The June 15 issue of the Bangor Daily News carried an article by Michael O’D. Moore to the effect that Maine’s Certificate of Need Unit was recommending to Commissioner Kevin Concannon of the Department of Human Services that the state obtain two mobile PET scanners while denying Eastern Maine Medical Center its request for a permanent one at its facility.

To some readers, the question that likely comes to mind is: What in the world is a PET scanner? A reasonable question. Medical imaging, roughly a century old, has come so far from the original X-ray scans that many of today’s technologies resemble something out of “Star Trek.” This brief review of medical-imaging techniques is derived from several sources including three books: Bettyann Kevles’ “Naked to the Bone,” Howard Sochurek’s “Medicine’s New Vision” and Alan Bleich’s “The Story of X-Rays.”

On Nov. 8, 1895, University of Wurzburg physics professor William Roentgen was experimenting with a cathode ray tube on a bench with a fluorescent screen at one end. He was amazed to see that, when he placed his hand between the tube and screen, a ghostly image of its bones were silhouetted on the screen. Roentgen had discovered the X-ray, an achievement that won him the Nobel Prize for physics in 1901. The first medical X-ray in the United States was in 1896 in the physics department at Dartmouth College. It was of a young boy who placed his broken arm on a photographic plate while X-rays from an unshielded tube were beamed on it for 15 minutes. Many improvements in standard X-ray machines have been made over the years but the basic principle has remained the same up to the present.

The next major advance in X-ray imaging came in the 1970s when British engineer Godfrey Hounsfeld and Tufts University physicist Alan Cormack came up with computerized axial tomography or the CAT scan. A CAT scan takes hundreds of cross-sectional X-rays through the body, which are then integrated into what amounts to a three-dimensional, high-resolution video image. The process is very similar to how a spacecraft beams back streams of data in digital code, which are then reconstructed into pictures. The first CAT scan covered only the head and took five minutes, while today a full-body scan can be done in a few thousandths of a second. Hounsfeld and Cormack shared the 1979 Nobel Prize in medicine for their discovery.

Magnetic resonance imaging, or MRI, does not involve X-rays at all. More than 60 percent of the atoms in the human body are hydrogen which, due to their spinning motion, have a small magnetic field. If a radio frequency of the same energy as the field is directed at the atoms, a few of them will change their direction of spin absorbing energy in the process. Hydrogen atoms do not all have the same spin energy because of their environments in the body. When a thin section of the body is scanned, various energies are absorbed by the atoms and can be measured either as energy absorption or emission. In the same way as a CAT scan, these slices can then be integrated together into a whole body scan. MRI scans are particularly valuable because they act in a reverse manner to X-rays in that the bones do not appear while soft tissue organs are emphasized. Many researchers were involved in the development of MRI, which was first introduced in the late 1970s.

Ultrasound imaging was a direct offshoot of the sonar devices used to detect submarines during World War II. They employ a piezoelectric crystal that expands and contracts as a variable electric field is placed on it. The transducer, the part of the instrument that is pressed to the body, sends out very high frequency sound waves that penetrate to, and then reflect back from, internal organs. The reflected sound waves are reconverted to electrical pulses by the transducer and these are used to form the image.

Tissues of high density, such as bone, reflect sound differently than soft organs such as the liver. These differences are what make an image possible. A British researcher, John Wild, constructed the first working ultrasound device in 1964. Today they are used for mammography and are particularly valuable in looking for arterial constriction and blood flow problems. But it is in the study of fetal development that ultrasound has proven its value. An estimated 70 percent of all pregnant women in the United States have ultrasonic exams.

Positron emission tomography, or PET, has its roots in nuclear physics. Most people are familiar with electrons, the negative particles that orbit the nucleus of an atom. The positron, a positive electron, is called the “antimatter” analogue of the electron. If an electron and positron come together, they are destroyed in a burst of energy that appears as two gamma ray photons speeding away from the point of contact. A PET scan is done by producing radioactive isotopes that emit positrons and combining them with a sugar that concentrates in the body where metabolic rates are highest. These are usually the brain and nervous system. The isotopes emit positrons that last for only a fraction of a second before being destroyed and the gamma rays detected. PET is used for brain and spinal cord tumors and to study brain diseases like epilepsy, Alzheimer’s and schizophrenia.

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.