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.