X-ray production

5 main criteria for the x-ray unit

1. must have enough power to eventually produce x-rays

2. must be able to increase or decrease the power selections as needed

3. power must travel in the same directions through the x-ray tube

4. must be a way to produce free electrons with enough energy to produce x-rays

5. must be an efficient way to dissipate heat resulting from interaction of photons and the anode

x-ray tube components

• produces x-rays

• consists of cathode and anode

• has a special heat resistant glass enclosure housing for cathode and anode that is shielded with metal to restrict radiation from exiting where not desired (the x-ray tube port)

The cathode

• two filaments made of tungsten, small and large for small and large body parts/pets

• filament used determined by MA setting on generator by the operator

The anode

• partner to the cathode

• consists of tungsten target that electrons (now photons of energy) are focused

• Two types of anodes: rotating and stationary

Rotating anode

• used for small animal installed x-ray units

• disperse heat more evenly, dose efficacy, and image quality stay in tact

• better for longer and larger scans

• more expensive, larger units

stationary anode

• anode does not move

• used for large animal portable units

• heat is always dissipated in the same spot, causing damage over time

• saves space, less expensive, portable

• best for short quick scans of small area

anode heel effect

• radiation intensity great on cathode side than the anode side

• useful when patients have one end of body thicker than the other (thicker end should be placed under cathode to use more intensity for the x-ray, like for deep chested dogs)

line focus principle

• describes how the electrons interact with the anode and change direction so the x-rays are actually directed toward the patient

• caused by the angle of the bevel on the outer edge of the anode

focal spot bloom (heat bloom)

• not usually an issue in vet med

• occurs when anode is angled less than 15 degrees from the vertical causing a very narrow beam and higher resolution images

• less than 15 degrees and multiple exposures (x-rays) cause the area around focal spot to heat which enlarges the focal spot causing lower resolution

off focus radiation

• caused by heat bloom

• electrons occasionally bound off the target anode and are attracted back to it but at a spot beyond the focal spot

• causes things outside the focal spot to be on x-ray (ex. if focal spot is throat, with this you may see the ears show up on the x-ray as well)

• will happen if KV is too high and collimation is not adequate

circuits

• electricity always flows in circuits or circles

• Any interruption in this circle means electricity stops flowing

Technical factors (important factors in transmitting electricity to x-ray tube)

• current (milliamperage, MA)

• voltage (kV) (KVP is what denise lists as our technical factor)

• resistance

• time

Milliamperage (MA)

• the unit used to measure the electric current that activates the x-ray tube

Voltage

• the speed at which the electrons in the electric current transfer energy along the current

• high voltage = short wavelengths, high frequency = highly penetrating x-ray

• low voltage = long wavelengths, low frequency

Resistance

• factor that slows the current as it travels through the wire

• wire large in diameter = low resistance

• wire small in diameter = high resistance

• very long or large diameter cables used in the x-ray unit can leak electric current (line loss)

time

• timer controls length of time x-rays are produced. determined on the settings control panel

Potential difference

• difference in the charge between the anode and cathode

line voltage compensator

• method of increasing or decreasing the incoming power line voltage

• technician should always check the line voltage meter on an older unit before beginning x-ray process

• if adjustments are needed they should be done before setting technical factors