Department of Radiology, Victoria Hospital, Blackpool and tRegional Department of Medical Physics and. Bioengineering, Christie Hospital & Holt Radium ...
FEBRUARY
1987, The British Journal of Radiology, 60, 167-173
1987
An investigation into the levels of radiation exposure in diagnostic examinations involving fluoroscopy By K. A. Rowley, *tS. J. Hill, B.Sc.; C.Phys., M.lnst.P., tR. A. Watkins and tB. M. Moores, B.Sc. Ph.D. Department of Radiology, Victoria Hospital, Blackpool and tRegional Department of Medical Physics and Bioengineering, Christie Hospital & Holt Radium Institute, Manchester
{Received December 1985 and in revised form August 1986)
ABSTRACT
In order to investigate the levels of radiation exposure resulting from fluoroscopic examinations, area-exposure product measurements were performed on 6532 patients whilst undergoing a variety of examinations at a large district general hospital. Results for both the same and different types of examinations, performed in two different X-ray rooms by a number of different radiologists, are compared in order to highlight some of the factors which influence the wide variations in patient exposure which frequently occur in radiological examinations. Variations in exposure of patients of different weights are also presented.
Approximately 12% of the total radiation dose to the UK population is due to man-made irradiation (NRPB, 1985). Of this, about 94% is due to medical procedures. Current and future legislation requires this irradiation to be kept as low as reasonably achievable (ALARA principle), taking social and economic factors into consideration. Kendall et al (1980) suggest that about 21 million radiodiagnostic examinations are carried out annually in National Health Service hospitals in the UK. Fluoroscopic examinations, although comprising only about 6% of all radiological procedures (approximately 1.2 million examinations per year), are often high-dose investigations and, as such, represent an appreciable fraction of the total somatic population dose. Also, since a large proportion of all fluoroscopy concerns the abdominal region, and the frequency of use of gonad shields is still low (Wall et al, 1980), these types of examination contribute a large fraction of the genetically significant dose. Indeed, all examinations which are likely to give high doses are worthy of special attention; it may be more important to consider the risk to the
•Present address: Mersey Regional Health Authority, Radiation Protection Service, 42 Rodney Street, Liverpool LI 9AA. Address for reprints. Dr B. M. Moores, Regional Dept of Medical Physics & Bioengineering, Christie Hospital and Holt Radium Institute, Wilmslow Rd, Withington, Manchester M20 9BX
individual patient concerned, rather than the population as a whole (Moores et al, 1982). Various reports have been published indicating the ranges of doses delivered for particular examinations (Wall et al, 1980; Harrison et al, 1983; Kearton, 1983; Faulkner & Bramhall, 1985), but none of these has yet covered a range of fluoroscopic examinations. Maximum permissible exposures have been defined for radiation workers and members of the general public, but no numerical guidance has been formulated as regards limits of exposures for patients undergoing standard radiological examinations. The results of this survey lend no support to the practicality of such a concept, albeit a measure of greater uniformity might well be achieved for simple radiographic procedures by the wider introduction of automatic exposure control (AEC) systems. Reject analyses have shown a fall in film reject rate after the introduction of a correctly installed AEC (Lewis, 1984, unpublished), and this in itself represents a reduction in the exposure per patient. For the more elaborate investigations of clinical problems requiring fluoroscopic control, the situation is more complex because of the multiplicity of factors influencing the doses delivered. These include not only the whole range of equipment from X-ray generator through intensifier to TV display but also the ability of patients to co-operate, and other variations in the clinical situation. While a properly implemented quality assurance programme should maintain equipment performance within appropriate limits, differences in patients and, perhaps, radiologists will always give rise to large and unpredictable variations in doses. There is no doubt that at present patients may receive vastly different radiation doses for the same examination under different circumstances, and some reports have considered possible causes for this. For example, Henshaw and Kennedy (1975) have reported wide variations in exposure rates to different intensifies under automatic control, and Harrison et al (1983) suggest that the differences in technique adopted by different radiologists may be of significance in explaining doses which.vary by as much as three orders of magnitude (Wall et al, 1980).
