Frequency
To appreciate why we choose a particular transducer for the specific subject you want to image, it is important to appreciate the fundamental ultrasound physics principles behind the choice of frequency. It helps to understand the relationship between the speed of sound (c), the frequency (f) and the wavelength (λ – lamda) of the sound wave:
The basic equation is c = f λ (speed of sound = frequency x wavelength)
- As a sound wave travels through soft tissue, it produces vibrations which cause the molecules to push up against one another. The frequency selected on the ultrasound system is the number of vibrations per second created by the crystals at the transducer surface i.e., for a sound (acoustic) wave, the ultrasound frequency transmitted is the number of cycles of compression and rarefaction which occur in 1 second.
- Frequency (f) is measured in hertz (Hz) i.e., one cycle per second is 1Hz, 1000 000 cycles per second is 1 megahertz (MHz) which is the typical unit for the frequencies we use in diagnostic ultrasound
- Wavelength (λ) is the length of one of these cycles and is usually measured in mm. The longer the wavelength, the deeper it can travel into the body.
- Given that the speed of sound in tissue is assumed to be constant 1540m/s ie C doesn’t change it is clear from the equation that if the frequency f increases, the wavelength λ must decrease and vice versa to balance the equation.
C (↔) = f↑↓ x λ↓↑
In simple terms, this equation is telling you that you choose a lower frequency if you are going deep into a patient – canine liver for example and a higher frequency if you are looking at something superficial eg feline stomach or a palpable lump in a patient
- The typical frequency range for diagnostic ultrasound is between 2—20MHz and modern transducers are multi-frequency e.g. 3-5MHz or 8-12MHz.
- The resolution of our system is the ability to distinguish two adjacent reflectors as separate finite structures, so it is essential that we choose the correct transducer to maximise the image quality.
- As the frequency of the transducer increases, so does the axial (longitudinal) resolution so the image quality and detail is better in the direction of the main beam (axis).
- The higher the frequency, the thinner is the piezoelectric crystal layer which vibrates to produce the sound wave. Since higher frequency transducers send out pulses of shorter wavelength, the depth of penetration into the body is reduced as sufficient sound cannot reach the deeper structures.
- Basically, there is a trade-off between resolution and depth of penetration/visualisation so you must always select the highest possible frequency to give the best image resolution for the required image depth.
- The ability to change the frequency (if this is an option), and how you do it will depend on your machine. On some machines there will be a control called frequency, on others it may be Gen/Pen/Res or High/Medium/Low (HML)
- Your equipment specification will determine if it transmits and receives the sound using multifrequency or broadband transducers (this is the kind of information your applications specialist will be able to help you with).
- Multifrequency transducers allow us to select a range of fundamental stand-alone ultrasound transmission frequencies, e.g., select a frequency of 5MHz for abdominal scans and 10MHz for superficial scans.
- Broadband transducers are a little more complex. They generate ultrasound at a fundamental frequency i.e., the original frequency of the sound wave emitted from the transducer but are programmed to detect or receive echoes from both the fundamental frequency and a harmonic or multiple of this fundamental frequency. The transducer subsequently receives a higher frequency value when compared to solely receiving echoes from the fundamental frequency. The ultrasound computer removes the fundamental frequencies to generate a sharper image – This process is known as tissue harmonic imaging (THI).
Tissue Harmonic Imaging (THI):
- The basic premise of THI is that it will give you a cleaner or ‘sharper’ image with better contrast resolution. It does this by taking advantage of the penetration of the lower frequency and the improved resolution of the higher frequency.
- THI reduces image clutter/scatter and noise (good for imaging larger patients) and by reducing the dynamic range improves the contrast and edge enhancement (great for fluid-filled structures).
- The detected signal is weaker than traditional echo and the pulse lengths are longer so increased power may be required for deeper depths.
- THI is a function that is incorporated into the pre-set of certain examinations but disabling it is a simple thing that can make a big difference to your image. Look for the THI control on your system and practice switching it on and off when scanning fluid-filled and soft tissue structures to see if there is a perceivable benefit.
Transducer footprint:
- The transducer footprint refers to the size of the surface in contact with the patient and determines the resultant beam shape. Matching the transducer footprint to the suitability of the region to be scanned is integral to good image production
- Although a high frequency linear transducer will give good image resolution, if it is too big and cumbersome for accessing small feline structures, it doesn’t matter how good the resolution is, you won’t get good images
- A small micro convex probe would be ideal for intercostal and abdominal scanning because curved or convex transducer footprints will generate a diverging beam shape and field of view, perfect for angling into body cavities
- A linear transducer footprint will generate a straight-sided beam shape and a rectangular shaped field of view, ideal for superficial or near-field structures
- A phased array transducer is used in cardiac scanning because it creates a point source diverging beam, ideal for fitting between rib spaces whilst providing a wide far field to enable the long axis of the heart to be assessed
- Manufacturers have designed small high-frequency hockey-stick linear transducers which are perfect for examining very small structures in detail, such as musculo-skeletal structures and small feline lymph nodes
In essence, for the best quality image you should always select the highest frequency transducer for the tissue depth you want to assess.
If increasing or decreasing the frequency as you scan is not an option or doesn’t improve your image, it is a good idea to change probes mid scan to help with visualisation.
If you have a brain block and can’t remember if high or low frequency equates to long or short wavelength imagine two runners one a sprinter and the other long distance:
The sprinter doesn’t have to travel so far (he is travelling only in the near field) so can afford for his feet to hit the ground at a much higher frequency than the marathon runner who has a much greater distance (depth) to travel!