Abstract
Both the axial and lateral resolutions of ultrasonography have recently been greatly improved, owing to complete digitalization of the machine as well as narrowing of the beam width and adoption of high frequency band scanners. Particularly for ultrasound examination of areas close to the skin surface, high frequency band scanners are useful because they enable detection of minute lesions only a few millimeter in size.
On the other hand, the treatment of breast cancers has generally tended to focus on minimizing the extent of surgical resection, because of the introduction of sentinel node biopsy, popularization of minor surgery preserving the breast, and tailor-made individualization of postoperative adjuvant therapy. Thus clinicians in this field have made increasing efforts to detect breast cancers at an early stage, including DCIS, and high-quality ultrasonography is expected to be a useful tool for this purpose.
In order to detect very tiny lesions, an extremely high-quality ultrasonography machine and skilled operation are required. Examiners can now finely control functions that are important for quality control, such as the receiving frequency, gain and frame rate, and images are displayed on a monitor and recorded as a printed picture.
The completely digitalized ultrasonography machine now mainly used employs electric focusing, by which signals sent from receiving probes can be managed by controlling the timing through a delay circuit, instead of focusing by means of an acoustic lens. By using this mechanism, the quality of images has improved. The delay circuit fixes the received sound speed at about 1,530m/s or 1,540m/s, calculates the delay time, and forms an electrically focused image. The fat content of the breast differs among women ; sound speed is slower when the breast is rich in fat and higher when fat is sparse. Thus the grade of the delay circuit changes according to differences in breast fat content, and image blurring can occur. The type of machine that has been widely adopted has a “beam compound system”, which radiates sound in many directions to make an image, and therefore blurring is increased.
We have found experimentally that controlling the speed of sound received actually yields a change in image quality. By comparing blurred images due to differences in sound speed with improved-quality images as a control, we wish to stress the usefulness of this newly developed function, by which sound speed correction can be performed, and employed to detect tiny lesions during ultrasound scanning of the breast.
