Therapeutic Ultrasound

Machine Settings to Know

• There are 4 main settings that you need to know :

1. Frequency: How many ultrasound sound wave cycles occur per second, measured in megahertz (MHz)

• • 1 MHz: Used to target deep tissues (more than 2 cm below the surface)

  • 3 MHz: Used to target superficial tissues (0.5-2 cm deep)

2. Duty Cycle: How much of the time the ultrasound is sending out sound waves

• Machines either represent this as a percentage or ratio

  • For example,  a 25% duty cycle means sound waves are being sent out 25% of the time

    • 25% duty cycle is equivalent to 1:3 pulse ratio - the machine will send 1 unit of sound waves and 3 units of rest (1/4 in total=25%)

• Lower percentages or cycles are best for more acute injuries (20%/1:4 or 25%/1:3) as the tissues respond better to less intensity

• Medium percentages or cycles are best for more subacute injuries ( 33%/1:2 or 50%/1:1)

• Higher percentages or cycles are best for more chronic injuries (50%/1:1 or continuous)

3. Intensity: The strength of the ultrasound energy delivered per unit area, measured in W/cm²

• Lower intensities are used for sensitive or acute conditions (0.1-0.3 W/cm²)

• Medium intensities are used for sub-acute conditions (0.2-0.5 W/cm²)

• Higher intensities for chronic issues or deeper tissues (0.5-1.0 W/cm²)

4. Time: The amount of time the machine will be on 

• A general rule is to apply ultrasound for 1 minute per area equal in size to the ultrasound head

  • For example, if the treatment area is three times the size of the ultrasound head, the ultrasound should be applied for approximately 3 minutes

• However,  the duty cycle (continuous or pulse) must also be taken into account as a lower duty cycle will require more time as sound waves aren't being sent out the entire time

• Therefore, the final general rule taking treatment area and duty cycle into account is: 

  • Treatment time = (Number of times head fits onto treatment area) x (pulse factor)

    • Pulse factor would be written as 1:2 = 3 in total for example, or 1:4 = 5 for example 

• Putting it all together: Let's say you are performing therapeutic ultrasound on a patient with acute lateral epicondylitis

  • The ultrasound head would fit approximately 3 times over the treatment area (extensor bulk at lateral epicondyle of humerus)

  • The duty cycle we will choose is 20% or 1:4 - therefore 1+4 = 5

  • Using the treatment time equation: Treatment time = 3 x 5 = 15 minutes

  • You do not need to take into account frequency (depth of tissues) when calculating time - 3 MHz would be appropriate for targeting extensor bulk in this example

Contraindications

• Active infection

• Reduced sensation

• Over plastic or metal implants

• Over the ends of bones (epiphyses) that are still growing

  • Can interfere with growth plates

• Malignancy - cancer in treatment area

• Over a pacemaker

• Non-healed fractures