Medical physics

Medical physics is the application of physics to medicine. Physics in mainly used in the process of imaging, such as X-Rays, CT and MRI; but it may also be used in the treatment of disease, such as radiotherapy. Many of the applications of physics in medicine use radiation or radioactivity.

X-rays are a form of electromagnetic radiation. They are formed by colliding fast-moving high energy electrons into a dense metal. As the the electrons interact with the metal they slow down, losing energy which is radiated as x-rays. The importance of x-rays in medicine is that the rays are stopped by bone much more than soft body tissues. So by placing a body part between the x-ray source and some photographic film an image of the patient's bones is formed. This is very useful for diagnosing fractures or other bone problems, but it is difficult to see detail in the soft tissues. To image the blood vessels or other soft structures, contrast media may be injected. These are substances which absorb the x-rays, so appear on the image. For example, a vein containing contrast media will appear on an x-ray, so by injecting the contrast into a patient, their blood vessels can be studied.

CT imaging uses x-rays to obtain 3D images of a patient. X-ray images are obtained at angles all around the patient and then reconstructed to produce slices through the body.

To understand how MRI works requires a degree in particle physics. Hydrogen atoms have only one electron, so produce their own small amount of magnetic field. Normally these fields are pointing in all different directions, but when they are placed near a much stronger magnet (the large tube used in MRI), all the fields align with the external field. When the small fields are hit by a radio-frequency pulse, they change direction and it takes some time for the fields to go back into allignment with the large magnet. The time it takes for the re-alignment to occur depends on what substances the hydrogen is chemically bonded to. So fat with have a different timing to muscle (which is mostly water). Images can then be formed based on how long it takes the atoms in each region to realign.