This Article Abstract Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kemerink, G J Articles by van Engelshoven, J M A Search for Related Content PubMed PubMed Citation Articles by Kemerink, G J Articles by van Engelshoven, J M A

The effect of equipment set up on patient radiation dose in conventional and CT angiography of the renal arteries

¹ Department of Radiology, University Hospital Maastricht, P. Debijelaan 25, 6229 HX Maastricht and ² Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands

	Abstract

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Patient radiation dose in angiography of the renal arteries was assessed and optimized after installing new radiological equipment. In three separate studies (n=50, 25 and 20) patient exposure was monitored in detail. For the first study default factory settings were used, for the second the number of digital subtraction angiography (DSA) images was halved and the X-ray beam filtering during fluoroscopy was increased, and for the third study filtering during DSA was increased as well. Standard projections were derived and used in Monte Carlo simulations to derive dose conversion coefficients to calculate effective dose from the dose–area product (DAP). Dose conversion coefficients were also calculated for CT angiography (CTA). Using default factory settings on the new angiography system, DAP, number of images and effective dose were much higher than on the replaced unit. For the studies given above, DAP was reduced from 144 Gy cm² to 65 Gy cm² to 32 Gy cm², and effective dose from 22 mSv to 11 mSv to 9.1 mSv, respectively. Effective dose due to CTA was 5.2 mSv. It is concluded that modern angiography systems, resulting in high customer satisfaction, may readily cause much higher patient exposure than older systems. These doses may also be much higher than necessary. Optimization before putting such systems into use is absolutely essential. Internationally accepted recommendations for image quality and technique factors in angiography would be of great help.

	Introduction

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
International organizations recommend that radiological procedures be optimized with respect to patient dose and image quality [1–3]. There are sound reasons for these recommendations, and although it may seem counter-intuitive, they certainly hold true for new and technologically advanced systems. In many respects such new equipment is far more powerful than systems of the past, virtually eliminating previously existing limitations in X-ray output, number of images to be taken and frame rate. Thus instrumental barriers that used to inherently limit exposure are now almost absent, and in this situation it is tempting to acquire more images of better quality than previously. This is expedited by detector systems that have a large dynamic range, which yield less noisy and thus better images instead of overexposed images when the dose is increased. Moreover, it appears that manufacturers of X-ray equipment try to obtain high customer satisfaction by implementing default settings that guarantee excellent image quality, even when it is not strictly needed. It is ironic that the technical improvements in imaging equipment and image processing, which in theory create scope for dose reduction with respect to exposure in the past, easily lead to the opposite. In this study we will see that this happened in our hospital, and therefore might be happening at many other institutions as well. Other investigators also observed that using digital instead of conventional equipment did not necessarily lead to lower patient doses [4]. In our hospital the X-ray equipment used for angiography has recently been replaced. The increase in dose was evident because dosimetric studies had previously been performed on the older angiography system [5]. This circumstance once more illustrates the benefit of evaluating radiological equipment and protocols whenever possible.

In the efforts undertaken to optimize the diagnostic process, evaluation of other imaging modalities may be worthwhile. At present the classical X-ray projection technique, often applied in the form of digital subtraction angiography (DSA), is still considered as the gold standard in angiography [6]. However, new methods have emerged, for example CT angiography (CTA) and MR angiography (MRA) [6]. Promising results on the diagnostic accuracy of these methods have been reported, where MRA has the advantage that no ionizing radiation is used at all [7]. Since the clinical value of these new methods had not yet been established in a large prospective study, our institution, in cooperation with several other hospitals, decided to evaluate the three methods, i.e. DSA, CTA and MRA, by applying them all to the same group of patients in a multicentre trial. This project, called RADISH (Renal Artery Diagnostic Imaging Study in Hypertension) aims at comparing the diagnostic value and cost of DSA, CTA and MRA in the workup of patients suspected of having renovascular hypertension. The present article reports on the dosimetric aspects of this project, including the optimization of exposure to ionizing radiation for a new angiography system.

