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Key words: radiation measurement, free-air chamber, half-value layer
To be able to verify the correct operation of an instrument used for Mammography Quality Standards Act (MQSA) inspections, it is necessary to know its operating characteristic when too little or too much filtration is present in the mammography unit. There are, however, only national standards for mammography beams for the normal amount of added filtration, 30 µm of molybdenum and 25 µm of rhodium. To be able to determine the response for the abnormal case, it was necessary to implement a free-air chamber (FAC) at our laboratory. Thus, the research efforts of RMB have focused on implementing an Attix Variable-Length Free-Air Chamber designed specifically for x-ray beams in the mammography region. This instrument is capable of making absolute measurements of exposure rates after certain corrections are applied. The basic principle of an FAC is to measure the charge collected from a known volume of air exposed to an x-ray beam. In an Attix Variable-Length FAC (see figure 19), the area of the known volume is defined by an aperture and the length is defined by a rod that moves one plate of the chamber with respect to the other for an accurately known distance. Measurements are made for different lengths of collecting volume and the results subtracted to eliminate the effect due to fringe electric fields. Knowing the charge and volume, there are a number of small corrections that need to be made before the exposure rate can be determined. The most important of these corrections is for air attenuation between the defining aperture and the center of the collecting volume. The Attix Variable-Length FAC was specifically designed to measure this correction by having a second positioning rod to precisely control the position of the collecting volume from the defining aperture. This correction is up to 7% for mammography beams but can be accurately measured. Other corrections are for scatter in the chamber (about 0.2%) and ion recombination (up to 0.8%) for lightly filtered beams with the x-ray generator operated at maximum mA).
To demonstrate that FDA was making correct measurements with its FAC, calibrations were made on a reference ion chamber (Exradin A11TW) which had preciously been calibrated at the National Institute of Standards and Technology (NIST) in their newly established mammography beams and with the German national mammography beams at the Physikalisch Technische Bundesanstalt (PTB). The results are shown in figure 20. In each case the calibration factor was determined with the institutions own free air chamber. The difference of each point for the average value for the beams available at that institution. For most beams, the spread is within ±1/3% is plotted.
During calibrations, the stability of the x-ray beam is measured with transmission monitor. The calibration factor of the transmission monitor as a function of HVL is given in figure 21. The results are shown for the entrance beams and exit beams (ending in "x") for both molybdenum and rhodium anode x-ray tubes. The transmission monitor calibration factor was determined with the FAC. The calibration factor for beams with different anode and added filter material and different operating potentials falls on a straight line with the possible exception of the MoRh beam (molybdenum anode, rhodium added filter) which is about 1% higher. It is hoped that the transmission monitor will be able to be used as the reference ion chamber in a "simultaneous" method of ionization chamber calibration, i.e., the transmission monitor is used as the reference for measuring the exposure rate, and the test instrument's reading is taken at the same time. In this method, one would do a daily quality control check to the transmission monitor calibration factors and then use some "historical" value, i.e., the FAC-determined calibration factor, for the actual calibration. There are two concerns:
To investigate both effects, figure 22 shows the ratio the calibration factor determined with the FAC (e.g., the "historic" value) and the calibration factor determined using the NIST-calibrated reference ion chamber used for each test instrument calibration between May and October of this year. Several things are evident:
One of the oddities in the measurements is that for the entrance beams at 25, 30, and 35 kV, the HVLs agree well with the values from NIST and Wisconsin, all of which are about 4% higher than PTB. However, for our exit beams, the other three institutions agree well and our exit beams are up to 5% low at 35 kV. Figure 23 shows the HVL values for the MoMo exit beams for various kV. The exit beams are the normal entrance beams plus an additional 2 mm of aluminum to simulate the spectra exiting the breast. CDRH has three sets of entrance beams with added filter thickness 16, 33 and 42 µm of Mo. Additionally, there is a beam of 33 µm of Mo plus a 3-mm Lucite "paddle." The NIST and Wisconsin entrance beams are 32 µm of Mo, while PTB specifies its entrance beam as 0.03 mm of Mo. Note for the exit beams, the HVL decreases as the Mo thickness increases. This is opposite to the entrance beam. In addition to the normal 2 mm of aluminum, another set of measurements were made with a piece of aluminum 2.2 mm thick in the beam to produce exit beam conditions. These results agree much better with other laboratory's exit beam results. A linear regression was fit to each family of added filter thickness. Then the constant and linear terms of the resulting regression analysis were fit giving the following equation for HVL as a function of kV and added molybdenum filter thickness, t:
HVL = 0.0797 + 0.00320t + 0.0207V - 0.00017t*V
Where t is in µm and V is in kV.
