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Chapter 6 Imaging
X-Ray Tube
| Question | Answer |
|---|---|
| The X-Ray tube consists of what three parts? | The support structure, The protective housing, The glass/ metal enclosure. |
| The x-ray tube is a what? | a vacuum contained in a glass or metal enclosure |
| Pyrex glass can withstand what? | large amounts of heat |
| the vacuum allows for more what? | efficient flow of electrons during x-ray production |
| As a glass enclosure/envelope tubs ages, this alters the what? | electrical properties of the tube, allowing tube current (electron flow) to stray and interact with glass enclosure resulting in arcing and tub failure. |
| Most common cause of tube failure is what? | tungsten vaporization |
| cathode is what? | negative electrode or negative side of the x-ray tube |
| cathode has two primary parts what are they? | -filament -focusing cup |
| filament is what? | coil of wire, ~2mm in diameter and 1-2 cm long, made of thoriated tungsten, 1 to 2 % thorium is added to the tungsten to increase the efficiency of thermionic emission and prolongs tub life |
| filament has a source of what? | electrons |
| why is the filament made of tungsten? | due to its high melting point and the fact the W does not evaporate easily |
| when the current is high ,the outer-shell electrons are what? | "boiled off" and ejected from the filament known as thermionic emission. |
| name for "boiled off" and ejected from the filament? | Thermionic emission |
| Most rotating anodes have what? | have two filaments, side by side |
| 2 filaments side by side are what? | dual filments |
| Rotating ahode plus what? | station |
| X-rays are emitted how? | isotropically (all directions) |
| Photons (x-rays) emitted through what? | a window |
| Photons(x-rays) emitted through a window are called what? | the useful or primary beam |
| leakage radiation levels may only equal what? | 100mR/hr at 1 meter when operated at maximum conditions |
| Focusing cup surrounds the filament and is made of what? | metal, usually nickel/molybdenum (low or slight negative charge) |
| the focusing cup makes sure what? | Electrons are sent as a beam to the anode |
| Filament current (amperes) works how? | a low current is passed through the filament to warm it up before the higher voltage is applied for x-ray production |
| A filament current is what? | no tube current (milliamperes) because the filament doesn't get hot enough for thermionic emission. |
| A high filament current is what? | thermionic emission |
| X-ray tube current is controlled by what? | a filament current |
| mA settings determine the amount of what? | current being sent to the filament |
| the relationship between filament and tube current depends on what? | tube voltage (kVp) |
| the cloud of electrons around the focusing cup is referred to as the? | "space change" |
| Most x-ray tubes have two focal spots what are they? | one large and one small |
| Space charge effect means what? | no more electrons can be boiled off |
| small focal spot is used when? | better spatial "detail" resolution ( the ability to image small objects that have high subject contrast) is needed. |
| large focal spot is used when? | large body parts are image or when images using high technique, that will produce more heat, are needed. |
| selection of the small or large focal spot is made on the control panel when? | selecting the mA station |
| either filament can be used with what mA? | 300 mA or less |
| only the large focal spot can be used with what mA? | 400 mA or up |
| small focal spots range from what? | .1 to 1mm |
| large focal points range from what? | .3 to 2mm |
| Anode is what? | the positive electrode or the positive side of the x-ray tube |
| there are two types of anode what are they? | rotating and stationary |
| what are the three functions of the anode? | electric conductor, mechanical support, thermal dissipater |
| what does the electric conductor of the anode do? | receives electrons emitted by the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator |
| what does the thermal dissipater of the anode do? | when the projectile electrons interact with the anode, 99% of their kinetic E " E=energy" is converted to heat. |
| In the thermal dissipater of the anode why must the heat be dissipated quickly? | so copper, molybdenum, and graphite are usually the materials anodes are made of due to high melting points. |
| projectile elements comes from where to where? | cathode to anode at a high rate of speed. |
| X-ray are what % | 99% heat and 1% xray |
| the target is what? | the area the anode struck by electrons from the cathode |
| stationary anodes - targets are made of? | a W "tungsten" alloy embedded the in copper anode |
| rotating anodes the entire rotating disc is the? | target |
| rotating targets composition can what? | vary. |
| alloying with W "tungsten" (usually rhenium) gives? | added strength to withstand rotation. |
| high-capacity tubes have? | molybdenum and graphite layered under the W target. |
| molybdenum and graphite are what? | lighter than W and help the anode to be lighter and easier to rotate |
| tungsten is used for? | anode targets |
| what is the atomic number for the anode target and what does it do? | Z number of 74, helps with efficient production and high E x-rays |
| the high melting point of the anode target? | melting point high and can withstand high heat without pitting or bubbling. |
| rotating anode does what? | allows the x-ray beam to interact with much larger target area so the heat is not confined to one small spot like in a stationary anode. |
| the actual target size for a stationary tube is? | 4mm2 |
| the actual target size for a rotating tube is? | 1760mm2 |
| the rotating anode provides almost what? | 500 times more area with the electron beam to interact |
| most rotating anodes rotate at? | 3600 rpm |
| high capacity anodes rotate at? | 10,000 rpm |
| rpm= | revolutions per minute |
| an electromagnetic induction motor is used to? | turn the anode |
| the induction motor has two main parts what are they? | stator and rotor |
| what is the stator of the induction motor? | a series of electromagnets on the outside of the enclosure spaced around the neck of the tube. |
| what is the rotor of the induction motor? | a shaft made of bars of copper and soft iron fabricated into one mass inside the enclosure |
| the induction motor works through? | electromagnetic induction like a transformer |
| on the induction motor engaging the exposure switch does what? | fires the stator to move in a circular motion of the rotor in the tube through electromagnetic induction. |
