Preclinical SPECT
Preclinical or small-animal Single Photon Emission Computed Tomography (SPECT) is a radionuclide based molecular imaging modality for small laboratory animals[1] (e.g. mice and rats).
Unlike in clinics, preclinical SPECT outperforms preclinical coincidence PET in terms of resolution (best spatial resolution of SPECT - 0.25mm,[2] PET ≈ 1 mm[3][4] ) and, at the same time, allows to perform fast dynamic imaging of animals (less than 15s time frames[5]).
Among major areas of its applications are oncology, neurology, psychiatry, cardiology, orthopedics, pharmacology and internal medicine.
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.
[7] 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.
[10] 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: 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.
However, when combined with moving the animal (the so-called "scanning-focus method" [16]) a larger area of interest can still be imaged with a good resolution and sensitivity.
[17] At the same time, dedicated high-sensitivity collimators can allow >1% detection efficiency and maintain sub-mm image resolution.
[18] Multiple pinhole SPECT system designs have been proposed, including rotating gamma camera, stationary detector but rotating collimator, or completely stationary camera[19][20] in which a large number of pinholes surround the animal and simultaneously acquire projections from a sufficient number of angles for tomographic image reconstruction.
With block-iterative methods, every iteration of the algorithm is subdivided into many subsequent sub-iterations, each using a different subset of the projection data.
An example of a widely used block-iterative version of MLEM is the Ordered Subsets Expectation Maximization algorithm[25] (OSEM).
Nevertheless, depending on the energy of γ-photons and the size of the animal that is used for imaging, correction for photon attenuation and scattering might be required to provide good quantification accuracy.
Two examples of common multi-isotope tracer combinations used for SPECT imaging are 123I-NaI/99mTc-pertechnetate (thyroid function[27]) or 99mTc-MAG3/111In-DTPA (assessment of renal filtration).
Another important characteristic of SPECT is the simplicity of tracer radiolabeling procedure that can be performed with a wide range of commercially available labelling kits.
Preclinical SPECT and PET are two very similar molecular imaging modalities used for noninvasive visualization of biodistribution of radiolabel tracers that are injected into an animal.
When a radioactive tracer is labeled with primary gamma-emitting isotopes (e.g. 99mTc, 123I, 111In, 125I), the acquired images provide functional information about the bio-distribution of the compound that can be used for multiple diagnostic purposes.
