CT & MRI

Computed Tomography and Magnetic Resonance Imaging



Computed Tomography

A CT apparatus (which surely you've ever seen in a hospital, in reality or on TV or film) consists basically of a ring (like a donut) which enters the patient, a transmitter and a receiver X-ray after the walls of the ring can rotate around it, and a computer that analyzes the data collected by the detector. Not to be confused with magnetic resonance scanners, which are the long tube-shaped and often cause problems for the claustrophobic. That's not a problem in a TAC.

To emit X-rays using a small particle accelerator, electrons are accelerated and made impact with a metal target. When the electrons hit the metal and hard braking, kinetic energy were emitted in the form of electromagnetic radiation (photons). Since the electrons are moving very fast, these photons have energy, and therefore frequency, very large, and a very short wavelength (about 10 -10 meters), are X-rays. Depending on the speed that had the electrons and the metal used (some, such as tungsten, the brakes faster than others such as molybdenum) to adjust the frequency of radiation.

On leaving the X-ray gun that emits a cone of radiation, is placed a screen of lead (lead is an excellent X-ray shielding) with a thin slit. What the cross is, therefore, a "slice" of the cone, fan-shaped fine: Obviously, the finer the slit, the greater the precision of the process. Depending on where the X-ray gun, it will in one direction or another. Suppose that the barrel is right on the CT ring pointing down (of course, if you're in do not see it because you're inside the ring and the canyon just across the wall of the ring). Then, X-rays travel from top to bottom, going through your head and below, through a thin section of your skull.

T: X-ray tube

D: detector.

X: X-ray beam R: rotation

Naturally, not all photons pass through the body and reach the other side of the ring: some are absorbed. Those who go through denser material are absorbed more frequently, while passing through soft areas are more likely candidates to reach the other end. Obviously, this means that you are absorbing ionizing radiation, which has its problems, but this talk later. Across the canyon (in the example, just below your head) is an X-ray detector: there are many substances that can serve this purpose, since ionizing radiation is fairly easy to detect. Throughout history it has used photographic plates, matches photostimulable, rare earth screens. Whatever the particular system employed, the detector registers a line of X-ray photons, just cut the projection of your body on him. Some points of the line will be brighter than others, depending on where bone, cartilage, air, water ... when the beam passed through your body.

Then the gun and the detector, which are mounted on a turntable, rotate a small angle. Suppose in the example turn 1, so that X-rays do not reach from just above your head, but almost. The detector records the X-ray photons that arrive, and the ring containing the gun and the ...