Rapid determination of linearity in a dose calibrator

A method of testing dose calibrator linearity comprising the steps of serially interposing a plurality of shields between the sample and detector, each shield having a given thickness to absorb a desired fraction of the radiation; measuring the activity of the sample as attenuated by the shields; calculating the actual activity of the sample at each level of attenuation; and comparing the measured activities with the calculated activities. For carrying out the method several embodiments of shields are described.

FIELD OF THE INVENTION 
The present invention relates to a method and to means for utilization of 
said method in the test for linearity of a dose calibrator. The linearity 
test disclosed herein is time independent, and may be conducted in minutes 
as opposed to prior art procedures which span at least 24 hours, and 
oftentimes more than 48 hours. 
BACKGROUND OF INVENTION 
Radioactive materials are in widespread use in the treatment of cancer, and 
are also employed in photography, as tracer materials in the chemical 
industry and in the structural analysis of welded members. Particularly in 
the field of radiopharmaceuticals, a radionuclide or radioisotope must be 
assayed before the patient is provided with treatment. The instrument used 
to measure the strength or activity of the radiopharmaceutical is a dose 
calibrator. 
The dose calibrator must be checked for accuracy prior to initial use, 
after each repair and periodically thereafter. The applicable tests for 
calibration of the dose calibrator are described in U.S. Nuclear 
Regulatory Guide 10.8, Appendix D, Section 2, PP. 10.8-27 to 29 (Rev. 1, 
October 1980). One such test is that for linearity. That is, the dose 
calibrator must perform accurately to within .+-.10% over the entire range 
of radionuclide activity contemplated during the use of the instrument. In 
most instances this range will be from about 500 millicuries to about 0.1 
millicurie (100 microcuries), but may include activities up to 2 curies or 
as low as 1 microcurie. 
The procedure in current use measures the initial activity of a test 
sample, and allows the sample to decay until the activity has decreased to 
the low end of the anticipated range of use, several measurements being 
taken through the period. Knowing the decay constant and initial activity, 
the actual activities of the decaying sample at various time increments 
may be calculated. Actual activities are then compared to measured 
quantities, linearity being confirmed by error deviations of less than 
.+-.10% for each comparison. If the initial activity is unkown, the 
procedure is the same except that one of the intermediate measurements is 
selected as a base point. Alternatively, one may calculate and compare 
time at incremental activities. Because the half life of technetium -99m 
(Tc-99m), the isotope most often used in linearity determinations, is 6.04 
hours, a complete test from 500 to 0.1 millicurie takes about 74 hours. 
Obviously, this time consuming procedure is inconvenient at best, and may 
seriously delay appropriate patient treatment. 
SUMMARY OF INVENTION 
It is an object of this invention to provide a method for rapidly 
determining linearity of a dose calibrator. 
A further object is to provide a rapid method of dose calibrator 
linearization which is suitable for radioisotopes having relatively long 
half lives. 
Another object of this invention is to provide a plurality of shields 
adaptable for use with a dose calibrator to effect a linearity 
determination thereof rapidly. 
These and other objects and advantages of the invention will be more fully 
understood upon a reading of the detailed description, a summary of which 
follows. 
A plurality of shields of known material are serially interposed between 
the radioactive sample (in its non-metallic container) and the detector, 
and the radiation as attenuated by the shields is measured. The number of 
such measurements thus made depends upon the width of the range of 
activities over which the dose calibrator is to operate during use. Each 
shield is characterized by being of a discrete thickness, which thickness 
is adapted to absorb a known fraction of the radiation emanating from the 
source sample. Knowing the amount of radiation absorbed by the shields, it 
is possible to calculate the activity value which should have been 
perceived by the calibrator. By comparing the measured to the calculated 
activities, confirmation of linearity of the calibrator can be obtained, 
an error deviation of less than a given standard for each comparison being 
required for acceptance of the unit. 
The shields are in the form of disks with a projecting lip, the lip 
receiving the sample container. In another embodiment, each shield is a 
cylindrical sheath whose bottom is of the desired thickness. Typically, 
the thickness of the shield will be accurate to within +0.5%.

DETAILED DESCRIPTION OF INVENTION 
For a more complete appreciation, attention is invited to FIG. 1 which 
shows a hollow cylindrical sheath 10 of lead or other suitable radiation 
attenuating material that is open at one transverse end 11. The sheath 10, 
moreover, has dimensions that are suitable to receive a radioactive 
sample, or source (not shown in the drawing), for instrument calibration 
purposes. 
