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Should Plain Films of the Lumbar Spine be Taken in the Posterior-to-Anterior or Anterior-to-Posterior Position? A Study Using Decision Analysis
SHOULD PLAIN FILMS OF THE LUMBAR SPINE TAKEN IN THE POSTERIOR-TO-ANTERIOR OR ANTERIOR-TO-POSTERIOR POSITION? A STUDY USING DECISION ANALYSIS
BE
Kenneth J. Young, DC
ABSTRACT Objective: The objective of this study was to mathematically propose the best position for frontally oriented lumbar spine radiographs. Methods: Decision analysis with a decision tree was used in this study. Factors used in the analysis were radiation dose, ease of implementation, image quality, cost, and patient comfort. Data on these factors were gathered by reviewing the literature. Results: Radiation dose was found to be the only factor with significant differences between the posterior-to-anterior (PA) position and the anterior-to-posterior (AP) position, and PA films gave lower doses to sensitive organs as compared with AP films. There is also some evidence that PA films show better overall image quality as compared with AP films. Conclusions: Based on the information used in this study, lumbar spine plain film radiographs should be obtained in the PA position. (J Manipulative Physiol Ther 2007;30:200-205) Key Indexing Terms: Radiography; Lumbar Vertebrae; Decision Support Techniques; Evidence-Based Medicine
I
t is now well established that every dose of ionizing radiation is harmful1,2 and that certain anatomical structures are more sensitive to radiation than others.3 It is also recognized that even small doses of radiation from medical investigations carry risk.4 In the United Kingdom, 100 to 250 annual deaths are now realized to be attributable to diagnostic radiation.5 Therefore, the goal in radiography of any body area is to obtain optimum visualization of the anatomical structures while minimizing the dose of ionizing radiation to patients. It is common practice for many hospitals, private health care clinics, and teaching institutions to take lumbar spine films in the anterior-to-posterior (AP) orientation. In the author’s experience, the most common—and often the only—justification for this, as opposed to the posterior-to-anterior (PA) position, is to minimize magnification and distortion of the spine by placing it as close to the film as possible. The literature was reviewed to obtain information for a decision analysis to Director, Young Radiology Consulting, Nottingham, England. Submit requests for reprints to: Kenneth J. Young, DC, Young Radiology Consulting, PO Box 9125, Nottingham, NG2 9EG, UK (e-mail: [email protected]). Paper submitted September 8, 2006; in revised form October 10, 2006; accepted November 25, 2006. 0161-4754/$32.00 Copyright D 2007 by National University of Health Sciences. doi:10.1016/j.jmpt.2007.01.013
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develop an evidence-based rationale for choosing an orientation in which to take lumbar spine radiographs. The method of positioning a body part for radiography is often based on the conscious weighing by health care professionals of various factors, including optimizing visualization of the structures, patient comfort, minimizing dose, practical issues such as whether a patient is ambulatory or not, and others. However, procedures and policies are sometimes undertaken or perpetuated because of habit, unfamiliarity with new information, or resistance to change. A MEDLINE search for articles on this topic revealed none that undertook a formal mathematical analysis of the rationale behind which position to use for lumbar spine radiography, although there is evidence of study and debate. Studies on lumbar spine plain film radiographs comprise a significant portion of the dose of ionizing radiation delivered to patients in hospital imaging departments, up to 15% of the collective dose6 and second only to computed tomography.7
METHODS The method used in this study was a formal decision analysis using a decision tree with probabilities and utilities, or quality-of-life factors, added to arrive mathematically at a conclusion on whether it is better to obtain radiographs of the lumbar spine in the AP or PA orientation.
Journal of Manipulative and Physiological Therapeutics Volume 30, Number 3
Decision analysis is an increasingly common method of making policy-level decisions in fields as diverse as manufacturing, economics, medical diagnosis, and coral reef management. Its advantage is that it can take in many elements, including subjective ones, to reach a best overall decision. Although decision analysis is not appropriate for all health care situations owing to its time-consuming nature, it can be used to help develop policies. These policies can then be implemented rapidly and efficiently with patients. Five areas were identified as relevant to the study of frontal lumbar spine films: radiation dose, ease of implementation, image quality, cost, and patient comfort. Qualitative and quantitative investigations from the literature were examined, because it is possible, although unlikely, that all qualitative studies support one position and all quantitative ones support the other. However, only the quantitative results will be of use in the calculations for this decision analysis.
