Uses of gamma rays in medical imaging

1. Gamma Rays in Biomedical
Engineering
By: Eslam Ehab
To: Dr. Mohammad Hesham

2.  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.

3. Medical imaging with gamma rays
Medical 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.

4. 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 reactionwith 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.