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Chapter 6 (Varkey)
Exam 1
| Question | Answer |
|---|---|
| What are the various support systems used for x-ray tubes? | (All found on pg. 105) 1. Ceiling Support System 2. Floor to Ceiling System 3. C-Arm System |
| What does "x-rays are emitted isotropically" mean? | They are emitted with equal intensity in all directions. |
| Explain "useful beam" and "leakage radiation": | X-rays are emitted through a special section of the tube called the window. Useful Beam - The beams that are emitted through the window. Leakage Radiation- X-rays that escape through the protective housing. (results in unnecessary exposure to the tech/Pt) |
| What is the function of the protective housing? | *reduces the level of leakage radiation to less than 1mGya/hr when operated at max conditions. *incorporates specially designed high voltage receptacles to protect again electric shock. *Provides mechanical support and protects it from damage. |
| Protective Housing continuation... | Some x-ray tubes contain an oil, it serves both as an insulator (for electric shock) and as a thermal cushion (dissipate heat). If the heat gets to high, the oil expands and a microswitch is activated, so the x-ray tube won't function until it's cool. |
| Advantages of Metal vs. Glass enclosures | Glass-made of pyrex glass, this enables it to withstand the tremendous heat. Metal - maintain a constant electric potential between electrons of the tube current and the enclosure. (therefore, has a longer life and less likely to fail) |
| An x-ray tube is an? | Electronic vacuum tube with components contained within a glass or metal enclosure. |
| Modern x-ray tubes are___________ | Coolidge tubes, is a vacuum tube. (*It can become gassy, causing x-ray production to fall, and the tube can fail) |
| Early x-ray tubes were_____________ | modifications of the Crookes Tube, which were not vacuum but rather controlled quantities of gas within the enclosure. |
| What are the 2 primary parts of the cathode assembly? | Filament and focusing cup. |
| What is a filament? | A coil of wire similar to that of a kitchen toaster, just smaller. Electric current is conducted through the filament. The filament emits electrons when heated. |
| What element is used to make the filament of an x-ray tube? | Thoriated Tungsten |
| What are the advantages of Tungsten? | Tungsten provides for higher thermionic emission than other metals. It's melting point is 3410*C, therefore; it's not likely to burn out. It also doesn't vaporize easily. The Thorium added enhances the efficiency of thermionic emission and tube life. |
| What is Thermionic Emission? | When the current that runs through the filament is sufficiently high, the outer shell electrons of the filament atoms are "boiled off" and ejected from the filament. |
| What is the most common cause of tube failure? | When tubes with glass enclosures age, some of the tungsten vaporizes and coats the inside of the glass. This alters the electrical properties of the tube, allowing current to stray and interact with the glass. This causes arcing & tube failure (Arcing) |
| What is the function of the focusing cup? | The filament is embedded in a metal shroud called the focusing cup. The focusing cup is negatively charged so that it electrostatically confines the electron beam to a small area on the anode. |
| Side Note... | Most x-ray tubes contain 2 filaments mounted in the cathode "side by side", creating a large and small focal spot. Biangle x-ray tubes have them "end to end", so small focal spot above the large. They are made of nickel. |
| What is a grid-controlled x-ray tube? | They are designed to be turned on/off very rapidly. They are used for multiple exposure systems for precise exposure times. The focusing cup is the grid and therefore the exposure switch. |
| The x-ray tube current is adjusted by controlling ______________ | the filament current |
| What is filament current? | Heats the filament to emit electrons. Once emitted from the filament, the electrons form an electron cloud around the filament known as a space charge. |
| What is a space charge? | Space charge is a cloud of electrons, it makes it difficult for subsequent electrons to be emitted by the filament because of the electrostatic repulsion. Also referred to as Space Charge Effect |
| What is saturation current? | An increase in kVp does not result in a higher mA because all the available electrons have been used. |
| What is meant by dual focus x-ray tube? | They have 2 focal spots (small/large). Small ranges from .1 to 1mm and large ranges from .3 to 2mm. Large focal spot is used for large body parts and small typically for extremities. |
| What is a layered anode? | A layered anode consists of a target surface backed by one or more layers to increase heat capacity. (Tungsten, Molybdenum, and Graphite) |
| Benefits of a layered anode? | Molybdenum and Graphite have lower mass density than Tungsten, making the anode lighter and easier to rotate. |
| What is the advantage of a rotating anode? | Rotating anodes are capable of producing high intensity x-rays beams in a short time. It allows for the electron beam to interact with a much larger target area. The heating of the anode is not confined to one spot, it provides 500x more area. |
| How fast does a rotating anode go? | The heat capacity can be improved by increasing the speed of the anode rotation. Most rotate at 3400 rpm, while high intensity x-ray tubes rotate at 10,000 rpm. |
