Ultrasound is a term used to describe mechanical vibrations which are ‘sonic’ or ‘sound’ waves but at a higher frequency.
Technically ‘ultrasonic energy’ or ‘ultrasound’ are ways of describing vibrations above the range of human hearing.
Medical ultrasound is normally thought of as being in the 0.5 to 5 mhz range (low and Reed 1994).
Ultrasound is really a series of compressions (and rarefactions) which are produced by, and move away, from the ‘face’ of the treatment (or diagnostic) head. Put together these are called longitudinal waves. Although we can’t see them in normal substances they can pass through solids, liquids and gases by the simple compression and separation of molecules. They can be seen in water if a obstacle is put in their way.
In real life this means the ultrasound waves will hit molecules and make them move.
This movement is normally around their average position (average is used as all molecules are in constant random motion Low and Reed 1994).
If the molecule can not move linearly it may rotate instead.
Some molecules move and become pulled out of shape.
As most molecules are close to others the movement will have a subsequent effect/s – this can be equated to the ‘mouse trap effect’. Moving one molecule will lead to the movement of molecules around it as they collide.
Some molecules, and hence tissues, are affected more than others by ultrasound.
This is dependent on many factors but in short the effects on many molecules (and tissues) can be increased or decreased by adjusting the ultrasound wave. However some molecules (and tissues) can not be affected by ultrasound in any meaningful way.
Because molecules and tissues react differently to different ultrasound waves ultrasound machines are very flexible.
Most ultrasound waves ‘interact’ with molecules and tissues in predictable ways:
Ultrasound waves can be:
Reflected– think diagnostic ultrasound – however this does happen in therapy as well.
Here the waves are not of sufficient energy to pass through the molecules and are bounced back.
Refracted – the wave hits one molecule and is partly absorbed (having a treating effect whilst also loosing some of it’s own energy). It may then be reflected as a new wave (very common in mixed tissues e.g. treatment to a ligament over a bone)
Absorbed – the waves energy is taken up by the molecule/s passing their energy to the molecules. If this energy is high enough heating will occur.
Attenuated – this is the most common in therapy – here some of the energy is given up to the most susceptible molecules (not necessarily the most superficial as shown here). As the wave moves deeper, the deeper molecules gain less energy and so on.
Dissipated – also very common in therapy the waves are absorbed in part whilst the rest of the energy passes onwards (absorbed + attenuated + refracted).
So what determines if a molecule and hence a tissue is susceptible to ultrasound waves?
Largely the wave it’s self and particularly the speed of the waves.
All molecules constantly move but at varying speeds. The closer the ultrasound wave matches this the more energy it will add to the molecule (Low and Reed 1994 describe it as superimposed).
Hence if a molecule is hit by another molecule rather than by the wave it will move at a sub optimal speed thus gaining less energy.
Then ultrasound can be used to ‘target’ certain molecules and hence tissues at defined settings.
So what parameters of the wave can we control?
Wave length – this is the distance from one wave peak to the next. In essence the longer the distance generally the further (or deeper) the wave will go.
Frequency – this is how many waves are generated in a given time frame this is measured in MHz. Please note frequency is often used to describe ‘pulsing’ in ultrasound but the two are different. The wave below demonstrates frequency in a continuous ultrasound treatment.
Velocity is the two factors combined i.e. Velocity is frequency x wavelength
Velocity of ultrasound waves is higher in more dense materials (this would appear to be counter intuitive but is true). As an example in air sound waves have a velocity of 343 m/s (Low and Reed 1994) where as in salt water (this is very similar to human soft tissues) the speed is 1500 m/s (Low and Reed 1994). Hence velocity is higher in human soft tissues than air (this is why we use gel to eliminate air gaps).
What this means is in therapy we often combine frequencies and wavelengths to achieve a velocity that is desirable.
