Ultrasound is a very valuable diagnostic tool, almost an extension of a physical exam, that helps us with non-invasive visualisation of internal structures of the body. It uses very high frequency sound waves, that are generated by the probe and pulsed into the body where they interact with tissues. Returning signals – echoes – are received back by the probe and processed by the computer to generate an image of organs, that is seen on a monitor. We must be aware, that the image we get, does not necessarily represent specific microscopic or macroscopic anatomy and cannot be the only tool to assess the functionality of the organs. Sometimes organs may look almost normal on the ultrasound in the presence of their dysfunction or can appear altered while functioning properly. Ultrasound examinations are very interactive and unlike with other imaging modalities, interpretation should be done at the time of the study. Making meaningful interpretation from recorded images or video clips is very difficult.
To be able to interpret normal images and detect pathological changes, the sonographer must be familiar with normal 3D anatomy, physiology and pathophysiology of the organs, differences between dogs and cats, know the basic principles of ultrasound, its interaction with tissue, capabilities and limitations and recognize generated artifacts.
For abdominal scanning one should develop a standard and systematic protocol, which ensures that no organ or structure is missed. The chosen order of exam is not that important, important is to make it always the same. It is better to always do a complete abdominal examination rather than restricting it to one organ system. Exams should have standard image orientation and recording of findings to make a meaningful study.
Usually, the animal is placed to the sonographer’s right and facing to the left, in dorsal or lateral recumbency. The position can be altered in order to maximize visualisation or use gravity to avoid gas filled intestinal loops and evaluate the possible mobility of structures within fluid (bladder, gallbladder). The probe is held in sonographer’s right hand with its marker facing patient’s head. That way, when scanning the abdomen longitudinally, the reference mark on the screen will be on the left, meaning what is left on the screen is cranial part of the patient, to the right is caudal, on the top is abdominal wall and dorsum on the bottom.
Ultrasound examination requires clipping of the hair coat on ventral abdomen in order to obtain a good quality image. Abdomen should be clipped from the costal arch cranially to the inguinal region caudally and laterally along the body wall. For good visualisation, generous amount of acoustic gel is applied to the skin. If possible, the patient should be fasted and if urinary system is the primary reason for the exam, the patient should not be allowed to urinate before the examination.
For good resolution selection of the highest frequency available, that still penetrates the depth of the patient’s liver is required. 8 – 10 MHz transducers can provide high-quality images up to 8 cm depth, so small dogs and cats can be examined with 7.5 MHz or higher but in large dogs 5 MHz may be necessary. For general scanning focus is set to the deepest level to maximize image quality. When viewing a specific area, the focal point is set to the depth of interest. The depth of the field should be adjusted in a way, that allows visualisation of entire organ of interest, but pay attention that the organ does not cover only small portion of the picture. The image should appear uniform throughout the field. This can be achieved using time-gain compensation controls (TGC), which enable reduction of near-field gain and preservation of far-field gain. Once the image is balanced, overall image brightness can be increased or decreased.
Usually, the exam begins at the liver, as it is the largest abdominal organ, located within the deepest portion of the cranial abdomen. Imaging of it requires most depth of the field, gain and in some large patients even lower frequency than the remaining abdominal structures. As the rest of abdomen is scanned, reduction of gain and depth should be made to optimize the image. Many sonographers choose the order: liver and gallbladder, spleen, stomach, duodenum, pancreas, kidneys, adrenal glands, bladder, prostate, medial iliac lymph nodes, and finish with remaining intestinal tract and additional lymph nodes.
Mattoon J. S., Nyland T. G.: Small Animal Diagnostic Ultrasound, 3rd Ed.