ABSTRACT: In diagnostic ultrasonography, high-frequency sound waves generated by a transducer are reflected back from within the patient to produce a two¬dimensional image of internal organs and structures. The image is displayed as a grey-scale, cross-sectional image. Blood flow can be demonstrated either as a grey-scale velocity profile (spectral Doppler) or as a colour map (colour Doppler). Ultrasound transducer construction is adapted to allow evaluation of superficial structures and images subcostally or intercostally. even in small patients. Recognition of the ultrasonographic appearance of normal structures is an important part of the diagnostic process, as disease may present as either focal or diffuse change.

Ultrasonography has been employed in the diagnosis of abdominal and thoracic disease in small animals for almost 40 years. Once the preserve of referral institutions, it is now widely available in general practice (Figures 1a & 1b). This article briefly reviews the principles of diagnostic ultrasound in small animals and focuses on aspects and goals in the clinical setting, especially those of particular significance to – veterinary nurses.

Figures 1a & b: The ultrasound machine on the left (a) is an example of a trolley-based, high-end hospital machine. The machine on the right lb) is a smaller portable machine. 

Rapid advances in transducer technology and digital processing have narrowed the gap between the two types of machine

Basic ultrasound principles

A detailed assessment of the physical principles upon which diagnostic ultrasound is based is beyond the scope of this article. The purpose here is to focus on those factors which are important to understand in the practical clinical setting.

Ultrasound uses high-frequency waves, which are generated by electrical stimulation   of crystals within a transducer. As long as there is good contact between the transducer – achieved by clipping the coat and applying liberal amounts of an ultrasound gel – these waves are able to penetrate Basic through the skin and internal tissues.

Although these ultrasound waves are weakened as they penetrate deeper into the tissues, a proportion of the waves is reflected by the tissues, back to the transducer. This allows the ultrasound computer to generate an image based on the strength of these echoes, and on the time it takes these echoes to return from the patient to the transducer.

It is important to appreciate that ultrasound waves are pressure (or sound waves) and that there is no radiation involved. Although some heating occurs within the tissues – caused by the ‘friction’ of the ultrasound waves – this is usually limited and explains why ultrasound is considered safe and non-invasive.

Ultrasound transducers are of variable construction (Figure 2). This is because there is a trade-off between image quality and the depth to which ultrasound waves penetrate. Transducers which produce fine-detail images can only penetrate several centimetres; whereas transducers which are able to penetrate to great depth (10 – 25cm) produce coarser images. Transducer selection for abdominal ultrasonography based on patient size is summarised in Table 1.

Figure 2: Different types of ultrasound transducer.

Linear transducers (A) are used to examine superficial structures, such as the thyroid or the eye. They are high-frequency transducers which permit the fine detail of superficial structures to be assessed.

Curvilinear or micro-convex transducers (B) have a small surface area or footprint' and are useful for positioning under the ribs or behind the sternum. This type of transducer is appropriate for examining the abdomen. The breadth of the sector produced by this transducer is wide so that as much as possible of the organ being examined can be displayed.

Phased transducers (C) are also sector transducers, used particularly for scanning the heart as the small footprint allows scanning between the ribs

Most organs and structures have a characteristic appearance on ultrasound (Figure 3); though age, body condition (obesity) and breed may have an effect on this typical appearance. Disease is recognised when there is change to the normal anatomical organisation or where there is a change in the brightness (echogenicity) or texture (echotexture) of the structure.

Focal changes, such as nodules or masses (Figure 4A), are easier to recognise than diffuse disease changes that are present throughout the organ (Figure 4B). Some diseased structures may show no change on ultrasound.

Figure 3: Stomach (A), kidney (B), liver (C) and spleen (D).

Normal features may relate to: shape |A – the stomach (arrows! of a cat resembles a wagon wheel'; internal anatomy 1B – for example, recognising the cortex and medulla of a cat s kidney); echotexture (C – the liver has a characteristic coarse speckled appearance; and echogenicity ID – the spleen is normally brighter lechogenic) than the left kidney cortex)

Figure 4: The effect of disease on ultrasonic images. Focal disease (A), liver. A large mass (arrowheads) occupies most of one of the liver lobes. The mass is hypoechoic' or darker in appearance to normal liver, a small segment of which can be seen adjacent to the mass (white arrow). The mass has a similar appearance throughout and is, therefore, termed homogeneous'.

Diffuse disease (B), liver. The liver lobe (arrowheads) is also more hypoechoic' or darker than normal, but all of the liver is affected. Normal structures, such as the portal vessels (*), which carry blood into the liver, are more conspicuous and the normal 'speckle' pattern is also more apparent. Compare this liver to Figure 3C. Diffuse patterns of disease can be difficult to recognise untess settings are optimal and the patient co-operative

In addition to identifying and characterising changes within tissue
s and organs, ultrasound can be used to evaluate blood flow using Doppler ultrasound (Figure 5A – C). There are two main types of Doppler: spectral Doppler which is used to measure the direction and velocity of blood flow, particularly within the heart; and colour Doppler in which the direction of blood flow is characterised as red or blue, depending on the direction of flow or as turbulent flow. Colour Doppler is frequently used in examination of the heart (echocardiography) and abdomen.

Figure 5: Doppler studies that display information relating to blood flow speed (velocity) and direction. In (A) the blood flow is displayed as a trace and the maximum speed (arrow) can be accurately measured. This is called a spectral Doppler study and is particularly useful in echocardiographic (heart) studies. In this case the flow is through a heart defect (ventricular septal defect).

Images (B) and (C) are both colour Doppler studies in which the direction and velocity are also shown as the colours red or blue or green superimposed on the picture. The convention is that blood which flows towards the top of the image is coded as red and flow away as blue (this can be remembered as BART – Blue Away, Red Towards) and does not have anything to do with arterial or venous flow.

In the image (B) of the liver, flow from the intestine into the liver appears red (arrowhead) and flow draining away from the liver is blue (arrow). In (C) some of the flow appears green and this indicates turbulent flow (arrows). Turbulent flow is seen with some heart defects, or narrowed heart valves and narrowed vessels in the abdomen. In this case the puppy has a heart defect, patent ductus arteriosus, and where blood from the aorta flows back into the pulmonary artery it is turbulent and appears green

Author

Andrew Holloway BVSc CertSAM DVDI DipECVDI MRCVS

Andrew Holloway qualified from the University of Pretoria in 1992. He spent eight years in general practice before joining the University of Cambridge as a visiting resident in radiology.

In 2006. Andrew moved to the Animal Health Trust as clinical radiologist. He holds the RCVS and European Diplomas in Veterinary Diagnostic Imaging.

To cite this article use either

DOI: 10.1111/j.2045-0648.2012.00145.x or Veterinary Nursing Journal Vol 27 pp 46-48 

• VOL 27 • February 2012 • Veterinary Nursing Journal