Fish radiology (Proceedings)

As aquatic medicine becomes more and more popular, diagnostic tools commonly used with traditional pets become part of the diagnostic workup for fish cases. In order to make radiology a successful part of the clinical workup, one needs to have a basic understanding of normal fish anatomy (e.g. one chambered swim bladder vs. bi-lobed swim bladder). While the setup to take radiographs of a fish might be different from the domestic animal, the general principles of radiology still apply (e.g. always use 2 views for evaluation, read the picture in an systematic way) as in any other species.

Restraining and Anesthesia

The majority of fishes can be radiographed quickly, safely, and efficiently out of water without tranquilization or anesthesia. Most fish are relatively quiet when initially placed on the cassette. If manual restraint is chosen, it is important to remember to prevent damage to the patient. Ideally the handler should wear latex gloves with the talc powder removed by rinsing them in water prior to handling the patient. The patient is quickly placed directly upon a plastic covered cassette.

An additional restraint procedure for fish involves putting the fish in a plastic bag with sufficient water to allow respiration and then placing the bag and fish onto the film cassette. The biggest disadvantage of this technique is that the addition of water between the patient and the cassette decreases radiographic detail.

Chemical Restraint

Certain fishes may require sedation to avoid injury to the fish and to make the procedure faster . Tricane methanesulfonate (MS -222) is the anesthetic agent recommended for use in fishes and it is the only agent approved for use in food fishes by the U.S. Food and Drug Administration. A stock solution can be prepared by mixing 10 g. of the crystalline compound into one liter of dechlorinated water. This produces an acidic solution; therefore 10 g. of sodium bicarbonate should be added to the solution as a buffer to bring it back to pH neutral range. MS-222 is light sensitive and should be stored in a dark bottle. The stock solution should be discarded if a precipitate forms.

The author uses also eugenol (Clove oil) as an anestheic agent. A stock solution is made by adding 1 ml of pure clove oil with 9 ml or ethanol. This solution will have about 100 mg/ml of eugenol. However, this drug is not approved by the FDA.

Radiographic Equipment

Standard radiograph equipment used for small animal patients is suitable for pet fish radiography. High detail intensifying screens and detail film are recommended to optimize radiograph detail. Placing the cassette in a plastic bag or putting the fish in a bag or on it protects the radiographic equipment from water.

Many ornamental pet fish have unusual body conformations that make vertical beam DV/VD positioning difficult. For larger fish that will maintain straight sternal (DV) positioning, a routine DV view is acceptable. The use of sandbags to maintain correct positioning is acceptable if they do not obstruct anatomical structures.

A second option for the lateral view is to lay the fish directly on a moistened radiographic plate. After a short initial period of 'flopping' the fish will remain still long enough for even a high detail exposure, requiring a long exposure time. If the radiographs are taken with a mobile unit near the pond, and the fish could possibly be harmed from uncontrolled movements, it is be best to anesthetize the fish for the procedure. If the use of contrast material for GI series is indicated, the author recommends the use of a iodine based dye instead of barium. Barium can seriously harm the gills if it accidentally comes in contact with the gills via reflux or spillage.

The goldfish should be used as a model for "standard" fish anatomy to familiarize oneself with basic fish anatomy. The gold fish is most likely the most commonly kept fish and directly related to the koi fish.

However, there are species differences between the many varieties of pet fish. It is important to understand anatomic differences when evaluating radiographic studies. In general, the coelomic detail in most pet fish is poor, therefore familiarity with normal anatomic structures is useful when subtle coelomic changes are identified radiographically. If a 'novel' species is presented as a sick patient, I recommend to take a radiograph of a 'normal' fish of the same species at the same time to use as a 'normal' comparison.

In addition, knowledge of normal variations should decrease the misdiagnosis of normal structures, as abnormal. However, in many diseased fish , the coleomic cavity is highly abnormal and organs can not be differentiated. Very often a coelomic ultrasound exam is needed to differentiate between organs. The radiograph serves as a great screening tool.

Conclusion

Conventional radiography of the pet fish can be a useful diagnostic aid. Standard radiographic equipment and radiographic techniques can yield high quality images of fish patients. Radiography appears to be especially useful in the diagnosis of swim bladder and skeletal diseases. Due to poor coelomic detail in most pet fishes, evaluation of the coelomic soft tissues is limited. Conventional radiographic contrast procedures and alternate imaging procedures such as ultrasound, and computed tomography may enhance evaluation of soft tissue coelomic structures.

For a good reference and part of the text has been taken from the following publication

Love NE, Lewbart GA. Pet fish radiography: technique and case history reports. Vet Radiol Ultrasound. 1997 Jan-Feb;38 (1):24-9.