Ultrasound Therapy

Anatomy and Physiology

Thermal Energy

Some of the earliest uses of therapeutic ultrasound relied on the transference of thermal energy through supersonic waves. The energy of these waves causes vibration and heating of the tissues under the probe. The thermal energy absorbed, in turn, can induce dilation of blood vessels, which can increase cellular metabolism through oxygen and nutrient delivery. At low powers, many physical therapists can treat stretch pain and shoulder pathologies with the thermal energy produced by the ultrasound[9]. Advancements in ultrasound equipment have also created exciting new uses for the thermal energy created by ultrasounds. In high-intensity focused ultrasound (HIFU), the probe has a curved surface, which creates a focal point of the waves at a controllable depth. Gikelobia et al. provide one example of how this technology may be paired with imaging, such as an MRI, to treat uterine fibroids. HIFU has been adapted to treat prostate cancer, skin abnormalities, and aberrant atrial tissue in patients with atrial fibrillation.

Cavitation

Cavitation is the major mechanism that drives procedures like extracorporeal shockwave lithotripsy (ESWL) and ultrasound assistive thrombolysis. Cavitation occurs when pressure variations exist within a liquid. The supersonic waves conducted from the ultrasound probe create pressure differences within fluids. As a result, bubbles form. Research models suggest that these bubbles burst as they run into solids, and this interaction can create a shockwave. These shockwaves exert force on the surface of solids and create motion within the fluid. Shockwaves in ESWL break up kidney stones and makes them easier to eliminate through voiding, while additional motion in the fluid can enhance the effectiveness of thrombolytic therapies.[10][11]

Conversion

In ultrasound diathermy, ultrasound energy is converted to heat energy. The effect increases the temperature of the deeper tissues (muscle, fascia) for therapeutic purposes such as muscle relaxation. [12]

Indications

Engineering advancements and dedicated research have paved the way for FDA approval of ultrasound therapy in various disciplines. Overall, ultrasound therapy can be divided  into "low" and "high" power therapies. [13]

High power - Lithotripsy and HIFU

Low Power - bone healing, sonophoresis, and diathermy

Contraindications

As each indication uses a specific modality, contraindications depend upon the modality. Several contraindications for each modality are listed below.

• ESWL - Infection, stone burden greater than 2.5 cm; coagulopathies, untreated hypertension, pregnancy-ESWL
• Magnetic Resonance-guided Focused Ultrasound Surgery (MRgFUS) - Cardiac pacemaker or other implantable devices
• Ultrasound diathermy - Bone fracture, malignancy, arteriosclerosis, application to eye, spine, active infection, or ischemic tissues

Equipment

All ultrasound therapies utilize electrical current through a probe to exert force through thermal energy and cavitation to produce the desired effect. The probe is typically a handheld device with piezoelectric crystals that produce the ultrasound waves. Hypoallergenic contact gel is applied to the patient’s skin and helps translate the ultrasound waves into the body’s tissues. HIFU procedures rely on curved probe heads, which focus the ultrasound waves a distance deep to the surface on which they rest and get further enhanced with an imaging modality.

Imaging modalities like ultrasound and MRI can increase the safety, accuracy, and effectiveness of therapeutic ultrasound when used in tandem.[14] Magnetic resonance imaging in MRgUS monitors the temperature of surrounding tissues while simultaneously focusing the ultrasound on the fibroid or tumor of interest. This action can prevent the ablation of viable, non-cancerous tissue. Ultrasound imaging has also been paired with HIFU because it is cheaper, obtains live images, and has wide compatibility with implantable devices. However, MRI is a preferable imaging modality for many HIFU procedures due to its more extensive field of view and image resolution.[14]

Personnel

In most US therapy, physicians and support staff are relevant to the procedure. Clinicians are the primary care provider for therapeutic ultrasound, while support staff often assist in monitoring patients and equipment. Musculoskeletal applications of low-intensity ultrasound are an exception to this rule. Due to the limited safety risks for low-intensity ultrasound, many physical therapists and physicians can provide this treatment with little or no assistance.

Preparation

For soft tissue tumors requiring MRgUS, MRI is necessary before the procedure; this will help physicians localize the tumor and position the patient for the procedure. Prior imaging will be compared and utilized to position the patient. Musculoskeletal uses of ultrasound therapy require short preparation and include examining the overlying skin for burns, rashes, infections, and active bleeding. Personnel should always inspect equipment before the start of the procedure. 

Technique or Treatment

Within the variety of FDA-approved therapeutic ultrasound procedures, there are a few similarities in protocols. All ultrasound therapies transmit their waves from a transducer to the target tissues by being placed directly on the skin through a coupling medium. The goal of the coupling medium is to conduct the waves to the target area in an efficient, predictable pattern. Various mediums have undergone testing, but hypoallergenic gels seem to produce strong transduction of waves with minimal interference.[15] Step-by-step outlines for some FDA-approved therapies are listed below.

