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12.Phantoms
Probably one
of the most important considerations in the use of a whole body counter
is the method of calibration. One may spend a considerable amount of effort
on the collection of data from subjects, however, if the calibration data
are poor, much of the effort is wasted. Some of the factors affecting the
calibration of a whole body counter include the number of different gamma
energies emitted by the radionuclide, the number of gamma photons at a
given energy per disintegration of the source, the distribution of the
radionuclide in the human body, (i.e., the chemical and physical characteristics
of the source in the body media), the size and shape of the phantom (i.e.,
the size and shape of the people who are to be measured).
12.1.1.Administration of Radionuclides to Human Volunteers
12.1.2.REMAB and REMCO Phantoms Some phantoms are available which have been built to resemble the human body very closely. These may be obtained either with or without a skeleton. These phantoms, built by Alderson Laboratories, can be filled with solutions containing radioactive materials to be distributed throughout the entire body, or in particular organs. A “REMAB” phantom is shown in Figure 12-1 on the next page. These phantoms were quite expensive, starting around $3,500.00, but that was in 1965.
Another type
of phantom, called the “Rando” phantom, is made of a rubber-like material,
the composition of which has been adjusted both physically and chemically
to give the desired values of atomic number and specific gravity. They
may be obtained with or without bones and with various organs. This phantom
has been cut into one inch slabs allowing placement of sources between
the layers to effect the type of distribution of which the investigator
is seeking. These phantoms are also quite expensive, but certainly are
desirable if one wishes to do research work of a highly technical nature.
The Rando phantom sold for approximately $3,800.00 in 1965.
Because the measurement of low energy X-rays, particularly from the isotopes of plutonium, is extremely difficult, the Nuclear Regulatory Commission and the Department of Energy decided to have a phantom fabricated which matched the human torso very closely, both in physical dimensions as well as in its absorption characteristics for low energy X and gamma-rays. The result of this work by Griffith, et al, (Gr79) has been well documented in the literature. Since it was built at the Lawrence Livermore National Laboratories, it is frequently called the “LLNL Realistic Phantom” and is commercially manufactured by Humanoid Systems. We shall discuss the LLNL Phantom is greater detail later in this chapter.
12.2.Other Types of Phantoms
Cofield (Co60) uses the phantom shown in Figure 12-2, below. Quoting directly from his paper in the Health Physics Journal:
A number of investigators
have found that for most whole body counting purposes it is not necessary
to have an expensive phantom with which to perform the calibrations. Helgeson
Scientific Services started with the Cofield lung phantom, then expanded
the idea into a complete phantom, the torso portion of which is shown in
Figure 12-3. This phantom is made up of one fourth inch thick pieces of oil impregnated Masonite, and has the general dimensions of the human body. It is five foot seven inches tall, weighs slightly more than the standard man, (182 pounds), and has
a density slightly greater than 1.0. (For information on the “standard man” see reference ICRP59.) Figure 12-3 also shows a typical method of calibration. Multiple individuals sources, all of which have the same amount of activity, are distributed uniformly throughout the Masonite phantom. One set of these sources consists of approximately two hundred individual sources. With a large number of sources one may
distribute them in the phantom body very closely to the manner in which they would be distributed in the human body. When calibrating for uniform body distribution, they are distributed such that there is a constant amount of mass of Masonite surrounding each of the sources.
Figure 12-3, above, shows how the sources may be distributed uniformly in the phantom lung to simulate the deposition of an insoluble radioisotope. Since there are many layers of lung cavities, this allows a good approximation at a uniform lung distribution. It is very useful for determining the variation of lung calibration factors as a function of total chest thickness and chest wall thickness. One of the disadvantages
of using a phantom of fixed dimensions is that one cannot account for the
various sizes of individuals who would be counted. A number of investigators
have shown that the calibration factor is not constant with weight. Some
investigators use a relationship plotting the calibration factor against
weight alone, while others plot the calibration factor against the ratio
of weight divided by height. Still another group plot the calibration factor
as a function of the square root of the ratio of the weight to the height
(Ha66). In the work of Helgeson Scientific Services the calibration factor
is plotted as a function of the weight of the subject only, such as the
curves shown in 12-5, and
is quite adequately described by the sum of two exponential expressions
of the form:
Factor = A*exp (BW) + C*exp(DW)
where W = the subject weight and
Not only are the calibration factors described in this manner, but also
the scatter factors from one radioisotope to another are quite well described
by the same type of expression. Honstead (Ho67) has found that over the
weight range for his studies, the calibration factor for a shadow shield
counter can be described by one exponential. Thus, it is obvious that the
careful calibration of a whole body counter is far more complex than using
a single point source in a single weight phantom. 12.2.1.Sugar Phantoms
In order to obtain
calibration factors as a function of weight one needs phantoms of variable
sizes. A typical phantom could be made up of sugar boxes since sugar is
relatively inexpensive, comes already packaged in one or two pound boxes,
and is very pure and free of naturally occurring radioisotopes. Figure 12-6
shows a typical 72 box sugar phantom
made from one pound boxes of sugar. Thus, it is quite simple to build phantoms
of various sizes to take into account the different sizes and shapes of
humans which one will encounter.
