Medical imaging began with the discovery of X-Rays by Wilhelm Conrad Röntgen in 1895 and his work showing that these rays could be used to peer inside the body and visualize bony structures. From this Nobel Prize winning discovery to the modern day, there are now many means by which the internal structures of the body can be assessed without the need for cutting it open.
X-Ray Imaging
Many people have had simple medical X-Rays taken. These images are generated by exposing a body region to X-Rays and detecting the differential absorption of their energy after they have passed through. X-Rays are a high frequency, high energy wave that can be slowed down to a lesser or greater extent as they pass through materials of different densities. Thus, for example, bones are of a much higher density material than muscle so when an X-Ray is taken of someone’s arm, the bones show up as clearly definable, whiter images and the muscles show up as more shadowy figures. Because of their higher density bones are said to be more “radiopaque”.
A more recent use of X-Ray imaging is the CT or CAT scan. Known as computerized axial tomography, CT scans also utilize X-Rays to image the body. However, in these scans many X-Ray images are taken from multiple angles around the body. The detectors that are built into the machine assess the differential absorption of the X-Rays and this information is processed by computer to generate a highly detailed image of the body’s interior. These images are able to provide a much greater level of detail than do “plain” X-Ray images.
Magnetic Resonance Imaging or MRI
MR scanning is a much different method of looking into the body’s interior that does not use X-Rays, but rather the ability of strong magnetic fields to make atoms inside the body generate radio signals. The large tube into which a patient slides in an MR scanner houses a very strong electromagnet and a radio tube.
When exposed to a strong magnetic field, protons associated with hydrogen atoms are made to align in the magnetic field. Then, they are exposed to radiofrequency waves. When they move in the radio waves the protons emit a signal that is picked up by the antennas associated with the scanner. Because the protons can “relax” from their enforced alignment in different manners, the signals generated can be detected and manipulated by computer algorithms to generate an image. Newer generation MR techniques include functional MRI (fMRI) which can be used to assess real time changes in tissue behavior in response to specific stimuli.
Positron Emission Tomography (PET) and Radionuclide Scanning
Another means of imaging the inside of the human body is to inject chemicals such as glucose, which are routinely used by cells, and to label them with short-lived radioactive elements. In positron emission tomography, a radiopharmaceutical compound is injected which can be “seen” by specialized detectors built into the scanner. As the radiopharmaceutical undergoes radioactive decay it emits positrons which lead to molecular actions that can be detected by the scanner. This method can measure the metabolic activity of specific tissues or regions of tissues as well as to assess the metabolic activity and spread of cancers, especially when coupled with coincident CT scanning as is the most common approach used today.
Other radionuclide scans have been used clinically for many years and include techniques such as bone scans, thyroid scans, and some cardiac scans. Compounds that have been radioactively labeled using isotopes of technetium or thallium, amongst several others, are taken up by the body, and their distribution is detected by the use of a specialized detector known as a “gamma camera”. Computers assess the signals that are received and generate the image for viewing. The advantage to these techniques is that they can measure the metabolic activity or function of specific tissues essentially in real time.
Ultrasonography Imaging
Using sound waves is another means to peer inside of the body. In ultrasound (frequencies of sound beyond the range of normal hearing) studies, sound waves are transmitted into the body and the probe detects the echos of the sound waves as they bounce back off of the structures they traverse. This occurs most efficiently at the boundary between two areas of different density. Ultrasound works best when there is an interface between tissue and fluid filled cavities, such as in the uterus of a pregnant woman carrying a fetus.
There are numerous other medical imaging methods available and there is even mobile medical imaging but as always consult your medical professional as it pertains to questions regarding your health. To learn more about these techniques visit the Radiology Guide at the U.S. Uniformed Services University of the Health Sciences (USUHS).
Kenneth Rosen - I am a medical research doctor with more than 20 years of experience studying how the nervous system and skeletal muscle develop and ...
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