Principles of Ultrasonography and its Applications in Animals

Ultrasonography (USG) has evolved as a very important non-invasive tool in veterinary practice and an alternative diagnostic aid.  Ultrasound (US) is a sound of frequency above that perceived by the human ear. Normal human ear can hear about 20 to 20000 hertz, ultra sound lies in 1-10 Mega hertz region.

Normally a sound waves travel in a pulse and when it is reflected back it becomes an echo. Based on the pulse echo principle, it is used for ultrasound imaging.

A pulse is generated by one or more piezo electric crystals in the transducer (scan head). When this crystal is stimulated electrically, it changes its shape and produces sound waves of a particular frequency.  The frequency of a transducer determined by the times the crystal expands and contracts per second.

 The pulse travels through the body until it reaches a reflecting surface, at which time a portion of the ultrasound pulse (the echo) is reflected back towards the transducer.

Read > Parts and mechanism of ultrasonography machine

The transducer emits pulse less than 0.1 % of the time, allowing it to spend the remaining time more than 99.9% for listening the returning echoes.  The time needed for the pulse to travel is directly proportional to the distance it has travelled and this allows the determination of the reflecting surface position, which will be displayed as a dot on a ultrasound screen at the appropriate depth.  The brightness of the dot is determined by the amount of ultrasound pulse that is reflected. As ultrasound travels through tissue, it becomes weaker; this phenomenon is called as attenuation. Like audible sound, ultrasound cannot be propagated in a vacuum and in gas, the transmission is poor and so a coupling agent is needed to bridge the small gap between the transducer space and the patient.  This agent is a coupling gel applied to the contact area of the patient. Since hairs traps air, it is important to clip the hairy parts of the patient and shave closely prior to applying gel.

Applications of ultrasonography in Animals

1. Ultrasonography has the advantage of being safe, non-invasive procedure that can be performed quickly and easily with proper knowledge of the equipment.
2. Ultrasonography is used as a complementary method after clinical examination; laboratory examination and radiography have been performed.
3. Ultrasonography is used as primary diagnostic technique in pregnancy diagnosis.
4. Ultrasound is often closest thing, one can offer to exploratory surgery.
5. With the use of appropriate equipment and training, almost any soft tissue structures can be viewed.
6. Ultrasonography is commonly used for examination of plural and peritoneal effusions, prostatic disease, urinary tract diseases, Cystocentesis, cardiac disease, Mass lesions and ultrasonic guided aspiration and biopsy and pregnancy diagnosis and evaluation of female genital tract.
7. Doppler technique allows measuring of velocity and flow pattern of blood within the heart and vessels. Transducer is displaced after processing by the computer either in A or B or M mode.

Interpretation of ultrasonography radiographs in animals

1. The structures are evaluated on the basis of their position, size, shape and Echogenicity. Unlike radiography, ultrasound images must be interpreted as they are acquired and hard copies are made only for further documentation.
2. Hyper-echoic/Echogenic: Brighter echoes and appear in white on conventional scans. for example- Bone and air.
3. Hypoechoic: Grey images or dark screen. eg. soft tissues (moderate reflection)
4. Anechoic / Echolucent: Absence of echo, the image is black.  E.g. fluids.  There is often a normal bright area immediately deeper to fluid, this phenomenon is called Acoustic enhancement. Similarly bone or gas or mineral deposits reflect sound waves totally and image seen is bright with no visible structure beneath it. This phenomenon is called Acoustic shadowing. This helps in detection of urinary or biliary stones. Soft tissues present an image of mixed shades of grey depending upon their proportion of fat, fibrous tissue and fluid.
5. Artifacts: Interaction between sound waves and tissues there are no true representation of the underlying structure called artifacts. 

Common artifacts in ultrasonography radiographs

1. Acoustic shadow
2. Distant enhancement
3. Refraction
4. Reverberation: largest source of artifacts that are not real. The echo bounces back to the transducer and is again reflected through the patient and back to transducer. This process of echo bouncing back and forth between the two interfaces is known as reverberation
5. Mirror image
6. Comet tail: air-fluid interface

Cardiac Ultrasonography (Echocardiography)

Cardiac Ultrasonography or Echocardiography is a useful non-invasive tool for imaging the heart and surrounding structures. It is used to evaluate chamber size, wall thickness, wall motion, valve configuration and motion and the movement of the great vessel.

Doppler image allows evaluation of blood flow patterns and velocity, ‘M’ mode two dimensional (2D real time).