Scan Time Reduction and Scatter Rejection in Dual Modality Breast Tomosynthesis

The dual modality tomosynthesis (DMT) system is an investigational scanner that combines x-ray breast tomosynthesis (DBT) and gamma ray emission breast tomosynthesis (MBT) in a single gantry. In breast tomosynthesis, several two-dimensional (2D) images are taken at varying angles over a circular arc within a limited angular range and combined together by a reconstruction algorithm to create a three-dimensional (3D) reconstructed volume of the breast. A combination of the modalities that are employed in the DMT reveal the anatomy of the breast (DBT), which can be correlated with a map of the molecular activity within the breast (MBT). Despite the success of the DMT pilot study in illustrating the benefit of combining both modalities for improving cancer detection, DMT DBT, as it currently stands, must be optimized.
DMT x-ray images are acquired through the step-and-shoot (SNS) method where the gantry comes to a complete stop between projection angles. The resulting DBT scan time can take up to two minutes, a long process that raises the probability of patient discomfort and motion artifacts in the x-ray images. In addition to long scan times, as in the case of most clinical DBT systems, there is currently no method for rejecting scattered x-rays from DMT DBT images. Scattered radiation introduces errors in the estimated attenuation coefficients of breast structures, represented by voxels in the reconstructed images. These errors degrade DBT image quality by reducing lesion detectability and contrast. Quantification of scattered radiation relative to primary rays in DMT DBT emphasizes the need for x-ray scatter rejection. The geometry of the DMT systems raises the possibility of implementing an anti-scatter grid for removing scattered x-rays.
The objectives of this work were to reduce the DBT scan time and to introduce a method for removing x-ray scatter from DBT images. To reduce scan time, a hybrid motion profile was developed where the gantry moves continuously for a portion of the scan and uses a slightly modified version of SNS for the remainder of the scan. For more efficient scatter removal, a novel method of reciprocating a 2D focused anti-scatter grid is presented.
The effects on image quality under conditions of fixed radiation dose resulting from the implementation of a hybrid motion profile and a reciprocating grid are discussed. Modification of the gantry motion profile made it possible to reduce the total DMT DBT scan time to 20 seconds. Finally, inclusion of the reciprocating grid improved lesion contrast of phantoms images under conditions of fixed radiation dose, which is conventionally raised for grid-in full-field digital mammography (FFDM) acquisitions. These adjustments will be incorporated for future DMT human subject clinical trials.