Nuclear Medicine is the application of nuclear energy (obtained in the form of radioactive materials) to the diagnosis and treatment of patients and to the study of human disease.
The impact of Nuclear Magnetic Resonance (NMR) and methodologies using stable isotopes and mass spectrometry (in the isotope dilution process), have led to a-new definition of Nuclear Medicine as “the medical speciality that utilizes the nuclear properties of radioactive and stable nuclides to make diagnostic evaluations of the anatomic and/or physiologic conditions of the body and to provide therapy with unsealed radioactive sources.”
For treatment, the radioactive material is administered in relatively large doses to exploit the lethal action of ionizing radiation upon living cells. For diagnosis and research the radioactive material is administered in doses which have no observable adverse biological effects. The tracer may be given orally, by inhalation, or most commonly, by intravenous injection. Its course through the body may be followed by radiation detectors placed externally over the body, or may be followed by taking samples of blood, urine, stools, expired air or tissue and analyzing them for radioactivity. Pictorial representation of distribution of radioactivity as a function of time is also obtainable, by scintigraphy.
By measuring the spatial distribution of the radiotracer at various points in time we can determine what is occurring within the body with respect to specific physiologic or biochemical processes, how fast and where such processes are occurring. The use of multiple tracers, each measuring a specific body function, adds the concept of functional resolution to spatial and temporal resolutions. These three concepts are fundamental to Nuclear Medicine.
Radio tracers are used in in vitro procedures like saturation analysis and neutron activation analysis of material obtained from patients. Since no radioactivity is administered to the patient in these procedures, their use can be extended to pregnant women and children as well, in whom one is reluctant to administer radioactive material unless it is absolutely essential.
TRACER PRINCIPLE
The most fundamental principle of Nuclear Medicine is the tracer principle invented by George Hevesy for which he was awarded the Nobel Prize in 1944. In the early 1920's Hevesy and his Coworkers used the then available naturally occurring radioisotopes of elements (thorium, bismuth lead, etc.) and studied their distribution in plants and animals. When artificially produced radioisotopes became available in the early 1930's, Hevesy again was the first to study the distribution of radioactive phosphorus in animals. His study strongly supported the concept of “dynamic equilibrium”: the formation of bone is a dynamic process; the bone is continuously taking up phosphorus atoms which are partly or wholly lost and again replaced by other phosphorus atoms. The concept of dynamic equilibrium was further expanded by Schoenheimer with the use of stable isotope of hydrogen-deuterium. The tracer principle and the dynamic state of body constituents occupy the central core of nuclear medicine.
MOLECULAR BIOLOGY AND NUCLEAR MEDICINE
The union of biology with physics and chemistry was the outstanding development of 20th Century Science. Physical and chemical approaches to problems in biology became increasingly productive giving rise to 2the new concepts of molecular biology and molecular medicine. The confluence of several powerful methods of observation—chemical analysis, electron microscopy, X-ray crystallography, electron spin resonance (ESR) nuclear magnetic resonance spectroscopy eventually led to the determination of the precise double helix architecture of DNA, the three dimensional configuration of protein molecules and the amino acid sequences of their constituent polypeptide chains and the precise characterization of most biologically active molecules. The synthesis of complex lipids and carbohydrates, the function of cell membranes and other cellular organelles and the accumulation and partitioning of inorganic ions occur as a secondary consequence of the action of specific proteins. Many of those proteins are enzymes that catalyze the biochemical conversion of one molecule into another. Some are structural proteins such as collagen and elastin, still others are regulatory proteins that dictate how much of each enzyme and each structural protein is made, when and where.
The human body is made up of trillions of cells which are constantly communicating with each other through recognition molecules. Molecular recognition is the fundamental feature of all biological processes encompassing ligand-receptor, substrate enzyme and antigen-antibody reactions. Nuclear medicine is uniquely placed to study biological events at the molecular level. As Dr Henry Wagner has been emphasizing for over three decades, the orientation of nuclear medicine is biochemical and not anatomical. Molecular nuclear medicine offers the opportunity of studying physiology and biochemistry at the molecular level, in the living human body including the brain. Hence nuclear medicine is closest to the body of general medicine than any other speciality in medicine. Now that the entire human Genome has become available for study, molecular nuclear medicine will enable us to connect genotype to phenotype via chemotype (Henry Wagner 1995).