| the focal spot is the area of the target from which? | the X-RAYS are emitted. the actual x-ray source. |
| the smaller the focal spot the better the what? | spatial resolution on the image BUT the heating of the target is concentrated onto a smaller area. (this is the downside of small focal spot). |
| rotating anodes use the line-focus principle how? | by angling the target. |
| the effective area of the target is what? | much smaller than the actual area of the electron interaction |
| effective target area or effective focal spot size is? | is the area projected onto the patient and IR. this is the value given when large or small focal spot size is used. |
| when the target angle is made smaller, the effective focal spot size is? | also made smaller |
| when the target angle is made larger, the effective focal spot size is also made? | larger |
| anode angles can vary from what degrees? | 5 to 20 degrees |
| the intensity of the x-rays that are emitted through the? | "heel" of the target are reduced because they have a longer path through the target. * this is called the heel effect.* |
| X-rays more intense on the? | cathode side |
| cathode has more of a what? | more of a angle |
| so the smaller the anode angle, the larger the? | heel effect. (less to go through, so more intense) |
| the heel effect is important when? | imaging structures that differ greatly in thickness |
| X-ray tubes are designed so that the? | projectile electrons from the cathode interact with the target only at the focal spot |
| some electrons bounce (rebounding electrons) off the focal spot and land on other areas of the target. this causes what? | x-rays to be produced from outside of the focal spot |
| what can off focus radiation do? | can image outside the collimators better known as "ghosting" |
| using minimum mA, kVp, and time appropriate to the exam and faster IR's will results in? | longer tube life |
| X-ray tube failure has several causes mostly related to what? | heat |
| heat can be dissipated by 3 ways what are they? | radiation, conduction, and convection |
| when the temperature of the anode is excessive in a single exposure, the localized surface can what? | melt and "pitting of the anode" can occur. this results in variable and reduced radiation output. |
| 2nd type of tube failure is called what? | bearing damage |
| 2nd type of tube failure results from what? | keeping the anode at elevated temperatures for prolonged periods |
| the W vaporized from the cathode along with the anode can disturb the what | electric balance of the tube,causing abrut intermittent changes in tube current, and lead to arcing. |
| the most frequent causes of abrupt tube failure is what? | electron arching from filament to enclosure due to vaporized W |
| 3 types of charts to guide the use of xray tubes are? | radiographic rating chart, anode cooling chart, housing cooling chart |
| tube rating chart does what? | provide a guide of the maximum technical factor combinations that can be used without over loading the tube. |
| anode cooling chart does what? | calculation of the time necessary for the anode to cool enough for additional exposure to b made. |
| The vacuum allows for more efficient flow of ________ during xray production. | electrons |
| Spatial Resolution is ________ | detail |
| What focal spot is used for large body parts using high technique that will produce more heat? | large focal spot |
| What is the stator? | a series of electromagnets on the outside of the enclosure spaced around the neck of the tube |
| The heel effect is important when imaging structures that will differ greatly in _________. | thickness |
| Does the use of a grid reduce off focus radiation? | No, it does not. |
| How does bearing damage occur? | results from keeping the anode at elevated temperatures for proonged periods. |
| What is used when better spatial resolution is needed? | Small focal spot |
| What is the most common external component? | Ceiling support system |
| "cloud of electrons" is referred to as the | space charge |
| What is thermionic emission? | when the current is high, and the outer shell electrons are boiled off and ejected from the filament. |
| What is the "stem?" | the shaft between the anode and the rotor |
| What is the area of the target from which the xrays are emitted? | the focal spot |
| The ____ side has more of an angle | cathode |
| Anode angles can vary from ___ to ___ degrees | 5 to 20 |
| What charge is the focusing cup? | low negative charge |
| mA settings determine the amount of current | being sent to the filament |
| 2 filaments side by side? | Dual filament |
| What does "rpm" stand for? | revolutions per minute |
| Single 0= | 1 |
| 30,6= | 1.35 |
| 30,12= | 1.41 |
| High= | 1.45 |
| maintain a constant electric potential between tube current and the enclosure? | metal enclosure |
| what is the focusing cup made of? | Nickel/ Molyhdenum |
| what does the focusing cup do? | confines electrons boil off. confines electrons boil off. |
| When electrons are emitted from the filament, what do they want to do? | spread out |
| the effectiveness of the focusing cup is determined by its________&__________also the filament_______&________and the position of ________in the focusing cup. | size & Shape, charge, filament size & shape, pos |
| A low filament current equals_______ | no tube current (MA) |
| A high filament current equals_______ | Thermionic emission |
| space charge effect is what happens when it is difficult for more electrons to be emitted due to? | Electrostatic repulsion |
| Saturation current is at any_____ | given filament |
| What is filament current? | Amperes |
| What is X-ray tube current | MA |
| What is it called when , even if the voltage is up it will not raise the MA because all the available electrons have been used? | saturation current |
| what are the 3 functions of the anode | electric conductor, mechanical support, thermal dissipater. |
| _____is the area of the anode that is struck by electrons from the cathode | target |
| ________anodes targets are made of _____shredded in the copper | stationary |
| what are the 3 reason tungsten is used for anode targets | atomic # or 2#, thermal conductivity, high melting point. |
| the___ of the anode is the shaft between the anode and the rotor | stem |
| the stem or shaft is made of | molybdenum |
| an _____ induction motor is used to turn the anode | electromagnetic |
| the induction motor works through _______ induction | electromagnetic |
| what is the cause x-ray be produced outside of the focal spot | rebounding or bounching |
| ___ radiation can image tissue outside of the focal spot | off focus |
Created by:
jackiblaylock