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All ultrasound probes have a reference marker to allow for proper orientation. The reference marker may be a raised dot or line molded into the plastic, or possibly a small LED light. On the image screen, there will be a symbol, often the company's logo, that corresponds with the probe's reference marker. The marker on the screen is commonly referred to as the “reference icon” (see Figure 4.3).

Sonographers should familiarize themselves with the various types of ultrasound probes (also called transducers): phased‐array (also called sector), linear, and curvilinear. Furthermore, they should know that by looking at the shape of the ultrasound image, the probe is readily apparent as follows: pie‐shaped pointed near‐field (phased‐array or sector), rectangular (linear), and pie‐shaped with curved concave near‐field (curvilinear) (Figure 4.4).

Pearl: You can tell what probe was used for the published image by looking at the image's near‐field as follows: pie‐shaped pointed near‐field (phased‐array or sector), rectangular (linear), and pie‐shaped with curved concave near‐field (curvilinear).

Most veterinarians are taught that when scanning the abdomen in long axis, the probe's reference marker is pointed toward the patient's head. Therefore, to maintain convention, the reference icon on the screen will also be positioned on the left‐hand side of the screen (screen left = cranial, screen right = caudal). When the probe is turned into the transverse orientation, the reference marker is pointed toward the patient's right, making a counterclockwise motion (“turning left”) if one views the probe from its tail or cable end (screen left = right side of patient, screen right = left side of patient). Again, the orientation is consistent with lateral and ventrodorsal radiographic orientation.

Some veterinarians are trained to orient the reference marker of the probe to be pointed caudally. Therefore, to maintain convention, the reference icon on the screen will be positioned on the right‐hand side of the screen. When switching into the transverse orientation, the probe is still rotated counterclockwise. The reference marker will be pointed towards the patient’s left, and the reference icon on the screen will still be on the right‐hand side of the screen.

No matter which training a veterinarian has received, the image on the screen should follow proper orientation convention with the patient's head to the left (screen left is cranial) and the patient's tail to the right (screen right is caudal) as with a lateral radiograph, and the patient's right is to the left of the screen as with a ventrodorsal radiograph.