“Molecular Nuclear Medicine can be the basis of both diagnosis and therapy. More and more, disease will be viewed as biochemical manifestations of biological processes. We may be able to diagnose some diseases biochemically before symptoms appear, with the potential to arrest or prevent the disease”. This statement of Dr Henry Wagner is best illustrated in the chapter 14 in this book which deals with the causation, prevention and reversal of vascular endothelial dysfunction, which along with insulin resistance causes the metabolic syndrome—visceral obesity, hypertension, Type 2 Diabetes mellitus and atherosclerosis. N-13 ammonia PET has shown diminished coronary flow reserve as an indicator of endothelial dysfunction in patients with angiographically and IVUS- normal coronary arteries. Endothelial dysfunction begins early in life, long before structural changes of atherosclerosis are seen. Lifestyle modification through diet, exercise and stress control can prevent and reverse endothelial dysfunction as shown by improvement in coronary flow reserve by N-13 ammonia PET, or Rubidium PET.
Radiochemists have developed stereo-specific ligands with the proper change, shape and lipophilicity for transport across cell membranes or the blood brain barrier. Growth promoting and growth-suppressing factors produced under the direction of genetic coding by proto-oncogene, oncogenes and antioncogenes can now be examined as they interact with cell membrane or intracellular receptors an enzymes in cancer cells as well as normal cells.
Table 1.1 lists the 20 intracellular signaling pathways known today and (Fig. 1.1) provides an illustrative example of one such pathway.
Molecular imaging techniques offer the possibility of monitoring the location, magnitude and persistence of reporter gene expression in intact living animals or humans (Gambhir SS 2000).
Scope of Diagnostic Nuclear Medicine Procedures
The diagnostic applications of radionuclides can be described under two major headings (a) procedures in which radioactivity is administered to the patient (b) procedures in which no radioactivity is administered to the patient but samples obtained from the patient are analysed by techniques involving the use of radioactive isotopes.
The procedures in which radioisotopes are administered to the patients can be broadly described under four categories.
- Dilution studies
- Volumes and spaces
- Turnover rates
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- Gastrointestinal blood loss
- Gastrointestinal protein loss.
- Dynamic function studies
- Uptake tests (Radioiodine in thyroid)
- Absorption tests
- Clearance tests (e.g. renal clearance)
- Breath analysis of C-14 CO2 : Radiorespirometry
- Blood flow to organs
- Organ function
- Cardiac function
- Hepatic function
- Renal function.
- Pulmonary ventilation and perfusion
- Cerebral perfusion and function.
- Organ imaging: Selective concentration of a gamma-emitting or positron radiopharmaceutical in the organ of interest. An image of the distribution (scintigraphy) provides information regarding the position, size, boundaries and relationships of the organ. For most organs, the radiopharmaceuticals localize in normal functioning tissues. Diseased sites consequently are detected as areas of decreased or absent function. Brain lesions, bone lesions, myocardial infarcts, functioning adenomas of thyroid and adrenals, and functioning thyroid metastases are notable exceptions in which the area of abnormality appears as a locus of increased activity.
- Autoradiography: The action of radiation on a photographic emulsion is used to locate radio-activity in the specimen. Information can thus be obtained about the distribution of radioactivity on electrophoretic strips and chromatograms, in tissue sections, in individual cells, in cell nuclei and even in individual chromosomes. This technique is used in small animals for basic and pre-clinical research and drug development.
Radioactive Tracer Tests in vitro
- Neutron activation analysis
- Sample material bombarded with thermal neutrons in a nuclear reactor or cyclotron
- Induced radioactivity helps identification and quantification of a variety of trace elements by scintillation spectrometry.
- Saturation analysis and derivative analysis
- Competitive protein-binding assay
- Radioimmunoassay, radioreceptor assay
- Immunoradiometric assay.
In vitro Radiorespirometry
- Biochemical reactions in isolated tissues can be assessed by measuring C-14 CO2 liberated when the tissues are incubated with C-14 labelled substrates. The released HCO2 can be captured in a vial or trapped in a filter paper soaked with potassium or sodium hydroxide.
- The presence of live bacteria in blood, urine or sputum samples can be detected by incubating samples with C-14 glucose or other substrate added to the usual culture media. If C-14 CO2 is evolved subsequently the test is positive, for this will not occur under sterile circumstances. This technique is particularly useful for tubercle and lepra bacilli. Drug sensitivity tests can be done much more rapidly than by conventional methods.
The Structure of the Atom
An understanding of radioactivity and of the interaction of radiation with matter is possible only if we have some concept (albeit oversimplified) of the structure of the atom.
