Ultrasound Therapy

Article Author:
Michael Matthews
Article Editor:
Michael Stretanski
Updated:
5/21/2020 2:26:54 PM
PubMed Link:
Ultrasound Therapy

Introduction

The first documented and published use of ultrasound began in 1958 when a team led by Ian Donald in Glasgow first utilized sonography as a diagnostic tool in the practice of obstetrics and gynecology.[1] Today, patients benefit from a wide range of sonographic uses. Due to the lack of radiation, low cost, and portability, fetal monitoring, joint injections, arterial line placements, and diagnoses of joint pathology are some of the many uses of ultrasound as an imaging and diagnostic modality. However, technological advancements and engineering have also allowed ultrasound to become a therapeutic modality. Most classically, continuous therapeutic ultrasound has been used to treat a variety of musculoskeletal pathologies like osteoarthritis, soft tissue shoulder pathology, and myofascial pain.[2][3] Ultrasound use has also expanded into atrial ablation, extracorporeal shockwave lithotripsy, accelerate fracture healing, limited rhytidectomy, and, when paired with MRI, can be used to treat benign and malignant soft tissues tumors.[4][5][6][7] Within this activity, we will focus on the physical principles and theories upon which form the basis for ultrasound therapies, the interactions between the ultrasound and human anatomy, indications of use, safety concerns, and it’s utility amongst the healthcare team.

Therapeutic ultrasound relies on a variety of power settings and comes from numerous manufacturers, but the principles remain the same; the ultrasound machine conducts an electrical signal through crystals found in the head of the ultrasound probe. The crystals vibrate and create mechanical waves at frequencies outside the range of human hearing (20 hertz to 20000 hertz). This phenomenon is known as the ‘piezoelectric effect.’ The waves produced transfer energy to the surface of the human body or maybe focused to target soft tissue deep to the epidermis. Tissues containing a higher content of proteins (muscle and bone) absorb the energy from the mechanical waves at a higher rate than tissues with higher water content (fat).[8] Additionally, the energy from the ultrasound may be focused to hit tissues deep to the surface without causing harm to more superficial tissues. Extracorporeal shockwave lithotripsy is a common procedure that demonstrates this mechanism.

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]

Indications

.Engineering advancements and dedicated research have paved the way for FDA approval of ultrasound therapy in a variety of disciplines. Below this article covers a few of the most common indications for ultrasound therapy.

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
  • MRgUS - 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 to 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 safety, accuracy, and effectiveness of therapeutic ultrasound when used in tandem.[12] 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.[12]

Personnel

In most US therapy, physicians and support staff are relevant to the procedure.  Physicians are the primary provider of care 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

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[13]. 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 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 with the patient lying supine or laterally 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
  • 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 gets positioned comfortably
  • Expose skin over the area to be treated
  • Turn on the Ultrasound machine and set 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

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 temperatures of the tissues receiving treatment. When used correctly, this can prevent 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.[13]

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 a variety of tumors and aberrant tissues.

Enhancing Healthcare Team Outcomes

Overall, studies of ultrasound therapies have demonstrated that it is 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 the use of 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.[14] 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.[15] Typically, breast cancer receives treatment with breast-conserving surgery or mastectomy. However, these procedures suffer from 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 in women who are no longer interested in bearing children.[16] Stewart Et al. found 71% of women experienced symptom reduction at six months with a low incidence of adverse events.[17] 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.[18] 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.[19] 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.[20] Shanks et al. also found limited evidence for ultrasound therapy benefits in lower limb musculoskeletal conditions.[21] More robust studies with clear indications and procedural techniques need to be conducted to make broad conclusions on low-intensity ultrasound.

This research was supported (in whole or part) by HCA Healthcare and/or an HCA Healthcare affiliated entity. The views expressed in this publication represent those of the author(s) and do not necessarily represent the official views of HCA Healthcare or any of its affiliated entities.


References

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