Radiation Dose The PA projection has been advocated in preference to the AP projection by the British Institute of Radiology8 b[w]hen it is inevitable that the gonads are within the primary beam. . .the gonad dose may be somewhat reduced particularly in the female. . .Q Nic an Ghearr and Brennan9 calculated dose reductions of approximately 60% for female patients with the use of PA views as opposed to AP views. Heriard et al10 found that they were able to reduce part thickness by 11% by placing patients in the prone, rather than supine, position on the table and were therefore able to reduce the milliampere values by 50%. Then, using a phantom with dosimeters placed at various critical areas within it, radiation doses were measured and found to be much higher in the AP position: 1300% for eyes, 216% for ovaries, 489% for uteri, and 900% for testes. Even the thyroid dose was 17% higher, although this was not found to be statistically significant. Only the bone marrow dose was found to be higher in the PA projection, by 28%.10 Brennan and Madigan11 showed through the use of dosimeter readings from within radiographic phantoms that there was a significant reduction of dose (~39%) by using the PA projection. Real patients showed nearly a 39% decrease in entrance surface dose measurements when the PA position was used, as compared with the AP position. This was a result of an overall reduction in the milliampere values used because the PA position (ie, prone on the x-ray table) compressed patients by 9.6% in diameter. Therefore, the PA prone position not only increases the distance from the x-ray source to the radiosensitive organs but also achieves an overall reduction of dose. The change in this case was from an average diameter of 18.8 to 17.0 cm.11 Because decreasing body thickness necessitates decreasing
Kenneth J. Young PA vs. AP Radiographs
milliampere values to maintain optical density, a reduction in radiation dose is given by PA prone positioning. Although no reference was found in the literature, it is reasonable to speculate that similar reductions might be gained in the upright weight-bearing position, if patients were instructed to rest firmly against the wall grid, or if compression bands were used. Likewise, Gray et al12 found significant dose reductions to testes, ovaries, and uteri for lumbar spine radiographs performed in the PA rather than the AP position using identical technical exposure factors. Weatherburn13 also found lower doses to the thyroid and breasts in PA projections; specifically, AP positions gave 20 times the dose of PA positions in adult phantoms and 10 times the dose with an in vivo study on children. Numerous authors who examined techniques for scoliosis evaluation found reduced doses to the thyroid, eyes, and breasts when using a PA upright position as compared with an AP position.14-21 In particular, Nash et al22 found increased breast carcinogenesis in young female patients with scoliosis and a 500% to 800% increase in radiation dose with the use of AP rather than PA projections. Gregg23 noted that spinal bone marrow exposure was increased in the PA position but that the risk-benefit ratio strongly favored PA views because bone marrow has one sixth the radiosensitivity of breast tissue. Investigations relating to abdominal radiology doses were also deemed to be relevant because the same area of the body is irradiated in abdominal radiographs as that in lumbar spine radiographs, although collimation may vary. Marshall24 found that bAP projections give rise to higher effective doses than to PA projections because the majority of irradiated radiosensitive organs within the abdomen, such as the small intestine and others, lie closer to the entrance surface for the AP views in comparison to the PA views.Q In addition, the dense paraspinal musculature of the back will attenuate radiation better than would the subcutaneous fat of the anterior trunk, unless the fat is very thick. The evidence that using PA views results in less radiation dose to radiosensitive organs appears convincing. The argument that the radiation dose to these structures could be minimized with carefully placed gonad protection may be made; however, this creates artifacts on the radiographs that may obscure the pathology, and other sensitive organs, such as the intestines, would be impossible to shield without obscuring all other structures.
Ease of Implementation Implementation of the procedure is easy and requires no additional equipment, and because radiographers commonly use PA techniques with other body areas, such as the chest, they are familiar with positioning patients in this orientation.11
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Kenneth J. Young PA vs. AP Radiographs
It has been noted that PA views may provide for easier positioning of spinal anatomy because bplacing the patient in a prone position exposes the spinous process to easy palpation and localization.Q10 Aikenhead et al25 found that filters were useful for reducing doses to radiosensitive organs in patients with scoliosis but that the PA position also reduced radiation doses. Posterior-to-anterior positioning used in conjunction with filters was even more effective, except to bone marrow.25 Filters are easy to implement with minor curves, but severe scolioses make effective filter implementation nearly impossible; it is very difficult to shield radiosensitive organs without masking spinal structures. The PA position can easily be used for patients with even the most severe curvatures.
Image Quality Before a new health care procedure can be introduced to human beings, it must show some evidence of safety. In the case of the implementation of a new radiographic position, it must be shown not to diminish image quality. Brennan and Madigan11 showed that the PA projection actually improved image quality, albeit not to a statistically significant degree in their study. This makes sense in that a thinner body part requires a lower milliampere setting, which means less scatter radiation is generated and therefore less fog is seen on radiographs. Heriard et al10 had radiologists certified by the American Board of Radiology evaluate PA radiographs against AP ones. The results showed the vertebral bodies to be better aligned in the PA projections as compared with AP views taken with the patients’ knees flexed. This was ascribed to the divergent beam aligning better with the lumbar lordosis in the PA position. When they measured L3 transversely on both types of images, the PA views showed a magnification factor 1.031 times greater than that with the AP views. However, there was no visual increase in magnification except with obese patients. Even with obese patients, however, the reviewing radiologists did not deem the magnification to adversely affect the quality of the radiographs. Their synopsis of the situation was that10 b[e]ndplates and interspaces are better defined because of corresponding part alignment to the divergent central ray with a minimal, non-effective increase in magnification.Q It would logically follow that the sacroiliac joints, if included on the film, would also be better visualized on a PA-oriented film because, anatomically, they are wider anteriorly than posteriorly and would therefore be more parallel with a divergent x-ray beam than if patients were facing the tube. Ahmad26 stated that the prone position for lumbar spine radiographs gives better image quality because it allows the divergent rays of the primary beam to travel at angles that more closely parallel the intervertebral disk spaces. This would make sense, although the lumbar lordosis is reduced
Journal of Manipulative and Physiological Therapeutics March/April 2007
in recumbent positions as compared with weight bearing,27 and the difference may even be better shown on upright films. Schock et al28 examined differences between AP and PA positioning for geometric alterations by measuring the angles of the long axis of the vertebral endplates. They found no difference in the angles on the different projections. Tsuno and Shu29 judged lumbar vertebrae to have less shape distortion in the PA than in the AP position and concluded that the PA view would give a lower dose to radiosensitive organs. Overall, PA-oriented films seem to give at least as much diagnostic information as AP films and may even be better.
Cost There is no known increase in financial costs associated with positioning a patient in the PA orientation as compared with the AP orientation.
Patient Comfort No information was found regarding patient comfort in the supine vs prone position, so it was not included in the analysis; however, it seems unlikely to be a major factor.
