Since the 1940s, when Dr. George Lewig first used an A-mode ultrasound of the abdomen to detect gallstones in dogs, ultrasonography has become an invaluable diagnostic and therapeutic tool with a wide range of clinical applications. Uniquely safe, portable, and noninvasive, ultrasound provides a means of evaluating internal anatomy unlike other imaging modalities that it frequently complements.
Overview of Transabdominal Ultrasound
Ultrasound provides a unique role in evaluating internal anatomy. The ability to visualize cross-sectional and sagittal anatomy and to clearly differentiate cystic structures from solid masses makes it a critical tool in assessing the pathologic changes in the abdomen prior to operative intervention. Ultrasonography is a safe, portable, and noninvasive technique without deleterious side effects.
Diagnostic ultrasound uses repetitive pulses of sound with high frequencies, which penetrate soft tissues once they are concentrated into a narrow, focused column. As these waves traverse the abdomen, they are partially reflected at tissue interfaces. The sectional anatomy that is displayed needs to be accurately conceptualized to interpret the images meaningfully. There are two standard orientations: longitudinal (eg, imaging the aorta along its length, so that it appears as two parallel lines) and transverse (eg, imaging the aorta across its width, so that it is visualized as a circle) both must be acquired to properly perform a transabdominal ultrasound.
Understanding Gray Scale Imaging
An ultrasound image comprises thousands of pixels. Each pixel is a particular shade of gray depending on the strength of the returning ultrasound signal. Tissues that are identified as strong reflectors will return ultrasound images that range from brilliant gray to white. Weak reflecting tissues will be seen as a dark shade of gray to black. Ultrasound examination is reported in terms of six categories of this gray scale imaging (Table 6-1).
Table 6-1 Categories of Gray-Scale Imaging on Ultrasound |Favorite Table|Download (.pdf)
Table 6-1 Categories of Gray-Scale Imaging on Ultrasound
|Hyperechoic||Brighter than surrounding structures/bright white (eg, renal sinus compared to cortex)|
|Hypoechoic||Darker than surrounding structures|
|Anechoic||Black in color, eg, a simple cyst|
|Echogenic||Bright white appearance against a black background|
|Homogenous||Uniform shade of gray throughout the organ denotes normal appearance of the organ|
|Heterogenous||Nonuniform shades of gray throughout the organ, denotes abnormal appearance of the organ|
On an ultrasound examination a cyst characteristically appears to be anechoic with thin walls. A complex cyst will appear anechoic with thickened wall, and may contain debris.
Posterior shadowing is associated with calculi. This refers to acoustic impedance between the calculus and the surrounding tissue or fluid (in the case of bile).
It is important to remember that solid masses may present on ultrasonography ...