Ultrasound Imaging

Abstract

Ultrasound is defined as sound waves whose frequency is above 20 kHz, the upper limit of the human audible range. The past century has witnessed the growth of an imaging technology that uses ultrasound as the information carrier. A physical medium is required to sustain longitudinal and transverse ultrasound waves. The imaging can be passive, when the object to be detected is the source of sound, or active, when an external deliberately controlled source of sound is used and the echoes from objects in the medium are used to generate an image. State‐of‐the‐art active systems can launch waves into a medium and nondestructively collect echo signals and display information pertaining to the inside of an optically opaque three‐dimensional object. In medical applications, the propagation medium is soft tissue. Present day technology permits real‐time (20 or more image frames per second) visualization of the gross anatomy and pathology. Medical ultrasound has found applications throughout the breadth of diagnostic investigations that includes obstetrics, gynecology, abdomen, ophthalmology, cardiovascular system, breast, and superficial structures. Besides medical applications, ultrasound is also used in nondestructive evaluation (NDE) and sound navigation and ranging (SONAR). NDE is commonly used in the aerospace and nuclear power industries to inspect safety‐critical parts for flaws during in‐service use. The medium here is generally solid material. SONAR pertains to imaging under the sea. The imaging techniques and technologies used in these different application areas are quite similar, at least for medical and NDE. Therefore, the main body of the discussion centers on medical imaging. An ultrasound imaging system usually conveys information about echo generating objects. The image can be thought of as a display of the multidimensional spatial distribution of some object‐dependent entity. Visualization of this information is one of the objectives, and perhaps the major objective in medical ultrasound. The image value is generally related to the mechanical or elastic property of the object. A secondary objective therefore is to understand and quantify the relationship between the image value and the object property, also known as quantitative imaging or tissue characterization in medical ultrasound. This area still remains a subject of active research. This article deals only with the first objective. The three steps involved in the imaging process, namely, transduction and reception by the transducer, two‐way propagation in the medium to some depth, and the scattering/reflection phenomenon are considered in detail. Different modes of scanning and information display employed in medical diagnosis and NDE are considered.