Feasibility study of shutter scan acquisition for region of interest (ROI) digital tomosynthesis

Abstract Dose reduction techniques have been studied in medical imaging. We propose shutter scan acquisition for region of interest (ROI) imaging to reduce the patient exposure dose received from a digital tomosynthesis system. A prototype chest digital tomosynthesis (CDT) system (LISTEM, Wonju, Korea) and the LUNGMAN phantom (Kyoto Kagaku, Japan) with lung nodules 8, 10, and 12 mm in size were used for this study. A total of 41 projections with shutter scan acquisition consisted of 21 truncated projections and 20 non‐truncated projections. For comparison, 41 projections using conventional full view scan acquisition were also acquired. Truncated projections obtained by shutter scan acquisition were corrected by proposed image processing procedure to remove the truncation artifacts. The image quality was evaluated using the contrast to noise ratio (CNR), coefficient of variation (COV), and figure of merit (FOM). We measured the dose area product (DAP) value to verify the dose reduction using shutter scan acquisition. The ROI of the reconstructed image from shutter scan acquisition showed enhanced contrast. The results showed that CNR values of 8 and 12 mm lung nodules increased by 6.38% and 21.21%, respectively, and the CNR value of 10 mm lung nodule decreased by 3.63%. COV values of the lung nodules were lower in a shutter scan image than in a full view scan image. FOM values of 8, 10, and 12 mm lung nodules increased by 3.06, 2.25, and 2.33 times, respectively. This study compared the proposed shutter scan and conventional full view scan acquisition. In conclusion, using a shutter scan acquisition method resulted in enhanced contrast images within the ROI and higher FOM values. The patient exposure dose of the proposed shutter scan acquisition method can be reduced by limiting the field of view (FOV) to focus on the ROI.

Thus, the accuracy of the ROI reconstruction method by the conventional algorithm may be insufficient, especially for clinical diagnosis. [5][6][7][8][9][10][11] Several studies have examined artifact reduction in images reconstructed from truncated projections. 12-20 Ruchala et al. (2002) reported an algorithm to reduce truncation artifacts caused by the limited field of view using a priori information. 21 Dennerlein et al. (2013) suggested the approximate truncation robust computed tomography (ATRACT) filter method to reduce truncation artifacts in reconstructed images and improve the accuracy of the image. 22 However, most studies do not focus on improvements in the image quality of the overall anatomy, particularly outside the ROI.
We propose a shutter scan acquisition method based on ROI imaging using a digital tomosynthesis system. The projections obtained by the shutter scan acquisition method consist of both non-truncated and truncated projections, so information outside the ROI can be obtained. The application of shutter scan acquisition for clinical practice is rarely studied. Therefore, this study is expected to be a feasibility study showing the clinical feasibility of ROI based imaging technology that can reconstruct the outside the ROI information applied to digital tomosynthesis system.
In this study, the shutter scan acquisition method for advanced ROI imaging was proposed to reduce the patient exposure dose received from a digital tomosynthesis system. The reconstructed image from shutter scan acquisition was corrected using the proposed image processing to remove the truncation artifacts. We applied shutter scan acquisition to a digital tomosynthesis system and evaluated the quality of the reconstructed image. The purpose of this study was to investigate the feasibility of proposed shutter scan acquisition for a digital tomosynthesis system.

2.A | Experimental set-up
We used a prototype chest digital tomosynthesis (CDT) R/F system (LISTEM, Wonju, Korea).The source-to-detector distance (SDD) and the source-to-object distance (SOD) were 1100 mm and 1000 mm, respectively. The X ray tube moved linearly at a speed of 160 mm/s. The phantom used in this study was a multipurpose chest phantom (LUNGMAN, Kagaku, Japan) with a normal anatomical structure of the human chest and lung nodules of 8, 10, and 12 mm. A photograph of the prototype CDT system and a LUNGMAN phantom with a lung nodule are shown in Fig. 1. Table 1 lists the major acquisition parameters of the prototype CDT system.  After removing the edges, we performed inverse Laplace transform on the image.
The 2D Laplace operator can be expressed as follows (1): where g 1 ðk; u; vÞ is the projection at location ðk; u; vÞ and g 2 ðk; u; vÞ is the projection after applying the Laplace operator. Three-point second derivatives along u and v were used for discretization of the Laplace operation. The image-processed projections were used to obtain the reconstructed images by applying the FBP algorithm.
After removing the truncated portion, we reduced the truncation artifact in the reconstructed images as shown in Fig. 5.

2.C | Image reconstruction process
A small number of projections are used to obtain reconstructed image from a limited angle of a digital tomosynthesis system. Existing reconstruction algorithms in digital tomosynthesis system can be divided into three categories: back projection (BP), filtered back projection (FBP), and iterative reconstruction algorithm. 23 In this study,  A flowchart for the overall reconstruction process is shown in Fig. 6.
Subset of non-truncated projections was simply back projected after Scaling correction was performed by the following eq. (2): where PV 0ðtruncatedÞ and PV ðtruncatedÞ denote the corrected truncated

2.D | Data analysis
The reconstructed image had a volume of 1000 9 1000 9 50 voxels.
We set the ROIs and background on the 37th slice. In this study, image quality was investigated using the contrast-to-noise ratio where ROI organ and ROI background are the mean values, and SD organ andSD background are the standard deviations of the object and background regions, respectively.
Second, the COV, which represents the image noise property, was defined as the ratio of the standard deviation to the mean as described by eq. (4): To compare the differences between the two acquisition methods, both the CNR and COV values were normalized.
Third, the total dose area product (DAP) values were measured and compared. This was to assess how much the patient's area dose could be reduced using shutter scan acquisition. We measured the DAP values using a DAP meter (

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The FOM values were calculated by the following eq. (5):   Furthermore, a study of scatter radiation analysis is also needed to clarify the cause of improved image quality using shutter scan acquisition. We are going to also conduct further study of the clinical application for proposed shutter scan acquisition method to demonstrate applicability of this method for clinical cases.

CONFLI CT OF INTEREST
The authors declare no conflict of interest.