# Diagnostic Imaging in Amphibian and Fish Medicine

## Quick Answer

- Radiography and ultrasonography are the first-line imaging tools for amphibians and fish, with CT and MRI reserved for specific diagnostic questions that require cross-sectional detail.
- Patient size, aquatic respiration, and unique anatomy such as tooth loss in many frog species and species-specific corneal structure determine which modality is practical and which views are diagnostic.
- Anesthesia or sedation is often required for computed tomography and magnetic resonance imaging in aquatic patients, and image interpretation depends on species-specific reference knowledge that general practitioners must verify before acting on findings.

Imaging amphibians and fish presents a distinct challenge in veterinary practice. These vertebrates have small body sizes, high metabolic variability, and anatomical features that differ substantially from mammals and birds. Water dependence, ectothermy, and the need to maintain gill or skin respiration during procedures constrain how imaging can be performed safely. This article reviews the utility and limitations of radiography, ultrasonography, computed tomography, and magnetic resonance imaging in aquatic patients, with attention to practical decisions that affect image quality and diagnostic yield.

The primary intent of this article is to help veterinarians select and interpret imaging modalities for amphibians and fish. The content is organized around the anatomical and physiological factors that influence imaging choices, the technical adjustments required for each modality, and the clinical scenarios where each tool provides the most value. Species-specific variation is substantial, and the evidence base for imaging in these taxa is thinner than for companion mammals. Practitioners should therefore combine published findings with careful patient assessment and, when uncertainty persists, seek consultation with colleagues who have advanced training in zoological medicine.

## Anatomical and Physiological Considerations That Shape Imaging Decisions

### Body Size and Tissue Density

Amphibians and fish span an enormous size range, from larval zebrafish that measure a few millimeters to large teleosts and caudates that may exceed one meter. This range directly affects which imaging modalities are feasible. Radiography requires sufficient tissue thickness to produce contrast, and in very small patients the spatial resolution of standard radiographic systems may be inadequate to resolve clinically meaningful structures. Ultrasonography requires transducer contact and an acoustic window, which is difficult to achieve in patients smaller than the footprint of the transducer. CT and MRI offer superior soft tissue contrast and cross-sectional imaging, but they require general anesthesia or deep sedation to prevent motion artifact, and the equipment is not universally available.

The anatomical organization of fish and amphibians also differs from mammals in ways that affect image interpretation. Fish have a swim bladder that creates a large gas-filled structure within the coelomic cavity, which can obscure adjacent organs on radiographs and create acoustic shadowing on ultrasound. Amphibians have a three-chambered heart, a lymphatic system that is more developed than in mammals, and skin that participates in respiration and fluid balance. These features alter the expected appearance of thoracic and coelomic structures on imaging studies.

### Dentition and Feeding Anatomy

Tooth presence and arrangement vary widely across amphibians, and this variation has direct relevance for imaging the head and oral cavity. A rigorous assessment of dentition across all major amphibian lineages, based on phenotypic data collected from over 500 genera using micro-computed tomography, demonstrated that teeth are invariably present in caecilians and salamanders, but teeth have been lost completely more than 20 times in frogs. This repeated loss of teeth in anurans is associated with a specialized diet of small invertebrate prey and with shortening of the lower jaw, but it is not correlated with a reduction in body size. For the imager, this means that the expected radiographic or tomographic appearance of the jaws varies by species, and the absence of teeth in a frog is not necessarily an abnormal finding. Conversely, the presence of teeth in a caecilian or salamander is expected, and their absence should prompt further investigation.

The association between tooth loss and jaw shortening in frogs also has implications for positioning and view selection. The shortened lower jaw changes the alignment of the temporomandibular region, and standard mammalian skull views may not be directly transferable. Micro-CT studies that map dentition across amphibian clades provide a reference framework for interpreting skull imaging in these species, but the published data are primarily phylogenetic and developmental in focus instead of clinical.

### Corneal and Ocular Structure

The eye is a common target for imaging in aquatic patients, particularly when evaluating for trauma, infection, or mass lesions. The structural organization of the cornea differs across vertebrate groups, and these differences affect how ocular imaging should be performed and interpreted. Studies using multiphoton and second harmonic generation microscopy have characterized the collagen architecture of the corneal stroma across vertebrates. In bony and cartilaginous fish, the cornea is composed of orthogonally arranged, rotating collagen sheets that extend from limbus to limbus with little or no interaction between adjacent sheets, a structural paradigm analogous to plywood. In amphibians and reptiles, these sheets are broken down into broader lamellae that begin to show branching and anastomosing with adjacent lamellae, while maintaining their orthogonal, rotational organization.

