Radiographic Image Quality - BIS 475

Radiographic Image Quality Chapter 10  OBJECTIVES At the completion of this chapter, the student should be able to do the following: 1. Define radiographic image quality, resolution, noise, and speed. 2. Interpret the shape of the characteristic curve...

Radiographic Image Quality Chapter 10  OBJECTIVES At the completion of this chapter, the student should be able to do the following: 1. Define radiographic image quality, resolution, noise, and speed. 2. Interpret the shape of the characteristic curve. 3. Identify the toe, shoulder, and straight-line portion of the characteristic curve. 4. Distinguish the geometric factors that affect image quality. 5. Analyze the subject factors that affect image quality. 6. Examine the tools and techniques available to create high-quality images.  Radiographic Image Quality Fidelity: A radiograph that faithfully reproduces structure and tissues is identified as a high-quality radiograph. The most important characteristics of radiographic image quality are: Spatial Resolution, Contrast Resolution, Noise, Artifacts.  Radiographic Image Quality Spatial resolution refers to the ability to image small objects that have high subject contrast, such as a bone–soft tissue interface, Contrast resolution is the ability to distinguish anatomical structures of similar subject contrast such as liver–spleen and gray matter–white matter.  Radiographic Image Quality Radiographic Noise is the random fluctuation in the Optical Density (OD) of the image. Screen-film radiographic noise has four components: Film graininess, the distribution in size and space of silver halide grains in the emulsion. Structure mottle, refers to the phosphor of the radiographic intensifying screen Quantum mottle, refers to the random nature by which x-rays interact with the image receptor. Scatter radiation.  Radiographic Image Quality - Film Factors Sensitometry: the relationship between the intensity of exposure of the film and the blackness after processing. Characteristic Curve: describes the relationship between OD and radiation exposure.  Radiographic Image Quality - Geometric Factors Magnification: For most medical images, the smallest magnification possible should be maintained. Minimizing Magnification Large SID Small OID Radiographic Image Quality - Geometric Factors Distortion: 1. Object thickness 2. Object position 3. Object shape Radiographic Image Quality - Geometric Factors Focal-Spot Blur: Focal-spot blur occurs because the focal spot is not a point. Object position Object shape. Focal-spot blur is the most important factor for determining spatial resolution. Radiographic Image Quality - Subject Factors Subject contrast depends on: Patient thickness thick body section attenuates a greater number of x-rays than does a thin body section. Tissue mass density Different sections of the body may have equal thicknesses yet different mass densities. Effective atomic number When the effective atomic number of adjacent tissues is very much different, subject contrast is very high. Radiographic Image Quality - Subject Factors Subject contrast depends on: Object shape a structure that has a form that coincides with the x-ray beam has maximum subject contrast Kilovolt peak kVp is the most important influence on subject contrast. A low kVp results in high subject contrast (short grayscale), high kVp results in low subject contrast (long grayscale). Radiographic Image Quality - Subject Factors Motion Blur : Loss of radiographic quality due to movement of the patient or the x-ray tube during exposure. Patient motion of two types may occur: Voluntary motion of the limbs and muscles is controlled by immobilization. Involuntary motion of the heart and lungs is controlled by short exposure time. Reduction of Motion Blur Use the shortest possible exposure time. Restrict patient motion by providing instruction or using a restraining device. Use a large source-to-image receptor distance (SID). Use a small object-to-image receptor distance (OID)  TOOLS FOR IMPROVED RADIOGRAPHIC IMAGE QUALITY Patient Positioning Anatomical structure under investigation be placed as close to the image receptor The axis of this structure should lie in a plane that is parallel to the plane of the image receptor Central ray should be incident on the center of the structure Patient must be immobilized effectively  TOOLS FOR IMPROVED RADIOGRAPHIC IMAGE QUALITY Image Receptors Standard type of screen-film image receptor is used throughout a radiology department for a given type of examination Selection of Technique Factors Select the optimum radiographic technique factors, that is, kVp, mAs, and exposure time. Time of exposure should be as short as possible Scatter Radiation Chapter 11  OBJECTIVES At the completion of this chapter, the student should be able to do the following: 1. Identify the x-rays that constitute image-forming radiation. 