Preclinical or small-animal Single Photon Emission Computed Tomography (SPECT) is a radionuclide based molecular imaging modality for small laboratory animalsMeikle SR, et al. Small animal SPECT and its place in the matrix of molecular imaging technologies. Phys Med Biol. 2005; 50(22):R45-61. (e.g. mice and rats). Although SPECT is a well-established imaging technique that is already for decades in use for clinical application, the limited resolution of clinical SPECT (~10 mm) stimulated the development of dedicated small animal SPECT systems with sub-mm resolution. Unlike in clinics, preclinical SPECT outperforms preclinical coincidence PET in terms of resolution (best spatial resolution of SPECT - 0.25mm,Ivashchenko et al. Quarter-Millimeter-Resolution Molecular Mouse Imaging with U-SPECT+. Mol Imaging. 2014. PET ≈ 1 mmWalker MD, et al. Performance Assessment of a Preclinical PET Scanner with Pinhole Collimation by Comparison to a Coincidence-Based Small-Animal PET Scanner. J Nucl Med. 2014; 55(8):1368-1374.Tai Y, et al. Performance Evaluation of the microPET Focus: A Third-Generation microPET Scanner Dedicated to Animal Imaging. J Nucl Med. 2005; 46(3):455-463. ) and, at the same time, allows to perform fast dynamic imaging of animals (less than 15s time framesVaissier PEB, et al. Fast spiral SPECT with stationary γ-cameras and focusing pinholes. J Nucl Med. 2012; 53(8):1292-9.).
SPECT imaging requires administration of small quantities of γ-emitting radiolabeled molecules (commonly called "tracers") into the animal prior to the image acquisition. These tracers are biochemically designed in such a way that they accumulate at target locations in the body. The radiation emitted by the tracer molecules (single γ-photons) can be detected by gamma detectors and, after image reconstruction, results in a 3-dimensional image of the tracer distribution within the animal. Some key radioactive isotopes used in preclinical SPECT are 99mTc, 123I, 125I, 131I, 111In, 67Ga and 201Tl.
Preclinical SPECT plays an important role in multiple areas of translational researchBernsen MR, et al. The role of preclinical SPECT in oncological and neurological research in combination with either CT or MRI. Eur J Nucl Med Mol Imaging. 2014; 41(1):36-49. where SPECT can be used for non-invasive imaging of radiolabeled molecules, including antibodies, peptides, and nanoparticles. Among major areas of its applications are oncology, neurology, psychiatry, cardiology, orthopedics, pharmacology and internal medicine.
Basic imaging principle of preclinical SPECT
Due to the small size of the imaged animals (a mouse is about 3000 times smaller than a human measured by weight and volume), it is essential to have a high spatial resolution and detection efficiency for the preclinical scanner.
Spatial resolution
Looking at spatial resolution first, if we want to see the same level of details relatively to e.g. the size of the organs in a mouse as we can see in a human, the spatial resolution of clinical SPECT needs to be improved by a factor of or higher. Such an obstacle forced scientists to look for a new imaging approach for preclinical SPECT that was found in exploiting the pinhole imaging principle.Beekman FJ, et al. The pinhole: gateway to ultra-high-resolution three-dimensional radionuclide imaging. Eur J Nucl Med Mol Imaging. 2007; 34(2):151-61.
A pinhole collimator consists of a piece of dense material containing only a single hole, which typically has the shape of a double cone. First attempts to obtain SPECT images of rodents with a high resolution were based on use of pinhole collimators attached to convectional gamma cameras.Weber DA, et al. Pinhole SPECT: an approach to in vivo high resolution SPECT imaging in small laboratory animals. J Nucl Med. 1994; 35: 342-8.Jaszczak RJ, et al. Pinhole collimation for ultra-high-resolution, small-field-of-view SPECT. Phys Med Biol. 1994; 39: 425-37. In such a way, by placing the object (e.g. rodent) close to the aperture of the pinhole, one can reach a high magnification of its projection on the detector surface and effectively compensate for the limited intrinsic resolution of the detector.
The combined effects of the finite aperture size and the limited intrinsic resolution are described by:
Rentmeester MCM, et al. Optimizing multi-pinhole SPECT geometries using an analytic model. Phys Med Biol. 2007; 52:2567-2581.
de - effective pinhole diameter, Ri - intrinsic resolution of the detector, M – projection magnification factor.
The resolution of a SPECT system based on the pinhole imaging principle can be improved in one of three ways:
by decreasing the effective diameter of the pinhole by increasing the magnification factor by using detectors with higher intrinsic resolution
The exact size, shape and material of the pinhole are important to obtain good imaging characteristics and is a subject of collimator design optimization studies via e.g. use of Monte Carlo simulations. Modern preclinical SPECT scanners based on pinhole imaging can reach up to 0.25 mm spatial or 0.015 μL volumetric resolution for in vivo mouse imaging.
Detection efficiency
The detection efficiency or sensitivity of a preclinical pinhole SPECT system is determined by:Metzler SD, et al. Resolution- versus sensitivity-effective diameter in pinhole collimation: experimental verification. Phys Med Biol. 2005; 50:5005-5017.
