Radionuclide imaging (or functional imaging) is a branch of medicine which provides the only means of assessing physiologic changes that is a direct result of biochemical alterations. In most cases, the information is used by physicians to make a quick, accurate diagnosis of the patient's illness [1]. This imaging is based on the radiotracer method which assumes that radioactive atoms or molecules in an organism behave in a manner identical to that of their stable counterparts because they are chemically indistinguishable. Radiotracers allow measurement of tissue function in vivo and provide an early marker of disease through measurement of biochemical change. As, other X-rays imaging methods assess changes by their differential absorption in tissues (tissue electron density); only anatomical or structural changes -which are the later effects of some biochemical process- can be assessed by these methods. Nuclear medicine has applications in neurology, cardiology, oncology, endocrinology, lymphatic's, urinary functions, gastroenterology and pulmonology. Some forms of radiation therapy are administered by nuclear specialists and these include radioisotope administration.

Isotopes have the same number of protons but different number of neutrons and these elements have same atomic number but differ in atomic mass. These unstable element decay by emission of energy in the form of alpha, beta (electron)/beta plus (positron) and gamma rays. Such isotopes, which emit radiation, are called radioisotopes [2] [3]. These radioisotopes are also known as radionuclides. These isotopes are used in various sectors like industries, agriculture, healthcare and research centres because of their characteristic nature of emitting radiation and their energies.

Radioactive products which are used in medicine are referred to as radiopharmaceuticals [4]. Radiopharmaceuticals differ from other medically employed drugs since they generally elicit no pharmacological response (owing to the minute quantities administered) and they contain radionuclide. They are prepared by tagging the chosen carrier component with an appropriate radioactive isotope. The carrier component of the radiopharmaceutical is a biologically active molecule used to localize the drug in a specific organ or group of organs to provide diagnostic information about those tissues such as pyrophosphate and methylene diphosphonate (MDP) compounds in skeleton bone tissues.

In 1898, discovery of polonium by Pierre and Marie Curie introduced the term "radioactive" [5]. Radium was discovered by the Curie six months after the discovery of polonium with the collaboration of the chemist G. Bemont [6]. Radium played by far a more important role than polonium. Its separation in significant amount opened the way to its medical and industrial application and also its use in laboratories. Later 'uranic rays' was discovered by Henri Becquerel in 1900 [5].

Overall 1800 isotopes are present, but at present only up to 200 radioisotopes are used on a regular basis, and most of them are produced artificially. Radioisotopes can be manufactured in several ways.

The most common is by neutron activation in a nuclear reactor. This involves the capture of a neutron by the nucleus of an atom resulting in an excess of neutrons (neutron rich) which leads to the production of desired radioisotope [2] [3] .

Some radioisotopes are manufactured in a cyclotron, devised by Lawrence and Livingston in 1932 [7] [8] in which charged particles such as protons, deuterons and alpha particles are introduced to the nucleus resulting in a deficiency of neutrons (proton rich). These particles are accelerated to high energy levels and are allowed to impinge on the target material. 11C, 13N, 18F, 123I, etc. are some of the isotopes that can be produced in a cyclotron.

Various isotopes used in medical field as in diagnostic and therapeutic aspects (Table 1) and (Table 2) [9] [10].

In diagnostic application depending upon the type of production these isotopes can listed as follows reactor isotopes and cyclotron isotopes (Table 3).

Mode of administration

These isotopes either during diagnostic or therapeutic can be administered by inhalation (xenon, argon, nitrogen), oral (iodine) or intravenous (thallium, gallium).

The most commonly used liquid radionuclides are technetium-99m, iodine-123, iodine-131, thallium-201, and gallium-67.

The most commonly used gaseous/aerosol/radionuclides are xenon-133, krypton-81m, Technetium- 99M and DTPA (diethylene-triamine-pentaacetate).