These structural differences have practical consequences. The fish cornea is relatively uniform in its collagen organization, which may affect how ultrasound waves propagate through the eye and how optical coherence tomography images are interpreted. The amphibian cornea, with its broader lamellae and branching pattern, may produce different echogenicity on ultrasound compared to the fish cornea. The corneal stroma is the major refractive element of the eye, and the mechanisms controlling corneal shape and hence visual acuity remain unknown, which limits the ability to predict how structural variation translates to functional differences. For the clinician, the key point is that ocular imaging findings in fish and amphibians should be interpreted with species-specific anatomy in mind, and extrapolation from mammalian ophthalmology is not reliable.

### Muscle and Skeletal Development

Imaging of the musculoskeletal system in amphibians and fish is relevant for evaluating developmental abnormalities, trauma, and mass lesions. The spatial dynamics of calcium signaling during skeletal muscle development have been studied in Xenopus embryos and zebrafish, and these studies have used imaging techniques that provide direct visualization of calcium signals in intact, normally developing organisms. While these studies are primarily developmental biology research instead of clinical imaging, they establish that the muscle architecture of amphibians and fish can be visualized with appropriate imaging techniques and that the subcellular organization of muscle differs from mammals in ways that may affect image interpretation.

The relevance of muscle imaging in clinical practice is illustrated by studies of muscular dystrophy in zebrafish models. In one study, knockdown of the popdc3 gene in zebrafish using splice-site blocking morpholinos resulted in larvae with tail curling and dystrophic muscle features. The same study used muscle magnetic resonance imaging in human patients with POPDC3 variants to reveal fat replacement of paraspinal and proximal leg muscles. This translational approach demonstrates that MRI can detect muscle pathology in fish models and that the imaging findings in fish correlate with those in mammals. For the veterinary imager, this suggests that MRI is a viable tool for evaluating muscle disease in larger fish and amphibians, provided that the patient can be safely anesthetized and positioned within the scanner.

## Radiography in Amphibians and Fish

### Indications and Patient Preparation

Radiography is the most widely available imaging modality in veterinary practice, and it is often the first imaging study performed in amphibians and fish. Common indications include evaluation of the coelomic cavity for organomegaly, free fluid, or masses, assessment of the skeletal system for fractures, deformities, or metabolic bone disease, and evaluation of the swim bladder in fish for buoyancy disorders. Radiography is also useful for detecting radiopaque foreign bodies, such as ingested gravel or metal objects, and for evaluating the reproductive tract in gravid females.

Patient preparation for radiography in amphibians and fish is less involved than for mammals, but it is not trivial. The patient should be removed from water and placed in a container that allows positioning without causing trauma to the skin or fins. For fish, the gills must remain moist, and the total time out of water should be minimized. For amphibians, the skin must remain moist to support cutaneous respiration, and desiccation is a real risk during positioning. Sedation or anesthesia may be required for patients that are active or that resist positioning, and the choice of anesthetic protocol should be made with species-specific considerations in mind.

### Technical Factors and Image Quality

The technical factors for radiography in amphibians and fish differ from those used in mammals. The small body size of many patients requires the use of high-detail film-screen combinations or digital detectors with small pixel sizes. The kilovoltage peak should be reduced to increase contrast in low-density tissues, and the milliampere-seconds should be adjusted to provide adequate exposure without motion blur. The use of a grid is generally not necessary in patients smaller than 10 centimeters in thickness, and the grid may actually degrade image quality by absorbing scattered radiation that would otherwise contribute to the image.

Positioning is critical for diagnostic radiographs in aquatic patients. For fish, the standard views are dorsoventral and lateral, with the lateral view obtained with the fish in a shallow water bath or on a radiolucent board with the gills kept moist. The swim bladder creates a large gas-filled structure that can obscure the kidneys and gonads on the lateral view, and oblique views may be necessary to separate the swim bladder from other structures. For amphibians, the standard views are dorsoventral and lateral, with the limbs extended away from the body to avoid superimposition. The lungs of amphibians are relatively simple sacs, and they may not be visible on radiographs unless they are distended with air.

### Common Findings and Interpretation Pitfalls

The most common radiographic findings in amphibians and fish include organomegaly, coelomic effusion, skeletal abnormalities, and swim bladder abnormalities. Organomegaly may be detected as an increase in the size of the liver, spleen, or kidneys, and the normal size of these organs varies by species and by reproductive status. Coelomic effusion appears as a generalized increase in soft tissue opacity with loss of the normal serosal detail, and it may be caused by hepatic disease, renal failure, or cardiac disease. Skeletal abnormalities include fractures, which are often the result of trauma or handling, and metabolic bone disease, which is common in amphibians maintained on inadequate diets or with insufficient ultraviolet B exposure.

