Understanding Ultrasound

Nonthermal ultrasound occurs when the heating effects of ultrasound are reduced, whether applying low-intensity or by pulsing the ultrasound. Nonthermal effects reduce edema, pain and muscle spasms, improve blood flow and induce nonunion bone repair, tissue regeneration and soft tissue repair. To promote soft tissue repair from acute injury, use pulsed ultrasound at 3MHz at a 20 percent duty cycle with an intensity of 0.5. Treat the area for one to two minutes per zone.

When using ultrasound, you also must consider another factor, the beam nonuniformity (BNR) ratio, which is the maximal/average intensity (w/cm2) found in the ultrasound field. Each transducer produces sound waves in response to the vibration of the crystal. This vibration has different intensities at points on the transducer head, having peaks and valleys of intensity. The smaller the ratio of highest intensity/average intensity, the more homogeneous the beam. This ensures patients are getting uniform and consistent ultrasound treatment.

Conversely, the greater the ratio difference in the BNR, the more likely the transducer will have hot spots. High intensities have been shown to retard tissue repair or cause unstable cavitation effects, such as microvessel trauma. Try to keep the BNR as low as possible. Low BNRs are more comfortable for the patient and have a more focused treatment area. Even with a low BNR, you still need to make an effort to ensure ultrasound is heating the affected area. To achieve the desired effect, consider the following factors:

Depth of penetration. The depth depends on the absorption and scattering of the beam as it travels through the tissue. Depth of penetration varies due to the medium being used (gel/lotion) and contact between the transducer, treatment surface and tissue type (fat, skin, muscle). The coupling medium, for example, should allow for high transmission and low absorption, and the sound head should be at an 80-degree to a 90-degree angle. If it isn't, sound wave transmission is significantly reduced.

The portion of the sound head that produces the sound wave is referred to as the ERA-the effective radiating area. The ERA is always smaller than the transducer area, but the closer the ERA and transducer surface areas are, the better the penetration and accuracy of the therapeutic dose. An example of a good ERA is the following: Surface area of the ultrasound head is 7 cm2 and the ERA is 5 cm2. An example of a poor ERA is the following: The ultrasound head is 7 cm2, and the ERA is 2.5 cm2. Have the machine checked every six months to make sure output indicators are accurate.

Absorption. The uptake of heat converts from acoustic energy through the tissues, absorbing into the surrounding tissues. To ensure the most absorption, you must understand that each tissue has its own resistance to wave propagation. Scar tissue, tendon and ligament demonstrate the highest absorption. Ultrasound also transmits relatively well through water and fat with little energy.

On the other hand, bone has high acoustic impedance, which means you'll need to deliver ultrasound at a higher intensity to achieve a thermal effect in tissue close to bones. Other areas that require higher intensity ultrasound include periosteum, tendinous and aponeurotic attachments of skeletal muscle, cartilaginous covering of joint surfaces, and peripheral nerves lying close to bones.

Tissues that are selectively heated by ultrasound include superficial bone, joint capsules, tendon, scar tissue, peripheral nerves, myofascial interface and cell membranes. The more homogeneous the tissue, the less ultrasound energy is absorbed. For example, fat, metallic and synthetic implants are homogeneous, producing very little temperature increase. In addition, poor blood supply in these tissues, which occurs with peripheral vascular disease, offers little heat dissipation by circulation, which can lead to pain.

Ultrasound usually isn't a stand-alone treatment. Used in conjunction with other manual techniques, it can effectively treat quadriceps muscle acute inflammation. To decrease the pain and tenderness of this condition, use ultrasound at 0.16 w/cm2 pulsed, 20 percent duty cycle for 10 minutes.

It's clear that ultrasound can be complex, which is why knowledge is so important. It isn't simply a heat modality, but an effective therapeutic tool that facilitates a tissue response. In untrained hands, ultrasound will produce little value. Or worse, it could hurt patients. By educating ourselves about this modality, we can ensure successful outcomes.

Laurie Kendall-Ellis is president of Allied Health Rehabilitation in West Haven, CT.

Understanding Ultrasound

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