In accordance with a feature of the invention, wall thicknesses 12, 13 of 
the bottom section, or transverse closed end 12 of the sheath 10 and the 
longitudinal wall 13 are of predetermined thickness 14, as described 
subsequently in more complete detail. 
Turning now to FIG. 2 a lead or other suitable radiation attenuating shield 
in the form of a disk 15 is illustrated. The disk 15 has a projecting lip 
16 with dimensions that are suitable to fit across one end of a 
radioactive sample container (not shown in the drawing). Further in this 
respect, predetermined thickness 17 of the disk 15 is established in the 
manner subsequently described. 
The linearity test disclosed herein is convenient for use with all dose 
calibrators, particularly for use with dose calibrators of the ionization 
type. Typical ionization type units are the commercially available 
RAD/CAL.TM. II unit (TM Victoreen, Inc.); the MEDIAC.RTM. dose calibrator 
Model No. 6362 (Nuclear-Chicago Corporation); and the CRC-16 and CRC-30 
radioisotope calibrators (Capintec, Inc.). 
Linearity may be defined as accuracy of the dose calibrator over a wide 
range of radioisotope activity. A plot of the natural log of activity 
versus decay time for a given isotope would, for a linearly accurate 
calibrator, provide a straight line according to the equation: 
EQU A=A.sub.o e.sup.-.lambda.t (1) 
where 
A=activity at time t 
A.sub.o =initial activity at time t=0 
.lambda.=decay constant, and 
t=time. 
A check for linearity is necessary because the same instrument may be used 
to assay samples having a wide range of activities. Thus, in nuclear 
medicine, one instrument may measure samples with an activity as high as 2 
curies or possibly as low as 1 microcurie. Generally, however, the range 
of activities normally encountered falls between approximately 500 to 
approximately 0.1 millicuries. The U.S. Nuclear Regulatory Commission 
Guide 10.8 (Rev. 1, October 1980) states at page 10.8-6 that all 
radiopharmaceuticals should be assayed to an accuracy of .+-.10% of the 
true value prior to administration to patients. Many hospitals have 
stricter internal standards adopted for safety of the patients. 
A range of activities is obtained under linearity test procedures of the 
prior art by allowing a sample of a short-lived isotope, usually Tc-99m to 
decay over time, measurements being taken and compared to activities 
calculated by equation (1). To linearize a calibrator over the range from 
500 to 0.1 millicuries takes about 74 hours; from 2 curies to 1 
microcurie, about 126 hours. 
To eliminate the time element, the present method repetitively measures the 
activity of the sample with shields of known materials and thickness, each 
shield being interposed serially between the sample (in its non-metallic 
container) and the detector or counter. Each shield thus interposed 
absorbs a discrete fraction of the radiation emanating from the source 
according to the equation: 
EQU I=I.sub.o e.sup.-.mu. l.sup.x (2) 
where 
I.sub.o =initial intensity without shield 
I=intensity with shield thickness=x 
.mu..sub.l =linear attenutation coefficient of the shield material, and 
x=shield thickness. 
Thus, a plurality of shields may be provided which absorb pre-determined 
amounts of radiation. For example, with Tc-99m as the source, .mu..sub.l 
for lead is 3.465 mm.sup.-1, and a lead shield of 1.129 mm thickness would 
by necessity absorb 98% of all radiation emanating from said source. That 
is, only 2% of the radiation would be measured by the detector. To obtain 
the corresponding decay of the Tc-99m sample such that the initial 
activity decreased by 98%, the technician, physicist or radiologist 
calibrating the instrument by the prior art procedure would have to wait 
about 34 hours. However, with the shield in place the value perceived by 
the instrument could be compared immediately to the value for a 98% 
reduction. 
To fully linearize the instrument over the range, it would be necessary to 
have at least several shields of known thickness, each shield being 
serially interposed to provide a new reading at a different level of 
perceived radiation. The number of measurements to be made would of course 
depend on the width of the range and accuracy desired. Typically, four to 
six measurements would be sufficient. Reproduced in tabular form below are 
the thicknesses of lead shields necessary for achieving a required level 
of radiation as measured by the detector, and the activity thus measured 
from a 2 curie and a 500 millicurie Tc-99m source. 
______________________________________ 
Radiation 
Reaching Detector (mCi) 
% Radiation Thickness (2 Ci. (500 m. Ci. 
