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Comparative Study
. 2015 Apr;42(4):1973-81.
doi: 10.1118/1.4915079.

Evolution of spatial resolution in breast CT at UC Davis

Peymon M Gazi  1 , Kai Yang  2 , George W Burkett Jr  3 , Shadi Aminololama-Shakeri  3 , J Anthony Seibert  4 , John M Boone  4
Affiliations
Comparative Study

Evolution of spatial resolution in breast CT at UC Davis

Peymon M Gazi et al. Med Phys. 2015 Apr.

Abstract

Purpose: Dedicated breast computed tomography (bCT) technology for the purpose of breast cancer screening has been a focus of research at UC Davis since the late 1990s. Previous studies have shown that improvement in spatial resolution characteristics of this modality correlates with greater microcalcification detection, a factor considered a potential limitation of bCT. The aim of this study is to improve spatial resolution as characterized by the modulation transfer function (MTF) via changes in the scanner hardware components and operational schema.

Methods: Four prototypes of pendant-geometry, cone-beam breast CT scanners were designed and developed spanning three generations of design evolution. To improve the system MTF in each bCT generation, modifications were made to the imaging components (x-ray tube and flat-panel detector), system geometry (source-to-isocenter and detector distance), and image acquisition parameters (technique factors, number of projections, system synchronization scheme, and gantry rotational speed).

Results: Characterization of different generations of bCT systems shows these modifications resulted in a 188% improvement of the limiting MTF properties from the first to second generation and an additional 110% from the second to third. The intrinsic resolution degradation in the azimuthal direction observed in the first generation was corrected by changing the acquisition from continuous to pulsed x-ray acquisition. Utilizing a high resolution detector in the third generation, along with modifications made in system geometry and scan protocol, resulted in a 125% improvement in limiting resolution. An additional 39% improvement was obtained by changing the detector binning mode from 2 × 2 to 1 × 1.

Conclusions: These results underscore the advancement in spatial resolution characteristics of breast CT technology. The combined use of a pulsed x-ray system, higher resolution flat-panel detector and changing the scanner geometry and image acquisition logic resulted in a significant fourfold improvement in MTF.

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Figures

FIG. 1.
FIG. 1.
Photograph of the latest UC Davis prototype of dedicated breast CT.
FIG. 2.
FIG. 2.
Photographs of the gantry assembly designed for the latest UC Davis prototype of dedicated breast CT are pictured. The top view of the gantry and the filter/collimator changer assembly are shown in (a) and (b), respectively. The gantry components of the scanner are marked as follows: x-ray tube (1), flat-panel detector (2), central post (3), PET scanner heads (4), collimator changer (5), and filter changer (6).
FIG. 3.
FIG. 3.
(a) Focal spot of the x-ray tube used in Cambria and Doheny (M-1500, Varian Medical Systems, Salt Lake city, UT), (b) line profile representative of the distribution of the focal spot at 60 kV and different mA values along the horizontal axis passing through the projection of the focal spot on the detector.
FIG. 4.
FIG. 4.
Detector MTF (projection imaging) results measured for three different pixel binning modes. The edge phantom was placed at two different locations on the surface of the detector. In each binning mode, the edge phantom was imaged and averaged with 300 views at 60 kV, 160 mA, and 3 ms of pulse width. The detector was set to work at LFW mode with pixel binning set to 1 × 1 (a), 2 × 2 (b), and 4 × 4 (c).
FIG. 5.
FIG. 5.
Detector MTF results at different binning modes with edge phantom placed at (a) center of detector and (b) isocenter.
FIG. 6.
FIG. 6.
The effect that the wire diameter has on the coronal plane MTF. The detector was set to work at high-gain 1 × 1 binning mode with a readout rate of 23.25 fps. The same technique factors and methods were used to measure the system MTF.
FIG. 7.
FIG. 7.
The coronal plane MTF for eight different radial positions from the axis of rotation of the reconstructed image. A total of 800 projection images were acquired at a low full well, 2 × 2 binning mode, and a 50 fps frame rate. All images were reconstructed using the Shepp-Logan filter.
FIG. 8.
FIG. 8.
The change in the MTF by switching to different reconstruction filters is shown. The detector was set to work at low full well, 2 × 2 binning mode with a 50 fps rate. Eight hundred views were used for the acquisition. CT images were reconstructed at a cone angle of less than 1°.
FIG. 9.
FIG. 9.
System MTF characterization with different numbers of projection images. The detector was set to LFW, 2 × 2 binning mode with a frame rate of 50 fps. Scans were performed with identical x-ray technique factors.
FIG. 10.
FIG. 10.
A comparison between system MTFs with different detector binning modes is shown. Eight hundred images were acquired at LFW mode, with identical x-ray technique factors.
FIG. 11.
FIG. 11.
A system MTF comparison between different generations of UC Davis dedicated breast CT is shown. The system MTF at the isocenter and the periphery of the FOV of each scanner is shown in (a) and (b), respectively.
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