- Email: [email protected]
A comparison of tomography and zonography during intravenous urography
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oF A P H E R S
Radiography (1996) 2, 99-109
A C O M P A R I S O N OF T O M O G R A P H Y A N D ZONOGRAPHY DURING INTRAVENOUS UROGRAPHY S. J. Daniels and P. C. Brennan* School of Diagnostic Imaging, St Anthony's, Herbert Avenue, Dublin 4, h'eland (Received 13 September 1995; accepted 17 February 1996)
Purpose: Wide angle tomography is used regularly during excretion urography to overcome the problem of bowel shadows obscuring the kidneys. Because of the increased focal depth zonography often requires fewer exposures. This study explores the possibility of employing zonography regularly for excretion urography with emphasis on radiation dose and image quality. Methods: A specially constructed renal phantom was used to establish radiation dose levels for tomography and zonography. Subjective analyses on subsequent images were carried out. Clinical trials compared both techniques to assess image quality objectively and to determine the optimum number of exposures. Ten patients were employed for each technique. Results: Statistical analysis demonstrated that fewer exposures per intravenous urography examination were necessary for zonography (1.6) compared with tomography (2.5) P=0.002. No differences were observed between the techniques for image quality or radiation dose per exposure. It was estimated that excretion urography employing zonography instead of tomography would reduce th~ effective dose equivalent by approximately 10 per cent. Conclusions: This investigation suggests zonography should replace tomography for routine excretion urography with potential benefits for patient dose and examination costs. Key words: radiation; image quality; slices; kidneys.
INTRODUCTION Intravenous urography (IVU), remains widely used as an imaging technique for the urinary tract [1]. It demonstrates the anatomy of the whole urinary system and, to some extent, renal function. This technique can demonstrate most lesions of the renal drainage system, providing renal function is maintained. The examination is both versatile and cost effective and the basic structure has seen little change since it was first described in the 1920s. One problem associated with imaging the kidneys arises from visual obstruction of these organs due to gas and faecal material in the overlying portions of the small and large intestines. Attempts have been made to overcome this problem by abdominal preparation. The application of bowel cleansing has been supported by a number of workers with some specifying that the laxatives should be given up to two nights preceding the examination [2, 3]. However, abdominal preparation is not without *Author to whom correspondence should be addressed. 1078-8174/96/020099 + 11 $12.00/0
© 1996 The College of Radiographers 99
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Daniels and Brennan
limitations. It has been argued that laxatives have an insignificant effect on the resultant image quality since the section of bowel most likely to obscure the kidneys, the transverse colon, is rarely cleared adequately, even with the most vigorous preparation regimens. Laxatives can also lead to gas filled bowels, resulting in equally poor visualization of the kidneys [4]. Bowel cleansing can offer considerable discomfort to the patient [5]. Therefore an effective technical solution to the problem of bowel shadowing is often adopted: renal tomography [6]. The principles and applications of tomography are well described. However, for the purposes of the current investigation, it is important to note that the radiation doses employed with abdominal tomography are quite substantial, with a tomographic exposure delivering a skin dose as high as 23 mSv [7]. A more recent study reported a dose increase of 117 per cent when tomography was employed [8]. Therefore it is a safe assumption that if renal area tomography is included within an IVU examination it will contribute significantly to the overall radiation dose, particularly if multiple sections are required. Clearly research into dose limitation for renal tomography is potentially beneficial for patients. Zonography, first described by Ziedses Des Plantes [9], is a subcategory of tomography employing an exposure angle of less than 10°. As a result of this smaller angle the thickness of the focal plane is considerably increased when compared with that of a wider tomographic angle of 3 0 4 0 °. This may remove the need for multiple sections and therefore potentially reduce the radiation dose delivered to a patient undergoing tomographic exposure. However the reduced angle employed with zonography may have implications for image quality factors, such as reduced bowel blurring efficacy [10, 11], which supports the widespread application of larger angles for excretion urography. There does, however, appear to be a paucity of published information directly comparing radiation doses and image quality between tomography and zonography for a particular type of examination. The aims of this study were therefore to: • record entrance and exit radiation dose levels with tomography and zonography of the renal area using a specially constructed phantom; • apply each technique to a number of patients to assess image quality and optimum number of exposures.
