Technology has brought about remarkable changes in imagining. 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 at work on cognitive tasks with functional MRI (fMRI), and that 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.