Radio-Imaging Effectiveness

Radiation can be used in both diagnosis and therapeutic manners. The radiation emitted from radioisotopes can destroy tissue and in the therapeutic use: the destruction of cancerous and other dangerous tissues. And with diagnosis the gamma particle pass through the body tissues with minimal damage to a gamma camera.

To evaluate the effectiveness of PET, SPECT, MRI and X-rays, we first need some basic knowledge on how each of them work.

PET stands for positron emission tomography and works by an instrument collecting radiation emitted from a radioisotope injected the patient’ body. The strengths of emission are recorded by a gamma camera, which has a series of scintillation crystals, each connected to a photomultiplier tube. The crystals convert the gamma rays, emitted from the patient, to photons of light, and the photomultiplier tubes convert and amplify the photons to electrical signals. These electrical signals are then processed by a computer to generate images. The table is then moved, and the process is repeated, resulting in a series of thin slice images of the body over the region of interest (e.g. brain, breast, liver). These thin slice images can be assembled into a three dimensional representation of the patient's body

Nowadays, PET scanning devices are most often used in conjunction with CT scanners, so that a more accurate image can be observed by the doctor for easier diagnosis of diseases or disorders.

SPECT (Single Photon Emission Computed Tomography) works in a way much the same to PET. But the radioactive substances used in SPECT (Xenon-133, Technetium-99, Iodine-123) have longer decay times than those used in PET, and emit single instead of double gamma rays

MRI has a more complex principle for its function; it works by creating a magnetic field so strong that the hydrogen protons in the body are forced into alignment with the magnetic field. Short bursts of radio waves are sent from the scanner into your body. The radio waves knock the protons from their position. When the burst of radio waves stops, the protons go back into position. They realign back to being in parallel with the magnetic field. As the protons realign, in a process known as relaxation, they emit tiny radio signals. A receiving device in the scanner detects these signals. The type of tissue can be interpreted from the strength of the signal emitted.

Most of the hydrogen atoms in the body are in water molecules. Each type of tissue has different water content. So, the strength of the signal emitted from different body tissues varies. The computer creates a picture based on the strength and location of the radio signals emitted from the body. (A different colour or shade for each strength of signal.)

CT scans work by having many X-ray shots taken. The X-ray tube and detectors are situated oppositely each other and they can take numerous X-ray images as they rotate at all angles around the patient. The machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. It's not usually necessary to scan the entire body, of course. More often, doctors will scan only a small section.

X-ray images are essentially photos that use X-rays instead of visible light to expose the film. When the X-rays hit the film, they expose it just as light would. Since bone, fat, muscle, tumours and other masses all absorb X-rays at different levels; the image on the film lets you see different (distinct) structures inside the body because of the different levels of exposure on the film. An x-ray tube releases the x-rays at the patient while an x-ray camera on the other side of the patient records the pattern of X-ray light that passes all the way through the patient's body. The X-ray camera uses the same film technology as an ordinary camera, but X-ray light sets off the chemical reaction instead of visible light. Doctors can bring different materials into focus by varying the intensity of the X-ray beam.

The provision of materials used, that is the isotopes needed for the diagnosis and their half lives greatly effects peoples judgement on how effective the mechanism is. MRI requires no radioisotopes for its use, which is a major benefit. Xrays and CT scans also do not require a radioisotope for injection.

PET and SPECT machines, however, do require radioactive isotopes for diagnosis. The isotopes used in PET generally are ones with extremely short half lives whilst those used in SPECT are usually ones that have longer half lives, but are still relatively short.

Isotopes