MATERIALS AND METHODS Methods
Phantom tests using both tomographic (30 °) and zonographic (8 °) exposure angle settings to assess radiation dose and establish the feasibility of a patient trial were carried out. In addition, since zonography had not been employed previously in the department where this study was carried out, the resultant image quality was checked subjectively before seeking ethical approval for the patient trial. Patient trials evaluating both techniques for image quality and optimum number of sections per examination were carried out. Phantom tests. No commercial model was available for this work, therefore, a phantom was designed and built to simulate a human kidney after injection of contrast. Pigs' kidneys were placed within a water bath, and with the help of a plinth positioned
Tomography vs Zonography During IVUs
l=cm
101
x
Figure 1. Diagrammatic representation of the simulated human kidney used in the phantom experiments.
Figure 2. Photographof the simulated human kidney used in the phantom experiments.
in their true antero-posterior position as described in Gray's Anatomy [12] (Figs 1 and 2). Calyceal systems were moulded from a silicon adhesive and placed in suitable locations within the kidneys to resemble contrast-enhanced kidneys (Fig. 3). A consistent thickness of polythene bubble wrap was placed between the kidneys and the water surface to simulate bowel shadowing. The distance from the anterior surface to the posterior surface of the phantom was 18 cm, a distance which correlates well with commercially available models. Five images of this phantom were then acquired at 0.5 cm intervals using tomography and zonography. This procedure was carried out three times. The exposure used, 60 kVp and 50 mA, was only slightly less than that employed for a typical patient in the department and the doses recorded correlated well with that of patient doses [13]. The resultant images resembled those from an actual examination (Fig. 4). The images from both techniques were assessed subjectively by an evaluation panel to ensure that the quality achieved was adequate.
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Danie& and Brennan
Figure 3. Mouldedsilicon adhesive used to mimic the ureteric pelvis in the phantom experiments.
The entrance and exit doses were recorded from projections providing images with evidence of the calyceal system. These measurements were carried out by placing lithium fluoride thermoluminescent dosemeters just above the water level and on the under surface of the phantom to establish entrance and exit doses for each exposure. The data was subjected to statistical analysis as described below. Patient trial. Following ethical approval 10 adult patients were chosen for each group, so that the mean weight of both samples lay within 10 kg of 70 kg [14]. individuals meeting these criteria were chosen consecutively. In selecting patients, the initial anterio-posterior abdomen projection was also considered. For a valid comparison between patients it was necessary to ensure that the degree of bowel shadowing was comparable and any patients with excessive or absent bowel shadowing were therefore eliminated from the study. This elimination was carried out by an independent observer who did not know which technique was to be employed. Patients who had conditions such as nephroblastomas which grossly affected the function or structure of the kidneys were not included. Finally, all individuals chosen for the study were consenting patients with no possibility of pregnancy. A record of the number of sections necessary to provide adequate diagnostic information of the renal area was kept for each patient. In addition, each image produced was assessed using the following qualitative criteria: image contrast, image
Tomography vs Zonography During IVUs
103
Figure 4. Zonogram of the renal area from the phantom experiments. Calcyesand the upper portion of the ureter can be seen clearly.
sharpness and successful blurring of the bowel. A scoring system was set for each of the three categories: • • • • •
excellent good adequate poor inadequate
5 4 3 2 l
points points points points point
All tomograms/zonograms were taken at the same stage of the investigation i.e. immediately after the 5-min film.
Evaluation panel A panel of senior radiographers and radiologists was formed. The films were sorted randomly and all markings and legends obscured so the assessors could not distinguish between tomograms and zonograms. The films were presented individually to the members of the panel who scored each film in the three categories already outlined. The films were viewed under the same conditions: against a single light box in a darkened room. Inter-observer variation was checked statistically.
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Figure 5. Entrance and exit surface doses for tomography and zonography. D0 Entrance surface dose; m, exit surface dose.
Statistical analysis The data from the dose measurements, the number of slices, and the image quality scoring were all analysed using a one-way analysis of variance (ANOVA) and comparisons between pairs of means using the Scheffe-F method [15]. A probability level of P<0.05 was regarded as significant.
