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·         Nuclear Medicine Applications

    

In nuclear medicine, medical professionals inject a tiny amount of a radioisotope—a chemical element that produces radiation—into a patient’s body. A specific organ picks up the radioisotope, enabling a special camera to take a detailed picture of how that organ is functioning.   In modern nuclear medicine, PET and SPECT are two widely used imaging techniques.

PET stands for Positron Emission Tomography. It is a nuclear medicine tomographic imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is injected into a patient’s body. Images of tracer concentration in 3-dimensional space within the body are then reconstructed by computer analysis.  The patient’s body will never come in contact with scanner itself.  PET scans can be used to measure metabolic activity and molecular function by using a radioactive glucose injection such as F-18 FDG (Fluorine-18 Fluorodeoxyglucose).  Although all cells use glucose as an energy source, cancer cells grow faster than normal healthy cells and they use glucose at much higher rate than normal cells. This is the basis of imaging with F-18 FDG for cancer detection in PET scan.

A PET Scan Machine

Source:
http://www.med.nyu.edu/di/images/pet1.jpg

A schematic diagram of a PET acquisition

process

Source:
http://upload.wikimedia.org/wikipedia/com
mons/c/c1/PET-schema.png

SPECT stands for Single Photon Emission Computed Tomography. SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body. SPECT is similar to PET in its use of radioactive tracer material and detection of gamma rays. In contrast with PET, however, the tracer used in SPECT emits gamma radiation that is measured directly, whereas PET tracer emits positrons which annihilate with electrons up to a few millimeters away, causing two gamma photons to be emitted in opposite directions. A PET scanner detects these emissions "coincident" in time, which provides more radiation event localization information and thus higher resolution images than SPECT (which has about 1 cm).  SPECT scans, however, are significantly less expensive.

A Lung SPECT / CT Fusion image

Source:

http://upload.wikimedia.org/wikipedia/commons/c/ca/Lung_SPECT
-CT_keosys_format_dicom.JPG
Radionuclides used in nuclear medicine are mostly artificial ones. They are primarily produced in a reactor or cyclotron and supplied by commercial companies to individual nuclear medicine departments and institutions. On the other hand, some radionuclides, in particular short-lived ones, are available at any time due to the availability of appropriate radionuclide generators. By far the most important generator in nuclear medicine is the 99Mo/99mTc generator, which has led to the almost unlimited availability of 99mTc.

Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e. it does not change into another element (transmutate) upon its "decay". It is a gamma ray emitting isotope used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body. It is well suited to the role because it emits readily detectable 140 keV gamma rays (these are about the same wavelength emitted by conventional X-ray diagnostic equipment), and its half-life for gamma emission is 6.01 hours (meaning that about 93.7% of it decays to 99Tc in 24 hours). The short half life of the isotope allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low.

     

These kinds of diagnostic procedures involve very small amounts of radioisotopes. In higher doses, radioisotopes also help treat disease. For example, radioactive iodine’s widespread use in therapy for thyroid cancer results in a lower recurrence rate than drug therapy. It also avoids potentially fatal side effects, such as the destruction of bone marrow.

     

Sealed sources of radiation placed inside the body, or radiation directed from external sources, are effective in treating various cancers. Nearly half of all cancer patients in the United States receive radiation treatment at some point in their therapy.

Hospitals also use radiation to sterilize materials, thus helping to prevent the spread of diseases. Exposing these materials to radiation does not make them radioactive.

     

References:

     

Ø      Christiaan Schiepers, Diagnostic Nuclear Medicine, Springer Berlin Heidelberg , ISBN 978-3-540-42309-6 (Print) 978-3-540-30005-2 (Online)

Ø      http://www.nei.org/howitworks/medicineandscientificresearch/

Ø      http://www.emeraldinsight.com/Insight/ViewContentServlet?Filename=/published/emeraldfulltextarticle/pdf/0870180405.pdf

Ø      http://www.osti.gov/bridge/servlets/purl/840065-Jhd7iT/native/840065.pdf

Ø      http://en.wikipedia.org/wiki/Positron_emission_tomography

Ø      http://en.wikipedia.org/wiki/Single_photon_emission_computed_tomography