What is the difference between them?
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An X-ray shoots X-rays through you once. A CT scan takes a 3D picture by shooting X-rays through you over and over again from a bunch of different angles and working out the density of each point in your body.
MRIs are really magic though, they basically excite different atoms in your body and then see how long it takes them to relax. As the atoms relax, they release radio waves. The MRI scanner picks these up and can tell you the intensity of the radio signals.
X-ray is shooting rays through the body to see what’s inside in one or two projections. CT is the same rays but shot multiple times (like dozens or hundreds) from different directions to construct a 3D picture. MRI is using magnets to excite atoms in the body in specific ways to also construct a 3D picture. MRI differs from CT in that it doesn’t involve potentially dangerous radiation, and also the kind of body structures and anomalies that can be seen in CT and MRI are somewhat different.
How do MRI, Magnetic Resonance Imagers work? https://youtu.be/hlFGbiZRR5I
A CT or Computerised Tomography and CAT or Computerised Axial Tomography use X-rays to scan various parts of the body to give a three dimensional computer generated image of the part of the body being scanned. https://youtu.be/4Y617cFGHCg
Xray is a 2D image of your body. Usually from the side or front and back. You stand in front of an Xray sensitive film, the X rays shoot through your body and affect the film.
CT scan is still using X rays to take pictures of your body, although it does rotate around your body to construct a 3D image. Imagine if you took a video camera and panned it 360 degrees around a person. Even the though video is 2D, the information from the video is enough to construct a 3D shape.
MRI images a body similar to a CT scan but it doesn’t use Xrays. Instead it uses a strong magnetic field to align the magnetic moment of certain chemicals in the body to respond to certain electromagnetic waves.
* An x-ray is 100-year-old technology of shooting a bunch of high energy photons through the body from one angle and then into a piece of film where an image is captured. It’s one view of the body taken from one angle.
* A CT/CAT (computerized axial tomography) uses x-ray technology, but essentially shoots many different angles and then uses a computer to reconstruct the image.
* An MRI (magnetic resonance image) is a different and much newer technology, where the body is immersed in a strong magnetic field, and then hit with radio photons that cause the molecules in the body to vibrate in the magnetic field. This vibration gives off signals that are then reconstructed by computer to form the image.
They are all ways of seeing the internals of a body, but somewhat different in implementation.
X-ray shines light (x-rays, our eyes can’t see them), and the picture is the shadow. The x-ray picture is a photo negative, so the shadows are white and the bits where light shines through are dark. The denser stuff that blocks light, like bones, are white.
A CT (computed tomography) scan is x-rays again, but you take lots of pictures from different angles and use a computer to stitch them together into a 2-D view so you get an x-ray that looks like you sawed a person in half and are peering down on the slice. This is useful to get a detailed view to look at the size and shape of things.
MRI (magnetic resonance imaging) is completely different. It uses a giant magnet and radio waves to cause certain atoms to spin, and then uses a ring of antennas to detect when they pop back (they “squeak” a little radio signal). A computer then creates an image from the radio signals kind of like the CT scan computer does. MRI shows much greater detail for softer tissue- the sort of stuff x-rays mostly pass through), and can show sharper images than CT scans. It’s great for looking at squishy things like brains.
Many years ago I worked on all three systems.
X-rays are one of the simplest forms of medical imaging. An x-ray tube converts high voltage into X-rays by sending an intense stream of electrons into a spinning tungsten disc. The X-rays produced from the disk passes through soft tissue and bones differently. This is recorded on film, special cassettes or imaging sensors as varying degrees of light, soft tissues appear darker and bones appear lighter, although this can be reversed depending on how it is used. X-rays are typically done in a fixed position but there are 2D and 3D applications where the X-ray tube is moved while imaging.
CT (Computerized Axial Tomography) scans use a similar X-ray tube, but instead of being in one location it is moved around the area of interest in a circle and a computer records the changes. Here is where mathematics are used, as the information is processed the amount of density varies based upon each angle. A fast computer takes all of this information and can determine areas that are soft tissue and other areas that are bone. A sufficiently smart program can determine specific density values and assigns a value to each ‘pixel’, where the computer arranges the information on the monitor. The more information the computer can store and process determines the density of the tissue but cannot determine the actual material of the area.
This is where an MRI (Magnetic Resonance Imaging) can excel, MRI can determine with pretty good accuracy exactly what atoms are in a precise location. An MRI consists of a very large and powerful magnet, within this magnet is a series of electrical coils (field coils) that can send and receive information from the atoms. When a person is placed in the tunnel of the MRI magnet, the atoms that make up our bodies are aligned to the big magnets magnetic field. When the field coils are turned on it temporarily shields the atoms from the big magnets magnetic field so they return to their original position. When the computer turns the field coils off the field coils start listening to the atoms, as the atoms realign to the big magnet they emit a small radio signal as they fight to stay in their normal alignment. The field coils record these signals and their positions into a large database. This is repeated many times, the field coils are energized and turned off and they listen to the atoms going back into alignment.
The neat thing is different atoms give off different radio signals when changing orientation to the big magnet, the computer recording these signals knows the difference between them and can assign a value to a precise location. After enough information is gathered the computer can make a pretty good guess as to what atoms are within your body. This information is pieced together and displayed on a monitor. Colors can be assigned to a known tissue value and the computer can generate a fair representation of what is inside.
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