Physics and Knobology

A basic understanding of the physics underpinning ultrasound is crucial for undertaking a formal exam in echo and also (more importantly!) be able to obtain quality images when undertaking a scan. Some of the pitfalls leading to interpretation errors stem from a poor understanding of how optimising depth, frame rate, gain etc… can influence overall quality. All artefacts on the echo image can be explained by the physical principles of ultrasound and so a good understanding is a necessity!

Recent technological advances in acquiring 2D and 3D echo images alongside improvements in processing power and the miniaturisation of probe technology have improved image quality dramatically giving the user the ability to identify even subtle abnormalities. New(ish) modalities such as tissue doppler imaging, speckle tracking and 3D reconstruction have allowed comprehensive assessment as well as opening new possibilities for interventional procedures such as Transcatheter edge-to-edge repair of the mitral valve.

Piezoelectric Effect

Ultrasonic sound waves are generated by the probe via the piezoelectric properties of certain ceramic materials located within the probe itself. This effect was originally described in 1880 when quartz was used. Piezo-electric materials deform mechanically when an electrical current is applied to them. This deformation can be controlled via an applied electrical current to oscillate at a resonant frequency generating a sound wave.

High frequency sound waves exist in the “ultrasonic” range and for most probes this is in the 5MHz range. The machine can alter periods of compression and rarefaction within the ultrasound wave. The overall pattern of oscillation is in parallel to the direction of the beam (unlike waves in the sea which are transverse to the direction of travel) This concept can be confusing as many diagrams (including the one below) show sound waves represented in a transverse fashion which is technically incorrect.

The Ultrasound Wave

Sound waves are emitted in a longitudinal manner where oscillations of compression and rarefaction progress in parallel to the wave direction  (unlike ripples on a pond which oscillate at right angles).

Once emitted from the transducer the ultrasound wave embarks on a journey first through the “matching layer” on the ultrasound transducer, next through the ultrasonic gel medium and next through any tissues, bony structures etc... that it may encounter within the patient. On it’s journey the ultrasound wave can undergo one of the following:

1. Reflection
2. Refraction
3. Scattering - this determines the “texture” of a given structure in the ultrasound image.
4. Absorption - energy lost as heat

Reflection

Reflected sound waves are interpreted and processed by the ultrasound probe to create the ultrasound image. The probe is able to detect how long the wave has taken to “return to base” and cause a further mechanical deflection of the piezoelectric crystal resulting in a voltage. Voltage signals are then processed by the echo machine to determine the depth at which the reflection occurred (derived from its time taken to return) as well as the amplitude of the reflection (how “strong” the reflection is).

Highly reflective structures tend to be those containing calcium such as bone or calcified vessels; these structures appear a bright white on the ultrasound image. Tissue reflects some of the ultrasound wave energy but also absorbs and scatters some resulting in a grey (of varying shade) Air-filled structures do not reflect any ultrasound waves and appear black.

The average propagation speed of ultrasound waves in soft tissue is 1540m/sec. This has practical relevance as increasing the frequency of the probe, say to 10MHz improves axial resolution by halving the wavelength of the beam, this is at a cost of decreased penetration. This explains why intracoronary ultrasound used in “IVUS” (which only has to travel a few millimetres from the source)  operates at 20Mhz or higher to improve axial resolution. By the same token “low frequency” ultrasound probes are used to image deeper abdominal structures.

Reflection of ultrasound signal occurs best when structures of interest are “in-line” with the ultrasound beam and reflection occurs back towards the probe. Structures at the peripheries of the echo sector scan window are often poorly visualised, the sector scan should therefore be narrowed as far as possible (within reason so that the echo operator can maintain orientation) whilst trying to centralise the object of interest.

Refraction

As the ultrasound beam moves between tissue planes of differing densities the incident angle of the beam will also change, this is known as “Snell’s Law”. The two major factors affecting this change in angle are the incident angle (angle upon entering the tissue plane interface) and the change in velocity of the sound wave between the two tissue mediums which in turn relates to differences in density between the two tissues.

Frequency

Frequency is what determines the “pitch” of the sound i.e. high frequency sounds tend to a have a high pitch. Ultrasonic waves are inaudible to human ears but the same principle applies. Frequency is measured in “cycles per second” which are known as Hertz (Hz) with 1Hz being 1 cycle per second. It is worth noting that as the frequency increases (i.e. number of peaks per unit time increases) the time period decreases.

Using a surfing analogy the frequency will be the number of waves that pass underneath you on your surfboard in a given period of time. Most transthoraic ultrasound probes operate in the 2.5Mhz range with intravascular ultrasound (IVUS) used by cardiology in the cardiac catheterisation laboratory in the 20MHz range as the depth of penetration required is so low; only the thickness of the arterial wall.

Time period is the time taken for two successive reference points on the wave to pass an arbitrary but fixed point in space. Frequency is therefore the inverse of the time period of a wave.