Interpretation pitfalls are common in aquatic radiography. The swim bladder of fish can mimic a mass lesion if it is asymmetrically distended or if it contains fluid. The gas-filled intestine of amphibians can create artifacts that mimic free air in the coelomic cavity. The lack of body fat in many fish and amphibians reduces the natural contrast between organs, making it difficult to distinguish adjacent structures. Finally, the normal radiographic anatomy of many species has not been described in the peer-reviewed literature, and the practitioner must rely on knowledge of comparative anatomy and on consultation with colleagues who have experience with these taxa.

## Ultrasonography in Amphibians and Fish

### Indications and Patient Preparation

Ultrasonography is a valuable complement to radiography in amphibians and fish because it provides real-time imaging of soft tissue structures without the use of ionizing radiation. Common indications include evaluation of the coelomic organs, assessment of the heart and major vessels, detection of free fluid, and guidance for fine-needle aspiration or biopsy. Ultrasonography is particularly useful for evaluating the liver, spleen, kidneys, gonads, and swim bladder in fish, and for evaluating the heart, liver, and reproductive tract in amphibians.

Patient preparation for ultrasonography in aquatic patients requires careful attention to the interface between the transducer and the patient. The patient should be placed in a shallow water bath or on a wet surface, and the transducer should be covered with a standoff pad or a water-filled glove to provide acoustic coupling. The use of coupling gel is generally not recommended because it may be toxic to the skin of amphibians and to the gills of fish. The patient should be restrained gently to minimize movement, and sedation may be required for active patients.

### Technical Factors and Image Quality

The technical factors for ultrasonography in amphibians and fish differ from those used in mammals. The small body size of many patients requires the use of high-frequency transducers, typically in the range of 7.5 to 15 megahertz, to provide adequate spatial resolution. The depth of penetration is limited with high-frequency transducers, and this may be a problem in larger patients. The acoustic window is often limited by the presence of gas in the swim bladder or intestine, and the transducer may need to be repositioned to avoid gas-filled structures.

The echogenicity of tissues in amphibians and fish differs from mammals. The liver of fish is often more echogenic than the liver of mammals, and the spleen may be difficult to distinguish from the surrounding tissue. The kidneys of fish are located retroperitoneally and may be difficult to image because of their position relative to the swim bladder. The heart of amphibians is three-chambered, and the normal echocardiographic appearance differs from the mammalian heart.

### Common Findings and Interpretation Pitfalls

The most common ultrasonographic findings in amphibians and fish include hepatomegaly, splenomegaly, renomegaly, coelomic effusion, and mass lesions. Hepatomegaly may be detected as an increase in liver size with a change in echogenicity, and it may be caused by fatty liver disease, infection, or neoplasia. Splenomegaly is often associated with infection or neoplasia, and it may be detected as an increase in spleen size with a change in echogenicity. Renomegaly may be caused by hydronephrosis, neoplasia, or infection, and it may be detected as an increase in kidney size with a change in echogenicity.

Interpretation pitfalls are common in aquatic ultrasonography. The swim bladder of fish creates a strong acoustic shadow that can obscure the kidneys and gonads, and the transducer may need to be positioned to avoid the swim bladder. The gas-filled intestine of amphibians can create artifacts that mimic masses or free fluid. The lack of body fat in many aquatic patients reduces the acoustic contrast between organs, making it difficult to distinguish adjacent structures. Finally, the normal ultrasonographic anatomy of many species has not been described in the peer-reviewed literature, and the practitioner must rely on knowledge of comparative anatomy and on consultation with colleagues who have experience with these taxa.

## Computed Tomography in Amphibians and Fish

### Indications and Patient Preparation

Computed tomography provides cross-sectional imaging with superior spatial resolution and the ability to reconstruct images in multiple planes. In amphibians and fish, CT is particularly useful for evaluating the skull, the axial skeleton, and the coelomic cavity. CT is also useful for detecting small pulmonary nodules, for evaluating the swim bladder in fish, and for planning surgical procedures. The use of micro-CT in research has provided detailed anatomical reference data for many species, and this information can be used to guide clinical imaging.