ESWL

• The patient gets positioned on a fluoroscopic table
• X-rays undergo review, and the patient adjusted with fluoroscopic images to target stone within the kidney
• The patient may be given a sedative or spinal anesthesia for pain control as shockwaves can cause discomfort
• Vertical adjustments of the table are made to position the stone within the "focus" of the ultrasound wave propagation
• Bursts of shockwaves get delivered at approximately 120 shockwaves per minute

HIFU

• The patient is supported while lying supine, prone, or on their side on a table.
• An immobilizer is an option on tissues such as breast
• Local anesthetic (1% lidocaine, 0.5% bupivacaine with epinephrine) may be used for pain control
• The tissue of interest is visualized and outlined with ultrasound or MRI
• The head of the HIFU probe gets centered upon the tissue of interest
• A test dose gets delivered to the center of the tissue of interest
• If necessary, adjustments to power are made based on the depth of tissue
• Bursts of ultrasonic waves are delivered to the tissue
• Destruction of tissue (hyperechoic for U/S or thermometry for MRI) is verified with an imaging modality
• A burst of ultrasonic can continue to be delivered circumferentially around the tissue until adequate tissue destruction

Musculoskeletal

• The patient is positioned comfortably
• Expose skin over the area to be treated
• Turn on the ultrasound machine and set the range at 1 to 3 MHz with an intensity of 1.0W/cm
• Apply hypoallergenic gel to the skin
• Apply ultrasound to skin utilizing a stroking motion
• Ultrasound should remain moving while applied to the skin
• Treatment time should be approximately 10 minutes
• After 10 minutes of treatment, wipe skin clean of ultrasound gel
• The provider can change the frequency to affect the tissue depth. 1 MHz would treat deeper tissues, while 3 MHz would treat more superficial tissues.

Complications

Ultrasound therapies are alternative therapies due to their low risk of complications. Low-intensity ultrasound can cause superficial burns due to prolonged exposure times. Therefore, the ultrasound wand should remain in motion when in contact with the skin. HIFU paired with MRI has the advantage of monitoring the temperatures of the tissues receiving treatment. When used correctly, this can prevent the burning and necrosis of healthy tissue. ESWL presents some of the more severe complications. Internal hemorrhage and scarring can occur. Procedural techniques like duration of shockwaves and total shockwave exposure to prevent scarring of renal tissue during ESWL.[15]

Clinical Significance

Ultrasound has expanded from its diagnostic origins to become a robust, adaptable therapeutic technology for physicians in a variety of specialties. Low-intensity ultrasound has become a mainstay therapy for many acute and chronic musculoskeletal conditions in the outpatient or inpatient setting due to its ease of use and limited complication rate. Meanwhile, HIFU has emerged as a non-invasive therapy used for the ablation of many types of tissues. Though many applications are still experimental, it is likely to be widely accepted as a safe, alternative therapy for various tumors and aberrant tissues.

Enhancing Healthcare Team Outcomes

Overall, studies of ultrasound therapies have demonstrated it to be a safe, non-invasive, and effective procedure that clinicians can offer as primary or adjuvant therapy to traditional treatments.

There is level III evidence for the use of HIFU for atrial ablation in patients with atrial fibrillation. Ninet et al. found a cure rate of 80% at six months when HIFU was used to ablate aberrant atrial tissue causing paroxysmal and persistent atrial fibrillation.[6] Additionally, this procedure can be completed without cardiopulmonary bypass and without the ultrasound wand contacting blood. Complications like ARDS, SIRS, and hemodilution are common and well documented with the use of cardiopulmonary bypass, making HIFU a potentially safer alternative for atrial fibrillation cure.[16] However, this procedure is still experimental, and further studies focusing on effectiveness and safety are necessary.

Numerous studies have shown level II and level III evidence for MRgFUS that demonstrate its safety and effectiveness for breast and uterine soft tissue masses. A systematic review conducted by Peek Et al. found the absence of a tumor or residual tumor in 95.8% of patients undergoing HIFU treatment for breast cancer.[17] Typically, breast cancer receives treatment with breast-conserving surgery or mastectomy. However, these procedures suffer from an increased risk of bleeding, infection, pain, and cosmetic issues. Thus, the need for alternative therapies, like HIFU, has increased. Additionally, some patients with significant co-morbidities may not be surgical candidates, or the surgery may. Hysterectomy can be curative for uterine fibroids but should only be an option for women who are no longer interested in bearing children.[18] Stewart Et al. found that 71% of women experienced symptom reduction at six months with a low incidence of adverse events.[19] Though no longitudinal studies examining the long-term effects of HIFU exist, Bohlmann Et al. reviewed 100 cases and found no increased risk of miscarriage or other obstetrical outcomes.[20] FDA approval of MRgFUS for uterine fibroids occurred in 2004, and numerous studies have demonstrated its effectiveness and safety

Lastly, there is level III evidence for the use of low-intensity ultrasound for pain in degenerative musculoskeletal disorders. Muftic et al. compared two groups receiving low-intensity ultrasound treatment at varying power settings and found visual analog scores for the pain to decrease by approximately 4 points in both groups.[21] This study included men and women suffering from chronic limb or spine pain. However, a systematic review conducted on ultrasound therapy for a variety of musculoskeletal conditions found limited benefits.[22] Shanks et al. also found limited evidence for ultrasound therapy benefits in lower limb musculoskeletal conditions.[23] More robust studies with clear indications and procedural techniques need to be conducted to make broad conclusions on low-intensity ultrasound.

Interprofessional teamwork is an optimal approach to treating patients receiving ultrasound therapy. This team can include clinicians, specialists, mid-level practitioners, and physical therapists, depending on the condition being treated. All members of the interprofessional team should coordinate their activities and share information about the patient with the rest of the team to optimize patient outcomes.