Equipment calibration tests To ensure reproducibility of the results the tomographic equipment was tested each morning for consistency before any experiments were carried out. The angle of exposure, fulcrum height, depth of focal plane and tube output were measured using the fan, the pin tool and focal plane thickness tests [16]. For all aspects of this study the same tomographic unit a Philips BTS4 (Philips Medical systems, Hammersmith, London, U.K.), the same film/screen combination (Quanta Super Rapid screens with H R - L X R F film) and the same X-ray processing unit a Du Pont daylight system (Du Pont [U.K.] Ltd., Stevenage, Hertfordshire, U.K.) were employed.
RESULTS
Phantom tests The entrance and exit doses for the phantom using both the tomographic and zonographic techniques are illustrated in Fig. 5. No statistically significant difference was observed between the two techniques. In addition, no subjective differences between the two techniques were evident following comparison of the resultant images. In the wake of the satisfactory outcome of these results, the patient trial was considered to be feasible and ethical approval was granted.
Tomography vs Zonography During IVUs
105
2
©
¢5
Tomography Figure 6.
c~
4
Zonography
Slices required per examination for tomography and zonography.
q-
3 09
2 0~
1
0
Tomography
Zonography
Figure 7. Qualitycriteria ibr tomography and zonography. [], Contrast; [], sharpness; l , bowel blurring.
Patient trials Following statistical analysis of the data the average number of exposures per patient was Shown to be significantly higher with t o m o g r a p h y P = 0.002 (Fig. 6). Analysis of the image quality scores for each film, both for the individual categories (Fig. 7) and for the total summation, demonstrated no statistical difference between the two techniques (Fig. 8). No significant inter-observer error was noted.
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Zonography
Figure 8. Summation of quality criteria scores for tomography and zonography. Table 1. Raw data from the phantom experiments Radiation entrance dose (mSv)
Radiation exit dose (mSv)
2.08 2.13
0.41 0.39
Tomography Zonography
Table 2. Raw data from the patient experiments Image sharpness
Image contrast
Image blurring
Summation of image scores
No. of slices per patient*
3"09 3'39
2.98 3'20
3.62 3.39
9.69 9.98
2.53 1.6
Tomography Zonography *Significant difference
The raw data for both p h a n t o m and patient experiments are summarized in Table 1. DISCUSSION The overall aim of this work was to establish the possibility of employing a zonographic technique for intravenous urography instead of tomography as currently practised in m a n y departments in the U.K. and Ireland. The advantage of reduced slices per examination with zonography due to an increased focal depth must be balanced against
Tomography vs Zonography During IVUs
107
any possible reduction in image quality. It is also essential to consider the implications for radiation dose. Initially it was necessary to ensure that dose to the patient would not increase, or image quality substantially decrease, as a result of a change in technique. Preliminary work was therefore carried out using a phantom designed and built by the authors. Although not ideal, the resultant images were quite realistic, and the exposure used and dose administered suggested that the degree of attenuation within the phantom was similar to that within a live individual. "The quality of the resultant phantom images produced by both techniques was considered to be comparable. The doses recorded using the two techniques were very similar and no significant differences were seen. In the patient trial, the number of exposures required per examination was reduced significantly using zonography, therefore, it is safe to assume that patients undergoing IVUs with zonography will receive a substantially lower radiation dose. Following an estimation of the effective dose equivalent using the data provided in the National Radiological Protection Board Report 262 [17] and the entrance surface measurements recorded with this investigation, it is proposed that for a complete, average IVU examination there will be approximately a 10 per cent reduction in radiation dose using zonography. It is reassuring to note that the dose delivered for both zonographic and tomographic projections has reduced significantly from that recorded in the late 1970s [7]. A larger dose technique may be considered acceptable if the images resulting from this are more informative. However in this study the objective assessment of the evaluation panel found no such advantage. This appears to be in direct contrast to the work of Dure-Smith and McArdle in 1972 [18], which states that zonography does not dispense with bowel shadowing adequately. Factors that may contribute to the acceptability of zonography in the current work may be the emergence of higher quality receptors over the past two decades, along with more consistent and accurate tomographic equipment and shorter exposure times. It is interesting to note that some of the earlier workers did advocate zonography, which in an attempt to reduce the number of tomographic cuts was laudable, however the benefits were merely linked to time and expense with no emphasis placed on radiation dose and no objective assessment made to the resultant image quality [19 21]. The use of a thicker slice in renal zonography also maximizes subject contrast and information in an area where inherent contrast is low [1 l]. These factors have led to the adoption of small exposure angles for other regions of the body [22]. It must be acknowledged that this study did not examine the ability to detect specific pathologies as a discriminant between the two techniques. The authors, although proposing zonography as the optimal technique, cannot therefore eliminate the need to employ wide angle tomography as an accessory or even alternative technique for some patients. Dure-Smith and McArdle [18] concluded that the optimum method of imaging the kidneys required a minimum of three radiographs and rejected the use of zonography because bowel shadowing was not totally eliminated. Some authors, notably Chapman and Nakielny [23] and Whitehouse [24], base their descriptions of IVU tomography on the 20-year-old work of Dure-Smith and McArdle [18]. Consequently many practitioners employ large angles for renal examination. Recent advances in technology, the need for reduction in radiation dose and the financial and time savings achieved by taking fewer radiographs support the use of zonography as described in this paper rather than the earlier conclusions of Dure-Smith and McArdle.
