Key terms are highlighted in italics.
Ultrasound imaging uses sound waves to form an image. The sound waves are produced inside the ultrasound transducer, by applying a current across the face of an arrangement of ceramic crystals, causing the crystals to vibrate; this in turn produces a pressure wave. When the pressure wave is applied to soft tissue via a couplant (ultrasound gel), the sound wave can penetrate the organs of the animal.
The crystals also act as receivers: when the initial compression wave hits the surface of a more dense structure, some of the sound is reflected or refracted. It is bounced away from the new surface and some of it will return to hit the crystals. The crystals have the ability to turn the returning pressure waves into electric currents, which are then displayed as a range of grays on the screen. This ability of the crystals to produce a compression wave when an electrical current is applied, and convert a returning wave back into an electrical current, is called the piezoelectric effect.
These crystals are the most expensive, delicate part of your transducer. Great care is required when the transducer is not in use.
Ultrasound Transducers
Electrical Linear Array, Curved Array
The crystals that produce the compression wave are arranged in a linear fashion and are fired sequentially in groups. This serves to minimise the diversion of the ultrasound beam.
For both these probe designs, there is plenty of useful information in the near field of the image – i.e. the top section of the image – close to the skin surface.
Mechanical Sector
These transducers contain fewer ceramic crystals which are mechanically rotated within an oil filled housing; the mechanical vibrations can be felt whilst using the equipment. The image produced by this design of probe is pizza shape, demonstrating minimal information in the near field.
Production and journey of an ultrasound wave
Above: An utrasound wave produced by the ceramic crystals is seen entering the tissue surface via a couplant. The returning echo is converted to a digital signal displayed on the monitor. The attenuated wave is shown as not returning to the transducer at all.
Properties of a compression wave (ultrasound wave)
As the ceramic crystals vibrate, they change their dimensions, becoming thicker and thinner, thicker and thinner, producing an oscillation – a compression wave.
This pattern of pressure oscillations over time can also be represented as a sine wave:
Clinical ultrasound typically uses frequencies 1.5MHz – 12MHz, sometimes higher. In order for the vibrations to travel, a medium (e.g. soft tissue) is needed. The vibrations of the sound waves cause the particles of the medium near to the wave to also vibrate. This is how the energy is transferred – in the direction that the wave is travelling. This makes sound a longitudinal wave.
The frequency of the ultrasound wave determines the penetration of the ultrasound wave and the image resolution.
With this in mind, a 7MHz linear probe is ideal for transrectal imaging of the bovine reproductive system: the probe is introduced in to the bowel cavity via the rectum. The colon is directly adjacent to the uterine body and uterine horns. Very little sound penetration is therefore required, and the image resolution is excellent.
Using the same probe for scanning a bulldog transabdominally, minimal sound will penetrate to the depth of the uterus, and the majority of the sound would be attenuated by the fatty tissues and bowel gas anterior to the uterus; hence, the penetration is poor. For this reason a lower frequency is required e.g. 3.5 MHz curved linear probe, providing the penetration required. The
payoff is less detail (poorer image resolution).
In other words, it is necessary to use the right equipment for the right job.
Attenuation
As the ultrasound passes through the tissues of the animal, the amplitude of the wave reduces (the height of the wave in the previous diagram). The further from the source the wave travels, the more reduced the amplitude becomes. This is known as attenuation. Different substances attenuate the beam more or less depending on their density (how closely together particles are arranged in a material) and stiffness (the elasticity of the material).
From the table above we can see that sound travels fastest through bone, but is attenuated due to bone’s high density.
Sound travels fast through fat and muscle, the wave losing little of its energy as it travels deeper.
Sound travels slowly through air/gas, losing its energy very quickly. Note that the lungs are full of many pockets of air!
Ultrasound and image production
So far we have covered transducers, how ultrasound is made and the properties of the ultrasound wave. Now we can look at how the beam of ultrasound behaves when we apply it to the area of interest and image production.
The image we see is composed of many reflections – echoes received back from the probe. Some of these are useful and others are not (noise and artifact).
An ultrasound wave will travel in a straight line through tissues until it hits a medium of a different density, for example, travelling through muscle and into fat. We know that sound travels through these two mediums at similar speeds because they have similar attenuation coefficients, so at the borders of these two tissues, there will be very little sound reflected back to the probe and the wave continues on its journey. However, if the ultrasound wave then hits bone, there is a great difference in the
acoustic impedance – the bone is very dense compared to fat and muscle – and there will be a big reflection back to the probe (this is why bone shows up as bright white on an ultrasound image). The ultrasound energy that remains travelling forward is greatly reduced, hence information beyond the bone surface is very poor.
If the ultrasound wave traveling through the tissue then hits gas in the bowel, there is a massive difference in the acoustic impedance, hence – like bone – the sound will be bounced back to the transducer. Imaging beyond the gas is minimal.
Couplant
Having established that the sound from the transducer travelling through air and hitting a skin surface will result in a lot of reflection at the skin surface because of the difference in density between air and soft tissue, a couplant is needed. This is a water-based gel, between the transducer surface and the skin surface – excluding air from the connection between the two surfaces.
Scanning animals transabdominally, gel is applied liberally to the skin surface to displace the air at the base of the hair of the animal’s coat. If this cannot be achieved then clipping the coat is required.
Transrectal scanning requires the use of a lubricant to pass the gloved hand of the operator and the transducer into the rectum. Once within the rectum, the moisture in the cavity is enough to exclude any gas/air from between the mucosal surface and the transducer, providing there is good contact with the surface of the bowel.
Ultrasound waves and fluid filled structures
Ultrasound passes easily through fluid filled structures – for example, the amniotic fluid surrounding a fetus, or a full bladder. The fluid itself returns no echoes to the transducer, hence the fluid is echo free and appears black on the screen.
Ultrasound is highly reflected by bone, cartilage or gas. The results are a bright image on the screen, beyond which is black. This is because very little sound is transmitted through the bone or gas.
Above: Amniotic fluid surrounding an advanced bovine gestation. The ribs are more dense and hence reflect the sound, the acoustic shadow (absence of sound transmitted) can be seen as dark gray stripes passing vertically at the rib ends.
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