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60, No. 710 K. A. Rowley, S. J. Hill, R. A. Watkins and B. M. Moores
Because of the importance of examinations involving fluoroscopy in relation to total population dose, both somatic and genetically significant, the present study was set up to assess the extent and range of patient exposure and the influence, if any, of some factors upon it. This was a large-scale survey conducted over a period of several years covering a wide range of such examinations and involved a number of different radiologists. Because data were recorded from a very large number of examinations, variations in such factors as size of patients and their ability to co-operate were evenly distributed amongst the radiologists and may be disregarded for comparison purposes. METHOD
The survey was carried out in the main X-ray department at Blackpool Victoria Hospital, a large district general hospital serving a population of about 300000, with a relatively high proportion of elderly people. During the period of data collection, fluoroscopic examinations were carried out by seven radiologists, working in a busy department with a high throughput of patients, suitable for a comprehensive study of this kind. The two main fluoroscopy rooms at the time, referred to here as Rooms 1 and 2, were equipped with CGR and GEC X-ray installations, respectively. The CGR equipment in Room 1 was designed for remote control use, with an undercouch intensifier. Fluoroscopic exposures could be controlled either manually or automatically, but manual control was rarely used. Radiographic exposures could be recorded using either medium-speed calcium tungstatefilm-screencassettes or a 105 mm camera. AEC was used for both these methods. In Room 2, the GEC equipment was of the "traditional" undercouch tube/overcouch intensifier design. Both manual and automatic control were available for fluoroscopy. All films were produced by use of medium-speed film-screen cassettes, and radiography was manually controlled. It was anticipated that the study would provide useful comparative figures for patient exposures in the two rooms. The numbers and types of examinations carried out in these rooms were typical of those found in a busy district general hospital. They were distributed evenly between the two rooms, with the exception of barium enema examinations, which were rather more common in Room 2. Although each of the radiologists concerned worked predominantly in one room or the other, from personal preference, they all carried out a representative proportion of each type of examination. Since the main aim of the study was to investigate the variations in radiation exposure to patients, and only to a lesser extent the magnitude of the exposures, it was felt that the most convenient method of measurement would be to use area-exposure product (Ron2) meters. Both rooms were fitted with Diamentor Ron 2 meters. These are flat-plate ionisation chambers which are 168
attached to the light beam/diaphragm housing. They are optically transparent so that where a light beam is used in positioning this is not adversely affected (Pychlau, 1984). The use of such equipment is recommended by the International Commission on Radiological Protection (ICRP, 1982), particularly for training purposes. An advantage of using this type of meter in the survey was that it gives a measure of total energy imparted to the patient (Shrimpton et al, 1984), and, neglecting attenuation in the air, this is the same at all distances from the tube. Thus variations in focus-skin distances are accounted for. Also, the size of X-ray field, a very important but often neglected factor, is taken into account automatically, including any variations taking place during the fluoroscopic procedures. The Diamentor meters used were capable of recording fluoroscopic and radiographic exposures independently. They were calibrated and checked periodically by the Regional Medical Physics Department. Subsequent references in this paper to exposure refer to the area-exposure product in units of Rcm2. For a period of approximately 3^ years, readings were taken from these meters for most fluoroscopic examinations in either room. For each examination, the following data were recorded: sex, age, weight, date and type of examination, X-ray room number, radiologist, fluoroscopic exposure (Rcm2), radiographic exposure (Rcm2), screening time, films taken on 105 mm camera or cassette (choice available in Room 1 only), and automatic or manual control of fluoroscopy. All these data were recorded on data sheets by the radiographers at the time of examination. The data were not analysed until the end of the period of collection. They were then numerically coded onto a second set of data sheets and entered manually into a digital minicomputer. RESULTS
At the end of the survey period, data from 6531 examinations had been recorded. The mean age of patients was 56 years and their mean weight was 65 kg. Of these examinations, 3405 were carried out on females, with mean age 57 years and mean weight 58 kg, and 3126 on males, mean age 55 years and mean weight 68 kg. The distributions of these parameters are shown in Figs 1 and 2. It can be seen that both are approximately normal distributions, with the age histogram negatively skewed. Table I presents the number of patients examined by each radiologist in the two rooms. The personal preference of each radiologist for a particular room is apparent. Although a total of 28 separate procedures were identified, the seven most common of these accounted
FEBRUARY 1987
Exposure in examinations involving fluoroscopy 1400 Male Mole
700
1200Female
Dr "emale
600
1000
500800 •? 400
600
i 300-
400 200-
100-
0-10
r 11-20
200
21-30 41-50 61-70 81-90 31-40 51-60 71-80 91-100 Age range (years) FIG.
