Gamma Rays in Biomedical Engineering

By: Eslam Ehab

To: Dr. Mohammad Hesham

Nature of Gamma rays:Gamma radiation, also known as gamma rays, and denoted by the

Greek letter γ, refers to electromagnetic radiation of an extremely high frequency and therefore consists of high-energy photons. They are classically produced by the decay of atomic nuclei as they transition from a high energy state to a lower state known as gamma decay.

It was discovered in 1900 by the French  chemist and physicist Paul Villard while he was studying the radiation of radium. In 1903, the new radiation was named "gamma rays" by Ernest Rutherford.

Natural sources of gamma rays on Earth include gamma decay from naturally occurring radioisotope, that is, isotopes that have nuclear radioactivity. there is many other sources but here we are only discussing the main source used in biomedical engineering.

Gamma rays typically have frequencies above 10  exahertz  (or >10E19 Hz), and therefore have energies above 100 keV  and wavelengths less than 10 picometers (10E−11 meter), which is less than the diameter of an atom.

Main uses in biomedical field:

1) Gamma radiation is used to, in a process called irradiation, sterilize medical equipment, remove decay-causing bacteria from many foods, and prevent the sprouting of fruit and vegetables to maintain freshness and flavor.

2) Gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells.

3) Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques. A number of different gamma emitting radioisotopes

are used. This is the main topic of this research. 

Medical imaging with gamma raysMedical imaging is the technique and process of creating visual representations of the interior of a body for clinical analysis and medical intervention. The must medical imaging processes that use gamma rays are Scintigraphy, Positron emission tomography (PET), and Single-photon emission computed tomography (SPECT).

1) Scintigraphy:Radioisotopes are taken internally and the emitted radiation is

captured by gamma cameras to form two-dimensional images. They image nuclear events provoked by collisions  among nuclear particles or ionizing radiation and atoms which result in a brief, localized pulse of electromagnetic radiation, usually in the visible light range due to a phenomenon called Cherenkov radiation. This pulse is detected and amplified by a photomultiplier and its resulting electrical waveform is processed by computers to provide two-dimensional images . Cherenkov radiation, briefly, is an electromagnetic radiation  emitted when a charged particle passes through a dielectric  medium at a speed greater than the phase velocity (the velocity of the light in this medium).

2) Positron emission tomography (PET) :A positron-emitting radioactive isotope (tracer) is introduced into the

body on a biologically active molecule, and the indirect gamma radiation is detected by the system, and 3D images of tracer concentration within the body are produced. As the radioisotope undergoes positron emission decay, it emits a positron, an antiparticle of the electron with opposite charge. The emitted positron travels in tissue for a short distance, losing kinetic energy and decelerating to a point where it can interact with an electron. The encounter annihilates (that is, both of them vanishes) both electron and positron, producing a pair of gamma  photons moving in approximately opposite directions. These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier  tubes and constructed by computer analysis into images.

3) Single-photon emission computed tomography (SPECT) : It is able to provide true 3D information and scan monitors level of

biological activity. Emissions from the radionuclide indicate amounts of blood flow in the capillaries. The technique requires delivery of a gamma-emitting radioisotope into the patient, normally through injection into the bloodstream. SPECT is similar to PET in its use of a radioactive tracer material and detection of gamma rays. In contrast with PET, however, the tracers used in SPECT emit gamma radiation that is measured directly. PET allows higher spatial resolution images than SPECT (which has about 1 cm resolution). SPECT scans are significantly less expensive than PET scans, because they are able to use longer-lived more easily obtained radioisotopes than PET.

Body reaction with gamma rays:We use some units to express the equivalent dose, which is the measure of the biological effect of radiation on human tissue. For gamma rays, it is equal to the absorbed dose.

1- The sievert (Sv) is the SI unit of equivalent dose, which for gamma rays is numerically equal to the gray (Gy = J/kg), which is the SI unit of absorbed dose, and is the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.

2- The rem is the deprecated CGS unit of equivalent dose. For gamma rays it is equal to the rad or 0.01 J of energy deposited per kg. 1 Sv = 100 rem.

Gamma rays cause damage at a cellular level and are penetrating, causing diffuse damage throughout the body. If an individual is exposed to small doses of gamma radiation, they may develop a mild case of radiation poisoning. The skin, hair and gastrointestinal tract cells are most likely to be affected first in radiation exposures.