Therapeutic ultrasound. Physical characteristics and biological mechanisms. Continuous and pulsed modes. Application techniques, dosage, indications, contraindications, and safety considerations

At a glance

1. Therapeutic ultrasound is a mechanical wave above hearing, usually at 1 MHz (deeper, about 2 to 5 cm) or 3 MHz (superficial, about 1 to 2 cm), made by a piezoelectric crystal in the treatment head.
2. It needs a coupling medium (gel, oil or degassed water), because air reflects almost all of it. It is absorbed most by collagen-rich tissue: tendon, ligament, capsule, scar and the periosteum.
3. Continuous ultrasound heats; pulsed ultrasound (often 20 per cent duty cycle) works mainly by mechanical effects: stable cavitation, acoustic streaming and micromassage.
4. Intensity is set in W/cm² (usually 0.5 to 2), objectively, from the device. The head keeps moving slowly; sessions are about 5 to 10 minutes per area.
5. Used for tendinopathies, enthesopathies, bursitis, scars and contractures, as an adjunct. Good trials often find little more benefit than a sham; one notable exception is calcific shoulder tendinitis.
6. Never over the pregnant uterus, the eyes, the gonads, a malignant tumour, a thrombosed vein, or the spinal cord after laminectomy. Unlike diathermy, a metal implant is not a contraindication.

01Physical characteristics

Therapeutic ultrasound is sound, a pressure wave in which particles push their neighbours forward and back, like a ripple of shoves passing down a queue. We hear up to about 20 kHz; anything faster is ultrasound. Therapy uses 0.8 to 3 MHz, and most devices offer 1 and 3 MHz. Because it travels by particles bumping into particles, it cannot cross a vacuum, and it crosses air very badly.

How it is made

The head holds a thin disc of crystal (once quartz or barium titanate, today mostly lead zirconate titanate). Such a crystal changes thickness when voltage is applied across it, the reverse piezoelectric effect. Fed with high-frequency alternating current, it swells and shrinks a million times a second and pushes the gel and tissue in front of it, like a tiny loudspeaker. Run in air, the crystal has nothing to push into, overheats and can crack, so the head is never left running in air.

speed of sound in soft tissue

1540m/sspeed of sound in soft tissue

wavelength at 1 MHz

1.5mmwavelength at 1 MHz

deep / superficial targets

1 / 3MHzdeep / superficial targets

usual upper limit of continuous output

3W/cm²usual upper limit of continuous output

The beam

• Effective radiating area (ERA): the part of the head that actually emits, a little smaller than its face; heads of about 1, 5 and 10 cm².
• Intensity: power divided by ERA, in W/cm². A head of 10 cm² putting out 10 W gives 1 W/cm².
• The beam is uneven. Close to the head (the near field) it has hot spots several times stronger than the average. The ratio of the peak to the average is the beam non-uniformity ratio (BNR); a good head has one below about 5 or 6. This is the main reason the head is kept moving.

Absorption, reflection and depth

Ultrasound is absorbed most by tissue rich in protein and collagen, and least by water and fat. This is the mirror image of diathermy.

When the beam meets a boundary between tissues with different acoustic impedance, part of it bounces back.

• Air and skin: almost total reflection, hence the coupling medium.
• Soft tissue and bone: about a third reflected. The returning energy meets the incoming beam just in front of the bone and heats the periosteum, which is full of pain fibres. A deep ache during treatment is a sign of overdose.
• Soft tissue and metal: strong reflection, but metal conducts heat away so fast that it does not overheat.

The higher the frequency, the more is absorbed near the surface, so the shallower the effect: 1 MHz reaches roughly 2 to 5 cm, 3 MHz roughly 1 to 2 cm. The book's school also used 0.8 to 1 MHz only, which is why older texts give a single frequency.

02Biological mechanisms

Ultrasound acts in two ways, and the mode decides which one dominates.

Thermal effects

Absorbed sound becomes heat, most in collagen-rich tissue. Continuous ultrasound can raise tendon or capsule temperature by about 1 °C (mild), 2 to 3 °C (moderate) or 3 to 4 °C (vigorous), enough to make collagen more extensible, ease muscle spasm and pain, and open vessels. The heat follows the rules of endogenous heat: it is made inside the tissue, so the skin feels little.

Mechanical (non-thermal) effects

1. 1

   Micromassage

   Tissue is compressed and stretched a million times a second, a fine shaking of cells and fluid.

2. 2

   Stable cavitation

   Tiny gas bubbles in tissue fluid grow and shrink with each wave without bursting. Their rhythmic motion stirs the fluid next to cell membranes.

3. 3

   Acoustic streaming

   The pressure of the beam pushes fluid one way, a slow current along cell surfaces.

4. 4

   Cell responses

   Membranes become more permeable, calcium enters cells, mast cells release their contents, and fibroblasts make more collagen. The idea is that this speeds the normal course of repair rather than suppressing inflammation.

Two mechanical effects are harmful and must be avoided:

• Unstable (transient) cavitation: at high intensity, bubbles collapse violently and damage cells. Low intensities and a moving head prevent it.
• Standing waves: with a stationary head over a reflecting surface such as bone, the outgoing and returning waves add up into fixed bands of high pressure; in vessels, blood cells collect in bands and flow stops. Moving the head prevents this too.

