British Journal of Radiology (2006) 79, 162-164
© 2006 British Institute of Radiology
doi: 10.1259/bjr/17920806
Excessive leakage radiation measured on two mobile X-ray units due to the methodology used by the manufacturer to calculate and specify the required tube shielding
I A Tsalafoutas, PhD
Medical Physics Unit, 'Konstantopoulio-Agia Olga' Hospital, 3-5 Agias Olgas, Nea Ionia, 142 33, Athens, Greece
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Abstract
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During the quality control (QC) procedure of a new mobile X-ray unit, it was revealed that the leakage radiation was well in excess of the current limit of 1 mSv h–1. As a result, this unit was returned to the vendor company and it was replaced by a new unit of the same brand and model. Leakage measurements revealed that the second unit presented the same problem. After consulting the vendor company and the tube manufacturer, it was discovered that the excessive leakage identified in these two X-ray units was not due to a defective construction, but due to the methodology with which the maximum permissible leakage and therefore the tube shielding had been determined. In this study, the implications of using such methods to the radiation protection of personnel and public are discussed.
Case history
Upon the arrival of a new mobile X-ray unit, a thorough quality control (QC) procedure was carried out in order to measure its performance. For leakage testing, the tube head was initially covered by radiographic cassettes and, with the collimator diaphragms completely shut, an exposure was performed with a tube potential of 100 kVp and a tube loading of 10 mAs. After the films were developed, large areas of maximum optical density were identified in all films, except for one film positioned on the top of the tube. By positioning a dosemeter on various points on the front face of the tube and the collimator, it was verified that the leakage was arising from the tube and not the collimator.
In an effort to measure the leakage radiation, a survey meter with measuring range from 0.5 µSv h–1 to 1000 µSv h–1 was initially employed and exposures with a tube potential of 100 kVp and a tube loading of 50 mAs (2 s) were performed. However, even at a distance of 3 m from the tube the leakage radiation was exceeding the maximum measurable dose rate of the instrument.
In order to determine as accurately as possible the actual leakage, a solid state dosemeter with measuring dose range from 20 nGy to 10 Gy andmeasuring dose rate range from 40 nGy s–1 to 185 mGy s–1 was positioned by a nearby wall at a distance 1 m away from the tube. For an identical exposure (100 kVp and 50 mAs (2 s)) the dosemeter reading was 8.2 µGy. By reducing the leakage measured in this single exposure to that assuming continuous operation for 1 h with tube current 5 mA, a value of 2.95 mGy was obtained. This is about three times the current limit for leakage and it would be even larger if the measurements were made at the maximum tube potential of the unit (i.e. 115 kVp).
As a result of these measurements, the mobile X-ray unit was returned to the vendor company, which a few weeks later supplied us with a new unit (the same brand and model) that unfortunately presented the same problem. For the second unit, the leakage was measured with tube potential 110 kVp and tube loading 50 mAs (2.5 s). The dosemeter reading at 1 m from the tube was 12.1 µGy and thus, assuming continuous operation for 1 h with tube current 4 mA, a leakage of 3.5 mGy was derived.
In view of these results, the available certificates and documentation concerning this X-ray unit were scrutinized and an anomaly was apparent. The tube QC certificate stated that the maximum value measured for leakage was 1750 mR h–1 (15.3 mGy h–1), with the limit set at 3.4 R h–1 (30 mGy h–1). Furthermore, the technical specifications section of the operator manual stated that the leakage radiation limit had been defined as 3448 mR h–1 (30 mGy h–1) for a duty cycle of 12 exposures per hour, with a 5 min time interval between exposures. On the other hand, in the collimator certificate (given with the serial number of the specific tube), the maximum leakage (presumably from the collimator alone) had been measured at less than 34 mR h–1 (0.3 mGy h–1) with exposure factors 125 kVp and 4 mA, while the limit has been set at 40 mR h–1 (0.35 mGy h–1).
