The usefulness of imaging in fish has been demonstrated.4 However, there is limited information regarding imaging protocols or the normal anatomy of fish using conventional radiography or alternate imaging modalities. The strict environmental factors required by fish limit the availability of diagnostic tests available to the practitioner. With proper preparation and planning, imaging can be a beneficial non-invasive addition to the diagnostic plan. The purpose of this presentation is to (1) present selected modalities that can be used to image fish, (2) describe selected successful imaging protocols for fish, and (3) present an example that highlights the strength of each modality discussed. Imaging modalities that have been utilized in fish include conventional radiography, digital fluoroscopy, ultrasonography, computed tomography (CT), nuclear medicine and magnetic resonance imaging (MR).1,2,4 While protocols have been published for non-contrast conventional radiography in fish, there is limited information on the imaging protocols of fish using CT and MR. Three normal koi (Cyprinus carpio) were imaged to determine normal anatomy and to develop and optimize imaging protocols for koi pairing conventional radiography and oral contrast, CT, and MR.
Conventional radiography is excellent for looking at bone and swim bladder abnormalities.4 Indications for use in clinical cases include suspected swim bladder abnormalities, bone changes (e.g., fractures, scoliosis), suspected mechanical ileus and identification of coelomic or superficial masses. To better evaluate abdominal coelomic structures, an upper gastrointestinal (UGI) contrast examination may be needed. This is due to the inherent poor soft tissue detail fish have on radiographs. Oral administration of positive contrast media can be used to improve visualization of the GI tract and aid in the evaluation of GI transit times. Fish for this project were imaged in right to left lateral, dorsoventral, and left lateral decubital views. A red rubber catheter was used for p.o. administration of the contrast media, Iohexol (Omnipaque 240, Nycomed, Princeton, NJ).
CT provides good soft tissue information and excellent bony detail. The advantages of CT over MR include cost, time of scan, patient handling, and identification of osseous changes. Both modalities are excellent for reducing anatomic complexity and providing anatomic detail. Helical whole body CT scans were used because of the short imaging times and increased flexibility for image reformatting and manipulation. Indications for CT use in clinical patients include detailed examination of bone, swim bladder problems and visualization of abdominal masses.
MR provides superior soft tissue detail compared to the other modalities, but is complicated by the time required for each scan, logistics of anesthesia, and cost. MR imaging presents novel difficulties. Ferromagnetic metals are not allowed in the MR chamber and the time needed for MR scans can be lengthy, therefore we adapted a previously described anesthesia machine for this function.3 Life support and anesthesia were achieved using an extended, out of room, open circuit system. Fish were imaged using a resin coated human wrist radiofrequency (RF) coil. Clinical indications for MR imaging include evaluation of abdominal masses, detailed soft tissue visualization, and swim bladder abnormalities.
Immobilization and anesthesia for imaging procedures was achieved with tricaine methane sulfonate or MS-222 (Finquel®, Argent Chemical Laboratories, Redmond, WA), which has been proven to be efficacious in a variety of fish.5 Many fish will not need anesthesia for conventional radiography examinations. Immobility is imperative for CT examinations, for CT scans less than 4 min, anesthesia was achieved by inducing the fish in 200 ppm of MS-222 until cessation of opercular movement. Species susceptibility to MS-222 varies widely, therefore dosages are somewhat species specific.5 Fish were placed on the CT gantry and imaged in sternal recumbency. Recovery was achieved by placing the fish in fresh water. For procedures lasting more than 4 min (MR) fish were managed using the previously described general anesthesia machine.3
Project supported by Paul Fisher, Director of the Biomedical Imaging Resource Facility, North Carolina State University. A special thank you to Cecil Charles, Magnetic Resonance Imaging Center, Duke University .
1. Bakal, R., N. Love, G. Lewbart, and C. Berry. 1998. Imaging a spinal fracture in a Kohaku koi (Cyprinus carpio): Techniques and case history report. Veterinary Radiology and Ultrasound 39(4): 318–321.
2. Blackband, S. and M. Stoskopf. 1990. In vivo nuclear magnetic resonance imaging and spectroscopy of aquatic organisms. Magnetic Resonance Imaging 8: 191–198.
3. Harms, C. and R. Bakal. 1995. Techniques in fish anesthesia. Journal of Small Exotic Animal Medicine 3: 19–25.
4. Love, N. and G. Lewbart. 1997. Pet fish radiography: technique and case history reports. Veterinary Radiology & Ultrasound 38(1): 24–29.
5. Noga, E. 1996. Fish Disease: Diagnosis and Treatment. Mosby Publishing. St. Louis, Missouri. 17.