A rather simple technique may be used for placing the source inside the sugar box, as shown in Figure 12-4, above. A small slit is cut in the side of a sugar box and a cardboard pouch sealed on three sides is inserted into the slit. The cardboard pouch is made very simply by taking a standard 3" x 5" filing card, cutting it to the approximate dimensions of the source, taping up three sides to assure that the sugar cannot leak out of the box and inserting it straight into the box and taping it into position. In this manner one can slip sources into the pouch, which is surrounded by sugar, and retrieve the source without loosing any sugar. The small “dot” source, about 4 mm in diameter, is taped to a smaller piece of cardboard so the source and cardboard may be inserted and removed easily. This makes for rapid interchange of sources and many calibrations can be performed in a relatively short period of time. For example, the shadow shield whole body counter was calibrated using ten different radioisotopes, seven different phantom sizes, and duplicate measurements in a number of instances. The data collection was completed in approximately 200 man-hours. The results provide an excellent knowledge of the response of the shadow shield whole body counter to gamma photons ranging from energies as low as 60 keV (from americium-241) to as high as 1.46 MeV (from potassium-40). We find that almost without exception the calibration curves could be described quite well as the sum of two exponential expressions, as has been previously shown in Figure 12-5. These data are being reduced through computer curve-fitting routines to a system of equations which describe the shape of the photopeak(s), the Compton Scatter region, and the variations as a function of the weight of the individual.
Figure 12-7, below, shows the photopeak counting efficiency of one of our 8" x 4" crystals as a function of energy and phantom weight. These data are from early work, before we had any sources with gamma rays below 100 keV. Later in this document you will see more current work with energies below 100 keV.
12.3.LLNL Realistic Phantom
The LLNL Realistic Phantom simulates a limbless human male torso terminated above and below the fifth cervical and fourth lumbar vertebrae, respectively. It is composed of materials known to be approximately tissue equivalent in their linear attenuation of uranium L X Rays. The phantom consists of a tissue equivalent polyurethane torso shell with sternum, rib cage, and vertebrae embedded therein. Tissue equivalent lungs, heart, and liver were cast in molds prepared from the thorax and organs of a cadaver 1.77 meters in height and 76 kilograms in weight. These organs, along with tissue equivalent material simulating intestines and body fluids, fill the phantom abdominal cavity. An important
feature of the LLNL phantom is that, due to its realistic anatomical structure,
it can be used to reproducibly simulate the counting geometry of uniform
internal radionuclide depositions.
The mean anterior chest wall thickness in the lung vicinity is 19 millimeters. To simulate the geometry and chest wall attenuation of the range of stature seen by male radiation workers, close-fitting chest plates of various thicknesses may be overlaid on the phantom. Two sets of overlays are provided in order to simulate tissue attenuation provided by lean muscle or by a combination of muscle and adipose tissue (50% each by weight). The Humanoid Phantom does not contain a skeleton as such but contains a bone simulating material to simulate the attenuation of these low energy X-rays by the bone. Figure 12-8, above, shows the LLNL phantom on the left.
12.4.The HSS Masonite Phantom, Model SLP
The phantom has 3 major body parts, plus torso spacers. The major body parts are (1) head, neck and lungs, (2) gut, and (3) lower gut and legs. The assembled height of these three parts is 175 cm (5 foot 9 inches). There are three body spacers with heights of 2.54, 5.08, and 10.16 cm, (1", 2", and 4") which may be inserted between major body parts for more height. This is especially important in the “Quicky” In Vivo Counter. Cavities for placing nuclides are located in the head, thyroid, chest, gut, thigh, and lower leg regions.
Figure 12-11, below,
shows the relative calibration efficiencies of the two phantoms.
