9.3 Radiographic Technique Basics

Radiographic Technique Basics

Producing a diagnostic radiograph requires an understanding of how X-ray machine settings affect the finished image. It also requires proper handling of recording media—whether traditional film or digital sensors—and the ability to identify and correct image artifacts.

X-ray Machine Settings: kVp and mAs

Three main variables are adjusted on the X-ray control panel to produce a radiograph: kilovoltage peak (kVp), milliamperage (mA), and exposure time (seconds).

1. Kilovoltage Peak (kVp)

kVp controls the electrical potential (voltage) applied between the cathode and anode in the X-ray tube. This potential determines the speed and kinetic energy of the electrons, which in turn determines the wavelength and energy of the X-ray photons.

• Penetrating Power (Quality): Higher kVp produces higher-energy X-rays with greater penetrating power. Thicker or denser tissues (such as bone) require higher kVp settings to penetrate the tissue and reach the recording medium.
• Contrast: kVp is the primary factor controlling radiographic contrast.
  • Low kVp produces high contrast (short scale of contrast), meaning there are distinct differences between black and white with few grey shades. This is ideal for bone radiography.
  • High kVp produces low contrast (long scale of contrast), meaning there are many subtle shades of grey. This is ideal for soft tissues (like the thorax) where small density changes must be visible.

2. Milliamperage (mA) and Exposure Time (seconds)

• Milliamperage (mA): mA controls the electrical current sent to the cathode filament, heating it and releasing electrons. mA controls the quantity (number) of X-rays produced.
• Exposure Time (s): The duration of the exposure in seconds.
• Milliamperage-Seconds (mAs): The product of mA and exposure time ($mA \times s = mAs$). This represents the total quantity of X-rays generated.
• Density (Blackness): mAs controls the overall density of the image.
  • Overexposed (too dark): An image with too high mAs will be excessively dark, losing detail in black areas.
  • Underexposed (too light): An image with too low mAs will be too light, lacking detail and exhibiting a grainy appearance called quantum mottle.

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The Technique Chart and Sante's Rule

To ensure consistent quality, clinics use a standardized technique chart. In many practices, the initial baseline settings are calculated using Sante's Rule. This formula estimates the kVp required based on patient thickness in centimeters:

$\text{kVp} = (2 \times \text{patient thickness in cm}) + \text{focal-film distance (FFD)} + \text{grid factor}$

• Focal-Film Distance (FFD): The distance from the X-ray tube to the cassette, typically standardized to 40 inches (100 cm) in small animal clinics.
• Grid Factor: If the body part measures greater than 10 cm, a grid (a series of lead strips under the table) must be used to absorb scatter radiation. Using a grid requires adding a grid factor (typically 8 to 10 kVp) to the calculation to compensate for the absorption of some primary X-rays.

For example, if a dog's abdomen measures 12 cm, the FFD is 40 inches, and the grid factor is 10, the estimated kVp would be:

$\text{kVp} = (2 \times 12) + 40 + 10 = 74\text{ kVp}$

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Film and Image Processing Technologies

Image processing has evolved from manual wet chemistry to digital systems. The veterinary assistant must understand how these systems work.

1. Manual Processing (Wet Tank)

Manual processing is a temperature-sensitive method where film is manually loaded onto metal hangers in a darkroom and submerged in a series of chemical tanks:

1. Developer: Converts exposed silver halide crystals in the film emulsion to black metallic silver. The standard time is 5 minutes at 68°F.
2. Rinse Bath (Water): Halts the development process.
3. Fixer: Clears unexposed silver halide crystals from the film and hardens the emulsion. Standard time is 10 minutes.
4. Wash Bath (Water): Removes residual chemicals to prevent staining.
5. Dryer: Film is hung to dry in a dust-free cabinet.

2. Automatic Processing

Automatic processors transport the film through developer, fixer, wash, and dry chambers using a motorized roller system. This takes approximately 90 seconds. The chemicals are maintained at high temperatures (around 95°F) to accelerate chemical reactions. Daily cleaning of rollers and replenishment of chemistry are essential to prevent processing artifacts.

3. Digital Radiography (CR vs. DR)

• Computed Radiography (CR): CR utilizes a cassette containing a photostimulable phosphor (PSP) plate. X-rays create a latent image on the plate. The cassette is inserted into a reader, where a red laser scans the plate, releasing the latent image energy as blue light, which is digitized. The plate is erased with intense white light and reloaded into the cassette.
• Direct Digital Radiography (DR): DR uses a flat-panel detector plate built into the X-ray table. The plate instantly converts X-ray energy into electrical signals, displaying the image on a computer monitor within 3 to 5 seconds. DR eliminates cassettes and readers, speeding up workflows.

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Cassette Care and Handling

Traditional film and CR cassettes contain intensifying screens that convert X-ray energy into visible light, which exposes the film or plate. This significantly reduces the radiation dose (mAs) required.

• Artifact Prevention: Dust, hair, or scratches on intensifying screens block light, causing white artifacts on the final image.
• Cleaning: Screens must be cleaned regularly using an approved antistatic cleaner and lint-free gauze. Cassettes should be stored upright in a clean area when not in use.

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Radiographic Artifacts

An artifact is any unwanted mark or distortion on a radiograph. Common artifacts include:

• Motion Blur: Caused by patient movement. Corrected by shortening exposure time (increasing mA to keep mAs constant).
• Grid Lines: Fine lines across the image caused by misalignment between the X-ray tube and the grid under the table.
• Static Electricity: Tree-branch-like black marks on film caused by static discharge when pulling film from the cassette in dry conditions.
• Processing Artifacts: Underdevelopment (too light), yellowing of film over time (insufficient washing), crescent-shaped black marks (creasing film before exposure), and roller scratches (dirty processor rollers).
• Double Exposure: Occurs when the same film or CR plate is exposed twice.
• Digital Halo (Uberschwinger Effect): A digital artifact appearing as a dark line around dense implants (like orthopedic plates), simulating lucency/infection.