	Materials and methods

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Patients
Selection criteria for patients to participate in this study were persistent hypertension with a diastolic blood pressure higher than 95 mm Hg and age between 18 years and 75 years. All patients had to exhibit at least one clinical symptom that suggested the presence of renal artery stenosis. Exclusion criteria were pregnancy, all contraindications for contrast-angiography and MRI, and contraindications for intervention. Dosimetric data were collected in three studies, including 50 patients in the first study, 25 in the second and 20 in the third, thus yielding in total 95 patients. Between the studies the image acquisition protocols were optimized (the brief overview of these changes in Table 3 in the three columns under "Acquisition protocol" may be helpful in the following). The medical ethics committee of our institution approved the study and all patients gave written informed consent.

Angiographic equipment
DSA was performed with a fully digital C-arm system. It is equipped with an all metal X-ray tube with spiral groove bearing, an anode angle of 11° and two focal spots of nominally 0.3 mm and 1.0 mm. The high-voltage generator is of the high frequency type. The image intensifier has a selectable field of view (FOV) with a diameter () of 38 cm, 31 cm, 25 cm, 20 cm or 17 cm. In nearly all cases the 31 cm FOV was used.

There are three manually selectable X-ray beam filters: Filter 1 gives a filtration equivalent to 0.6 mm Cu+4.3 mm Al, Filter 2 to 0.15 mm Cu+4.3 mm Al; and Filter 3 to 4.0 mm Al. The selection of a different filter is not coupled with changing any other system setting. There is one more filter (Filter 0), corresponding to 3.0 mm Al equivalent, which can only be set by software. This Filter 0 was used during DSA in the first two studies, while Filter 2 was used for this purpose during study number three. The additional filtration of approximately 0.5 mm Al equivalent caused by the flat ionization chamber that measured DAP has been included in the figures given above.

Fluoroscopy is performed in a pulsed mode with 12.5 frames per second. In the first study Filter 2 was generally used during fluoroscopy, and the heavier Filter 1 in the second and third study. These filters were the default settings, which could be changed by the operator, but this happened rarely. The system has automatic exposure control both during fluoroscopy and DSA. An image matrix of 1024 x 1024 was used. The initial setup of the equipment, e.g. of the filter settings, and the acquisition protocols was performed by the manufacturer. This setup was used during the first study and in clinical practice before this dosimetric study was initiated.

The dose rates on the face of the image intensifier during fluoroscopy and the doses required for DSA images, are given in Table 1. These doses and dose rates were measured with an ionization chamber on the surface of the image intensifier and using a 1.5 mm Cu absorber that was mounted against the diaphragm housing on the X-ray tube. In the presence of the 1.5 mm Cu plate the X-ray beam filter setting had a small influence on the entrance dose and dose rate of less than 3%. These measurements give an impression of the sensitivity of the image receptor system. The entrance dose rate during fluoroscopy was also measured using a 20 cm thick perspex phantom that simulated a patient. This was done for the three manually selectable X-ray beam filters (Filters 1–3). The entrance doses during DSA were measured for the two filters that had been used in patient studies (Filters 0 and 2). These data are a measure of the overall sensitivity of the system. It is to be noted that in all experiments the anti-scatter grid was present on the entrance surface of the image intensifier.

On both X-ray systems (X-ray stand and CT) contrast medium (Omnipaque (iohexol); Nycomed-Amersham, Oslo, Norway) was injected using a remotely triggered power injector (Medrad Spectris, Indianola, PA).

Angiographic procedures
Following selective catheterization of the renal vessels, blood samples for the determination of active plasma renin were drawn from the aorta and both renal veins. Fluoroscopy was used during catheter positioning, but in this part of the study no contrast medium was applied because this might affect renal physiology.

Subsequently, mean renal blood flow for each kidney was assessed with the ¹³³Xe-washout technique. These studies required some additional fluoroscopy, because the renal arteries and veins had to be catheterized selectively for this purpose. After the ¹³³Xe procedure, a standard DSA procedure was performed, with the contrast medium administered in the aorta slightly above the renal arteries or, in selective imaging, in a renal artery. In the first study, DSA runs usually consisted of about 20 images acquired with an initial frame rate of 3 per second, reduced to 2 frames per second after the arterial phase and still later to 1 per second. In the second and third study of the present investigation the frame rates were 2 and 1 per second, respectively. Typical projections, characterized by the midfield location, beam direction and field size, are shown in Table 2.

