Radiology Lecture 3.docx

Lecture 3 Chapter 5 – Radiation Characteristics Radiation characteristics include x-ray beam quality, quantity, and intensity. Variations in the character of the x-ray beam influence the quality of the resulting images Wavelength determines the energy and penetrating power of radiation. X-ra...

Lecture 3 Chapter 5 – Radiation Characteristics Radiation characteristics include x-ray beam quality, quantity, and intensity. Variations in the character of the x-ray beam influence the quality of the resulting images Wavelength determines the energy and penetrating power of radiation. X-rays with shorter wavelengths have more penetrating power, whereas those with longer wavelengths are less penetrating and more likely to be absorbed by matter. In dental imaging, the term quality is used to describe the mean energy or penetrating ability of the x-ray beam. The quality, or wavelength and energy, of the x-ray beam is controlled by kilovoltage. Voltage is a measurement of force that refers to the potential difference between two electrical charges. Inside the dental x-ray tubehead, voltage is the measurement of electrical force that causes electrons to move from the negative cathode to the positive anode. Voltage determines the speed with which they move. When voltage is increased, the speed of the electrons is increased. The electrons strike the target with greater force and energy, resulting in a penetrating x- ray beam with a short wavelength. Voltage is measured in volts or kilovolts. Most radiographic units operate using kilovolts; 1 kilovolt (kV) is equal to 1000 volts. The term kilovoltage (kV) is the maximum voltage, or peak voltage of an alternating current. Kilovoltage (kV) and kilovoltage peak (kVp) are often synonymous terms. In the past, dental x-ray units were available with adjustable settings ranging from 65 to 100 kV. <65 kV = inadequate penetration of the x-ray beam >100 kV = overpenetration of the x-ray beam The use of higher kV produces more penetrating dental x-rays with greater energy, whereas the use of lower kV produces less penetrating dental x-rays with less energy. Current intraoral x-ray units include adjustable settings that range from 60 to 70 kV, or else a fixed setting of 70 kV. The quality, or wavelength and energy, of the x-ray beam is controlled by the kilovoltage. The kilovoltage regulates the speed and energy of the electrons and determines the penetrating ability of the x-ray beam. Increasing the kilovoltage results in a higher energy x-ray beam with increased penetrating ability. When kV is increased, electrons move from the cathode to the anode with more speed. Density is the overall darkness or blackness of an image. An adjustment in kilovoltage results in a change in the density of a dental image. If the kilovoltage is increased while other exposure factors (milliamperage, exposure time) remain constant, the resultant image exhibits an increased density and appears darker. If the kilovoltage is decreased, the resultant image exhibits a decreased density and appears lighter. Contrast refers to how sharply dark and light areas are differentiated or separated on an image. When lower kilovoltage settings are used, a high-contrast image will result. An image with “high” contrast has many black areas, many white areas, and few shades of gray. An image with high contrast is useful for detecting and determining the progression of dental caries. With higher kilovoltage settings, low contrast results. An image with “low” contrast has many shades of gray instead of areas that are predominantly black and white. An image with low contrast is useful for the detection of periodontal or periapical disease. A digital image can be adjusted so that the contrast is higher, which is desirable in caries interpretation, or so that the contrast is lower, which is desirable in evaluating periodontal disease. In comparison, if film is used and the contrast is incorrect, the image must be retaken with adjusted exposure factors. Exposure time refers to the interval of time during which x-rays are produced. An impulse is a term of measurement that refers to the fact that x-rays are created in a series of bursts or pulses rather than in a continuous stream. Kilovoltage and exposure time are inversely related. On older x-ray units, if the kilovoltage was changed, the exposure time needed to be adjusted in order to maintain the diagnostic density of an image. When kilovoltage was increased, the exposure time was decreased in order to compensate for the penetrating power of the x-ray beam. When kilovoltage was decreased, the exposure time was increased. Quantity of the x-ray beam refers to the number of x-rays produced in the dental x-ray unit. Amperage determines the amount of electrons passing through the cathode filament. An increase in the number of electrons available to travel from the cathode to the anode results in production of an increased number of x-rays. The quantity of the x-rays produced is controlled by milliamperage. Milliamperage regulates the temperature of the cathode filament. A higher milliampere setting increases the temperature of the cathode filament and consequently increases the number of electrons produced. An increase in the number of electrons that strike the anode increases the number of x- rays emitted from the tube. The quantity, or number of x-rays emitted from the tubehead, is controlled by milliamperage. Milliamperage, as with kilovoltage, has an effect on the density of a dental image. An increase in milliamperage increases the overall density and results in a darker image. Conversely, a decrease in milliamperage decreases the overall density and results in a lighter image. Milliamperage and exposure time are inversely related. On older units, if the milliamperage was changed, the exposure time also needed adjustment in order to maintain the diagnostic density of an image. When milliamperage was increased, the exposure time was decreased. When milliamperage was decreased, the exposure time was increased. All dental x-ray machines have three exposure factor settings: kV, mA, and time. On dental x-ray units, only the exposure time setting is always adjustable. Quality refers to the energy or penetrating ability of the x-ray beam; quantity refers to the number of x-ray photons in the beam. Quality and quantity are described together in a concept known as intensity. Intensity is defined as the product