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Daniels and Brennan CONCLUSIONS
(a) Zonography required fewer slices than tomography to demonstrate the kidneys during excretion urography. (b) The radiation dose per slice for each technique demonstrated no significant difference. (c) N o loss of quality was noted using zonography. (d) Zonography should be employed for routine excretion urography in preference to wide angle tomography.
Acknowledgements The authors are grateful to Ms Kate Matthews for discussing the results and helping to prepare the manuscript. Thanks are also due to Mr Stephen O'Connor who helped with the illustrations.
References 1. Baddeley H. Excretion urography. In: Radiological investigation. Chichester: John Wiley, 1984:254 64. 2. Gaylord GM. Urography, pyelography, cystography and urethrography. In: Diagnostic and interventional radiology--a clinical manual. Philadelphia: W. B. Saunders, 1989: 46-9. 3. Bryan GJ. Urinary tract. In: Diagnostic radiography. Edinburgh: Churchill Livingstone, 1981: 290-311. 4. George CD, Vinnicombe SJ, Balkissoon ARA, Heron CW. Bowel preparation before intravenous urography: is it necessary? Br J Radiol 1993; 66" 17-19. 5. Coombs BM. Laxatives as colonic preparation for barium enema: the patient's viewpoint. Radiography 1983; 49: 221-2, 6. Bates E. Renal tomography in neonates. Radiography 1982; 48" 104-5. 7. Meredith WJ, Massey JB. Tomography. In: Fundamentalphysics of radiology. Bristol: John Wright, 1977: 358--76. 8. Cade WJS, Haddaway MJ. Measurement of absorbed skin dose in tomography. Radiography 1983; 49: 24-6. 9. Ziedes des Plantes PG. Planigraphy~ a roentgenographic differentiation method. Doctoral thesis, 1934. 10. Short AJ. Tomography: finding a formula for layer thickness. Radiography 1982; 48" 207. 11. Christensen EE, Body section radiography. In: Christensen'sphysics of diagnostic radiology. Philadelphia: Lea & Febiger, 1990:242 56. 12. Williams PL, Warwick R. The urinary organs. In: Gray's anatomy. Edinburgh: Churchill Livingstone, 1980: 1387-1409. 13. Brennan PC. A dose reducing technique in examination of the pelvis in pregnant women. Radiogr Today 1985; 61: 31~4. 14. National Radiological Protection Board (NRPB). Number and choice of patients. In: National protocol Jor patient dose reduction in diagnostic radiology. Prepared by the Dosimetry Working Party of the Institute of Physical Sciences in Medicine, Chilton: NRPB, 1992:8 9. 15. Sokal RR, Rohlf FJ. Biometry: The principles and practice of statistics in biological research. San Francisco: WH Freeman, 1981. 16. Milner SC, Case J. Tomographic exposure angle: a comparison of two tests. Radiography 1984; 50: 177-9. 17. National Radiological Protection Board (NRPB). In: Estimation of effective dose in diagnostic radiology J?om entrance surface dose and dose area product measurements. Chilton: NRPB, 1992: 8-9. 18. Dure-Smith P, McArdle GH. Tomography during excretory urography: technical aspects. Br J Radiol 1972; 45: 896901. 19. Gillespie HW, Maille WBD. Routine tomograms with excretion urography: a report on 140 cases. Br J Radiol 1954; 27: 344-9.
