Ultrasound

CHAPTER 1 Ultrasound



Ultrasound is an interactive modality that integrates the art of the physical examination with modern high-resolution imaging technology. In some ways the evolutionary offspring of the stethoscope, ultrasound is a handheld diagnostic tool that is only as powerful as the experience and expertise of its user.


Ultrasound is often the first-line modality for imaging patients suspected of having abdominal or pelvic disease. It provides comprehensive anatomic detail for many abdominal and pelvic organs, yet remains less costly than the other cross-sectional imaging modalities, making it widely available. Because sonography also lacks the risks associated with ionizing radiation and iodinated contrast media, little disadvantage exists to imaging patients with ultrasound.


In recent years, significant advances have been made in the field of sonography. Current systems make use of sophisticated technology to produce high-resolution images incorporating anatomy and pathology. Applications such as color and power Doppler, tissue harmonic imaging, and speckle reduction imaging have helped to revolutionize the role of sonography as a problem solver in medical imaging. The ready availability and standard use of real-time imaging is unique to ultrasound among cross-sectional imaging modalities. Contrast-enhanced sonography is emerging in the twenty-first century as an imaging modality that will likely play an important role in the management and follow-up of disease progression in patients with cancer.


Of course, sonography does have some limitations. The patient’s body habitus often determines the image quality and visibility that can be achieved. Beyond size, the location of a lesion or anatomic structure is critical because sonography is unable to visualize structures that lie deep to bone or air. Perhaps the most critical limitation is that the diagnostic yield is highly dependent on the skill of the individual performing the examination.



GENERAL PRINCIPLES OF ULTRASOUND TECHNIQUE


Ultrasound scanners use the piezoelectric effect to transform electricity into acoustic energy. The transducer serves to generate and transmit energy into the body, as well as to receive acoustic energy as it returns after being altered by tissues. Some of the many factors that influence image quality are listed in Box 1-1.






Transducer Array


Several basic types of transducers are available, and advantages and disadvantages are associated with each. These include sector scanners, curved arrays, linear arrays, and phased arrays.




Curved Array


Curved array transducers have become the mainstay for most abdominal and pelvic applications. The convex surface results in a mildly diverging beam, optimizing anatomic coverage from a given window (Fig. 1-2). The smooth convexity also provides for patient comfort as the transducer is moved along the skin surface. Endocavitary transducers rely on small, curved arrays with high frequency to provide high-resolution images of the pelvic organs.





Phased Array


Phased array transducers orchestrate multiple elements activated in sequence to create a beam that can be steered electronically. Phased array transducers usually provide a cone-shaped field of view that appears similar to a sector scanner (see Fig. 1-1) but are more versatile because the beam can be steered from side to side. A small footprint makes these transducers useful when only a small sonographic window is available (e.g., intercostal approach, neonatal heads, postsurgical patients with wounds/bandages).







COMMON ARTIFACTS


English dictionaries define the word artifact to mean something created by humans, whereas medical dictionaries define it as something that results from technique. In fact, every sonographic image is a human creation born of technical innovation. Perhaps the more common radiologic use of the word describes the unintended production of an image because of technical factors. This parsing of words may seem trivial, but despite the often negative connotation associated with the word artifact in the field of imaging, artifacts are sometimes a key diagnostic finding in sonography. Because the presence and recognition of an artifact can lead to diagnosis and the lack or recognition may lead to misdiagnosis, a basic understanding of the most common artifacts is critical to the effective use of sonography.



Acoustic Shadowing


Acoustic shadowing results when tissues absorb or reflect the incident ultrasound beam. Because there is no signal representing the tissues distal to that point, the image is replaced with a dark band along the projected course of the beam (Fig. 1-7). Shadowing is commonly seen with urinary or biliary calculi.



Calcium deposits and their interface with soft tissues can reflect and/or absorb ultrasound waves effectively. This has two main impacts on the ultrasound image: (1) a highly echogenic appearance at the near margin of the calcified object, and (2) a loss of signal beyond that object. The presence and type of posterior acoustic shadowing produced is related to the size and surface morphology of a structure. In the case of small calcium deposits, color Doppler can be used to increase conspicuity of tissue reflectivity in the form of the color comet tail artifact (CCTA), also called twinkle artifact (see Fig. 23-37 on CD-ROM).


