Page 224 - Adams and Stashak's Lameness in Horses, 7th Edition

190 Chapter 3

Kilovoltage determines the energy of X‐rays being X‐ray detector being used. Collimating the primary beam
produced. Kilovoltage potential should range from 70 is a safety practice that must be monitored for each
VetBooks.ir when a portable X‐ray machine is used. A 10% increase Limiting the size of the primary X‐ray beam is a major
exposure and must be reset if the cassette size changes.
to 90 for equine limb radiographs on mature horses
factor in reducing scatter radiation, which keeps radia­
or decrease in kilovoltage potential effectively doubles
or halves the radiation exposure. tion exposure to personnel as low as possible.
The focal spot‐film distance (FFD) is the distance Collimation also improves image quality, particularly
from the X‐ray tube focal spot to the detector. The focal in large body parts such as the caudal cervical spine.
spot location is usually marked on the outside tube Fixed cylinders or cones and adjustable light‐beam col­
housing; if it is not, the approximate center of the tube limators are available (Figure 3.2). The disadvantage
housing can be used for its location. A constant FFD is of fixed primary beam restrictors is that they do not
necessary to minimize improper exposures because the conform to different cassette sizes. Adjustable light‐
intensity of X‐rays that expose the film is inversely beam collimation is recommended for equine radiogra­
proportional to the square of the FFD. Therefore, a phy and is standard to many X‐ray machines. They
small difference in the FFD can dramatically change the come with an internal light source, preferably 40 W or
48
exposure on the X‐ray film. greater, so the limits of the primary X‐ray beam are
Some method of measuring the FFD before each expo­ projected on the X‐ray cassette/detector as visible light.
sure should be employed. Measuring devices with The type of X‐ray machines best suited for equine
converging light beams at the correctly set FFD are easy to practice depends on the type of practice: out of hospital
use and less cumbersome than a measuring tape or string. versus in hospital. The features of the X‐ray machines
The FFD for equine limb radiography should be between must be matched to fit different practice situations.
36 and 40 inches (85 and 100 cm) and no less than Some compromises or trade‐offs must be made, e.g. less
24 inches (60 cm). When the FFD is less than 24 inches, the milliamperage or kilovoltage potential for more porta­
object is magnified, and spatial resolution is reduced. FFDs bility. An ideal X‐ray machine for equine radiography
greater than 40 inches can be used if exposure times are has the following features:
not excessively long. When making radiographs of larger 1. Easily and quietly movable, with a tube head that can
body parts such as necks, it is important to remember that
extend to the floor or ground surface
the distance needs to be set from the X‐ray tube to the 2. Adjustable milliamperage and kilovoltage potential
detector, not to the skin surface, a concept that is more
setting
easily forgotten when the plate is not well seen by the 3. Electronic, two‐step timer capable of accurate
person holding the X‐ray generator.
Collimation is the process of limiting or restricting exposure times of 0.1 second or faster
the primary X‐ray beam to the appropriate size to 4. Line voltage compensator and compensation meter
cover the anatomical region of interest within the

Figure 3.2. Adjustable light‐beam collimator. The primary beam
is limited by adjustable leaves in the collimator. A light source within
Figure 3.1. Control panel for a 30‐mA portable X‐ray machine. the collimator defines the limits of the primary beam (white arrows).