Patient preparation for CT in aquatic patients requires general anesthesia or deep sedation to prevent motion artifact. The patient should be positioned in a radiolucent container that allows imaging without causing trauma to the skin or fins. For fish, the gills must remain moist, and the total time out of water should be minimized. For amphibians, the skin must remain moist to support cutaneous respiration. The CT scanner should be programmed with appropriate exposure parameters for the size of the patient, and the field of view should be adjusted to include the entire region of interest.

### Technical Factors and Image Quality

The technical factors for CT in amphibians and fish differ from those used in mammals. The small body size of many patients requires the use of a small field of view and a high-resolution reconstruction algorithm. The slice thickness should be minimized to provide adequate spatial resolution, and overlapping slices may be necessary to detect small lesions. The use of intravenous contrast material may be helpful for evaluating vascular structures and for characterizing mass lesions, but the dose and injection rate must be adjusted for the size of the patient.

The interpretation of CT images in amphibians and fish requires knowledge of species-specific anatomy. The swim bladder of fish appears as a gas-filled structure within the coelomic cavity, and it may be confused with a pulmonary lesion if the interpreter is not familiar with fish anatomy. The lungs of amphibians are relatively simple sacs, and they may not be visible on CT unless they are distended with air. The skeletal anatomy of amphibians and fish differs from mammals, and the interpreter must be familiar with the normal appearance of the skull, spine, and appendicular skeleton in these taxa.

### Common Findings and Interpretation Pitfalls

The most common CT findings in amphibians and fish include skeletal abnormalities, coelomic masses, and swim bladder abnormalities. Skeletal abnormalities may include fractures, deformities, and metabolic bone disease, and CT is particularly useful for evaluating complex fractures and for detecting subtle abnormalities that are not visible on radiographs. Coelomic masses may be detected as soft tissue attenuating lesions, and CT can be used to determine the extent of the mass and its relationship to adjacent structures. Swim bladder abnormalities may include overinflation, underinflation, and fluid accumulation, and CT can be used to characterize these abnormalities and to guide treatment.

Interpretation pitfalls are common in aquatic CT. The small body size of many patients can result in partial volume averaging, which can obscure small lesions. The lack of body fat in many aquatic patients reduces the contrast between organs, making it difficult to distinguish adjacent structures. The presence of gas in the swim bladder or intestine can create streak artifacts that degrade image quality. Finally, the normal CT anatomy of many species has not been described in the peer-reviewed literature, and the practitioner must rely on knowledge of comparative anatomy and on consultation with colleagues who have experience with these taxa.

## Magnetic Resonance Imaging in Amphibians and Fish

### Indications and Patient Preparation

Magnetic resonance imaging provides superior soft tissue contrast compared to CT and is particularly useful for evaluating the brain, spinal cord, and musculoskeletal system. In amphibians and fish, MRI is used for evaluating intracranial lesions, spinal cord compression, and muscle disease. MRI is also useful for characterizing mass lesions and for evaluating the reproductive tract. The use of MRI in research has provided detailed anatomical reference data for many species, and this information can be used to guide clinical imaging.

Patient preparation for MRI in aquatic patients requires general anesthesia or deep sedation to prevent motion artifact. The patient should be positioned in a radiolucent container that allows imaging without causing trauma to the skin or fins. For fish, the gills must remain moist, and the total time out of water should be minimized. For amphibians, the skin must remain moist to support cutaneous respiration. The MRI scanner should be programmed with appropriate pulse sequences for the size of the patient, and the field of view should be adjusted to include the entire region of interest.

### Technical Factors and Image Quality

The technical factors for MRI in amphibians and fish differ from those used in mammals. The small body size of many patients requires the use of a small field of view and a high-resolution acquisition matrix. The signal-to-noise ratio is lower in small patients, and the use of a surface coil may be necessary to improve image quality. The choice of pulse sequences should be tailored to the clinical question, and T1-weighted, T2-weighted, and fluid-attenuated inversion recovery sequences are commonly used.

The interpretation of MRI images in amphibians and fish requires knowledge of species-specific anatomy. The brain of fish and amphibians differs from mammals in its organization and in the relative size of its components. The muscle of fish and amphibians has a different fiber type composition and a different pattern of fat infiltration compared to mammals, and these differences affect the appearance of muscle on MRI. The presence of the swim bladder in fish creates a susceptibility artifact that can degrade image quality, and the interpreter must be aware of this artifact when evaluating the coelomic cavity.

### Common Findings and Interpretation Pitfalls

The most common MRI findings in amphibians and fish include intracranial lesions, spinal cord compression, and muscle disease. Intracranial lesions may include neoplasia, infection, and hemorrhage, and MRI can be used to characterize these lesions and to guide treatment. Spinal cord compression may be caused by vertebral fractures, intervertebral disc disease, or mass lesions, and MRI can be used to determine the location and extent of the compression. Muscle disease may include muscular dystrophy, myositis, and neoplasia, and MRI can be used to detect fat replacement and inflammation.