Dosimetry
The X-ray angiography system is equipped with a flat ionization chamber that determines the dose–area product (DAP) separately for fluoroscopy and imaging. This system was calibrated by measuring air kerma free in air and the field size, and comparing the product of these two quantities with the reading on the console. Total fluoroscopy time is logged automatically. Each angiographic procedure was attended by an observer who recorded the number of DSA runs, the number of images per run, DAP per run, DAP for fluoroscopy, rotation and angulation of the C-arm, beam position, peak tube voltage, the distance between X-ray focus and image intensifier and the size of the input field chosen on the image intensifier. Also, the age and sex of the patient were recorded, as well as the name of the radiologist who performed the procedure. From the data on the projections, five characteristic standard views were derived. These were used in Monte Carlo simulations, together with parameters characterizing X-ray beam quality. We used for DSA 75 kVp and a beam filter of 3.0 mm Al (in studies 1 and 2), and 75 kVp and 0.15 mm Cu+4.3 mm Al (in study 3). For fluoroscopy, beam quality parameters were 75 kVp with a filter of 0.15 mm Cu+4.3 mm Al (in study 1), and 97 kVp with 0.6 mm Cu+4.3 mm Al (in studies 2 and 3). The latter two X-ray tube peak voltages were averages since the peak tube voltage is automatically adapted to the attenuation caused by the patient, while the peak tube voltage for DSA was a fixed setting. Note that the additional filtration caused by the flat ionization chamber (approximately 0.5 mm Al) has been included. The effective dose was calculated as the sum over all projections of the product of the measured DAP and the corresponding dose conversion coefficient (DCC) resulting from the Monte Carlo simulations, where the contributions from fluoroscopy and DSA were estimated separately.

In CTA a single scan protocol was followed strictly. There only existed a small variation in the length of the interval that was scanned, and therefore this interval was determined from the number of CT slices that actually had been acquired.

Finally, note that our dosimetry implicitly assumes that patients have a size similar to the phantoms ADAM or EVA.

Monte Carlo calculations
The present calculations of DCCs that relate DAP to effective dose were performed at the Department of Radiation Technology of the Interfaculty Reactor Institute using the standard radiation transport code Monte Carlo N-particle (MCNP) version 4C [8]. As in similar calculations [9], the mathematical phantoms ADAM and EVA represent male and female adult patients [10]. Taking the technical parameters specified above, input X-ray spectra were generated with the IPEM-78 spectrum processor [11]. The fluoroscopy and DSA projections that were simulated are given in Table 2.

Exposure to X-rays during the CTA procedure was quantified in the following way. The X-ray beam profile within the fanbeam was measured using thermoluminescence dosimeters (TLD 100 (LiF), Harshaw, Solon, OH). As a check, measurements were also performed with a pencil ionization chamber, yielding virtually identical results (difference between the profiles was well below 5% of the maximum of the profile). Monte Carlo radiation transport calculations have been performed for 120 kVp X-ray beams and tomographic scans with sections of 5 mm thickness covering the whole trunk region, using the same mathematical phantoms and MCNP code as used for fluoroscopy and DSA. These calculations have also been performed at the Interfaculty Reactor Institute. Exposure due to the projection scan, used for planning of the scan interval, was also simulated. The effective dose for a scan interval of any length may thus be calculated by adding the effective doses for the required number of slices. When the scan interval did not exactly coincide with a set of simulated slices, appropriate fractions of the effective dose of simulated slices at the boundaries of the scan interval were taken.

	Results

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Table 1 shows entrance doses, entrance dose rates and the corresponding peak tube voltages. Together they characterize the sensitivity of our imaging system.

Standard projections derived from the exposure conditions encountered during the angiographic procedures are presented in Table 2. Effective DCCs, the effective dose per unit DAP calculated with the Monte Carlo simulations, are given for five different projections, for three different X-ray beam qualities and the two sexes, i.e. 30 different combinations.

The number of patients included in the three consecutive studies is shown in Table 3. In total 95 patients participated. The average age of the patients was 52 years in all three studies. A summary of changes in the acquisition protocols that distinguished the three studies is also presented in Table 3 under "Acquisition protocol". Finally, Table 3 contains a (small) selection of the large amount of experimental data that has been collected; shown are the number of DSA series (runs), the number of DSA images per run, fluoroscopy time, total DAP and effective dose, and the contribution of fluoroscopy to DAP and effective dose. The highest contribution to both DAP and effective dose came from the central projection (PA-M; data not shown). For comparison, results obtained with our previous angiography system (Philips Arc C, equipped with a film exchanger and DSA facility) are shown also.

