MicroCAT II + SPECT Hybrid Scanner

MicroCATIIThe microCAT II +SPECT is housed in the Molecular Imaging and Translational Research Program (MITRP) and Preclinical and Diagnostic Molecular Imaging Laboratory of the Graduate School of Medicine.

is a fully shielded high-resolution hybrid x-ray (CT) and single photon emission computed tomography (SPECT) in vivo imaging system.  The x-ray source is a 40 W, tungsten anode, 35-80 kVp source mounted on a rotating gantry capable of step-and-shoot acquisition over the full 360° around the subject positioned on the animal bed.  The x-ray detector is a 2048 ´ 3096 pixel CCD coupled to a high-resolution phosphor screen via a fiber optic taper.  It boasts a 61 ´ 91 mm field of view and a resolution of 27 mm.  The digitized images are 12 bit depth and the detector dynamic range when using 2 ´ 2 or 4 ´ 4 binning (standard in our mode of operation) is 72 dB.  The CT data are reconstructed using the cone-beam Feldkamp algorithm, although iterative reconstruction methods are being developed and evaluated in our program.  Real time reconstruction is performed using the Cobra software package (Exxim Computing Corporation, Pleasanton, CA) running on a dual-processor Dell workstation with 8 GB of memory.  A standard 512 ´ 512 ´ 768 (4 ´ 4 binned image) volumetric reconstruction onto 77 mm3 voxels usually completes within 2 minutes after the final projection of a 360-projection acquisition (1° azimuths).  Offline reconstruction can alternatively be conducted using an in-house developed parallel implementation of the Feldkamp algorithm.  This code is capable of providing volumetric images of the same dimensions as above in less than 5 mins using a small group of PCs.

The SPECT detectors are mounted on stepper motor driven stages so that they can be readily retracted form the CT field of view.  Detectors are available optimized for either high or low energy photon emitting nuclides and are housed within a 10 mm-thick casing of 6% antimonial lead.  For low energy nuclides (< 170 KeV; such as 125I, 123I, and 99mTc) we employ two large-area detectors mounted facing each other on the rotating gantry and at 90° to the x-ray source and detector.  The detector heads are 150 ´ 150 mm2 and composed of 5 mm-thick sodium iodide (NaI) crystals with a 1.5 ´ 1.5 mm or 1.25 ´ 1.45 mm crystal spacing.  For high energy photon imaging (up to 300 KeV, e.g. 111In) similarly sized detectors (150 ´ 150 mm2) are available with 10 mm long NaI crystals and a crystal spacing of 2.2 ´ 2.2 mm.

For both sets of detectors parallel, pinhole and multi-pinhole collimator configurations are available.  Composed of 12 mm-thick 6% antimonial lead, the collimators can be fitted with pinholes made of 15 mm-thick tungsten containing either a 0.5, 1.0, 2.0, or 3.0 mm diameter aperture.  For our standard mouse scans using ~ 100 mCi of tracer we use the 2.0 mm pinhole collimators on both detectors, which provides a modest degree of magnification and statistically good count rates (even when each detector is reconstructed independently).  The SPECT data from each detector may be reconstructed either independently or following summation as a single (“multiple-detector”) file. Reconstruction is performed post-acquisition using a dedicated ordered subset expectation maximization (EM) algorithm.  A 52 ´ 52 ´ 68 image typically computes in approximately 2 minutes.  Further upsampling of the image to smaller voxel sizes is normally performed post-reconstruction using the Lanczos filtering kernel in Amira.

Equipment location:
The Molecular Imaging and 
Translational Research Program (MITRP)
Preclinical and Diagnostic Molecular Imaging Laboratory
Graduate School of Medicine
 
Contact information:
Molecular Imaging and Translational Research Program
Phone: 865-305-6181
Preclinical and Diagnostic Molecular Imaging Laboratory
Phone: 865-305-8497