The success in the practice of any system of medicine (modern human medicine /veterinary medicine, traditional medicine, Ayurveda, or homeopathy etc.) rests on timely exact diagnosis and rational therapeutic approach. Clinical examination of the patient, laboratory investigations, electrocardiography, and imaging modalities (radiography, ultrasonography including echocardiography, magnetic resonance imaging, computerized tomography etc.), are the pillars of diagnosis. None of these diagnostic modality is perfect in isolation and has its own advantages and fall outs.

During recent years ultrasonography has become an indispensable diagnostic modality in human and veterinary medicine because of its non-invasive nature, providing visualization of internal organs, and quick results. Birth of ultrasound took place in 1794 when an Italian biologist Lazzaro Spallanzani discovered that bats used sound waves for navigation. Since then continuous developments and advancements in the technology have made the sonography a dependable and an easily accessible diagnostic modality.

Ultrasonography or sonography is a diagnostic medical procedure that uses echoes of ultrasound pulses to produce images, of the objects or of the areas of different densities in the body, on the screen. This diagnostic procedure is performed through ultrasound machine. Sonogram is a visual image produced from ultrasonography.

Doppler ultrasound is a non-invasive diagnostic tool being employed in evaluating blood flow in major vessels, veins and arteries. Information regarding perfusion in an organ or an area can also be obtained through Doppler ultrasonography. Doppler ultrasound is an important part of echocardiographic examination in dogs and cats. Its more recent application is in the investigation of the tissue wall motion while evaluating the heart. B-mode ultrasound is used to examine cardiac structures.

Ultrasound is an excellent diagnostic modality for detecting visceral abnormalities. Detection of masses, nodules or effusions through sonography provides non-specific diagnosis. For specific/definite diagnosis tissue samples are necessary for histopathological examination. The techniques of ultrasound guided fine needle aspiration and core biopsy are of great help in establishing etiological diagnosis.

Liver is situated in cranial abdomen. Scanning of abdomen is started with the examination of liver. Hepatic ultrasonography is a non-invasive diagnostic modality to evaluate liver parenchyma. Differentiation of focal disease from diffuse disease, cystic lesions from solid masses, and obstructive jaundice from non-obstructive jaundice is possible by hepatic ultrasonography.

Spleen is an elongated solid organ located on the left side along cranial and ventral abdominal wall. The size of spleen is variable in dogs. The spleen is larger in German Shepherds and Greyhounds. In cats the spleen is smaller and has fixed size. It is parallel to greater curvature of stomach within greater omentum. The position of spleen in dogs is variable as it is only attached by gastrosplenic ligament.

Ultrasonographic examination of gastrointestinal tract is not very much rewarding in dogs because of gas in the tract but it is crucial for a correct diagnosis. Imaging of stomach plays a significant role in the diagnosis of gastric diseases such as gastric dilatation, gastric foreign bodies, pyloric outflow obstruction, or gastric tumours.

Ultrasound and survey radiography are two important diagnostic modalities regularly employed in the diagnosis of intestinal diseases. Both modalities are complementary to each other in investigating the problems related to intestines. It is always better to have plain survey radiograph of abdomen first and then ultrasound to rule out obstruction or radio-opaque foreign bodies. Wherever barium meal radiograph is indicated, it should be done after sonographic examination as barium may degrade image quality of the sonogram. Assessment of intestinal motility can be done in real time sonography.

The pancreas is a thin and elongated ‘V’ shaped organ having two lobes (right and left) and body. It lies in the right cranial quadrant of the abdomen. It is difficult to visualize normal pancreas sonographically because of its echogenicity similar to that of adjacent mesentery and fat.

The urinary tract is the body’s drainage system. It includes kidneys, ureters, urinary bladder and urethra. Ultrasound is a most preferred imaging tool for the diagnosis of diseases of urinary tract not only in humans but also in dogs and cats. Little adipose tissue in urinary system of dogs makes the contrast between different planes and anatomical structures poor.

Application of ultrasonography has become a routine practice in canine and feline medicine to detect pregnancy, to ascertain foetal wellbeing, to ascertain live/dead status of the foetus, ovarian dynamics, and to diagnose reproductive ailments (uterine, ovarian,) in females. Sonography is also being preferred in males for the diagnosis of diseases of prostate gland, cryptorchid, and testicular diseases.

