The human body is composed of materials that have their own speed of sound (SOS), which is the speed that sound travels through them. Because the harder the material, the greater the SOS, the SOS through each tissue can provide data on the elasticity of the tissue. For the pathological diagnosis of tumors, palpation is used in clinical medicine to provide important information about the tumors because most sarcomas are softer than carcinomas and scirrhous carcinomas are harder than medullary carcinomas. However, manual palpation is subjective and depends on experience, while data on SOS through tissues are objective and comparable among lesions.
A scanning acoustic microscope (SAM) is a device that uses ultrasound (frequency, 80–400 MHz) to image an object by plotting SOS through tissues on the screen (Figure 1). For SAM imaging, we used a microscope supplied by Honda Electronics Co., Ltd., Toyohashi, Japan, and a 120-MHz transducer. SAM functions by directing focused sound from a transducer to a small area of the target object on a glass slide. The sound emitted by the acoustic transducer hits or penetrates the tissue and is reflected by the surface of the tissue or glass. It is then returned to the receiver, which is coincident with the transducer. SOS through the tissue is automatically calculated by comparing the time of flight of the pulse from the surfaces of both the tissue and the glass.
SAM needs thick and flat sections (10–15 µm) that are of good quality, and its resolution depends on the frequency. At 120 MHz, the resolution is 13 µm, which can barely detect single cells. Since the 1980s, the acoustic properties of many organs and disease states, such as myocardial infarctions1, kidneys2, aortic atherosclerosis3, ligaments4, lungs5, and lymph nodes6, have been investigated with SAM.