There are only two naturally occurring isotopes of iodine that are I - 127 (stable) and I - 129 (radioactive). Other radioactive iodine isotopes (e.g., I - 131) do not occur in nature [3,4]. The radioactive isotope I - 123 is considered the agent of choice for brain, thyroid, and renal imaging and uptake measurements. I - 125 is used as a cancer therapeutic, and as a brain, blood, and metabolic function diagnostic. I - 131 is used as a brain, pulmonary, and thyroid [3]. Iodine isotopes above 127I decay by beta particle emission, and energy is shared between the beta particle and the gamma ray. A total of 72% of uranium fissions and 75% of plutonium fissions lead directly, or by beta decay, to iodine isotopes. For example, 2.89% of 235U and 3.86% of 239Pu fission atoms lead to the formation of a series of isobar 131 isotopes, including 131In, 131Sn, 131Sb, 131Te, 131I, and 131Xe. In 90.4% of the decays, a beta particle is emitted. The remaining excess energy is emitted as either a gamma ray or a pair of gamma rays [6]. Beta decay happens when a proton decays into a neutron, a positron (the antiparticle of the electron) and a neutrino. The positron and the neutrino are emitted and the radioactive particle is the positron (fig.2.a). Gamma decay is high-energy electromagnetic waves, which are emitted from the nucleus (fig.2.b). These waves are photons. A gamma decay can happen after an alpha decay or a beta decay [9].
Fig 2.a. Beta decay [Adapted from 10] Fig 2.b. Gamma decay [Taken from 10]
Isotopes of mass less than 127 are produced in particle accelerators (common examples are 123I and 125I), while those with a mass greater than 127 are formed in neutron generators, such as nuclear reactors (common examples are 129I and 131I), or cyclotrons (fig.2). The cyclotron uses high voltages and electrical fields to accelerate hydrogen atoms through a vacuum chamber. When they collide with a target substance they produce radioactivity. It is more difficult to make a radioisotope in a cyclotron than in a reactor. Cyclotron reactions are less productive and less predictable than nuclear reactions performed in a reactor [7].
Fig 2. A Cyclotron [Taken from 8]
The detector most commonly used with radioactive tracers is the Anger scintillation camera, invented by Hal Anger in the late 1950s. Gamma radiation causes crystals
of sodium iodide to emit photons of light. This is called scintillation. The process of obtaining an image from a radioactive tracer is called scintigraphy. Other imaging techniques (computerized tomography, CT; magnetic resonance imaging, MRI) give anatomical information. Scintigraphy gives information on the movement of compounds through tissues and vessels. Tomography uses computer technology to convert numerous planar images into a three-dimensional slice through the object. This data processing is also used with CT and MRI. With radioactive tracers, it is called emission computed tomography, which includes single photon emission computed tomography (SPECT) and positron emission tomography (PET). With SPECT scans, anger scintillation cameras obtain numerous images by rotating around the patient (fig.1). Computers then form the images that provide data, and information, about the area of body being diagnosed or treated [5].
Fig 1. Schematic diagram of an anger scintillation camera. Illustration by Hans & Cassidy. Courtesy of Gale Group. [Taken from 5]
Iodine-131 can be very useful in medicine, but does not come without risks. The radioactive iodine that is not taken up by the thyroid is rapidly eliminated through body fluids such as urine, saliva, and perspiration. This means that, for a number of days after the iodine has been administered, everything that the patient touches could become contaminated with radioactive iodine. In order to prevent the spread of this contamination, it is necessary for the patient to remain in a specially prepared room.
References
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O'Mathúna, Dónal, P. (2005). “Radioactive Tracers”, Gale Encyclopaedia of Science [online]. Available from: http://find.galegroup.com/ips/infomark.do?&type=retrieve&tabID=T001&prodId=IPS&docId=CX3418501903&source=gale&srcprod=EB00&userGroupName=lowcoll&version=1.0 [Accessed: 27/11/2005]
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