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. 2008 May;35(5):1978-87.
doi: 10.1118/1.2903425.

Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system

Bo Zhao  1 , Wei Zhao
Affiliations

Imaging performance of an amorphous selenium digital mammography detector in a breast tomosynthesis system

Bo Zhao et al. Med Phys. 2008 May.

Abstract

In breast tomosynthesis a rapid sequence of N images is acquired when the x-ray tube sweeps through different angular views with respect to the breast. Since the total dose to the breast is kept the same as that in regular mammography, the exposure used for each image of tomosynthesis is 1/N. The low dose and high frame rate pose a tremendous challenge to the imaging performance of digital mammography detectors. The purpose of the present work is to investigate the detector performance in different operational modes designed for tomosynthesis acquisition, e.g., binning or full resolution readout, the range of view angles, and the number of views N. A prototype breast tomosynthesis system with a nominal angular range of +/-25 degrees was used in our investigation. The system was equipped with an amorphous selenium (a-Se) full field digital mammography detector with pixel size of 85 microm. The detector can be read out in full resolution or 2 x 1 binning (binning in the tube travel direction). The focal spot blur due to continuous tube travel was measured for different acquisition geometries, and it was found that pixel binning, instead of focal spot blur, dominates the detector modulation transfer function (MTF). The noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector were measured with the exposure range of 0.4-6 mR, which is relevant to the low dose used in tomosynthesis. It was found that DQE at 0.4 mR is only 20% less than that at highest exposure for both detector readout modes. The detector temporal performance was categorized as lag and ghosting, both of which were measured as a function of x-ray exposure. The first frame lags were 8% and 4%, respectively, for binning and full resolution mode. Ghosting is negligible and independent of the frame rate. The results showed that the detector performance is x-ray quantum noise limited at the low exposures used in each view of tomosynthesis, and the temporal performance at high frame rate (up to 2 frames per second) is adequate for tomosynthesis.

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Figures

Figure 1
Figure 1
(a) Geometry of the prototype breast tomosynthesis system. (b) A photograph of the prototype tomosynthesis system used in our investigation.
Figure 2
Figure 2
Diagram showing the image sequence used in lag measurement: An offset image was acquired before x-ray exposure, and Nd image frames were acquired after a single x-ray exposure. The x-ray exposure varied from 0.5 to 15 mR. The time intervals between subsequent image views are 0.5 and 0.8 s, respectively, for binning and full resolution modes.
Figure 3
Figure 3
Diagram for ghosting measurement; x-ray sensitivity was measured at a reference exposure before (a) and after (b) ghosting exposures. The exposure for ghosting ranged from 10 to 1000 mR. Ghosting is quantified as ratio of sensitivity at reference exposure before and after ghosting.
Figure 4
Figure 4
(a) Presampling MTF measured from the system; (b) a comparison of MTF due to detector inherent resolution, focal spot blur, and oblique entry angle; and (c) calculated and measured MTF due to oblique x-ray entry alone.
Figure 5
Figure 5
Pixel response of the detector in full resolution and binning modes as a function of x-ray exposure. Measured data are shown with symbols and linear fitting shown with straight lines. ADU∕mR=58.5 for full resolution and 114 for binning mode.
Figure 6
Figure 6
(a) Measured electronic noise NPS (in orthogonal directions of the detector) in both full resolution and pixel binning modes; (b) NPS at different detector exposures for both modes. For clarity, only NPS in the data-line (binning) direction is shown. The Nyquist frequencies fNY of NPS are 5.88 and 2.94 cycles∕mm, respectively, for full resolution and binning modes.
Figure 7
Figure 7
Measured detector DQE at different exposures in full resolution and binning modes. Individual symbols are used for the results in full resolution mode, and lines are used for the binning mode.
Figure 8
Figure 8
Lag as a function of frame number measured at different detector exposures for (a) full resolution and (b) binning mode.
Figure 9
Figure 9
Ghosting measurement, quantified as sensitivity at reference exposure as a function of ghosting exposure.
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