Modern CT Scanners also typically have many detectors and X-Ray sources that work in parallel, improving scan performance and reducing scan times. To improve tissue contrast with CT Scanning, a special type of X-Ray dense contrast dye can be administered, either intravenously or by mouth, which makes any tissues or blood vessels it is in appear brighter on the resulting CT Scan.
Although CT Scans represent a major technological and diagnostic advance compared with simple X-Rays, the advent of CT Scanning has also dramatically increased the amount of radiation that patients are exposed to. (See “What are the Radiation Risks of a CT (CAT) Scan?”)
An MRI is made in a completely different way, without X-Rays or any other type of ionizing radiation. Magnetic Resonance Imaging involves some truly amazing (and pretty hard to follow) physical principles. Protons inside your body are constantly spinning around, and the axis of this spin (like the axis of a planet’s spin) is reasonably random. However, if you place a body part (and it’s protons) inside a really strong magnetic field, the protons start spinning with their axes aligned relative to the magnetic field. As if this isn’t complex enough, the next step is to send in a pulse of radio frequency (RF) energy, that causes some of the protons to shift the angle of their spin axis. When this “RF pulse” is turned off, the shifted protons “relax” their spin axes back to the original state, and when they do this, RF energy is emitted that can be measured by the MRI machine. The ‘coils’ that pull off this trick in the MRI machine often have to be arranged in a fashion that closely surrounds the body part of interest, as shown in the picture: