Ultrasound is a valuable tool for diagnosing soft tissue injuries in horses. Transducer frequency, however, greatly affects diagnostic accuracy when evaluating suspensory ligament branches (SLB) and distal sesamoidean ligaments (DSL), said Mathias Ankjaer Dinesen, DVM, an intern at Dubai Equine Hospital, in the United Arab Emirates. He presented his team’s recent research on the topic at the 2022 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.

Compared to MRI, ultrasound lacks diagnostic accuracy. When MRI is unavailable, however, an ultrasound exam might be the only option. So Dinesen and colleagues conducted a trial to determine the diagnostic accuracy of low (9 MHz) and high (16 MHz) ultrasound for DSLs and SLBs.

Dinesen and his colleagues used client-owned horses in the prospective, blinded study, with lameness located to the fetlock via diagnostic analgesia (nerve block). They examined each horse with both transducers and the same machine and software in both the transverse and longitudinal planes. They then performed an MRI—considered the gold standard against which they compared both ultrasound techniques—and measured the cross-sectional area (CSA) of the ligaments in nine predetermined zones of the ligaments and branches.

The study involved 13 horses with a median age of 10 years used primarily for show jumping and dressage.

“The main finding of the study was that the 16-MHz transducer had better diagnostic value in all zones,” said Dinesen. “The most noticeable difference was for distal sesamoidean ligaments.”

Specifically, the 9-MHz transducer allowed veterinarians to identify eight out of 17 SLB injuries and only four of 16 DSL injuries. In contrast, the 16-MHz transducer helped them detect 10 of 17 SLB injuries and eight of 18 DSL injuries.

The CSA measurements did not differ between the two transducers, but the calculated CSAs differed from those assessed via MRI.

“This means that switching frequency does not affect the measured cross-sectional area; however, by switching between ultrasound and MRI, the measured CSAs differ. Thus, CSA measurements from MRI studies/examinations cannot be used as a reference value for CSAs measured on ultrasound, as they will be different,” said Dinesen. “An increased CSA typically comes before the actual injury is visible on ultrasound and is one of the indications that the ligament/tendon is abnormal either due to an acute injury or chronic.”

In summary, said Dinesen, “the 16-MHz transducer increased diagnostic accuracy of both DSLs and SLBs and is therefore preferable to a lower-frequency transducer for SLBs and DSLs.”