The atom consists of a small central nucleus, about 10−12 cm in radius, very dense and positively charged. Surrounding the nucleus is a spherical region that is mostly empty space, occupied by negatively charged electrons, moving about rapidly and held in the atom by the pull of the positive charge of the nucleus. The “atmosphere” of the electrons extends about 10−8 cm outward from the nucleus. An oversimplified picture of the atom shows its electrons arranged in concentric shells that are identified by quantum numbers, N = 1, 2, 3, 4, 5, 6 and 7 or by letters K, L, M, N, O, P and Q beginning with the innermost shell. It is more realistic to show a “probability cloud” whose density represents the probability of finding the electron at a given position in the atom (Fig. 1.2).
The chemical identity of an atom is determined by the number of orbital electrons which it possesses; ordinary chemical reactions between different atoms involve interactions only between electrons in their outermost electron shells (Fig. 1.3).
Fig. 1.2: The probability cloud model is somewhat nearer to the true model of atom. The blurred image represents a time exposure of electron motion. The reader visualized atom as having spherical shape
Fig. 1.3: The subshell arrangement and the sizes of atoms. Hydrogen and chlorine are given as examples. Open circles represent that occupy each subshell
The proton has a mass of approximately 1 unit on the scale of mass used in atomic and nuclear physics, and carries a positive electric charge equal in magnitude to that of the electron. The number of protons in the nucleus is symbolized by 2, called by two names, the proton number and the atomic number.
The neutron has a mass similar to that of the proton but carries no electrical charge (hence the name). The number of neutrons in the nucleus is symbolized by N the neutron number. The sum of neutrons and protons is called A, the mass number (A = N + Z).
For most but not all atoms, the number of protons and neutrons in the nucleus are nearly equal; the mass number is then approximately twice the atomic number.
The number of stable arrangements of protons and neutrons is rather limited, and any other configuration of those particles results in an unstable nucleus. Such nuclei will* tend to assume a more stable configuration losing mass and electrical charges and disposing of excess energy by the emission of radiation. This process is called radioactive decay or disintegration.
Minute amounts of radioisotopes C-14 and H-3 (tritium) occur naturally in the garth's biosphere; they are formed as a result of the interaction of cosmic radiation with the elements in the atmosphere. A few naturally occurring radioisotopes are weakly radioactive such as uranium and thorium but are continually decaying into the stronger radionuclide Radium-226.
A nuclide is a species of atom characterized by its atomic number (Z), mass number (A) and the nuclear energy state.
Isotopes are nuclides of an element, characterized by having the same atomic number (Z) and hence the same chemical properties, but different mass number (different number of neutrons).
Stable isotopes: Since the neutron carries no electrical charge, the addition of an extra neutron will result in an increase of 1 unit in the mass number, without altering the chemical identity. Most naturally occurring elements exist as mixtures of two or more forms which differ in the number of neutrons in their nuclei.
Radionuclides are nuclides which are radioactive, i.e. which undergo spontaneous disintegration or transformation with the emission of radiation.
Radioisotopes are isotopes which are radioactive.
Illustrative example: Carbon, the most important element in biology, exists in the following forms (Fig. 1.4).
- C-11—six protons and five neutrons.
- C-12—six protons and six neutrons.
- C-13—six protons and seven neutrons.
- C-14—six protons and eight neutrons.
Carbon-11 is a neutron-deficient radioisotope which emits positrons; hence it is useful in positron emission tomography (PET).
Carbon-14 is a neutron-excess radioisotope which decays with beta emissions. Although it is unsuitable for use in the living humans due to its prolonged half-life (5000 years) it has played a crucial role in tracer studies in biochemistry and intermediate metabolism, and is currently utilised in a procedure designated as in vivo radiorespirometry.6
Fig. 1.4: Examples of isotopes (top row) isobars (center row), and isotones (botton row). Protons in the nuclei are represented by black circles. The subdivision of the L-shells is not shown in this figure
Carbon-13 is a stable isotope of the ubiquitous carbon-12 but heavier than C-12. With the application of mass spectrometry “heavy” isotopes have found important applications in biochemical tracer work.
Carbon-13 has yet another nuclear property shared by nuclei with an odd number of protons and even number of neutrons (and vice versa)—that is intrinsic angular momentum or spin, which can be utilized for NMR (nuclear magnetic resonance) spectrometry and NMR imaging.
Nuclear Magnetic Resonance
Intrinsic angular momentum or spin is one of the fundamental properties of matter, beginning at the level of elementary particles. If a number of these particles (protons, neutrons) are grouped together to form a nucleus, their respective spins will add (by vector) and the nucleus will have a net nuclear spin. As a general rule, those nuclei with an even number of protons and neutrons will have zero net spin. Those nuclei with an odd number of protons or neutrons will have a net nuclear spin. The nucleus of hydrogen consists of a single proton, with a single orbital electron. Like a spinning top which also wobbles around its vertical axis, the protons of hydrogen also spin and have a wobble called precession. They behave like very tiny bar magnets of a definite strength or magnetic moment. If they are placed i n a magnetic field, they will line up more or less parallel to that field and will precess around it. The rate or frequency of this precession is proportional to the magnetic field strength in which they are placed.
