British Journal of Radiology (2005) 78, 130-134
© 2005 British Institute of Radiology
doi: 10.1259/bjr/50195548
Dose implications of fluoroscopy-guided positioning (FGP) for lumbar spine examinations prior to acquiring plain film radiographs
M Saunders, BMRS
A Budden, BMRS
F MacIver, BMRS
M Teunis, BMRS
and
H Warren-Forward, BSc, PhD
Medical Radiation Science, University of Newcastle, Newcastle, NSW, Australia
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Abstract
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Fluoroscopy is increasingly being used as a positioning device prior to obtaining plain film radiographs. This is particularly true for those examinations where the type of projection and habitus of the patient present difficulties. An example is the examination of the lumbar spine; especially the L5/S1 projection. The purpose of this study was to determine the effect of fluoroscopy-guided positioning (FGP) on patient dose. The study assessed the difference in dose–area product (DAP) between conventional film–screen radiography (FSR) and a FGP assisted series of the lumbar spine. DAP values were monitored on 102 patients (50 FSR, 52 FGP) over 7 (4 FSR, 3 FGP) study sites. The median values for all FGP and FSR procedures were 8.3 Gy cm2 and 12.5 Gy cm2, respectively. The differences in doses were attributed to lower mAs and tighter collimation used in FGP assisted procedures. The study has demonstrated that it is possible to achieve lower DAP values using FGP. What now has to be asked is whether FGP should be acknowledged and further introduced into clinical practice. If so, there is a need for careful monitoring and reporting of dose so that strict protocols can be set in place to ensure the ALARA principle is enforced.
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Introduction
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Advanced technology and demand for procedures that require fluoroscopy has allowed the introduction of screening units to be affiliated with conventional X-ray tubes within the general X-ray room. Observations at radiology sites have shown that fluoroscopy is being used to aid positioning of the patient, particularly in lumbar spine examinations, prior to conventional film–screen radiography (FSR).
Medical radiation, in the form of X-rays has long been recognized as a benefit to the community. The risk of biological effects following irradiation has also been well established. It is the professional and ethical responsibility of those administering ionizing radiation to the community, to follow the professional guidelines of justification, optimization and limitation set by the International Commission on Radiological Protection (ICRP) to reduce the risk of radiation induced effects [1].
In the past, large accumulated doses have been the result of repeated projections associated with positioning and patient habitus. It has been suggested that fluoroscopy-guided positioning (FGP), prior to plain radiographs, will reduce these repeat examinations, and reduce patient dose. Observation at radiology centres which use FGP techniques have given the subjective impression that exposure factors, and more importantly the time used in screening, do not produce lower patient doses.
Compared with other common X-ray examinations, i.e. extremities, chest or abdomen X-ray procedures, lumbar spine examinations result in comparatively larger radiation doses. Lumbar spine examinations contribute approximately 15% of the collective dose of all medical radiation exposure received by the community [2]. The lateral L5–S1 projection gives one of the highest radiation doses to the patient for a given single radiograph [3] and is frequently repeated due to positioning inaccuracies and variations in patient habitus, adding to patient dose. Martin et al conducted a study comparing the doses produced by medical imaging departments to the reference levels set by the National Radiological Protection Board (NRPB) [4]. They reported that thoracic and lumbar spine examinations were of most concern, with doses from 20–30% of units above the reference level. The projection of the L5–S1 joint space was particularly poor, with more than 50% of departments exceeding the reference level of 40 mGy. It is therefore important to reduce the dose from lumbar spine examinations.
The implications of reduced patient dose through the reduction of repeat rates formed the basis of this study. The amount of research previously done in the use of fluoroscopy to guide positioning of a patient for a plain film radiographic examination is very limited. This is probably due to the use of FGP being forbidden in a number of countries. A study in Belgium evaluated the justification of using an ultrashort fluoroscopic pulse to aid patient positioning [5]. It was noted that in difficult examinations, a short fluoroscopic pulse, to guide the radiographer in positioning before the exposure is made, could result in an overall reduction in dose to the patient.
