Article Figures & Data
Figures
- FIGURE 1.
Dose calibrator. Electronics module almost always sits separately from cylindric ionization chamber, as shown in this figure. Ionization chamber is surrounded by lead shield, which blocks external radiation from entering and also causes photons from source being measured to be backscattered into gas space. Dipper handle can be seen coming out of measurement space. (Photo courtesy of Biodex Medical Systems, Inc.)
- FIGURE 2.
Linearity attenuation shields. Source is placed in black tube, which is in turn placed within measuring space of dose calibrator. Other tubes, which have colored bands on upper edge, are then placed into dose calibrator around black tube. Known attenuation factors of each combination of tubes allow operator to simulate activity levels over wide range, thus assessing dose calibrator’s ability to measure accurately at different activity levels. (Photo courtesy of Capintec, Inc.)
- FIGURE 3.
Survey meters. (A) Small Geiger counter intended for personal use. (B) Geiger counter with pancake probe, used to locate areas of radioactive contamination. (C) Geiger counter with Geiger-Müller tube attachment, commonly used to monitor ambient radiation levels. (D) Ionization survey meter. This instrument operates at lower electric potential than Geiger counter, making it less sensitive and therefore not appropriate for identification of areas of contamination. (A and B courtesy of Capintec, Inc.; C and D courtesy of Biodex Medical Systems, Inc.)
- FIGURE 4.
Thyroid probe/well counter. This instrument contains 2 detectors, organ probe at upper left and well counter at lower left, sitting atop base/wheel assembly. Both detectors use sodium iodide, a scintillation crystal, as the detection material, and both produce an energy spectrum as their output. Scintillation detectors have an inherent advantage over gas-filled detectors, in that the detecting material is much denser and therefore more likely to interact with γ rays and other high-energy photons. (Photo courtesy of Capintec, Inc.)
- FIGURE 5.
Energy spectrum. This screenshot from system in Figure 4 shows energy spectrum for 137Cs. x-axis of graph is channel number, with each channel representing small portion of energy scale. y-axis indicates number of counts registered in each channel. Given that energy of 137Cs photopeak is 662 keV, and location of photopeak on graph is at about channel 330, each channel is approximately 2 keV wide. Note also real-time and live-time numbers in upper right. Difference between them represents dead time incurred during this measurement, which is expressed as percentage of live time. (Photo courtesy of Capintec, Inc.)
- FIGURE 6.
Surgical probe. This instrument is designed to identify small foci of radioactivity such as lymph nodes in operative setting. The detecting material in this case is semiconducting material, cadmium-zinc-telluride. Semiconductor radiation detectors interact with photon or particulate radiation in ionization interactions, creating electrons that are sent directly into electronic circuit. No bulky PMT is required as in organ probe shown in Figure 4, making this a better instrument for surgical operation. (Photo courtesy of Capintec, Inc.)
Tables
Radionuclide Half-life Principal x- or γ-ray energy Surrogate for… Common uses 137Cs 30 y 662 keV Dose calibrator constancy, accuracy Scintillation detector calibration, energy resolution 68Ge 287 d 511 keV (from 68Ga daughter) Positron-emitting radionuclides Well counter constancy, accuracy Dose calibrator constancy 133Ba 10.7 y 356 keV 131I Dose calibrator constancy Scintillation detector efficiency 57Co 272 d 122 keV 99mTc Dose calibrator constancy Scintillation detector energy resolution Semiconductor probe constancy Adapted from Zanzonico (8).
Determination of… Tube insert Reading (mCi*) Calibration factor Corrected reading Percent deviation Calibration factors Black only (source holder) 209 1.0000 Black and red 120.30 1.737 Black and orange 66.50 3.142 Black and yellow 19.40 10.773 Black and green 5.59 37.388 Black and blue 1.89 110.582 Black and purple 0.45 465.517 Dose calibrator linearity† Black only 124.5 1.0000 124.5 Black and red 71.40 1.737 124.0 −0.24% Black and orange 40.10 3.142 126.0 1.01% Black and yellow 11.50 10.773 123.9 −0.49% Black and green 3.31 37.388 123.8 −0.60% Black and blue 1.11 110.582 122.7 −1.40% Black and purple 0.26 465.517 122.7 −1.45% Mean 123.9 −0.49% Type of use Pitfall Routine Source not centered in dipper Contamination of dipper or liner Incorrect isotope button selected Extraneous source affecting reading High voltage not as needed for correct operation Negative reading due to rezeroing when contamination is present Not enough time allowed for reading of low-activity source QC Required QC testing not performed in regulatory time frames Inconsistent geometry for QC testing Incorrect correction factors (linearity testing) Incorrect source information (constancy, accuracy testing) Type of use Pitfall Routine Battery power insufficient for measurement Insufficient time allowed for reading to settle Cable wires or thin end-window broken Covering of thin end-window not removed for particulate radiation measurement Unit contaminated with radioactivity High radiation source causing dead time Unit not close enough to source (e.g., for contamination survey) Shielding material between source and detector QC Geometric configuration for operational check not according to protocol Not calibrated within regulatory time frame Type of use Pitfall Routine Geometry of measurement not according to protocol Calibration performed incorrectly Incorrect energy window chosen Extraneous radiation source present (patient, contamination, etc.) Background not subtracted Source activity too high; measurement incurs dead time Efficiency factor not applied when needed Incorrect detector chosen (well counter vs. thyroid probe) QC Daily calibration not performed or performed incorrectly Geometry of constancy measurement not according to protocol χ2 test affected by extraneous sources of radiation Efficiency factor out of date or not determined according to protocol Current source activity not calculated correctly for efficiency factor determination Energy window incorrect for given QC test Type of use Pitfall Routine Incorrect energy window chosen Audible signal level not adjusted correctly Directionality of probe not considered Probe not properly cleaned and sterilized QC Daily constancy test not performed with correct energy window Battery power insufficient for probe operation
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