The methodology used by the tube manufacturer to determine the maximum permissible leakage was as follows. The limit for the leakage radiation in 1 h had been set to 1 mGy (air kerma) and this had been converted to its equivalent of 115 mR. The X-ray unit had been assumed to have a duty cycle of 12 radiographs per hour and thus the maximum leakage for each exposure had been calculated as 115 mR/12 = 9.58 mR (0.084 mGy). By assuming a tube loading of 200 mAs (20 mA x 10 s) per exposure, the manufacturer calculated that the maximum leakage would be (9.58 mR/10 s) x (3600 s h–1) = 3448.8 mR h–1 (30 mGy h–1). In this way, the limits of 3.4 R h–1 and 30 mGy h–1 given in the operator manual were derived. If the value of 3.4 R h–1 (30 mGy h–1) is multiplied by 120 s, that is the cumulative exposure time in 1 h for the duty cycle assumed, the limit of 115 mR (1 mGy) in 1 h is obtained and, therefore, according to the manufacturer the X-ray tube was complying with the relevant norms.
This methodology, although seeming reasonable at first glance, is incorrect. It could lead to significant doses being received by both employees and members of the public, as is illustrated in the following discussion. For a quick overview of the differences between the two methodologies for calculating leakage radiation, the results of the measurements made at the hospital and the factory are summarized in Table 1
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Table 1. The measurement conditions and assumptions used at the hospital and the factory for calculating the leakage radiation. Measured and calculated dose values are for a distance of 1 m from the tube focus
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Discussion
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In the recently published report No. 147 of the NCRP [1] it is stated that "manufacturers are currently required by regulation to limit the leakage radiation to 0.876 mGy h–1 (100 mR h–1) at 1 m. Compliance with this requirement is evaluated using the maximum X-ray tube potential and the maximum beam current at that potential for continuous tube operation". These maximum tube potential and current ratings (kVpmax and Imax, respectively) are usually quoted as leakage technique factors. Imax depends on kVpmax and the values typically assumed for Imax are 3.3 mA, 4 mA and 5 mA for kVpmax of 150 kVp, 125 kVp and ![{els]](/math/els.gif)
100 kVp, respectively [1, 2].
Since in radiation protection a number of dosimetric quantities are simultaneously used and often confused (as it was seen in the manufacturer calculations where a limit of 115 mR instead of 100 mR was used), their relationships should be clarified. An exposure of 1 R corresponds to an air kerma of 8.76 mGy that for shielding calculations is traditionally assumed to result in an absorbed dose in tissues of 10 mGy and an equivalent dose of 10 mSv [1, 3, 4]. Within this context the leakage limit is also given as 1 mSv h–1 [1], in terms of ambient dose equivalent. Thus, while the quantity usually measured with dosemeters is the exposure (in R) or the air kerma (in Gy), the aforementioned correspondence between units is used to convert the measured or theoretically estimated values of exposure and air kerma to equivalent dose (ambient dose equivalent). To find the required shielding thickness or assess the adequacy of the existing shielding, the resulting value of ambient dose equivalent is compared with the respective limits of effective dose for the personnel and public and, in certain cases, with the equivalent dose limits for the skin and the lens of the eye.
To determine the shielding requirements of the given diagnostic tube, the methodology of Tsalafoutas et al [4] was employed, assuming leakage technique factors of 4 mA and 115 kVp and using published data on the X-ray output [2] and the attenuation properties of lead [5]. To conform to the current limit for leakage, lead shielding of 2.07 mm Pb is required, whereas according to the manufacturer-derived limit of 30 mGy h–1 the respective value is only 0.72 mm Pb. According to the measurements made at the hospital, the leakage for the second unit was 3.5 mGy h–1 and thus the shielding of the tube should be about 1.44 mm Pb equivalent.