Time Period

This is the time taken for two identical locations on a given wave to pass an arbitrary position in space. It is the reciprocal of frequency and is described in units of time (e.g. milliseconds) Using a surfing analogy it can be thought of as the time you have to wait on your surfboard until the next wave comes along (to pummel you onto the seabed!)

Bandwidth

Bandwidth refers to the range of frequencies that the probe is able to transmit and interpret. Various filters can be placed on bandwidths to be analysed, for instance focussing on only “high velocities” when examining turbulent blood flow or “low velocities” when using tissue doppler analysis and filtering out frequencies outside of this range.

Wavelength

At any fixed time the wavelength is the distance between two identical corresponding points on successive waves i.e. the distance between two peaks. This is measured in units of distance e.g. micrometers. Frequency and wavelength are related by the speed of sound within the medium. In tissue of “average” density this is 1540m/sec. Using a final surfing analogy this is the physical distance from the wave you are currently duck diving underneath and the next one in the set appearing behind it about to hit you on the head. Waves with a longer time period will therefore have a longer wavelength.

Propogation speed through biological tissues is quite uniform only varying slightly with different tissue types. The main  factor determining acoustic impedance to the ultrasound wave becomes the density of tissue that it is travelling through. The wavelength for a standard 5Mhz ultrasound probe is approximately 0.3mm

Anatomy of the Ultrasound Probe

The echo probe itself contains four main components which are required to generate an image for interpretation. These components are all fixed at the point of manufacture but remain a popular topic for exam questions to show candidates understand the principles underlying image acquisition.

1. Backing material

Provides a damping effect to oscillations created by the piezoelectric crystals. This prevents the crystals from “ringing” like a fingertip rubbing over a wine glass. This effect serves to increase the bandwidth of frequencies that the probe can emit which improves axial resolution for shallower structures of interest. The enhanced damping provided by the backing material serves to decrease the “Q-Factor” (see below)

2. Piezoelectric crystal array

These are ceramic crystals which exert unique electromechanical  properties conferring them the ability to transduce electrical into mechanical energy. The piezoelectric effect is derived from the Greek “Piezen” meaning “to squeeze” referring to the mechanical contraction and expansion of the crystal that occurs when an electrical current is applied.

This unique property results in oscillation at a certain (ultrasonic) resonant frequency when a given electrical current is applied to the crystal (and vice versa). This resonant frequency is determined by the crystal material as well as the thickness of the piezoelectric crystal layer. The crystals can be “stimulated” with an electrical current resulting in the generation of an ultrasonic wave. They can also “detect” ultrasonic waves reflected from the tissues creating mechanical deformation resulting in a small electrical current which can be detected by the echo machine.

Quartz (silicon dioxide) is the crystal classically described as having piezoelectric properties and zinc oxide was used successfully in early “crystal radios” which work by harnessing the piezoelectric effect. Most ultrasound probes use lead zirconate for the piezoelectric crystal layer.

3. Matching Layer

This layer serves to optimise the transmission efficiency of ultrasound waves into the tissues as well as facilitating their optimal transfer back into the piezoelectric detector crystals. There is a large difference in impedance from the ultrasound source (33MRayls) to the tissues (1-3MRayls).  The overall purpose of the matching layer is to optimise energy transfer into the tissues by gradually decreasing impedance in a stepwise and controlled manner.

This layer results in an overall energy transfer of around 90%, much of which would otherwise have been lost in a large step down in impedance from the piezoelectric crystals to the tissues without it. Optimal thickness of the matching layer is 1/4 of the operating frequency of the transducer elements, this is extremely thin and is one of the more challenging parts of the probe manufacturing process. Multiple matching layers may be used in some transducers. Thicker matching layers are cheaper but increase attenuation and therefore result in poor signal quality.

4. Acoustic Lens

This serves to attenuate beam width as well as focussing depth. The lens is made from a specialised rubber which has low impedance and matches well with body tissue velocities (around 1000m/sec) It serves to focus the ultrasound beam and improve image quality by reducing scatter.

The above information outlines the components of the simplest possible form of ultrasound “probe” which only allows imaging along one individual scan line. Modern probes are organised into an “array” of piezoelectric crystals which work synchronously to create a real-time image across a “sector” of a given width. TOE machines use a “phased array” which refers to the way in which adjacent groups of crystals are “activated” after very short time delays. Time delays allow for angulation of the beam away from the perpendicular face of the probe improving the width which can be scanned.

3D-capable TOE probes utilise a “matrix array” of crystal units within the probe tip allowing for a pyramidal sector with depth, width and also elevational width characteristics, all of which can be adjusted and optimised.

Ultrasound Beam “Physiology” and Characteristics

Constructing an image for interpretation requires a pulse of ultrasound to make a “round trip” from the transducer, to reflect off of the object of interest and then return back to the transducer for analysis. In order to avoid range ambiguity (not knowing how far away a given  signal is) this whole round trip process must be completed before the next cycle can start. This round trip takes longer if objects of interest are further away from the probe and this is one reason why deeper structures are often poorly visualised using ultrasound.