Nuclear Medicine Physics PDF

Introduction

In 1973, the WHO defined nuclear medicine (NM) as a discipline

"...embracing all applications of radioactive materials in diagnosis and treatment or in medical research, with the exception of the use of sealed radiation sources in radiotherapy."

Apart from medical knowledge, NM includes physics, pharmacy, biology, chemistry, mathematics, computer science, and all branches of engineering concerned with the development of devices used in NM.

NM embraces a vast set of applications such as imaging methodologies using radionuclides (ranging from autoradiography to positron emission tomography [PET]), in vitro analysis, metabolic radiotherapy, and medical research studies and techniques involving radioactive tracers.

The relative contribution of these applications has varied considerably over the years, although at present imaging methodologies play a leading role. Despite this, it is still inaccurate to reduce the definition of NM to nuclear imaging applications, as is frequently claimed, even when this includes all the functional information resulting from imaging studies. NM is not simply a medical imaging technique, although in this area it does have very special characteristics.

It is perhaps important, at this stage, to raise the following question: Why NM?

There are several important reasons:

1. Of all the available methodologies,NMis best at supplying functional information.

2. It has the most sensitive detection methods (detecting masses below picomole level), enabling studies to be performed in physiological conditions without any interference fromthe processes being studied.

3. Most biological molecules can be labeled and used as radiotracers, making the information supplied by this technique multiparametric and unique in comparison with other techniques that provide information about just one or a small number of properties.*

4. Direct image analysis can provide functional results in relative terms.

5. With some additional steps, it can provide quantitative results.

6. Studies can be repeated within short periods of time.

Certain negative aspects of NM techniques may also be cited:

1. They require a nonconventional environment and certain precautions (radioactivity cannot be switched off as an x-ray machine can).

2. In NM imaging, it is always necessary to administer radiopharmaceuticals, which emit ionizing radiation.

3. The spatial resolution ofNMimages is almost always worse than that of morphological images.

Focusing our attention on the second point, the effective doses received by patients in the vast majority ofNMexams fall within the medium dose range for radiological studies, that is, 0.5–6 mSv, corresponding to a few months to about 2 years of average background irradiation.

The various types of NM images can be classified as

• Static planar scintigraphy
• Dynamic planar scintigraphy
• Whole-body scintigraphy
• Single photon emission computerized tomography (SPECT)
• Gamma camera coincidence detection
• Computerized PET
• Hybrid systems

Generally speaking, the current NM imaging techniques using external detection provide local functional information that is specific, easily presented in relative terms, and very useful in a large number of situations. Quantitative analysis is also possible if the appropriate corrections are applied and, eventually, additional data are obtained.

In some situations, the limitations associated with the unavailability of appropriate tracers (if existing ones fail to enhance the structures being studied) or the inadequacy of the spatial or temporal resolution of the available NM imaging systems can make nuclear images an unattractive option in comparison with direct sampling of biological material and measurement of radioactivity, or the possible application of other techniques.

Important developments are currently taking place in most areas of NM. Newradio-labeled biological drugs, new cell labeling techniques, new technical concepts in radiation detection, improvements in instrumentation, access to multimodal techniques, the rapid increase in computer power, and a much better understood partnership with the clinical disciplines are all combining to create a new profile for NM.

In addition, the sequencing of the human genome and the ever-increasing knowledge of proteomics, systems biology, and the pathogenesis of human disease have created unprecedented opportunities for molecular imaging in NM and have also provided opportunities for understanding the molecular basis of normal and diseased cellular functions.

These developments have had a very positive effect in terms of improved diagnostic quality and, it is hoped, in reducing patient exposure to radiation.

This book, Nuclear Medicine Imaging Physics, aims at providing a series of contributions, mainly in areas of physics that are related to the theoretical bases of NM and its applications, some of which cannot be currently found in text books or, at least, in the perspective and detail that new or advanced approaches may require.

The subjects have been selected according to their direct potential interest toNMtoday or to disciplines particularly concerned with the applications of this science.

A reasonable background in mathematics and radiation physics; the basics of biology and human physiology; and an understanding of linear and nonlinear system theory, model building, and simulation (linear and nonlinear) are recommended.

The book covers the following themes: the production of radioisotopes and radiopharmaceuticals, positron physics and positron applications in medicine and biology, modern radiation detectors and measuring methods, systems in NM, imaging methodologies, dosimetry, and the biological effects of radiation.

* Fluorescence techniques rival NM in this area but are generally only applicable to surface organs or tissues.