of the quantity (number of x-ray photons) and quality (energy of each photon) per unit of area per unit of time of exposure. Intensity of the x-ray beam is affected by a number of factors, including kilovoltage, milliamperage, exposure time, and distance. Kilovoltage regulates the penetrating power of the x-ray beam by controlling the speed of the electrons traveling between the cathode and the anode. Higher kilovoltage settings produce an x-ray beam with more energy and shorter wavelengths; higher kilovoltage levels increase the intensity of the x-ray beam. Milliamperage controls the penetrating power of the x-ray beam by controlling the number of electrons produced in the x-ray tube and the number of x-rays produced. Higher milliampere settings produce a beam with more energy, increasing the intensity of the x-ray beam. Exposure time, as with milliamperage, affects the number of x-rays produced. A longer exposure time produces more x-rays. An increase in exposure time produces a more intense x-ray beam. The distance traveled by the x-ray beam affects the intensity of the beam. Target-surface distance: The distance from the source of radiation (tungsten target in anode) to the patient’s skin Target-object distance: The distance from the source of radiation (tungsten target in anode) to the tooth Target-receptor distance: The distance from the source of radiation (tungsten target in anode) to the receptor The distance between the source of radiation and the receptor has a marked effect on the intensity of the x-ray beam. As x-rays travel from their point of origin or away from the target anode, they diverge like waves of light and spread out to cover a larger surface area. As x-rays travel away from their source of origin, the intensity of the beam lessens. Unless a corresponding change is made in one of the other exposure factors (kilovoltage), the intensity of the x-ray beam is reduced as the distance increases. The inverse square law is stated as follows: The intensity of radiation is inversely proportional to the square of the distance from the source of radiation. According to the inverse square law, when the target-receptor distance is doubled, the resultant beam is one-fourth as intense. When the target-receptor distance is reduced by half, the resultant beam is four times as intense. To reduce the intensity of the x-ray beam, aluminum filters are placed in the path of the beam inside the dental x-ray tubehead. Aluminum filters are used to remove the low-energy, less penetrating, longer-wavelength x-rays. Aluminum filters increase the mean penetrating capability of the x-ray beam while reducing the intensity. When placed in the path of the x-ray beam, the thickness of a specified material (e.g., aluminum) that reduces the intensity by half is termed the half-value layer (HVL). Chapter 18 – Introduction to Dental Imaging Examinations The purpose of this chapter is to introduce the dental radiographer to the different intraoral imaging examinations used in dentistry, to define the complete mouth series, and to describe in detail the diagnostic criteria of intraoral images. In addition, the extraoral imaging examinations used in dentistry are introduced. Three types of intraoral imaging examinations are used in dentistry: Periapical examination Interproximal examination Occlusal examination Periapical examination Used to examine the entire tooth (crown and root) and supporting bone. The periapical receptor is used in periapical examination; usually #1 sensor/film for anterior teeth, #2 for posterior teeth Periapical images show the terminal end of the tooth root and surrounding bone as well as the crown Two methods are used for obtaining periapical images: (1) the paralleling technique and (2) the bisecting technique Periapical images must show the entire crowns and roots of the teeth being examined, as well as 2 to 3 mm beyond the root apices. Interproximal examination Used to examine the crowns of both maxillary and mandibular teeth on a single image. The bite-wing receptor is used in interproximal examination; #2 film/receptor for adults, #0 for small kids Size 1 in preteens often works well The bite-wing receptor has a “wing” or tab attached to it; the patient “bites” on the wing to stabilize the receptor. The bite-wing technique is used in interproximal examination. Bite-wing images must show open contacts, or interproximal tooth surfaces that are not overlapped. Occlusal examination Used to examine large areas of the maxilla or the mandible on one image The occlusal receptor is used in occlusal examination Patient occludes or bites on the receptor A size 4 receptor is used for this projection on adults. Full mouth series (FMX, FMS, CMS) A series of intraoral dental images that show all the tooth-bearing areas of both jaws. Tooth-bearing areas are the regions of the maxilla and the mandible where the 32 teeth of the human dentition are normally located. Tooth-bearing areas include dentulous areas, or areas that exhibit teeth, as well as edentulous areas, or areas where teeth are no longer present. The CMS consists of periapical images alone or a combination of periapical (PA) and bite-wing (BW) images. Includes every tooth and all tooth-bearing areas; 14 to 20 images The patient without teeth, 14 periapical images are usually sufficient The number of periapical images varies, depending on which technique—paralleling or bisecting—is used. Receptor size is also dictated by the technique used. The complete mouth series (CMS) must include images that show all tooth-bearing areas, including dentulous and edentulous regions. WHAT IS AN EDENTULOUS PATIENT? A PATIENT WITHOUT TEETH Extraoral imaging examination An inspection used to examine large areas of the skull or jaws. Common extraoral images include the panoramic image as well as the lateral jaw, lateral cephalometric, posteroanterior, Waters, submentovertex, reverse Towne, transcranial, and tomographic projections. Prescribing Dental Images A CMS is appropriate when a new adult patient presents with clinical evidence of generalized dental disease or a history of extensive dental treatment. Otherwise, a combination of bite-wings, selected periapicals, and/or a panoramic image should be prescribed on the basis of a patient's individual needs. WHAT PROJECTION WOULD BE USED TO SEE THE MANDIBLE OF A PATIENT? OCCLUSAL A NEW PATIENT HAS NO SIGN OF DISEASE AND NO RESTORATIONS, WHAT IMAGES DO YOU TAKE? BITE-WINGS