0-10 21-30 41-50 61-70 81-90 101-110 121-130 11-20 31-40 51-60 71-80 91-100 111-120 Weight range (Kg)
1.
FIG.
2.
Frequency distribution of patients' ages for both males and females undergoing screening procedures.
Frequency distribution of patients' weights for both males and females undergoing screening orocedures. procedures.
for more than 95% of all examinations in this survey. Accordingly, Table II presents the number of patients undergoing each of these seven examinations in the two rooms, and all other examinations are grouped together. The examinations were distributed fairly evenly between the two rooms except for barium enemas: the difference here accounts almost exactly for the difference in total examination numbers. Although not shown in the table, it may be noted that the various types of examination were also evenly distributed among the seven radiologists.
Because the distributions of the dose data were found to be extremely skewed it was decided that the median, 2\ and 97^ percentile values were the most suitable values to quote. However, for completeness, mean values have in most cases also been quoted. Table III presents the median, 2\ and 9 7 | percentile values of fluoroscopic and radiographic exposures for each radiologist and each room. It is immediately apparent that exposures are consistently lower in Room 1 and TABLE II NUMBER OF PATIENTS PER EXAMINATION IN EACH ROOM
TABLE I NUMBER OF PATIENTS EXAMINED BY EACH RADIOLOGIST IN EACH
Examination
Room 1 Room 2 256 1538 585 155
216 1530 1313 170
472 3068 1898 325
62
104
166
107 76 144
48 50 177
155 126 321
2923
3608
6531
Both rooms
ROOM
Radiologist
Room 1
Room 2
Both rooms
1 2 3 4 5 6 7
— — 1088 485 1025 — 325
177 364 13 1137 336 1504 77
177 364 1101 1622 1361 1504 402
Barium swallow Barium meal Barium enema Barium swailow + meal Barium meal + follow-through T-tube cholangiogram Micturating cystogram Others
Total
2923
3608
6531
Total 169
VOL.
60, No. 710 K. A. Rowley, S. J. Hill, R. A. Watkins and B. M. Moores TABLE III MEDIAN FLUOROSCOPIC AND RADIOGRAPHIC EXPOSURES ( R C M 2 ) DELIVERED BY EACH RADIOLOGIST IN EACH ROOM
Radiologist
Room 2
Room 1 Fluoroscopic Median
2i
Radiographic
97i
Median
percentiles
97i
2i
_
_
_
_
_
2
—
—
—
—
—
—
3
510 (610) 390 (560) 430 (510) —
80
1720
30
2230
20
1990
10
2400
80
1440
40
2550
—
—
240 (440) 190 (460) 330 (600) —
—
—
5 6
Median
percentiles
1
4
Radiographic
Fluoroscopic
7
340 (430)
40
1400
400 (819)
40
3420
All
440 (540)
220
1630
280 (540)
270
2580
1130 (1420) 880 (1110) 1040 (1206) 680 (840) 1040 (1380) 1020 (1180) 700 (830) 890 (1090)
97i 2± percentiles
Median
150
4900
190
3520
290
2890
70
2700
90
5150
140
3240
160
2890
1760 (1950) 1240 (1270) 690 (850) 720 (760) 1440 (1600) 1040 (1180) 940 (1250)
100
3390
2i
97i
percentiles
980 (1120)
270
4640
140
2490
240
1940
10
1730
70
3690
120
2810
90
3740
70
2980
Values in parentheses are means.
