation in medical imaging techniques such as SPECT and PET. In contrast, radiation dose estimates of SPECT and PET imaging of small animals are not very well established. For that reason we have estimated the whole-body radiation dose to mice and rats for isotopes such as 18F, 99mTc, 201Tl, (111)In, 123I, and 125I that are used commonly for small animal imaging. We have approximated mouse and rat bodies with uniform soft tissue equivalent ellipsoids. The mouse and rat sized ellipsoids had a mass of 30 g and 300 g, respectively, and a ratio of the principal axes of 1:1:4 and 0.7:1:4. The absorbed fractions for various photon energies have been calculated using the Monte Carlo software package MCNP. Using these values, we then calculated MIRD S-values for two geometries that model the distribution of activity in the animal body: (a) a central point source and (b) a homogeneously distributed source, and compared these values against S-value calculations for small ellipsoids tabulated in MIRD Pamphlet 8 to validate our results. Finally we calculated the radiation dose taking into account the biological half-life of the radiopharmaceuticals and the amount of activity administered. Our calculations produced S-values between 1.06 x 10(-13) Gy/Bq s and 2.77 x 10(-13) Gy/Bq s for SPECT agents, and 15.0 x 10(-13) Gy/Bq s for the PET agent 18F, assuming mouse sized ellipsoids with uniform source distribution. The S-values for a central point source in an ellipsoid are about 10% higher than the values obtained for the uniform source distribution. Furthermore, the S-values for mouse sized ellipsoids are approximately 10 times higher than for the rat sized ellipsoids reflecting the difference in mass. We reviewed published data to obtain administered radioactivity and residence times for small animal imaging. From these values and our computed S-values we estimated that the whole body dose in small animals ranges between 6 cGy and 90 cGy for mice and between about 1 cGy and 27 cGy for rats. The whole body dose in small animal imaging can be very high in comparison to the lethal dose to mice (LD50/30 approximately 7 Gy). For this reason the dose in small animal imaging should be monitored carefully and the administered activity should be kept to a minimum. These results also underscore the need of further development of instrumentation that improves detection efficiency and reduces radiation dose in small animal imaging.     

A submillimeter resolution fluorescence molecular imaging system for small animal imaging

Most current imaging systems developed for tomographic investigations of intact tissues using diffuse photons suffer from a limited number of sources and detectors. In this paper we describe the construction and evaluation of a large dataset, low noise tomographic system for fluorescence imaging in small animals. The system consists of a parallel plate-imaging chamber and a lens coupled CCD camera, which enables conventional planar imaging as well as fluorescence tomography. The planar imaging data are used to guide the acquisition of a Fluorescence Molecular Tomography (FMT) dataset containing more than 106 measurements, and to superimpose anatomical features with tomographic results for improved visual representation. Experimental measurements exhibited good agreement with the diffusion theory models used to predict light propagation within the chamber. Tests of the instrument's capacity to quantitatively reconstruct fluorochrome distributions in three dimensions showed less than 5% errors between actual fluorochrome concentrations and FMT findings, and suggested a detection threshold of approximately 100 femptomoles for small localized objects. Experiments to assess the instrument's spatial resolution demonstrated the ability of the system to resolve objects placed at clear distances of less than 1 mm. This is a significant resolution increase over previously developed systems for animal imaging, and is primarily due to the large dataset employed and the use of inversion methods. Finally, the in vivo imaging capacity is showcased. It is expected that the large dataset collected can enable superior imaging of molecular probes in vivo and improve quantification of fluorescence signatures.     

