Cross-sectional imaging of liver, biliary, and pancreatic disease: Introduction and basic principles

Introduction

Cross-sectional imaging of the liver, biliary tree, and pancreas can be performed with ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI). Positron emission tomography (PET) is most often performed in combination with CT (PET-CT) and has more recently been performed in conjunction with MRI (PET-MRI). This chapter will emphasize basic principles of ultrasound, CT, and MRI that highlight the strengths and limitations of each technique for the ordering clinicians.

Ultrasound

Ultrasound as an imaging modality has tremendous versatility, is low-cost, and has real-time capability and portability. Ultrasound is considered safe at clinical, diagnostic levels, with no confirmed harmful biologic effects on the operator or patient. Use of Doppler ultrasound also allows for the assessment of blood flow dynamics (see Chapter 5 ). Despite these advantages, certain limitations influence the applicability of ultrasound. Ultrasound waves are unable to penetrate bone or air, which can obscure lesions and limit the field of view. The quality of ultrasound imaging and its interpretation are also operator dependent, which means they are influenced by skill and experience.

Different ultrasound transducers are optimized for specific frequencies. Lower-frequency transducers have poorer resolution with greater depth of penetration and thus are used to image deeper structures such as abdominopelvic tissues. Higher-frequency transducers have better spatial resolution, but higher-frequency sound attenuates rapidly and has poorer tissue penetration. Higher frequency ultrasound is best used for superficial soft tissues, such as the thyroid, and it can also be used to assess liver surface nodularity.

Echogenicity of tissue refers to the reflection or transmission of ultrasound waves relative to surrounding tissues. Based on gray scale imaging, a structure on the image display can be characterized as anechoic (uniformly black), hypoechoic (dark gray), or hyperechoic (light gray) ( Fig. 13.1 ). Acoustic artifacts often occur, many of which are clinically useful. For example, acoustic enhancement is described when there is increased through-transmission of sound waves in fluid-containing structures, making tissue behind the fluid appear artificially bright, a characteristic of cystic structures (see Fig. 13.1 ). Certain artifacts are useful to hepatobiliary imaging in particular. Acoustic shadowing occurs when a structure attenuates sound more rapidly than surrounding tissues and casts a dark acoustic shadow beyond the object. This occurs with strong reflectors, such as calcifications, or strong attenuators, such as dense tumors. Acoustic shadowing is a feature of gallstones and, in conjunction with mobility, aids in distinction between gallstones and gallbladder polyps (see Chapter 33 ). Comet tail artifact is a type of reverberation that occurs when two reflective interfaces are closely spaced, such as within a punctate crystal, producing posterior echoes that are parallel, evenly spaced echogenic bands with a triangular tapered shape. Comet tail artifact allows for the identification of surgical clips and is also a feature of gallbladder adenomyomatosis. Twinkle artifact is a color Doppler artifact that helps to detect and verify crystals and calcifications, particularly if a calculus does not demonstrate acoustic shadowing. Twinkle artifact occurs posterior to strong reflectors and appears as turbulent color Doppler flow with a mix of red and blue pixels; however, spectral Doppler tracing demonstrates noise. Additional artifacts have been described and are outside the scope of this chapter; the reader is referred to specialized texts.