Interpretation pitfalls are common in aquatic MRI. The small body size of many patients can result in a low signal-to-noise ratio, which can degrade image quality. The presence of gas in the swim bladder or intestine can create susceptibility artifacts that obscure adjacent structures. The lack of body fat in many aquatic patients reduces the contrast between organs, making it difficult to distinguish adjacent structures. Finally, the normal MRI anatomy of many species has not been described in the peer-reviewed literature, and the practitioner must rely on knowledge of comparative anatomy and on consultation with colleagues who have experience with these taxa.

## At a Glance

| Modality | Best Use in Amphibians and Fish | Key Limitation | Anesthesia Requirement |
| --- | --- | --- | --- |
| Radiography | Skeletal evaluation, swim bladder assessment, coelomic screening for organomegaly or free fluid | Low soft tissue contrast, swim bladder and gas-filled intestine obscure adjacent organs | Usually none, but sedation may be needed for active patients |
| Ultrasonography | Real-time evaluation of coelomic organs, heart, and reproductive tract, guidance for aspiration or biopsy | Limited acoustic window from gas-filled structures, high-frequency transducers limit penetration in larger patients | Usually none, but sedation may be needed for active patients |
| Computed Tomography | Skull and axial skeleton evaluation, detection of small pulmonary nodules, surgical planning | Requires general anesthesia, limited availability, radiation exposure | General anesthesia or deep sedation required |
| Magnetic Resonance Imaging | Brain, spinal cord, and muscle evaluation, characterization of mass lesions | Requires general anesthesia, limited availability, susceptibility artifacts from gas-filled structures | General anesthesia or deep sedation required |

## Modality Selection by Clinical Presentation

| Clinical Presentation | First-Line Modality | Complementary or Advanced Modality | Key Anatomical Consideration |
| --- | --- | --- | --- |
| Buoyancy disorder in fish | Radiography, lateral and dorsoventral views | Ultrasonography for swim bladder wall assessment, CT for complex cases | Swim bladder gas creates contrast but can obscure adjacent organs |
| Skull trauma or oral mass in amphibian | Radiography for screening | CT for detailed bone evaluation, micro-CT reference data for dentition | Tooth loss is normal in many frogs but not in caecilians or salamanders |
| Coelomic mass or effusion | Ultrasonography for real-time evaluation | CT for extent of disease, MRI for soft tissue characterization | Lack of body fat reduces organ contrast in many aquatic species |
| Suspected muscle disease | Ultrasonography for screening | MRI for fat replacement and inflammation detection | Muscle fiber composition and fat infiltration patterns differ from mammals |
| Ocular disease | Ultrasonography with high-frequency transducer | Optical coherence tomography where available | Corneal collagen organization differs between fish and amphibians |

## Practical Implementation Steps for Imaging Aquatic Patients

### Step 1: Assess Patient Stability and Suitability

Before any imaging study, the veterinarian must assess whether the patient is stable enough to tolerate the procedure. Patients with severe respiratory distress, cardiovascular compromise, or severe debilitation may not tolerate the stress of handling, positioning, or anesthesia. The veterinarian should perform a thorough physical examination, including assessment of the skin, gills, eyes, and coelomic cavity, and should review the patient's history and husbandry. If the patient is unstable, the veterinarian should stabilize the patient before proceeding with imaging.

### Step 2: Select the Appropriate Modality

The choice of imaging modality should be guided by the clinical question, the size of the patient, and the availability of equipment. Radiography is the first-line modality for most skeletal and coelomic indications. Ultrasonography is the first-line modality for soft tissue evaluation and for guidance of aspiration or biopsy. CT is indicated when cross-sectional imaging is needed for surgical planning or for evaluation of complex skeletal abnormalities. MRI is indicated when superior soft tissue contrast is needed for evaluation of the brain, spinal cord, or muscle.

### Step 3: Prepare the Patient and Equipment

Patient preparation includes fasting for patients that will undergo general anesthesia, and removal of any external devices that could interfere with imaging. The imaging equipment should be prepared with appropriate settings for the size of the patient, and the imaging table should be covered with a radiolucent or MRI-compatible material. For fish, a water bath or moist towel should be prepared to keep the gills moist during the procedure. For amphibians, a moist environment should be maintained to support cutaneous respiration.

