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Probes, or transducers, come in two basic types, mechanical and electronic. Mechanical probes are on many counts considered outdated but there are still some around with their working parts visibly rotating or rocking under their translucent covers. Newer ultrasounds have electronic probes as standard. Electronic probes come in various arrangements. Probes are generally described by the size and shape of their face, referred to as their “footprint,” which is represented by the gray rubber probe covering (see Figure 4.13). Selecting the right probe is essential to getting good images, although there may be times when more than one probe may be appropriate for a given exam.

Three basic types of probes are used in general practice, emergency, and critical care POCUS: linear, curvilinear, and phased‐array (also known as sector) (see Figure 4.4). Linear probes are typically of higher frequency and have a rectangular footprint. Curvilinear probes are arranged along a convex face and are typically of lower frequency than linear probes. A phased‐array (sector) probe generates an image from an electronically steered beam in a close array, generating an image that comes from a point and is good for getting between ribs, such as in cardiac ultrasound. Both curvilinear and phased‐array probes generate sector or pie‐shaped images, narrow in the near‐field and wide in the far‐field. Phased‐array probes are typically lower frequency. Because of their smaller footprint, “pie‐shaped” image, and commonly used frequencies, curvilinear probes are generally the most versatile and ideal for POCUS and FAST studies.

Probes are generally named for the primary frequency they emit. For example, a General Electric (GE) 8C probe indicates that 8 MHz is its primary frequency and the C represents the probe’s curvilinear footprint. A GE 9L probe indicates a 9 MHz primary frequency in a linear (L) probe, and a GE 7S has 7 MHz as its primary frequency in a sector (S) probe. However, modern probes are capable of emitting a range of frequencies, known as bandwidth. In choosing the best frequency, we need to go back to basics. Remember that higher frequencies are attenuated more and that means less penetration but better detail. Lower frequencies are attenuated less and that means deeper penetration but less detail. Another rule of thumb is that the smaller the patient or area of interest, the higher the frequency (MHz) versus the larger the patient or area of interest, the lower the frequency (MHz).