A comparison of MR-based attenuation correction in PET versus SPECT

Attenuation correction (AC) is a critical step in the reconstruction of quantitatively accurate positron emission tomography (PET) and single photon emission computed tomography (SPECT) images. Several groups have proposed magnetic resonance (MR)-based AC algorithms for application in hybrid PET/MR systems. However, none of these approaches have been tested on SPECT data. Since SPECT/MR systems are under active development, it is important to ascertain whether MR-based AC algorithms validated for PET can be applied to SPECT. To investigate this issue, two imaging experiments were performed: one with an anthropomorphic chest phantom and one with two groups of canines. Both groups of canines were imaged from neck to abdomen, one with PET/CT and MR (n = 4) and the other with SPECT/CT and MR (n = 4), while the phantom was imaged with all modalities. The quality of the nuclear medicine reconstructions using MR-based attenuation maps was compared between PET and SPECT on global and local scales. In addition, the sensitivity of these reconstructions to variations in the attenuation map was ascertained. On both scales, it was found that the SPECT reconstructions were of higher fidelity than the PET reconstructions. Further, they were less sensitive to changes to the MR-based attenuation map. Thus, MR-based AC algorithms that have been designed for PET/MR can be expected to demonstrate improved performance when used for SPECT/MR.     

A multipinhole small animal SPECT system with submillimeter spatial resolution

Single photon emission computed tomography (SPECT) is an important technology for molecular imaging studies of small animals. In this arena, there is an increasing demand for high performance imaging systems that offer improved spatial resolution and detection efficiency. We have designed a multipinhole small animal imaging system based on position sensitive avalanche photodiode (PSAPD) detectors with the goal of submillimeter spatial resolution and high detection efficiency, which will allow us to minimize the radiation dose to the animal and to shorten the time needed for the imaging study. Our design will use 8 x 24 mm2 PSAPD detector modules coupled to thallium-doped cesium iodide [CsI(Tl)] scintillators, which can achieve an intrinsic spatial resolution of 0.5 mm at 140 keV. These detectors will be arranged in rings of 24 modules each; the animal is positioned in the center of the 9 stationary detector rings which capture projection data from the animal with a cylindrical tungsten multipinhole collimator. The animal is supported on a bed which can be rocked about the central axis to increase angular sampling of the object. In contrast to conventional SPECT pinhole systems, in our design each pinhole views only a portion of the object. However, the ensemble of projection data from all of the multipinhole detectors provide angular sampling that is sufficient to reconstruct tomographic data from the object. The performance of this multipinhole PSAPD imaging system was simulated using a ray tracing program that models the appropriate point spread functions and then was compared against the performance of a dual-headed pinhole SPECT system. The detection efficiency of both systems was simulated and projection data of a hot rod phantom were generated and reconstructed to assess spatial resolution. Appropriate Poisson noise was added to the data to simulate an acquisition time of 15 min and an activity of 18.5 MBq distributed in the phantom. Both sets of data were reconstructed with an ML-EM reconstruction algorithm. In addition, the imaging performance of both systems was evaluated with a uniformity phantom and a realistic digital mouse phantom. Simulations show that our proposed system produces a spatial resolution of 0.8 mm and an average detection efficiency of 630 cps/MBq. In contrast, simulations of the dual-headed pinhole SPECT system produce a spatial resolution of 1.1 mm and an average detection efficiency of 53 cps/MBq. These results suggest that our novel design will achieve high spatial resolution and will improve the detection efficiency by more than an order of magnitude compared to a dual-headed pinhole SPECT system. We expect that this system can perform SPECT with submillimeter spatial resolution, high throughput, and low radiation dose suitable for in vivo imaging of small animals.     

Curved array photoacoustic tomographic system for small animal imaging

We present systematic characterization of a photoacoustic imaging system optimized for rapid, high-resolution tomographic imaging of small animals. The system is based on a 128-element ultrasonic transducer array with a 5-MHz center frequency and 80% bandwidth shaped to a quarter circle of 25 mm radius. A 16-channel data-acquisition module and dedicated channel detection electronics enable capture of a 90-deg field-of-view image in less than 1 s and a complete 360-deg scan using sample rotation within 15 s. Measurements on cylindrical phantom targets demonstrate a resolution of better than 200 microm and high-sensitivity detection of 580-microm blood tubing to depths greater than 3 cm in a turbid medium with reduced scattering coefficient mu(s) (')=7.8 cm(-1). The system is used to systematically investigate the effects of target size, orientation, and geometry on tomographic imaging. As a demonstration of these effects and the system imaging capabilities, we present tomographic photoacoustic images of the brain vasculature of an ex vivo mouse with varying measurement aperture. For the first time, according to our knowledge, resolution of sub-200-microm vessels with an overlying turbid medium of greater than 2 cm depth is demonstrated using only intrinsic biological contrast.     