The aim of this pilot study was to determine the effect of FGP techniques, prior to conventional FSR of the lumbar spine on patient dose in comparison with standard FSR techniques.
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Methods
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Following ethics clearance, a letter of invitation was sent to 35 radiology departments in the Sydney, Central Coast and Hunter regions of New South Wales. Included with the initial letter was an information request form for the radiographic technique used. This requested information on the number and type of projections routinely taken and whether the department used screening for positioning.
Patient selection
Patient selection included all adult patients (>18 years) who required routine lumbar spine examination as specified by the departments. Patients requiring additional projections were excluded, as their resultant DAP could not be compared with the DAP from a routine series of projections. The study included three categories to determine a weight distribution of the patients. These were 50–60 kg, 61–80 kg and 81–90 kg.
Equipment and calibration
The measurement of radiation dose in fluoroscopy is more difficult than the measurement of the radiation dose for plain film examinations. Technique factors are under automatic control and the area being exposed changes continually during the examination. As such, the dosemeter of choice for both FSR and fluoroscopy examinations was the dose–area product (DAP) meter. The DAP meter was individually calibrated on each X-ray unit [6] and each tube underwent a full quality control (QC) assessment as recommended in the radiation guidelines published by the Environment Protection Authority, NSW, Australia [7].
Completion of patient data sheets
Each centre was given data sheets for recording patient age, body habitus, exposure factors for each projection and the total DAP values for the whole examination. The data sheets for FGP also recorded screening time. Instructions on how to fill in data sheets were positioned near the electrometer for ease of use for the radiographer. Repeat views for the examinations were also recorded.
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Results
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Four sites replied indicating that FGP techniques were used for lumbar spine examinations. It was known that a number of other sites routinely used FGP practice, but failed to indicate its use. The reason for this is that the practice is unacceptable to some members of the radiography profession. Only three of these sites were selected; the fourth centre used the technique too infrequently. There were 11 replies from centres using conventional FSR. Of these sites, four sites were selected. The selected centres consisted of a country private practice, an outer suburb private practice, an inner suburb private practice and a public hospital. These centres were selected to offer a range different clinical sites and potential techniques.
Results for 106 patients undergoing lumbar spine examination were examined. Four patients were excluded from analysis: three due to incomplete datasets and one for being too young. In total, analysis was performed on 50 patients examined with FSR (represented by sites A–D) and 52 patients were examined using FGP (represented by sites E–G).
Patient doses
Summary DAP values for each study site are given in Table 1
. The DAP values received by patients ranged from 2.2 Gy cm2, recorded by an FGP site, to 66.2 Gy cm2, which was recorded by an FSR site. Thus the patient with the highest DAP value received 30 times more radiation than the patient who received the lowest DAP value.
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Table 1. The table shows summary dose–area product (DAP) values and coefficient of variation (CV%) for the whole examination by each study site. Summary values have been given for all conventional film–screen techniques (FSR) and fluoroscopy-guided positioning techniques (FGP)
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The aim of the study was to investigate the differences in dose between patients investigated with FSR and FGP procedures. Plotting a frequency distribution of all DAP values received by patients for both FSR and FGP techniques resulted in a positively skewed distribution, as expected (Figure 1). The distribution shows the majority of FGP examinations were conducted with lower DAP values (solid line). The median DAP value for FSR (12.5 Gy cm2) was higher than the FGP (8.3 Gy cm2) value. A graphical distribution of median DAP value per site (Figure 2) shows values ranging from 6.2 Gy cm2 to 14.8 Gy cm2.
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Figure 1. Double histogram of dose–area product (DAP) values for conventional film–screen radiography (FSR) and fluoroscopy guided positioning (FGP). The best line of fit shows that the peak for FGP (solid line) is higher and to the left (lower dose) of the peak of the FSR (dotted line).