### Step 4: Position the Patient and Acquire Images

Positioning should be performed gently to avoid trauma to the skin, fins, or limbs. The patient should be positioned in a container that allows imaging without causing stress or injury. For radiography, the standard views are dorsoventral and lateral, with additional oblique views as needed. For ultrasonography, the transducer should be positioned to provide an acoustic window that avoids gas-filled structures. For CT and MRI, the patient should be positioned within the scanner with the region of interest centered in the field of view.

### Step 5: Interpret Images and Communicate Findings

Image interpretation should be performed with knowledge of species-specific anatomy and with attention to the limitations of each modality. The veterinarian should compare the imaging findings with the clinical presentation and with the results of other diagnostic tests. The findings should be communicated to the owner or caretaker in clear terms, and the implications for treatment and prognosis should be discussed. If the imaging findings are inconclusive or if the veterinarian is uncertain about the interpretation, consultation with a colleague who has advanced training in zoological medicine should be sought.

## Records and Measurements for Imaging Studies

### Standardized Image Labeling and Storage

Accurate records are essential for imaging studies in amphibians and fish. Each image should be labeled with the patient identification, the date and time of the study, the modality used, the views or sequences obtained, and the name of the veterinarian who performed the study. Images should be stored in a format that allows retrieval and comparison with future studies. Digital imaging and communications in medicine (DICOM) format is the standard for CT and MRI, and it should be used for radiography and ultrasonography when possible.

### Measurement of Lesions and Organs

Measurements of lesions and organs should be recorded in a standardized manner to allow comparison with future studies. The size of a mass lesion should be measured in three dimensions, and the location should be described in relation to anatomical landmarks. The size of organs such as the liver, spleen, and kidneys should be measured and compared with published reference values when available. The thickness of the swim bladder wall and the volume of the swim bladder should be recorded in fish with buoyancy disorders.

### Documentation of Technical Parameters

The technical parameters used for each imaging study should be documented to allow reproduction of the study if needed. For radiography, the kilovoltage peak, milliampere-seconds, and focal film distance should be recorded. For ultrasonography, the transducer frequency, gain settings, and depth should be recorded. For CT, the slice thickness, reconstruction algorithm, and contrast dose should be recorded. For MRI, the pulse sequences, repetition time, echo time, and field of view should be recorded.

### Comparison with Reference Data

The imaging findings should be compared with published reference data when available. The micro-CT data on amphibian dentition provide a reference for evaluating the skull in frogs, salamanders, and caecilians. The studies of corneal structure provide a reference for evaluating the eye in fish and amphibians. The studies of muscle development and disease provide a reference for evaluating the musculoskeletal system in zebrafish and Xenopus models. When published reference data are not available, the veterinarian should rely on knowledge of comparative anatomy and on consultation with colleagues who have experience with these taxa.

## Common Failure Patterns in Imaging Aquatic Patients

### Failure to Maintain Moisture and Respiration

The most common failure pattern in imaging aquatic patients is failure to maintain moisture and respiration during the procedure. Fish removed from water for prolonged periods may develop gill damage and hypoxia. Amphibians removed from a moist environment may develop skin desiccation and impaired cutaneous respiration. The veterinarian should minimize the time out of water, keep the gills and skin moist, and monitor the patient closely for signs of respiratory distress.

### Failure to Adjust Technical Parameters for Patient Size

The second most common failure pattern is failure to adjust technical parameters for the size of the patient. Radiographic exposure factors that are appropriate for a cat or dog will produce overexposed or underexposed images in a fish or amphibian. Ultrasound transducers that are appropriate for a cat or dog will not provide adequate resolution in a small fish or amphibian. CT and MRI protocols that are appropriate for a cat or dog will produce images with inadequate spatial resolution in a small patient. The veterinarian should adjust the technical parameters for the size of the patient and should use the smallest field of view that includes the region of interest.

### Failure to Recognize Species-Specific Anatomy

The third most common failure pattern is failure to recognize species-specific anatomy. The swim bladder of fish can be mistaken for a mass lesion, and the gas-filled intestine of amphibians can be mistaken for free air. The absence of teeth in many frog species can be mistaken for an abnormal finding, and the presence of teeth in caecilians and salamanders can be mistaken for a normal finding. The corneal structure of fish and amphibians differs from mammals, and ocular imaging findings should be interpreted with species-specific anatomy in mind. The veterinarian should review the normal anatomy of the species being imaged before interpreting the study.