Ultrasound has been used to image thyroid and parathyroid glands. It can assist in the diagnosis of hyperthyroidism and hyperparathyroidism in dogs and cats. Recently it has also been used to differentiate cases of primary hypothyroidism or euthyroid syndrome.

Ophthalmic or ocular ultrasound can provide important diagnostic information that is readily not available with other diagnostic techniques. Ocular ultrasound is emerging as an important diagnostic modality in veterinary ophthalmology.

Sonography of the nervous system in dogs dates back to 1972 when it was employed in experimental studies. Thick bony cage of the brain limits the sonographic evaluation of the brain. It has an important place in human paediatric medicine. Sonography of the brain and the nervous system is not very common in veterinary medicine.

Imaging of muscles, tendons, joints and bones is not common practice in canine and feline medicine. However, scanning of tendons and ligaments is extensively done in race horses to see their fitness. Adhesions, ruptures/ tears and inflammation of tendons and ligaments can be detected by sonographic imaging of these structures. These structures are relatively superficial so a high frequency linear or curvilinear transducer (7.5 to 13.0 MHz) is used to scan these structures. For scanning of joints acoustic window is relatively small so micro convex or sector probes are more appropriate.

Cardiac ultrasound is also known as echocardiography and it has a very important place in the diagnosis of cardiac diseases of the humans as well as animals. Echocardiography is an imaging of the heart using ultrasound. The diagnosis of congenital heart diseases, shunts, complex malformations, valvular heart diseases, different cardiomyopathies, endocarditis, pericardial effusion, cor pulmonale and cardiac tumours has been possible by cardiac ultra ultrasound that was otherwise difficult.

Beside heart, ultrasound of the thorax facilitates the imaging of lungs, pleura, mediastinum, chest wall and diaphragm. Sonographic examination of the heart is discussed in chapter16. Because of nearly total reflection of sound waves at gas interfaces (Fig. 176) thoracic ultrasound has limitations. Pulmonary and mediastinal lesions located deep are difficult to image. Peripheral pulmonary lesions and pleural fluid can be visualized.

Abdominal organs in different species of animals have distinctive features with respect to their shape, size, location and echogenicity and echo texture. This is true for dogs and cats also. Therefore, information with respect to sonographic features of dogs cannot be taken for granted for cats in totality

Focused assessment with sonography for trauma (abdominal and thoracic) refers to the limited ultrasound examination with the main focus on quick identification of the presence of free fluid within peritoneal, pleural and pericardial spaces in trauma cases in intensive care units. Despite limitations, findings of FAST are helpful in stabilizing the emergency patients.

Architecture of the organ is affected in the primary or secondary diseases of the organs. Ultrasonography plays a significant role in detecting the changes in the organs in various diseases.

For the sake of convenience ruminants are broadly divided into two categories as large ruminants and small ruminants. Large ruminants consist of cattle and buffaloes and small ruminants consist of goats and sheep. Sonography is becoming an important part of diagnostic modalities in ruminants also. Its major use in ruminants is in reproduction and lameness.

During last two decades significant advances have been made in diagnostic ultrasound .These advances have positively impacted on diagnostic imaging across all species of animals and have made ultrasound as an indispensable diagnostic modality in veterinary medicine. Since equines have been more a sport and pack animals, the ultrasound technique has long been utilized in the diagnosis of equine musculo-skeletal disorders. Recent years have witnessed its utility in the management of equine reproduction beside its use in the diagnosis of colic and other diseases.

Ultrasonography is being routinely practiced in canine and feline medicine for the diagnosis of diseases of the organs and also to confirm pregnancy. Its routine use in ruminant medicine is mainly restricted to pregnancy diagnosis. Recently the potential of ultrasonography is also being explored in zoo and wild animals as well as in exotic pets. Information available in companion and other domestic animals cannot be applied blindly to zoo and wild animals.

Utility of ultrasound has extended from diagnostic realm to alternate therapies. Therapeutic ultrasound is also known as ultrasound physiotherapy. It is a commonly used modality for the rehabilitation of the soft tissues of the musculoskeletal system.

Sonograms of animals suffering from different diseases are presented along with other investigations as case studies to provide a comprehensive picture of the disease.