The phantom is made of one-eighth inch sheets of “Masonite,” a wood product. In its original form, waste wood products were reduced to sawdust consistency, combined with a glue as a binding agent, and were subjected to intense pressure and heat to cure the glue. These one-eighth inch sheets are then cut to the appropriate dimensions, assembled into the forms which we require for our phantom work, then glued, cured, and finished with a clear varnish to produce the end product. As was stated earlier in this document, there has been no attempt to try to make it useable for nuclides such as 241-americium or 238,9 plutonium. 12.4.1.Source Fabrication Essentially all of the sources used by Helgeson Scientific services are fabricated to our dimensional specifications by an experienced vendor of radioactive sources. Because we wish to make calibrations for different source distribution geometries, it is necessary to order many “point” sources. Typically, we may use as many as 240 individual point sources in a set for a particular radionuclide. The vendor provides us with a calibration certificate. When the sources are received from the vendor they are re-calibrated against our NIST sources. This re-calibration may consist of measuring After calibration the sources are taped to a small card measuring about 1 inch by 3 inches. The card is stamped with the name of the radionuclide, the source identification number, the activity per point source, and the date on which that value of radioactive content was made. Figure 12-12, below, is a full scale picture of one of the depleted 238-uranium cards.
12.4.2.Description of the Styrofoam Sizes and Phantom Cavities The radioactive sources must be distributed in the phantom in a defined pattern so consistency will be maintained for all calibrations, even between different types of in vivo counters. The phantom cavities may be filled with sheets of styrofoam on which the individual sources have been fastened. The sources are located in the “slots” between the sheets of styrofoam to best simulate a uniform distribution in the cavity and to produce results which match as closely as achievable the results obtained with the Lawrence Livermore phantom or the bottle phantom. The “slots” are numbered from the front of the phantom. We now tape all of the styrofoam sheets for a particular “organ” so the relative positions of the sources and styrofoam do not change. This is one of our standard practices to ensure reproducible results. The details of the loading are given below. A typical lung "slot" is shown in Figure 12-13, below.
The sources are distributed in the phantom according to the manner in which they would most likely be distributed in the human body. For example, the nuclides of the cesiums, potassium, sodium etc., i.e., the transportables, would be distributed throughout the whole body as follows: 7 % in the head 31 % in the lung 32 % in the stomach 21 % in the lower GI 9 % in the legs These percentages are based on the relative muscle masses in these portions of the body. Conversely, the cobalts, manganese, zirconium-niobium, etc., i.e., the non-transportables, would be found in the lungs. In any event, multiple sources must be evenly distributed side-to-side and evenly distributed from front-to-back in each cavity. The recommended distributions are given in the next section.
The thyroid cavity
has been left as an open section in the phantom neck. It is designed to
receive a masonite source holder. The actual cavity in the neck is 96 mm
high by 125 mm wide by 21 mm deep. The source holder is 90 mm high by 130
mm wide by 16 mm deep and has a well in it which is 76 mm high (measured
from the top to the bottom of the well) by 98 mm wide by 6 mm deep. The
sources are fastened to a thin but sturdy card and may be distributed in
the shape of the two thyroid glands, or may be distributed uniformly over
the surface of the card. The card is then inserted into the source holder
and the source holder is inserted into the neck cavity. Because the source
holder is slightly wider than the width of the cavity in the neck (130
mm for the holder versus 125 mm for the neck cavity), it is easy to remove
the source holder when you wish to perform other calibrations, such as
for lung or total body distributions.
Some of our clients want to have their in vivo counters calibrated with real 131-iodine, rather than use “mock iodine,” which is a mixture of 133-barium and 137 cesium. This source is also supplied by a commercial source manufacturer and the activity is distributed as a series of points in an area which simulates the front profile of the thyroid. Figure 12-13, above, shows a typical card which contained 131-iodine. 12.4.2.2.Head Cavity The head cavity
is approximately 175 mm high by 88 mm wide by 148 mm deep (6.875 inches
high by 3.4375 inches wide by 5.875 deep).
11 styrofoam sheets are used for the head cavity. Each sheet is 6.5 inches by 5.75 inches (165 mm by 146 mm). The source placement is described in Table 12-1,below. With 11 sheets of styrofoam there are 10 “slots” within which 7 sources may be placed.