Reaching Detector 
(mm) Source) Source) 
______________________________________ 
100 0 2000 500 
75 0.083 1500 375 
50 0.200 1000 250 
20 0.464 400 100 
10 0.664 200 50 
1 1.329 20 5 
0.1 1.994 2 0.5 
0.01 2.696 0.2 0.05 
0.001 3.323 0.02 0.005 
0.00005 4.187 0.001 0.00025 
______________________________________ 
Not all of these shields are necessary for each check of linearity, but a 
complete set would suffice for most applications. In certain instances 
where great accuracy is desired or a very wide range is encountered, 
additional shields can be used. In addition, several of the finer shields 
may possibly be concatenated to produce the effect of a thicker shield. 
Finally, different materials having lower linear attenuation coefficients 
may be employed to obtain thicker shields having greater durability, but 
of equivalent attenuation. Shield thickness should be such that the level 
of accuracy in the performance of the procedure is commensurate with 
accuracy requirements of the user. When the dose calibrator is used to 
assay radiopharmaceuticals a thickness accuracy of -0.0 to .+-.0.5% is 
believed necessary to insure proper calibration. Other materials suitable 
for use as shields are selected from the ferrous and non-ferrous metals, 
including aluminum, iron, tin, cadmium, and each of their alloys. 
Non-metallic materials, for example, plastic materials such as Lexan.TM. 
nylon, and the like, could also be used, but would result in shields of 
extreme thickness. Use of metallic and non-metallic materials having low 
linear attenuation coefficients may be advantageous, however, where only a 
small fraction of the radiation is to be attenuated. The thicker shield 
would be easier to work with, and less subject to breakage. 
The shields may be in the form of disks 15 (FIG. 2) adapted for insertion 
between the sample container and detector. Most conveniently, however, the 
disk type shield will be provided with a lip 16 about its periphery, the 
lip receiving the container. In this way there will be no errant radiation 
entering the detection zone. In another embodiment the shields are 
cylindrical sheaths 10 (FIG. 1), the sample container being received 
thereinto. In each embodiment, the bottom section 12 of the shield, e.g., 
the portion interposed between the sample container and detector, would be 
of proper thickness as indicated from the above discussions. In another 
embodiment, particularly with respect to the finer shields, the base of 
the shield would comprise a relatively thick, highly absorbent exterior 
annular portion and an interior circular portion of proper thickness. The 
annular portion provides the requisite strength for the shield, and 
absorbs essentially all of the radiation incident to its surface. The 
circular portion allows that fraction of the radiation associated with the 
shield to pass through and be detected. 
Because the procedure described herein is not dependent on time, and, 
indeed may be performed in 10 to 20 minutes, it is suitable for use with a 
wide variety of isotopes, including those of long half lives, for example, 
sodium-24 (14.97 hours), iron-59 (45.1 days), iodine-131 (8.08 days), 
xenon-133 (5.27 days), cobalt-57 (270days), and cobalt-60 (5.24 years). 
Tabulated below is a hypothetical example illustrative of the present 
method. The table is predicted on the use of a Tc-99m sample having an 
initial activity of 250 mCi. 
______________________________________ 
Equiv- Activity 
Shield % alent Activity 
Calcu- % 
Thickness 
Radiation Time Measured 
lated Devi- 
(mm) Measured (hrs) (mCi) (mCi) ation 
______________________________________ 
0 100 0 250 -- -- 
0.083 75 2.5 192 187.5 2.4 
0.200 50 6.0 127 125 3.2 
0.464 20 13.9 56 50 12.0 
0.644 2.0 19.9 5.32 5.0 6.4 
1.329 1.0 39.9 2.55 2.5 2.0 
______________________________________ 
The calculated activity in the table is merely the "% radiation measured" 
times the initial sample activity inasmuch as the shields have been 
fabricated to a pre-set thickness which defines the actual fraction of 
radiation perceived by the detector. Because the activity measured at 80% 
attenuation (20% passing through shield) is in error by more than 10%, the 
dose calibrator would not pass the linearity test under the Nuclear 
Regulatory Commission standards. Note that time is not a factor in the 
above example. 
The above disclosure is intended to be exemplary of the invention and is 
not to be construed to be limited except as provided in the appended 
claims. For example it is readily seen that the shields may be fabricated 
for use with a particular radioisotope, the shield thickness being such as 
to provide attenuation corresponding to equivalent decay time. Thus, in 
the example, shields could have been provided to mirror the activity after 
5, 10, 20 and 40 hours of decay when used with a particular radioisotope.