Sound waves are created in a transducer by back and forth displacement. They are sent out by the transducer and the transducer then “listens” for returning echoes. The position of a structure on the screen is a function of how long it takes the wave to return to the transducer. Reflection occurs when the ultrasound beam hits two tissues (areas) having different acoustic impedance (flow per unit of area). The combination of these reflections producing a 2-dimensional image, and a combination of these images can help better identify a 3-dimensional structure.
The basic equipment includes an ultrasound machine and a transducer. The machine allows for adjustments to optimize the image (knobology); depth, gain, frequency/resolution, amplitude/brightness, focus/field of view. Addition features allow for further characterization of structures; color and power doppler, measurements, etc.
Ultrasound technique is of the utmost importance, as inaccurate probe position cause falsify an image, known as artifact. The three basic maneuvers with the probe are tilt, toggle, and heel-toe movements.
The use of diagnostic ultrasound is becoming more prevalent, as it provides high quality information that can be attained quickly and at a much lower cost that other imaging modalities such as CT or MRI. In addition, it can be used in real time to evaluate structures dynamically; as seen in visualizing rotator cuff impingement, elbow laxity with a collateral ligament injury, subluxing peroneal tendons, or a subluxing ulnar nerve.
Studies show ultrasound to be as accurate as MRI in diagnosing rotator cuff pathology. It can be used to visualize elbow injuries, such as a UCL tear. It can clearly identify changes from chronic tendinitis. It also very accurately identifies nerve entrapments, such as carpal tunnel and cubital tunnel, through easily made cross sectional area measurements.