Hyperspectral and multispectral bioluminescence optical tomography for small animal imaging

For bioluminescence imaging studies in small animals, it is important to be able to accurately localize the three-dimensional (3D) distribution of the underlying bioluminescent source. The spectrum of light produced by the source that escapes the subject varies with the depth of the emission source because of the wavelength-dependence of the optical properties of tissue. Consequently, multispectral or hyperspectral data acquisition should help in the 3D localization of deep sources. In this paper, we describe a framework for fully 3D bioluminescence tomographic image acquisition and reconstruction that exploits spectral information. We describe regularized tomographic reconstruction techniques that use semi-infinite slab or FEM-based diffusion approximations of photon transport through turbid media. Singular value decomposition analysis was used for data dimensionality reduction and to illustrate the advantage of using hyperspectral rather than achromatic data. Simulation studies in an atlas-mouse geometry indicated that sub-millimeter resolution may be attainable given accurate knowledge of the optical properties of the animal. A fixed arrangement of mirrors and a single CCD camera were used for simultaneous acquisition of multispectral imaging data over most of the surface of the animal. Phantom studies conducted using this system demonstrated our ability to accurately localize deep point-like sources and show that a resolution of 1.5 to 2.2 mm for depths up to 6 mm can be achieved. We also include an in vivo study of a mouse with a brain tumour expressing firefly luciferase. Co-registration of the reconstructed 3D bioluminescent image with magnetic resonance images indicated good anatomical localization of the tumour.     

Design of field-cycled magnetic resonance systems for small animal imaging

This paper presents a design study for a field-cycled magnetic resonance imaging (MRI) system directed at small animal imaging applications. A field-cycled MRI system is different from a conventional MRI system in that it uses two separate and dynamically controllable magnetic fields. A strong magnetic field is used to polarize the object, and a relatively weak magnetic field is used during signal acquisition. The potential benefits of field-cycled MRI are described. The theoretical dependences of field-cycled MRI performance on system design are introduced and investigated. Electromagnetic, mechanical and thermal performances of the system were considered in this design study. A system design for imaging 10 cm diameter objects is presented as an example, capable of producing high-duty-cycle polarizing magnetic fields of 0.5 T and readout magnetic fields corresponding to a proton Larmor frequency of 5 MHz. The specifications of the final design are presented along with its expected electromagnetic and thermal performance.     

Optimizing in vivo small animal Cerenkov luminescence imaging

In vivo Cerenkov luminescence imaging is a rapidly growing molecular imaging research field based on the detection of Cerenkov radiation induced by beta particles when traveling though biological tissues. We investigated theoretically the possibility of enhancing the number of the detected Cerenkov photons in the near infrared (NIR) region of the spectrum. The analysis is based on applying a photon propagation diffusion model to Cerenkov photons in the tissue. Results show that despite the smaller number of Cerenkov photons in the NIR region, the fraction exiting the tissues is greater than in the visible range, and thus, a charge-coupled device detector optimized for the NIR range will allow to obtain a higher signal. The comparison was performed considering Cerenkov point sources located at different depths inside the animal. We concluded that the improvement can be up to 35% and is more significant when the Cerenkov source to be imaged is located deeper inside the animal.     

Micro-PET imaging and small animal models of disease

Positron emission tomography (PET) has been used clinically to measure enzyme reactions, ligand-receptor interactions, cellular metabolism and cell proliferation. Until recently, however, PET has not been suitable for small animal models because of resolution limitations. Development of micro-PET instrumentation for small animal imaging and the availability of positron-emitting tracers has made this technology accessible for the non-invasive, quantitative and repetitive imaging of biological function in living animals. The development of new probes and positron-imaging based reporter genes has extended micro-PET applications to investigations of metabolism, enzyme activity, receptor-ligand interactions, protein-protein interactions, gene expression, adoptive cell therapy and somatic gene therapy. Because small animal PET is immediately extrapolatable to the clinic, laboratory advances should rapidly be translated to clinical practice.     