| What is the metal used in the construction of anode? | The metal used in the construction of the anode would be Tungsten, Copper, Molybdenum and, Graphite. |
| What is a target? | It is the area of the anode struck by the electrons from the cathode. |
| How pitting and cracks occur on anode? What are the consequences? How can you avoid or prolong it from happening? | Pitting and Cracks occur on the anode when the rotating mechanism of the anode becomes overheated. |
| The rotating anode is powered by ____________________ | electromagnetic induction motor (More info on Pg. 113) |
| Understand Rotor and stator | Stator – Consist of a series of electromagnets equally spaced around the neck of the tube. Rotor – Inside the enclosure is a shaft made of bars of copper and soft iron fabricated into one mass. |
| What is meant by coast time? | The time it takes for the anode to come to a complete stop. It's approximately 60 sec. for new x-ray tubes. |
| Explain line focus principle ... | To allow for a large area for heating, the principle states by angling the target one makes the effective area of the target much smaller than the actual area of electron interaction. |
| What are actual focal spot and effective focal spot? | Effective focal spot – Is what is projected onto the patient Actual Focal Spot – Is what hits the anode |
| ____________ is the actual source of the x-ray. | The focal spot |
| What is the difference between a tube current and filament current? | Tube current is controlled by changing the filament current because of thermionic emission, a small change in filament current result in a large change in tube current. |
| Diagnostic x-ray tubes have target angle that vary from approximately ___ to ____ degrees. | 5 - 20 |
| When anode angle increases the effective focal spot size ________ | Increases |
| Anode angle and the size of the effective focal spot is ____________ proportional | Directly |
| Explain anode heel effect... | Anode Heel Effect states that the radiation intensity on the cathode side of the x-ray field is greater than that on the anode side. |
| When anode angle decreases the anode heel effect ______________ | Increases |
| Anode angle and heel effect is _____________ proportional. | Indirectly |
| The smaller the anode angle, the ___________ the heel effect. | Larger |
| Which side of the x-ray tube has stronger x-ray beam? | If the radiation intensity along the central ray is designated as 100%, then the intensity on the cathode side may be as high as 120%, and that on the anode side may be as low as 75%. The stronger x-ray beam is in the cathode side. |
| How can you take advantage of it? | Positioning the cathode side of the x-ray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor. |
| The usual shape of a ___________ is the double banana. | focal spot |
| Is the size of effective focal spot same at cathode side and anode side? Explain | The size of effective focal spot is less on the anode side of the x-ray beam. This happens because the effective spot changes size and shape across the projected x-ray field. |
| What is off focus radiation? How can you reduce it? | Off Focus Radiation can be described as electrons that bounce off of the focal spot and landing on other areas of the target, causing x-rays to be produced from outside of the focal spot. |
| How can you reduce it? | By using variable aperture collimators. Also by designing a fixed diaphragm in the tube housing near the window of the x-ray tube. also using a metal enclosure x-ray tube. Electrons are reflected from the focal spot are extracted and conducted away. |
| What are the causes of tube failures? | Mostly occur because of Thermal Characteristics (Radiation, Convection and, Conduction. Vaporization and constant heating without allowing the temperature to drop. |
| Tube Failures continued... | - Wrong use of mA and kVp - Wrong use of image receptor - Not warming up the tube at the beginning of the day and whenever the anode is cold. |
| What is an open filament? | Excessive heating of the filament caused by mA operation for prolonged periods, more tungsten is vaporized. The filament becomes thinner and eventually breaks, producing and open filament. |
| What is the most frequent cause of abrupt tube failure? | The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure because of vaporized tungsten. |
| What is the function of a tube rating chart? | A guide regarding the most common technical factor combinations that can be used without overloading the tube. |
| Describe the importance of the radiographic rating chart... | The radiographic rating chart is the most important because it conveys which radiographic techniques are safe and which techniques are unsafe for x-ray tube operation. |
| Describe the importance of the anode cooling chart... | Anode cooling chart help us to determine how long we have to wait to make further exposures after the anode has reached its maximum or near maximum capacity. |
| Describe the importance of the housing cooling chart... | It is used in precisely the same way as the anode cooling chart. |
| LEARN HOW TO USE THE RATING CHART... | |
| LEARN TO CALCULATE THE HEAT UNIT AND TO DO THE MATH PROBLEMS USING COOLING CHART... | HU = kVp * mA * time * constant Generator Type Constant Single-phase 1.00 Three-Phase, Six Pulse 1.35 Three-Phase, Twelve Pulse 1.41 High-Frequency 1.45 |
Created by:
ROOD33