Sonography of the Breast

by Cynthia L. Rapp BS, RDMS
Radiology Imaging Associates - Swedish Medical Center
Englewood, Colorado
Reprinted with author's permission.

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From: Official Proceedings - Society of Diagnostic Medical Sonography's 17th Annual Conference. 
September 14-17, 2000, Dallas Texas, p. 57-67

One in nine women will develop breast cancer during her lifetime, provided she lives to age 85. Most breast cancer is NOT genetically linked – less than 16%. In the study by Stavros et al., July 1995, – over 80% of all sonographic, biopsy proven, solid nodules were benign.

Indications
Breast ultrasound is a diagnostic rather than a screening procedure; it is targeted to a specific clinical or focal mammographic finding in the vast majority of patients. Breast ultrasound should be performed on palpable lumps when the mammogram in the area of the lump is negative or nonspecific. Although, there are now a few studies on whole breast screening ultrasound.

Equipment
Breast ultrasound must have excellent spatial and contrast resolution. Both the axial and lateral components of spatial resolution must be good. Broadband, high frequency linear electronically focused probes currently offer the best combination of spatial and contrast resolution for breast ultrasound.

Axial Resolution
Excellent axial resolution is important in identifying normal structures which course parallel to the skin (such as mammary ducts and the fascial planes surrounding the mammary zone) and in identifying the characteristics of the capsules around cysts and solid nodules.

Lateral Resolution
Lateral resolution at all depths within the breast is important in order to minimize volume averaging of surrounding normal breast tissues with pathological lesions. Such volume averaging may cause mischaracterization of small cystic lesions as solid and may even cause small solid lesions to be indistinguishable from surrounding tissues. Lateral spatial resolution is also a complex subject. For linear probes there are two planes which determine lateral resolution; the long axis and short axis (elevation plane focus).

The long axis of the linear probe can be electronically focused. Continuous electronic focusing may be done on receive or transmit phases. The degree of electronic focusing upon receive depends upon many factors, including:

In general lateral resolution improves with increasing; the number of channels, aperture size, number of elements in the transducer and scan lines. Electronic focusing on transmit depends on many of the same factors as receive focusing, but is more limited. It depends upon the number of transmit zones. In general, the more transmit zones, the better the lateral resolution. However, increasing the number of transmit zones, decreases the frame rate. In general, multiple transmit focal zones in the first 2cm are very beneficial in breast sonography.

Elevation Plane
The elevation plane (short axis) of the probe cannot currently be electronically focused. The elevation plane is focused at a fixed depth by an acoustic lens. The manufacturer decides how deeply to focus this plane before the probe is built. Elevation plane, focal lengths are usually decided by the application for which the probe will primarily be used. Dedicated small parts or near field probes should be focused at about 1.5cm or even more superficially.

5MHz linear array probes were designed with peripheral vascular applications in mind and are focused in the elevation plane of about 3 to 4cm. This is too deep for most breast imaging, the elevation plane would be focused in the pectoralis muscle in most patients. In general a 7.5 to 13MHz transducer, with an elevation plane of about 1.5cm, is the best breast ultrasound transducer currently available.

 

Above figure: If a 5MHz transducer is used and the lesion of interest is small and superficial, volume averaging of adjacent normal tissues will be a problem. Cysts may fill-in and be misclassified as solid. Small solid lesions may be completely missed.

 

If a palpable nodule is pea or bee-bee sized, then it is very small and near the skin. In such cases, even optimally focused transducers may have difficulty resolving and characterizing the lesion. In such circumstances, the sonographer should use a 1cm standoff pad or a large "glop" of gel in order to move the elevation plane focus closer to the skin. Remember if too thick of a standoff pad (3cm pad) is used, then the elevation plane will be centered in the pad and not in breast tissue.

Annotation
Labeling your films can be very time consuming. Most ultrasound departments utilize the clock method. In our lab we also use the ABC and 123 to label the exact location of a lesion. This is particularly helpful if you need to follow-up a lesion of is the patient is sent to another facility for a biopsy.

First a clock position is stated. Secondly, the location of the lesion is noted. There are five possible choices for the 123, location. 1 through 3 is divided into 3 concentric rings. If a lesion is near the nipple, this location is 1, mid way out in the breast is 2 and in the periphery is 3. If a lesion is under the nipple we label this area SA for subareolar and lesions in the axilla are labeled AX.

For the ABC; A, is if a lesion is near the surface or close to the transducer. B is mid way down (and represents the mammary zone) in the breast and C is against the chest wall.

Diagram of a right breast lesion about 4 cm directly superior to the nipple and 3 cm deep which was scanned in a radial plane would typically be described as "Rt 12 2C RAD". The left breast lesion in the upper outer quadrant about 6 cm from the nipple and near the chest wall which was scanned in an anti-radial plane would be labeled "Lt 1:30 3C AR". This cryptic method of annotating ultrasound images is very advantageous. It saves keystrokes and shortens the examination time.

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