REVIEW:
Nuclear medicine is critically dependent on the accurate, reproducible performance of clinical radionuclide counting and imaging instrumentation. Quality control (QC), which may be defined as an established set of ongoing measurements and analyses designed to ensure that the performance of a procedure or instrument is within a predefined acceptable range, is thus a critical component of routine nuclear medicine practice. An extensive series of parameters has been developed over the years for acceptance testing and performance characterization of γ-cameras, SPECT and PET scanners, and other nuclear medicine instrumentation. And detailed data acquisition and analysis protocols for this purpose have been promulgated by the National Electrical Manufacturers Association (NEMA), the American Association of Physicists in Medicine (AAPM), and other regulatory, advisory, and professional organizations (1–9). In practice, however, less time-consuming and less rigorous procedures often suffice for day-to-day QC. The current article is a brief overview of such routine QC procedures for current nuclear medicine instrumentation, including the survey meter, dose calibrator, well counter, intraoperative probe, organ (“thyroid”) uptake probe, γ-camera, SPECT and SPECT/CT scanner, and PET and PET/CT scanner. The far more rigorous and more extensive acceptance-testing procedures performed for γ-cameras, SPECT and SPECT/CT scanners, and PET and PET/CT scanners are beyond the scope of this article, which is not intended to supersede or replace manufacturer-recommended acceptance-testing, QC, and preventive-maintenance procedures.
Nuclear medicine is critically dependent on the accurate, reproducible performance of clinical radionuclide counting and imaging instrumentation. Quality control (QC), which may be defined as an established set of ongoing measurements and analyses designed to ensure that the performance of a procedure or instrument is within a predefined acceptable range, is thus a critical component of routine nuclear medicine practice. An extensive series of parameters has been developed over the years for acceptance testing and performance characterization of γ-cameras, SPECT and PET scanners, and other nuclear medicine instrumentation. And detailed data acquisition and analysis protocols for this purpose have been promulgated by the National Electrical Manufacturers Association (NEMA), the American Association of Physicists in Medicine (AAPM), and other regulatory, advisory, and professional organizations (1–9). In practice, however, less time-consuming and less rigorous procedures often suffice for day-to-day QC. The current article is a brief overview of such routine QC procedures for current nuclear medicine instrumentation, including the survey meter, dose calibrator, well counter, intraoperative probe, organ (“thyroid”) uptake probe, γ-camera, SPECT and SPECT/CT scanner, and PET and PET/CT scanner. The far more rigorous and more extensive acceptance-testing procedures performed for γ-cameras, SPECT and SPECT/CT scanners, and PET and PET/CT scanners are beyond the scope of this article, which is not intended to supersede or replace manufacturer-recommended acceptance-testing, QC, and preventive-maintenance procedures.
SAFETY AND ELECTROMECHANICAL INSPECTION
For those nuclear medicine instruments that “interface” directly with patients—the intraoperative probe, organ uptake probe, γ-camera, SPECT and SPECT/CT scanner, and PET and PET/CT scanner—safety features should be regularly inspected. Such features include manual emergency-off switches (“panic buttons”), collision-detection switches that immediately stop all motion if a collision occurs (e.g., between the rotating γ-camera detector and the patient during a SPECT acquisition), and interlocks that immediately turn off the x-ray tube of a SPECT/CT or PET/CT scanner if a primary-barrier door is opened during a CT scan. All position displays on the gantry and computer console and all alignment lasers should likewise be visually inspected. All manual motion-control functions (e.g., gantry rotation, detector radial motion, and table translation) should be checked as well. Finally, as with all electromechanical devices, intraoperative probes, organ uptake probes, γ-cameras, SPECT and SPECT/CT scanners, and PET and PET/CT scanners should be inspected regularly for frayed wires and broken or otherwise damaged electrical insulation, loose electrical or mechanical connections (including missing or visibly loose screws, nuts, or bolts), and dents, sharp edges, or other physical damage.