Shadowing often results in a uniform dark stripe distal to the object, but this is not always the case. Although the authors prefer to avoid the term dirty shadowing, it is sometimes used in the literature to describe less uniform absorption, usually at soft-tissue interfaces or from gas (see Fig. 1-7). Although this has been reported to be helpful to identify structures or processes including lung, bowel, pneumobilia, and portal venous gas, caution must be applied. The sign is neither sensitive nor specific because there can be overlap with the more traditional shadowing.



Reverberation


Reverberation occurs when acoustic energy “bounces” between two or more adjacent highly reflective surfaces. The result is a series of bands perpendicular to the beam, each representing a different delay in the return of energy to the transducer (Fig. 1-8). The effect is more pronounced when these surfaces are in the near field and in the center of the field of view. Reverberation artifact can be mistaken for sludge within the gallbladder, although examining the gallbladder from a different angle should eliminate the artifact. Reverberation is useful in creating the “comet tail” artifact characteristic of adenomyomatosis of the gallbladder wall. Color Doppler can be used to accentuate the reverberation effects of adenomyomatosis. The result is another form of CCTA (twinkle artifact) (Fig. 1-9).








Acoustic Enhancement


Acoustic enhancement refers to a zone of increased signal (usually a bright stripe) seen deep to a structure with low acoustic impedance. This is most commonly seen deep to the gallbladder, urinary bladder, simple cysts, or large blood vessels (Fig. 1-13). Acoustic enhancement is usually omitted from lists of ultrasound artifacts, perhaps because of its utility in characterizing cystic structures. Nonetheless, acoustic enhancement is merely an unintended technical phenomenon, and although it is often useful, it can also lead to errant diagnosis if unrecognized. For example, ascites in the hepatorenal space can make the kidney appear echogenic, prompting an inaccurate diagnosis of diffuse renal parenchymal disease (Fig. 1-14).





ULTRASOUND OF THE UPPER ABDOMEN



Ultrasound of the Liver


The liver is best imaged with the patient in the supine and left lateral decubitus positions, starting with 3- to 7-MHz curved array transducers. In most cases, harmonic imaging improves image quality in the liver. A subcostal acoustic window should be attempted first, supplemented with intercostal scanning as necessary. The liver surface (usually the ventral left lobe) should be evaluated for nodularity with a near-field, optimized, 5- to 12-MHz linear array transducer. It is easier to appreciate subtle nodularity during real-time examination or on video clips than on hard-copy images. Furthermore, surface characteristics are easier to appreciate when ascites is present (Fig. 1-15) Angling the transducer so that it is not perpendicular to the liver surface may falsely simulate nodularity.



Routine color flow imaging is useful in patients with suspected liver disease. Optimal color flow and spectral Doppler sonography of the liver generally require relatively low-frequency (2-3 MHz) scanning and good acoustic access. Spectral Doppler sonography can help distinguish arterial from venous flow and detect abnormal hemodynamics within vessels.




Steatohepatitis


Severe fatty infiltration of the liver often results in hepatomegaly and diffusely increased echogenicity (Fig. 1-17). Acoustic penetration may be decreased, resulting in indistinctness of blood vessels and the diaphragm. Hepatic echogenicity is usually equal to or greater than that of the renal cortex. Although subjective assessment of the renal cortex and liver parenchyma echogenicity is useful in severe fatty infiltration, this approach is error prone and insensitive. Although unproven, we prefer to use the relative echogenicity of the kidneys compared with the spleen and liver (Fig. 1-18). The normal spleen is slightly more echogenic than the normal liver. Therefore, if the difference in echogenicity between the liver and right kidney is greater than the difference between the spleen and left kidney, the liver parenchyma has abnormally increased echogenicity. This approach assumes, of course, that the echogenicity of the kidneys is equal bilaterally.




Fat deposition and sparing are not always uniform, with focal (Fig. 1-19) and regional (Fig. 1-20) differences possible within the liver. Patterns of fat deposition and sparing are discussed in more detail in Chapter 12.





Cirrhosis


Cirrhosis is the end stage of chronic hepatocellular injury, characterized by bridging fibrosis and regeneration. The most common sonographic findings in cirrhosis are listed in Table 1-1. Unfortunately, many of these signs are insensitive and insufficiently specific for cirrhosis to be diagnosed reliably with sonography. Furthermore, the extent of sonographic changes is not a reliable predictor of disease severity. Nevertheless, evaluation of the liver surface with a high-resolution linear array transducer is useful because surface nodularity may be the only sonographic sign of cirrhosis (Fig. 1-21). Be aware that metastases to the liver can also result in a nodular liver contour, and this nodularity may be the only indication of metastatic disease in patients with a known primary malignancy (Fig. 1-22).