Two practical examples:
- 1mhz (frequency) with 1.5 (mm) wavelength – 1500m/s
- This would go deeper (further) due to the longer wave length
- 3mhz (frequency) with 0.5 (mm) wavelength = 1500m/s
- This would be more superficial due to the shorter wave length
Amplitude is just how much a molecule is moved by each wave – the greater the movement the higher the amplitude.
How is ultrasound made?
Obviously we need a way of generating the the ultrasound in the first place. Traditionally crystals that vibrated (when exposed to electricity) were used (these had the disadvantage of needing different treatment heads per frequency and were vulnerable to breaking if dropped), but in modern machines the ultrasound is generated by transducers. Modern machines can produce different frequencies from the same treatment head.
Treatment head considerations:
This then brings us to treatment heads and particularly head size – in most therapy machines the treatment heads are a circle (unlike in diagnostic ultrasound where most are rectangles). And are much larger than the wavelength i.e. wave lengths are in mm and treatment head sizes are often up to 3cm wide. This means ultrasound waves are not uniform as they leave the treatment head, some amplify, whilst others cancel or reflect. This makes ultrasound waves variable even before they enter the body. Couple to this most treatments are performed using the zone nearest the treatment head where the waves are most irregular (called the Fresnel zone or near field zone). Together all of these factors can lead to very irregular waves leaving the head and entering the body.
To counteract this the treatment head is commonly moved throughout treatment as this evens out the differences in the waves.
However, it is not the only reason to keep the head moving.
Keeping the head moving helps to even out reflections and refractions whilst allowing tissues to absorb the energy in a meaningful way. It helps to reduce ‘hot spots’ and limits unstable cavitation.
How susceptible to ultrasound are human tissues?
Real tissues – so if we chose the correct parameters human tissues will be affected by ultrasound in different ways and more importantly by different amounts. This is know as attenuation. In essence this is how readily a tissue absorbs ultrasound and ‘uses up’ the energy.
Frizzel and Dunn (1982) gave a simple scale from least to most absorption.
Depth- combining all of the above in real human tissue how deep can we go and still have a good effect?
This is usually stated as the ‘half depth value’. That is how deep will you get and still have half of the energy left. This is a contentious area of debate, however here is a general guide from Low and Reed (1994).
Half depth values in mm (aggregated values)
| Tissue | Skin | Fat | Muscle | Tendon | Cartilage | Bone |
| 1 MHz | 11 | 50 | 9 | 6 | 6 | 2 |
| 3 MHz | 4 | 16 | 3 | 2 | 2 | 0.5 |
Pulsed ultrasound:
Pulsing of ultrasound is different to it’s frequency. Modern machines can be set to turn on and off the ultrasound waves giving a pulsed effect. Most manufacturers use a ratio method of describing the pulses but it can also be expressed as a percentage.
For example –
- 1:1 ratio is on 50% of the time or a 50% duty cycle
- 1:4 ratio is on 20% of the time or a 20% duty cycle
You can then vary the time the cycle is active which is normally stated in ms.
Why is this popular?
In theory you could allow any heat generated to dissipate by using pulses (remembering some tissues dissipate heat faster than others e.g. muscle due to better blood flow) hence delivering higher doses of ultrasound without heating the tissues.
Intensity or power:
Overall intensity or power is when you add up the treatment head size the frequency and the wavelength you get an average measurement known a watts per centimetre squared written as W/cm2 This is the average intensity over the treatment head when the machine has been set.
Most treatments will be described in a value of W/cm2 not as frequency or wavelength.
Time averaged intensity is normally used in pulsed treatments. This takes into account the amount of on/off time and is referred to as space-averaged time-averaged or SATA for short. These measurements are normally much smaller than the normal W/cm2.
Obviously the overall treatment time affects total treatment dose and there are many methods for determining the optimal amount of treatment time.
Low and Reed (1994) recommended 1-2 mins per 10cm2
As a general rule I recommend 1 min per treatment head size – this makes most treatments 1-5 mins in length.