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Westra D. Zonographie die Tomography mit sehr geringer Verwischung. Rontgenstrahlen 1962; 97:605 18. 21. Ettinger A, Fainsinger MH. Zonography in daily radiological practice. Radiology 1966; 87: 82 6. 22. Yamamoto Y, Yoshida S, Maeda T. An experimental and clinical study of zonography of the lung by means of Fuji computed radiography. Nippon Igaku Hoshasen Gakkai Zasshi 1992; 52:611-22. 23. Chapman S, Nakielny R. Excretion urography, in: A guide to radiological procedures. London: Bailli6re Tindall, 1986: 80-7. 24. Whitehouse GH. The urinary tract. In: Techniques in diagnostic radiology. Oxford: Blackwell Scientific, 1983: 215~5. 20.
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Radiography (1996) 2, 99-109
A C O M P A R I S O N OF T O M O G R A P H Y A N D ZONOGRAPHY DURING INTRAVENOUS UROGRAPHY S. J. Daniels and P. C. Brennan* School of Diagnostic Imaging, St Anthony's, Herbert Avenue, Dublin 4, h'eland (Received 13 September 1995; accepted 17 February 1996)
Purpose: Wide angle tomography is used regularly during excretion urography to overcome the problem of bowel shadows obscuring the kidneys. Because of the increased focal depth zonography often requires fewer exposures. This study explores the possibility of employing zonography regularly for excretion urography with emphasis on radiation dose and image quality. Methods: A specially constructed renal phantom was used to establish radiation dose levels for tomography and zonography. Subjective analyses on subsequent images were carried out. Clinical trials compared both techniques to assess image quality objectively and to determine the optimum number of exposures. Ten patients were employed for each technique. Results: Statistical analysis demonstrated that fewer exposures per intravenous urography examination were necessary for zonography (1.6) compared with tomography (2.5) P=0.002. No differences were observed between the techniques for image quality or radiation dose per exposure. It was estimated that excretion urography employing zonography instead of tomography would reduce th~ effective dose equivalent by approximately 10 per cent. Conclusions: This investigation suggests zonography should replace tomography for routine excretion urography with potential benefits for patient dose and examination costs. Key words: radiation; image quality; slices; kidneys.
INTRODUCTION Intravenous urography (IVU), remains widely used as an imaging technique for the urinary tract [1]. It demonstrates the anatomy of the whole urinary system and, to some extent, renal function. This technique can demonstrate most lesions of the renal drainage system, providing renal function is maintained. The examination is both versatile and cost effective and the basic structure has seen little change since it was first described in the 1920s. One problem associated with imaging the kidneys arises from visual obstruction of these organs due to gas and faecal material in the overlying portions of the small and large intestines. Attempts have been made to overcome this problem by abdominal preparation. The application of bowel cleansing has been supported by a number of workers with some specifying that the laxatives should be given up to two nights preceding the examination [2, 3]. However, abdominal preparation is not without *Author to whom correspondence should be addressed. 1078-8174/96/020099 + 11 $12.00/0
© 1996 The College of Radiographers 99
1O0
Daniels and Brennan
limitations. It has been argued that laxatives have an insignificant effect on the resultant image quality since the section of bowel most likely to obscure the kidneys, the transverse colon, is rarely cleared adequately, even with the most vigorous preparation regimens. Laxatives can also lead to gas filled bowels, resulting in equally poor visualization of the kidneys [4]. Bowel cleansing can offer considerable discomfort to the patient [5]. Therefore an effective technical solution to the problem of bowel shadowing is often adopted: renal tomography [6]. The principles and applications of tomography are well described. However, for the purposes of the current investigation, it is important to note that the radiation doses employed with abdominal tomography are quite substantial, with a tomographic exposure delivering a skin dose as high as 23 mSv [7]. A more recent study reported a dose increase of 117 per cent when tomography was employed [8]. Therefore it is a safe assumption that if renal area tomography is included within an IVU examination it will contribute significantly to the overall radiation dose, particularly if multiple sections are required. Clearly research into dose limitation for renal tomography is potentially beneficial for patients. Zonography, first described by Ziedses Des Plantes [9], is a subcategory of tomography employing an exposure angle of less than 10°. As a result of this smaller angle the thickness of the focal plane is considerably increased when compared with that of a wider tomographic angle of 3 0 4 0 °. This may remove the need for multiple sections and therefore potentially reduce the radiation dose delivered to a patient undergoing tomographic exposure. However the reduced angle employed with zonography may have implications for image quality factors, such as reduced bowel blurring efficacy [10, 11], which supports the widespread application of larger angles for excretion urography. There does, however, appear to be a paucity of published information directly comparing radiation doses and image quality between tomography and zonography for a particular type of examination. The aims of this study were therefore to: • record entrance and exit radiation dose levels with tomography and zonography of the renal area using a specially constructed phantom; • apply each technique to a number of patients to assess image quality and optimum number of exposures.