Various factors need to be taken into account when deciding on the usefulness of a given diagnostic procedure. In this case, 5 factors were included in the initial analysis: relative radiation dose, which essentially equates to the risk of inducing disease, ease of implementation, diagnostic quality of the radiographs, cost of implementation, and patient comfort. The professional literature seemed to find no difference in PA vs AP positioning for most of these factors. Specifically, it was not deemed to be more difficult to obtain radiographs of the lumbar spine with the patient in the prone, rather than the supine, position on the table. There was no reduction in the diagnostic quality of the radiographs obtained when the patient was prone on the table; if anything, a small and statistically insignificant increase in quality was found. For the purpose of this article, differences in ease of positioning were discounted and both positions were treated as equal in the quality of the radiographs that they produce. There was no difference in costs between the PA position and the AP position; neither was information on patient comfort in the supine vs prone position discovered. Therefore, these two factors were not included in the calculations. Therefore, the only factor necessary to account for was radiation dose. An intuitive impression may be formed from reading the literature, but it was deemed worthwhile to formalize it in a decision analysis and to quantify it, however roughly. The exact probabilities about the risk of inducing cancer or birth defects in offspring of patients receiving a single
Journal of Manipulative and Physiological Therapeutics Volume 30, Number 3
dose of diagnostic ionizing radiation are not known at this time but are known to be very low.1 The important conclusion from the decision analysis in this case is relative probability, not exact probability. Therefore, the risk of inducing cancer in any given tissue from a single dose of ionizing radiation was arbitrarily set at 1:1000 (or 0.001 for the purpose of the calculations). This was also the figure used for the probability of inducing a genetic change that will cause a birth defect in the child of a patient receiving a single dose of radiation to the gonads, male or female. Because the literature found that the gonads are more heavily irradiated in AP views, birth defects and cancers of the abdominal structures were considered together in the decision analysis and are labeled bACBDQ on the decision tree. The only negative factor for the PA position is radiation dose to the bone marrow. Only one numerical figure was found in the literature, a 28% increase. A 28% increase of a 1:1000 chance is very small indeed, and with the information that bone marrow is only half as sensitive as gonads3 and one sixth as sensitive as breast tissue23 to the effects of radiation, this increase becomes so small as to be essentially negligible and will be treated as such in this analysis. Two states of health are possible after a single dose of ionizing radiation: (1) healthy and (2) inducement of abdominal cancer/inducement of birth defects in offspring (ACBD). These conditions are known as outcome states. The decision tree was then very simple, with only these two outcome states possible. Decision analysis requires utilities, or quality-of-life factors, to be assigned scores out of 100 to the different states of health that are possible after receiving a single dose of diagnostic radiation. Healthy, described for the purposes of this study as lack of induced disease, is usually ascribed a utility score of 100, and that was the case in this study. It is labeled bNO ACBDQ on the decision tree. Immediate death is usually given a score of 0, but that was not an option in this study. Inducement of abdominal cancer or giving birth to a child with birth defects was assigned a utility score of 60. These numbers are subjective and would vary with each individual. Some people would rate their life higher than 60 if they have cancer, and of course it would depend on the type of cancer and prognosis. Perhaps facing death from cancer within weeks would only rate a 0.5 utility; conversely, a mild cancer requiring only a single course of chemotherapy leading to complete cure may rate a 95 utility. Certainly, it seems reasonable however that having a disease reduces the quality of life. Similarly, giving birth to a child who is not entirely healthy also impacts the quality of life and reduces utility from 100; in this case, it was again arbitrarily assigned a utility score of 60. Therefore, 60 was used here as the utility score for a patient who received a single dose of radiation that led to any type of cancer or for a patient giving birth to a child with birth defects.
Kenneth J. Young PA vs. AP Radiographs
Fig 1. Decision tree. The following utilities were used for the decision analysis: NO ACBD (healthy; absence of abdominal cancer or not having a child with birth defects) = 100 and ACBD (presence of abdominal cancer or having a child with birth defects) = 60. The radiation dose data for the differences between AP and PA views vary widely in the literature, from 39% increased for AP views11 to 1000% to 2000%.13 Therefore, the highest and lowest figures will be discounted, and the rest will be averaged. Nic an Ghearr and Brennan9 found a 60% increase. Heriard10 found increases of 216% for ovaries, 489% for uteri, and 900% for testes. Nash22 found a 500% to 800% increase (average = 650%). The average of all these [(60 + 216 + 489 + 900 + 650)/5] is 463%—or an approximately 4.5 times greater dose to the abdominal organs when patients are placed in the AP rather than the PA position. In decision analysis, the results are termed expected utilities and calculated as follows: (Chance of Outcome Utility for That Outcome) + (Chance of Opposing Outcome Utility for That Outcome). In this case, the formula was as follows: (Chance of ACBD Utility for ACBD) + (Chance of Healthy Outcome Utility for Healthy Outcome). This was calculated for both PA and AP positioning. The results were then compared, and the higher number indicated the better method. For PA positioning, the chance of ACBD is 0.001, and the utility of ACBD is 60. Furthermore, the chance of a healthy outcome is 1 0.001 or 0.999, and the utility of a healthy outcome is 100. Therefore, the calculation was as follows: (0.001 60) + (0.999 100) = 99.9600. For AP positioning, the chance of ACBD is 4.5 times greater than that for PA positioning and hence was calculated as 0.001 4.5, which equals 0.0045. Again, the utility of ACBD is 60. The chance of a healthy outcome is 1 0.0045 or 0.9955, and its utility is 100. Therefore, the calculation was as follows: (0.0045 60) + (0.9955 100) = 99.5527. The higher number found as
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Kenneth J. Young PA vs. AP Radiographs
the expected utility for the PA position indicated an advantage for that position.
RESULTS Figure 1 shows the decision tree. The expected utility for the PA position was 99.9600, whereas that for the AP position was 99.5527. A single AP- or PA-positioned radiograph of the lumbar spine has an extremely small detrimental effect on the quality of life of a patient, but the PA position has now been mathematically shown to be less detrimental. With all other factors being essentially equal, it is better to use the PA position for lumbar spine radiographs.
