Difficulty in obtaining adequate sonographic images is a challenge for many emergency physicians and can be a major hindrance to incorporating ultrasound into clinical practice.

Ultrasound waves are poorly transmitted through air and adipose tissue, and are reflected by bone. For this reason, visualization of structures within the thoracic cage, the peritoneal cavity, or adjacent to the bony skeleton may be compromised.

This article will review simple approaches such as probe selection, image-enhancing settings of the machine, patient position changes, and the utilization of sonographic windows that contribute to image quality and the sensitivity of ultrasound examinations.

Emergency physicians encounter many challenging patient scenarios, including large body habitus, limited mobility, the patient in pain, and others. Nonetheless, in almost all cases, simply pressing a button on the machine, switching to a different probe, or changing the patient’s position is all that is necessary to improve image quality.

Probe

A probe (Fig. 1) with low frequency (2-5 MHz), such as a curvilinear probe, enables greater tissue penetration at the expense of anatomic image resolution.

A probe with higher frequency (5-8 MHz), such as a linear array probe, offers better detail but only to superficial structures. Simply stated, high-frequency probes are used to visualize in better detail superficial structures, such as peripheral vessels and soft tissue foreign bodies.

Low-frequency probes can be used to visualize deep structures, such as the aorta or gallbladder, but do not provide fine detail.

Many probes will have the frequency they use represented as a range. These “multifrequency” probes are commonly used in the emergency department because they are the most versatile. Setting a multifrequency probe to the lower part of its frequency range can be done for larger patients, and using the upper range of frequency can be done for smaller patients (Fig. 2).

The probe footprint ranges in shape and should be selected based on the clinical application. A linear array probe sends waves down in parallel lines, creating a rectangular image on the screen, whereas a phased-array probe creates a fan-shaped image.

A phased-array probe can image through a small sonographic window, such as the heart between intercostal spaces.

Buttons and Knobs

The following knobs are contained within the majority of machines being used in today’s emergency departments:

• Gain (Fig. 3). The gain control changes the amplification of the waves and brightness of the image overall. If it is set too low, the image will appear dark and structures will be indiscernible. If it is set too high, extraneous echoes can appear and the image is too bright and white. Gain should be adjusted to accentuate the appearance and borders of the adjacent structures.
• Time-gain compensation (Fig. 4). Echoes returning from deeper tissues (far field) will be weaker than those returning from tissues closer to the transducer (near field) because they have to travel through much more tissue. Time-gain compensation can amplify echoes returning from deeper tissues to make the image more uniform.
• Depth (Fig. 5). Depth can be decreased to enlarge superficial areas or increased to visualize deeper structures. To image the same ture, depth may need to be increased for obese patients and decreased for thinner patients. Depth should be adjusted to include the area of interest and center it on the screen.
• Focus (Fig. 6). The focus control can be used to adjust the ultrasound’s waves to converge on the area of interest. At any given depth, focus can be adjusted to get the best possible image.
• Tissue harmonics (Fig. 7). When sound waves of a given frequency pass through tissue, harmonics are produced at multiples of the initial frequency.

The tissue harmonics setting interprets one of these harmonics, filtering out reverberation echoes allowing for a cleaner image with better contrast and less artifact.

Certain applications, such as visualizing comet tails when performing lung ultrasound, utilize these tissue artifacts and in these instances tissue harmonics should be turned off.

Patient Positioning

Changing patient position can mean the difference between a technically acceptable study and not being able to answer the focused question of interest for your clinical scenario.

Turning the patient into the left lateral decubitus position can facilitate cardiac or gallbladder scanning.

Sitting the patient upright may improve imaging of the gallbladder.

In the trauma abdominal examination, using the Trendelenberg position increases the sensitivity for fluid collections in the hepatorenal space.

Bending the patient’s knees can relax the abdominal wall musculature and facilitate scanning of the aorta and aid in subxiphoid scanning of the heart.