	Discussion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

The present angiography stand had been installed and tested by the manufacturer and was then put into clinical use without further evaluation of the protocols and their settings. Only later, in the present study, and in a study related to neurointerventions [12], patient dose and protocols were evaluated. The angiographic procedure performed in the present study is a combination of parts of the procedures AANR (intra-arterial arteriography of the renal arteries) and AANB (intra-arterial aortography, including sampling of blood in the renal arteries and veins) that had been monitored on the old system in our hospital [5]. Assuming an overlap between AANR and AANB of 25% in terms of exposure, one would expect, on the basis of the old data, that the number of images per procedure should be about 39, while in the new practice 84 were made. Similarly, the extrapolated DAP is 60 Gy cm² (now in fact 144 Gy cm²), fluoroscopy time 8.6 min (now 8.9 min) and effective dose 8.0 mSv (now 22 mSv). The values for DAP and effective dose found in study 1 are also high compared with the values reported in the literature, as may be seen from Table 5. It should be noted, however, that this latter comparison is somewhat hampered by the uncertainty in the exact nature of the procedures that were monitored in the various studies and by the fact that in some studies median instead of mean values were reported.

On the old system the total X-ray beam filtration was fixed to 6 mm Al, so increasing the filter thickness during DSA was also considered. In consultation with the manufacturer it was decided to increase the filter thickness from 3.0 mm Al (Filter 0) to 0.15 mm Cu+4.3 mm Al (Filter 2). Again, the images contained slightly more noise, but the quality remained adequate, as comparison of images with the two filters showed. The modified filtration reduced DAP again by about 50%, but the reduction in the effective dose was less. Nevertheless, this reduction in DAP is important, because in the same angiography suite interventions are performed that are complicated and very time consuming. For such procedures, several radiation accidents have been reported in the literature [14]. In the reported cases, high entrance doses caused deterministic effects in the skin and sometimes in the underlying tissue. Assuming that exposure parameters, such as field size and distance of the patient to the X-ray focus, remained the same after changing filters, the entrance dose depends approximately linearly on DAP, so that the additional filtering for DSA, which is now standard, reduces the risk for skin damage substantially. A comparison of our results with data reported in previous studies is presented in Table 5. The final DAP in our study 3 is the lowest of all published values, showing the effect of using relatively hard radiation and limiting the number of DSA images. The effective dose is in the midrange. Our ratio of effective dose and total DAP then is, as to be expected, relatively high, at 0.286 mSv (Gy cm²)^-1.

It is likely that several similar angiography units are in use without having been optimized, while the present results underline the need for critical evaluation and optimization. It would clearly be advantageous if recommendations were available concerning image quality, X-ray beam quality and acquisition protocols to be used in angiography. For a number of common examinations the European Commission has specified guidelines for technique factors as well as image quality criteria. We hope to have provided useful data for promoting future extensions of such guidelines to angiography.

	Conclusion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
A modern, highly appreciated angiography system caused a much higher patient exposure than its predecessor. By increasing X-ray beam filtration and by halving the number of DSA images, patient exposure was lowered to an acceptable level, while adequate diagnostic information was maintained. It seems essential that new systems be optimized before putting them into service. Such optimization could be greatly facilitated by international recommendations for technique factors in angiography.

	Footnotes

 
This study was financially supported by The Dutch Health Care Insurance Board (Grant number OG 97-003).

Received for publication October 31, 2002. Revision received March 17, 2003. Accepted for publication May 29, 2003.

