Radiography of Animals

Fundamentals of Image Formation

• Physical Basis: Radiographic images are created when x-ray photons (high-energy light particles) pass through a subject; different materials (gas, fat, fluid, bone, or metal) absorb or scatter these photons differently based on their radiopacity.

• Primary Exposure Factors:

    ◦ Kilovoltage potential (kVp): Controls the energy and penetrating power of x-rays. Higher kVp increases penetration but decreases image contrast.

    ◦ Milliamperes (mA): Controls the number of x-rays produced; increasing mA increases image contrast and heat loading on the tube.

    ◦ Exposure Time: Controls the duration of the beam; longer times darken the image.

    ◦ mAs (Milliampere-seconds): The product of mA and time. High mA and short exposure times are used to minimize motion artifacts.

• Technique Management:

    ◦ Technique Charts: Standardized protocols based on animal size and body part ensure consistent image appearance.

    ◦ Automatic Exposure Control (AEC): A system that terminates exposure at an optimal time, though it is used less in veterinary medicine due to variations in animal body size.

II. Equipment and Scatter Control

• Collimators: Adjust the beam size to fit the area of interest, which improves image contrast and decreases radiation exposure to operators.

• Grids: Thin plates of lead and plastic strips used for subjects ≥10 cm thick (15 cm for digital) to remove scattered radiation before it reaches the detector.

• Intensifying Screens: (Used in film systems) Phosphorescent crystals that convert x-rays to light to expose film more efficiently, drastically reducing radiation exposure to the subject and operator.

Filmless (Digital) Radiography

• Categories of Digital Systems:

    ◦ Computed Radiography (CR): Uses a semiconductor plate in a cassette that must be processed in a separate electronic reader.

    ◦ Direct Digital Radiography (DR): Uses a scintillator array to capture x-rays and convert them to light pulses detected by photodiode elements. It is faster, has better contrast resolution, and has largely replaced CR.

• Data Standards and Storage:

    ◦ DICOM (Digital Imaging and Communications in Medicine): The standard medical format; it includes a hidden header that records all manipulations to prevent data corruption or trace malicious changes.

    ◦ PACS (Picture Archiving and Communication System): Programs used to securely store, distribute, and display DICOM images.

    ◦ Processing Algorithms: Critical computer-generated instructions that create diagnostic images based on the specific body region being radiographed.

Obtaining Quality Radiographs

• Positioning Standards:

    ◦ Thorax: Usually requires three views (left lateral, right lateral, and ventrodorsal/dorsoventral) taken at peak inspiration.

    ◦ Abdomen: Includes three views (left/right lateral and ventrodorsal) taken at peak expiration; dorsoventral views are discouraged due to visceral crowding.

    ◦ Musculoskeletal: Always requires at least two orthogonal radiographs (at 90-degree angles to each other). Long bone studies must include the joint above and below the area of interest.

• Animal Restraint:

    ◦ Chemical Restraint: Sedation or anesthesia is highly desirable to decrease patient anxiety, control pain, and minimize motion artifacts.

    ◦ Manual Restraint: Should be minimized; excessive attempts to improve positioning during manual restraint can increase radiation exposure to personnel.

Radiation Safety

• Protective Apparel: Personnel must wear lead-impregnated aprons and gloves to protect against scatter radiation.

    ◦ Thyroid shields are mandatory due to the gland's sensitivity.

    ◦ Eye shields (leaded glasses) are recommended, especially for large animal radiography.

• ALARA Principle: Safety protocols must ensure exposure is "as low as reasonably achievable".

• Monitoring: Individuals involved in radiography should wear devices to monitor their radiation exposure levels over time.

Interpretation and Reporting

• Interpretive Skills: Requires a solid understanding of anatomy; changes are evaluated based on size, shape, location, opacity, and margination.

• Systematic Approaches:

    ◦ Inside-out/Outside-in: Evaluating structures in a specific order (e.g., heart first or extrathoracic tissues first) to ensure nothing is missed.

    ◦ Systems-based: Groups of organs are evaluated together to identify abnormalities.

• The Imaging Report:

    1. Imaging Description: Lists all abnormal findings.

    2. Imaging Conclusion: Synthesizes findings into the most likely differential diagnoses.

    3. Recommendations: Suggests further tests or imaging to reach a final diagnosis.

Specialized Modalities and Procedures

• Contrast Procedures: Radiopaque compounds (like barium for the GI tract or iodine for vessels/kidneys) are used to delineate structures from surrounding tissues. Nonionic agents are preferred for IV use due to lower toxicity.

• Teleradiology: The transmission of digital images via the internet to board-certified radiologists for rapid interpretation.

• Artificial Intelligence (AI): An emerging field for AI-guided image interpretation, though legal and ethical issues remain unresolved.