Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. Review for Rad Physics 50 questions, 21 over old material Old material How does x-ray interact with matter? Know this cold! Chapter 13, p 149 - 160 X-ray beam photons interact with matter in 5 ways 1. Classical scatter – aka Thomson scatter – low energy The net result of classical scattering is a change in direction of the x-ray without a change in wavelength ( energy ). Involves low energy x-rays which contribute little to the radiograph. Classical scatter occurs throughout th diagnostic range but is more prevalent at lower energies ( At 70 kVp 3% classical scatter contributes to small amount of film fog ) 2. Compton effect - high energy, diagnostic range. Is the cause of all scatter. Takes away contrast results in longer gray scale. X-ray photon comes in, approaches the atom, … knocks outer shell electron recoil electron. Photon deviates and results in scatter. The photon is not used up in the process. It interacts with the film and results in a low contrast film. Occurs during the diagnostic range and begins to dominate above 180 kVp. Almost all the scatter radiation that we encounter in diagnostic radiology comes from Compton scattering. This scatter contributes no useful information to the image. Characteristics of Compton scatter Most likely to occur with loosely bound outer shell electrons As x-ray energy increases the probability of penetration through tissue without interaction also increases, there is increased probability of Compton relative to the photoelectric effect, there is reduced probability of Compton scattering. As mass density of pt increases there is a proportional increase in x-ray attenuation(partial absorbtion of energy) there is more Compton scatter 3. Photoelectric effect - characteristic radiation – not many x-rays 1. electron – at the anode 2. photon – in the patient’s body; photon comes in, approaches the atom, K shell electron is weaker than the photon energy interaction probability is high: K shell electron is knocked out of its shell, photon is used up, the electron that got kicked out is the photoelectron. Result of photoelectric effect is 1 electron and 1 photon. The general process of the photoelectric effect is ionization. This occurs when an incident electron is totally absorbed during the ionization of an inner shell electron. The incident photon disappears, the K shell electron = photoelectron – is ejected from the atom with kinetic energy equal to the difference b/w the energy of the incident electron and the binding energy of the k-shell electron. A photoelectric interaction cannot occur unless the incident x-ray has energy equal to or greater than the electron binding energy. Most likely to occur with inner shell electrons With tightly bound electrons When x-ray energy is just higher than electron binding energy As x-ray energy increases increased probability of penetration through tissue without interaction Less probability of photoelectric effect relative to Compton scatter Reduced absolute photoelectric effect 4. Pair production Occurs only with high energy x-rays and gamma rays above 1.02 meV and predominates above 24 meV. The incident X-ray interacts with the nuclear force field and is completely absorbed. From this, 2 particles are produced: 1. One electron 2. One positron This is bad if the patient is undergoing radiation therapy. 1 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999 DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. The positron is a positively charged electron, a form of antimatter. Therefore, it will only exist until it meets another form of matter. When this reaction occurs, both particles will mutually destroy each other in a process called Annihilation reaction. When this reaction occurs 2 photons are emitted in opposite directions of each other with an energy of .51 meV each This energy is the result of Einstein’s equation E=MC 2 If the energy of an electron is calculated in this equation, the result is .51 meV Pair production rarely occurs in the diagnostic range, it is more common in therapeutic radiology 5. Photodisintegration Occurs only with high energy X-rays and gamma rays above 10 meV. Some of these high energy photons can penetrate the nuclear force field and strike the nucleus. When this happens, the photon is completely absorbed and a particle ( nucleon ) from the nucleus is emitted. Rarely occurs in diagnostic range Summary Photoelectric effect is most important for diagnostic radiology because it is responsible for differential absorption and therefore subject contrast. Compton effect is important because it is responsible for creating scatter radiation. Coherent or unmodified scatter could produce fog on a film if no filtration is used to remove the low energy photons. Pair production and photodisintegration are of no importance in diagnostic radiography. Scatter increases film fog, decreases image contrast. Grids clean up scatter. Binding energy An electron from a higher shell drops down, has higher energy, needs to loose some, gives it off. The difference in energy b/w the 2 shells is called binding energy. Energy which binds an electron to a given shell. Binding energy is higher in shells closer to the nucleus. Since the nucleus of each element is different (atomic