X-Ray Beam Limitation Devices

Study Notes

The primary x-ray beam should be collimated to the size of the image receptor (IR) being used for the examination.
This is achieved using a light-localizing variable-aperture rectangular collimator, which adjusts the size and shape of the x-ray beam.
The collimator is the most popular x-ray beam limitation device.

These devices confine the primary beam before it enters the area of clinical interest, limiting the quantity of body tissue irradiated.
They reduce scattered radiation in the tissue and prevent unnecessary exposure to tissues not under examination.

Exposure reproducibility refers to the consistency in output radiation intensity for identical generator settings.
Variance of 5% or less is acceptable.
Exposure linearity is the ratio of the difference in mR/mAs values between two successive generator settings to the sum of those mR/mAs values.
It must be less than 0.1.

A radiographic grid is a device made of parallel radiopaque strips alternately separated with low-attenuation strips.
The grid's purpose is to reduce scattered radiation and improve image quality.

The radiographer's responsibility is to minimize radiation exposure by using correct technical exposure factors.
The radiographer can manipulate the image to avoid overexposure.
CR phosphor sensitivity is approximately equal to a 200-speed screen-film combination.
CR has greater kilovoltage flexibility than conventional screen-film radiography.
X-ray beam collimation and centering of the body part on the CR cassette are essential.
Radiographic grids should be used frequently, except for the majority of pediatric patients.

Fluoroscopic procedures require intermittent, or pulsed, fluoroscopy to reduce patient dose.
The last-image-hold feature is beneficial.
Limiting fluoroscopic field size reduces patient dose.
Technical exposure factors for adult patients include:
- kVp range: 75 to 110 kVp, depending on the body area being examined
- SSD not less than 38 cm (15 inches) for stationary fluoroscopes; not less than 30 cm (12 inches) for mobile fluoroscopes
Selection of technical exposure factors for children involves:
- Decreasing kVp by up to 25%
- Decreasing technical exposure factors, maintaining SSD, and minimizing the height of the image intensifier entrance surface above the patient
Filtration is required, with a half-value layer (HVL) of 3 to 4.5 mm aluminum.
SSD should be no less than 38 cm (15 inches) for stationary fluoroscopes; no less than 30 cm (12 inches) for mobile fluoroscopes.
Cumulative timing devices are required to monitor fluoroscopic exposure time.
Exposure rate limitation is essential, with a maximum of 100 mGy per minute (10 R/min) for general-purpose intensified fluoroscopic units, and 200 mGy per minute (20 R/min) for units equipped with high-level control (HLC).