that there are also differences in the exposures delivered by each radiologist. Table IV shows the medians of fluoroscopic and radiographic exposures and screening time for each of the seven most common examinations. These values are further subdivided in order to observe variation between examinations of males and females. As might be expected, the exposures are higher for male patients
in almost every case although the screening times show little difference. For convenience of display, the patients' weights were divided into 10 kg cells and in Fig. 3, the median fluoroscopic exposure rates (measured exposure divided by recorded screening time) are shown for male patients together with the 2\, and 97^ percentile values. There is seen to be a general increase in dose rate with
TABLE IV MEDIAN FLUOROSCOPIC AND RADIOGRAPHIC EXPOSURES ( R C M 2 ) AND SCREENING TIME FOR THE SEVEN MOST COMMON EXAMINATIONS
Examination
Fluoroscopic Male
Barium swallow
550 (670) Barium meal 720 (880) Barium enema 960 (1270) Barium swallow + meal 720 (890) Barium meal + 530 follow-through (740) T-tube cholangiogram 330 (410) Micturating cystogram 160 (450)
Radiographic
Screening time (min)
Female
Male
Female
Male
Female
380 (500) 530 (690) 850 (1050) 450 (660) 410 (600) 260 (350) 110 (200)
400 (600) 690 (840) 1190 (1390) 650 (830) 430 (580) 330 (380) 80 (190)
330 (510) 520 (670) 1120 (1250) 620 (740) 590 (610) 270 (370) 80 (170)
3.0 (3.4) 3.0 (3.7) 2.5 (3.0) 3.0 (3.5) 2.5 (2.7) 1.5 (2.1) 2.5 (2.8)
3.0 (3.2) 3.0 (3.8) 2.5 (2.9) 3.0 (3.6) 2.0 (2.2) 2.0 (2.2) 2.0 (2.3)
Values in parentheses are means.
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FEBRUARY
1987
Exposure in examinations involving fluoroscopy categories was noted for females but not for males and vice versa. The equipment in Room 1 offered the alternative of radiographic recording either using a film-screen cassette or on film in a 105 mm camera. In order to compare exposures by these two methods the values for the most common examination, barium meals, were examined. These results are presented in Table V. Most fluoroscopy in Room 1 was under automatic mA control; however, in Room 2 the alternatives of manual and automatic control were investigated. Again using the example of barium meal examinations, the exposures are compared in Table VI. Only those examinations in which the use of automatic or manual selection was actually recorded are included in Tables V and VI.
1400
1200 -
1000
—
BOO
600
C)
400
o
DISCUSSION
C)
C)
200
Q
Table III demonstrates that exposures to patients in Room 2 are much higher than in Room 1. The ratio of fluoroscopic exposures in these two rooms is 2.0, and the corresponding value for radiographic exposures is 3.5. By reference to Tables I and II, it is apparent that these differences are not due to the distribution of examinations or radiologists. The relevant factor is that of the equipment. Since the Diamentor readings are independent of the differences in geometry between the two installations (although if an exact comparison of skin doses were required, a correction would be necessary to account for the fact that the table top is between detector and patient in one room (Shrimpton & Wall, 1982)), it is concluded that the different fluoroscopic exposures are due primarily to the different input dose rates at which the intensifiers are operating. Faulkner and Bramhall (1985) have presented similar results from a limited survey. Although the equipment in Room 1 was clearly operating at a lower exposure to the patient, it should be remembered that, as has been pointed out by Faulkner and Moores (1982), the radiation dose to any staff at the couch side is considerably higher for this type of equipment. The overcouch tube/undercouch intensifier configuration is designed for remote control use, but fluoroscopy must on occasions be carried out with personnel in the vicinity of the patient. During this survey, maximum use of the distance factor in staff protection was made in this circumstance.
o i 0-10
21-30 41-50 61-70 81-90 101-110 11-20 31-40 51-60 71-80 91-100
Weight range (Kg) FIG.
3.
