Sonographic Equipment and Selection of the Appropriate Transducer
Sonographic units used today can be operated with different types of transducers (see below) and are mobile for use in the sonography suite as well as in the intensive care unit or emergency room (Fig. 8.1). The transducers are generally stored on the storage shelf on the right side of the unit.
Precautions should be taken when moving the sonographic unit. Avoid having a dangling transducer cable being caught on a door knob, stretcher, etc., and do not drop a transducer on the floor. Replacing a damaged transducer can be quite expensive! For the same reason, the transducer should never be left unattended on the patient's abdomen when the examination is interrupted, for instance by a phone call. Furthermore, the transducer should be placed upside down to hang with the cable straightened and not pinched or kinked where it enters the transducer (danger of breaking the wires in the cable).
Selection of the appropriate transducer:
Of the many types of transducers only the applications of the three most important ones will be described here.
The linear array transducer emits sound waves parallel to each other and produces a rectangular image. The width of the image and the number of scan lines are constant at all tissue levels (Fig. 8.2, center). An advantage of the linear array transducers is good near-field resolution. They are primarily used with high frequencies (5.0-7.5 MHz) for evaluating soft tissues and the thyroid gland. The disadvantage of these transducers is their large contact surface, leading to artifacts when applied to a curved body contour due to air gaps between the skin and transducer. Furthermore, acoustic shadowing (45) as caused by ribs can deteriorate the image (Fig. 8.2). In general, linear array transducers are not suitable for visualizing organs in the thorax or upper abdomen.
A sector transducer produces a fan-like image that is narrow near the transducer and increases in width with deeper penetration (Fig. 8.2, left). This diverging propagation of sound can be achieved by moving the piezo elements mechanically. This is the less expensive solution but has the inherent risk of wear and tear. The electronic version (phased array) is more expensive but has become established primarily in cardiology with frequencies of 2.0-3.0 MHz. The interference of the sound-reflecting ribs can be avoided by applying the transducer to the intercostal space and by taking advantage of the beam's divergency to a 60°- or 90°-sector with increasing depth (Fig. 8.2). The disadvantages of these types of transducer are poor near-field resolution, a decreasing number of scan lines with depth (spatial resolution), and handling difficulties.
Curved or convex array transducers are predominantly used in abdominal sonography with frequencies from 2.5 MHz (obese patients) to 5.0 MHz (slim patients), with the mean value around 3.5-3.75 MHz. As a compromise of both preceding types, it offers a wide near and far zone and is handled easier than a sector scan. However, the density of the scan lines decreases with increasing distance from the transducer (Fig. 8.2, right). When scanning the upper abdominal organs, the transducer has to be carefully manipulated to avoid acoustic shadowing (45) of the lower ribs.
Sector (phased array)
Sector (phased array)
Linear Convex (curved array)
Cognizance of the physical properties of sound that can mimic pathologic findings is mandatory for the correct interpretation of a sonographic image. The most important artifacts include so-called distal shadowing. An acoustic shadow (45) appears as a zone of reduced echogenicity (hypoechoic or anechoic = black) and is found behind a strongly reflecting structure, such as calcium-containing bone. Ilius the visualization of soft-tissue structures in the upper abdomen is impeded by overlying ribs, and those of the lower pelvis by the pubic symphysis. This effect, however, can be exploited to reveal calcific gallstones (49) (Fig, 9.2), renal stones (49) (Figs. 42.1, 42.2), and atherosclerotic plaques (49) (Fig. 9.3). Similar shadowing can be caused by air in the lungs or intestinal tract. Evaluating structures behind air-containing bowel loops (46) is often precluded by acoustic shadowing (45) or echogenic comet-tail artifacts (Figs. 9.2-9.4).
The air artifacts interfere primarily with the evaluation of retroperitoneal organs (pancreas, kidneys, and lymph nodes) behind air-containing stomach or bowel. Adequate visualization, however, is still possible by following the approach described on page 79.
Another characteristic finding is the so-called edge shadowing (45) behind cysts (64), principally occurring behind all round cavities that are tangentially hit by sound waves (Fig. 9.1). Edge shadowing is caused by scattering and refraction and can be seen behind the gallbladder (14). Figure 9.4 requires careful analysis to attribute the acoustic shadow (45) to edge shadowing caused by the gallbladder.
rather than falsely attribute it to focal sparing of fatty infiltration (62) in the liver (9).
Relative distal acoustic enhancement (70) is found wherever sound waves travel for some distance through homogeneous fluid. Because of decreased reflection in fluid, the sound waves attenuate less and are of higher amplitude distally in comparison with adjacent sound waves. This produces increased echogenicity that is seen as a bright band (70) behind the gallbladder (14) (Fig. 9.4), behind the urin-ary bladder (38) (Figs. 10.1-10.3), or even behind major vessels such as the aorta (15) (Fig. 9.3). This increased echogenicity is a physical phenomenon unrelated to the true characteristics of the underlying tissue. The acoustic enhancement, however, can be applied to distinguish renal or hepatic cysts from hypoechoic tumors. Fig. 9.1
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