DISCUSSION Errors associated with inadequate data are smaller than errors associated with no data –Charles Babbage (1792-1871)
The intentions of this study were to review the literature regarding frontal lumbar spine radiographs and to use the data to mathematically determine the best orientation in which to obtain them. It had been anticipated that this would create a complex decision tree with a large number of utilities to calculate. However, 4 of the 5 factors (ie, ease of implementation, image quality, cost, and patient comfort) emerged to be almost exactly equivalent for both types of positioning; therefore, these elements were not included in the decision analysis, which reduced the necessary size of the decision tree. In addition, the extremely small difference in bone marrow dose between the AP position and the PA position meant that it was essentially negligible in the calculation and hence reduced the size of the tree even further. Because the real risk of inducing disease from a single dose of ionizing radiation is not known, albeit known to exist, an absolute number could not be achieved from this decision analysis. However, even choosing the arbitrary 1:1000 risk serves the purpose by allowing a demonstration of the relative risks of AP and PA positioning. In combination with the radiation dose information that has been published, these relative numbers are quite useful and clearly show the increased risk associated with AP positioning. In other words, although the absolute numbers have little meaning, the result is nonetheless clear because of the relationship of the numbers relative to each other. From the data synthesized in this study, the single most important number that emerges is the estimate that AP positioning gives a dose 4.5 times greater to the abdominal organs, including the gonads, which are the most radiosensitive of all organs.3 Ultimately, the question concerning qualitative vs quantitative studies did not provide any difficulty because all the articles found in the literature search had similar findings.
Journal of Manipulative and Physiological Therapeutics March/April 2007
Further studies on several topics would help optimize the safety and efficacy of frontal lumbar spine radiographs. A survey of patient comfort in the prone and supine positions would provide data for a fuller analysis of the subject. It would be useful to obtain more information regarding whether reduction of the diameter of the abdomen in the upright position is practical, with or without compression bands, and how much reduction in milliampere values would be achievable. A study on collimation for the lumbar spine would also be valuable. Some practitioners take frontal lumbar spine films with 14 17-in (35 43 cm) collimation to visualize abdominal soft tissues along with the spine, others with much narrower collimation, close to the spine itself. Occasionally, a significant but clinically silent pathology is found on these wider-collimated films.30 It is unknown at this time whether the additional incidental pathology revealed by the larger collimation is offset by the additional radiation dose to the population. Finally, it would be useful to compare the distortion of osseous structures on radiographs between patients in the prone and those in the standing PA position because the lordosis is altered in the prone vs standing position.
CONCLUSIONS Although the decision tree and resulting calculations ultimately were quite simple, a thorough search of the literature was necessary to discover that many potential factors for the decision analysis were not necessary to enter into the calculations. It was found that the PA position was just as easy to implement, caused no reduction in image quality, and incurred no additional cost as compared with the more traditional AP position. No information was found regarding patient comfort in one position vs the other; therefore, this factor could not be included in the analysis. Although radiation dose findings revealed widely varying numbers, a clear advantage in dose reduction was shown for the PA position, and doses to the abdominal organs averaged 4.5 times greater with the AP position. The PA
Practical Applications ! Factors that may influence the orientation in which to take frontal lumbar radiographs include dose, ease of implementation, image quality, cost, and patient comfort. ! Of these factors, only dose is significantly different between the PA and AP positions. ! Dose to radiosensitive organs is significantly lower in the PA position. ! Based on the information used in this study, lumbar spine radiographs should be taken in the PA position.
Journal of Manipulative and Physiological Therapeutics Volume 30, Number 3
position for taking lumbar spine films clearly shows an overall advantage over the AP position because of its lower dose of ionizing radiation to the radiosensitive tissues of patients and the equivalency of the other factors.
REFERENCES 1. Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council. Health risks from exposure to low levels of ionizing radiation: BEIR VII—phase 2. Washington, DC7 National Academy of Sciences; 2006. 2. Cardis E, Vrijheid M, Blettner M, et al. Risk of cancer after low doses of ionizing radiation: retrospective cohort study in 15 countries. BMJ 2005;331:77. 3. Canadian Centre for Occupational Health and Safety [homepage on the internet]. Hamilton, Ontario, Canada: OSH Answers; n1997-2006 [updated 1999 May 23; cited 2006 August 15]. Available from: http://www.ccohs.ca/oshanswers/ phys_agents/ionizing.html. 4. Moilanen A, Kokko ML, Pitkanen M. Gonadal dose reduction in lumbar spine radiography. Skeletal Radiol 1983;9:153-6. 5. Royal College of Radiologists and National Radiological Protection Board Documents of the National Radiological Protection Board. Patient dose reduction in diagnostic radiology, Volume 1: No. 3. London: Health Protection Agency; 1990. 6. Halpin SFS, Yeoman L, Dunda D. Radiographic examination of the lumbar spine in a community hospital: an audit on current practice. BMJ 1991;303:813-5. 7. RCR Working Party. Making the best use of a department of clinical radiology: guidelines for doctors. 4th ed. London7 Royal College of Radiologists; 1998. 8. British Institute of Radiology (BIR). Radiation protection of the patient: a manual of good practice. London7 Chameleon Press Ltd; 1980. 9. Nic an Ghearr FA, Brennan PC. The PA projection of the abdomen: a dose reducing technique. Radiography 1998;4: 195-203. 10. Heriard JB, Terry JA, Arnold AL. Achieving dose reduction in lumbar spine radiography. Radiol Technol 1993;65:97-103. 11. Brennan PC, Madigan E. Lumbar spine radiology: analysis of the posteroanterior projection. Eur Radiol 2000;10:1197-201. 12. Gray JE, Ragozzino MW, VanLysel MS, et al. Normalized organ doses for various diagnostic radiologic procedures. AJR Am J Roentgenol 1981;137:463-70. 13. Weatherburn GC. Reducing radiation doses to the breast, thyroid and gonads during diagnostic radiography. Radiography 1983;49:151-6.
