Nuclear Medicine

Nuclear Medicine - What is Nuclear Medicine?

Nuclear medicine is a branch or specialty of medicine and medical imaging that uses radioactive isotopes (radionuclides) and relies on the process of radioactive decay in the diagnosis and treatment of disease.

In nuclear medicine procedures, radionuclides are combined with other chemical compounds or pharmaceuticals to form radiopharmaceuticals.

These radiopharmaceuticals, once administered to the patient, can localize to specific organs or cellular receptors.

This property of radiopharmaceuticals allows nuclear medicine the ability to image the extent of a disease-process in the body, based on the cellular function and physiology, rather than relying on physical changes in the tissue anatomy.

In some diseases nuclear medicine studies can identify medical problems at an earlier stage than other diagnostic tests.

Treatment of disease, based on metabolism or uptake or binding of a ligand, may also be accomplished, similar to other areas of pharmacology.

However, radiopharmaceuticals rely on the tissue-destructive power of short-range ionizing radiation.

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History of Nuclear Medicine

The history of nuclear medicine is rich with contributions from gifted scientists across different disciplines in physics, chemistry, engineering, and medicine.

The multidisciplinary nature of Nuclear Medicine makes it difficult for medical historians to determine the birthdate of Nuclear Medicine.

This can probably be best placed between the discovery of artificial radioactivity in 1934 and the production of radionuclides by Oak Ridge National Laboratory for medicine related use, in 1946.

Many historians consider the discovery of artificially produced radioisotopes by Frédéric Joliot-Curie and Irène Joliot-Curie in 1934 as the most significant milestone in Nuclear Medicine.

Although, the earliest use of I-131 was devoted to therapy of thyroid cancer, its use was later expanded to include imaging of the thyroid gland, quantification of the thyroid function, and therapy for hyperthyroidism.

Widespread clinical use of Nuclear Medicine began in the early 1950s, as knowledge expanded about radionuclides, detection of radioactivity, and using certain radionuclides to trace biochemical processes.

Pioneering works by Benedict Cassen in developing the first rectilinear scanner and Hal O. Anger's scintillation camera (Anger camera) broadened the young discipline of Nuclear Medicine into a full-fledged medical imaging specialty.

In these years of Nuclear Medicine, the growth was phenomenal. The Society of Nuclear Medicine was formed in 1954 in Spokane, Washington, USA.

In 1960, the Society began publication of the Journal of Nuclear Medicine, the premier scientific journal for the discipline in America.

There was a flurry of research and development of new radionuclides and radiopharmaceuticals for use with the imaging devices and for in-vitro studies5.

Among many radionuclides that were discovered for medical-use, none were as important as the discovery and development of Technetium-99m.

It was first discovered in 1937 by C. Perrier and E. Segre as an artificial element to fill space number 43 in the Periodic Table.

The development of generator system to produce Technetium-99m in the 1960s became a practical method for medical use.

Today, Technetium-99m is the most utilized element in Nuclear Medicine and is employed in a wide variety of Nuclear Medicine imaging studies.

By the 1970s most organs of the body could be visualized using Nuclear Medicine procedures. In 1971, American Medical Association officially recognized nuclear medicine as a medical specialty.

In 1972, the American Board of Nuclear Medicine was established, cementing Nuclear Medicine as a medical specialty.

In the 1980s, radiopharmaceuticals were designed for use in diagnosis of heart disease. The development of single photon emission tomography, around the same time, led to three-dimensional reconstruction of the heart and establishment of the field of Nuclear Cardiology.

More recent developments in Nuclear Medicine include the invention of the first positron emission tomography scanner (PET).

The concept of emission and transmission tomography, later developed into single photon emission computed tomography (SPECT), was introduced by David E. Kuhl and Roy Edwards in the late 1950s.

Their work led to the design and construction of several tomographic instruments at the University of Pennsylvania. Tomographic imaging techniques were further developed at the Washington University School of Medicine.

These innovations led to fusion imaging with SPECT and CT by Bruce Hasegawa from University of California San Francisco (UCSF), and the first PET/CT prototype by D. W. Townsend from University of Pittsburgh in 1998 .

PET and PET/CT imaging experienced slower growth in its early years owing to the cost of the modality and the requirement for an on-site or nearby cyclotron.

However, an administrative decision to approve medical reimbursement of limited PET and PET/CT applications in oncology has led to phenomenal growth and widespread acceptance over the last few years.

PET/CT imaging is now an integral part of oncology for diagnosis, staging and treatment monitoring.

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Nuclear Medicine Analysis

The end result of the nuclear medicine imaging process is a "dataset" comprising one or more images.

In multi-image datasets the array of images may represent a time sequence (i.e. cine or movie) often called a "dynamic" dataset, a cardiac gated time sequence, or a spatial sequence where the gamma-camera is moved relative to the patient.

SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera are reconstructed to produce an image of a "slice" through the patient at a particular position.

A collection of parallel slices form a slice-stack, a three-dimensional representation of the distribution of radionuclide in the patient.

The nuclear medicine computer may require millions of lines of source code to provide quantitative analysis packages for each of the specific imaging techniques available in nuclear medicine.