In nuclear resonance, one makes use of this precession to study the atomic nuclei and their surroundings by exposing them to RF (radio-frequency) electromagnetic energy of exactly the same frequency as their precession. At that frequency they absorb energy from the radiation—resonance absorption—and change their alignment relative to the applied magnetic field. It is possible to turn them to 90° or 180°. After the RF field is removed the absorbed energy is released by the nuclei at particular frequencies—the strength being proportional to the proton density. The nuclei eventually return to their original orientation. This process is known as relaxation. The variables T1 and T2 are called the spin-lattice and spin-spin relaxation times. T1 is a measure of the connection between the spin system and its molecular environment and T2 is a measure of the connection between the spins themselves. By the use of those parameters important microscopic information about the human body is obtainable.
Hydrogen nuclei (protons), because of their high abundance (10 percent) in the human body and the strong NMR signal they generate, have been the focus of NMR imaging. The majority of hydrogen is formed in water molecules, although a significant proportion is present in fats and proteins. For this reason most of the NMR signal is derived from tissue water.
Another important atom with magnetic properties is phosphorus-31. The concentration P-31 which offers insight into basic intracellular metabolic process is 7several orders of magnitude lower than that of hydrogen (protons).
Fluorine-19 has excellent NMR characteristics and this may be exploited through fluorinated Pharmaceuticals and with fluoro-carbon blood substitutes.
Several other interesting atoms have nuclei with magnetic properties—deuterium H2, carbon-13, nitrogen-14 and nitrogen-15, oxygen-1 7, sodium-29, sulphur-31, sulphur-33, chlorine-35 and potassium-39. Unpaired electrons may also be imaged using electron paramagnetic resonance (EPR).
H-1, C-13 and P-31 have been fully used in NMR spectroscopy C-13 NMR spectroscopy has been used to measure the concentration of intracellular metabolites non-invasively such as glucose and glycogen in muscles (Shulman 1990). Details of such studies will be found in Chapter 14 (insulin resistance syndrome/metabolic syndrome and chapter 7 (Small animal PET and SPECT for basic research and drug development).
Nature of Radiation Energy
When an unstable nuclide reorders its nuclear components towards stability, the rearrangement leads to release of energy. Radionuclides disintegrate by six distinctly different nuclear processes, as shown in Table 1.2. In each case the atom loses discrete amount of energy that must be carried off by one or more atomic particles or by quanta of photons without mass, each type of disintegration can be indicated by a decay scheme, which is a combination of graph and an energy level diagram, (See appendix II).
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An alpha particle is essentially a helium nucleus consisting of two protons and two neutrons. It has relatively large mass, and low velocity and small penetrating powers, indeed a few inches of air or a sheet of paper will stop most alpha particles. Their ionizing affect is therefore sharply local, and is effectively exploited for therapy. It also means that alpha particles are of no possible use when the aim is to detect their presence with detectors placed outside the body.
A beta particle, an electron, has a negative charge, 7000 times smaller mass but more velocity compared to alpha particles, with enough energy to penetrate upto several millimeters of water. With sufficiently sensitive equipment beta radiation can be detected at the surface of the tissue if not too much tissue intervenes, but its range of ionization is still local enough for absorption of undesirably high radiation doses.
Positron decay, emission of positively charged electrons (or anti-electrons) is one way in which stability is restored in a nucleus with excess of positive charge. The positron will very shortly collide with an electron, and the two oppositely charged particles will disappear as matter, being converted into energy. The energy output of this transmutation consists of two gamma rays of 511 Kev energy emerging in almost opposite directions. The ability of detectors to locate the precise point in space where the transmutation took place is an important feature of PET (positron-electron transmutation tomography or positron emission tomography).
Electron Capture: In a radionuclide with excess protons stability is achieved by the shift of an electron from an inner orbit into the nucleus, thereby transforming an excess proton into a neutron. Energy released from this transaction of electron capture, is a photon known as characteristic X-ray,
Metastable State and Internal Conversion: A nucleus in a metastable state is under constant bombardment of the K shell electrons, to which the nucleus may transfer all its energy of excitation. The electron will then break off as a conversion electron. Following internal conversion, the atom is left with a vacancy in orbital electrons which will be filled by an electron, from a higher energy level—in this cascade the last 8vacancy will be filled by a “free electron” picked up by the atom to complete its full complement of electrons. Each time one of these electron transactions takes place, the atom loses energy either (1) by generation of a photon, known as characteristic X-ray, or (2) by freeing additional electrons from the atom through the Auger process.