Kenneth J. Young PA vs. AP Radiographs
14. Butler PF, Thomas AW, Thompson WE, et al. Simple methods to reduce patient exposure during scoliosis radiography. Radiol Technol 1986;57:411-6. 15. Andersen PE, VanDerKooy P. Dose reduction in radiography of the spine in scoliosis. Acta Radiol Diagn 1982;23: 251-3. 16. Ardran GM, Coates R, Dickson RA, Dixon-Brown A, Harding FM. Assessment of scoliosis in children: low dose radiographic technique. Br J Radiol 1980;53:146-7. 17. Drummond D, Randallo F, Lonstein J, Brooks HL, Cameron J. Radiation hazards in scoliosis management. Spine 1983;8:741-8. 18. Raia TJ, Kilfoyle RM. Minimizing radiation exposure in scoliosis screening. Appl Radiol 1982;46-54. 19. Gray JE, Hoffman AD, Peterson HA. Reduction of radiation exposure during radiography for scoliosis. J Bone Joint Surg (Am) 1983;65:5-12. 20. Hellstrom G, Irstam L, Nachemson A. Reduction of radiation dose in radiologic examination of patients with scoliosis. Spine 1983;8:28-30. 21. Palmer SH, Starritt HC, Paterson M. Radiation protection of the ovaries in young scoliosis patients. Eur Spine J 1998;7: 278-81. 22. Nash CI, Gregg EC, Brown RH, Pillai K. Risks of exposure to x-rays in patients undergoing long-term scoliosis. J Bone Joint Surg Am 1979;61:371-4. 23. Gregg EC. Radiation risks with diagnostic x-rays. Radiology 1977;123:447-53. 24. Marshall NW, Faulkner K, Busch HP, Marsh DM, Pfennig H. A comparison of radiation doses in examinations of the abdomen using different radiological imaging techniques. Br J Radiol 1994;67:478-84. 25. Aikenhead J, Triano J, Baker J. Relative efficacy for radiation reducing methods in scoliotic patients. J Manipulative Physiol Ther 1989;12:259-64. 26. Ahmad N, The lowdown on lumbar spine imaging [article on the internet]. San Francisco, CA: AuntMinnie.com n2006 [updated 6/19/2003, cited 2006 October 5]. Available from:http://www.auntminnie.com/index.asp?Sec=sup&Sub= xra&Pag=dis&ItemId=58461. 27. Stephens GC, Yoo JU, Wilbur G. Comparison of lumbar sagittal alignment produced by different operative positions. Spine 1996;21:1802-6. 28. Schock CC, Brenton L, Agarwal KK. The effect of PA versus AP x-rays on the apparent scoliotic angle. Orthop Trans 1980;32. 29. Tsuno MM, Shu GJ. Posteroanterior versus anteroposterior lumbar spine radiology. J Manipulative Physiol Ther 1990;13: 144-51. 30. Young KJ, Johnson J. Porcelain gallbladder. J Manipulative Physiol Ther 2002;25:534-43.
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Kenneth J. Young, DC
ABSTRACT Objective: The objective of this study was to mathematically propose the best position for frontally oriented lumbar spine radiographs. Methods: Decision analysis with a decision tree was used in this study. Factors used in the analysis were radiation dose, ease of implementation, image quality, cost, and patient comfort. Data on these factors were gathered by reviewing the literature. Results: Radiation dose was found to be the only factor with significant differences between the posterior-to-anterior (PA) position and the anterior-to-posterior (AP) position, and PA films gave lower doses to sensitive organs as compared with AP films. There is also some evidence that PA films show better overall image quality as compared with AP films. Conclusions: Based on the information used in this study, lumbar spine plain film radiographs should be obtained in the PA position. (J Manipulative Physiol Ther 2007;30:200-205) Key Indexing Terms: Radiography; Lumbar Vertebrae; Decision Support Techniques; Evidence-Based Medicine
I
t is now well established that every dose of ionizing radiation is harmful1,2 and that certain anatomical structures are more sensitive to radiation than others.3 It is also recognized that even small doses of radiation from medical investigations carry risk.4 In the United Kingdom, 100 to 250 annual deaths are now realized to be attributable to diagnostic radiation.5 Therefore, the goal in radiography of any body area is to obtain optimum visualization of the anatomical structures while minimizing the dose of ionizing radiation to patients. It is common practice for many hospitals, private health care clinics, and teaching institutions to take lumbar spine films in the anterior-to-posterior (AP) orientation. In the author’s experience, the most common—and often the only—justification for this, as opposed to the posterior-to-anterior (PA) position, is to minimize magnification and distortion of the spine by placing it as close to the film as possible. The literature was reviewed to obtain information for a decision analysis to Director, Young Radiology Consulting, Nottingham, England. Submit requests for reprints to: Kenneth J. Young, DC, Young Radiology Consulting, PO Box 9125, Nottingham, NG2 9EG, UK (e-mail: [email protected]). Paper submitted September 8, 2006; in revised form October 10, 2006; accepted November 25, 2006. 0161-4754/$32.00 Copyright D 2007 by National University of Health Sciences. doi:10.1016/j.jmpt.2007.01.013
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develop an evidence-based rationale for choosing an orientation in which to take lumbar spine radiographs. The method of positioning a body part for radiography is often based on the conscious weighing by health care professionals of various factors, including optimizing visualization of the structures, patient comfort, minimizing dose, practical issues such as whether a patient is ambulatory or not, and others. However, procedures and policies are sometimes undertaken or perpetuated because of habit, unfamiliarity with new information, or resistance to change. A MEDLINE search for articles on this topic revealed none that undertook a formal mathematical analysis of the rationale behind which position to use for lumbar spine radiography, although there is evidence of study and debate. Studies on lumbar spine plain film radiographs comprise a significant portion of the dose of ionizing radiation delivered to patients in hospital imaging departments, up to 15% of the collective dose6 and second only to computed tomography.7
METHODS The method used in this study was a formal decision analysis using a decision tree with probabilities and utilities, or quality-of-life factors, added to arrive mathematically at a conclusion on whether it is better to obtain radiographs of the lumbar spine in the AP or PA orientation.
