Veterinary Diagnostics: Diagnostic Imaging and X-Ray Imaging Notes

Diagnostic imaging is crucial for patient diagnosis and treatment.
Radiography and ultrasonography are common modalities; CT, MRI, and NM are used in referral hospitals.
Veterinary technicians often operate imaging equipment; understanding the physics is important for quality studies.

X-Ray Generation

X-rays are electromagnetic radiation with shorter wavelengths, higher frequency, and higher energy than visible light.
High energy makes x-rays dangerous.
X-rays are generated when electrons from the cathode collide with the anode.
Electron energy converts to heat (99\%) and x-rays (1\%).
Heat generation is a limiting factor in x-ray production.

X-Ray Tube Anatomy

The x-ray tube contains a cathode (-) with a tungsten filament, which generates electrons when heated.
A focusing cup focuses the electron beam on the anode's focal spot.
The anode (+) contains a rotating tungsten target where x-rays are generated, angled at 11 to 20 degrees.
The angle directs x-rays downward through a window.
The anode and cathode are in a vacuum glass or metal envelope.
A beryllium window provides minimal inherent filtration (1 mm Al eq).
An aluminum filter outside the window adds filtration (1.5 mm Al eq), absorbing low-energy x-rays.
X-ray tubes over 70 kVp must have a collimator for 2.5 mm Al equivalent total filtration.
Lower-energy x-rays increase patient dose without contributing to image formation.
Oil surrounds the tube as an electrical barrier and heat absorber.
A lead housing protects from stray radiation and external damage to the glass envelope.

The Heel Effect

The heel effect is the unequal distribution of x-ray beam intensity along the cathode-anode axis.
Lower target angles (e.g., 11 degrees) quickly decrease intensity on the anode side due to absorption.
Can place the patient’s head toward to anode side so the higher intensity (cathode side) is directed to the thickest part of the patien such as the thorax.
Most noticeable with large film sizes, low kVp, and long focal-film distance.

Radiographic Image Quality

Radiographic density refers to the degree of blackness on a radiograph. Dark areas are black metallic silver deposits where x-rays penetrated.
mAs (milliamperage multiplied by time) intensifies radiographic density by increasing the number of x-rays or electron travel time.
Higher kVp increases radiographic density by increasing x-ray beam penetrating power.

Radiographic Contrast

Radiographic contrast is the density difference between adjacent areas.
A long scale of contrast film has many shades of gray; short scale film has few.
Long scale of contrast is generally desirable.
Contrast depends on subject density, kVp level, film contrast, and film fogging; computers can adjust contrast in digital radiographs.
Subject density is the tissue's ability to absorb x-rays. Air/lung tissue appears radiolucent (black), and dense tissue appears radiopaque (white).
Bone (calcium and phosphorus) absorbs more x-rays than muscle (hydrogen and nitrogen).
Thickness affects absorption; thicker areas absorb more x-rays.
Increasing kVp lengthens the scale of contrast, while higher kVp also provides more latitude for technique errors.
Long-latitude film has more technique variation tolerance.
With digital radiographs, the computer can change the scale of contrast.

Film Fogging

Film fogging decreases radiographic contrast by decreasing density differences.
Film fogging may result from light leaks, scatter radiation, heat, or improper processing.
CR cassettes can fog due to environmental radiation exposure and should be reset if there is a long time between uses.

Radiographic Detail

Diagnostic quality radiographs are those with sharp tissue and organ interfaces.
Patient motion and penumbra effect significantly influence radiographic detail.
This can be controlled by using the shortest possible exposure time
The smallest focal spot size prevents this effect whenever possible.
The penumbra effect is fuzziness from stray x-rays.
Increasing SID decreases the penumbra effect, and is limited by the inverse square law.
mAs must increase four times if SID doubles.

Object-Image Distance

Decreasing OID minimizes penumbra.
Foreshortening of long bones occurs when the object is not parallel to the recording surface.
When vertebral column is radiographed the midcervical vertebrae tend to sag; use padding.

Distortion

Distortion occurs when the x-ray beam is not perpendicular to the recording surface.
This increases in distance from the center of the primary beam.
Center the primary beam over complex joints.

Scatter Radiation

Scatter radiation fogs the film, decreasing the contrast and posing a safety hazard.
High kVp produces more scatter radiation, with body parts measuring 10 cm or more.
Beam-limiting devices (collimators, cones, diaphragms, filters) decrease scatter radiation.
Collimators limit the size of the primary beam and filters absorb soft rays as they leave the tube head.

Grids

Grids decrease scatter radiation and increase contrast when imaging areas 10 cm or more in thickness.
Grids consist of thin lead strips (radiodense) and plastic, aluminum, or fiber spacers(radiolucent).
They are placed under the table to minimize scatter radiation since it diverges in all directions.
Increasing mAs by up to 6.6 times is required to compensate for the loss of usable x-rays.
Parallel grids have perpendicular lead strips and exhibit grid cutoff (x-ray beam absorption in the periphery).
Focused grids have angled lead strips to match the x-ray beam divergence, eliminating cutoff.
Grid manufacturer supplies the grid focal distance.

