600
| Although enhanced sonographic visibility of other organs after
fasting has not been critically assessed as of the writing of this re- view, fasting may also improve imaging access to other organs, such as the liver. Circumstances obviously prevent a pre- examination fast in acute colic, but all horses undergoing scheduled full abdominal sonography should be fasted (with free access to water) for at least 12 h prior to the examination. If ultrasonography and gastroscopy are being performed at the same visit, ultrasonography would ideally be performed first to avoid the potential interference from insuf- flated gas in the stomach and small intestine. The instillation of fluid did not have the effect of enhancing small
intestine visibility in one investigation (Norman et al., 2010 ), but it does change the appearance of the stomach in ways that may be diagnostically advantageous (Epstein & Hall, 2022 ). Instillation of 6 litres of water via nasogastric tube increased the number of inter- costal spaces in which the stomach was visible, making more of the stomach visible for evaluation (Epstein & Hall, 2022 ). The author has also used nasogastric delivery of intragastric water for contrast to enhance the appearance of gastric wall lesions. The astounding advances in imaging technology have opened
up a variety of options to the equine practitioner. Portable, laptop configuration ultrasound units are common in practice and pro- vide high- quality images and a range of software options. Recently, hand- held or ‘pocket’ devices designed to work with a cell phone or tablet interface have been validated for point- of- care use in horses (Bevevino et al., 2023 ; Deacon et al., 2021 ; Eberhardt & Schwarzwald, 2022 ). These devices offer several advantages for point- of- care evaluations, including low cost, independence from an electricity source and simplicity (Deacon et al., 2021 ). In order to perform a comprehensive abdominal examination, a
low- frequency transducer of either curvilinear or sector configura- tion is ideal. The advantage of these configurations is the pie- shaped image, which provides a wide image window through a relatively small
transcutaneous footprint. Linear configuration transduc-
ers (e.g. ‘tendon’ or ‘rectal’ transducers) are high frequency, which provides greater tissue detail. These are appropriate for thoracic evaluation and can be used to obtain limited information about the abdominal cavity but cannot penetrate the deeper tissues of the abdomen (Haardt et al., 2022 ). Because the pleural surface is rela- tively close to the body surface, higher frequency transducers can be very useful for thoracic evaluation, but a curvilinear configuration remains the preferred choice due to the increased width of anatomy that can be evaluated at one time (Figure 2 ). A linear transducer is often the ideal choice for evaluating the ribs when fractures are sus- pected due to increased skin contact and the proximity of the ribs to the skin surface. Many ultrasound systems have pre- set examination settings,
which are a great starting place, but which cannot account for all of the physical (overall size, breed, adiposity) and physiological (hy- dration, ambient temperature) differences between patients. This means that the examiner is required to ‘finesse’ the settings to ac- count for these differences and to account for the varying depths and densities of various organs.
NORMAN Settings that are frequently adjusted include depth, frequency,
focus, gain and time/gain compensation. Depth should generally be adjusted so that the organ of interest just fills the screen; this maximises the information contained in the viewing area (Figure 3 ). Because of the various sizes and locations of abdominal organs within the body, depth is constantly adjusted in the course of any abdominal sonographic examination. Modern transducers offer a range of frequencies through which they operate. The ‘fre- quency’ refers to the number of sound waves that pass a fixed point per second. As frequency increases, the level of detail in the image increases, but the ability of the sound waves to penetrate tissue decreases. Therefore, frequency should be adjusted to the highest frequency that yields a diagnostic image at the required depth. Like depth, frequency is adjusted up and down to accom- modate variable depths and tissue densities throughout the ab- dominal scan. Gain is the brightness of the image.
It should not be altered
until after the frequency has been optimised. Time/gain compen- sation changes the gain throughout the depth of the image, which is especially important to use when evaluating deeper structures to produce an image with even brightness throughout and not overly bright in the superficial structures and not bright enough in the deeper ones. This principle can also be used in reverse in cases of effusion, when a fluid interface causes acoustic enhancement that results in excessive brightness of the deeper structures. Thoracic so- nography generally requires less manipulation of the images during the examination due to the fact that the pleural surface is generally the sole object of interest and occurs at a roughly consistent depth. In the case of pathology, however, adjustments will need to be made as in abdominal sonography. Regardless of what structure is being evaluated, labels are re-
quired to identify (at least) the anatomic location in the image and the orientation of the transducer with respect to the patient ' s body. The anatomic location seems logical and obvious when the anatomy is normal but is often neither in pathologic states. The label should indicate the side of the horse and the general region and provide more information if relevant. For thoracic imaging, it is useful to di- vide the trunk into thirds from ventral to dorsal and indicate that and the intercostal space number on the label. For the sonographic examination, as for any examination, the
practitioner should follow the same pattern each time to maxi- mise muscle memory and to avoid errors. This pattern should be logical and easy to the operator; no one system is perfect for ev- eryone. The author begins every evaluation on the left side in the most caudal and dorsal imaging window; for the abdomen, this is the left paralumbar fossa, and for the thorax, this is the left 15th intercostal space just ventral to the epaxial musculature. The ex- amination proceeds by sliding down each intercostal space from dorsal to ventral, and then, for the abdominal evaluation, sliding from cranial to caudal (e.g. with the hair) along the ventral abdo- men. When evaluating the thorax, each intercostal space is inter- rogated starting dorsally at the epaxial muscles and terminating at the diaphragm. Care is needed to orient the transducer parallel in
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