Ultrasound Dose Calculations
Ultrasound Dose Calculations
One of the advantages of ultrasound therapy remains the reasonably broad range of trails
from which effective treatment doses can be established.
In principle, there is no need for the often used 'recipe book' in which a list of conditions is
produced alongside the treatment dose. One of the reasons for this is that the 'best' recipe for
all the conditions one might encounter does not exist, certainly not from the evidence base.
Secondly, there is effectively no need to learn a whole list of such formulas for successful
application, one needs to apply the principles to the particular tissue in question, taking into
account the relevant parameters.
The following 'dose calculations' offer one method (by no means the only one) by which the
most likely clinically effective dose can be established. There is no guarantee the resulting
dose will work, but it does offer a mechanism by which the dose which is most likely to work
can be estimated.
Dose Calculation Stages
The first steps involve the decision as to which machine settings are most appropriately
applied to the patients particular problem. The second stage is to bring these into an effective
treatment combination.
Machine settings :
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 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 2-3cm depth is often taken as a useful boundary. Hence, if the target tissue is
within 2-3cm (or an inch) of the skin surface, 3MHz treatments will be effective whilst
treatments to deeper tissues will be more effectively achieved with 1MHz ultrasound.
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.
It is worthy of note that some machines offer pulse ratios for which no evidence can be
identified to ascertain the effectiveness of the intervention. Pulse ratios of 1:9 or 1:20 for
example can be found on machines but with no trial evidence to support their use.
The table below indicates the ratios and duty cycle percentages for commonly encountered
settings
The selection of the most appropriate pulse ratio essentially depends on the state of the
tissues. The more acute the tissue state, the more energy sensitive it is, and appears to
respond more favourably to energy delivered with a larger pulse ratio (lower duty cycle). As
the tissue moves away from its acute state, it appears to respond preferentially to a more
'concentrated' energy delivery, thus reducing the pulse ratio (or increasing the duty cycle).
It is suggested that pulse ratios of 1:4 are best suited to the treatment of acute lesions,
reducing this as the tissue moves towards the chronic state moving through 1:3 and 1:2 to end
up with 1:1 or continuous modes. Some machines are unable to deliver the most effective
treatment modes and the therapist will need to compromise the treatment dose according to
the facilities that are available.
It is of note that it is the state of the tissue that determines the most appropriate pulse ratio
rather than simply the duration since the onset of the lesion. In a similar way to the clinical
decision making process in manual or other therapies, tissue reactivity is the key. If the tissue
in question behaves in an acute manner on assessment, then the lesion is effectively treated
with an 'acute' dose. If it behaves as a chronic, less responsive tissue, then treat with a
'chronic' dose. Some patients will present several weeks after an injury or lesion onset, yet the
problem exhibits 'acute behaviour' and should treated accordingly. Similarly, some lesions
appear to move swiftly into chronic behaviour mode, and these are best managed with a dose
estimated for chronic lesions. The key here is to treat what you find at assessment, rather than
what the timescale says should be there.
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 smaller the 'strength' of the
ultrasound that is required 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.
One important factor is that some of the ultrasound energy delivered to the tissue surface
will/may be lost before the target tissue (i.e. in the normal or uninjured tissues which lie
between the skin surface and the target). In order to account for this, it may be necessary to
deliver more at the surface than is required, therefore allowing for some absorption before the
lesion, and allowing sufficient remaining ultrasound to achieve the desired effect.
The intensity required at the lesion can be determined from the following table :
The rate at which ultrasound is absorbed in the tissues can be approximately determined by
the half value depth - this is the tissue depth at which 50% of the ultrasound delivered at the
surface has been absorbed. The figures used for these estimates are average values in that it
absolute values will vary with the thickness of various tissues (e.g. skin, fat, muscle etc). The
average 1/2 value depth of 3MHz ultrasound is taken at 2.5cm and that of 1MHz ultrasound as
4.0 cm though there are numerous debates that continue with regards the most appropriate
half value depth for different frequencies..
The tables below indicate the intensity required at the skin surface in order to achieve a
particular intensity at depth. It is suggested that the intensity required at depth is established
first (from the table above), then the most appropriate frequency selected and these two
factors are used to determine the surface intensity required.
Table to indicate the surface intensity (W/cm2) required to achieve a particular intensity at
depth using 3MHz ultrasound
Table to indicate the surface intensity (W/cm2) required to achieve a particular intensity at
depth using 1MHz ultrasound
Size of the Lesion
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.
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 not 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.
Compiling the treatment dose.
The final compilation of the treatment dose which is most likely to be effective is based on the
principle that one needs to deliver 1 minutes worth of ultrasound energy (at an appropriate
frequency and intensity) for every treatment head that needs to be covered.
The size of the treatment area will influence the treatment time, as will the pulse ratio being
used.
The larger the treatment area, the longer the treatment will take. The more pulsed the energy
output from the machine, the longer it will take to deliver 1 minutes worth of ultrasound energy
(there is a greater proportion of time during which the machine gives no output).
Using the tables above, it is possible to estimate the surface intensity required at a particular
frequency to achieve sufficient ultrasound at the required depth to gain the desired effect.
Using the information in the previous sections, the following examples may serve to illustrate
the point :
Example 1
Ultrasound treatment for an acute lesion of the medial collateral ligament of the knee
Assuming that on examination, the primary focus of the lesion is determined to be at the
central portion of the ligament (as it crosses the joint line), 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
• 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 is a guaranteed dose, 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
Example 3
Ultrasound treatment of a chronic lesion of the anterior capsule of the shoulder (glenohumeral
joint
Assuming that on examination, the primary focus of the lesion is determined to be at the
anterior capsule of the glenohumeral joint, the following clinical decisions are made :
• The lesion is not superficial, hence a 1MHz frequency would be most appropriate
• The lesion is chronic, thus an intensity of 0.5 W/cm2 should be sufficient to treat the lesion
• There is a need to increase the surface dose to allow for loss of ultrasound at depth, and
using the tables above, it is estimated that the required surface dose will need to be 0.75
W/cm2
• The lesion is chronic, therefore a pulse ratio of 1:1 will be most appropriate
• Using the large treatment head, 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 (2) = 4 minutes.
The final treatment dose will therefore be
1MHz ; 0.75 W/cm2 ; Pulsed 1:1 ; 4 minutes
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