Grid Cutoff

Cutoff of the primary beam occurs if the grid is not perpendicular to or centered with the x-ray tube
Grids produce thin white lines.
These can be decreased by making the lead strips as thin as possible while retaining the ability to absorb scatter radiation effectively.
Grid visibility can be decreased by increasing grid lines per inch (typically 80 to 100 lines).
Potter-Bucky diaphragm (Bucky) sets the grid in motion, blurring the lines.
A disadvantage of using the Bucky mechanism in veterinary medicine is the noise and vibration it produces.

Exposure Variables

mAs, kVp, focal-film distance, and object-film distance control radiographic density, contrast, and detail.
Changing one factor requires adjusting another.

mAs

mAs is the product of the milliamperage and the exposure time.
The milliamperage controls the number of electrons and filament temperature, which dictates intensity.
Varying the exposure time controls the number of x-rays.
300 mA \text{ at } 1/60 \text{ seconds} = 5 mAs
200 mA \text{ at } 1/40 \text{ seconds} = 5 mAs
100 mA \text{ at } 1/20 \text{ seconds} = 5 mAs
Exposure time and mA are inversely related. It is best to use the fastest exposure time to allow less movement on the film
mAs can be used to adjust the radiographic density according to the following rules:
- To double the radiographic density, double the mAs.
- To halve the radiographic density, halve the mAs.

Kilovoltage Peak

kVp is the voltage applied between the cathode and the anode.
Increasing the kVp increases the positive charge and speeds electrons, increasing collision force and beam penetration.
The thicker the part, the higher the kVp setting because more penetration is needed.
A higher kVp produces a longer scale of contrast and more exposure latitude allowing for more variation in exposure factors, while still producing a diagnostic radiograph.
Rules when changing radiographic density with kVp:
- To double the radiographic density, increase the kVp by 20%.
- To halve the radiographic density, decrease the kVp by 16%.

Source Image Distance

SID is the distance from the target to the recording surface (film). This distance is held constant for most radiographic procedures, around 36 to 40 inches.
The inverse square law formula says that the intensity of the x-ray beam is inversely proportioned to the square of the distance from the source of the x-ray.
If the SID is doubled, the mAs must be increased four times to maintain radiographic density.
The same number of x-rays must diverge to cover an area that is four times as large
Changing the SID does not affect the penetrating power of the beam, so kVp remains constant.

Object-Image Distance

OID (aka object film distance aka OFD) is the distance from the object being imaged to the recording surface (film or digital recording plate).
OID should be as short as possible to minimize the penumbra effect magnification with a long OID.

Radiographic Film

X-ray film consists of a thin protective layer, an emulsion containing silver halide crystals, and a polyester film base.
The first layer is a thin clear gelatin protective coating.
The second layer is the emulsion that contains 90% to 99% silver bromide crystals in a gelatin base giving the film greater sensitivity, increasing the speed, density, and contrast.
By increasing speed the exposure required to produce an image can be decreased, thus decreasing the exposure of the patient and vet staff/personnel.
The film base is in the film’s center, giving it support. A blue tint has been added to ease eyestrain and does not produce a visible light pattern or absorb the light.
When the silver halide crystals are exposed to electromagnetic radiation they become sensitive to chemical change, making up the latent image.
When the film is placed into the developer, the latent image is reduced to black metallic silver and the remaining silver halide crystals are removed in the fixer, producing varying shades of black metallic silver and a clear film base.
The film is sensitive to all types of electromagnetic radiation, heat, and light and sensitive to excess pressure.

Types of Film

Used in veterinary radiography are screen-type film and direct-exposure film.
Screen-type film contains blue-sensitive (sensitive to blue-light-emitting phosphors like calcium tungstate) and green-sensitive (sensitive to green-light-emitting rare-earth phosphors) options.
Direct-exposure film is more sensitive to direct x-rays and requires higher mAs, often used for extremities and dental radiology.
film speeds are rated as high (regular or fast), average (par), and slow (detail).
Film Latitude is the film’s ability to produce shades of gray.

Intensifying Screens

Intensifying screens containing fluorescent crystals bound to a cardboard or plastic base. When exposed to x-rays, they emit foci of light.
95% of radiographic density results from screen fluorescence, 5% from direct x-ray exposure.
Each x-ray photon emits 1000 light photons amplifying the photographic effect of the x-rays.
High-speed screens require less exposure time but decrease detail.
Changing from high speed to par speed the mAs must be increased two times, when changing from high speed to slow speed the mAs must be increased four times to maintain radiographic density.
Routine cleaning with lint-free cloth and screen-cleaning solution is necessary. Don't use denatured alcohol or abrasive products and allow for drying to complete before reloading.
Do not drop or set heavy objects on the cassettes which can result in poor film-screen contact and blurring of one area of the image.

