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This concept is very important to understand and be aware of while scanning. The angle of insonation is the angle at which the probe is held in relation to the area of interest. It defines the angle of incidence at which the ultrasound waves strike the object.

The clearest image is obtained when the angle of incidence is zero degrees (Figure 4.5). This results in the highest number of sound waves being reflected back to the probe. Much beyond three degrees, the image quality will diminish (Bahner et al. 2016). A curvilinear probe will lead to numerous different angles of incidence with the same angle of insonation. This is one reason why curvilinear probes do not produce as detailed an image as a linear probe. While the curvilinear probe can be considered the workhorse in veterinary ultrasound, adding a linear probe to the line‐up can significantly improve your images. In terms of POCUS and FAST scans, however, the more maneuverable curvilinear probes remain the most commonly prescribed.

Figure 4.5. Angle of insonation. The angle at which the ultrasound beams strikes the organ or structure of interest is known as the angle of insonation. The best echo quality, and hence the best image quality, is achieved when the angle of insonation equals the angle of reflection. In (A), if the structure of interest is parallel to the transducer footprint, it will be best imaged. In contrast, in (B) if the area of interest is oblique then the probe must be adjusted for the optimal angle of insonation.

Source: Illustration courtesy of Randi Taggart, Richmond, VA.