Record Keeping
Documenting of QC procedures and organized recording of the results of such procedures are requirements of a sound, compliant QC program. A written description of all QC procedures, including the acceptable (or tolerance) range of the results of each such procedure and the corrective action for an out-of-tolerance result, should be included in the procedure manual of the facility; for each procedure, the written description should be signed and dated by the facility director, physicist, or other responsible individual. For each QC test performed, the following data, as well as the initials or signature of the individual performing the test, should be recorded on a structured and suitably annotated form: the test performed; the date and time of the test; the make, model, and serial number of the device tested; the make, model, and serial number of any reference sources used; the results of the test; and a notation indicating if the test result was or was not acceptable (i.e., was or was not within the specified tolerance range). Such records should be archived in chronologic order in a secure but reasonably accessible location. It is generally helpful to track the results of QC tests longitudinally (e.g., in the form of a graph of the numeric result vs. date of the test). In this way, long-term trends in instrument performance, often imperceptible from one day to the next, may become apparent. Increasingly, of course, such records are maintained in electronic form (i.e., are computerized). In many jurisdictions, records must still be maintained in hard-copy form—and it is advisable to do so in any case, both as a backup and for convenient review by regulators and other inspectors.
Reference Sources
In many instances, QC tests of nuclear medicine instrumentation are performed not with the radionuclides that are used clinically but with longer-lived surrogate radionuclides in the form of so-called reference sources. Such standards are commercially available in various activities and geometries, depending on the application. Importantly, the certified activities of such reference sources must be traceable to the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards. NIST traceability helps ensure the accuracy of the calibrated activity. Because such reference sources are long-lived, a single standard may be used for months to years, avoiding the need to prepare sources on a daily or weekly basis and thereby avoiding possible inaccuracies in dispensing activity as well as the possibility of radioactive contamination. As with all sealed sources, however, reference sources must be checked for leakage of radioactivity (i.e., wipe-tested) at least annually, and an up-to-date inventory of such standards must be maintained. Reference sources are still radioactive at the end of their useful life span and must therefore be returned to the vendor or an authorized third party or otherwise disposed of as radioactive waste.
A long-lived radionuclide comprising a reference source must match, in the frequency and energy of its x- and γ-ray emissions, the clinical radionuclide for which it acts as a surrogate to ensure that instrument responses to the clinical radionuclide and to its surrogate are comparable. Surrogate radionuclides commonly used in nuclear medicine and their physical properties and applications are summarized in Table 1.
View this table:
TABLE 1
Long-Lived Radionuclides Comprising Reference Sources for Instrumentation QC
Survey Meters
Survey meters, an essential component of any radiation safety program, are portable, battery-operated gas-filled ionization detectors or solid-state scintillation detectors used to monitor ambient radiation levels, that is, exposure rates (e.g., in mR/h) or counting rates (e.g., in counts per minute [cpm]). Among ionization-detector survey meters, so-called cutie-pies are relatively low-sensitivity ionization chambers (i.e., are operated at a relatively low potential difference between the anode and cathode) and are designed for use where high fluxes of x- and γ-rays are encountered. The more familiar Geiger counters (or Geiger-Mueller counters) are operated at a high potential difference, providing a high electron amplification factor and thus high sensitivity. Geiger counters are therefore well suited for low-level surveys, for example, checking for radioactive contamination. Generally, cutie-pies are calibrated in exposure rates (mR/h) and Geiger counters in counting rates (cpm). As an ionization chamber, the cutie-pie has an electron signal that depends on the energy of the detected x- or γ-rays and is therefore directly related to the exposure for all radionuclides. For Geiger counters, the amplitude of the signal pulses is independent of the energy of the incoming radiation. Therefore, if calibrated for exposure rates (mR/h), Geiger-counter calibration results apply only to the particular radionuclides comprising the reference source used in the calibration procedure. Solid-state detectors use a non–air-equivalent crystal as the detection medium and thus cannot measure exposure rates; they can measure only counting rates.
QC tests of survey meters generally include a daily battery check, with a display indicating whether the voltage supplied by the battery is within the acceptable operating range. To confirm that the survey meter has not been contaminated (i.e., yields a reproducibly low exposure or counting rate in the absence of radioactivity), the background exposure or counting rate should be measured daily in an area remote from radioactive sources within the nuclear medicine facility, if such an area is reasonably accessible. In addition, survey meters should be checked daily for constancy of response by measuring the exposure or counting rate of a long-lived reference source in a reproducible measurement geometry. Aside from the short-term decay of the reference source, the measured day-to-day exposure or counting rates should agree within 10%; if not, the meter should be recalibrated.