number), each element will have its own characteristic binding energies for the various shells Potential energy: Ability to do work due to position. Electrons farther out from the nucleus have more potential energy. Ionization – an electron goes to a higher state, looses the energy and drops back down. As it goes back down, it gives off an electron – deionization. Excitation – electron goes to a higher state. Air is radiolucent Basic principle of x-ray machines: Electrons are boiled off, thermoionic emission. Amount of electrons is determined by the current going through the filament – mAs. The overall purpose of the X-ray machine is to convert potential energy to electromagnetic energy. A high electric potential (high voltage) is created between the cathode and the anode of the X-ray tube – the electrons are boiled off. A high amperage filament circuit at the cathode provides free electrons The high voltage causes the free electrons to accelerate and stream across the gap. KVp is the potential difference b/w the cathode and the anode – potential energy Thus the electric potential energy is converted to the kinetic energy of the electrons When the electrons collide or interact with the atoms of the anode, some of the kinetic energy is converted to electromagnetic energy in the form of X-rays The X-ray tube must meet the following criteria: 1. Generate a source of electrons 2 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999 DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. This is provided by the high amperage filament circuit 2. Rapidly accelerate the electrons to the anode Provided by the high voltage tube circuit 3. Must rapidly decelerate the electrons Provided by the high atomic number atoms in the tungsten-alloy target in the anode Bremstrahlung – most x-rays Also called Brems, general, braking or white radiation, this accounts for 70-90% of all x-rays produced. Also responsible for the heterogeneity of the X-ray beam. The higher the atomic number, the higher the energy of the X-ray produced. Characteristic Radiation Accounts for 10-30% of all X-rays produced. The energy of the characteristic X-rays depends on the material used as a target. Each orbit will produce its own specific energy of X-ray. The higher the atomic number, the higher the energy. Half-wave rectification Represents a condition in which the voltage is not allowed to swing negatively during the negative half of its cycle. If a rectifier is placed in an AC circuit, only 1/2 the pulses will pass through the circuit Electrons flow only during one-half of the input waveform cycle By eliminating 1/2 the pulses, you lose 1/2 the energy of the original AC current Since the resulting DC current rises and falls, it is called pulsating DC current Yields 60 positive pulses per second (60 hz system) because AC is 120V rectified by ½ wave 60 hz Full-wave rectification page 101 In full wave rectified circuit the negative half cycle corresponding to the inverse voltage is reversed so that a positive voltage is always directed across the x-ray tube. In order to avoid losing 1/2 of the pulses, a rectifier bridge can be installed in the AC current Requires a system of 4 rectifiers This is better but there is still fluctuation ( peaks and valleys, kvp ) Ripple factor This is the variation in the voltage across the X-ray tube expressed as a percentage of the maximum value. A low ripple factor is desirable, high = undesirable Example: In a single wave rectified circuit the ripple factor is 100% because the voltage goes from zero to a maximum value with each cycle. If you select the X-ray machine for 70 kVp, the resulting X-rays will range from 0 to 70kVp 3 - Phase Machines (Triple phase ) A way to decrease the ripple factor is to use 3 separate AC current sources each of which is 120 degrees out of phase to the other waves. With this type of wave pattern, the voltage never drops to zero and Xrays are constantly being produced. This is more efficient because, although still slightly pulsed, it is almost direct current Advantages of triple phase machines: Short exposure times can be achieved The X-rays produced have a higher average energy and more uniform Special transformers are used with delta windings on the primary and star windings on the secondary side. Triple phase units produce more heat in the tube, so high speed anodes must be used Produces very small ripple 3 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999 DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. High Frequency Machines Uses a single source of alternating current which is sent to a microprocessor which changes the frequency to 10,000 HZ and uses capacitors to store electricity to be discharged at a precise time. Generate almost pure D.C current with almost no ripple Heel effect – uneven distribution of beam … due to angle results in the primary beam being absorbed. An uneven distribution of radiation from cathode side to anode side of the X-ray field. Increase in radiation at the cathode end Decrease in radiation at the anode end There can be as large as a 40% difference between these two ends. The effect is caused by absorption of radiation in the heel of the anode and by the angle of the anode. A smaller anode angle will increase the amount of the heel effect, because it