The implications of the duty cycle concept used by the tube manufacturer to calculate the tube shielding requirements could be made obvious, if one were to initially accept that the leakage limit is 1 mGy h–1 (air kerma) and only 12 exposures of 200 mAs can be realised in 1 h, and then assume that the operator of this mobile unit was performing only these 12 examinations within a ward during a working day, standing at 1 m away from the tube. Under these assumptions and even if the scattered radiation from the patient is ignored, the operator would be exposed to 1 mGy air kerma corresponding to an equivalent dose of about 1.15 mSv. Assuming 22 working days per month and 10 months per year, the cumulative equivalent dose would be 25 mSv per month and 250 mSv per year. It is obvious that these values are too high compared with the annual effective dose limit for occupationally exposed persons (20 mSv) and the annual equivalent dose limits for the lens of the eye and the skin (150 mSv and 500 mSv, respectively).
Good practice requires that the operator should wear a protective lead apron and should be 2 m or more away from the tube or behind a wall, while the tube potential and tube loading routinely used are less than that assumed and therefore the actual dose would be much less. The above simplistic calculations, however, illustrate that the duty cycle concept is by definition dangerous since, except for the operator, one must also take into account the patients on the nearby beds and the other medical staff within the ward or within the nearby unshielded rooms. It is worth also mentioning that since a 400 speed class screen–film combination obtains a net optical density of 1 with about 2.5 µGy, special care would be required to shield the cassettes from leakage radiation when transporting them with that mobile unit.
Since the radiographic and fluoroscopic mobile units used in a fixed location or frequently in the same location may also require structural shielding [1], the implications of the duty cycle concept on the shielding requirements should also be mentioned. Whilst stationary X-ray units are able to operate at higher tube loadings than mobile units, as far as the required structural shielding is concerned there is no essential difference between stationary and mobile units, if the weekly workload, operating potential etc. are the same. This is because when calculating the shielding requirements of a room the weekly workload is assumed in mA min without differentiating if a workload of 300 mA min, for example, will be obtained with 1 h continuous fluoroscopy and 5 mA tube current, intermittent fluoroscopy with tube current 1 mA and cumulative fluoroscopy time of 300 min, or with radiographic exposures of 300 mA and total beam-on time of 1 min made up by many short exposures with duration of a few milliseconds. Thus, it is easily understood that if a tube were shielded according to the duty cycle concept, the structural shielding requirements would be considerably determined by the leakage radiation. Therefore, the tube shielding should be designed for the maximum possible duty cycle, not for a typical duty cycle.
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Conclusion
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Proper shielding of any X-ray tube, using the standard methodology and leakage limit, is mandatory for the radiation protection of the operators, medical personnel, patients and public. Indeed, most tube manufacturers shield their tubes so as to comply with stricter limits than 1 mSv h–1. Tubes with shielding calculated in ways similar to that reported in this study should be considered as a potential radiation hazard and should be recalled in order to be properly shielded. The proper shielding of the tube is imperative for mobile radiographic units and fluoroscopic C-arm units used for interventional procedures, as in these cases the operator and the rest of the medical staff do not enjoy the radiation protection offered by the shielded walls of a common fluoroscopic or radiographic facility.
Concerning this specific mobile X-ray unit, it must be mentioned that after negotiations the unit was recalled to the factory in order to be properly shielded. After it was returned to the hospital, the leakage radiation had been reduced to about 1/8 of its previous value, thus conforming to the current leakage radiation limit.
Received for publication May 16, 2005.
Revision received August 3, 2005.
Accepted for publication August 10, 2005.
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References
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- National Council on Radiation Protection and Measurements. Structural shielding design for medical X-ray imaging facilities. NCRP Report 147. Bethesda, MD: NCRP, 2004.
- Simpkin DJ, Dixon RL. Secondary shielding barriers for diagnostic X-ray facilities: scatter and leakage revisited. Health Phys 1998;74:350–65.[Medline]
- Archer BR, Fewell TR, Conway BJ, Quinn PW. Attenuation properties of diagnostic X-ray shielding materials. Med Phys 1994;21:1499–507.[CrossRef][Medline]
- Tsalafoutas IA, Yakoumakis E, Sandilos P. A model for calculating shielding requirements in diagnostic X-ray facilities. Br J Radiol 2003;76:731–7.[Abstract/Free Full Text]
- Simpkin DJ. Transmission data for shielding diagnostic X-ray facilities. Health Phys 1995;68:704–9.[Medline]
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