	References

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 

1. International Commission on Radiological Protection, ICRP publication 73. Radiological protection and safety in medicine. Oxford: Pergamon Press, 1996.
2. International Commission on Radiological Protection, ICRP publication 60, 1990 Recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press, 1991.
3. Council directive, 97/43/Euratom of June 30 1997 on "Health Protection of Individuals against the Dangers of Ionising Radiation in Relation to Medical Exposure" Publication L. 180, Official Journal of the European Communities, Luxembourg, 1997.
4. Hart D, Wall BF. Potentially higher patient radiation doses using digital equipment for barium studies. Br J Radiol 1995;68:1112–5.[Abstract]
5. Kemerink GJ, Kicken PJH, Schultz FW, Zoetelief J, van Engelshoven JMA. Patient dosimetry in abdominal arteriography. Phys Med Biol 1999;44:1133–45.[CrossRef][Medline]
6. Aitchison F, Page A. Diagnostic imaging of renal artery stenosis. J Hum Hypertens 1999;13:595–603.[CrossRef][Medline]
7. Vasbinder GBC, Nelemans PJ, Kessels AGH, Kroon AA, de Leeuw PW, van Engelshoven JMA. Diagnostic tests for renal artery stenosis in patients suspected of having renovascular hypertension: A meta-analysis. Ann Intern Med 2001;135:401–11.[Abstract/Free Full Text]
8. Briesmeister JF, editor. MCNP, A general Monte Carlo N-particle transport code, version 4C. Manual LA-13709-M. Los Alamos, NM: Los Alamos National Laboratory, 2000.
9. Schultz FW, Geleijns J, Zoetelief J. Calculation of dose conversion factors for PA chest radiography of adults with a relatively high-energy X-ray spectrum. Br J Radiol 1994;67:775–85.[Abstract]
10. Kramer R, Zankl M, Williams G, Drexler G. The calculation of dose from external photon exposures using reference human phantoms and Monte Carlo Methods. Part I: The male (Adam) and female (Eva) adult mathematical phantoms. GSF-Bericht S-885. GSF - National Research Center for Environment and Health, Neuherberg, Germany, 1986.
11. Catalogue of diagnostic X-ray spectra and other data. The Institute of Physics and Engineering in Medicine. Report No. 78. York: IPEM, 1997.
12. Kemerink GJ, Frantzen MJ, Oei K, Sluzewski M, Van Rooij WJ, Wilmink J, et al. Patient and occupational dose in neurointerventional procedures. Neuroradiology 2002;44:522–8.[CrossRef][Medline]
13. Barkhausen J, Schoenfelder D, Nagel HD, Stöblen F, Müller R-D. Optimierung von Zusatzfilterung, Durchleuchtungskennlinie und Bildverstärker-Eingangsdosisleistung zur Reduktion der Strahlenexposition bei der Angiographie. Fortschr Röntgenstr 1999;171:391–5.[CrossRef]
14. Koenig TR, Mettler FA, Wagner LK. Skin injuries from fluoroscopically guided procedures: Part 2, Review of 73 cases and recommendations for minimizing dose delivered to patient. AJR Am J Roentgenol 2001;177:13–20.[Free Full Text]
15. Steele HR, Temperton DH. Patient doses received during digital subtraction angiography. Br J Radiol 1993;66:452–6.[Medline]
16. Vañó E, Gonzalez L, Fernandez J M, Guibelalde E. Patient dose values in interventional radiology. Br J Radiol 1995;68:1215–20.[Abstract]
17. Ruiz-Cruces R, Perez-Martinez M, Martin-Palanca A, Flores A, Cristofol J, Martinez-Morillo M, et al. Patient dose in radiologically guided interventional vascular procedures: Conventional versus digital systems. Radiology 1997;205:385–93.[Abstract/Free Full Text]
18. Williams JR. The interdependence of staff and patient doses in interventional radiology. Br J Radiol 1997;70:498–503.[Abstract]
19. Mini RL, Schmid B, Schneeberger P, Vock P. Dose–area product measurements during angiographic X ray procedures. Radiat Prot Dosim 1998;80:145–8.[Abstract]
20. McParland BJ. A study of patient radiation doses in interventional radiological procedures. Br J Radiol 1998;71:175–85.[Abstract]

This article has been cited by other articles:

				F. A. Mettler Jr, W. Huda, T. T. Yoshizumi, and M. Mahesh Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog Radiology, July 1, 2008; 248(1): 254 - 263. [Abstract] [Full Text] [PDF]

				W.H. Backes and R.J. Nijenhuis Advances in Spinal Cord MR Angiography AJNR Am. J. Neuroradiol., April 1, 2008; 29(4): 619 - 631. [Abstract] [Full Text] [PDF]

				F. W. Schultz and J. Zoetelief Dose conversion coefficients for interventional procedures Radiat Prot Dosimetry, December 1, 2005; 117(1-3): 225 - 230. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Kemerink, G J Articles by van Engelshoven, J M A Search for Related Content PubMed PubMed Citation Articles by Kemerink, G J Articles by van Engelshoven, J M A