Key words: x-ray system inspection, instrument calibration, laboratory accreditation, radiation measurement, kVp meters
One of OST's main service functions is providing the OC with the calibrated instruments that are necessary to enforce compliance with mandatory radiation safety standards for ionizing-radiation-emitting electronic products, including diagnostic x-ray systems. A crucial element in any compliance program is to ensure that inspectors making field measurements have properly calibrated instruments. The entire calibration probes must be defensible, if the inspector's measurements are challenged in court. The quality manuals, quality control procedures, uncertainty analysis, and on-site evaluations provide assurance that routine calibrations meet the accuracy goals required for a successful and defensible compliance program. In addition to providing calibrated x-ray instruments, OST also provides technical guidance to field inspectors on radiation measurement issues, provides expendable supplies to inspectors, orders replacement instruments, and maintains an inventory of all x-ray instruments assigned to State radiation control programs. The high quality of the instruments calibrated is recognized through accreditation of the OST calibration activity under the Secondary Calibration Laboratories program operated by the National Voluntary Laboratory Accreditation Program (NVLAP). Accreditation is a key element in the credibility and defense of x-ray measurements made by field inspectors.
OST calibrated about 1,200 radiographic probes for measuring diagnostic x-ray beams. Field personnel were kept supplied with expendable items in the Compliance Inspection Kits. Since there are presently no national standards for x-ray beams used in mammography, OST also calibrated about 180 mammography probes in radiographic beams operating at the same voltage as mammographic beams. These beams are the only ones presently available for calibration by the medical physicists doing on-site evaluation of mammographic facilities. Electrical pre-calibration checks and linearity tests were performed on 500 instruments. Of the total probes calibrated, 50% were for FDA-owned instruments, 40% for State-owned instruments, and 10% for other Federal facilities. In addition, OST calibrated 100 noninvasive kVp meters used to verify compliance with the x-ray generator voltage requirement in the performance standard. OST tested nine night vision devices for the Office of Compliance. Figure 15 shows the types in calibrations performed last year and figure 16 shows for whom the instruments were calibrated. [Enf]
Calibrations performed by OST last year, categorized by type.
Internal and external customers of OST=s calibration program. OST=s contribution in this area is crucial to the successes of the Center=s compliance program.
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Key words: mammography, x-ray tubes, ionization chambers, MQSA, free air chamber, instrument calibration
To be able to calibrate instruments that FDA inspectors use to verify compliance with the Mammography Quality Standards Act (MQSA) in x-ray beams comparable to those used in mammography, a new calibration laboratory had to be set up. This is because the x-ray anode material used for mammography (usually molybdenum or rhodium) is different from the anode material used in normal diagnostic procedures (tungsten). This means that the spectrum of the x rays is different and hence the energy response of the mammographic probes (ionization chambers) could be different. At present, NIST does not have U.S. national standards for these beams; so OST has entered into an Interagency Agency Agreement (IAG) with NIST to develop national standards for a limited number of mammographic x-ray beams. When these beams are available, OST will have in-house reference ionization chambers calibrated directly against the U.S. national standards. Also, when these standards are available, the Accredited Dosimetry Calibration Laboratories (ADCL) (accredited by the American Association of Physicists in Medicine) will also have their reference chambers calibrated directly against the national standards. Then, the medical physicists making the yearly on-site evaluation of mammography facilities will be able to have their instruments traceable to the national standards. Until NIST makes this service available, OST is sending its reference ionization chambers to NIST's counterpart in Germany, thePhysikalisch Technische Bundesanstalt (PTB). PTB offers calibrations in a limited number of x-ray beams with molybdenum anodes. OST has also purchased a Free-Air Chamber (FAC) designed for mammographic x-ray beams identical to the one NIST will use as the national standard and which was developed at the University of Wisconsin's ADCL. OST is also participating with this ADCL in a joint study of the response of typical mammography probes in both our molybdenum and rhodium beams. This effort also provided the initial calibration factors of the monitor used to measure the exposure rate from these x-ray tubes.
OST has the responsibility of supplying inspection kits for the inspectors verifying compliance with MQSA. To use existing resources, only mammography ionization chambers (i.e., probes) were purchased rather than complete x-ray monitors. These are being used with the x-ray instruments currently employed for the existing x-ray compliance program. About 180 complete test kits were shipped to certified MQSA inspectors. OST coordinated the policy decision between the Office of Health and Industry Programs (OHIP), OC, and the Office of Regulatory Affairs (ORA) on how the existing pool of exposure-rate measuring instruments should be assigned to the States to meet the needs of both regulatory programs.
