X-Ray Production Study Guide - Chapter 3 PDF

Study Guide- Chapter 3 ▪ How fast do electrons travel? While x-rays travel at the speed of light, moving electrons travelling from cathode to anode travel at approximately half the speed of light. ▪ The electrons interact with the top 0.5mm surface of the anode target. ▪ What are the two...

Study Guide- Chapter 3 ▪ How fast do electrons travel? While x-rays travel at the speed of light, moving electrons travelling from cathode to anode travel at approximately half the speed of light. ▪ The electrons interact with the top 0.5mm surface of the anode target. ▪ What are the two types of x-ray production interactions? Two types of interactions are responsible for x-ray production: Bremstrahlung and Characteristic interactions. ▪ Bremstrahlung is a German word that means “braking” or “slowing down radiation”. o In this type of interaction, the electrons completely avoid the orbital electrons in the tungsten atom and travel very close to its nucleus. The closest the electron travels to the nucleus, the higher the attraction, which means the incoming electron loses energy and changes its direction. The loss of energy reappears as an x-ray photon. The more energy that is lost by the electron, the stronger the x-ray photon will be (higher energy). o Conversely, if the incoming electron travels farther from the nucleus, the weaker the attraction, which means there is less loss of energy. The electron changes direction and the emerging x-ray photon is weaker (lower energy). o How is x-ray energy measured? X-ray energy is measured in kiloelectron volts (keV). 1 keV is equal to 1000 electron volts. o How do you calculate the energy of a bremsstrahlung x-ray photon? Subtract the energy that the projectile electron exited the atom with from the energy it entered the atom with. For example: the electron exited with an energy of 40keV, and entered with 100keV, the resulting photon would have an energy of 60keV. ▪ Characteristic interactions are termed this way because the resulting x-rays have energies characteristic of the tungsten element and its binding energy values. o In this type of interaction, the projectile electron interacts with an electron from the inner shell (K-shell) of a tungsten atom. The projectile electron must possess an energy equal to or higher than the binding energy of the electron shell to be able to eject an electron. When the electron is ejected from the K shell, a vacancy is created. Electrons from outer shells (L or M) will cascade in to fill the vacancy. This is called electron transition and creates an energy difference that results in the x-ray photon. o How do you calculate the energy of a characteristic x-ray photon? The shells involved and their binding energies must be known and then subtracted from each other. For example: a projectile electron removes a K-shell electron, and an L-shell electron fills the vacancy. Binding energy of K shell: 69.5keV and L-Shell: 12.1keV To calculate the energy of the emerging photon: subtract 69.5 – 12.1 = 57.4keV ▪ What is the diagnostic energy range? 30keV to 150keV. ▪ Most x-rays interactions are bremsstrahlung in the diagnostic range. If the kVp is below 70, the entire x- ray beam (100%) results from bremsstrahlung interactions. If the kVp is 70 or higher, about 85% of the beam will result from bremsstrahlung, and about 15% from characteristic interactions. ▪ In the x-ray emission spectrum the lowest energies range from 15 to 20 keV, while the highest energy cannot exceed the kVp selected by the operator. ▪ What type of switches are used to make an x-ray exposure? Deadman switches. o Old systems have two switches, the first is a rotor or prep switch, and the second is the exposure. o In newer systems these two switches are combined into one. o These types of switches require you to apply positive pressure (pushing down) during the entire exposure process If the pressure is removed, the exposure is terminated immediately. o Pushing the prep button, causes an electrical current to be induced across the filament. o The filament current is about 3-5 Å, and operates at about 10 V. ▪ What determines the amount of current at the filament? mA selected at the panel. ▪ What is the space charge? A cloud of electrons around the filament that results from thermionic emission.  ▪ What is the space charge effect? The cloud of electrons around the filament prevents more electrons from being boiled off the filament. o Imagine a bunch of people crowded in a small room. The more people (charges) there are, the more they push and influence each other. This "crowding" makes it harder for new people (charges) to enter or move around freely. ▪ What is tube current? It is the flow of electrons from cathode to anode and it is measure in milliamperes (mA). ▪ How can the quality and quantity of x-rays be altered? By controlling the kVp, mA, and exposure time. Quantity is how many x-ray photons are in the x-ray beam and quality is their penetrating power. ▪ What determines the speed of the electrons? The kilovoltage set by the operator. o The kV is applied across the tube from cathode to anode and determines the speed of the electrons. The speed is directly related to the Kv set, but not proportional. This means that if Kv increases, the energy also increases and vice versa. The reason it is not proportional is because when the speed is doubled, the energy level will increase by its square. ▪ A kVp meter measures the actual kilovoltage. This number can vary no more than +/- 5%. If the variability is greater, x-ray quality will be affected. Generators To provide a sufficient potential difference (kVp) for x-ray production, a generator is required to convert low voltage to high voltage. There are three types of generators: 1) Single-phase: voltage ripple 100% 2) Three-phase:13% 3) High Frequency (HF):1% Each system produces a waveform which represents the voltage supplied to the x-ray tube. In a single-phase system, the voltage drops from 100 to 0 and goes up again, hence the voltage fluctuation is 100%. In three phase systems, there are three separate waveforms offset 120 degrees from each other, so the voltage never drops to zero, rather it fluctuates, which creates a ripple of 13%. In High frequency generators, there is one waveform but in much higher frequencies (kHz to MHz). ▪ What controls the number of electrons in the beam? mA, selected at the console. As the mA increases, the number of electrons increase proportionately. mA does not impact the energy or quality of the electrons. o Exposure time determines the length of time over which the x-ray tube produces x-rays. Therefore, a higher exposure time will lead to a greater flow of electrons, therefore more x-rays. Exposure time is expressed in seconds (s) or milliseconds (ms). 1 second= 1000 milliseconds o To calculate mAs= mA x seconds ▪ For example: 200mA X 0.25s= 50 mAs ▪ To convert milliseconds to seconds= divide by 1000 ▪ To convert seconds to milliseconds= multiply by 100 ▪ A digital timer device measures the actual exposure time. The variability cannot be +/-5% for times >10ms and +/-10% for times