Journal of Manipulative and Physiological Therapeutics Volume 30, Number 3
Decision analysis is an increasingly common method of making policy-level decisions in fields as diverse as manufacturing, economics, medical diagnosis, and coral reef management. Its advantage is that it can take in many elements, including subjective ones, to reach a best overall decision. Although decision analysis is not appropriate for all health care situations owing to its time-consuming nature, it can be used to help develop policies. These policies can then be implemented rapidly and efficiently with patients. Five areas were identified as relevant to the study of frontal lumbar spine films: radiation dose, ease of implementation, image quality, cost, and patient comfort. Qualitative and quantitative investigations from the literature were examined, because it is possible, although unlikely, that all qualitative studies support one position and all quantitative ones support the other. However, only the quantitative results will be of use in the calculations for this decision analysis.
Radiation Dose The PA projection has been advocated in preference to the AP projection by the British Institute of Radiology8 b[w]hen it is inevitable that the gonads are within the primary beam. . .the gonad dose may be somewhat reduced particularly in the female. . .Q Nic an Ghearr and Brennan9 calculated dose reductions of approximately 60% for female patients with the use of PA views as opposed to AP views. Heriard et al10 found that they were able to reduce part thickness by 11% by placing patients in the prone, rather than supine, position on the table and were therefore able to reduce the milliampere values by 50%. Then, using a phantom with dosimeters placed at various critical areas within it, radiation doses were measured and found to be much higher in the AP position: 1300% for eyes, 216% for ovaries, 489% for uteri, and 900% for testes. Even the thyroid dose was 17% higher, although this was not found to be statistically significant. Only the bone marrow dose was found to be higher in the PA projection, by 28%.10 Brennan and Madigan11 showed through the use of dosimeter readings from within radiographic phantoms that there was a significant reduction of dose (~39%) by using the PA projection. Real patients showed nearly a 39% decrease in entrance surface dose measurements when the PA position was used, as compared with the AP position. This was a result of an overall reduction in the milliampere values used because the PA position (ie, prone on the x-ray table) compressed patients by 9.6% in diameter. Therefore, the PA prone position not only increases the distance from the x-ray source to the radiosensitive organs but also achieves an overall reduction of dose. The change in this case was from an average diameter of 18.8 to 17.0 cm.11 Because decreasing body thickness necessitates decreasing
Kenneth J. Young PA vs. AP Radiographs
milliampere values to maintain optical density, a reduction in radiation dose is given by PA prone positioning. Although no reference was found in the literature, it is reasonable to speculate that similar reductions might be gained in the upright weight-bearing position, if patients were instructed to rest firmly against the wall grid, or if compression bands were used. Likewise, Gray et al12 found significant dose reductions to testes, ovaries, and uteri for lumbar spine radiographs performed in the PA rather than the AP position using identical technical exposure factors. Weatherburn13 also found lower doses to the thyroid and breasts in PA projections; specifically, AP positions gave 20 times the dose of PA positions in adult phantoms and 10 times the dose with an in vivo study on children. Numerous authors who examined techniques for scoliosis evaluation found reduced doses to the thyroid, eyes, and breasts when using a PA upright position as compared with an AP position.14-21 In particular, Nash et al22 found increased breast carcinogenesis in young female patients with scoliosis and a 500% to 800% increase in radiation dose with the use of AP rather than PA projections. Gregg23 noted that spinal bone marrow exposure was increased in the PA position but that the risk-benefit ratio strongly favored PA views because bone marrow has one sixth the radiosensitivity of breast tissue. Investigations relating to abdominal radiology doses were also deemed to be relevant because the same area of the body is irradiated in abdominal radiographs as that in lumbar spine radiographs, although collimation may vary. Marshall24 found that bAP projections give rise to higher effective doses than to PA projections because the majority of irradiated radiosensitive organs within the abdomen, such as the small intestine and others, lie closer to the entrance surface for the AP views in comparison to the PA views.Q In addition, the dense paraspinal musculature of the back will attenuate radiation better than would the subcutaneous fat of the anterior trunk, unless the fat is very thick. The evidence that using PA views results in less radiation dose to radiosensitive organs appears convincing. The argument that the radiation dose to these structures could be minimized with carefully placed gonad protection may be made; however, this creates artifacts on the radiographs that may obscure the pathology, and other sensitive organs, such as the intestines, would be impossible to shield without obscuring all other structures.
Ease of Implementation Implementation of the procedure is easy and requires no additional equipment, and because radiographers commonly use PA techniques with other body areas, such as the chest, they are familiar with positioning patients in this orientation.11
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It has been noted that PA views may provide for easier positioning of spinal anatomy because bplacing the patient in a prone position exposes the spinous process to easy palpation and localization.Q10 Aikenhead et al25 found that filters were useful for reducing doses to radiosensitive organs in patients with scoliosis but that the PA position also reduced radiation doses. Posterior-to-anterior positioning used in conjunction with filters was even more effective, except to bone marrow.25 Filters are easy to implement with minor curves, but severe scolioses make effective filter implementation nearly impossible; it is very difficult to shield radiosensitive organs without masking spinal structures. The PA position can easily be used for patients with even the most severe curvatures.