Techniques for Dealing With Tissue Artifacts Various types of tissue artifacts include:

• Air. There are large differences in density between air and soft tissue. When ultrasound waves encounter these large differences, the energy is scattered. Little energy returns to the transducer to be analyzed into an image. An adequate amount of transmission gel placed between the footprint of the probe and the imaging surface will reduce the number of difficult air-filled pockets the waves encounter, such as when scanning over the umbilicus.
• Air, lung. Parasternal windows depend on visualizing the heart through the cardiac notch of the left lung. In patients with hyperinflated lungs from obstructive pulmonary disease or positive pressure ventilation, this window may be obscured. Images can be improved by keeping the probe as close as possible to the left sternal border or placing the patient in a left lateral decubitus position to move the heart closer to the anterior thoracic wall. The subxiphoid view of the heart can be improved by moving the probe a bit to the patient’s right, under the costal margin, and directing the probe toward the patient’s left shoulder. In this way, the liver is used as a sonographic window. Moving the probe to the patient’s left, which may seem more anatomically correct, results in attempting to obtain images through an air-filled stomach or transverse colon.
• Air, bowel. For structures viewed from the right and mid-upper abdomen (such as the proximal aorta, gall bladder, right kidney, and sub­xiphoid view of the heart), the homogeneous, blood-filled liver makes a good acoustic window. A deep inspiration moves the diaphragm and liver caudad, pushing the transverse colon inferiorly to improve visualization of structures beneath. In addition, patient comfort permitting, a process of slow graded compression or a rocking motion may be used to push bowel gas out of the way.
• Bone. When ultrasound waves encounter a highly reflective surface such as bone, the waves are returned right back to the transducer. There is very little energy left to travel to deeper structures and create an image. This hypoechoic area deep to the bone is referred to as shadowing.
• Bone, thorax. Emergency ultrasound of the heart includes evaluation for pericardial effusion and cardiac activity. The heart is encased in the bony thorax, so the ribs cast sonographic shadows. A phased-array cardiac probe (2-4 MHz) or a low-frequency curvilinear probe with a small footprint can be used, which allow the ultrasound waves to be directed between the rib interspaces.
• Bone, upper abdomen (Fig. 8). Using the inferior displacement of the abdominal viscera with a deep inspiration can also bring structures out from beneath rib shadows. If these techniques do not allow for adequate visualization, the patient can be moved into the left lateral decubitus position and the probe placed in the right midaxillary line, scanning toward the patient’s left side. This position can bring structures out from behind the rib cage, and the echogenic liver used as a window to view deeper structures. Similarly, but to a lesser extent, when imaging the left upper abdomen, the spleen can be used for left kidney visualization. Variations in the respiratory cycle can also be used to move the kidneys out from the shadows of overlying ribs.
• Adipose tissue. Abdominal imaging includes evaluation of the aorta, biliary system, focused assessment with sonography for trauma (FAST), and kidneys and bladder.

A low-frequency (2-5 MHz) curvi­linear probe should be used to image abdominal structures.

In an obese patient, the ultrasound waves have farther to travel and are attenuated along the way. The lower end of the frequency range should be used in obese patients, which can compromise detail but allows for better penetration.

Further Reading

1. Ma OJ, Mateer JR, Ogata M, et al. Prospective analysis of a rapid trauma ultrasound examination performed by emergency physicians. J. Trauma 1995;38:879-85.
2. Durham B, Lane B, Burbridge L, et al. Pelvic ultrasound performed by emergency physicians for the detection of ectopic pregnancy in complicated first-trimester pregnancies. Ann. Emerg. Med. 1997;29:338-47.
3. Plummer D, Brunnette D, Asinger R, et al. Emergency department echocardiography improves outcome in penetrating cardiac injury. Ann. Emerg. Med. 1992;21:709-12.
4. Kuhn M, Bonnin RL, Davey MJ, et al. Emergency department ultrasound scanning for abdominal aortic aneurysm: accessible, accurate, and advantageous. Ann. Emerg. Med. 2000;36:219-23.
5. Rosen C, Brown DFM, Chang Y, et al. Ultrasonography by emergency physicians in patients with suspected cholecystitis. Am. J. Emerg. Med. 2001;19:32-6.
6. Hoffner RJ, Chan D, Esekogwu VI, et al. The role of emergency department ultrasonography versus intravenous pyelography in the evaluation of suspected ureteral colic. Acad. Emerg. Med. 1997;4:392.