MRI Scanning

MRI Scanning relies on radio frequency electromagnetic waves, and the fact that some atomic nuclei behave like small magnets in an external magnetic field. The patient lies within this magnetic field.

The property that allows some nuclei to behave like magnets is called Spin. Hydrogen nuclei are used in MRIs, because Hydrogen is present in all tissues. A Hydrogen atom is a Proton. These Protons have small N and S poles, and they are in random directions under normal circumstances (gravitational field).

When a strong magnetic field is used, the protons are forced to align itself with the field with the N facing the S pole of the field. Occasionally, a proton is in a flipped position compared to the rest (S pole to S pole) - this is an unstable higher energy state.


The protons rotate in the direction of the magnetic field rather than align exactly with it. The rotation action is called Precession.

The angular frequency of the precession is known as the Larmor Frequency, which depends on the nucleus' involved and the magnetic flux density of the field (the `frequency` is measured using Radians per second). The stronger the external magnetic field is, the faster the protons precess.

The protons in the external magnetic field are occasionally subject to pulses of (radio frequency) waves. This frequency equals the frequency of the precession, so resonance occurs (nuclear magnetic resonance).

When the RF waves are switched off, the protons gradually revert back to the lower energy states - by releasing RF waves. We can detect these, and the rate of the `relaxation` of the protons can be used to tell us about the environment of the protons. For example, water/watery tissues have a relaxation period of several seconds, fatty tissues last several hundred milliseconds, but cancerous tissues has an intermediate period.

This allows an image of the patient to be built up.

PET Scanning

PET, or Positron Emission Tomography, is a technique using γ-radiation and positrons. The radiopharmaceuticals used emit positrons - β+.

For example -  
  Fluorine-18 (18/9 F) decays into 0xygen-18 (18/8 O) along with β+, V and γ.

When the positron is emitted, it can collide with an electron - which is known as Positron-Electron Annihilation - and two γ-photons are emitted. These photons are emitted at 180 degrees to each other.

  By surrounding the patient with γ detectors, the γ-rays are detected, and the time difference between the two rays helps determine where they were emitted. This can eventually build up a 3D image of the area.

On this image you can see that there would be Gamma detectors surrounding the patient.

Radiopharmaceuticals & Gamma Cameras

  To put it plainly, a Radiopharmaceutical, or Tracer, is a radionuclide (, or radioisotope) attached to a substance that will target a specfic part of a patient's body - for example, cancerous cells have an uncontrolled cell division, so take up more glucose to fuel the divisions. Therefore glucose would be a suitable substance to attach a radioactive marker to if you were looking for cancerous tissue.

  Once ingested or injected into the patient's body (and after a suitable time period so the radiopharmaceutical has `zoned in` on the targeted tissue or area), the patient is screened using a Gamma Camera.

A diagram showing a Gamma Camera taking in Gamma radiation, γ.

The γ-radiation emitted from the patient is projected in every direction. The Collimator, shown on the diagram, ensures only the γ-radiation travelling at the right plane enters the camera. The collimator is usually made from lead to stop other planes of radiation entering.
  The Scintillator, shown in the diagram as the crystal, only detects photons travelling along the axis of the collimator, so any radiation at a slight angle is cut out. The γ-photons that are dead on strike the crystal, and electrons are emitted on the other side (the Photoelectric effect). The electron travels up the photomultiplier tube, and impacts involving the dynode release more electrons. Soon there is a huge number of electrons and an electrical pulse is noticeable.
   Those pulses from the photomultiplier tubes are processed electronically by an attached computer and reassembled to produce an image.