Image Quality Before a new health care procedure can be introduced to human beings, it must show some evidence of safety. In the case of the implementation of a new radiographic position, it must be shown not to diminish image quality. Brennan and Madigan11 showed that the PA projection actually improved image quality, albeit not to a statistically significant degree in their study. This makes sense in that a thinner body part requires a lower milliampere setting, which means less scatter radiation is generated and therefore less fog is seen on radiographs. Heriard et al10 had radiologists certified by the American Board of Radiology evaluate PA radiographs against AP ones. The results showed the vertebral bodies to be better aligned in the PA projections as compared with AP views taken with the patients’ knees flexed. This was ascribed to the divergent beam aligning better with the lumbar lordosis in the PA position. When they measured L3 transversely on both types of images, the PA views showed a magnification factor 1.031 times greater than that with the AP views. However, there was no visual increase in magnification except with obese patients. Even with obese patients, however, the reviewing radiologists did not deem the magnification to adversely affect the quality of the radiographs. Their synopsis of the situation was that10 b[e]ndplates and interspaces are better defined because of corresponding part alignment to the divergent central ray with a minimal, non-effective increase in magnification.Q It would logically follow that the sacroiliac joints, if included on the film, would also be better visualized on a PA-oriented film because, anatomically, they are wider anteriorly than posteriorly and would therefore be more parallel with a divergent x-ray beam than if patients were facing the tube. Ahmad26 stated that the prone position for lumbar spine radiographs gives better image quality because it allows the divergent rays of the primary beam to travel at angles that more closely parallel the intervertebral disk spaces. This would make sense, although the lumbar lordosis is reduced
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in recumbent positions as compared with weight bearing,27 and the difference may even be better shown on upright films. Schock et al28 examined differences between AP and PA positioning for geometric alterations by measuring the angles of the long axis of the vertebral endplates. They found no difference in the angles on the different projections. Tsuno and Shu29 judged lumbar vertebrae to have less shape distortion in the PA than in the AP position and concluded that the PA view would give a lower dose to radiosensitive organs. Overall, PA-oriented films seem to give at least as much diagnostic information as AP films and may even be better.
Cost There is no known increase in financial costs associated with positioning a patient in the PA orientation as compared with the AP orientation.
Patient Comfort No information was found regarding patient comfort in the supine vs prone position, so it was not included in the analysis; however, it seems unlikely to be a major factor.
Various factors need to be taken into account when deciding on the usefulness of a given diagnostic procedure. In this case, 5 factors were included in the initial analysis: relative radiation dose, which essentially equates to the risk of inducing disease, ease of implementation, diagnostic quality of the radiographs, cost of implementation, and patient comfort. The professional literature seemed to find no difference in PA vs AP positioning for most of these factors. Specifically, it was not deemed to be more difficult to obtain radiographs of the lumbar spine with the patient in the prone, rather than the supine, position on the table. There was no reduction in the diagnostic quality of the radiographs obtained when the patient was prone on the table; if anything, a small and statistically insignificant increase in quality was found. For the purpose of this article, differences in ease of positioning were discounted and both positions were treated as equal in the quality of the radiographs that they produce. There was no difference in costs between the PA position and the AP position; neither was information on patient comfort in the supine vs prone position discovered. Therefore, these two factors were not included in the calculations. Therefore, the only factor necessary to account for was radiation dose. An intuitive impression may be formed from reading the literature, but it was deemed worthwhile to formalize it in a decision analysis and to quantify it, however roughly. The exact probabilities about the risk of inducing cancer or birth defects in offspring of patients receiving a single
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dose of diagnostic ionizing radiation are not known at this time but are known to be very low.1 The important conclusion from the decision analysis in this case is relative probability, not exact probability. Therefore, the risk of inducing cancer in any given tissue from a single dose of ionizing radiation was arbitrarily set at 1:1000 (or 0.001 for the purpose of the calculations). This was also the figure used for the probability of inducing a genetic change that will cause a birth defect in the child of a patient receiving a single dose of radiation to the gonads, male or female. Because the literature found that the gonads are more heavily irradiated in AP views, birth defects and cancers of the abdominal structures were considered together in the decision analysis and are labeled bACBDQ on the decision tree. The only negative factor for the PA position is radiation dose to the bone marrow. Only one numerical figure was found in the literature, a 28% increase. A 28% increase of a 1:1000 chance is very small indeed, and with the information that bone marrow is only half as sensitive as gonads3 and one sixth as sensitive as breast tissue23 to the effects of radiation, this increase becomes so small as to be essentially negligible and will be treated as such in this analysis. Two states of health are possible after a single dose of ionizing radiation: (1) healthy and (2) inducement of abdominal cancer/inducement of birth defects in offspring (ACBD). These conditions are known as outcome states. The decision tree was then very simple, with only these two outcome states possible. Decision analysis requires utilities, or quality-of-life factors, to be assigned scores out of 100 to the different states of health that are possible after receiving a single dose of diagnostic radiation. Healthy, described for the purposes of this study as lack of induced disease, is usually ascribed a utility score of 100, and that was the case in this study. It is labeled bNO ACBDQ on the decision tree. Immediate death is usually given a score of 0, but that was not an option in this study. Inducement of abdominal cancer or giving birth to a child with birth defects was assigned a utility score of 60. These numbers are subjective and would vary with each individual. Some people would rate their life higher than 60 if they have cancer, and of course it would depend on the type of cancer and prognosis. Perhaps facing death from cancer within weeks would only rate a 0.5 utility; conversely, a mild cancer requiring only a single course of chemotherapy leading to complete cure may rate a 95 utility. Certainly, it seems reasonable however that having a disease reduces the quality of life. Similarly, giving birth to a child who is not entirely healthy also impacts the quality of life and reduces utility from 100; in this case, it was again arbitrarily assigned a utility score of 60. Therefore, 60 was used here as the utility score for a patient who received a single dose of radiation that led to any type of cancer or for a patient giving birth to a child with birth defects.
