Christine Butts

Physics

Ultrasound operates on the pulse echo principle. Electrical energy created by the ultrasound machine causes crystals in the tip of the transducer to vibrate, also known as the piezoelectric effect. This vibration emits high-frequency sound waves that travel into the body. The sound waves are reflected back to the transducer at varying intensities and speeds, depending on the nature of the object that it encounters. The ultrasound machine is able to interpret this information and plot an image on the screen.

Objects in the body that are liquid or water-like, such as a full bladder, reflect very few sound waves and allow most of the energy to pass through them. These objects are presented on-screen as black by ultrasound and are described as anechoic, without echoes (Fig. 66-1). Conversely, objects that are dense and have very little water content, such as bones, reflect almost all the sound waves back. These objects are presented on-screen as white by ultrasound and are described as hyperechoic (i.e., producing a lot of echoes) (Fig. 66-2). Objects that lie between these two extremes are varying shades of gray, depending on the water content of the object. For example, the liver contains a large amount of blood, a water-like substance, but is not completely liquid and appears on-screen as a dark gray (Fig. 66-3). An object that contains less water but is not completely solid would appear as a lighter shade of gray.

These properties of objects also account for two important artifacts. Acoustic shadowing is an artifact that is encountered when dealing with hyperechoic objects. These objects reflect almost all the sound waves back to the transducer. As a result, the ultrasound machine “senses” an absence of information deep to the hyperechoic object. This absence of information is represented by a strong, dark vertical line emanating deep to the object (Fig. 66-4). This type of shadowing is referred to as a “clean shadow.” Although this shadow can be frustrating to the sonographer when attempting to obtain the best image possible, classically, when imaging over the ribs, it can also be helpful in identifying hyperechoic objects, such as gallstones or foreign bodies.

In contrast to a “clean shadow,” the presence of air may create a phenomenon known as “dirty shadowing” (Fig. 66-5). Air causes the ultrasound beam to scatter and creates a hazy, gray appearance on the image. This can be an anticipated finding, such as when viewing bowel gas within the abdomen, or an indication of an abnormality, such as when viewing gas within subcutaneous tissue.

Acoustic enhancement, or an acoustic window, is the effect created by an anechoic object. As noted earlier, sound waves pass through anechoic objects well and therefore lose less of their energy. This enables more ultrasonic energy to be available when the sound reaches the object on the other side of the fluid. This results in a brighter, clearer image immediately behind the fluid-filled object. As an example, a full bladder enables a clearer image of the pelvic organs (Fig. 66-6).

Indications and Contraindications

Although vascular access was one of the first uses of ultrasound in ED procedures, the list of procedures that can be facilitated by ultrasound is growing rapidly and continuously (Box 66-1). Even in cases in which the procedure cannot be directly observed with ultrasound, bedside ultrasound can frequently be used to diagnose the abnormality and to plan the approach for the procedure.

Ultrasound is a very safe modality for imaging at the bedside. It can be used in a variety of populations (pediatrics, pregnancy), without concern for excessive radiation exposure. The only absolute contraindication to using bedside ultrasound for procedural guidance is lack of training or experience in its use. Lack of adequate training or experience may result in an incorrect diagnosis and erroneous evaluation of the anatomy. This may result in harm and unnecessary complications in patients.

Equipment

The nature of the image obtained depends on the type of transducer used. In general, transducers fall into two categories: high frequency and low frequency, which refers to the type of ultrasound waves generated by the transducer.

High-frequency sound waves do not penetrate very far into the tissue but provide excellent resolution. A linear, or vascular, transducer and an intracavitary transducer are examples of specific types of high-frequency transducers (Figs. 66-7 and 66-8). Examples of types of examinations that are best evaluated with a high-frequency transducer are central or peripheral vascular access, evaluation of the pleura for pneumothorax, and evaluation of soft tissue for an abscess or foreign body. These examinations rely on a high degree of resolution (Fig. 66-9).

