Biomedical imaging has revolutionized medicine and biology by allowing us to see inside the body and to visualize biological structures and function at microscopic levels. Images are representations of measurable properties that vary with spatial position (and often time). Images can provide exquisitely detailed information about biological structures; the most powerful imaging modalities provide functional information as well, allowing the recording of molecular and cellular processes, or physical properties (such as elasticity or temperature). Methods to visualize and quantify these properties are now available at the macroscopic (i.e., of a size visible to the human eye) and microscopic level. This information can be used clinically for diagnosis and monitoring of treatment as well as scientifically for understanding normal and abnormal structure and physiology.
Technology has brought about remarkable changes in imaging (Figure 12.1). Gene expression can now be imaged using positron emission tomography (PET) imaging—an image creation method that depends on injection of special radioisotopes—coupled with methods from genetics. The brain can be imaged in real time during cognitive tasks with functional MRI (fMRI), generating information can be used to guide neurosurgery. The mechanical action of the heart can be mapped using high-frequency sound waves (ultrasound imaging); these maps identify areas of injury after a heart attack. Images are essential tools in medicine because they provide a spatial map, enabling physicians to localize the biological phenomena being examined in space as well as time.
The many different imaging modalities—or types of imaging methods—fill different scientific and/or clinical niches. Every modality has limitations: A particular method may be low in quality, slow in acquiring images, expensive, or not suitable for all patients. The set of advantages for a particular technology (e.g., high quality, faster, cheaper, or dynamic) will make it suitable in the right situations. Different modalities operate over different time or length scales and—because of the physics that underlie their operation—can measure different structures or functions (Table 12.1).
Virtually all imaging modalities are now digital; the images are acquired by a computer and are made up of individual picture elements, or pixels. Digital images can be readily processed to improve their quality, make measurements, and extract features of interest.