Nuclear Medicine—SPECT, PET, and Fusion Imaging

CHAPTER 11


Nuclear Medicine—SPECT, PET, and Fusion Imaging



It is not the intent of this book to present an in-depth explanation of nuclear medicine technology. The discussion will include only a brief introduction into the world of nuclear medicine and the use of single photon emission computed tomography (SPECT) and positron emission tomography (PET).


The term fusion imaging or fusion technology describes a hybrid of both computed tomography (CT) (transmission) and PET or SPECT (emission) scanning to present a complete picture of both the anatomic features and the metabolic information of a pathologic process. Imaging fusion occurs best in an advanced integrated system rather than having the patient undergo a CT scan and a PET or SPECT scan separately and attempting to correlate both studies manually. The integrated system uses software that aligns the images of the two scans to provide a more accurate diagnosis than individual PET/SPECT and CT scans.


The combination of PET and CT allows the physician to demonstrate not only anatomic information but also the functional activity of the tissue being studied. PET/CT is being used in the detection of a wide variety of pathologic processes including breast, lung, esophageal, thyroid, and head and neck cancers. Brain imaging and myocardial function studies are also being performed.


The integration of the two modalities reduces the time it requires to make a diagnosis as compared with performing each of the studies at different times. The patient must still have both examinations, but the fusion units have the advantage of integrating the images from both. The patient handling is effortless, and the improved data acquisition and registration of the information are improved because of the fusion of the images from the two separate examination. The procedure is extremely accurate in predicting location and pathologic condition of the tissues under investigation.



NUCLEAR MEDICINE


Nuclear medicine is a diagnostic tool that utilizes a radioactive substance to help diagnose a disease process from inside the body. These radiopharmaceuticals are organ-, tissue-, or even cell-specific in providing information about the pathologic process. The modality not only provides anatomic information but can assess the function of the organ, bone, or tissue that it is designed to identify. In fact, because the amounts of radioactive material are so small and the duration of the radioactivity so limited, it can provide a diagnosis without harm to nontargeted areas. In most cases the amount of radiation that a patient receives is no more than that received during CT scan or fluoroscopy.


Some of the disease processes that have been identified through nuclear medicine are ovarian, endocrine, colon, prostate, pancreatic, breast, bone, and lung cancers; diagnosis of heart disease is another area that has benefited from this modality through stress testing. Nuclear medicine has also been effective in the diagnosis of rheumatoid arthritis, joint disease, and meningitis, to name a few.



Indications and Contraindications


The indications using nuclear medicine imaging techniques are wide and varied. Each anatomic area has its own set of indications that apply to the type of organ study and the reasons for its performance. Some indications for the use of nuclear medicine studies are as follows:




This list is by no means complete; it is only a representation of the range of indications for nuclear medicine studies using SPECT and PET scanning. A nuclear medicine study can image any organ tissue or cell type that a radiopharmaceutical has been designed to target in the body.


The contraindications are grouped into three major categories: pregnancy, allergic reactions, and other.





Other Contraindications


As in routine radiography, it is well known that some radiologic procedures can affect the results of another procedure if they are not performed in the proper sequence. Some radiologic procedures can affect the result of the nuclear medicine procedure as well. Previous surgical procedures can also affect the outcome of the nuclear medicine study. Because of the potential adverse effect on the nuclear medicine procedure, these factors are considered as contraindications to performance of the examination. There is also the possibility that medications the patient is taking may affect the uptake of the radiopharmaceutical. In some cases the medications can be suspended prior to the examination, or if it is not possible this may become a contraindication for the procedure. The technologist should be aware of these situations and must alert the physician if the potential for contraindication exists.



Radionuclides


Terminology


It is important to understand some of the terminology associated with radionuclides. As in radiography, the radiation produced by the radionuclide is expressed by certain units. These units are the curie (Ci) and the becquerel (Bq). Unlike units in radiography, these represent units of activity. Activity (radioactivity) is defined as the number of disintegrations per unit time. The curie represents a radioactivity equal to 3.7 × 1010 disintegrations per second (dps). The term curie was derived from the name of the individual, Madame Marie Curie, who was a pioneer in the field of radioactivity.


The curie is generally expressed in very small units such as millicuries (3.7 × 107dps), microcuries (3.7 ×104dps), nanocuries (3.7 × 10 dps), and picocuries (3.7 × 10−2 dps). Obviously, the curie is a very large number, and the smaller units are used for expressing radioactivity in curies.


In the International System of units, activity is represented by the becquerel. One becquerel is equal to 1 dps or 27.03 × 10−11 Ci. This unit was named after Henri Becquerel, who in 1896 was the first to discover radioactivity.


As a radionuclide ages over time its activity is dependent upon the number of atoms of the radionuclide present at a specific time and a constant (K) that is specific to the radionuclide. This constant represents the probability that a single atom of a particular radionuclide will decay (the disintegration of the nucleus of an unstable radionuclide). It can also be referred to as the decay constant. The actual calculation of the decay constant is beyond the scope of this text and is referenced here expressly to the discussion of the concept of half-life.


The concept of half-life is an important one when dealing with radionuclides. This is the time interval for a specific number of unstable nuclei to decay (disintegrate) to one half their original number. Half-life can be considered in a physical and biologic sense. The physical half-life of a radionuclide is the amount of time it takes for one half of the activity to decay. The biologic half-life refers to the amount of time it takes for one half of the radionuclide to physiologically clear from the body. Both types of half-life are important in nuclear medicine. Radionuclides with long half-lives would not be very useful because of the radiation dose that would be given to the patient over the course of the examination. Long biologic half-lives also create the same danger. Of course, radionuclides that have too short a half-life would not be useful either. This is the same as the characteristic of persistence in the use of contrast agents in radiography. The substance must remain long enough for the examination to be completed but be eliminated from the body within a reasonable amount of time to avoid adverse effects. In the case of nuclear medicine the half-life must be long enough to provide for an adequate study but limit the patient dose to its as low as reasonably achievable (ALARA) levels.



Basics of Radionuclide Production


Radionuclides are produced utilizing a number of different methods. Two major processes are used: reactor-produced substances (fission from heavier nuclides) and accelerator-produced radionuclides.


Neutrons are essential to the fission process. Fission is the splitting of a heavier radionuclide into several different radionuclides having mass numbers smaller than the target nuclide. Naturally occurring radioactive substances are excellent sources of alpha particles. These alpha particles produce neutrons when they react, and neutrons are responsible for initiating the fission process.

Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Nuclear Medicine—SPECT, PET, and Fusion Imaging
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