X-Ray Interactions with Matter

in this video we will look at five types of interactions that x-rays have with matter by the end of this unit you will be able to describe each of the five interactions define differential absorption and describe its effect on image contrast explain the effect of kVp atomic number and tissue density on differential absorption and explain the difference between absorption and attenuation let's take a moment to review a little about electromagnetic energy x-ray photons are a type of electromagnetic energy that travel through space in a sinusoidal fashion photons that travel a shorter wavelength move with increased frequency and energy so the relationship between wavelength and energy is indirectly proportional in other words decreasing the wavelength increases the frequency and increases the energy x-rays have very short wavelengths ranging from 10 to the negative 8 to negative 9 meters if we look at three x-ray photons that travel with increasing energies you'll see how each interacts with an atom the first low-energy photon travels with a longer wavelength and interacts with the entire atom the second moderate energy photon travels with a shorter wavelength and interacts with electrons causing ionization this is the type of interaction we have in diagnostic radiology the third high-energy photon has a much shorter wavelength and completely disappears when it interacts with nuclei creating a nucleon fragment there are five specific mechanisms of interaction between x-ray photons and matter coherent scattering Compton effect photoelectric effect pair production and photodisintegration Compton effect and photoelectric effect are particularly important to diagnostic radiology and will be covered in greater detail in this video than the others incoherent scattering an x-ray photon interacts with a whole atom and causes it to become excited this excitation causes the atom to release an x-ray photon of equal wavelength however it is now traveling in a different direction since the wavelengths are equal there is no change in the energy level of the photon this occurs when the photon has less than 10 kilo electron volts of energy generally speaking coherent scattering has little effect on the x-ray image some coherent scattering does occur throughout the diagnostic energy range but it is a very small percentage and contributes only slightly to the image noise the Compton effect refers to interactions between an x-ray photon with an outer shell electron the photon causes the atom to lose an electron remember this is called ionization and the x-ray photon is scattered in a different direction with less energy this interaction may also be called Compton scattering since the x-ray is now traveling in a different direction the Compton effect occurs throughout the diagnostic energy range and has a great effect on the x-ray image though in a very negative way the scattered photons provide no useful information on the radiograph and caused a reduction in image contrast continuing with the Compton effect hearing this type of interaction the ejected electron known as the Compton electron and the scattered x-ray may have enough energy left to go on causing more ionization reactions before losing all of their energy you will learn in radio biology that ionization reactions are what increased potential harm to the patient in Compton interactions the x-rays can be scattered in any direction when they are scattered back in the same direction as the original photon this is called back scatter Compton scattered x-rays are the source of most of the occupational radiation exposure Arty's receive especially during fluoroscopic procedures it should also be noted that the probability of the Compton effect is inversely proportional to the x-ray energy and independent of atomic number meaning the atomic number of the atom has no effect on whether Compton effect occurs in the photoelectric effect which also occurs in the diagnostic energy range an x-ray photon interacts with an inner shell electron and completely disappears causing the electron to be ionized this electron is now called the photo electron interaction is essential for diagnostic radiology and we will discuss why later in the video these interactions may also be referred to as photoelectric absorption since the photon is completely absorbed if you recall from previous discussions in class characteristic x-rays are emitted when an outer shell electron fills an inner shell void since this can happen following an ejection of an inner shell electron during photoelectric absorption these characteristic x-rays are referred to as secondary radiation and act much like scatter radiation these low-energy x-rays do not exit the patient and have no effect on x-ray images it should also be noted that the effective atomic number of the material does have an effect on this type of interaction remember in Compton effect the atomic number did not with the photoelectric effect the higher the effective atomic number the greater the x-ray absorption for example the effect of atomic numbers of fat and bone are six point three and thirteen point eight respectively greater x-ray absorption will occur with bone and lead to brighter white areas on the radiograph however increasing the energy of the x-rays decreases the probability of the photoelectric effect occurring you can think of it this way if you set your kvp too high you will have very little absorption and your radiograph will be too black to read if an x-ray photon has enough energy it will bypass the electron shells and interact with the nuclear field in this interaction the x-ray photon disappears and a pair of electrons appear one positively charged called the positron and one negatively charged called the Negatron this is why this interaction is called pair production since the energy of the x-ray photon must be at least 1.0 to mega electron volts this interaction is unimportant in x-ray imaging it does however have great importance to positron emission tomography or PET imaging in nuclear medicine in photodisintegration an x-ray photon with energy above 10 mega electron volts is apes interaction with the electron shells and the nuclear field the x-ray photon is completely absorbed by the nucleus the nucleus is raised to an excited state and emits a nucleon or nuclear fragment this type of interaction does not occur in diagnostic radiology as you have seen only two of the five interactions are important to diagnostic radiology the Compton effect and the photoelectric effect more important that these interactions however are the x-ray photons transmitted through the body without interacting here you can see how these three types of interactions contribute to an x-ray image those that are absorbed create white areas on the image those that are transmitted through the patient create the dark areas on the image those x-rays that are scattered onto the image contain no diagnostic information but contribute to the film just the same we use techniques as RTS to reduce this type of x-ray as much as possible in order to image bony structures in detail you need to be able to see both the bones and surrounding soft tissue those x-rays that undergo photoelectric absorption within the bone allow for light areas on the film and those that pass straight through to the image receptor allow for the dark areas on the film the difference in these x-ray interactions is called differential absorption and is the sum of Compton scattering photoelectric effect and transmitted x-rays a high-quality radiograph will have maximum differential absorption to increase differential absorption you can decrease kvp however in doing so you will increase the patient dose it is important to find a compromise between the two differential absorption also depends on differences in effective atomic number imaging an area with bones and soft tissue a high atomic number and low atomic number will have a greater differential absorption than areas of strictly soft tissue where all of the atomic numbers are relatively the same in diagnostic radiology we artificially increase an area's atomic number by introducing higher atomic number contrast agents this allows for greater differential absorption and the visualization of structures that would otherwise be unseen on a radiograph lastly mass density also affects differential absorption you can think of mass density as how tightly the atoms are packed together intuitively we know the bony ribs are more dense than air filled lungs by increasing the mass density you increase the amount of x-ray interactions within the tissue and thus increase differential absorption finally we need to discuss the difference between two terms absorption and attenuation with absorption it is considered an all-or-none process either the x-ray is absorbed and disappear or it doesn't if an x-ray is partially absorbed it is considered a scattering interaction since the x-ray photon emerges with less energy and in a different direction attenuation is the product of absorption and scattering it is the total reduction in the number of x-ray photons remaining in the beam after passing through a given thickness of tissue let's wrap up by reviewing your objectives specifically you should know the name of the interaction the energy range at which it occurs the specifics of the process and whether or not it has an effect on diagnostic x-ray images you should be able to define differential absorption describe how it affects image contrast and explain how KDP atomic number and mass density affect differential absorption lastly you should be able to explain the difference between absorption and attenuation should you need further information your textbook has similar diagrams and explanations in the chapter on x-ray interactions with matter

Channel: Mary Cordray

Views: 152,360

Rating: 4.9481783 out of 5

Keywords: iMovie, Radiology, X-Ray Interactions, Radiation Physics

Id: d3n-kFMAtns

Length: 10min 34sec (634 seconds)

Published: Thu Apr 17 2014