In diagnostic aspects
In nuclear medicine with advances included as positron emission tomography (PET), imaging has value in cardiovascular, neurological, psychiatric, and oncological diagnosis. Positron emission tomography is a functional imaging modality that allows the measurement of metabolic reactions within the whole body. F-18 in FDG (fluorodeoxyglucose) which is an analog of glucose has become very important in detection of cancers and the monitoring of progress in their treatment, using PET. The combination of PET scan and computed tomography (CT) scan in a single device provides simultaneous structural and biochemical information (fused images) under almost identical conditions, minimizing the temporal and spatial differences between the two imaging modalities and is called Fusion imaging [11] [12].

Single-photon emission computed tomography (SPECT) imaging technique was developed as an enhancement of planar imaging enables the exact anatomical site of the source of the emission to be determined. This technique involves the detection of gamma rays emitted singly (single photon) from radionuclides such as technetium-99m and thallium-201.

Radioimmunodetection/radioimmunoassay is an in vitro nuclear medicine, is a very sensitive technique used to measure concentrations of antigens by use of antibodies.

In therapeutics aspects
The radiations given out by some radioisotopes are very effective in curing certain diseases such as radiocobalt (⁶⁰Co) is used in the treatment of brain tumor, radiophosphorus (³²P) in bone diseases and radioiodine (¹³¹I) in thyroid cancer [4].

Radioactive sources are also available for brachytherapy with many nuclides and in various shapes and size depending upon the type of their radiation energy and emission [13].

A new field is targeted alpha therapy (TAT) or alpha radioimmunotherapy, especially for the control of dispersed cancers. The short range of very energetic alpha emissions is targeted into cancer cells, with carrier such as a monoclonal antibody tagged with the alpha-emitting radionuclide. Lead-212 is being used in TAT for treating pancreatic, ovarian and melanoma cancers. An experimental development of this is boron neutron capture therapy (BNCT) using boron-10 which concentrates in malignant brain tumors. Radionuclide therapy has progressively become successful in treating persistent disease and doing so with low toxic side-effects. With any therapeutic procedure the aim is to confine the radiation to well-defined target volumes of the patient. The doses per therapeutic procedure are typically 20–60 Gy [14].

Radioimmunotherapy is dependent on three principal interdependent factors: the antibody, the radionuclide and the target tumor, and host and is most commonly employed in the management of hematopoietic neoplasms, especially non-Hodgkin lymphoma (NHL) [15].

Indications for radioisotope imaging in head and neck region
The following are the indications for radioisotope imaging in head and neck region [16]:

• In assessment of sites and extent of bone metastases as in tumor staging.
• Bone scan scintigraphy as it is sensitive and non-invasive technique for demonstrating osteoblastic lesions of the skeletal system.
• Investigation of salivary gland function especially in Sjögren's syndrome (technetium-99m pertechnetate)-salivary gland scintigraphy.
• Investigation of thyroid gland.
• Brain scans and assessment of breakdown of blood brain barrier.
• Growth assessment-evaluation of bone grafts
• In growth pattern-assessment of continued growth in condylar hyperplasia
• Blood flow and blood pool examination into the specific tissue/organ.

Advantages of radioisotope imaging

• Target tissue function is investigated.
• All similar target tissues can be imaged during one investigation. For example, whole skeleton can be imaged in one bone scan.
• It can display blood flow.
• Assessment of physiologic or functional change in tissues, because of disease process.
• Computer analysis and enhancement of results are available.

Disadvantages radioisotope imaging

• Poor image resolution-often only minimal information is obtained on target tissue anatomy.
• The radiation dose to the whole body can be relatively high.
• Images are not usually disease-specific.
• Difficult to localize exact anatomical site of source of emissions.

Imaging technologies have become increasingly sophisticated in recent years. Nuclear medicine and molecular imaging, which provides the only means of assessing physiologic changes that is a direct result of biochemical alterations at cellular and molecular levels, and in combination with traditional anatomic imaging such as computed tomography scan and magnetic resonance imaging (MRI) scan, provide precise localization of functional abnormalities. These imaging techniques are based on the radiotracer method, and allow the measurement of tissue function in vivo and provide an early marker of disease through measurement of biochemical change. Many elements which found on earth exists in different atomic configurations used in medicine are referred to as radiopharmaceuticals which are useful to get diagnostic and therapeutic information about those tissues.

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