page_title: Preclinical SPECT
section_title: Spatial resolution
hierarchical_section_title: Preclinical SPECT / Basic imaging principle of preclinical SPECT / Spatial resolution
caption_attribution_description: English: When pinhole imaging is used, the high magnification of the projection greatly reduces the resolution loss due to detector blurring.
context_page_description: Preclinical or small-animal Single Photon Emission Computed Tomography is a radionuclide based molecular imaging modality for small laboratory animals. Although SPECT is a well-established imaging technique that is already for decades in use for clinical application, the limited resolution of clinical SPECT 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 and, at the same time, allows to perform fast dynamic imaging of animals.
  SPECT imaging requires administration of small quantities of γ-emitting radiolabeled molecules 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 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.
context_section_description: 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 ∛3000≈15 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.
  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. 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:
  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 0.015μL volumetric resolution for in vivo mouse imaging.
caption: When pinhole imaging is used, the high magnification of the projection greatly reduces the resolution loss due to detector blurring
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