Median, 2\ and 97^ percentiles of the exposure rate (Rein 2 min" 1 ) for each weight range of male patients (mean values shown as O)-
increasing weight, as would be expected, but the spread of individual measurements as indicated by the 2\-91\ percentile range is large in each case. (A similar trend was noted for female patients.) A Mann-Whitney [/-test, the non-parametric equivalent of the unpaired /-test, was applied to the dose rate vs weight data for both males and females in order to assess the significance for the differences between the exposure rates of adjacent weight groups. Generally the results showed a significant difference between exposure rates of groups in the range 20-90 kg, while median values for groups outside this range were not significantly different. However, sometimes a significant level of difference between adjacent weight TABLE V MEDIAN, 2% AND 91\ PERCENTILES OF RADIOGRAPHIC EXPOSURES (RCM 2 ) FOR BARIUM MEAL EXAMINATIONS USING FILM-SCREEN CASSETTE AND 105 MM CAMERA IN ROOM 1
No. of Median exposure examinations (radiographic) Film-screen 375 cassette 105 mm 949 camera
TABLE VI MEDIAN, 2\ AND 97^ PERCENTILES OF FLUOROSCOPIC EXPOSURE (RCM2) FOR BARIUM MEAL EXAMINATIONS USING AUTOMATIC AND MANUAL DOSE CONTROL IN ROOM 2
2\ 91\ percentiles
930 (1140)
70
3840
210 (260)
50
840
Automatic Manual
Values in parentheses are means.
No. of Median exposure examinations (fluoroscopic)
2\ 91\ percentiles
386 507
90 3430 170 2810
820 (1020) 950 (1110)
Values in parentheses are means.
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60, No. 710 K. A. Rowley, S. J. Hill, R. A. Watkins and B. M. Moores
Several factors may have a bearing on the difference in radiographic exposures delivered in the two rooms. It is conceivable that fewer films were used per examination in Room 1 or that higher kilovoltages were employed resulting in lower doses, but by reference to Table V it can be seen that the differences are largely due to the regular use of the 105 mm camera with AEC, instead of film-screen cassettes. Another factor which may relate to this is the regular use of AEC for cassette films in Room 1. This is likely to have led to a reduction in the numbers of repeat films. Table III also highlights the variations in exposures delivered by the different radiologists. Since the distributions of patient size and examination type are similar for each, it is deduced that these variations are largely attributed to the different techniques employed by individual radiologists. It is noticeable that in Room 2, which was used by all the radiologists, those who delivered the highest fluoroscopic exposures also tended to deliver the highest radiographic exposures. Mean screening times are shown in parenthesis in Table IV for both males and females. The National Radiological Protection Board (NRPB) (Wall et al, 1980) have quoted values for three of the examinations presented here. In comparing these it is noted that the figures for barium enemas and cystography are similar, whilst the NRPB value of 2.94 min for barium meals is somewhat less than those recorded during this study. However, the relevance of the mean value needs to be considered carefully. From Table IV, it can also be seen that both median and mean exposures to male patients are consistently higher than to female for both the fluoroscopic and radiographic components of all the common examinations. The ratios of these are within the range 1.0 to 1.5. Since the median and mean exposure values are obtained from large numbers of patients, and the distribution of sexes amongst rooms and radiologists is unbiased, it is concluded that the consistently higher exposures to males are due entirely to the fact that the average weight of the males was greater than that of the females. Den Boer (personal communication, 1985) has shown that patient dose and image quality are related to a parameter which he defines as "obesity index", which depends on height and weight. Patient height was not recorded during this survey, nor was patient thickness measured with calipers, but the weight of each patient had been recorded and the correlation between this and the fluoroscopic exposure rate (measured exposure divided by recorded screening time) was calculated for each of the most common examinations. No significant correlation was found for any of the examinations. However, when the patient weights were divided into 10 kg groups, and the median (or mean) exposure rate within each group plotted on a graph, the result (Fig. 3) showed a clear trend (supported by results of a MannWhitney £/-test). It is concluded that the fluoroscopic
