Recent Applications of Acoustic Microscopy for Quantitative Measurement of Acoustic Properties of Soft Tissues

Abstract

High resolution biomedical imaging using high frequency ultrasound is possible because both wavelength and beamwidth are inversely proportional to ultrasonic frequency. Scanning acoustic microscopy (SAM) uses 100 MHz or higher frequency ultrasound. The spatial resolution achieved by a 100 MHz and 1 GHz ultrasound SAM are 15 and 1.5 μm, respectively. This level of detail enables cellular imaging. There are three unique features of SAM compared with other microscopy modalities such as optical, electron and atomic force microscopy. First, SAM can be applied for easy and simple histopathological examinations because it does not require special staining techniques. The contrast observed in SAM images depends on the acoustic properties (i.e., density, stiffness, and attenuation) and on the topographic contour of the tissue. Second, microscopic acoustic properties obtained with high frequency ultrasound can be used for assessing echo intensity and texture in clinical echography with lower frequency ultrasound. Third, SAM data can provide the basic data for assessing biomechanics of tissues and cells. Ultrasound is transmitted through a coupling medium and focused on the surface of the substrate. Transmitted ultrasound is reflected at both the surface of the biological material and the interface between the biological material and the substrate. The transducer receives the sum of these two reflections. The interference of these two reflections is determined by acoustic properties of the biological material. The interference signal as a function of the frequency depends on the thickness and sound speed of the sample. The interference signal as a function of the intensity depends on the amplitude of the surface reflection and attenuation of ultrasound propagating through the tissue. For bone, cartilage, tendon and cardiovascular tissues, microacoustic properties can provide important information on biomechanical properties. Biomechanic evaluation of these tissues is especially important for assessing the pathophysiology. Cells are considered to consist of viscoelastic materials and SAM has provided information on viscosity by ultrasonic attenuation estimates and information on elasticity by sound speed estimates. Instead of stretching cells or using atomic force microscopy for measuring biomechanical properties, SAM can be used to measure precise mechanical property distributions without contact to the cells. Thus, SAM provides a new paradigm of pathology that is based on the mechanical properties of is the object being imaged. Recent developments such as ultrasound speed microscopy, 3D ultrasound microscopy and high frequency array transducers may provide a clinically applicable SAM in the near future.