Chapter 4: Diagnostic X-Ray Production

Learning Objectives: Chapter 4 Understand the mechanical components both inside and outside of an x-ray unit List & describe the parts of an x-ray tube and trace the creation of an x-ray Describe the Line Focus Principle and its application in radiography Define the focal spot, and understand problems that can occur with it Define the anode heel effect

Steps in Creating a Radiograph 1. Getting power to the unit and tube 2. Heating the tube & producing energy 3. Converting energy to x-rays 4. Focusing the beam 5. Exposing the film

Getting Power to the Unit Power (W) = current (I) x volts (V) Unit of power is the watt Toady’s x-ray units use a max. of 300 milliamperes and 125,000 volts How many mA are in 1 ampere? How many watts is this? *Generators in clinics are rated at 30,000 W min. How many kilowatts is this?

Equipment Relating to Power Energy that is needed travels from a power plant to clinic generator Transformers boost power along the way Alternating current travels in 1 positive and then 1 negative pulse This provides low current, with high voltage Cheaper and more efficient than direct current Power goes to the circuit breaker first Accepts the current into the unit (milliamperage) Needs to have a ground wire for safety *When would the circuit be interrupted?

Equipment Relating to Power Wall switch- within reach of the x-ray unit Required by law On-off switch located on x-ray unit too Brings power directly to the x-ray unit Line voltage compensator Increases/decreases incoming voltage Won’t see this on new units *Radiology uses mA to regulate the number of electrons used to produce x-rays*

Getting Power to the X-Ray Tube 3 transformers in an X-ray circuit: 1. Autotransformer – increases incoming voltage Allows operator to select kV’s to produce a radiograph 2. High-voltage transformer Final increase of voltage From 220 volts to 125,000 volts (maximum) 3. Filament transformer Produces voltage to the x-ray tube filament in the form of heat Filament requires specific temp. for exposure to happen

X-Ray Unit 4 essentials to every unit: Control Panel/Generator X-Ray tube High-tension transformer Dedicated power line

How are X-Rays Produced? When electrons are slowed or stopped by the atoms of a target area, x-rays are produced. This happens inside the x-ray tube, which is a vacuum environment. Vacuum allows electrons to keep their energy Once the electrons strike the target area, an x-ray beam of photons is created.

X-Ray Tube Where X-rays are generated Transformers supply the power to here We need 5 elements to produce x-rays: Source of electrons A way to accelerate the electrons Obstacle free path Target area A heat-resistant glass envelope to provide a vacuum environment Produces not only x-rays but a lot of heat

The X-Ray Tube

Cathode Two filaments made of tungsten Provides a source of electrons and directs them towards a target area Overall holds a negative charge Circuit is activated  filament transformer sends electricity here  heats up  electrons leave the filament Two filaments made of tungsten Filaments emit electrons when heated electrons are held less tightly and become excited Filaments are found in the focusing cup of the cathode Point directly towards the target area on the anode

Rotating Anode Contains the target of the x-ray tube Where the electrons are being sent to Where are they coming from? Target is mounted on an angled disc Acts as an electrical conductor Transmits the electrons back to the generator Completes the circuit Made of tungsten, tungsten molybdenum or tungsten/rhenium alloy All heat resistant materials: which is best? Rotates so photons are not always focused on the same spot Dissipates the heat

Rotor Circuit Turns the rotating anode More than 3000 revolutions/minute! Activated when the filament transformer starts to heat the cathode Prepares the anode to receive electrons Could be the source of loud noise Happens at the bearings

Stationary Anode Anode is fixed in place Made of copper What is the concern here? Operator must follow the warning lights on the machine! Made of copper Has a wide stem Also composed of tungsten To handle the x-ray conversion When are these used?

The Line Focus Principle The change of direction of the radiation due to the angle of the bevel on the anode. Ideal bevel angle is <15° from vertical This makes the x-ray beam very narrow Narrow beam = high resolution image Purpose is to make a small “effective focal-spot size”.

Possible Focal Spot Issues: 1. Off-Focus Radiation: Produced by electrons without great aim (hit outside of the target) Bounce off the anode and to the patient outside of the focal spot Collar of lead around tube normally prevents this Can appear as artifact on film if the lead is not sufficient *Prevent by having machine maintenance performed

Off-Focus Radiation

Possible Focal Spot Issues 2. Heat Bloom: Target is exposed to radiation a great amount in a short time Anode gets too hot and is not allowed to cool Outer edges of focal spot expand Enlarges effective focal spot size Decrease in image resolution (Image loses sharpness) *Can be eliminated by checking your anode cooling chart

Focal Spot Bloom

The Anode Heel Effect Causes the intensity of radiation to be greater on the cathode side of the x-ray tube Bevel of the angled anode limits the amount of x-rays produced on anode side Place thicker end of patient on the cathode side Head usually to right

The Exposure Switch Usually 2-stages Sets the events of producing an x-ray in motion Usually 2-stages 1st stage activates the rotor, filament circuit, and transformers 2nd Stage causes the exposure through the tube Can be hand controlled or via foot pedal Exposure takes two movements You must follow the light signals “wait”, “ready”, etc

Exposure Concerns: You are now generating x-radiation! Thoughts? Be familiar with the noises They may scare your patient – be ready! May need to condition the patient Sound of boiling liquid is bad – tube could rupture