A method of image registration for small animal, multi-modality imaging

Many research institutions have a full suite of preclinical tomographic scanners to answer biomedical questions in vivo. Routine multi-modality imaging requires robust registration of images generated by various tomographs. We have implemented a hardware registration method for preclinical imaging that is similar to that used in the combined positron emission tomography (PET)/computed tomography (CT) scanners in the clinic. We designed an imaging chamber which can be rigidly and reproducibly mounted on separate microPET and microCT scanners. We have also designed a three-dimensional grid phantom with 1288 lines that is used to generate the spatial transformation matrix from software registration using a 15-parameter perspective model. The imaging chamber works in combination with the registration phantom synergistically to achieve the image registration goal. We verified that the average registration error between two imaging modalities is 0.335 mm using an in vivo mouse bone scan. This paper also estimates the impact of image misalignment on PET quantitation using attenuation corrections generated from misregistered images. Our technique is expected to produce PET quantitation errors of less than 5%. The methods presented are robust and appropriate for routine use in high throughput animal imaging facilities.     

A microPET/CT system for in vivo small animal imaging

A microCT scanner was designed, fabricated and integrated with a previously reported microPET II scanner (Tai et al 2003 Phys. Med. Biol. 48 1519, Yang et al 2004 Phys. Med. Biol. 49 2527), forming a dual modality system for in vivo anatomic and molecular imaging of the mouse. The system was designed to achieve high-spatial-resolution and high-sensitivity PET images with adequate CT image quality for anatomic localization and attenuation correction with low x-ray dose. The system also has relatively high throughput for screening, and a flexible gantry and user interface. X-rays were produced by a 50 kVp, 1.5 mA fixed tungsten anode tube, with a focal spot size of 70 microm. The detector was a 5 x 5 cm(2) photodiode detector incorporating 48 microm pixels on a CMOS array and a fast gadolinium oxysulfide (GOS) intensifying screen. The microCT system has a flexible C-arm gantry design with adjustable detector positioning, which acquires CT projection images around the common microPET/CT bed. The design and the initial characterization of the microCT system is described, and images of the first mouse scans with microPET/CT scanning protocols are shown.     

Technical considerations in longitudinal multispectral small animal molecular imaging

In a previous study, we investigated physical methods to reduce whole-body, diet-related autofluorescence interference in several mouse strains through changes in animal diet. Measurements of mice with an in vivo multispectral imaging system over a 21-day period allowed for the quantification of concentration changes in multiple in vivo fluorophores. To be an effective instrument, a multispectral imaging system requires a priori spectral knowledge, the form and importance of which is not necessarily intuitive, particularly when noninvasive in vivo longitudinal imaging studies are performed. Using an optimized spectral library from a previous autofluorescence-reduction study as a model, we investigated two additional spectral definition techniques to illustrate the results of poor spectral definition in a longitudinal fluorescence imaging study. Here we systematically evaluate these results and show how poor spectral definition can lead to physiologically irrelevant results. This study concludes that the proper selection of robust spectra corresponding to each specific fluorescent molecular label of interest is of integral importance to enable effective use of multispectral imaging techniques in longitudinal fluorescence studies.     

Imaging techniques for small animal imaging models of pulmonary disease: micro-CT

Microcomputed tomography (micro-CT) is ideal for quantifying pulmonary disease because of the inherent contrast between tissue and air that exists in the lungs. Both in vivo and in vitro studies can be performed using micro-CT. Live animal studies show function, while fixed specimen studies show structure. Through the use of image processing techniques, both acute and chronic lung diseases can be quantified. The information provided by micro-CT is complementary to histological evaluation, since CT is nondestructive. This paper discusses two examples, in vivo and in vitro, of how micro-CT can be used to assess pulmonary diseases in small animal models. With the use of micro-CT, we were able to quantify pulmonary fibrosis in the live rat and investigate the microstructure of the airway in fixed mouse lungs.     

Unsupervised analysis of small animal dynamic Cerenkov luminescence imaging

Clustering analysis (CA) and principal component analysis (PCA) were applied to dynamic Cerenkov luminescence images (dCLI). In order to investigate the performances of the proposed approaches, two distinct dynamic data sets obtained by injecting mice with (32)P-ATP and (18)F-FDG were acquired using the IVIS 200 optical imager. The k-means clustering algorithm has been applied to dCLI and was implemented using interactive data language 8.1. We show that cluster analysis allows us to obtain good agreement between the clustered and the corresponding emission regions like the bladder, the liver, and the tumor. We also show a good correspondence between the time activity curves of the different regions obtained by using CA and manual region of interest analysis on dCLIT and PCA images. We conclude that CA provides an automatic unsupervised method for the analysis of preclinical dynamic Cerenkov luminescence image data.     

