title: Design of a dual-resolution collimator for preclinical cardiac SPECT with a stationary triple-detector system.
abstract: OBJECTIVE
  One approach to preclinical single-photon emission computed tomography (SPECT) imaging that provides both high resolution and high sensitivity is based on imaging a mouse inside a collimating tube; many magnified pinhole projection images from a small target region, e.g., the heart, can be recorded simultaneously on multiple detectors with little multiplexing since each pinhole aperture's opening angle is restricted to view mostly the target organ. However, to obtain complete data for reconstruction, it may be necessary to scan the mouse through the target region of the tube. The authors are developing a different approach based on acquisition and reconstruction of both low-resolution and high-resolution projection data acquired sequentially through many pinholes embedded in two tungsten tube sections of different diameters, a "scout" section and a high-resolution section, placed end-to-end along the axis of a triple-head clinical SPECT scanner. This paper describes the design procedures used to determine the geometric parameters of two new collimator-tube sections, as well as one approach for joint reconstruction of data acquired from both sections.
  
  
  METHODS
  The high-resolution section was designed by projecting as many pinhole views of a simulated mouse heart as possible over each detector's camera, with no overlapping of heart projections and minimal overlapping between adjacent "hot" organ and cardiac projections. The authors then jointly optimized the geometric design of the scout section for a triple-detector camera system, as well as the number of maximum-likelihood expectation maximization (MLEM) iterations required to provide minimum mean-squared error of reconstructed voxel counts throughout a 7-cm axial range, with the constraints of fixed, 2.4-mm scout system resolution at the tube center for all apertures, limited multiplexing, and no detector motion. Simulated mouse projection data from both tube sections were then reconstructed to illustrate a simple approach for using high-resolution data to improve the whole-body scout images within a cylindrical region surrounding the heart.
  
  
  RESULTS
  The 2-cm-inner-radius high-resolution tube section accommodated 87 platinum-iridium pinhole inserts, each with a 0.3-mm square aperture; their radial distances from the centerline of the system ranged from 2.2 to 3.0 cm. The optimal radial distance to the closest scout pinhole and optimal number of MLEM iterations were 4.4 cm and 35 iterations, respectively, and the radial distances of the 39 scout pinholes ranged from 4.4 to 4.8 cm; aperture sizes ranged from 1.1 to 1.7 mm transaxially and 0.9-1.5 mm axially. After including data from the high-resolution section viewing the heart region into whole-body mouse reconstructions from scout data, the authors obtained high-resolution images of the heart, embedded within lower resolution images of the body, with minimal artifacts.
  
  
  CONCLUSIONS
  The authors have optimized a dual-resolution collimator tube that provides both whole-body projections of a mouse and more targeted projections centered on the heart that can be jointly reconstructed to obtain high-resolution images of the heart embedded within lower-resolution whole-body images.
authors.affiliation: Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Moore
authors.affiliation: Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Mi-Ae
authors.initials: MA
authors.lastname: Park
authors.affiliation: Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115.
authors.forename: Zhe
authors.initials: Z
authors.lastname: Liu
authors.affiliation: inviCRO, LLC, Boston, Massachusetts 02210.
authors.forename: Morgan C
authors.initials: MC
authors.lastname: Lyon
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
authors.forename: Lindsay C
authors.initials: LC
authors.lastname: Johnson
authors.affiliation: Solidscape, Inc., Merrimack, New Hampshire 03054.
authors.forename: Victor H
authors.initials: VH
authors.lastname: Lushear
authors.affiliation: Solidscape, Inc., Merrimack, New Hampshire 03054.
authors.forename: James G
authors.initials: JG
authors.lastname: Westberg
authors.affiliation: Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104.
authors.forename: Scott D
authors.initials: SD
authors.lastname: Metzler
mesh_terms: D000818:Animals; D004867:Equipment Design; D006321:Heart / Q000000981:diagnostic imaging*; D007091:Image Processing, Computer-Assisted; D051379:Mice; D015899:Tomography, Emission-Computed, Single-Photon / Q000295:instrumentation*