### Failure to Use Appropriate Restraint or Anesthesia

The fourth most common failure pattern is failure to use appropriate restraint or anesthesia. Active patients that are not adequately restrained will produce motion artifact that degrades image quality. Patients that are not adequately anesthetized for CT or MRI will move during the acquisition, and the images will be nondiagnostic. The veterinarian should use appropriate restraint techniques for radiography and ultrasonography, and should use general anesthesia or deep sedation for CT and MRI.

### Failure to Seek Consultation When Uncertain

The fifth most common failure pattern is failure to seek consultation when uncertain. The normal imaging anatomy of many amphibian and fish species has not been described in the peer-reviewed literature, and the practitioner may encounter findings that are difficult to interpret. The veterinarian should seek consultation with a colleague who has advanced training in zoological medicine when the imaging findings are inconclusive or when the interpretation is uncertain.

## Welfare and Safety Context for Imaging Aquatic Patients

### Minimizing Stress and Pain

Imaging procedures can be stressful for amphibians and fish, and the veterinarian should take steps to minimize stress and pain. The patient should be handled gently and minimally, and the environment should be quiet and calm. The water temperature should be appropriate for the species, and the water quality should be maintained during the procedure. Analgesia should be provided when the procedure is likely to cause pain, and the choice of analgesic protocol should be made with species-specific considerations in mind.

### Monitoring During Anesthesia

Patients that undergo general anesthesia for CT or MRI should be monitored closely during the procedure. The depth of anesthesia should be assessed regularly, and the heart rate, respiratory rate, and mucous membrane color should be monitored. The body temperature should be maintained within the species-specific range, and the patient should be kept moist to support cutaneous respiration in amphibians and gill respiration in fish. The veterinarian should be prepared to intervene if the patient shows signs of anesthetic complications.

### Radiation Safety

Radiography and CT involve exposure to ionizing radiation, and the veterinarian should follow the principles of radiation safety. The use of radiography and CT should be justified by the clinical need, and the exposure should be kept as low as reasonably achievable. The veterinarian and any assistants should wear lead aprons and thyroid shields, and the patient should be positioned to minimize the need for repeat exposures. The imaging equipment should be maintained and calibrated regularly to ensure that the exposure is accurate.

### Biosecurity and Infection Control

Imaging procedures can transmit infectious agents between patients, and the veterinarian should follow appropriate biosecurity and infection control practices. The imaging table and equipment should be cleaned and disinfected between patients, and single-use items should be discarded after each use. The veterinarian should wear gloves when handling patients, and the hands should be washed between patients. Patients with suspected infectious disease should be imaged in a separate area or after other patients to prevent cross-contamination.

### Professional Escalation Criteria

The veterinarian should seek consultation with a colleague who has advanced training in zoological medicine when the imaging findings are inconclusive, when the patient is unstable, or when the required imaging modality is not available. The veterinarian should also seek consultation when the patient requires general anesthesia and the veterinarian does not have experience with anesthesia in the species being imaged. The veterinarian should refer the patient to a specialty practice when the imaging study cannot be performed safely or when the interpretation requires expertise that is not available in the primary care setting.

## Frequently Asked Questions

### What is the best first-line imaging modality for a fish with a buoyancy disorder?

Radiography is the best first-line imaging modality for a fish with a buoyancy disorder. The swim bladder is a gas-filled structure that is well visualized on radiographs, and overinflation, underinflation, and fluid accumulation can be detected. The lateral view is most useful for evaluating the swim bladder, and oblique views may be necessary to separate the swim bladder from other structures. Ultrasonography can be used as a complement to radiography to evaluate the swim bladder wall and to detect free fluid in the coelomic cavity.

### How do I keep a fish's gills moist during radiography?

The fish should be placed in a shallow water bath or on a radiolucent board with the gills kept moist. A moist towel or sponge can be placed over the gills, and the water should be dripped over the gills periodically during the procedure. The total time out of water should be minimized, and the fish should be returned to water as soon as the images are acquired. The water temperature should be appropriate for the species, and the water quality should be maintained during the procedure.

### Why do many frogs not have teeth on imaging studies?

Teeth have been lost completely more than 20 times in frogs, and this repeated loss is associated with a specialized diet of small invertebrate prey and with shortening of the lower jaw. The absence of teeth in a frog is not necessarily an abnormal finding, and the expected appearance of the jaws varies by species. In contrast, teeth are invariably present in caecilians and salamanders, and their absence should prompt further investigation. Micro-CT studies provide a reference framework for interpreting skull imaging in these species.

### What transducer frequency should I use for ultrasound in a small amphibian?

A high-frequency transducer in the range of 7.5 to 15 megahertz should be used for ultrasound in a small amphibian. High-frequency transducers provide adequate spatial resolution for small patients, but the depth of penetration is limited. The transducer should be covered with a standoff pad or a water-filled glove to provide acoustic coupling, and coupling gel should not be used because it may be toxic to the skin of amphibians.

