DBT Physics and Instrumentation

DBT is a recently developed technique based on digital mammography (Palazuelos, Trujillo, & Romero, 2014). The image acquisition geometry of DBT is similar to that of FFDM with a moving X-ray tube at regular angular intervals, in a plane around the compressed breast which rests on static support. The X-ray tube movement range is different for each manufacturer. During exposure, the image receptor is either fixed or rotated to keep it perpendicular to the X-ray tube (Feng & Sechopoulos, 2012; Sechopoulos, 2013a). Several DBT prototype machines have been introduced by different manufacturers. Examples of these include GE Essential, Hologic Selenia Dimenssion, IMS GiottoTOMO, Philips MicroDose, Planmed Nuance Excel, and Siemens MAMMOMAT Inspiration. All of these machines have the same purpose but each has its specific characteristics. So far, the Hologic Selenia Dimensions machine is the only one that has been approved in America by the FDA, in 2011 (Destounis & Gruttadauria, 2015; Sechopoulos, 2013a).

3.5.1.1 Image Acquisition

The DBT machine has the same structure as FFDM units, consisting of an X-ray tube mounted within the arm, a breast compression paddle, a breast support and a digital image

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receptor (Sechopoulos, 2013a). The only difference between FFDM machines is that the DBT X-ray tube, as well as the image receptor in some DBT brands, can rotate across an arc of 11o to 50o during exposure, depending on manufacturer design, see Figure (3-4). This produces between 9 and 25 images (Lim & Maxwell, 2015). For instance, the Hologic Selenia Dimensions has a range of 15 degrees, between -7.5 and +7.5 degrees. It generates 15 images per view (Feng & Sechopoulos, 2012). The image acquisition process is achieved either by continuous exposure during X-ray tube movement, or pulsed exposure at each angle of tube movement. Shorter acquisition times are required for the continuous method, however lower image resolution can result due to motion blur. For both methods the acquisition time ranges from 3s to 25s per single view (Lim & Maxwell, 2015). The image acquisition time should be minimised to be as short as possible to avoid patient movement blur. For DBT imaging the required X-ray energy is slightly higher than that for FFDM. Therefore, the spectra of a tungsten target with Al, Rh, or Ag filters are typically used (Sechopoulos, 2013a; Yaffe & Maidment, 2014).

The Sectra MicroDose from Philips is a different DBT machine design which is based on the slit-scan photon counting technique. In this machine, a collimated fan shaped X-ray beam and multi-slit linear detector are used to scan the compressed breast across an arc (Sechopoulos, 2013a). The geometry of this machine is completely different to that of other DBT units. The focal-image receptor distance of this machine is 66 cm. Also, there is a 1.93 cm gap between the breast support and the image receptor. The rotation centre is located 104 cm below the focal spot level (Dance, Young, & van Engen, 2011). The main advantages of Sectra MicroDose unit over the traditional DBT units are that less electronic noise is produced, there‘s a lower patient radiation dose of about 1/20, and it can determine the energy of a transmitted X-ray photon which is useful for both breast absorption measurements and breast composition (Schmitzberger et al., 2011). However, the main disadvantage of this machine is that it cannot be used to acquire conventional two- dimensional images (Sechopoulos, 2013a).

Gilbert, Young, Astley, Whelehan, and Gillan (2010) documented in NHSBSP publication #69 that the breast compression in DBT is the same as in FFDM and they preferred to achieve them both in one compression. However, a Monte Carlo study by Saunders, Samei,

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Lo, and Baker (2009) found that a 12.5% reduction in breast compression during DBT examinations results in approximately the same MGD and constant lesion conspicuity for both breast microcalcification and mass. Work by Förnvik et al. (2010) investigated the effect of half compression force on image quality during DBT examination for 45 women. They reported equivalent image quality for both compression values. A comparable effect of half breast compression on image quality for 130 Malaysian women has been obtained by Suhaimi, Mohamed, and Ahmad (2015). Overall, more studies are required to consider the effect of breast compression reduction in DBT on both image quality and patient radiation dose.

3.5.1.2 DBT Image Reconstruction

The tomographic images are reconstructed from the series of projections through filtered back projection or iterative reconstruction algorithms (Sechopoulos, 2013b). The image reconstruction process is achieved by shifting the projection images with respect to the frame of the image. Therefore, the structures at the specific plane are visualised in the same place, at the frame. Then the images are added together, reinforcing the contrast of the structures‘ image in the selected plane and blurring out the structures in other planes. Figure (3-5) illustrates the tomosynthesis image reconstruction process, (Kotre & Reis, 2015). Tomosynthesis produces the highest spatial resolution in the planes and poorer resolution between them. The limited range of acquisition angles in DBT makes the data of the tomosynthetic projection less complete than computed tomography (CT). Therefore, the tomosynthesis reconstructed images are pseudo three-dimensional images of breast tissues (Yaffe & Maidment, 2014).

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Figure (3-4) Demonstrates the image acquisition process in DBT (Kotre & Reis, 2015).

Figure (3-5) Demonstrates the image reconstruction process in DBT (a) In the level of circle plane and (b) In the level of square plane (Kotre & Reis, 2015).