Small animal SPECT and its place in the matrix of molecular imaging technologies

Molecular imaging refers to the use of non-invasive imaging techniques to detect signals that originate from molecules, often in the form of an injected tracer, and observe their interaction with a specific cellular target in vivo. Differences in the underlying physical principles of these measurement techniques determine the sensitivity, specificity and length of possible observation of the signal, characteristics that have to be traded off according to the biological question under study. Here, we describe the specific characteristics of single photon emission computed tomography (SPECT) relative to other molecular imaging technologies. SPECT is based on the tracer principle and external radiation detection. It is capable of measuring the biodistribution of minute (<10(-10) molar) concentrations of radio-labelled biomolecules in vivo with sub-millimetre resolution and quantifying the molecular kinetic processes in which they participate. Like some other imaging techniques, SPECT was originally developed for human use and was subsequently adapted for imaging small laboratory animals at high spatial resolution for basic and translational research. Its unique capabilities include (i) the ability to image endogenous ligands such as peptides and antibodies due to the relative ease of labelling these molecules with technetium or iodine, (ii) the ability to measure relatively slow kinetic processes (compared with positron emission tomography, for example) due to the long half-life of the commonly used isotopes and (iii) the ability to probe two or more molecular pathways simultaneously by detecting isotopes with different emission energies. In this paper, we review the technology developments and design tradeoffs that led to the current state-of-the-art in SPECT small animal scanning and describe the position SPECT occupies within the matrix of molecular imaging technologies.     

In vivo small animal imaging: current status and future prospects

The use of small animal models in basic and preclinical sciences constitutes an integral part of testing new pharmaceutical agents prior to commercial translation to clinical practice. Whole-body small animal imaging is a particularly elegant and cost-effective experimental platform for the timely validation and commercialization of novel agents from the bench to the bedside. Biomedical imaging is now listed along with genomics, proteomics, and metabolomics as an integral part of biological and medical sciences. Miniaturized versions of clinical diagnostic modalities, including but not limited to microcomputed tomography, micromagnetic resonance tomography, microsingle-photon-emission tomography, micropositron-emission tomography, optical imaging, digital angiography, and ultrasound, have all greatly improved our investigative abilities to longitudinally study various experimental models of human disease in mice and rodents. After an exhaustive literature search, the authors present a concise and critical review of in vivo small animal imaging, focusing on currently available modalities as well as emerging imaging technologies on one side and molecularly targeted contrast agents on the other. Aforementioned scientific topics are analyzed in the context of cancer angiogenesis and innovative antiangiogenic strategies under-the-way to the clinic. Proposed hybrid approaches for diagnosis and targeted site-specific therapy are highlighted to offer an intriguing glimpse of the future.     

Design, calibration and evaluation of a robotic needle-positioning system for small animal imaging applications

A needle-positioning robot has been developed for image-guided interventions in small animal research models. The device is designed to position a needle with an error < or =100 microm. The robot has two rotational axes (pitch and roll) to control needle orientation, and one linear axis to perform needle insertion. The three axes intersect at a single point to create a remote centre of motion (RCM) that acts as a fulcrum for the orientation of the needle. The RCM corresponds to the skin-entry point of the needle into the animal. The robot was calibrated to ensure that the three axes intersected at a single point defining an RCM and that the needle tip was positioned at the RCM. Needle-positioning accuracy and precision were quantified in Cartesian coordinates at ten target locations in the plane of each rotational axis. The measured needle-positioning accuracy in free space was 54 +/- 12 microm for the pitch axis plane and 91 +/- 21 microm for the roll axis plane. The measured needle-positioning precision was 15 and 17 microm for the pitch and roll axes planes, respectively. The robot's ability to insert a needle into a tumour in a euthanized mouse was demonstrated.