Low-frequency sound waves penetrate farther into tissues but do not provide as much resolution as high frequency does. Curvilinear, phased-array, and microconvex transducers are examples of specific types of low-frequency transducers (Figs. 66-10 and 66-11). They are distinguished from each other by the shape of the footprint of the transducer, the part that directly touches the patient, and by the layout of crystals in the tip of the transducer. Examples of the types of examinations that are best evaluated with a low-frequency transducer are evaluation of the pleural or peritoneal cavities for fluid drainage, evaluation of the bladder for aspiration or suprapubic catheter placement, and evaluation of the pericardium for pericardiocentesis. These examinations are not as dependent on resolution but typically require greater penetration (Fig. 66-12).

The growing use of ultrasound for procedures has prompted the development of echogenic needles and needle guidance systems. Both of these advances are designed to improve the ease and accuracy of ultrasound-guided procedures, but this depends on the operator’s familiarity with their properties and correct use.

Additionally, many procedures require sterile conditions. Commercially designed sterile transducer covers are available and allow the operator to completely cover both the transducer and the cord. In the absence of these covers, the transducer can be placed inside a sterile glove. To further protect the field, a sterile drape can be wrapped around the cord.

General Approach

Although each procedure has unique techniques, there are several general principles that apply when using ultrasound. Selection of the correct transducer is of the upmost importance for adequate visualization of the anatomy and pathology. This process is described in the preceding section, but in general the degree of resolution and depth of penetration should guide selection of the transducer.

Once the correct transducer has been selected, evaluate the area of interest. This step is key when evaluating the nature of the pathology. For example, understanding the size and position of a subcutaneous abscess and the surrounding anatomy is essential. Determining the correct orientation of objects on the screen can be challenging, but it is crucial for success with ultrasound-guided procedures. All transducers have a dot or mark on one side (Fig. 66-13). This mark corresponds to an indicator on the left-hand side of the screen (Fig. 66-14). When the indicator on the transducer is pointing toward the patient’s right side, a transverse image will be generated (Fig. 66-15). Objects on the right of the screen are closer to the patient’s left side and objects on the left side, near the on-screen indicator, are closer to the patient’s right side. Objects near the top of the screen are closer to the skin and objects near the bottom of the screen are deeper in the body. When the indicator is turned toward the patient’s head, a longitudinal image is generated (Fig. 66-16). Objects on the right side of the screen are closer to the patient’s feet and objects on the left side of the screen (near the on-screen indicator) are closer to the patient’s head. As with the transverse image, objects near the top of the screen are closer to the skin surface and objects at the bottom of the screen are deeper in the body.

Once the initial evaluation has been completed, make a decision whether to use ultrasound to directly guide the procedure or to simply “mark the spot” beforehand. Some procedures, such as paracentesis of large fluid collections, may not require direct guidance. In this case, ultrasound can be used to locate the most optimal puncture site, away from important structures. Once that site has been marked, proceed with the procedure in the usual fashion. For other procedures, such as central line placement, direct visualization throughout the entire procedure may be more important.

When direct visualization is desired, the orientation of the needle to the transducer must be considered. When the needle is introduced at the midpoint of the transducer in the transverse approach, the tip of the needle may be difficult to visualize (Fig. 66-17). In this orientation the operator may have difficulty following the tip of the needle because only a small portion of the needle intersects the ultrasound beam at any given time (Figs. 66-18 and 66-19). Conversely, when the needle is introduced from either end of the transducer, the needle can be visualized in its entirety (Figs. 66-20 and 66-21). However, slight movement of the transducer may result in “losing” the view of the needle. Consider each potential approach in the context of the particular procedure.

The oblique approach is a combination of the transverse and longitudinal approaches. Primarily, this has been described for central venous access but can also be applied to other procedures as well.³ In this method, place the transducer on an oblique axis to the target object (Fig. 66-22). This allows an oblique view of the target object while enabling the operator to introduce the needle along the long axis of the transducer (Fig. 66-23).

Complications

In general, the incidence of complications should be reduced with the use of ultrasound, but complications may occur even when performed by an experienced sonographer. Complications may be related to one of several factors. To maximize success, obtain an optimal image of the structure in question. Evaluate the anatomic relationships to ensure that the target object (e.g., the femoral vein versus the femoral artery) is correctly identified and pursued. Once the procedure begins, pay constant attention to the orientation and location of the needle. Errors, including inadvertent arterial puncture, can occur when the position of the tip of the needle is not closely followed.4,5 Techniques to minimize these errors are addressed in the individual chapters.

References