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Principles of Imaging Science I (RAD119) Attenuation Radiographic Technique Attenuation • When x-ray photons interact with matter, the quantity is reduced from the original x-ray beam • Attenuation is the result of interactions between x-ray and matter that include absorption and scatter • Photoelectric absorption • Compton scattering • Coherent scattering • Differential absorption increases as kVp decreases Differential Absorption Three types of xrays are important to the making of a radiograph: those scattered by Compton ) interaction ((A); those absorbed photoelectrically (B); and those transmitted through the patient without interaction (C). 1 Interaction of xrays by absorption and scatter is called attenuation. In this example, the x-ray beam has been attenuated 97%; 3% of the x-rays have been transmitted. Attenuation • Contingent upon the thickness of the body part, the atomic number, and density • Thicker body parts attenuate more x-ray photons than the same body part that is thinner • Higher atomic number structures absorb more x-ray photons than lower atomic number structures • Due to higher # of electrons • Denser structures absorb more x-ray photons as compared with less dense structures (kg/m3) Human Body Tissue Substance Atomic # Density (kg/m3) Fat 6.3 910 Soft Tissue Water Muscle 7.4 7 4 7.5 1000 1000 7.6 1000 Bone 13.8 1850 2 Contrast Material Contrast Agent Atomic # Density (kg/m3) Air 7.6 1.3 Iodine 53 4930 Barium 56 3500 Radiographic Demonstration • Air – Easily penetrated – Increased density (dark) • Fat – Harder to penetrate than air – Lower atomic # and density than muscle – Easier to penetrate than muscle – Decreased density (grey) Radiographic Demonstration • Muscle – Harder to penetrate than fat – Higher atomic # and density compared to fat – Decreased density (grey) • Bone – H Hardest d tb body d substance b t tto penetrate t t – Highest atomic # and density – Decreased density (white) due absorption of x-ray photons • Subject contrast is achieved due to differences in photon attenuation 3 Conventional Radiography • • • • Method is film-based. Method uses intensifying screens. Film is placed between two screens. Screens emit light when x-rays strike them. • Film is processed chemically. • Processed film is viewed on lightbox. Digital Imaging • Broad term first used medically in 1970s in computed tomography (CT). • Digital imaging is defined as any image q p process that p produces an acquisition electronic image that can be viewed and manipulated on a computer. • In radiology, images can be sent via computer networks to a variety of locations. Computed Radiography • • • • • Uses storage phosphor plates Uses existing equipment Requires special cassettes Requires a special cassette reader Uses a computer workstation and viewing station and a printer • Method was slow to be accepted by radiologists. • Installation increased in the early 1990s. • More and more hospitals are replacing film/screen technology with digital systems. 4 Digital Radiography • Cassetteless system • Uses a flat panel detector or charge-coupled device (CCD) hard-wired to computer • Requires new installation of room or retrofit Digital / Conventional Radiography RADIOGRAPHIC DENSITY Conventional Radiography • One of the photographic properties that determines visibility of detail • Overall blackness or darkness of the entire radiographic image or a specific area • When evaluating an image for proper radiographic density, the density of the entire image is considered • Optical density vs Radiographic density 5 Optical Density Measurement Densitometer • Formula: OD = log10 (Io/It) – OD = Optical density – Io = Intensity of the original (incident) light – It = Intensity of the transmitted light Exposure Indicators Digital Imaging • The amount of light given off by the imaging plate is a result of the radiation exposure that the plate has received. • The light is converted into a signal that is used to calculate the exposure indicator number, which is a different number from one vendor to another. • The base exposure indicator number for all systems designates the middle of the detector operating range. Exposure Indicators • For Fuji, Phillips, and Konica systems, the exposure indicator is known as the S, or sensitivity, number. – The higher g the S number with these systems, y , the lower the exposure. 6 Exposure Indicators • Kodak uses exposure index, or EI, as the exposure indicator. – An EI number plus 300 (EI + 300) is equal to a doubling of exposure, and an EI number of minus 300 (EI − 300) is equal to a halving of exposure. (Direct relationship) Exposure Indicators • The term for exposure indicator in an Agfa system is the lgM, or logarithm of the median exposure. – Each step of 0.3 above or below 2.6 equals an exposure factor of 2. Exposure Indicators Summary • S, EI, and lgM are terms used by manufacturers to indicate the amount of exposure. • The exposure range numbers represent the maximum to minimum diagnostic exposures. • The middle value in that range represents the S, EI, or lgM number. 7 CONTRAST • The second photographic property that determines visibility of detail – Subject Contrast – Film Contrast CONTRAST • Ensures visibility of detail • Dependent upon adequate density • Density y difference between adjacent j structures • Changes in density affect image contrast CONTRAST • HIGH CONTRAST – – – – Low kVp Black & White Short scale contrast Used for skeletal anatomy • LOW CONTRAST – – – – High kVp Shades of gray Long scale contrast Used for Chest, KUB, or as warranted by M.D. 8 Digital Image Receptor Systems • There is no substitute for proper kilovoltage peak and milliampere-second settings. Images cannot be created from nothing; that is, insufficient photons, insufficient penetration, or overpenetration will result in loss of g information that cannot be manufactured by y diagnostic manipulation of the image parameters. • Exposure latitude is slightly greater with digital imaging than that of film/screen imaging because of the wide range of exposures recorded with digital systems. DENSITY • CONTROLLING FACTOR: mAs • • • • mAs = mA X time (sec) mA = mAs/time time = mAs/mA Reciprocity Law – The same radiographic film density will result from different mA and time selections, provided that the mAs totals are equal Calculations mAs mA Time 9 Calculations Density Influencing Factor • kVp – Affects the penetrability of x-ray photons through the patient – Affects the quality of the x-ray beam based upon the emission spectrum – Whole number increments (Major/Minor) Density Influencing Factor • SID – Based upon Inverse Square Law • Film/Screen Combination (RSS) – Slow, Medium, High 10 Density: INFLUENCING FACTORS • kilovoltage I1 == I2 I1: Beginning Intensity I2 : New Intensity kVp12 kVp22 kVp1: Beginning kilovoltage kVp2 : New kilovoltage Density: kilovoltage calculations Density: kilovoltage calculations 11 Density: INFLUENCING FACTORS • Source - Image Distance mAs1 mAs2 mAs1: = D12 D22 OR mAs 2 = mAs1 D22 D12 Beginning mAs mAs2 : New mAs D1: Beginning distance D2 : New distance Density: Distance Calculations Density: Distance calculations 12 General Rules New Distance (inches) mAs Change by General Rule Formula mAs change 30 40 60 72 80 96 0.56 1.0 2 25 2.25 3.24 4.0 5.76 ½ 1 2X 3X 4X 6X mAs – Round to tenth location when using seconds kVp - Utilize whole numbers Density: INFLUENCING FACTORS • Film/Screen Combination mAs1 mAs2 == RSS2 RSS1 mAs1: Beginning mAs mAs2 : New mAs RSS1: Beginning Film/Screen Speed RSS2 : New Film/Screen Speed Density: RSS calculations 13 Contrast • 15% Rule – A 15% increase in kilovoltage will double the exposure. This is comparable to doubling the mAs, exposure time, or mA. – A 15% decrease in kilovoltage will halve the exposure. This is comparable to halving the mAs, exposure time, or mA. – Kilovoltage should not be the primary factor used to change density – 2nd Semester: Applied to maintain density while altering contrast 15% Rule: Calculations 14