12.4.2.3.Lung Cavity The lung cavity is approximately 202 mm high by 88 mm wide by 271 mm deep ( 7.9375 inches high by 3.4375 inches wide by 10.75 deep). 11 styrofoam sheets are used for the lung cavity. Each sheet is 6.5 inches by 5.75 inches (165 mm by 146 mm). The source placement is described in Table 12-2, below. With 11 sheets of styrofoam there are 10 “slots” within which 31 sources may be placed.
12.4.2.4.Stomach Cavity The stomach cavity is approximately 242 mm high by 88 mm wide by 271 mm deep (9.3125 inches high by 3.4375 inches wide by 10.75 deep). 11 styrofoam sheets are used for the stomach cavity. Each sheet is 6.5 inches by 5.75 inches (165 mm by 146 mm). The source placement is described in Table 12-3, below. With 11 sheets of styrofoam there are 10 “slots” within which 32 sources may be placed.
12.4.2.5.Lower G.I. Cavity
The lower G.I. cavity is approximately 202 mm high by 162 mm wide by 271 mm deep (7.9375 inches high by 6.375 inches wide by 10.75 deep). 11 styrofoam sheets are used for the lower G.I. cavity. Each sheet is 6.5 inches by 5.75 inches (165 mm by 146 mm). The source placement is described in Table 12-5, below. With 20 sheets of styrofoam there are 10 “slots” within which 22 sources may be placed.
12.4.2.6.Leg Cavities The leg cavities are each approximately 202 mm high by 162 mm wide by 94 mm deep (6.875 inches high by 6.375 inches wide by 3.6875 deep). 11 styrofoam sheets are used for the leg cavity. Each sheet is 6.5 inches by 5.75 inches(165 mm by 146 mm). The source placement is described in Table 12-6, below. With 22 sheets of styrofoam there are 10 “slots” within which 4 sources may be placed.
12.4.3.Description of 40-Potassium Source Preparation Calibration for 40-potassium is simplified because it is not a licensed material. Thus, to obtain an excellent supply of 40-potassium, you only need to order C. P. Grade Potassium Chloride. The term “C.P. Grade” means “chemically pure grade,” which means that there are very few impurities in the product. The next task is to determine how much potassium chloride, hereafter referred to by its chemical symbol, “KCl,” must be placed in each phantom cavity. We need to know the specific activity of KCl, i.e., how many disintegrations per second come from one gram of natural potassium? This calculation is shown below and will be readily understood by anyone who can remember his high school chemistry lessons. One gram molecular weight (GMW) of potassium (or of any substance) is defined as the number of grams of the substance which is numerically equal to the molecular weight of the substance. Potassium has an atomic weight of 39.1. Therefore, one GMW is 39.1 grams. We know that each atom of an element has an integer GMW, however, potassium, as found in nature, is a mixture of several isotopes:
-->
(Editor's Note:
These data were taken directly from the “Radiation Hygiene Handbook, page
6-96. The error in the total, therefore, is theirs, not ours. Reference
Ba59.)
Continuing with our calculation: 1 gram molecular weight (GMW) of natural potassium weighs 39.096 grams and contains 6.061 x 1023 atoms, or, the concentration is 6.061x
1023 atoms per 39.1 grams = 1.55924 x 1022 atoms per gram.
However, since the 40-potassium isotope is only 0.0119 percent of the total, then we must make a correction for this:
1.55924 x 1022 x 0.000119 = 1.85549 x 1018 atoms per gram of 40-potassium.
The specific activity of a radionuclide is defined as the number of disintegrations
per second per unit quantity of the isotope. The half-life of 40-potassium is 1.26 x 109
years. Siince we want the specific activity in units of disintegrations per second, we must transform the half life in years to the half-life in seconds:
Half-life in seconds = 1.26 x 109 x 365.25 days/year x 24 hours/day x 3,600 seconds/hour = 3.98 x 1016 seconds.
The natural logarithm of 2 = 0.693147.
Therefore, the decay rate of one atom is the natural logarithm of 2 divided by the half-life in seconds, or 0.693147/ 3.98 x 1016 seconds = 1.74 x 10-17 disintegrations per second.
Now, putting all of this
together we find that the specific activity of 40-potassium is:
32.34525 disintegrations per second from one gram of total potassium.
Now we must determine
how strong we want the source to be. Since we are trying to obtain statistically
valid counting results in a relatively short period of time, we shall use
about 2,900 grams of C.P. Grade KCl. The calculations for each of the phantom
cavities are given in the tables on the next several pages.
This is the end of Chapter 12.
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