Kenneth J. Young PA vs. AP Radiographs
Fig 1. Decision tree. The following utilities were used for the decision analysis: NO ACBD (healthy; absence of abdominal cancer or not having a child with birth defects) = 100 and ACBD (presence of abdominal cancer or having a child with birth defects) = 60. The radiation dose data for the differences between AP and PA views vary widely in the literature, from 39% increased for AP views11 to 1000% to 2000%.13 Therefore, the highest and lowest figures will be discounted, and the rest will be averaged. Nic an Ghearr and Brennan9 found a 60% increase. Heriard10 found increases of 216% for ovaries, 489% for uteri, and 900% for testes. Nash22 found a 500% to 800% increase (average = 650%). The average of all these [(60 + 216 + 489 + 900 + 650)/5] is 463%—or an approximately 4.5 times greater dose to the abdominal organs when patients are placed in the AP rather than the PA position. In decision analysis, the results are termed expected utilities and calculated as follows: (Chance of Outcome Utility for That Outcome) + (Chance of Opposing Outcome Utility for That Outcome). In this case, the formula was as follows: (Chance of ACBD Utility for ACBD) + (Chance of Healthy Outcome Utility for Healthy Outcome). This was calculated for both PA and AP positioning. The results were then compared, and the higher number indicated the better method. For PA positioning, the chance of ACBD is 0.001, and the utility of ACBD is 60. Furthermore, the chance of a healthy outcome is 1 0.001 or 0.999, and the utility of a healthy outcome is 100. Therefore, the calculation was as follows: (0.001 60) + (0.999 100) = 99.9600. For AP positioning, the chance of ACBD is 4.5 times greater than that for PA positioning and hence was calculated as 0.001 4.5, which equals 0.0045. Again, the utility of ACBD is 60. The chance of a healthy outcome is 1 0.0045 or 0.9955, and its utility is 100. Therefore, the calculation was as follows: (0.0045 60) + (0.9955 100) = 99.5527. The higher number found as
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the expected utility for the PA position indicated an advantage for that position.
RESULTS Figure 1 shows the decision tree. The expected utility for the PA position was 99.9600, whereas that for the AP position was 99.5527. A single AP- or PA-positioned radiograph of the lumbar spine has an extremely small detrimental effect on the quality of life of a patient, but the PA position has now been mathematically shown to be less detrimental. With all other factors being essentially equal, it is better to use the PA position for lumbar spine radiographs.
DISCUSSION Errors associated with inadequate data are smaller than errors associated with no data –Charles Babbage (1792-1871)
The intentions of this study were to review the literature regarding frontal lumbar spine radiographs and to use the data to mathematically determine the best orientation in which to obtain them. It had been anticipated that this would create a complex decision tree with a large number of utilities to calculate. However, 4 of the 5 factors (ie, ease of implementation, image quality, cost, and patient comfort) emerged to be almost exactly equivalent for both types of positioning; therefore, these elements were not included in the decision analysis, which reduced the necessary size of the decision tree. In addition, the extremely small difference in bone marrow dose between the AP position and the PA position meant that it was essentially negligible in the calculation and hence reduced the size of the tree even further. Because the real risk of inducing disease from a single dose of ionizing radiation is not known, albeit known to exist, an absolute number could not be achieved from this decision analysis. However, even choosing the arbitrary 1:1000 risk serves the purpose by allowing a demonstration of the relative risks of AP and PA positioning. In combination with the radiation dose information that has been published, these relative numbers are quite useful and clearly show the increased risk associated with AP positioning. In other words, although the absolute numbers have little meaning, the result is nonetheless clear because of the relationship of the numbers relative to each other. From the data synthesized in this study, the single most important number that emerges is the estimate that AP positioning gives a dose 4.5 times greater to the abdominal organs, including the gonads, which are the most radiosensitive of all organs.3 Ultimately, the question concerning qualitative vs quantitative studies did not provide any difficulty because all the articles found in the literature search had similar findings.
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Further studies on several topics would help optimize the safety and efficacy of frontal lumbar spine radiographs. A survey of patient comfort in the prone and supine positions would provide data for a fuller analysis of the subject. It would be useful to obtain more information regarding whether reduction of the diameter of the abdomen in the upright position is practical, with or without compression bands, and how much reduction in milliampere values would be achievable. A study on collimation for the lumbar spine would also be valuable. Some practitioners take frontal lumbar spine films with 14 17-in (35 43 cm) collimation to visualize abdominal soft tissues along with the spine, others with much narrower collimation, close to the spine itself. Occasionally, a significant but clinically silent pathology is found on these wider-collimated films.30 It is unknown at this time whether the additional incidental pathology revealed by the larger collimation is offset by the additional radiation dose to the population. Finally, it would be useful to compare the distortion of osseous structures on radiographs between patients in the prone and those in the standing PA position because the lordosis is altered in the prone vs standing position.
CONCLUSIONS Although the decision tree and resulting calculations ultimately were quite simple, a thorough search of the literature was necessary to discover that many potential factors for the decision analysis were not necessary to enter into the calculations. It was found that the PA position was just as easy to implement, caused no reduction in image quality, and incurred no additional cost as compared with the more traditional AP position. No information was found regarding patient comfort in one position vs the other; therefore, this factor could not be included in the analysis. Although radiation dose findings revealed widely varying numbers, a clear advantage in dose reduction was shown for the PA position, and doses to the abdominal organs averaged 4.5 times greater with the AP position. The PA
Practical Applications ! Factors that may influence the orientation in which to take frontal lumbar radiographs include dose, ease of implementation, image quality, cost, and patient comfort. ! Of these factors, only dose is significantly different between the PA and AP positions. ! Dose to radiosensitive organs is significantly lower in the PA position. ! Based on the information used in this study, lumbar spine radiographs should be taken in the PA position.
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position for taking lumbar spine films clearly shows an overall advantage over the AP position because of its lower dose of ionizing radiation to the radiosensitive tissues of patients and the equivalency of the other factors.
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