Comparison of correction factors measured with the transmission chamber and the corresponding values from PTB x-ray tubes with both molybdenum and rhodium anodes have been made. Based on OST experience, NIST has purchased similar x-ray tubes for developing their reference x-ray beams. They are water-cooled, so they are capable of continuous operation without damage to the tubes. However, they do not have the high-intensity, short-pulse duration found in mammography units, so a rotating anode tube has been ordered.
A joint study with the University of Wisconsin's ADCL on the response of most commercially available mammography probes has been completed. The study was conducted in OST's new x-ray instrument calibration facility mammography beams using the university's FAC. Using the preliminary calibration factors for the transmission monitor obtained in this study, OST also investigated the response of ionization chambers as potential in-house reference instruments. The response of these chambers as a function of a number of variables was completed. This study includes the inter-comparison of preliminary OST results on measured calibration factors with the results from PTB for similar beams. Figure 17 shows that there was strong agreement.
This illustration depicts the intercomparison of correction factors measured with the transmission monitor and the corresponding value from the PTB.
All of the other calibration hardware for the mammography calibration facility has been built and installed. All of it can be operated remotely from the laboratory control computer. The Free Air Chamber (FAC) has been installed and aligned, so that the x-ray beam passes through its center and is aligned on the center of the axis of the calibration bench. The following figures show a schematic view of the FAC (figures 18a and 18b). Preliminary measurements have been made with the chamber. A contract has been established for a physicist to fully characterize the performance of the FAC for use as an absolute standard for evaluating mammography beams.
A schematic view of the University of Wisconsin's Free Air Chamber.
A secondary calibration laboratory for diagnostic x-ray instruments must have the ability to calibrate them in two quite different kinds of x-ray fields. The first are similar to the ones NIST established as its reference beams. These are typical of the x-ray fields used in radiation therapy, namely relatively low intensity and relatively long exposures, and they are the type that NIST is establishing for their new mammography reference beams. This is satisfactory for calibrating the reference instruments that the secondary calibration laboratory uses to establish the exposure rate from its reference x-ray beams. However, in mammographic procedures, x-ray exposures are typically relatively high intensity and short duration. So, the secondary calibration laboratory must also have the capability of calibrating mammographic instruments in this second type of x-ray beam. To be able to study the response of x-ray instruments in high intensity, short duration x-ray beams, a new rotating molybdenum-anode x-ray tube and x-ray generator were ordered and delivered. This system will now have the added advantage of it being possible to develop a procedure for testing and calibrating the x-ray duration timing circuitry in mammography x-ray instruments. [Enf]
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Key words: calibration, computer control, automation, real-time system, quality control
The calibration process for radiation-measuring instruments used for FDA compliance programs is sufficiently complex to require automating the process to reduce the chance of error. The existing computer partially controlling the calibration process uses interface hardware that is no longer supported by the manufacturer, so it must be replaced. The lack of hardware support means that later updates of the operating system can not be implemented: the current unit is using systems that are 10 years out of date. The new computer control system being implemented uses a modern computer system designed for industrial process control. The benefit is that equipment interface cards, specifically designed for it, are available thus eliminating much of the need for custom engineering to implement the system. The software system selected facilitates graphic interfaces to be designed, in-house, specifically to fulfill OST's particular needs. It also uses a real-time spreadsheet for controlling the equipment and analyzing the data. The upgraded calibration laboratory computer system will provide a number of significant benefits, including:
All calibration hardware (instrument position table, filter wheel, x-ray tube position plate, environmental probes, electrometers, safety and timing shutter, and x-ray generator) now can be controlled from the computer. There was a major modification of the calibration software to increase efficiency and streamline operations (by reorganizing the modular pages and their content in a more logical sequence). The real-time spreadsheet pages for controlling the electrometers were revised for automatic data collection depending on operator-selected conditions at the time of calibration. An enhancement was added to set up the major pieces of calibration hardware by simply selecting a beam code from a list and then setting the hardware according to the parameters for that code. For several of the windows, the graphical interface was modified to accommodate new requirements and to accelerate their display. A new calibration method was incorporated based on a substitution method, where the x-ray field is first measured with a reference ion chamber and then with a test instrument, thereby allowing the test instrument correction factor to be directly measured.