2. Recognize the relationship between scatter radiation and image contrast. 3. List three factors that contribute to scatter radiation. 4. Discuss three devices developed to minimize scatter radiation. 5. Describe beam restriction and its effect on patient radiation dose and image quality. 6. Describe grid construction and its measures of performance. 7. Evaluate the use of various grids in relation to patient dose.  Image-forming X-Rays X-rays that exit from the patient and interact with the image receptor Two types of x-rays are responsible for the optical density (OD) and contrast on a radiographic image: Those that pass through the patient without interacting, Those that are Compton scattered within the patient.  Scatter Radiation As scatter radiation increases, the radiographic image loses contrast and appears gray and dull. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, Field size, Patient thickness. The Effect of kVp on Scatter Radiation The Effect of Field Size on Scatter Radiation As field size is increased, scatter radiation also increases  The Effect of Patient Thickness on Scatter Radiation Imaging thick parts of the body results in more scatter radiation than does imaging thin body parts.  CONTROL OF SCATTER RADIATION Basically, three types of beam-restricting devices are used: The Aperture Diaphragm, Cones or Cylinders, and The Variable-Aperture Collimator  The Aperture Diaphragm The simplest Lead or lead-lined metal diaphragm. Attached to the x- ray tube head  Cones or Cylinders Considered modifications of the aperture diaphragm. The extended metal structure restricts the useful beam to the required size. The useful beam produced is usually circular Difficulty with alignment. Routinely used in dental radiography  The Variable-Aperture Collimator Most commonly used beam-restricting device. 2- Stages: Collimator Blades. Cross- and Long- Shutters Light localization.  Positive- Beam-Limiting (PBL) Devices Was mandated by the U.S. FDA in 1974. Under no circumstances should the x-ray beam exceed the size of the image receptor. When a film-loaded cassette is inserted into the Bucky tray and is clamped into place, sensing devices in the tray identify the size and alignment of the cassette. A signal transmitted to the collimator housing actuates the synchronous motors that drive the collimator leaves to a pre-calibrated position, so the x-ray beam is restricted to the image receptor in use.  Radiographic Grids A carefully fabricated section of radiopaque material (grid strip) alternating with radiolucent material (interspace material). Designed to transmit only x-rays whose direction is on a straight line from the x-ray tube target to the image receptor. Positioned between the patient and the image receptor.  Radiographic Grids has three important dimensions: the thickness of the grid strip (T), the width of the interspace material (D), the height of the grid (h).. Grid Ratio = h/D High-ratio grids are more effective in reducing scatter radiation than are low- ratio grids. Grid Frequency: The number of grid strips per centimeter. Radiographic Grids Performance  Radiographic Grids Types Parallel Grid All lead grid strips are parallel Grid Cutoff: the undesirable absorption of primary x-rays by the grid.  Radiographic Grids Types Crossed Grid lead grid strips that run parallel to the long and short axes of the grid. Crossed grids are much more efficient than parallel grids. The main disadvantage of parallel and crossed grids is grid cutoff. Three serious disadvantages: Positioning Tilt-table techniques. Exposure technique  Radiographic Grids Types Focused Grid The lead grid strips of a focused grid lie on the imaginary radial lines of a circle centered at the focal spot. Designed to minimize grid cutoff. More difficult to manufacture.  Radiographic Grids Types Moving Grid A major improvement in grid development to avoid the production of produce grid lines on the image. Two basic types of moving grid mechanisms are in use today: Reciprocating and Oscillating. A Reciprocating grid is a moving grid that is motor-driven back and forth several times during x-ray exposure. A Oscillating grid the grid oscillates in a circular fashion around the grid frame, Two disadvantages: Increased patient-receptor distance (increased Mag.) Introduce motion (Blurring).  