How To Detect Chronic Heel Pain With Musculoskeletal Ultrasound
Throughout the previous three decades, technological advances paved the way for the use of sonography in diagnosing and assessing musculoskeletal pathology. Continued innovations in this arena have led to affordable portable units that enable private office practitioners to utilize ultrasonography. Use of these units has grown over the last five years, especially in rheumatology. In fact, one leading ultrasound company has turned all of its advertising attention from the podiatry profession to rheumatology. However, podiatry still remains a formidable market for the portable ultrasound.
There are many benefits to ultrasound. Its safety profile is excellent. Ultrasound produces no ionizing radiation as evidenced by its use in obstetrics. It has no known side effects or contraindications. It is inexpensive as compared to magnetic resonance imaging (MRI). There is minimal patient preparation and it is usually painless. It can be used for purposes such as guided injections, biopsies and draining of cysts. Ultrasound also allows for real time imaging, which is indispensable for assessing tendon integrity.
However, ultrasound is also operator dependent and this becomes its leading limitation. In the educated hands, diagnostic ultrasound is a powerful tool. Being educated on this device involves more than knowing the anatomy and being able to recognize structures on the screen. Clinicians should also have a strong grasp of how the image is formed, the artifacts that can result and the variables that one controls while scanning. Accordingly, let us take a closer look at the technical aspects of ultrasound use.
Understanding The Physics Of Ultrasound
Sound waves travel through solids, liquids and air in sinusoidal waves. These waves really represent a mechanical disturbance (vibration) of the medium brought on by the acoustic energy. These vibrations are measured as cycles per second or hertz (Hz). Audible sound is in the range of 60 to 20,000 Hz. Anything above 20,000 is considered ultrasound. With musculoskeletal (MSK) diagnostic ultrasound, the vibrations are measured as MHz (1 million). Diagnostic ultrasound will fall in the range of 1.5 to 15 MHz while less than 3 MHz would be that used for therapy. The optimum range for MSK ultrasound is 7.5 to 13 MHz with 10 to 13 MHz being optimum for the foot and ankle. The higher the MHz, the less deep the sound waves will be able to readily penetrate. Conversely, the higher the frequency, the more detailed the image will be.
It obviously makes sense to scan the foot and ankle at 10 to 13 MHz. In contrast, it is better to scan the leg at 7.5 to 10 MHz because it is a larger anatomic structure. Detail is highly dependent on the frequency of the probe. Also keep in mind that the size of the probe will affect detail. Smaller probes (i.e. less radius) will have fewer detail capabilities compared to a probe that has a larger radius. Other factors also contribute to the degree of detail but are beyond the scope of this article.
The speed by which the sound wave will travel will change depending on the medium it propagates through. The average speed sound waves travel in human tissue is 1,540 m/sec. Sound waves will either travel through tissue and be attenuated or reflected. Those waves that return to the transducer (probe) will produce the signal on screen (white). When the sensor picks up no echoes, there will be no signal (black). When sound waves change speed, even minutely, the interface that exists between the tissues will appear as contrast to our eyes and be perceivable.