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Figure 2. Median dose–area product (DAP) value for each study site. Sites A–D used the conventional film–screen (FSR) technique; sites (E–G) used the fluoroscopy-guided positioning (FGP) technique. The median value for each technique has also been included.
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Patient parameter
Differences in DAP can be attributed to differences in patient habitus. It is important to ensure the same types of patients are being assessed by each technique. The distribution in genders (45% male and 55% female) was similar for both techniques. There was a slight difference in weight distribution between the two techniques. The patients in the FSR group had a smaller average weight than the patients in the FGP group. This difference was small and not significant enough (
2=0.87, p=0.65) to contribute to differences in patient dose.
Radiographic technique and projections
There was a difference in the number of projections used for the two techniques. Three projections (AP, Lat, L5/S1) were commonly used in FSR and four (AP, Lat, L5/S1, AP axial) in the FGP series. The reported film–screen sensitivity for each department was 400; however this was not independently assessed nor was the effect of processing, thus the actual sensitivities were not known. Applied potential (kVp) and mAs change according to the habitus of the patient and different radiographic projections depending on the thickness of the body region being examined. High kVp techniques are also used to reduce dose. Applied potential was recorded for all patients in the study, while mAs was only recorded for 72 patients. Median results of applied potential and mAs are given in Table 2.
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Table 2. The table shows the median applied potential (kVp) and time–current product (mAs) used by each study site. The number of patients (N) examined per site is given together with the repeat rates (RR%) for each projection and each site
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Repeat rates
Another reason for the lower DAP values in FGP is a smaller repeat rate compared with the FSR procedures. The repeat rates are shown in Table 2
. It must be appreciated that these repeat rates are obtained from small sample sizes and may not accurately represent the repeat rates obtained over a longer period of time. The largest repeat rates are seen for the L5/S1 examination, as this is the most difficult to obtain radiographically. While there were differences between the individual sites for the lateral projection, there was no difference between the two procedures (2% compared with 1.9%). The same cannot be said for the L5/S1 projection where the repeat rate for the FSR (10%) was nearly double that of the FGP procedure (5.8%).
Screening time
The median screening time for FGP sites E, F and G were 18 s, 24 s and 12 s, respectively. These values were well correlated (r=0.99) with the median DAP values. The variation in screening times and DAP for all patients within one site gave a correlation coefficient of r=0.50, while expanding this analysis over the three FGP study sites resulted in a correlation coefficient of r=0.21. This correlation coefficient is lower as it incorporates the effect of other influencing factors such as variations in field collimation.
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Analysis and discussion
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The average DAP was 36% lower for patients undergoing the FGP procedure. This was unexpected as the FGP procedures included screening time and an additional radiographic view. A t-test was applied to a logarithmic transformed dataset to test for significance. The calculated t-statistic was 3.97 representing a highly significant difference (p<0.0001) in the DAP values between the two techniques.
Radiographic technique
Analysis of the radiographic techniques showed that the tube potential used in FSR and FGP sites was almost equal. A more significant difference was seen in mAs values. The mAs used in the FSR sites were on average nearly 20% higher than those used in FGP sites. The lateral and L5/S1 projections resulted in considerably larger differences.
Comparing individual FSR sites, site A consistently used the highest median tube potential, whilst site B used the lowest. Site A had the lowest median mAs and site B had the highest values. These findings correlated well with the median DAP values for the sites, with site A having the lowest value and site B the highest. Similar comparisons can be made for sites that employed the FGP technique; however the dose readings do not appear as well correlated with the exposure factors.
