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Transcript
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) oUnit of power is the watt •Toady’s x-ray units use a max. of 300 milliamperes and 125,000 volts oHow many mA are in 1 ampere? oHow 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 oBrings power directly to the x-ray unit •Line voltage compensator oIncreases/decreases incoming voltage oWon’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. oVacuum 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 oTransformers supply the power to here •We need 5 elements to produce x-rays: 1. 2. 3. 4. 5. 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 Provides a source of electrons and directs them towards a target area oOverall holds a negative charge • Circuit is activated filament transformer sends electricity here heats up electrons leave the filament oTwo 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 oWhere the electrons are being sent to • Where are they coming from? oTarget is mounted on an angled disc •Acts as an electrical conductor oTransmits the electrons back to the generator • Completes the circuit •Made of tungsten, tungsten molybdenum or tungsten/rhenium alloy oAll heat resistant materials: which is best? •Rotates so photons are not always focused on the same spot oDissipates the heat Rotor Circuit •Turns the rotating anode oMore 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 oHappens at the bearings Stationary Anode •Anode is fixed in place oWhat is the concern here? oOperator must follow the warning lights on the machine! •Made of copper oHas a wide stem •Also composed of tungsten oTo 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 oThis makes the x-ray beam very narrow oNarrow 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 oAnode 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 20 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 oHead usually to right The Exposure Switch Sets the events of producing an x-ray in motion •Usually 2-stages o1st stage activates the rotor, filament circuit, and transformers o2nd Stage causes the exposure through the tube •Can be hand controlled or via foot pedal oExposure takes two movements •You must follow the light signals o“wait”, “ready”, etc Exposure Concerns: •You are now generating x-radiation! oThoughts? •Be familiar with the noises oThey may scare your patient – be ready! • May need to condition the patient •Sound of boiling liquid is bad – tube could rupture