For this fit, the coefficient of variation of the measured and fit values was ±0.5%. [Enf]
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Key words: dosemeters, calibration, standards, diagnostic x-ray
The standardization activities in ionizing radiation fall into three areas: laboratory accreditation efforts, work with the Council on Ionizing Radiation Measurements and Standards (CIRMS) in developing and implementing Measurement Program Descriptions (MPDs) for programs of national importance in metrology, and work in developing international standards on the performance and testing of radiation measuring instruments.
There were two major thrusts in the laboratory accreditation area. First, the OST laboratory continues to be accredited by the National Voluntary Laboratory Accreditation Program (NVLAP) for x-ray calibrations of diagnostic instruments, survey instruments, and reference instruments. Before the last on-site inspection of the laboratory, NVLAP's accreditation criteria was changed to include all the requirements in ISO/IEC Guide 25, "General Requirements for the Competence of Testing and Calibration Laboratories." The recommendations of the on-site team to bring the laboratory into complete compliance with the new accreditation criteria have been implemented. ISO Guide 25 is presently being revised to make it clearer that a laboratory meeting its criteria also meets the criteria of ISO Guide 9000. The second thrust is developing new criteria for laboratories calibrating diagnostic instruments explicitly including mammography instruments. At present, neither the NVLAP accreditation criteria nor the accreditation criteria of the American Association of Physicists in Medicine have requirements specifically related to the calibration of mammography instruments. OST is actively participating in the development of these criteria to help ensure that laboratories calibrating the instruments which medical physicists will use in their evaluations of mammography facilities will be adequate.
OST continues to provide the chair for the Medical Subcommittee of CIRMS. This subcommittee has developed four Measurement Program Descriptors (MPDs) in the area of radiation therapy, nuclear medicine and diagnostic radiology. These MPDs and those from the other three subcommittees were published in a report "National Needs in Ionizing Radiation Measurements." One of the original MPD is now completed: the national air-kerma standards for mammography. The effort in completing this MPD resulted from the successful collaboration between CDRH, NIST, and the University of Wisconsin's Accredited Dosimetry Calibration Laboratory. The use of these MPDs to help prioritize national needs in ionizing radiation metrology was demonstrated when the Director of NIST's Physics Laboratory gave testimony before the House Technology Subcommittee citing their importance in establishing internal priorities. The medical subcommittee is proposing three new MPDs. Of particular interest to FDA are the ones to develop
OST also provides a member to IEC 62C WG3, Performance of Dosemeters. This working group has completed the following draft international standards: "Medical electrical equipment Dosemeters with ionization chambers as used in radiotherapy" and "Medical electrical equipment Dosemeters with ionization chambers and/or semi-conductor connectors as used in radiography including mammography and fluoroscopy." These documents are awaiting their translation into French for a final vote. Each of these standards includes performance requirements and its associated test procedures for their respective dosimeters. The working group is currently working on a standard for "Medical electrical equipment - dosimetric instruments for noninvasive measurement of x-ray tube voltage in diagnostic radiology." It is anticipated that there will be a committee draft of this standard by the end of 1996. [Stds]
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Key words: laboratory, automation, computer control, graphical interface
The CDRH X-ray Calibration Laboratory uses computers in all aspects of the calibration process. During the past year there has been a three-part effort to upgrade computer systems and application software, to develop new databases and procedures for inventory control, and to implement a local area network to transfer data between the various Radiation Metrology Branch (RMB) computers.
The facilities for calibrating radiographic instruments presently are controlled by a VAX 750 computer. The interface to some of the computer-controlled calibration equipment is a parallel interface which the computer manufacturer no longer supports. The result is that upgrades to the VAX system are no longer possible, and the existing hardware is breaking down with increasing frequency. With the development of the new laboratory for calibrating mammography instruments, the decision was made to use a new computer system for the control of the calibration equipment and for the gathering and analysis of the data. This decision meant that it would be necessary to develop new database programs and procedures independent of those on the VAX. To be able to transfer data to/from the new databases to Branch PCs and to the calibration computer, a local area network was developed within the Branch.