Screening time
An explanation for the low correlation between tube potential, mAs and DAP values in the FGP procedure is differences in screening time. Site F had the highest screening time and the highest DAP measurement. Similarly, site G had the lowest total screening time and the lowest mean DAP reading. Image display systems varied between clinical sites using FGP techniques. In site G, the spot image taken remained on freeze frame after screening stopped. This image was used as a guide if an alteration of position was required. In sites E and F, there was no freeze frame facility, and the radiographer was required to re-screen to verify an alteration of position. These observations correlated with analysis of screening times (Figure 3a), where the site with the lowest median screening time (site G) used freeze frame and produced the lowest median DAP value (Figure 3b).
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Figure 3. The median screening times illustrated in (a) correlate with the use of freeze-frame in site G, which displays the lowest median screening time of the three units. There is excellent correlation between median screening time and median dose–area product (DAP) values (b), indicating the importance of minimizing screening time.
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The study aim was to determine if FGP techniques produced higher doses in comparison with FSR techniques, as such data were only collected on the total DAP value for each examination. A continuation of this study will monitor DAP due individually to fluoroscopy and radiographic modes, as well as the DAP value for each projection.
Repeat rates
It has been reported that approximately 19% of repeat radiographs are repeated due to incorrect positioning [8]. This implies that the use of FGP would reduce patient dose if FGP techniques were used instead of blind positioning. In this study, the use of FGP did not significantly reduce the repeat rate for the whole examination; the only difference was for the L5/S1 projection, where FSR produced nearly double the repeat rate.
Field size and effective dose
The use of smaller collimated fields used with FGP is another reason for the difference in DAP values. The collimation can be adjusted after the initial fluoroscopy spot film is displayed on the monitor, reducing the overall area of exposure in the subsequent FSR film. The reduction in field size demonstrates the reduction in energy imparted to the patient from a lumbar spine series. Depending on the anatomical area under investigation, there may be greater reductions in effective dose, as beam collimation will limit the amount of primary radiation and scattered radiation reaching the gonads. A more detailed analysis needs to be performed to draw more definite conclusions, and this was beyond the scope of the present study.
Comparison with the reference levels and other studies
The NRPB DAP reference level for a complete lumbar spine examination is 15 Gy cm2 [2]. The median value for the FGP techniques was 45% lower than this and the median value for FSR techniques 17% lower. Shrimpton et al reported the mean DAP reading for a lumbar spine series as 11.7 Gy cm2 [9]. A study in Western Australia measured a mean DAP of 11.4 Gy cm2 for the lumbar spine [10]. A study performed by Warren-Forward et al measured the mean DAP for a lumbar spine as 7.92 Gy cm2 [11]. The NRPB 2000 Review reports a third quartile level of 7.3 Gy cm2 for measurements on over 1000 patients and 50 hospitals [12]. This level was calculated from individual projections of the three common projections (AP, LAT, LSJ) used in the UK. If the results from the current study are to be compared with this new value, then only one centre produced a lower median dose (noting that three sites used four projections). For over a decade there has been a drive to assess and optimize radiation doses in the UK and Europe while there has been very little assessment in Australia. Doses can thus be expected to be higher in Australia as there has been no basis upon which to change practice.
Additionally, the current study included a large cohort of patients above 80 kg, while the latest NRPB figures are based on a weight range of 50–90 kg. Another explanation is the difference in applied potential. The range of applied potentials in this study (Table 1) are at the low end or even lower than Commission of the European Community (CEC) recommendations [13]. The CEC recommends between 75 kVp and 90 kVp for AP projections, between 80 kVp and 95 kVp for lateral and between 80 kVp and 100 kVp for L5/S1 projections. For the AP projections, six out of the seven departments used median values lower than the recommended value, with a mean value of 71 kVp compared with 76 kVp in the NRPB report. For the lateral projection the mean value (80.7 kVp) was 6 kVp lower than the NRPB mean value (87 kVp). Similarly, for the L5/S1 projection the mean value was 88 kVp compared with 95 kVp, with over half the departments using less than the CEC recommendation. Correspondingly the values of the mean mAs's were significantly higher in our study: AP (47% higher); LAT (53% higher) and L5/S1 (90% higher).