Time sequences can be further analysed using kinetic models such as multi-compartment models or a Patlak plot.

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Radiation Dose

A patient undergoing a nuclear medicine procedure will receive a radiation dose. Under present international guidelines it is assumed that any radiation dose, however small, presents a risk.

The radiation doses delivered to a patient in a nuclear medicine investigation present a very small risk of inducing cancer. In this respect it is similar to the risk from X-ray investigations except that the dose is delivered internally rather than from an external source such as an X-ray machine.

The radiation dose from a nuclear medicine investigation is expressed as an effective dose with units of sieverts (usually given in millisieverts, mSv).

The effective dose resulting from an investigation is influenced by the amount of radioactivity administered in megabecquerels (MBq), the physical properties of the radiopharmaceutical used, its distribution in the body and its rate of clearance from the body.

Effective doses can range from 6 μSv (0.006 mSv) for a 3 MBq chromium-51 EDTA measurement of glomerular filtration rate to 37 mSv for a 150 MBq thallium-201 non-specific tumour imaging procedure.

The common bone scan with 600 MBq of technetium-99m-MDP has an effective dose of approximately 3.5 mSv.

Formerly, units of measurement were the curie (Ci), being 3.7E10 Bq, and also 1.0 grams of Radium (Ra-226); the rad (radiation absorbed dose), now replaced by the gray; and the rem (Röntgen equivalent man), now replaced with the sievert.

The rad and rem are essentially equivalent for almost all nuclear medicine procedures, and only alpha radiation will produce a higher Rem or Sv value, due to its much higher Relative Biological Effectiveness (RBE).

Alpha emitters are nowadays rarely used in nuclear medicine, but were used extensively before the advent of nuclear reactor and accelerator produced radioisotopes.

The concepts involved in radiation exposure to humans is covered by the field of Health Physics.

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Jobs in Nuclear Medicine

Nuclear medicine scientist

The information below is adapted from the Society of Nuclear Medicine (SNM) website on a scientist career.

The nuclear medicine scientist works closely with the nuclear medicine physician. Some of the scientist's primary responsibilities are to:

  • Prepare and administer radioactive chemical compounds, known as radiopharmaceuticals
  • Perform patient imaging procedures using sophisticated radiation-detecting instrumentation
  • Accomplish computer processing and image enhancement
  • Analyze biologic specimens in the laboratory
  • Provide images, data analysis, and patient information to the physician for diagnostic interpretation.

During an imaging procedure, the scientist works directly with the patient. The scientist:

  • Gains the patient's confidence by obtaining pertinent history, describing the procedure and answering any questions
  • Monitors the patient's physical condition during the course of the procedure
  • Notes any specific patient's comments which might indicate the need for additional images or might be useful to the physician in interpreting the results of the procedure.

Nuclear medicine scientists work in a wide variety of clinical settings, such as

  • Community hospitals
  • University-affiliated teaching hospitals and medical centers
  • Outpatient imaging facilities
  • Public health institutions
  • Government and private research institutes.

The physician career in nuclear medicine

Nuclear medicine physicians are primarily responsible for interpretation of diagnostic nuclear medicine scans and treatment of certain diseases, such as cancer, thyroid disease and palliative bone pain.

There are a variety of reasons why physicians have chosen to specialize in nuclear medicine. Some became nuclear medicine physicians because of their interest in nuclear physics and medical imaging.

Others may have switched to nuclear medicine after training in other specialties, because of the regular work hours (on average about 8 to 10 hours a day).

Others have chosen nuclear medicine because of research opportunities in molecular medicine or molecular imaging.

Nuclear medicine physicians frequently interact with other specialties in medicine and consult on a variety of clinical cases.

A nuclear medicine report may save a patient from more invasive or high risk procedures, and/or lead to early disease diagnosis.

Nuclear Medicine physicians can be called upon to consult on complex or equivocal clinical cases.

Aside from consultations with other physicians, nuclear physicians may directly interact with patients through various nuclear medicine therapies (e.g.: I131 thyroid therapy, refractory lymphoma treatment, palliative bone pain therapy).

A disadvantage of a nuclear medicine career for a physician is that it suffers from low job turnover and a small job market, owing to the specialized nature of the field.

Advantages of the field include job satisfaction and more regular hours than many fields of medicine, since very rarely are the procedures in this field performed on an emergency basis.

Nuclear medicine residency/training (physicians)

The information below is adapted from the American Board of Nuclear Medicine (ABNM).

General professional education requirement in the United States of America: graduation from a medical school approved by the Liaison Committee on Medical Education or the American Association of Colleges of Osteopathic Medicine.

In USA the post-doctoral training in nuclear medicine can be approached from three different pathways:

  1. If the person has successfully completed an accredited radiology residency then additional ONE year of training in Nuclear Medicine is required to be eligible for ABNM board certification.
  2. If the person has successfully completed a clinical residency (e.g. Internal Medicine, Family Medicine, Surgery, Neurology, etc.) then an additional TWO years of training in Nuclear Medicine is required to be eligible for ABNM board certification.
  3. If the person has successfully completed one year of preparatory post-doctoral training (internship) then an additional THREE years of training in Nuclear Medicine is required to be eligible for ABNM board certification.

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