How often can you use ultrasound therapy? or Dose Calculation of Ultrasound therapy?

One of the advantages of ultrasound therapy remains the reasonably broad range of trials from which effective treatment doses can be established.

Dose Calculation Stages

The first steps involve the decision as to which machine settings are most appropriately applied to the patient's particular problem. The second stage is to bring these into an effective treatment combination

Machine settings :

There are effectively 4 things that you can change on your machine :

  1. Machine Frequency
  2. Pulse Ratio
  3. Ultrasound Treatment Intensity
  4. Duration of treatment

1. Machine Frequency

Taking into account that the most frequently available treatment frequencies are 1 and 3MHz, the option between them relates primarily to the effective treatment depth that is required

3MHz ultrasound is absorbed more rapidly in the tissues, and therefore is considered to be most appropriate for superficial lesions, whilst the 1MHz ultrasound energy is absorbed less rapidly with progression through the tissues, and can therefore be more effective at greater depth.

The boundary between superficial and deep lesions is in some ways arbitrary, but somewhere around the 2cm depth is often taken as a useful boundary. Hence, if the target tissue is within 2cm (or just under an inch) of the skin surface, 3MHz treatments will be most effective whilst treatments to deeper tissues (2cm+) will be more effectively achieved with 1MHz ultrasound.

2. Pulse Ratio

The pulse ratio determines the concentration of the energy on a time basis. The pulse ratio determines the proportion of time that the machine is ON compared with the OFF time. A pulse ratio of 1:1 for example means that the machine delivers one 'unit' of ultrasound followed by an equal duration during which no energy is delivered. The machine duty cycle is therefore 50%. A machine pulsed at a ratio of 1:4 will deliver one unit of ultrasound followed by 4 units of rest, therefore the machine is on for 20% of the time (some machines use ratios, and some percentages). The table overleaf shows their equivalence.

Ideally, try for 1:4 or 1:3 for the ACUTE lesions, 1:2 and 1:1 for the SUBACUTE lesions and 1:1 or Continuous for the CHRONIC lesions.

3. Ultrasound Treatment Intensity

In a similar way to the pulse ratio decision, the intensity of ultrasound required at the target tissue will vary with the tissue state. The more acute the lesion, the less strong the ultrasound needs to be in order to achieve/maintain the tissue excitement. The more chronic the tissue state, the less sensitive, and hence the greater the intensity required at the lesion in order to instigate a physiological response.

Tissue State Intensity required at the lesion (W/cm2 )

Acute 0.1 - 0.3

Sub Acute 0.2 - 0.5

Chronic 0.5 - 1.0

4. Duration of treatment

The greater the size of the lesion, the longer the duration of the ultrasound that will be required in order to achieve a particular effect. The most common method to take account of this factor is to estimate the number of times which the ultrasound treatment head to be utilised can be placed over the target tissue. Given that the intention is to apply 1 minutes worth of ultrasound energy per treatment head area covered (see below), there is a direct relationship between treatment area and treatment time. For example, if the large treatment head is used to treat the anterior capsule of the shoulder, it can be estimated that it will fit twice over the target. Similarly, if the small treatment head is applied over the lateral ligament of the elbow, it may only fit once. There is no need to measure the treatment head, it is a matter of estimating the number of time the head fits onto the target tissue rather than a millimetre by millimetre measurement.

Using the information in the previous sections, the following examples may serve to illustrate the point :

Example 1. 

Ultrasound treatment for a (very) acute lesion of the lateral ligament of the ankle Assuming that on examination, the primary focus of the lesion is determined to be at the anterior portion of the ligament (anterior talo-fibular), the following clinical decisions are made :

    • The lesion is superficial, hence a 3MHz frequency would be most appropriate
    • The lesion is acute, thus an intensity of 0.2 W/cm2 should be sufficient to treat the lesion
    • There is no need to increase the surface dose to allow for loss of ultrasound at depth (<0.5cm depth to reach the tissue)
    • The lesion is acute, therefore a pulse ratio of 1:4 will be most appropriate
    • Using the large treatment head, it is estimated that the target tissue is approximately the same size as the treatment head (i.e. the head fits on to the tissue once) Working on the principle of 1 minutes worth of ultrasound per treatment head area, the total time taken to treat the lesion will be (1 minute) x (number of times the treatment head fits over the lesion) x (the pulse ratio) which in this instance = (1) x (1) x (5) = 5 minutes.

The final treatment dose will therefore be 3MHz ; 0.2 W/cm2 ; Pulsed 1:4 ; 5 minutes

There is no 'proof' that this dose is guaranteed to work, but given the available evidence, it is the dose that is most likely to achieve the intended effect (i.e. activation of the tissue repair process).

Example 2

Ultrasound treatment of a subacute lesion of the lateral ligament complex of the elbow and superior radioulnar joint Assuming that on examination, the primary focus of the lesion is determined to be at the lateral ligament of the elbow joint itself together with the lateral portion of the annular ligament of the superior radioulnar joint, the following clinical decisions are made :

  • The lesion is superficial, hence a 3MHz frequency would be most appropriate
  • The lesion is sub-acute, thus an intensity of 0.4 W/cm2 should be sufficient to treat the lesion
  • There is no need to increase the surface dose to allow for loss of ultrasound at depth
  • The lesion is sub-acute, therefore a pulse ratio of 1:2 will be most appropriate
  • Using the small treatment head (due to the nature of the surface), it is estimated that the target tissue is approximately twice the size of the treatment head (i.e. the head fits on to the tissue twice)

Working on the principle of 1 minutes worth of ultrasound per treatment head area, the total time taken to treat the lesion will be (1 minute) x (number of times the treatment head fits over the lesion) x (the pulse ratio) which in this instance = (1) x (2) x (3) = 6 minutes.

The final treatment dose will therefore be 3MHz ; 0.4 W/cm2 ; Pulsed 1:2 ; 6 minutes