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.
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.