ABSTRACT
Medical imaging is an important diagnostic tool. There are a variety of medical imaging techniques, each imaging a different physical or physiological property of the body. For example, x-ray images, both planar and tomographic (x-ray CT), are images of the distribution of the linear attenuation coefficient at an average x-ray energy within the human body. This is a property which is largely a function of tissue density, at least for soft tissues, but is also a function of tissue composition. Thus bone has a higher attenuation coefficient than can be explained simply by increased density because it contains significant amounts of relatively high atomic number elements. X-ray images are largely images of anatomy. Radioisotope imaging (RI) produces images of the distribution of a chemical labelled with a γ-ray-emitting isotope. The chemical is distributed according to physiological function so the image is primarily an image of function, although as function is distributed anatomically it also produces recognizably anatomical images. Ultrasonic imaging (UI) produces images related to changes in the acoustic impedance of tissues, again largely anatomical in nature, although Doppler flow imaging (DFI) produces functional images of flow. Magnetic resonance imaging (MRI) produces images of proton density, largely a function of the water content of tissue, and images of relaxation times which depend on the environment of the protons. These are largely anatomical. Some functional images, such as flow images, can also be produced and there are prospects of images of other elements apart from hydrogen. Other less familiar imaging techniques produce images of other properties. For example, in electrical impedance tomography (EIT) the electrical properties of tissue are imaged. Each of the imaging techniques is unique, in the sense that a unique physical property is being imaged, although in some cases different techniques can be used to make the same diagnosis. Figure 11.1 illustrates the distinction between functional and anatomical imaging by comparing RI and MRI images of the head.
