Chapter3 The Gamma Camera Image quality and Performance

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Image quality • Image quality

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Basic Methods for characterizing and

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical

Chapter-3 The Gamma Camera Image quality and Performance Characteristics • Interaction of Photons with

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Intrinsic spatial resolution

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Chapter-3 The Gamma Camera Image quality and Performance Characteristics Important points in chapter 3 (chapter 14 and 15 from the book) • We discuss the essential elements of image quality in nuclear medicine and how it is measured and characterized • We describe the major factors that determine gamma camera performance Standard tests of gamma camera performance The general concepts are applicable as well to the tomo-graphic imaging techniques

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Important points in chapter 3 (chapter 14 and 15 from the book) Part-1 Image quality of gamma camera Part-2 Gamma camera performance characteristics

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Image quality • Image quality refers to the faithfulness with which and image represents the imaged object • The quality of nuclear medicine images is limited by several factors • The gamma camera is not capable of producing “perfect” images of the radionuclide distribution • Certain inherent imperfections arises from the performance characteristics of the detector, its associated electronic circuitry, and the collimator • Image artifacts also can be caused by malfunctions of various camera components

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Basic Methods for characterizing and evaluating image quality The first by means of physical characteristics that can be quantitatively measured or calculated for the image and image system • • Spatial resolution Contrast Noise Contrast-to-noise ratio (CNR) The second by means of observer performance studies using images obtained with different imaging systems or under different imaging conditions

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Basic Methods for characterizing and evaluating image quality Although they describe 3 different aspects of image quality these 3 factors cannot be treated as completely independent parameters because improvements in one of them frequently are obtained at the expense or deterioration of one or more of the others • Example; collimator resolution and efficiency Although observer performance can be characterized objectively, and certainly is related to the physical measures of image quality, the relationships are not well established due to the complexity of the human visual system and other complicating factors, such as observer experience

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Basic Methods for characterizing and evaluating image quality Hence; The two methods, though related, are some what independent

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution • Spatial resolution refers to the sharpness or detail of the image, or to the ability of the imaging instrument to provide such sharpness or details • Sharp edges or small, pointed objects produce blurred rather than sharply defined images • Part of the blurring arises from collimator characteristics and part arises in the detector and electronics

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Intrinsic spatial resolution The limit of spatial resolution achievable by the detector and electronics, ignoring additional blurring due to the collimator spatial resolution

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Intrinsic spatial resolution is limited primarily by two factors The multiple scattering of photons within the detector The statistical fluctuation in the distribution of light photons among PMTs from one scintillation event to the next

Chapter-3 The Gamma Camera Image quality and Performance Characteristics • Interaction of Photons with matter Compton scattering Photoelectric effect Predominant type of interaction for various combinations of incident photons and absorber atomic numbers Pair production

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Intrinsic spatial resolution

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Intrinsic spatial resolution Modern cameras are substantially improved over earlier versions because of • The use of more efficient PMs tubes and better techniques for optical coupling between the detector crystal and the PM tubes • The use of greater numbers of smaller PMTs (some gamma cameras have as many as 110 PMTs per head) • The use of improved electronics • The use of accurate corrections for nonlinearity and non-uniformity

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Intrinsic spatial resolution • The intrinsic spatial resolution makes a negligible contribution to the overall system resolution of the gamma camera, which is largely determined by the resolution of the collimator

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Collimator spatial resolution • Is perhaps the principle limiting factor when absorptive collimators are used for spatial localization Because collimator hole diameters must be relatively large (to obtain reasonable collimator efficiency), there is blurring of the image by an amount at least as great as the hole diameters Collimator resolution also depends on source-to-detector distance

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Collimator spatial resolution

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Collimator efficiency Source-to-collimator distance

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution System resolution (extrinsic resolution)

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Methods for evaluating spatial resolution

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Other factors could affect spatial resolution • Image sharpness (resolution) can be affected by patient motion

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Other factors could affect spatial resolution

Chapter-3 The Gamma Camera Image quality and Performance Characteristics Evaluating image quality using physical characteristics 1. Spatial resolution Other factors could affect spatial resolution The image resolution also can be affected by the display system