Fundamental Mathematics and Physics of Medical Imaging

Jack Lancaster, Bruce Hasegawa

• Publisher: CRC Press
• Year: 2016
• eBook ISBN:
  9781498751643
• Topic: Medicine
• Subtopic: Radiology, Radiotherapy & Nuclear Medicine

Authored by a leading educator, this book teaches the fundamental mathematics and physics concepts associated with medical imaging systems. Going beyond mere description of imaging modalities, this book delves into the mechanisms of image formation and image quality common to all imaging systems: contrast mechanisms, noise, and spatial and temporal resolution, making it an important reference for medical physicists and biomedical engineering students. This is an extensively revised new edition of The Physics of Medical X-Ray Imaging by Bruce Hasegawa (Medical Physics Publishing, 1991), and includes a wide range of modalities such as X-ray CT, MRI and SPECT.

II

Intermediate Concepts

CHAPTER4

Physical Determinants of Contrast

A MEDICAL IMAGE CAN BE roughly described in terms of the three basic features given in Chapter 1: contrast, spatial resolution, and noise. Spatial resolution or clarity refers to the spatial detail of small objects within the image. Noise refers to the precision within the image; a noisy image will have large fluctuations in the signal across a uniform object, while a precise signal will have very small fluctuations. The subject of this chapter, contrast, relates to the difference in signals between a structure and its immediate surroundings. For example, if circles are displayed against a black background, a white circle will have larger contrast relative to the background when compared to gray circles (Figure 4.1). One uses the differences in gray shades to “visually” distinguish different tissue types, determine anatomical relationships, and sometimes assess their physiological functions. The larger the contrast between different tissue types, the easier it is to make distinctions clinically. It is often the objective of an imaging system to maximize the contrast in the image for a particular object or tissue of interest, although this is not always true since there may be design compromises where noise and spatial resolution are also very important. The contrast in an x-ray image depends on both physical characteristics of the object and properties of the device(s) used to image the object. The focus of this chapter is x-ray image contrast and we discuss the physical determinants of contrast, including material properties, x-ray spectra, detector response, and the role of perturbations such as scatter radiation and image intensifier veiling glare. We include physical determinants of contrast for several other medical imaging modalities, including those used in nuclear medicine, magnetic resonance imaging, and computed tomography to round off the discussion.

4.1COMPONENTS OF X-RAY IMAGE CONTRAST

Contrast can be quantified as the fractional difference in a measurable quantity between adjacent regions of an image. Usually, when we say “contrast,” we mean image contrast, which is the fractional difference in signals between two adjacent regions of an image. In conventional radiography, contrast can be separated into three components: (1) radiographic contrast, (2) detector contrast, and (3) display contrast (Table 4.1 and Figure 4.2).

FIGURE 4.1Contrast relates to the relative difference between an object (circle here) and its surrounding background. Here the background gray level is set to black and the object gray level increased from a low-contrast ...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Table of Contents
• Series Preface
• Preface
• Acknowledgments
• Author
• Introduction
• Section I Basic Concepts
• Section II Intermediate Concepts
• Section III Advanced Concepts
• Section IV Dynamic Imaging
• Section V Tomographic Imaging
• Index