Authors: Katsutoshi Miura & Seiji Yamamoto
A scanning acoustic microscope (SAM) imaging system calculates and color codes the speed of sound (SOS). Because the harder the tissue and thus the greater the SOS, SAM can provide data on the elasticity of tissues and lesions. Areas with greater SOS correspond to those with higher concentrations of collagen or muscle fibers. Cell-poor or myxoid-degeneration areas demonstrate less SOS than the surrounding tissues.
SAM offers the following benefits: (1) images are acquired in only a few minutes and do not require special staining, (2) repeated observations, even after the staining of the same section, are possible, (3) digital imaging from SOS exhibits high resolution that is comparable to that of light microscopy, (4) analysis by SAM systems is helpful for understanding echographic imaging, and (5) digitized SOS data can be statistically compared among different lesions.
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.
We thank Dr. K. Kobayashi (Honda Electronics Co., Ltd., Toyohashi, Japan) for his technical support and advice on scanning acoustic microscopy and S. Okamoto, T. Kato, Y. Kawabata, and N. Suzuki for preparing the tissue sections. This work was supported in part by grants from the Japan Science and Technology Agency (AS232Z01789F) and the Ministry of Education, Culture, Sports, Science, and Technology of Japan (24590445).
Figure 1: Principles of scanning acoustic microscopy (SAM)
Ultrasonic waves, which are irradiated from the transducer, reflect off both surfaces of the glass slide and the section and return to the transducer. These waves pass through the 10-µm sample sections with different ultrasonic properties. The transducer automatically scans the section to calculate the speed of sound (SOS) through each area. The section is placed on the transducer upside down, and distilled water is applied between the transducer and the section as a coupling fluid. SOS only through water is 1,500 m/s, and this measure is used as a control SOS.
Figure 2: Screenshot of ultrasonic data
On the screen, the vertical bar on the left and the horizontal bar at the bottom of each figure indicate the distance (mm) on the slide. The vertical colored column on the right side of the Sound Speed figure indicates the average SOS of each square area on the section. Similarly, the vertical colored column in the Attenuation and Thickness figures indicates the attenuated intensity of sound per mm (dB/mm) and the thickness of each square area (µm), respectively.
Katsutoshi Miura, Scanning acoustic microscope Lab, Hamamatsu University School of Medicine
Seiji Yamamoto, Medical Photonics Research Center,Hamamatsu University School of Mediccine
Correspondence to: katsutoshi Miura ([email protected])
Source: Protocol Exchange (2013) doi:10.1038/protex.2013.040. Originally published online 15 April 2013.