Survey meters should be calibrated—that is, checked for accuracy—using suitable long-lived reference sources at installation, annually, and after any repair. If the source is small (compared with the mean free path of its emitted x- and γ-rays within the material comprising the source) and the distance between the source and meter large (compared with the dimensions of the source), then a point-source geometry is approximated and the expected exposure rate (in mR/h), 
, in air is given by the inverse-square law:
Eq. 1where Ao is the activity (in MBq) of the reference source at calibration, λ is the physical decay constant (in /d) of the radionuclide comprising the reference source, Δt is the time interval (in d) between the calibration of the reference source and the current measurement, Γ is the specific γ-ray constant (in mR/h/cm2/MBq) of the radionuclide comprising the reference source, and d is the distance (in cm) between the reference source and the meter. The reference-source activity should be sufficient to yield an exposure rate of ∼1,000 mR/h under the foregoing measurement conditions, and the exposure rates should be measured on each scale and, by appropriate adjustment of the source–meter distance, at 2 readings (∼20% and ∼80% of the maximum) on each scale. For all readings, the expected and measured exposure rates should agree within 10%.
, in air is given by the inverse-square law:Eq. 1where Ao is the activity (in MBq) of the reference source at calibration, λ is the physical decay constant (in /d) of the radionuclide comprising the reference source, Δt is the time interval (in d) between the calibration of the reference source and the current measurement, Γ is the specific γ-ray constant (in mR/h/cm2/MBq) of the radionuclide comprising the reference source, and d is the distance (in cm) between the reference source and the meter. The reference-source activity should be sufficient to yield an exposure rate of ∼1,000 mR/h under the foregoing measurement conditions, and the exposure rates should be measured on each scale and, by appropriate adjustment of the source–meter distance, at 2 readings (∼20% and ∼80% of the maximum) on each scale. For all readings, the expected and measured exposure rates should agree within 10%.
Many nuclear medicine facilities have their survey meters calibrated by the institutional radiation safety office or by a commercial calibration laboratory. In addition to a calibration report (typically a 1-page document) specifying the reference sources used, the measurement procedure, and the measured and expected exposure rates, a dated sticker summarizing the calibration results should be affixed to the meter itself.
Dose Calibrators
The dose calibrator is a pressurized gas-filled ionization chamber for assaying activities in radiopharmaceutical vials and syringes and in other small sources. Among routine dose-calibrator QC tests, constancy must be checked daily and accuracy and linearity at least quarterly (7,18,19); daily checks of accuracy are recommended, however. At installation and after service of a dose calibrator, its geometry (position and volume)-dependent response for 99mTc must be measured and volume (from 2 to 25 mL)-dependent correction factors relative to the standard volume (e.g., 10 mL) derived.
For the constancy test, a reference source, such as 57Co, 133Ba, 68Ge, or 137Cs (Table 1), is placed in the dose calibrator, and the activity reading on each scale is recorded; day-to-day readings should agree within 10%. For the accuracy test (also sometimes known as the energy linearity test), NIST-traceable reference sources of at least 2 of the radioisotopes listed in Table 1are separately placed in the dose calibrator and the activity reading on each scale recorded. For each source, the measured activity on each scale and its current actual activity should agree within 10%.
For the quarterly check of linearity by the so-called decay method, one begins with a high-activity (∼37 GBq), independently calibrated 99mTc source and assays its activity at 12-h intervals over 3 consecutive days. Over that time, which is equivalent to 12 half-lives of 99mTc, the activity decays to about 11 MBq. The measured activities are then plotted versus time on a semilogarithmic graph, and the best-fit straight line drawn through the data points is plotted (either by eye or by using a least-squares curve-fitting algorithm). For each data point, the difference between the measured activity and the activity on the best-fit straight line at that point should be less than 10%. An alternative approach to checking linearity is the so-called shield method, in which lead sleeves of increasing thickness are placed in the dose calibrator with a 99mTc source (Fig. 1). By interposing increasing decay-equivalent thicknesses (as specified by the manufacturer of the set of lead sleeves) between the source and the sensitive volume of the dose calibrator, a decay-equivalent activity is measured for each sleeve. Although the shield method is much faster than the decay method for checking linearity (taking minutes instead of days), an initial decay-based calibration of the set of sleeves is recommended to accurately determine the actual decay equivalence of each shield.