X-Ray Equipment Types

Portable units (15-30 mA fixed, 40-90 kVp variable) are ideal for large-animal extremities.
Mobile units (up to 300 mA, 125 kVp) are used in-hospital and can be lowered or suspended.
Stationary units are installed in a shielded room and have varying capabilities, high-volume practices will benefit from a stationary unit.
Basic equipment include x-ray generator system, collimator, grid, table, tube stand, and positioning aids.
Cassettes and x-ray tubes should never be handheld.

Digital Imaging

DR involves obtaining and displaying images on computer monitors in grayscale.
Types include CR, DR, and CCD technologies.
Advantages are digitized image enhancement (contrast, brightness, zoom, pan), elimination of film and chemicals, retrofitting existing machines for DR, and fewer repeat radiographs.
Hard copy films can be made from DR.
CR uses cassettes and IPs with photostimulable phosphors, eliminating the film medium/scanner uses a scanning laser beam that emits light. The IP is then erased and reloaded for reuse.
DR uses IP arrays connected to a computer, translating x-rays into electrical signals.

Digital Storage

DR and other modalities use DICOM format, which includes patient information and is encrypted for security.
Storage options are CDs, DVDs, or PACS.
PACS advantage is multiple patient storage and availability to multiple computers/clinicians.

Fluoroscopy

Fluoroscopy visualizes dynamic structures by directing a beam through a patient onto a fluorescent screen or image intensifier.
This is commonly for GI studies, angiography and myelography.
Fluoroscopy is not commonly used modality because of economic limitations, referral clinics and university veterinary hospitals have access to one.

Radiation Safety

Ionizing radiation damages intracellular water and damages critical components of the cell, such as DNA. It may have carcinogenic effects, which means cancer may develop in body tissues.
Tissues most sensitive have rapidly growing or reproducing cells; reproductive organs, hematopoietic cells, thyroid gland, intestinal epithelium, eye lens, and developing fetus are radiosensitive.

Terminology

REM - roentgen equivalent man, expresses dose equivalent from ionizing radiation; also, Sievert (SV) - current terminology (1 SV = 100 REM).
Rad - radiation absorbed dose; also, Gray (GY) - current terminology (1 GY = 100 rads).
MPD - maximum permissible dose. NCRP recommends that occupationally exposed persons not exceed 5 REM per year.
ALARA - "as low as reasonably attainable." MPD for nonoccupational persons is 10% of the MPD for occupationally exposed persons, or 0.5 REM per year. A fetus should not receive more than 0.5 REM during the entire gestation period and pregnant employee should wear an additional badge at waist level for monitoring fetal dose (not to exceed 0.05 REM per month).

Methods to minimize occupational radiation exposure

Lead shielding - Gowns, gloves, and thyroid shields should contain at least 0.5 mm of lead and lead-based glasses can also be worn
Increase the distance from the primary beam/use mechanical restraints
Use the fastest film-screen combinations to reduce exposure time.
Each clinic should have a radiation protection supervisor that educate personnel to radiation safety.
A good radiation control program consists of safe x-ray equipment, low- exposure techniques, use of positioning aids, proper measuring of patients, proper positioning methods, shielding, and monitoring personal radiation exposure.

Darkroom Techniques

Designed to ensure the consistent production of high-quality radiographs.
“dry bench” area away from the“wet bench”area separated by a partition to prevent chemical splashes from damaging the dry films or sensitive intensifying screens.
The room must be light-tight to prevent the film from fogging.
The darkroom should have adequate ventilation to prevent volatile chemical fumes from accumulating in the room.
Cleanliness is essential to prevent leaving parts of the film unexposed and film hangers should also be cleaned regularly.

Film Identification

Permanent labeling is necessary and radiographs must be identified before processing for legal purposes and certification organizations.
The label should include the clinic name, date, owner’s name, address, patient’s name, and some patient data, such as age and breed.

Safelights

Necessary for darkroom processing to provide sufficient light to work in the room but does not cause film fogging.
Safelights can be mounted to provide light directly or indirectly directed at the workbench.
A blue-light-sensitive film requires a safelight that filters out blue and ultraviolet light while a a green-light-sensitive film requires a safelight to filter both green and blue light.
A white frosted 7½- to the 10-watt bulb is recommended for most safelight.

Film-Processing Chemistry

Developer converts sensitized silver halide crystals into black metallic silver with solvent, reducing agents, restrainer, activator, and preservative.
The liquid form requires dilution with water. The powder form should never be mixed in the darkroom.
Fixer removes silver halide crystals leaving the black metallic silver and hardens the film emulsion with solvent, fixing agent, an acidifier, a hardener, and a preservative.
Manual processing tanks must be large enough to accept 14 17-inch film hangers.
Developing depends on duration of immersion (5 minutes recommended) and temperature of chemicals (68°F or 20°C). The rinse bath removes the developer. The fixing time is double the developing time.
The wash tank should have fresh circulating water to decrease the time needed for the final wash for 30 minutes.

Automatic Processing

Develop film more quickly (90 to 120 seconds), consistently high-quality radiographs, and eliminate the need for repeat radiographs because of processing errors.
The rollers can be cleaned with mild detergent and a soft sponge and tanks at a 1:32 solution of laundry bleach.