Dosimetry: It is the determination or measurement of the amount of Radiation Exposure or dose in Diagnostic radiology for Medical or Dental purpose. With the increase in the number of Diagnostic tests which are quite helpful in Diagnosis are found to be equally harmful. There are studies linking excess exposure to X-ray Radiation and cancers of the skin and Eyes. There are harmful effects seen in both acute and chronic exposures. Hence it is very important to measure the amount of Exposure to Radiation in a Diagnostic or Clinical setting. Radio sensitivity of various organs of the human body plays a major role in determining the exposure of X-rays to the particular organ.
Radio sensitivity is the relative susceptibility of cells, tissues, organs to the harmful action of radiation. It denotes the level of harm which radiation can cause to certain types of cells in the body. This makes it quite important for technician, Doctors, patients and other staff in the Radiology center to know the amount of Exposure over a period of time to check if it is crossing harmful levels. It is very important in children and pregnant women who are more sensitive to radiation than normal people.
How is Radiation or Dosimetry measured:
MPD or Maximum permissible dose is formulated as the maximum dose of radiation that in the light of present knowledge would not be expected to produce any significant radiation effects in the life time of an individual.
The MPD for non-occupational person is 0.005 Sv/year while that of an occupational person is MPD = (Age – 18) x 5 rem which is around 0.05 Sv per year.
There are various units of measurements which are used to help measuring Radiation exposure:
- Curie (Ci) or SI unit becquerel (Bq): The amount of radiation emitted by a radioactive material.
- Rad or SI unit gray (Gy): The radiation dose absorbed by a person or the amount of energy deposited in human tissue by radiation.
- Rem or SI unit sievert (Sv): The biological risk of exposure to radiation
The amount of exposure dose in rem Dose in sievert (Sv): One dental x-ray 15 mrem 0.15 mSv
Measuring patients exposure to radiation is also important which can be done with certain devices.
Devices for measuring Radiation Exposure:
Film Badge dosimeter:
These are portable badges which contain a film to determine the amount of radiation exposure on being processed based on the amount of darkening. It is used to determine an individuals exposure limits. The badge consists of radiation-sensitive lithium fluoride crystal which trap the electrons on exposure to radiation, these are released on heating the crystals to high temperatures releasing visible light which is used to measure radiation dose. These are called as Thermoluminescent dosimeters.
The badges are of many types – body badge, ring badge, pen, etc which have to be worn on the neck or chest area, on your finger facing the radiation source. Make sure that the Badge is worn over the Lead apron and not beneath it. Wear the badge everyday on entering work without fail and place it in a radiation free zone after work.
It is used to determine the radiation exposure of a room and not individuals, these are also used to measure the output of X-ray tubes. These are available in the form of a box which is used to measure outputs of X-ray tubes and also act as photo timers in automatic exposure controls. They are also available as pocket ionization chambers which are portable form of dosimetry devices in the shape of large pens which are used to provide immediate readings.
Ionization chamber comprises of two oppositely charged plates which are separated by a certain amount of air. A standard charge is applied to the plates, which are connected to a Galvanotmeter to measure the charge. The Ionization chamber functions when the x-ray beam is exposed to the air, negative and positive ions are formed. The positive ions are attracted the negative plate and negative ions get attracted to the positive plate which results in partial discharges which is measured. The radiation exposure is based on the number of ion pairs produced and the drop in its potential.
Thermoluminescent Dosimeter (TLD):
As the name suggests, TLD’s use the principle of certain crystals having the ability to absorb x-ray photons and generate visible light. The crystals used are Lithium fluoride, LiF. The TLD dosimeter is used to measure a long term exposure of X-ray radiation, it is word in the forma of a badge for a period of time usually 3 months which is then processed to determine the amount of radiation exposure.
How does Thermoluminescent Dosimeter work: As TLD consists of either Lithium fluoride or Calcium Fluoride in a solid crystal structure, when exposed to ionizing radiation at ambient temperatures it reacts with phosphor crystals and deposits all or part of the incident energy in that material. Some of the atoms in the material absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.
On heating the crystal causes the crystal lattice to vibrate releasing trapped electrons in the process. The released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. The light released in the process is measured using photomultiplier tubes and the number of photons counted as proportional to the quantity of radiation striking the phosphor. The amount of light released versus the heating of the individual pieces of thermoluminescent is measured. The glow curve produced by this process is then related to the radiation exposure. This process can be repeated many times making it reusable.
One thing to remember is that, no amount of radiation is safe and hence proper Radiation protection is a must whenever you are being exposed to Radiation.