The advantages of the new system include:
The new computer system for controlling the mammography facility is an industrial processor, a GESPAC computer. One major advantage of this system is that all of the interface cards necessary for controlling the calibration equipment were available. This greatly cut down the time needed to engineer the interfaces with only custom controls needed for some of the hardware; then these controls would interface directly to the cards in the GESPAC. The initial effort in developing the equipment control and data analysis software was to develop a system that allowed the operator to fully control all aspects of the calibration process. This software system is referred to as the "manual" version. The software for the control and analysis is done in a real-time spreadsheet, ControlCalc. The communication between the operator and the computer is via a graphic interface (G-Windows). Figure 24 shows the graphic interface for the control panel. The manual version of the code is fully operational and has been used for calibrating mammography probes since May 1996. For routine calibrations, many of the operations which are now done manually could be included in automated sequences. This would enhance the quality and reliability of calibrations and the productivity of the laboratory staff. Thus, efforts are under way to automate potions of the manual version of the software. The laboratory has ordered two x-ray generators to replace the existing ones used for radiographic calibrations. The existing manual version of the code can be used to automate the new calibration system with changes only to those pages involving the control of the x-ray generator. <'p>
CDRH must maintain the inventory of calibrated instruments for the Federal, State, and local governments. The database application program, Paradox, will be used to generate applications for database management of the calibration data and inventory of the equipment that is calibrated by the CDRH X-ray Calibration Laboratory. Applications are being developed to create a consistent and user-friendly environment for the end user (calibration technician), while at the same time providing the maximum amount of flexibility in the use of information and data used. Data includes both historic and future information related to all aspects of the customer and CDRH calibration equipment. For example, pertinent information for equipment that is used by Federal and State field inspectors in enforcing compliance with radiation safety standards include the following:
Computers in the Radiation Metrology Branch (RMB) in OST are connected to one another using a local area network (LAN) and the Microsoft Windows NT network operation system. All systems needing to exchange data with the server or other connected systems are equipped with Digital Pathworks. This provides the means for connection to Center computer resources. The drawing in figure 25 shows a representation of the network.
Data obtained from calibration instruments is formatted and stored on the server computer, RMB_MQSA. Users can generate reports from this data using their own connected PCs or software that resides on a second server, SERVER2. Other CDRH components can also view the data. This network arrangement permits Center access to the data as it is developed in the RMB laboratories.
An RMB WEB-site is under development using the WEB-site development software that is shipped with the Windows NT operating system. Operation manuals for the calibration laboratory are being developed for viewing using a WEB browser such as Netscape or Explorer.
Reliability of the system is enhanced with this network operating system, because failure of the main server, RMB_MQSA, will cause the second server, SERVER2, to continue with server tasks without interruption. [Enf]
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Key words: inspection calibration, radiation measurement
The Radiation Metrology Branch (RMB) in OST continues to provide support to FDA field inspection programs by calibrating radiation measuring instruments. In FY 96, a total of 1,556 radiation-sensitive devices were calibrated by exposure to known x-ray fields. In addition, 553 electrical calibrations of associated read-out instruments were performed.
Most of the instruments calibrated are used in the enforcement of standards promulgated under the Radiation Control for Health and Safety Act and the Mammography Quality Standards Act. These instruments are owned either by FDA or by States contracted by FDA to do inspections. Of the remaining calibrations, some calibrations are done for other Federal agencies under interagency agreements; others are part of research projects.
Instruments calibrated include:
Figures 26, 27, and 28 show the breakdown of all the calibrations performed.
On May 15, 1996, routine calibrations began on a new mammography calibration range. This facility was added to the calibration laboratory in order to provide x-ray spectra with energies identical to the x-ray beams used in mammographic examinations. Of the 370 calibrations completed for the MQSA activities, 135 were done using the new range. Efforts are under way to automate the mammography facility further. This will enable CDRH to perform all its mammography calibrations on this range. The mammography facility is illustrated in figures 29 and 30 below.
The CDRH calibration laboratory continues to be accredited by the National Voluntary Calibration Laboratory Accreditation Program. As required under this program, during FY 96, CDRH successfully participated in a proficiency test administered by the National Institute of Standards and Technology and was inspected by a site evaluation team. Of the 1,556 radiation calibrations performed during the fiscal year, 1,223 fell under the scope of accreditation.
In order to continue to provide a high volume of high quality calibrations, the laboratory is updating the two calibration ranges used for the general radiography calibrations. The two currently used aging x-ray generators present several problems:
For these reasons, two new generators were purchased in FY 96 for installation in FY 97. [Enf]