Mammography is the only screening modality that has been proven to reduce mortality from breast cancer.1 The technique produces planar, projected images of the breast. Overlapping breast tissue on mammography’s projection views can, however, lead to lesions being obscured by overlaid parenchyma. Breast density may also affect the sensitivity of mammography.2
Unlike conventional mammography, which relies on the absorption of x-rays from a stationary tube to create a 2D projected image, digital tomosynthesis uses a moving x-ray source to generate 3D images. The technique is expected to overcome limitations related to superimposition of breast tissue, and thus is viewed as a promising adjunct to mammography.3,4
Although the basic principles of tomosynthesis have been known for many years, its development for mammography was hampered by the poor quality of available x-ray detectors. Advances in digital imaging, especially full-field digital mammography, have now allowed tomosynthesis to be implemented on clinical digital imaging units.
Systems for digital breast tomosynthesis are now available from several vendors, offering a range of angles and arc lengths. The motion of the tube may be linear, circular, or elliptical. The method of acquisition may be “step-and-shoot,” with one exposure taken at a series of discrete positions across the angular range, or continuous, where the exposure is pulsed throughout the motion of the x-ray source. Imaging may be performed with flat-panel detectors or multislit scanning systems. The multislit technique allows multiple projections to be acquired during a single scan, as the detector moves concurrently with the tube.5 The use of monochromatic x-ray sources has also been proposed.5
A set of low-dose source images is captured at various angles around the fulcrum during data acquisition while the breast is compressed. Images may be acquired in one or two views, typically without an antiscatter grid. Thin slices parallel to the detector plane are generated from the data sets to provide detailed visualization of the breast volume. Images are reconstructed using an algorithm usually similar to that used in CT reconstruction. Image data sets are sent from the acquisition workstation to the reading workstation.
A wide angular range allows thin-section reconstructions, which provides superior separation of sections. Depending on the range of the arc, between 30 and 80 sections, each 1 mm thick, can be obtained.
INITIAL EXPERIENCE
We performed breast tomosynthesis on a series of 150 patients who presented with clinical symptoms and whose mammograms or ultrasound scans revealed lesions categorized as BI-RADS 3, 4, or 5. All patients had craniocaudal (CC) and mediolateral oblique (MLO) digital mammography and tomosynthesis (MLO view) of the same breast. Ultrasound was performed at the discretion of the radiologist. Breast tomosynthesis was performed under protocols approved by the institutional review board. All women were at least 40 years old and all provided informed consent.
The tomosynthesis was performed on a prototype unit adapted from the Senographe DS (GE Medical Systems). Fifteen projection images were acquired over an angular range of 40º, using an acquisition time of 15 seconds. Acquisition parameters were selected manually for each patient according to a table indicating appropriate choices for a given breast density and breast thickness under compression. Resulting images were reconstructed in 1 mm increments using an iterative reconstruction algorithm.
