Soft tissue scanning
Good to excellent
Cost of equipment
Portability of equipment
Requirement of infrastructure
Deep structure scanning
Ultrasound provides a dynamic instant image of structures directly under the transducer. It is ideal to align the probe in a perpendicular manner to the structure of interest. Image quality directly correlates with the frequency of the transducer. High-frequency waves provide excellent superficial tissue resolution, while low-frequency waves provide better visualization for deeper structures.
Ultrasound scanning should include visualizing the target in long axis and short axis when applicable. First, use knowledge of anatomical landmarks to guide orientation. Next, it is important to minimize artifact, especially anisotropy, by ensuring the structure of interest is directly perpendicular to the transducer. When the sound beam is perpendicular to bone, the cortex will appear hyperechoic and well defined. The final step is analyzing the image for pathology. Ultrasound imaging can be performed over any area of concern. When evaluating tears in muscles, tendons, or ligaments, the structure in question can be actively and passively engaged to characterize the severity. A force can be applied to examine for joint space widening. Furthermore, a limb can be moved in various planes to mimic and reproduce symptoms under real-time image analysis.
Hyperechoic images are bright and can be visualized at the interfaces between bone and soft tissue as the sound beam is strongly reflected back to the transducer. Low or weak sound reflection produces hypoechoic or darker images. Deep to the bone is anechoic and appears completely black because no sound waves penetrate. Isoechoic is the term to describe a structure that is of similar echogenicity to adjacent tissue.
Anisotropy is the term used to describe ultrasound artifact that occurs secondary to the amount of sound beam that is reflected back to the transducer based on the angle the beam reflects off the structure of interest. Changing the angle as few as 2–3 degrees relative to the perpendicular can result in a hyperechoic structure (expected healthy tendon) appearing hypoechoic (pathologic) as fewer sound waves are reflected to the transducer.
Ultrasound takes advantage of the Doppler effect to distinguish objects moving away and toward the probe. Use this color flow feature before injecting to highlight vasculature.
Setting Up Injection
Injections can be performed either with the needle in-plane (parallel) or out-of-plane (perpendicular) to the transducer. Some injections based on physical anatomy and location will have a preferred orientation. Overall, in-plane allows optimal viewing of the needle course through structures completely to the target as well as uninterrupted identification of the needle tip.
The needle is best visualized directly parallel to and under the transducer. Therefore, when possible a shallow needle angle is preferred. Overall the needle is hyperechogenic and the image is diminished as depth increases. To improve visualization, rotate the bevel, inject fluid looking for spread, or cautiously move the needle in and out.
Out-of-plane involves the needle being entered perpendicular to probe. This allows a larger field of vision; however it impairs the ability to identify the tip of the needle from the shaft as the complete path of the needle cannot be tracked. When injecting out-of-plane, a “walk-down” approach can be utilized to track the needle tip by stepwise sliding the probe and then further advancing the needle. By repeating this technique, the tip of the needle is followed directly to the target.