Radiographic Grids Problems Off-Level Grid Off-Center Grid  Radiographic Grids Problems Off-Focus Grid Upside-Down Grid  Radiographic Grids Selection It is appropriate to design radiographic imaging around moving grids When moving grids are used, parallel grids can be used, but focused grids are more common. When focused grids are used, the indicators on the x-ray apparatus must be in good adjustment and properly calibrated. Selection of a grid with the proper ratio depends on an understanding of three interrelated factors: kVp, degree of scatter radiation reduction, and patient radiation dose. Radiographic Grids Selection Computed Radiography Chapter 15  OBJECTIVES At the completion of this chapter, the student should be able to do the following: 1.Describe several advantages of computed radiography over screen-film radiography. 2.Identify workflow changes when computed radiography replaces screen- film radiography. 3.Discuss the relevant features of a storage phosphor imaging plate. 4.Explain the operating characteristics of a computed radiography reader. 5.Discuss spatial resolution, contrast resolution, and noise related to computed radiography. 6.Identify opportunities for patient radiation dose reduction with computed radiography.  Introduction Recent technology trends, Conversion from screen- film radiography (analog) to digital radiography (DR) Digital radiography was introduced with the first commercial computed radiography (CR) imaging system. CR imaging eliminates some of the steps associated with screen-film radiographic technique and can produce better medical images at lower patient dose. The Computed Radiography (CR) Image Receptor A technology based on “Photostimulable Luminescence” phenomena. Some materials such as barium fluorohalide, the response to X-ray exposure appears as trapped electrons in a higher energy metastable state. (Energy trapping). Some time later, when exposed to a different light source, they emit light (Energy release)  The Computed Radiography (CR) Image Receptor Composition Image Plate (IP) Made of PhotoStimulable Phosphor (PSP) screen in a cassette. Handled in the same manner as a screen- film cassette (Advantage). The CR Image Receptor Work Theroy The CR Image Receptor Response Function  The Computed Radiography (CR) Reader Mechanical Features Precision drive mechanism. A deflection device such as a rotating polygon or an oscillating mirror deflects the laser. Optical Features Components of the optical subsystem include the laser, beam-shaping optics, light-collecting optics, optical filters, and a photodetector. Computer Control processed for amplitude, scale, and compression. digitized, with proper sampling and quantization The Computed Radiography (CR) Reader The Computed Radiography (CR) Reader Digital Radiography Chapter 16  OBJECTIVES At the completion of this chapter, the student should be able to do the following: 1.Identify five digital radiographic modes in addition to computed radiography. 2.Define the difference between direct digital radiography and indirect digital radiography. 3.Describe the capture, coupling, and collection stages of each type of digital radiographic imaging system. 4.Discuss the use of silicon, selenium, cesium iodide, and gadolinium oxysulfide in digital radiography.  Introduction Describing and identifying the DR imaging systems technology: Capture element: An element which sense X-ray and produce a proportional quantity of equivalent measurable signal such as current, voltage or light. Coupling element: An element which transfers the generated signal to the collection element Collection element. An element which displays the generated signal. DR imaging systems technology:  CHARGE-COUPLED DEVICE (CCD) Technology Is a light-sensing element is a silicon-based semiconductor. Highly Sensitive: is the ability to detect and respond to very low levels of visible light. High Dynamic range: is the ability to respond to a wide range of light intensity, from very dim to very bright. Very small, making it highly adaptable to DR in its various forms  CESIUM IODIDE/CHARGECOUPLED DEVICE (CsI/CCD) Technology CsI /CCD is an indirect DR process. CsI converts X-ray first to light. Fiber-optics transfers light to CCD CCD then converts light to electronic signal. CESIUM IODIDE/AMORPHOUS SILICON (CsI/a-Si) Technology CsI /a-Si is an indirect DR process. CsI converts X-ray first to light. a-Si then converts light to electronic signal. Fill Factor: The percentage of the pixel face that is sensitive to x- rays. (80%) AMORPHOUS SELENIUM (a-Se) Technology Identified as direct DR because no scintillation phosphor is involved. x-ray beam interacts directly with amorphous selenium (a-Se), producing a charged pair. The a-Se is both the capture element and the coupling element.