The other major section of the new calibration system is the implementation of a new database system to keep track of field and laboratory inventory, generate shipping labels, perform routine procedures for controlling the flow of the instrument at the different stages in the calibration process, generate reports on instruments due to be recalled for calibration, provide summary reports on the calibration process, and provide input variables to the calibration computer code on instrument specific information. The network connecting the new laboratory control computer system (GESPAC) and the existing control system (VAX) to PCs is now operational. The address database has been transferred from the VAX to a PC. A relational database has been written, duplicate addresses eliminated, and a new code nomenclature for identifying users, instrument owners, and shipping destinations has been developed. Another database has been developed for recording, ordering, and transferring the calibration parameters to the computer containing the calibration code. A third database has been developed to keep track of the specific instruments assigned to each MQSA inspector. [Enf]
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Key words: radiation measurement, MQSA, NVLAP, calibration laboratory
The best method of assuring that calibrations for radiation measuring instruments are traceable to national standards is for the calibration laboratory to be accredited by a nationally recognized program. In the United States, there are programs for laboratories in the private, State, and Federal sectors. The CDRH X-ray Calibration Laboratory is accredited by the National Voluntary Laboratory Accreditation Program (NVLAP) for x-ray calibrations of diagnostic instruments, survey instruments, and reference instruments. NVLAP is operated by NIST. One of the NVLAP programs is for Secondary Calibration Laboratories, i.e., those laboratories that have their reference standards calibrated directly against the primary national standards maintained by NIST and that meet other criteria specified in the accreditation criteria. The CDRH X-ray Calibration Laboratory was the first laboratory accredited by NVLAP for calibrations under this program. All of these accreditation programs require that the laboratories have quality manuals, an on-site evaluation, and pass a periodic proficiency test. These measures provide assurance that the calibrations made by the lab are defensible and of the highest quality.
There are four separate laboratory accreditation activities for which OST provides national leadership:
First, the CDRH laboratory continues to be accredited by NVLAP's program for Secondary Calibration Laboratories. OST participated in a proficiency test with NIST in which the Calibration Laboratory calibrated a transfer standard sent by NIST. The difference of the OST measurement from the national standard based on the results of the transfer instrument calibration for three x-ray beams is 0.0 percent, 0.6 percent, and 0.1 percent, well within the OST accreditation criteria of 5.0 percent. OST is working with NVLAP, so that NVLAP will develop accreditation criteria for calibration of diagnostic instruments in mammography beams. OST is revising its Quality Manual, which governs all aspects of the Calibration Laboratory, to make it comparable with the requirements in ISO Guide 25, "General Requirements for the Competence of Calibration and Testing Laboratories," which was recently incorporated into the NVLAP accreditation criteria. OST also has an interagency agreement instituted and funded by NIST to obtain an FDA technical expert for the NVLAP accreditation program.
Second, OST also provides a liaison to the American Association of Physicists in Medicine Subcommittee on Accredited Dosimetry Calibration Laboratories (ADCL). The ADCLs are primarily concerned with calibration of the ionization chambers medical physicists use for measuring the output of machines utilized for radiation therapy. However, they will also calibrate the instruments that the medical physicists will use for their yearly on-site evaluation of mammography facilities. One concern is to ensure that the types of radiation fields employed to calibrate the instruments that FDA inspectors and medical physicists use are comparable, so that instrument calibration issues will not arise during the enforcement of MQSA. OST is providing input to a working group developing new criteria for the ADCL subcommittee based on ISO Guide 25.
Third, OST provides a technical expert to the Health Physics Society's (HPS) Laboratory Accreditation-Assessment Committee. The HPS accreditation program is primarily interested in the accreditation of private sector institutions calibrating radiation protection instruments. However, the HPS also has accreditation criteria for diagnostic instruments, and firms manufacturing and repairing mammographic diagnostic instruments may wish to become accredited. For comparability of the programs, it is important that FDA remain aware of what is happening in these programs.
The fourth and final program is the accreditation program of the Conference of Radiation Control Program Directors (CRCPD). The CRCPD's Radiation Measurements Committee has developed accreditation criteria for State-operated laboratories. These laboratories calibrate instruments for enforcement of State and local radiation control programs. OST provides a resource person to the Radiation Measurements Committee to assist the States on issues involving radiation measurements for instrumentation used in their radiation compliance programs. OST provided a member of the assessment team that reevaluated the CRCPD accredited lab operated by the State of California. [Enf, Stds]
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