Transcript Chapter 11
Chapter 11/12 CREATION OF THE RADIOGRAPHIC IMAGE & PRODUCTION OF SUBJECT CONTRAST X-ray Beam Primary Radiation (PR)- portion of beam from tube to the patient; radiation before it enters the patient Remnant Radiation (RR)- radiation emerging from patient’s body to expose the film; image forming radiation Primary Beam The beam that is directed at the patient’s body Photons can be absorbed by the patient’s body part’s Photons could pass right thru the patient’s body and enter the film, putting density on the film. These photons are called exit radiation or remnant radiation Photons could hit something in the patient’s body, bounce off, and fly out in a new direction. This last event produces scattered radiation. Primary Beam Distribution 5% of primary beam passes through the patient without any interactions 15% of the primary beam interacts with atoms and produce secondary radiation, they make it out of the patient and expose the film. 80% will be totally absorbed by patient Distribution of Remnant Radiation 20% or 1/5 of the intensity of the original beam exposes the film With remnant radiation about 75% to 80% of the beam is made up of secondary radiation Refer to figure 1-12 pg. 14 Absorption of the Beam Attenuation- partial absorption of the x-ray beam, occurs when the beam is absorbed partially by tissue, bone Difference of x-ray absorption affects the density on the film Subject Contrast Subject Contrast refers to tissue differences Represent differences in x-ray absorption in the body Greater the absorption of tissues in relation to adjacent structures, the greater the subject contrast Review Types of Radiation Primary Radiation (PR) The beam from the focal spot to the object being examined, prior to absorption Remnant Radiation (RR) The beam emanating from the object and exposing the film CR/SID/OID CR- central ray is the center of the x-ray beam SID- source to image distance is the entire distance traveled by the x-rays from the focal spot the to the IR- image receptor OID- object to image distance is the distance from the object to the film ( Units in Radiology Roentgen is the primary unit used for the quantity of x-rays (R); this is the unit of exposure to an x-ray beam Rad and Rem are quantitatively equal to the Roentgen (R) Coulomb per Kilogram C/Kg internation unit for to measure exposure which is equal to 2.58 x 10 to the 18th R Prime Factors of Radiography mA- milliampere S- seconds time kVp- kilovoltage peak SID- source to image distance These are all controlled by the technologist Variables affecting quality Electrical factors Geometrical Variables Patient Status Image receptor system Processing Variables Viewing conditions Creating a Radiographic Image Creating a image by differential absorption requires that several processes occur: Attenuation Absorption transmission Differential Absorption The process of image formation where the x-ray beam interacts with the anatomic tissue and a portion of the remaining beam strikes the image receptor to give us the x-ray image. Differential Absorption In this process some of the x-ray beam will be absorbed by the tissue and some will pass through the body at a decreased energy, strike the film and be visible on the manifest image. What is the manifest image vs. latent image? Differential Absorption We use differential absorption when discussing how varying anatomy absorbs and /or attenuates the xray beam differently. Which anatomical parts will absorb more of the x-ray beam? bone or soft tissue Differential Absorption and Image Formation A radiographic image is created by passing an x-ray beam through the patient and the interactions that occur with the image receptor. What is an image receptor? Attenuation A partial absorption of the x-ray beam The reduction in x-ray intensity that occurs as the x-ray beam traverses the body part. Two distinct processes occur during beam attenuation: absorption and scattering Beam Attenuation As the primary beam passes through the patient it will loose some of its’ original energy. This reduction in the energy of the primary beam is known as attenuation. Determining Attenuation of the Beam Three essential aspects of tissues will determine their attenuation properties and the resulting subject contrast: Tissue Thickness Tissue density Tissue atomic number Tissue Thickness As a tissue area becomes thicker, its attenuation of the x-ray beam is greater. Changes in tissue thickness may cause the subject contrast to either increase or decrease, depending on how the tissue has changed. Tissue Density The physical density of a substance refers to the amount of physical mass that is concentrated into a given volume of space (concentration of atoms or molecules in the tissue). At higher tissue densities, there are more atoms or molecules packed into a given space. Visible radiographic contrast occurs between two tissues in the image that are extremely different in physical density. Tissue Atomic Number Contrast agents, bone, and metallic objects are visible on a radiograph primarily because of the difference between their atomic numbers (how large their atoms are on average). Tissue Atomic Number Larger atoms are not spatially larger in actual size, but rather they are denser with electrons. Elements with higher atomic numbers will have increased absorption of the x-ray photon concentration of electrons within the space of the atom’s diameter is refereed to as electron density Physical Density vs. Atomic Number Summary Review Physical Density Number of atoms concentrated into a volume of space Average Atomic Number Relates to the concentration of electrons within each atom Types of Body Tissues Three types of body tissues may be distinguished from each other on a radiograph primarily because of the effect of tissue density: Soft tissue (muscles, glands) Gas (air in the lungs or bowel) Fat (adipose tissue) What will the density appear like? Soft tissue (muscles and glands) absorb more x-rays than fat or gases thereby resulting in lighter shades of gray on the image Fat will appear slightly darker than the muscles and glands Gas will appear the darkest Transmission If the incoming x-ray photon passes through the anatomic part without any interaction with the atomic structures, it is called transmission. The combination of absorption and transmission of the x-ray beam will provide an image that represents the anatomic part. Density/Contrast Density(images brightness) overall blackness on the processed image. Contrast-Differences in brightness levels or densities in order to differentiate among anatomic tissue Subject contrast Production of Subject Contrast Contrast is essential for visibility of the manifest image Overall contrast on the image is due to the subject contrast produced by the interactions