Appropriate Probe selection

Frequency

To appreciate why we choose a particular transducer for a specific task or patient conformation you wish 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 of the tissue 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, 106 cycles per second is 1 megahertz (MHz) which is the typical unit for the frequencies used 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, it is clear from the equation that if the frequency increases, the wavelength has to decrease and vice versa to balance the equation.

                                           C (↔) =  f↑↓ x λ↑↓

• 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 with the image quality and detail being 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 structure (smaller wavelengths are attenuated more than larger ones!).

• 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 ultrasound system. 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 also 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).

Transducer footprint:

• The transducer footprint size and surface shape determines the resultant beam shape and refers to the size of the surface which comes into contact with the patient. 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 the footprint is too big and cumbersome for enabling good skin contact and needle access on smaller patients, then you may not get good images, despite the higher frequency. You may need to compromise with a smaller footprint probe such as a hockey-stick (small high frequency linear array probe) or a micro-convex probe.

• 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 (locoregional anaesthesia scans).

• A small micro convex probe is more suited to deeper locoregional sites or when working with patients of high adiposity / muscularity (dense and attenuating to on-coming ultrasound, subsequently degrading the image resolution). In this case, the divergent beam, wider field of view and lower frequency will work to provide better visualisation of these deeper structures. These probes are more frequently suited to  intercostal and abdominal scanning for this very reason

• 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. It is unlikely to be a first choice for locoregional anaesthesia.

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 see if this can 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 (better image resolution) than the marathon runner who has a much greater distance (depth) to travel (poorer image resolution) and can take slower, longer strides!

Correct Image Orientation

• All transducers have an orientation marker which represents the side of the probe from which elements are ‘fired’ first in any given frame to form the image. This marker is usually both palpable and visible and is a dot or raised line at one the end of the probe

• There will also be a visible mark on the ultrasound screen which correlates with the marker on the transducer (assuming probe orientation is correct).

• The convention for general scanning (the opposite of cardiac scanning!) is that the orientation mark corresponds to the left-hand side of the screen when scanning in a long (sagittal plane) axis i.e. towards the cranial aspect of the patient. So when scanning in a long axis and holding the probe the right way round means that anything to the left of the ultrasound image on screen will be more cranially positioned than anything on the right hand side of the image as the probe moves.

• A great deal of locoregional scanning requires scanning a short axis section of the neurovascular bundle (transverse plane), it is essential that the transducer is rotated correctly, (ie, anti-clockwise) through 90 degrees – meaning that the orientation marker is turned towards you, the operator. Never turn the probe in a clockwise direction. Aside from the risk of incorrect orientation to the patient, you will find this more uncomfortable and can cause damage to your wrist and arm.

• The convention in the transverse plane is that the left-hand side of the image/screen corresponds to the aspect of the patient situated to the right (i.e. most adjacent to the operator) and the right side of the screen/image represents structures sited further to the left (away from the operator).

• Correct orientation is important to establish before introducing a needle into the skin – the needle needs to appear at the corresponding aspect of the screen as it is inserted, thus avoiding confusion when guiding the needle with precision towards the desired target site.

• Practice moving the probe in a long and short axis section on a patient or pet to see the impact incorrect probe orientation has on the screen as the probe is moved!