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Conclusion
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The FGP technique for lumbar spine examinations produced an overall lower median DAP than an FSR technique. This is in agreement with the findings of Deprez et al [5]. This has potential implications for the future of radiographic practice, as the use of FGP has been discouraged in a number of countries. There is a presumption that FGP produces higher levels of radiation dose. What now has to be asked is "should FGP be further introduced into clinical practice for specific examinations?"
Before a definitive answer can be given, a more extensive study is needed involving a wider range of clinical sites both within and outside Australia and data collection incorporating other factors affecting DAP values and repeat rates, such as the use of automatic exposure control. One area of concern is the complacency that may develop if FGP is incorporated into routine clinical practice. The radiographers involved in the study may have been conscious of being "monitored" and thus may have reduced their screening times and deliberately have used tighter collimation than in usual practice. It is known that FGP is being introduced in a number of sites, so it is pertinent that its effect on patient dose is carefully monitored and reported so that strict protocols can be set in place.
Received for publication August 19, 2002.
Revision received July 5, 2004.
Accepted for publication September 24, 2004.
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References
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- International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. Report 60. Annals of the ICRP 21. Oxford: Pergamon Press, 1990.
- National Radiation Protection Board. Patient dose reduction in diagnostic radiology. Report by the Royal College of Radiologists and the NRPB. Documents of the NRPB. 1(3). Chilton: NRPB, 1990.
- Harrison RM, Clayton CB, Day MJ, Owen JP, York MF. A survey of radiation doses to patients in five common diagnostic examinations. Br J Radiol 1983;56:383–95.[Abstract/Free Full Text]
- Martin CJ, Darragh CL, McKenzie GA, Bayliss AP. Implementation of a programme for reduction of radiographic doses and results achieved through increases in tube potential. Br J Radiol 1993;66:228–33.[Abstract/Free Full Text]
- Deprez T, Schrijver L, Pauwels H, Bosmans H, Marchal G. Can patient positioning using an ultrashort fluoroscopic pulse be justified. IAEA International Conference on Radiological Protection of patients in diagnostic and interventional radiology, nuclear medicine & radiotherapy, Malaga, Spain 26–30 March 2001.
- Faulkner K, Busch HP, Cooney P, Malone JF, Marshall NW, Rawlings DJ. An international intercomparison of dose-area product meters. Radiat Prot Dosim 1992;43:193–5.[Abstract]
- Environment Protection Agency. Registration requirements and industry best practice for ionizing radiation apparatus used in diagnostic imaging, Radiation Guideline 6: Part 2, Fluoroscopy and Radiography. Environment Protection Agency, Sydney, Australia, 1999.
- Hill SJ. Reject Analysis in ERPET training course for medical physicist, "Quality Assurance and patient radiation protection in diagnostic radiolgy", Passau. 1996.
- Shrimpton PC, Wall BF, Jones DG, Fisher ES, Hillier MC, Kendall GM, et al. Doses to patients from routine diagnostic X-ray examinations in England. Br J Radiol 1986;59:749–58.[Abstract/Free Full Text]
- Gray N, Rafferty MW, Black JL, Hartley BM, Thompson RI, Stock B. A patient dose survey for three common diagnostic radiographic examinations using a dose-area product meter. Aust Phys Eng Sci Med 1994;17:79–83.
- Warren-Forward HM, Haddaway MJ, Temperton DH, McCall IW. Dose-area product readings for fluoroscopic and plain film examinations, including an analysis of the source of variation for barium enema examination. Br J Radiol 1998;71:961–7.[Abstract]
- Hart D, Hillier MC, Wall BF. Doses to patients from medical x-ray examinations in the UK - 2000 Review. NRPB-W14. Chilton: NRPB, 2002.
- Commission of the European Communities. Quality Criteria for Diagnostic Radiographic Images. Document XII/221/93 CEC, Brussels, 1990.
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Abstract
Introduction
