IMAGING MODALITIES: BASIC PRINCIPLES
Ultrasonography, or ultrasound (US), utilizes high-frequency sound waves to noninvasively interrogate structures within a body. The production of ultrasonographic images is based on transmission and reception of physical sound waves through a physical medium, both using a handheld transducer. A typical transducer converts electric pulses into sound waves ranging from 2 to 16 MHz using a piezoelectric material. The speed of sound in different tissues varies but is generally more than five times that in air. The time of flight of the sound wave to be received back to the transducer is used to determine the depth of an object. The amount of reflected signal is a function of the angle of incidence and the difference between the acoustic impedance of tissue on either side of an interface. The received echo signals are detected and interpreted into luminance to form an image with depth perception.
The spatial resolution in US has two distinct, axial and lateral, components. The axial resolution is defined as the ability to discern two objects that lie on top of one another. It is dependent on the length of the transmitted signal. The lateral resolution is the ability to discriminate between two adjacent objects, and is determined by the transducer beam width.
Variations in US techniques have been developed to tailor-specific applications. Tissue harmonic imaging refers to the use of the second (and higher) fundamental frequencies to improve the generated images by reducing artifacts and scatters particularly in the near field. This technique has been shown to improve lesion conspicuity in “difficult-to-scan” regions and obese patients.
Contrast-enhanced US utilizes microbubble contrast agents to visualize tissue perfusion, most commonly and successfully in evaluation of hepatic lesions. The principle is analogous to using intravenous contrasts in cross-sectional imaging, as is the ability to obtain multiphasic images.1 Contrast-enhanced US can be performed in conjunction with gray scale and Doppler examination in transabdominal and endoscopic settings. The first-generation air-based microbubble contrast agents had relatively short half-lives and rigid shells requiring high US output. The second-generation agents, which typically consist of more flexible shells filled with fluorinated hydrocarbons or sulfur hexafluoride, are able to provide real-time visualization of enhancement patterns of various benign and malignant lesions2 with a lower US output power.
A specialized US application that is particularly relevant in hepatobiliary imaging is endoscopic ultrasonography (EUS). EUS permits proximity of the transducer (typically 7.5 to 10 MHz) to improve visualization of deep and small organs such as pancreas and duodenal wall as well as the biliary and pancreatic ductal system.3 Tissue biopsy is also possible via fine aspiration or core needles. Newer therapeutic applications include endoscopic radiofrequency ablation of pancreatic lesions (see Chapter 148). Increasingly, EUS has been utilized to evaluate cystic pancreatic neoplasms and to screen individuals at high risk for pancreatic adenocarcinoma. One of the challenges with endoscopic US ...