will result in more absorption of the beam. The shorter the focal film distance ( FFD ) from the tube to the film, the greater the heel effect will be. At 40” the heel effect is quite large. The heel effect can be eliminated by the use of a wedge filter. Or place the head towards the anode ( thinner thoracics ) and the feet towards the cathode ( thicker thoracics ) to use the heel effect in our favor. Filtration – added vs inherent Removes the soft or low energy X-rays from the X-ray beam which can reduce skin dose by 90%. Consists of aluminum which is placed in between the X-ray tube window and the collimator. Compensation filter is a type of added filter Placed in the beam to modify the X-ray field distribution to compensate for wide variations in patient thickness NEW MATERIAL Use common sense – cut out the fluff Intensifying screens How do they work, how are they rated, factors Gurney Mott Hypothesis deals with the process of development of a latent image to a manifest image. Takes place in the development tank. Conduction Band Theory An X-ray strikes an outer shell electron which is then raised to an excited energy state. This creates a hole in the outer electron shell which is an unstable condition. The hole is filled when the excited electron returns to its normal state. To do this it emits energy in the form of visible light Crystal size Spatial resolution Speed Why are screens used? Phosphor layer Is the active layer of the intensifying screen. Phosphor layer emits light during stimulation by X-rays, converts the energy of the X-ray beam into visible light Common Phosphors Used: 4 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999 DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. Calcium tungstate – not much in use any more Zinc sulfide Barium lead sulfate Rare Earth phosphors – most often used now, but difficult to manufacture Gadolinium – also contrast material, green Lanthanum Yttrium - blue Properties of Intensifying Phosphors: 1. Should have a high atomic number, so that X-ray absorption is high 2. Should emit a large amount of light per absorption of X-ray photons, this is called conversion efficiency 3. The spectral emission of the screen must match the sensitivity of the X-ray film, this is called spectral matching 4. Should not be affected by heat, humidity, or other factors in order to keep them from spontaneously discharging. Luminescence Light is emitted by the phosphor layer by a process called luminescence Luminescence may occur by two different processes: NOTE Dr. Mestan made a mistake when he talked about this during the review, he reversed it! 1. Fluorescence Light is emitted within 10-8 seconds after being exposed to radiation In this process, light is emitted promptly and the stops Process similar to characteristic X-ray emission, BUT it involves outer shell electrons and is called the conduction band theory 2. Phosphorescence Light is emitted longer than 10-8 seconds after being X-rayed, which results in a delayed emission of light causing an afterglow or lag. This is undesirable! ( This is used in Fluoroscopy and TV screens. )The intensifying screens use phosphors that emit light by fluorescence, otherwise the film would turn black from too much exposure Spectral matching - film is sensitive to a specific color and is used with a screen with that color. Duplixing Developers 90 second Also called fast access Processes 300 film/hr with a 950F developer temperature 18 to 22 second developing time depending on brand Most common processor currently on the market The higher the temperature of the developer, the lower the contrast Transport roller system Series of rollers racks and drive chains powered by an electric motor, that transports the film through the various solutions This system is made up of 3 sub-systems Roller sub-assembly - Two types of rollers 1. Transport rollers - 1” diameter, more numerous, mounted in pairs 2. Master rollers - 3” diameter, normally found at the bottom of each tank when the film must bend to turn back upwards Replenisher Low volume practice – flood replenisher 5 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999 DISCLAIMER: Please be advised that the information contained within this text is extra study material and should NOT be deemed as completely correct without mistakes. It is the responsibility of the user to verify the correctness of ALL information contained within. High volume – automatic or continuous replenishment The pH, temp, how long the film is in there is what makes the developer work. Oxidation makes the developer old, after a certain period of time, the developer becomes exhausted or used up. This is governed by: Rate of oxidation Number of film processed Accumulation of by-products (Bromides) Know the properties of film Sensitive to light, etc. Silver reclamation, 2 types 1. Metallic replacement: Uses steel wool or iron impregnated from inside a plastic container The silver ions replace the iron and settle in the container in the form of silver sludge Efficiency is about 80% - 85%, designed for low volume operation 2. Electrolytic Uses electric current, yields high purity of silver Brings in more money than metallic replacement, but cost is between $150 and $2000 6 edited by Marie Paas Rad Physics Tri 2 Test 2 material July 1999