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

Abstract

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.

Figure 1

Photograph of the multi-pinhole collimation device with its main components identified. The patient table and gamma camera detectors of the SPECT scanner are visible.

Figure 2

Collimation device in use with a clinical SPECT-CT scanner. For SPECT acquisition (a), the device is placed on the patient table and visually centered in the field of view of the gamma camera detectors with clinical collimators removed. For CT acquisition (b), the animal bed is extended outside the cylindrical collimator, and the patient table is moved toward the rear of the gantry to position the animal for the CT scan.

Figure 3

Illustration of helical sampling versus ordered-subsets axial sampling. This example assumes two detectors, 180° rotation, 12 angular steps (24 projection views), and 4 subsets. The axis of rotation is indicated with an arrow in the transaxial view (a) and axial views (b) and (c). In the transaxial view, the central pinhole coordinates are drawn with a repeating sequence of 4 different symbols. The axial views show the pinhole's vertical position versus its axial position. For the case of helical sampling (b), the animal bed is moved in constant intervals between angular steps. For the case of ordered-subsets sampling (c), the animal bed is moved in a repeating sequence to 4 different axial positions.

Figure 4

Reconstructed transaxial images of a micro-Derenzo phantom. The SPECT image resolves all sectors of the Data Spectrum insert (a) with rod diameters of 2.4, 2.0, 1.7, 1.35, 1.0, and 0.75 mm. The co-registered CT image (b) is generated by the clinical CT scanner and illustrates image quality under low contrast conditions (water versus acrylic). The SPECT image of a custom-machined insert (c) having rod diameters of 1.1, 1.0, 0.9, 0.8, 0.7, and 0.6 mm is also shown.

Figure 5

Reconstructed coronal slice images of a micro-Defrise phantom for various axial sampling schemes: (a) circular sampling (no axial bed motion), (b) helical sampling (30 mm axial range), and (c) ordered-subsets sampling (20 mm axial range)

Figure 6

Contour maps of system efficiency for transaxial slices: (a) helical sampling, center slice, (b) helical sampling, 10 mm axial offset, (c) ordered-subsets sampling, center slice, (d) ordered-subsets sampling, 10 mm axial offset.

Figure 7

Reconstructed images of a uniform cylindrical phantom: (a) transaxial slices, (b) coronal slices.