exposure rate is only related to patient weight when median (or mean) values are obtained from a large sample. The extremely wide variations in exposure rate that exist within any weight category are caused by wide variations in field size, tissue thickness and other factors over the range of examinations. In particular, for patients of high or low weights, the exposure rate delivered is not significantly different from that delivered to a patient in the adjacent weight group. In other words, it is only at the central weight values that the system (either the radiologist/radiographer, or, in the case of automatic exposure control, the equipment) is able reliably to detect differences in patient weight and to deliver appropriate exposures. It seems likely that at the extremes of patient weights, the equipment, particularly the automatic exposure control, is reaching the limits of its capability. This became apparent in the examination of infants and young children in which manual control of fluoroscopic exposure produced a significant reduction. Table V shows clearly the benefits which may result in terms of patient dose from using 105 mm camera films during these procedures in place of the film-screen cassette method. As with all methods of dose reduction, this should be viewed carefully, bearing in mind the trade-offs which may result from reduced image quality. The value of automatic fluoroscopic dose control in Room 2 may be assessed by referring to Table VI. It is apparent that in this case only a small saving in exposure is achieved by using the automatic system. This suggests that, if image quality under both manual and automatic control is satisfactory but no better than necessary, the radiologists at this particular hospital are, on average, using the equipment in a dose-efficient manner. This does not necessarily mean that automatic systems are of no value. One has only to refer again to Table III to observe the exposures delivered by different radiologists. Whereas Radiologists 4 and 7 employ average fluoroscopic exposures which are significantly lower than the average exposure under automatic control, Radiologists 1 and 5 employ significantly higher average fluoroscopic exposure. Automatic control could help to reduce average screening exposures for individual radiologists. CONCLUSION
This study has examined some of the factors influencing the fluoroscopic and radiographic radiation exposures to patients undergoing common fluoroscopic procedures. Median and mean exposures in two rooms fitted with different types of X-ray equipment varied by a factor of two or more. The variation in fluoroscopic exposures was due to the different dose rates at which the two intensifies were operating, and the lower radiographic exposures were associated with the use of a 105 mm camera. The ratios between the highest and lowest median exposures given by individual radiologists are 1.7 for
172
FEBRUARY 1987
Exposure in examinations involving fluoroscopy fluoroscopy and 2.5 for radiography. While the authors do not presume to consider what comprises a satisfactory result from a clinical point of view, there is no doubt that some radiologists conduct their examinations with lower exposures to the patient than others. Whether or not this lower exposure indicates that there is less diagnostic information is an open question. The differences in technique may involve variations in fluoroscopic exposure time, number of films taken, exposure factors used, or beam size. It cannot be stressed too often that radiation doses can be kept low not only by keeping screening time and number of films low, but also by maintaining the image quality (TV or film) at a level no higher than necessary, and imaging an area no larger than the region of clinical interest. If any guidance is to be given in the future regarding the range of exposures which may typically be expected during specific fluoroscopic examinations, then this would obviously have to take account of many factors noted in this paper including the size of the patient. Patient weight alone has been shown to be insufficient for an accurate prediction of patient exposures. The concept of an index of obesity may prove to be of more help in relating exposure to patient size. Radiographic exposure savings of 75% or more have been achieved during the examinations studied here by use of the 105 mm camera. Clearly this should be used regularly where the image quality resulting is likely to prove clinically satisfactory. In this survey, an overall advantage of automatic control of fluoroscopy was not demonstrated. The results suggest that a skilful operator can achieve adequate results using manual control and in some circumstances it was manifest that an exposure to the patient significantly lower than that from an automatic system was achievable. Finally, when assessing the purchase of new equipment in the light of the above considerations, it is essential that the inter-relationship of costs, risks and benefits to the patient, staff and community at large, be fully examined. ACKNOWLEDGMENTS
The authors wish to thank the radiologists at Blackpool Victoria Hospital for their willing co-operation and the many radiographic and clerical staff for their efforts over a long period in recording and initial handling of the data. We also acknowledge the assistance of Mr R. Swindell, Department of Statistics, Christie Hospital, in the statistical analysis of the data.
173
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