How are radiological images produced?
The various types of radiological procedures may be grouped by the means in which their images are produced: transmission imaging, reflection imaging, or emission imaging.
- transmission imaging
X-rays, computed tomography (CT) scans, and fluoroscopy are radiological examinations whose images are produced by transmission. In transmission imaging, a beam of high-energy photons is produced and passed through the body structure being examined. The beam passes very quickly through less dense types of tissue such as watery secretions, blood, and fat, leaving a darkened area on the x-ray film. Muscle and connective tissues (ligaments, tendons, and cartilage) appear gray. Bones will appear white.
- reflection imaging
Reflection imaging refers to the type of imaging produced by sending high-frequency sounds to the body part or organ being studied. These sound waves "bounce" off of the various types of body tissues and structures at varying speeds, depending on the density of the tissues present. The bounced sound waves are sent to a computer that analyzes the sound waves and produces a visual image of the body part or structure. Ultrasound is an example of reflection imaging.
- emission imaging
Emission imaging occurs when tiny, nuclear particles or magnetic energy are detected by a scanner and analyzed by computer to produce an image of the body structure or organ being examined. Nuclear medicine studies use emission of nuclear particles from nuclear substances introduced into the body specifically for the examination. Magnetic resonance imaging (MRI) studies, for example, are obtained by using a large magnet to cause changes in the body tissue in order to detect magnetic energy in the body part or structure being examined.