S – detection efficiency (sensitivity), de-effective pinhole diameter with penetration, N – total number of pinholes, rc – collimator radius (e.g. object-to-pinhole distance).
The sensitivity can be improved by:
increasing the pinhole diameter Possible drawbacks: degradation of spatial resolution
decreasing the object-to-pinhole distance (e.g. placing the animal as close as possible to the pinhole aperture) using multiple pinholes that simultaneously capture projections from multiple angles Possible drawbacks: When multiple pinhole projections are projected on a single detector surface, they can either overlap each other (multiplexing projections) or be fully separated (non-overlapping projections). Although pinhole collimators with multiplexing projections allow reaching a higher sensitivity (by allowing to use a higher number of pinholes) when compared to non-overlapping designs, they also suffer from multiple artifacts in reconstructed SPECT images.Rowe RK, et al. A stationary hemispherical SPECT imager for three-dimensional brain imaging. J Nucl Med.1993; 34:474-80.Meike SR, et al. A prototype coded aperture detector for small animal SPECT. IEEE Trans Nucl Sci. 2002; 49:2167-71.Van Audenhaege K, et al. The Evaluation of Data Completeness and Image Quality in Multiplexing Multi-Pinhole SPECT. IEEE Trans Med Imag. 2015; 34: 474-86.Vunckx K, et al. Effect of Overlapping Projections on Reconstruction Image Quality in Multipinhole SPECT. IEEE Trans Med Imag.2008; 27: 972-83. The artifacts are cause by ambiguity about the origin of γ -photons detected in the areas of the overlap.
decreasing the size of the "field-of-view" Placing the animal close to the pinhole aperture comes at the cost of reducing the size of the area that can be imaged at a given time (the "field-of-view") compared to imaging at a lower magnification. However, when combined with moving the animal (the so-called "scanning-focus method" Branderhorst W, et al. Targeted multi-pinhole SPECT. Eur J Nucl Med Mol Imaging. 2001; 38(3):552-61.) a larger area of interest can still be imaged with a good resolution and sensitivity.
The typical detection efficiency of preclinical SPECT scanner lies within a 0.1-0.2% (1000-2000 cps/MBq) range, which is more than tenfold higher than the average sensitivity of clinical scanners.Madsen MT. Recent Advances in SPECT imaging. J Nucl Med. 2007; 48(4):661-673. At the same time, dedicated high-sensitivity collimators can allow >1% detection efficiency and maintain sub-mm image resolution.Ivashchenko, et al. Ultra-high-sensitivity sub-mm mouse SPECT. J Nucl Med. 2015; 56(3):470-5.
System design
Multiple pinhole SPECT system designs have been proposed, including rotating gamma camera, stationary detector but rotating collimator, or completely stationary cameraFurenlid LR, et al. FastSPECTII: A Second-Generation High-Resolution Dynamic SPECT Imager. IEEE Trans Nucl Sci. 2004; 51(3):631-635.Van der Have F, et al. U-SPECT-II: An Ultra-High-Resolution Device for Molecular Small-Animal Imaging. J Nucl Med. 2009; 50(4):599-605. in which a large number of pinholes surround the animal and simultaneously acquire projections from a sufficient number of angles for tomographic image reconstruction. Stationary systems have several advantages over non-stationary systems:
no need for repetitive system geometry recalibration Why: due to the stable position of the detector(s) and the collimator
unlike non-stationary systems, stationary systems are very well suited for dynamic SPECT imaging Why: because all required angular information is acquired simultaneously by multiple pinholes.
Modern stationary preclinical SPECT systems can perform dynamic SPECT imaging with up to 15s time-frames during total body and up to 1s time-frames during "focused" (e.g. focusing on heart) image acquisitions.
Multimodality imaging
Medical imaging encompasses many different imaging modalities, which can roughly be divided into anatomical and functional imaging. Anatomical modalities (e.g. CT, MRI) mainly reveal the structure of the tissues and organs, while the functional modalities (SPECT, PET and optical imaging) mainly visualize the physiology and function of the tissue. Because none of the existing imaging modalities can provide information on all aspects of structure and function, an obvious approach is to either alter one imaging modality to the task (e.g. special imaging sequences in MRI) or to try to image a subject using multiple imaging modalities. Following the multimodality approach, in recent years the combination of a SPECT/CT system became a standard molecular imaging modality combination in both the pre-clinical and clinical fields, where the structural information of CT complements the functional information from SPECT. Nevertheless, integration of SPECT with other imaging modalities (e.g. SPECT/MR, SPECT/PET/CTCherry SR. Multimodality In Vivo Imaging Systems: Twice the Power or Double the Trouble. Annu Rev Biomed Eng. 2006; 8:35-62.) is not uncommon.
Reconstruction
A SPECT measurement consists of 2-dimensional projections of the radioactive source distribution that are obtained with collimator(s) and gamma-detector(s). It is the goal of an image reconstruction algorithm to accurately reconstruct the unknown 3-dimensional distribution of the radioactivity.Qi J, et al. Iterative reconstruction techniques in emission computed tomography. Phys Med Biol. 2006; 51(15):R541-78.