### Is general anesthesia required for CT in a fish?

General anesthesia or deep sedation is required for CT in a fish to prevent motion artifact. The fish should be positioned in a radiolucent container that allows imaging without causing trauma to the skin or fins, and the gills must remain moist during the procedure. The choice of anesthetic protocol should be made with species-specific considerations in mind, and the fish should be monitored closely during the procedure.

### What are the limitations of MRI in amphibians and fish?

The limitations of MRI in amphibians and fish include the small body size of many patients, which results in a low signal-to-noise ratio, and the presence of gas in the swim bladder or intestine, which creates susceptibility artifacts that degrade image quality. The lack of body fat in many aquatic patients reduces the contrast between organs, and the normal MRI anatomy of many species has not been described in the peer-reviewed literature. General anesthesia is required, and the equipment is not universally available.

### How do I interpret the absence of teeth on a skull radiograph of a salamander?

The absence of teeth on a skull radiograph of a salamander is an abnormal finding. Teeth are invariably present in salamanders, and their absence should prompt further investigation. The absence of teeth may be caused by trauma, infection, neoplasia, or a developmental abnormality. The veterinarian should perform a thorough oral examination and should consider additional imaging, such as CT, to evaluate the jaws and to determine the cause of the tooth loss.

### When should I refer an amphibian or fish patient for advanced imaging?

The patient should be referred for advanced imaging when the required imaging modality is not available in the primary care setting, when the patient requires general anesthesia and the veterinarian does not have experience with anesthesia in the species being imaged, or when the imaging findings are inconclusive and the interpretation requires expertise that is not available in the primary care setting. The veterinarian should also refer the patient when the imaging study cannot be performed safely or when the patient is unstable and requires advanced monitoring and support.

## Related Veterinary Guides

- [Equine Diagnostic Imaging: Radiography, Ultrasound, and Advanced Modalities](/knowledge/veterinary-medicine/equine-care/equine-diagnostic-imaging-radiography-ultrasound-advanced-modalities)
- [Reptile Diagnostic Imaging: Radiography and Advanced Techniques](/knowledge/veterinary-medicine/reptile-care/reptile-diagnostic-imaging-radiography-advanced-techniques)
- [Avian Diagnostic Imaging: Radiography and Ultrasonography in Birds](/knowledge/veterinary-medicine/backyard-poultry/avian-diagnostic-imaging-radiography-ultrasonography-birds)
- [Imaging the Canine and Feline Spine: Radiography and Advanced Modalities](/knowledge/veterinary-medicine/diagnostic-imaging/imaging-canine-feline-spine-radiography-advanced-modalities)
- [Radiography, Ultrasound, CT, and MRI in Veterinary Medicine: Imaging Selection by Clinical Question](/knowledge/veterinary-medicine/clinical-methods/radiography-ultrasound-ct-mri-veterinary-imaging-selection-by-clinical-question)

## References and Further Reading

- [Pet Care](https://www.avma.org/resources-tools/pet-owners). American Veterinary Medical Association.
- [AAHA Guidelines](https://www.aaha.org/resources). American Animal Hospital Association.
- [Global Guidelines](https://wsava.org/global-guidelines). World Small Animal Veterinary Association.
- [Merck Veterinary Manual](https://www.merckvetmanual.com/). Merck Veterinary Manual.
- [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/). Cornell University.
- [Animal Health and Welfare](https://www.woah.org/en/what-we-do/animal-health-and-welfare). World Organisation for Animal Health.
- [Evolution of the vertebrate corneal stroma.](https://pubmed.ncbi.nlm.nih.gov/29398348). Progress in retinal and eye research, 2018.
- [Rampant tooth loss across 200 million years of frog evolution.](https://pubmed.ncbi.nlm.nih.gov/34060471). eLife, 2021.
- [Methylene Blue Assay for Estimation of Regenerative Re-Epithelialization In Vivo.](https://pubmed.ncbi.nlm.nih.gov/28228166). Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada, 2017.
- [Visualization of Ca²+ signaling during embryonic skeletal muscle formation in vertebrates.](https://pubmed.ncbi.nlm.nih.gov/21421918). Cold Spring Harbor perspectives in biology, 2011.
- [POPDC3 Gene Variants Associate with a New Form of Limb Girdle Muscular Dystrophy.](https://pubmed.ncbi.nlm.nih.gov/31610034). Annals of neurology, 2019.

> This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.