Isomeric transition: Two nuclei having the same values of Z and the same values of N, but different nuclear configurations (which occur when one is excited to a different energy level from the other), are called isomers. The excited state comes to ground state simply by rearrangement of the internal structure. Energy is lost in this isomeric transition in the form of a photon (gamma ray).
The gamma ray is characterized by no mass, a velocity equal to that of light, a wavelength generally considerably shorter than that of X-rays, and great penetrating power. Their ionizing capabilities, therefore, can be detected at considerable distance from their source, making them suitable, for diagnostic purposes, with little or negligible local damage to the tissues. The energy of gamma rays is measured in terms of thousand electron volts (KeV). Gamma rays with energy around 200 KeV are well suited for usa with existing instruments.
The ideal gamma-emitting nuclide should decay by isomeric transition or electron capture without internal conversion. Those nuclides which decay with beta emissions or significant internal conversion, and those with longer than a few hours of physical half life, are less desirable. For diagnostic purposes it is desirable to give as large a dose as safely permissible, of a gamma-nuclide of minimally appropriate longevity, with energy characteristics as high as currently available instruments can efficiently utilize.
Radioactive Decay
Nuclear decay is a random process: for any individual radioactive atom we cannot predict exactly when it will disintegrate. We can only express a probability that it will disintegrate in any given time interval. With a large number of identical radioactive atoms, we can say how many we expect to disintegrate in each second on an average. The number of disintegrations per second is a measure of the strength of radioactivity of sample of radionuclide. An activity of one disintegration per second is called a becquerel, Bq. In nuclear medicine we are usually dealing with activities of several million disintegrations per second, i.e. megabecquerels, (mBq). Traditional unit of radjoactivity, the curie was defined as the activity of one gramme of radium, which is equal to 3.7 × 1010 disintegrations per second. The submultiples of the curie are millicurie (mCi) and microcurie μCi).
The activity A, of a radioactive source, measured in disintegrations per second, is equal to the number of radioactive atoms present, N, multiplied by the probability that any one of them will decay in one second. If we call this probability as lamda (λ), then
A = AN. The activity may also be defined at the rate at which N is decreasing. Thus—
The solution of this differential equation gives the activity at any time as
Where A0 is the activity at time t = 0. This is known as exponential decay.
The rate of decay can also be specified by the half life (t½) as the time it takes for half of the given amount of radioactivity to decay, that is the time at A/A0 =. 54. It is related as follows:
The activity will halve in each half-life irrespective of when measurements are started. For example technetium — 99m has a half-life of 6 hours. A source which has an activity of 8 mCi at 8 a.m. will have only half of this value (4 mCi) at 2 p.m., and by 8 p.m. it will be 2 mCi. After 2 days the activity will be only 51 uCi (1/256 of the original activity) but it will still halve during the next 6 hours.
The development of instruments, choices of radionuclides, and effective use of gamma rays and X-ray sources in nuclear medicine depend upon our understanding of the behavior of the three interactions – alpha decay, isobaric transitions (beta decay, positron decay, electron capture) and isomeric transitions (gamma rays, internal conversion).
Pediatric Nuclear Medicine
Forty percent of the Indian population belong to pediatric and adolescent age group, hence it is useful to know the application of nuclear medicine in infant and children, keeping in mind the dosimetric considerations (Table 1.3).9
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BIBLIOGRAPHY
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- Henry Wagner. Molecular Nuclear Medicine: From genotype to phenotype via chemotype J Nucl Med 1995.
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- Hevesy G Radioelements as tracers in physics and chemistry Chem News 1913;108: 66.
- Hevesy G The absorption and translocation of lead by plants. A contribution to the application of the method of radioactive indicators in the investigation of the change of substance in plants. Bio Ch 1923; 17: 439–45.
- Krasak M. et al Intramyocellular lipid concentrations are correlated with insulin resistance in human – a H-1NMR spectroscopy study. Diabetologia 1999; 42: 113–16.
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- Schoenheimer R, Clarke HT The dynamic state of body constituents Harvard Univ. Press. Cambridge MA 1949
- Shulman GI et al Quantitation of muscle glycogen synthesis in normal subjects and subjects with non-insulin dependent diabetes mellitus by C-13 NMRS N Eng J Med. 1990;322: 223–28.