The Technologist’s Perspective

Training

Technologists, like physicians, require an initial 8 hours of formal training before being certified to perform tomosynthesis exams. Typically the technologists are trained during a session with the manufacturer’s applications specialist. A technologist who is trained and comfortable with the new tomosynthesis equipment can then successfully train other technologists. A useful tomosynthesis training tool for the technologist includes creation of a tomosynthesis checklist designed simply to list all the features of the tomosynthesis unit and to specifically differentiate applications that are unique to tomosynthesis, thereby facilitating technologist adaptation to the new equipment.

Radiation Exposure

Because tomosynthesis is based on x-ray technology, each tomosynthesis acquisition exposes the patient to radiation. In general the cumulative dose per tomosynthesis view is approximately the same or slightly more than a conventional 2D view, although this is variable depending on the manufacturer. For example, the dose of a combined tomosynthesis and 2D mammogram obtained by a commonly used tomosynthesis unit is approximately double the amount received during a conventional 2D mammogram. Despite the increased dose, it is important to understand that the dosage is still below Mammography Quality Standards Act (MQSA) limits. As per the American College of Radiology, 3 mGy is the upper limit for radiation dose of a single mammography exposure. This standard was established over 20 years ago for analog mammography. Since the development of digital mammography, the radiation dose for 2D mammography has decreased over time and is dependent on the equipment manufacturer. As per one vendor, the dose for 2D mammography is now approximately 1.2 mGy per average exposure, and for tomosynthesis, the dose is approximately 1.45 mGy; therefore a combination exam is approximately 2.65 mGy, less than the MQSA limit of 3 mGy.

Most patients do not understand dosage when described in units of mGy (radiation energy in a kilogram of matter) or mSv (biological effect of radiation energy in a kilogram of human tissue), so it can be useful to explain using comparisons with normal background radiation. Although a single exposure tomosynthesis combined with 2D mammography is slightly more than double the dose compared to conventional 2D mammography alone, the combined dose due to tomosynthesis is equivalent to only about 2 months of annual background radiation in the United States. Stated another way, the combined radiation dose of tomosynthesis is 50% less than the difference in annual background radiation dose in women living in the mountains of Colorado compared to the average background radiation dose for women living elsewhere in the United States.

Keep in mind that exact dosage for a tomosynthesis exam also varies among manufacturers and equipment. Because the tomosynthesis technique is different depending on the manufacturer of the unit, the dose per exposure and per bilateral exam is variable. For example, some manufacturers recommend performing simultaneous combination 2D plus tomosynthesis craniocaudal (CC) and mediolateral oblique (MLO) views for a routine screening exam. Other manufacturers recommend tomosynthesis only for the MLO views and 2D mammography for the CC views or vice versa. In an attempt to lower the dose of the tomosynthesis exam, some manufacturers offer or are developing a synthesized tomosynthesis mammogram. The synthesized mammogram acquires information from the tomosynthesis scan and reconstructs a 2D mammogram, eliminating the need for the conventional 2D mammogram. In addition, other manufacturers are hoping to develop an accurate tomosynthesis mammogram that would not require any additional 2D component. Technologists and radiologists should be aware of the extra radiation dosage for tomosynthesis, depending on the specific unit in use.

In addition to variations in dose and 2D/tomosynthesis image combinations, tomosynthesis equipment specifications also vary across different manufacturers. Detectors and filters, pixel size, scan angles, image time, and reconstruction algorithms are all vendor dependent, as shown in Table 3.1 .

Performing the Tomosynthesis Mammogram

Performing a mammogram with tomosynthesis is similar to performing a conventional 2D digital mammogram with a few differences. The compression paddles and tube position are basically similar to conventional mammography units, with slight manufacturer dependent variations. Patient positioning is also similar. One important consideration is the importance of positioning of the CC view. Many technologists have a tendency to slightly laterally exaggerate the CC views, trying to include more of the tissue in the lateral, especially upper outer quadrant. This, however, can result in distortion of the tissues in the CC view ( Fig. 3.1 ). When interpreting tomosynthesis images, radiologists find the CC view particularly useful, and consistency from year to year is therefore very important. For optimal positioning, the breasts should be pulled straight and the nipples centered.

The tomosynthesis acquisition time is typically around 5 seconds, but as noted in Table 3.1 , some units may require a longer exposure time. Depending on the manufacturer, tomosynthesis may be performed as a single exam or in combination with the 2D mammogram. Some units only allow a single tomosynthesis acquisition without a 2D component. Other units allow conventional 2D only, tomosynthesis only, or a combination 2D/tomosynthesis exam to be obtained in any single view during a single compression. With the combined 2D plus tomosynthesis image acquisition, the exposure time may be 4 seconds longer than a conventional mammogram. Faster and more efficient image processing helps balance the extra exposure time of the tomosynthesis image acquisition. For example, one type of unit allows the technologist to take an image and continue onto the next image without waiting to accept or reject the previous.

During tomosynthesis, the tube rotates in an arc as the exposures are acquired. The angle of the arc is variable across manufacturers, typically ranging from 11 to 50 degrees. The motion across the arc may be continuous or a stepwise “stop and shoot” scan. A notable difference compared with standard mammography is the face guard on the tomosynthesis unit, which protects the patient’s head while the tube is making its acquisition sweep. An important tip is to warn the patient that the tube will move. Although the compression paddle and receptor in which the breast is compressed is stationary, many patients may react by pulling back if they are not expecting the machine to move.

For combination 2D plus tomosynthesis exams, the total exposure time may be 10 seconds or more, with tomosynthesis exposures usually preceding the 2D exposure. Therefore attention to the patient’s breath-holding technique is important to eliminate motion artifact. It is usually not possible for a woman to hold her breast during an entire 2D plus tomosynthesis combination acquisition. Technologists vary in their approach to coaxing the patient through the exam. Some technologists allow the patient to breathe normally throughout the entire exam. Others have the patient breathe normally during the initial tomosynthesis exposure and then hold their breath during the final 2D exposure. Both approaches are successful, and which technique is used depends on both the technologist and the specific patient.

Currently, the most common tomosynthesis technique used in the United States is the combination 2D plus tomosynthesis exam. For screening mammography, typically bilateral MLO and CC combined 2D and tomosynthesis views are obtained. However, women with large breasts requiring tiling/multiple images per view, as well as women with implants, are already receiving additional radiation from the standard extra views, and performing tomosynthesis could increase radiation beyond the MQSA limits. For these women, use of the synthesized 2D mammogram instead of the additional full-field digital mammography (FFDM) 2D view enables the patient to reap the benefits of tomosynthesis at the same dose as the conventional mammogram. On the other hand, a combination of FFDM and synthesized 2D plus tomosynthesis could also be obtained. For example, in women with implants, the standard CC and MLO views can be obtained using the conventional FFDM 2D technique, and the implant displacement CC and MLO views could be obtained using tomosynthesis plus the synthesized 2D tomosynthesis views.