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Nuclear Medicine Primer
What is Nuclear Medicine?
Nuclear medicine is a branch of medical imaging and therapy which utilizes radioisotopes to diagnose
and treat illnesses. This primer will focus on medical imaging to diagnose pathologies.
What do we use?
In nuclear medicine imaging, we use radioisotopes (radioactive isotopes) and distribute them
throughout the patient’s body. Once appropriately distributed in the patient, we can view the radiation
emitted by the radioisotopes using a gamma camera.
Radiopharmaceutical
Radioisotopes are used in the form of a radiopharmaceutical. A radiopharmaceutical consists of a
radioisotope and a pharmaceutical ligand. The radioisotope determines the radioactive qualities of the
radiopharmaceutical, for example Technetium-99m will emit gamma radiation and has a half life of 6
hours. The pharmaceutical ligand will determine the biodistribution of the radiopharmaceutical. For
example, the ligand MDP (Medronate) accumulates into the bones and is used for bone imaging. The
ligand Sestamibi accumulates into myocardial muscle and is used for cardiac imaging. Changing the
ligand will determine what you are going to image while changing the isotope will determine the
radioactive qualities of your imaging agent. Sometimes you want an isotope which will remain
radioactive for a long time (long half life) because you want to image the patient periodically over a
number of days without having to continuously inject the patient with more radioisotopes “topping up”
(ie. Gallium-67 for gut transit studies – 78 hours). Additionally, not all radioisotopes can be used with all
ligands due to their chemical properties. Gallium 67 cannot be attached to MDP or Sestamibi.
Technetium-99m is the most widely used radioisotope because it can be easily attached to a vast array
of pharmaceutical ligands.
Gamma Camera
The gamma camera is the primary imaging apparatus in Nuclear Medicine. It consists of a bed, the table
(which the bed rests on), the gantry (large circular base) and camera heads (extend from the gantry). A
camera can have 1, 2 or 3 camera heads; the one below has 2. More camera heads are useful for doing
faster 3 dimensional orbital or SPECT (Single Photon Emission Computed Tomography) scans – similar to
CT. Single head cameras are much more flexible in terms of maneuverability and are ideal for localized
planar (2 dimensional) images – more like x-rays compared to CT. 2 camera heads are the most common
configuration as they are a compromise between 1 and 3. Regardless, all cameras are capable of 2D and
3D images, some are just more capable for certain images. All gamma cameras are also capable of full
body scans when required.
What are we imaging and how do we do it?
The gamma camera “sees” gamma radiation. In the camera heads are a set of scintillation crystals which
will actually interact with incoming gamma rays and convert them into rays of light. The rays of light are
then processed by an array of photomultiplier tubes and converted to electronic signals. The signals are
then interpreted by a computer and reconstructed as an image which can be displayed onto a computer
screen or printed onto hard copy.
Glossary
Isotope - An isotope is a form of a chemical element whose atomic nucleus contains a specific number of
neutron s, in addition to the number of proton s that uniquely defines the element. The nuclei of most
atom s contain neutrons as well as protons. (An exception is the common form of hydrogen, whose
nucleus consists of a lone proton.) Every chemical element has more than one isotope. For example:
Technetium-99m is a radioactive istotope (radioisotope) of the isotope atom Technetium-99 and the
naturally occurring atom Technetium-98. NB: the numbers 98 and 99 indicate how many neutrons the
atom has.
Photomultiplier Tubes - (photomultipliers or PMTs for short), members of the class of vacuum tubes,
and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible,
and near-infrared ranges of the electromagnetic spectrum.