Table 1-1 Sonographic Findings of Cirrhosis
























Finding Comments
Nodular liver surface More specific than sensitive; however, also consider metastatic disease if patient has known primary malignancy
Echogenic liver Nonspecific, commonly seen with fatty infiltration
Flow reversal in portal vein Specific finding, although transient reversal occasionally seen with cardiac disease
Collateral venous flow Specific finding when present (Fig. 1-23)
Enlarged hepatic arteries (Fig. 1-24) Also seen with portal vein thrombosis and arteriovenous shunting
Narrowing of the hepatic veins and intrahepatic IVC Dampens transmission of cardiac waveform (see Fig. 1-25)

IVC, Inferior vena cava.







Color Doppler sonography can show portal vein flow reversal or portosystemic collaterals, suggesting the diagnosis of portal hypertension (Fig. 1-26). Because considerable variation exists in the size of the portal vein with changes in respiration and patient position, absolute portal vein diameter is not a reliable predictor of portal hypertension. In fact, a lack of respiratory variation (an increase in portal vein diameter during inspiration and a decrease during expiration) can indicate portal hypertension. Other portal flow abnormalities include bidirectional flow and, rarely, nearly static blood flow. Be aware that transient portal flow reversal also occurs in some clinical conditions not related to liver disease such as tricuspid regurgitation and heart failure (Fig. 1-27).




Sonography also plays an important role in the evaluation and follow-up of patients with known liver cirrhosis. Although focal fatty infiltration, regenerating nodules, and other benign lesions can occur in a cirrhotic liver, any sonographically detected mass within a cirrhotic liver should be considered suspicious for hepatocellular carcinoma (HCC) and referred for additional diagnostic evaluation.



Focal Liver Lesions


Sonography excels at the detection and characterization of cystic lesions in the liver. These cystic lesions can be easily classified based on their sonographic features as simple or complex. Simple cysts are usually welldefined, echo-free, round or oval lesions with thin walls and enhanced through transmission. Thin septations are a frequent finding in simple hepatic cysts. Thicker septations or mural nodules are generally a sign of a cystic neoplasm such as biliary cystadenoma or cystic metastases.


The sonographic appearance of hydatid disease in the liver can range from simple cystic to complex, demonstrating calcifications, mural nodules, and the so-called water lily sign. The latter finding represents separation of the endocyst layer of the hydatid cyst. When a hydatid cyst has dense egg-shell calcification, the shadowing produced from the reflected echoes may simulate a granuloma or be confused with other solid lesions (Fig. 1-28).



In the United States, 85% of HCCs occur in patients with cirrhosis or precirrhotic conditions. Small HCCs (<5 cm) are often hypoechoic. With further progression, lesions become more numerous and heterogeneous, and often develop a hypoechoic peripheral rim (Fig. 1-29). Advanced HCC is often multifocal, making it difficult to distinguish from metastatic disease. Echogenic nodules are fairly common in multifocal HCC (Fig. 1-30). Fatty metamorphosis of HCC also causes increased echogenicity and can result in confusion with hemangioma. A mass associated with invasion of the portal or hepatic veins should suggest the diagnosis of HCC (Fig. 1-31), although other liver tumors occasionally invade venous structures. About three-fourths of HCCs have identifiable internal color flow, a feature present in only one-third of metastases.





Because patients presenting with right upper quadrant pain, fever, and leukocytosis usually undergo an ultrasound as the initial imaging examination, liver abscesses are frequently identified with ultrasound. The sonographic appearance of liver abscess is quite variable. Abscesses are usually round or oval and hypoechoic with irregular margins (Fig. 1-32). The abscesses can be homogeneous or inhomogeneous. They may also simulate solid masses, and acoustic enhancement is frequently absent. Often, the internal contents of a liver abscess can be seen to move during real-time sonography, distinguishing it from a solid tumor.



The presence of echogenic fine foci within the abscess usually indicates microbubbles (Fig. 1-33). Microbubbles can also simulate the appearance of blood flow within some abscesses on color Doppler. Large, confluent pockets of gas within an abscess can be confused with bowel gas, particularly in right lobe abscesses close to the diaphragm.



Cavernous hemangiomas are usually well-defined, echogenic lesions that blend with the normal liver without distinctive halos and have enhanced through transmission. Larger hemangiomas frequently diverge from this pattern, with mixed echogenicity or even hypoechoic lesions (Fig. 1-34

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Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Ultrasound

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