of the x-ray beam with the various tissues of the body. Subject Contrast Subject contrast is produced by the differential absorption between various tissues of the body. Exit Radiation When the attenuated x-ray beam leaves the patient, the remaining x-ray beam is known as exit radiation Exit radiation is composed of both transmitted and scattered radiation. The varying amounts of transmitted and absorbed radiation creates an image representing the anatomic area of interest. White and Black Areas on a Film Scatter radiation creates unwanted density on the image called fog (most common form of noise). The areas within the anatomic tissue that absorb incoming x-ray photons will create a white or clear areas on the image. The incoming x-ray photons that are transmitted will create black areas on the image. Scale of Grays Anatomic tissues that vary in absorption and transmission will create a range of dark and light areas. The scale of image densities are created by x-ray absorption and transmission of the x-ray beam as it passes through anatomic tissues. Interactions of X-rays within the Patient Let’s take a look at the interactions occurring within the patient when x-rays are taken. There are three distinct interactions we will be discussing: Photoelectric Effect Compton Effect Thompson Effect Photoelectric Effect Complete absorption of the incoming x-ray photon occurs when it has enough energy to remove (eject) an inner shell electron. The ejected electron is called a photoelectron. Photoelectron The ability to remove or eject an electron from an atom is a characteristic of x-rays. It refers to ionization of an atom. In the diagnostic range, this x-ray interaction is known as the Photoelectric Effect. Photoelectric Effect With the photoelectric effect, the ionized atom has a vacancy, or electron hole, in its inner shell. An electron from an upper level shell will drop down to fill the vacancy. Photoelectric Effect As a result of the difference in binding energies between the two electron shells, a secondary x-ray photon will be emitted. This secondary x-ray photon is a form of scatter radiation and may interact with other tissue electrons. Photoelectric Effect The secondary x-ray photon does not reach the film. The photoelectric effect is crucial to the formation of the radiographic image. The photoelectric effect is responsible for the production of contrast on the radiographic image. Photoelectric Effect During attenuation of the x-ray beam, the photoelectric effect is responsible for total absorption of the incoming x-ray photon. Photoelectric Effect Whether the incoming photon is totally absorbed depends on its energy and the atomic number of the anatomic tissue. After absorption of some of the x-ray photons, the overall energy of the primary beam will be decreased as it passes through the anatomic part. Scattering/ Compton Effect Some incoming photons will not be absorbed, but instead they will lose energy during interactions with the atoms composing the tissue, this process is called scattering and results from the diagnostic x-ray interaction with matter known as the Compton Effect. The loss of energy of the incoming photon occurs when it ejects an outer shell electron from the atom. Scattering/ Compton Effect An electron affected from an atom by the Compton Effect is called a recoil electron. The ejected electron is called a Compton electron or secondary electron. The remaining lower energy x-ray photon will change direction and may leave the anatomic part to interact with the image receptor. Scattering/ Compton Effect The Compton photon may be scattered in any direction. Scatter refers to any x-ray photon which has changed direction from the direction of the primary beam. The Compton Effect may be considered as scatter, since 99% of all scattered x-ray photons originate from Compton interactions in the patient. Thompson Effect When the energy of the incoming x-ray photon is less than the binding energy of the orbital electron, Thompson interaction may occur. The orbital electron absorbs the entire photon, but this additional energy is not sufficient to eject the electron from its orbit. Thompson Effect The orbital electron re-emits the photon with its original energy, but it may be emitted in any direction and is considered scatter. Thompson Effect accounts for 1% of all scatter produced. These photons have very low energies and are not likely to reach the film. Compton Interactions Compton interactions can occur within all diagnostic x-ray energies and are therefore an important interaction in radiography. Scattered radiation provides no useful information and must be controlled during radiographic imaging. Compton Interactions Compton interactions become the more prevalent interaction at higher kVp levels. Compton interactions may be considered as almost constant, regardless of atomic number or kVp. Photoelectric Interactions The greatest number of photoelectric interactions will be achieved when the kVp is low and the tissue atomic number is high. Where do interactions occur Compton interactions occur only in the outer shells of an atom. Photoelectric interactions occur only in the inner most shell of an atom. Other interactions After the ionization event of photoelectric and Compton effects, the atom is left with an orbital vacancy and soon pulls another electron into the orbit to fill it. Each time an electron falls into an orbit, energy is lost and is emitted in the form of electromagnetic radiation (characteristic radiation) and it does not make it out of the patient to expose the film. Other interactions The final quality of the radiographic image is determined by the photoelectric, Compton, and Thompson interactions occurring within the patient’s body. Scatter Radiation Can Affect Three Areas If a scattered photon strikes the image receptor, it will not contribute any useful information regarding the anatomic area of interest. If scattered photons are absorbed within the patient, they will contribute to the radiation exposure to the patient. Scatter Radiation Can Affect Three Areas In addition, if the scattered photon leaves the patient and does not strike the image receptor, it could contribute to the radiation exposure of anyone within close proximity to the patient Secondary Radiation vs. Scatter Radiation Secondary Radiation refers to any radiation resulting from interactions within the patient. Scatter radiation refers only to that secondary radiation which has been emitted in a direction different than the original x-ray beam. Most secondary radiation is scattered. Attenuation of the Beam All interactions within the patient, whether Compton, Thompson, or photoelectric, represent some degree of absorption of the overall x-ray beam. All interactions attenuate the x-ray beam.