Ultrasonography is an essential tool of the farm animal clinician and has indeed come a long way during the past 27 years. Previously, it often used to be reserved for the senior members of the farm animal team; it was heavy, operated by mains electricity and almost exclusively used for pregnancy diagnosis. Nowadays, it is accessible by all members of the farm animal team; it is battery operated, with interchangeable probes and with a myriad of options to readily integrate with various electronic platforms. More importantly, it is now used not only for pregnancy diagnosis, but also as a diagnostic tool during a routine internal or external examination of the farm animal patient.
The basics of ultrasound
Ultrasound scanners operate on the principle of piezoelectric effect. Piezoelectric derives from the Greek “piezein”, which means to press, and “electron”, which means amber, a material that is renowned for its static electricity properties.
Each scanner probe contains a number of piezo-crystals that convert electrical energy to sound that lies in the ultrasound range. The same crystals are acting not only as a transmitter, but also as a receiver by collecting sound energy and converting it back into electrical energy. The on-board computer measures the difference between the transmitted sounds and their echo, which represents deflection and dispersion, calculating the shape and density of the scanned tissue as a result.
Echogenicity, or the ability of a tissue to create an echo, depends on the density of the tissue that is being examined. On one end of the spectrum we have anechoic materials, such as the amniotic fluid, that return very little echo and are conventionally depicted as black on the image. At the other end of the scale we have hyperechoic materials, such as the foetal skull made mostly of cartilage or the pelvis consisting of bone tissue, that return most of the transmitted sound (Figure 1).
Special care must be taken with ultrasound when scanning through faeces, particularly dry dung, which is made up of a mixture of solids and gas. The routing of ultrasound waves through gas is unlikely to be linear and it becomes unpredictable as sound waves tend to disperse. As a result, this diminishes the piezo-crystals’ ability to receive the echo and the scanner’s on-board computer to calculate the difference between transmission and echo. The outcome is that no image is generated.
The various probes come with different properties. A linear probe transmits ultrasound waves of higher frequency (5MHz to 7.5MHz) and produces images with higher detail. The resulting scanned area is usually the size of a credit card. A curved linear probe transmits ultrasound waves of lower frequency (2.5MHz to 5MHz) and produces images of greater depth. The scanned area is usually the size of our hand.
General principles of pregnancy diagnosis in the bovine
There are four signs of pregnancy that we are interested in during ultrasonographic examinations in cattle: the foetus, the amniotic and allantoic fluid, the amniotic and allantoic membranes and the placentomes. The latter are constructed by the fusion of a cotyledon on the placental side and the caruncle on the uterine side of the embryonic membranes.
Ideally, we need to identify as many signs as possible, or a minimum of two, in order to declare with certainty that the animal is pregnant. Caution should be exercised when basing our diagnosis solely on the presence of amniotic or allantoic fluid, as these are indistinguishable from the clear mucous uterine content of the non-pregnant cow that is in the pro-oestral or the oestral phase of her reproductive cycle (Figure 2).
Estimating the age of a foetus
The size of the bovine foetus at a given time is affected by factors such as breed and nutrition, but the predominant one is the stage of pregnancy. There are a number of measurements and reference tools that we can utilise in order to ascertain the approximate age of the bovine foetus.
A practical way to estimate bovine foetal age is for a grid of known-sized squares to be superimposed over the scanned image that is projected onto our screen or our goggles. Usually, these squares have sides of 1cm or 2cm. A foetal measurement can then be carried out instantly by counting the number of whole, or parts of, superimposed grid squares that a specific foetal measurement can fit in.
The commonest measurements are crown to rump length (CRL), head diameter (HD), trunk diameter (TD), placentome diameter (PD) and eye diameter (ED) (Descôteaux et al., 2010; Figure 3). Their accuracy is dependent on how exactly our image fits to the sagittal, transverse or frontal plane.
Crown to rump length
This measurement refers to the distance between the foetal vertex (crown) and the foetal rump. It is worth remembering that the CRL at around 30 days is approximately 1cm, around 40 days is approximately 2cm, around 50 days is approximately 4cm, around 65 days is approximately 8cm, around 80 days is approximately 12cm and around 100 days is approximately 16cm (Descôteaux et al., 2010).
This measurement represents the distance between the two opposite parietal bones in the foetal skull. It is also referred to as biparietal diameter. This measurement at around 65 days is approximately 2cm, around 80 days is approximately 2.6cm and around 100 days is approximately 4cm (Descôteaux et al., 2010).
This measurement refers to the widest part of the foetal chest, particularly where the diaphragm attaches to the rib cage. This measurement at around 65 days is approximately 2cm, around 80 days is approximately 3cm and around 100 days is approximately 5cm (Descôteaux et al., 2010).
This measurement refers to the widest part of the biggest placentome in a particular image. Placentomes become more relevant in the middle trimester of pregnancy, but are used more often in the last trimester when no other foetal ageing measurements can be utilised. This measurement is incredibly inaccurate, not because the relation between placentome size and foetal age is tentative, but because the biggest of placentomes at a given stage of bovine pregnancy may not be captured by our ultrasound image at that time. This measurement at around six months is approximately 5cm, around seven months is approximately 6cm and around eight months is approximately 7cm. The author hardly ever uses this parameter in practice.
This measurement refers to the widest part of the eye socket of the bovine foetus. It is, if not the most precise measurement, one of the most accurate measurements of foetal age. This measurement at around 90 days is approximately 1cm.
Ultrasonic bovine health explorations
We should not forget, after making such a considerable investment that an ultrasound device is, that ultrasound technology should not be reserved only for internal examinations. Devices with interchangeable probes, where the linear probe can be swapped with the sector probe, or models that have the curved linear probe attached can also be used for external examinations of the patient. This is an essential complement to our clinical examination, with similar complementary benefits to endoscopic examinations. The liver, the kidney and the reticulum, to mention a few, can be scanned externally and relatively easily during routine examinations of sick animals in order to offer a diagnosis and more importantly a prognosis (Figure 4).