The Scope Of Nuclear Medicine Imaging

Have you ever heard of nuclear medicine? What is it all about? Well, be the end of this article you would be able to evaluate the importance of nuclear medicine in medical imaging. I would like to give you a brief historic background that led to the invention of nuclear medicine in diagnosis. In the years just before and during World War II, nuclear research focused mainly on the development of defense weapons. Later, scientists concentrated on peaceful applications of nuclear technology. After years of research, scientists have successfully applied nuclear technology to many other scientific, medical, and industrial purposes.

The basis of nuclear medicine uses radioisotope atoms which are able to release particulate radiation in other to become stable. Although they are tiny, atoms have a large amount of energy holding their nuclei together. Certain isotopes of some elements can be split and will release part of their energy as heat. This splitting is called fission. The heat released in fission can be used to help generate electricity in power plants.(Discovery et al., n.d.). Uranium-235 (U-235) is one of the isotopes that fissions easily. During fission, U-235 atoms absorb loose neutrons. This causes U-235 to become unstable and split into two light atoms called fission products. A series of fissions is called a chain reaction.

If enough uranium is brought together under the right conditions, a continuous chain reaction occurs. This is called a self-sustaining chain reaction. A self-sustaining chain reaction creates a great deal of heat, which can be used to help generate electricity. Nuclear powerplants generate electricity like any other steam-electric powerplant. Water is heated, and steam from the boiling water turns turbines and generates electricity.

The main difference in the various types of steam-electric plants is the heat source. Heat from a self- sustaining chain reaction boils the water in a nuclear powerplant. Coal, oil, or gas is burned in other powerplants to heat the water..(Discovery et al., n.d.)The Nuclear medicine is gaining greater and greater importance as a diagnostic tool, and one reason for that is the enormous progress made in the imaging of the tissue activity distribution and of its temporal variation after administration of radiolabeled agents to patients.(Giussani, n.d.).

The availability of hybrid systems, where SPECT or PET machines are combined with X-ray computed tomography (CT) or, more recently with magnetic resonance (MR) devices, enables to integrate together the functional information provided by the administered radiopharmaceutical with the superior anatomical details of CT and MRI.(Giussani, n.d.). Nuclear Medicine (e.g., PET, SPECT) is based on emission data from radioactive materials injected in to the body. Nuclear signals penetrated through the body are detected and reconstructed to form images.

Nuclear medicine and plain radiography both utilizes ionizing radiation to obtain clinical information. Information with nuclear medicine is related to organ function while in plain radiography information is related to anatomical structures. There are three branches of nuclear medicine. These are; imaging, therapeutics and radioimmuno assay. Let us first talk about the imaging aspect of nuclear medicine speciality. The imaging aspect of nuclear medicine has many sophisticated modalities use purposely of detection of an organ functioning and even in the molecular level for diagnosis. These modalities includes, Planar scintigraphy, SPECT(Single Photon Emission Computer Tomography), PET( Positron Emission Tomography), and . First of all, I would like to elaborate the functionality of Planar scintigraphy.

Planar scintigraphy; Planar imaging produces a 2D image with no depth information and structures at different depths are superimposed. The result is loss of contrast in the plane of interest.

SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT)

SPECT was developed as an enhancement of planar imaging. It detects the emitted gamma photons (one at a time) in multiple directions. Uses one or more rotating cameras to obtain projection data from multiple angles. SPECT displays traces of radioactivity in only the selected plane axial, coronal and sagittal. Computer manipulation of the detector radiation is also possible. SPECT is a method of acquiring tomographic slices through a patient. Most gamma camera have SPECT capability. In this technique either a single or multiple (single, dual or triple headed system ) gamma camera is rotated 360° about the patient. Image acquisition takes about 30 -45 minutes. Filtered back projection & most recently iterative reconstruction algorithms to form a number of contiguous axial slices similar to CT by X – ray process the acquired data.

The sensitivity of an SPECT system is ∞ to the number of detectors. After every 6° camera halts for 20 – 30 seconds & acquires the view of the patient. 60 views are taken from different directions. These data can then be used to construct multiplanar images of the study area. SPECT studies can be presented as either a series of slices or 3 D displays. By changing contrast & localization, SPECT imaging increases sensitivity & specificity of disease detection. Tomography enhances contrast & removes superimposed activity. SPECT images have been fused recently with CT images to improve identifying of the location of the radionuclide.

Positron Emission Tomography ( PET)

It is a nuclear medicine imaging technique which produces a three dimension image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer). In this, a proton in the nucleus is transformed into a neutron & a positron. Positron emission is favoured in low atomic number elements. Positron Annihilation The positron has short life in solids & liquids .Interactions with atomic electrons rapidly loses kinetic energy Reaches the thermal energy of the electron Combines with the electron undergoes annihilation.

Their mass converts into energy in the form of gamma rays. The energy released in annihilation is 1022 KeV. To simultaneously conserve both momentum & energy, annihilation produces 2gamma rays with 511 keV of energy that are emitted 180 degree to each other.

The detection of the two 511 keV gamma rays forms the basis for imaging with PET. Coincidence detection- simultaneous detection of the 2 gamma rays on opposite sides of the body. (bismuth germanates ), If both gamma rays can subsequently be detected, the line along which annihilation must have occurred can be defined. By having a ring of detectors surrounding the patient, it is possible to build a map of the distribution of the positron-emitting isotope in the body. PET employs electronic collimation.

Three types of coincidence detection. Sensitivity in PET Measures capability of system to detect ‘trues’ & reject ‘random. Radionuclides used in PET scanning are typically isotopes with short half-lives: Carbon-11 (~20 min), Nitrogen-13 (~10 min), Oxygen-15 (~2 min), and Fluorine-18 (~110 min). These radionuclides are incorporated either into compounds normally used by the body such as glucose (or glucose analogues), water or ammonia, or into molecules that binds receptors or other sites of drug action. Advantages of PET Sensitive method for imaging. It can investigate disease at a molecular level even in the absence of anatomical abnormalities.

It is possible to quantify the amount of tracer within a region of interest in the patients body ; possible to monitor the amount of tracer in mg/100ml of tissues. Disadvantages of PET imaging high cost of PET setup, Requires more space , electricity & air conditioning than conventional nuclear medicine. Requires an on – site cyclotron due to the short half life of the positron emitting . CT data better identifies the invasion of the oral carcinomas into the jaws than FDG PET. Major image quality degradation is due to the metallic dental implants therefore all removable artificial dentures & metal parts to be removed during scanning. PET & PET / CT like any other imaging technique is not able to identify micro metastasis i.e.; metastasis up to 2mm.

SUMMARY OF THE BENEFITS.

One unique aspect of a nuclear imaging test is its extreme sensitivity to abnormalities in an organ's structure or function. As an integral part of patient care, nuclear imaging is used in the diagnosis, management, treatment and prevention of serious disease. Nuclear medicine imaging procedures often identify abnormalities very early in the progression of a disease long before some medical problems are apparent with other diagnostic tests. This early detection allows a disease to be treated early in its course when there may be a better prognosis.

Although nuclear imaging is commonly used for diagnostic purposes, it also has valuable therapeutic applications such as treatment of hyperthyroidism, thyroid cancer, blood imbalances, and any bony pain from certain types of cancer.

References

Discovery, T., First, T., Reaction, S. C., Development, T., Energy, N., Applications, P., & References, S. (n.d.). The History of Nuclear Energy.

Giussani, A. (n.d.). Imaging in Nuclear Medicine.