03Continuous and pulsed modes

The duty cycle is the share of time the head is on. Pulsed ultrasound at 50 per cent warms a little; at 20 per cent, barely. The intensity shown may be the intensity during each pulse or the average over time, depending on the device, so the setting should be recorded as the device states it.

A special form is low-intensity pulsed ultrasound (LIPUS, 1.5 MHz, very low intensity, 20 minutes daily), used at home to stimulate fracture healing. A large sham-controlled trial in tibial fractures found it neither speeded healing nor improved function.

04Application techniques

1. 1

   Direct contact (the usual method)

   Gel on the skin, head flat on it. The head moves slowly, about 2 to 4 cm a second, in small circles or overlapping strokes (labile technique), covering an area no larger than about twice the ERA. The stationary (stable) technique is rare and used only at low intensity.

2. 2

   Under water (subaqual)

   For uneven, bony small parts such as the hand, wrist, ankle and foot. The part and the head go into a bowl of degassed water (boiled and cooled, so bubbles do not block the beam), with the head 1 to 3 cm from the skin. The water fills every hollow. Bubbles on the skin are wiped away.

3. 3

   Gel pad or water bag (standoff)

   For uneven areas that cannot go under water: a gel pad or a water-filled bag between head and skin, with gel on both sides.

4. 4

   Phonophoresis

   Ultrasound through a gel containing a drug, such as hydrocortisone, a non-steroidal anti-inflammatory gel (diclofenac) or a local anaesthetic, to push it into the skin by cavitation and streaming. Penetration gains in people are small and inconsistent. Compare iontophoresis, which drives charged drugs with direct current.

05Dosage

Unlike diathermy, the dose is set objectively: the device shows what the head delivers, whatever the patient feels. Tolerance grows over a course, so the intensity may rise a little.

The book gives strict upper limits over the spine (0.2 W/cm² for the neck, 0.4 W/cm² for the lumbar region). These are the old school's caution near the spinal cord and are lower than modern usage; the safe rule today is to avoid the cord after laminectomy and to dose by stage and depth.

06Indications

Ultrasound suits small, collagen-rich structures that the head can reach: tendons, ligaments, capsules, bursae and scars.

What the evidence shows

Ultrasound is one of the most studied physical agents, and most good trials find little difference from a sham:

• Reviews of placebo-controlled trials across musculoskeletal pain and injury find little evidence that it adds to other treatment.
• Chronic low back pain: a Cochrane review found the evidence too uncertain to show a benefit on pain and concluded that it does not support using ultrasound.
• Knee and hip osteoarthritis: a Cochrane review found it may help pain and function, but the evidence was of low quality.
• Calcific tendinitis of the shoulder: a well-known randomised trial found that pulsed ultrasound at about 0.9 MHz and a fairly high intensity, 24 sessions of 15 minutes over six weeks, helped the deposits resolve and eased symptoms in the short term.
• Fractures: low-intensity pulsed ultrasound did not speed healing of tibial fractures.

It is reasonable as a short adjunct to exercise in chronic tendon and capsule problems, not as treatment on its own.

07Contraindications

Each follows from heating, from mechanical energy, or from where the beam goes.

08Safety considerations

• Keep the head moving and in full contact; a stationary head makes hot spots, standing waves and cavitation.
• Never run the head in air: the crystal overheats and breaks.
• Use enough gel, and in water treatment degassed water without bubbles on the skin.
• Stop for pain: a deep ache means periosteal overheating, a burning means too much heat.
• Check sensation before continuous treatment.
• The therapist's hand stays out of the water bath during underwater treatment; the beam reflects in the water.
• Calibrate the machine regularly and clean the head between patients.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1A woman with a plated distal radius fracture has a painful, stiff scar over the wrist. Which heating modality may be used over the plate?

Shortwave diathermy, capacitiveMicrowave diathermy, centimetricTherapeutic ultrasound, with the head kept movingInductive shortwave at an athermic dose

2Why must there be gel or water between the ultrasound head and the skin?

Air reflects almost all ultrasound, so without a coupling medium nothing enters the bodyThe gel carries the electric current to the skinThe gel cools the skin to prevent burnsThe gel contains the drug that does the treating

3A pregnant woman, 20 weeks, has lateral epicondylitis. Which statement about ultrasound is correct?

Ultrasound is contraindicated anywhere in pregnancyThe elbow may be treated, but the beam must never be aimed at the pregnant uterusOnly pulsed ultrasound may be used, over any areaUltrasound is safe over the abdomen because diagnostic scans are

4During continuous 1 MHz ultrasound over a thin patient's shin, she reports a deep, dull ache. What does it mean and what do you do?

The treatment is working; continueThe gel has dried; add more and continue at the same doseThe crystal is faulty; change the headReflected energy is heating the periosteum; reduce the intensity, move faster or stop

5A footballer has a calf muscle strain from four days ago. If ultrasound is used, which setting fits the stage?

Continuous 1 MHz at 2 W/cm² for 15 minutes to heat the musclePulsed, 20 per cent duty cycle, at low intensity, for a few minutesContinuous 3 MHz at the highest outputStationary head at 1 W/cm² over the tear