MATERIALS AND METHODS Methods
Phantom tests using both tomographic (30 °) and zonographic (8 °) exposure angle settings to assess radiation dose and establish the feasibility of a patient trial were carried out. In addition, since zonography had not been employed previously in the department where this study was carried out, the resultant image quality was checked subjectively before seeking ethical approval for the patient trial. Patient trials evaluating both techniques for image quality and optimum number of sections per examination were carried out. Phantom tests. No commercial model was available for this work, therefore, a phantom was designed and built to simulate a human kidney after injection of contrast. Pigs' kidneys were placed within a water bath, and with the help of a plinth positioned
Tomography vs Zonography During IVUs
l=cm
101
x
Figure 1. Diagrammatic representation of the simulated human kidney used in the phantom experiments.
Figure 2. Photographof the simulated human kidney used in the phantom experiments.
in their true antero-posterior position as described in Gray's Anatomy [12] (Figs 1 and 2). Calyceal systems were moulded from a silicon adhesive and placed in suitable locations within the kidneys to resemble contrast-enhanced kidneys (Fig. 3). A consistent thickness of polythene bubble wrap was placed between the kidneys and the water surface to simulate bowel shadowing. The distance from the anterior surface to the posterior surface of the phantom was 18 cm, a distance which correlates well with commercially available models. Five images of this phantom were then acquired at 0.5 cm intervals using tomography and zonography. This procedure was carried out three times. The exposure used, 60 kVp and 50 mA, was only slightly less than that employed for a typical patient in the department and the doses recorded correlated well with that of patient doses [13]. The resultant images resembled those from an actual examination (Fig. 4). The images from both techniques were assessed subjectively by an evaluation panel to ensure that the quality achieved was adequate.
102
Danie& and Brennan
Figure 3. Mouldedsilicon adhesive used to mimic the ureteric pelvis in the phantom experiments.
The entrance and exit doses were recorded from projections providing images with evidence of the calyceal system. These measurements were carried out by placing lithium fluoride thermoluminescent dosemeters just above the water level and on the under surface of the phantom to establish entrance and exit doses for each exposure. The data was subjected to statistical analysis as described below. Patient trial. Following ethical approval 10 adult patients were chosen for each group, so that the mean weight of both samples lay within 10 kg of 70 kg [14]. individuals meeting these criteria were chosen consecutively. In selecting patients, the initial anterio-posterior abdomen projection was also considered. For a valid comparison between patients it was necessary to ensure that the degree of bowel shadowing was comparable and any patients with excessive or absent bowel shadowing were therefore eliminated from the study. This elimination was carried out by an independent observer who did not know which technique was to be employed. Patients who had conditions such as nephroblastomas which grossly affected the function or structure of the kidneys were not included. Finally, all individuals chosen for the study were consenting patients with no possibility of pregnancy. A record of the number of sections necessary to provide adequate diagnostic information of the renal area was kept for each patient. In addition, each image produced was assessed using the following qualitative criteria: image contrast, image
Tomography vs Zonography During IVUs
103
Figure 4. Zonogram of the renal area from the phantom experiments. Calcyesand the upper portion of the ureter can be seen clearly.
sharpness and successful blurring of the bowel. A scoring system was set for each of the three categories: • • • • •
excellent good adequate poor inadequate
5 4 3 2 l
points points points points point
All tomograms/zonograms were taken at the same stage of the investigation i.e. immediately after the 5-min film.
Evaluation panel A panel of senior radiographers and radiologists was formed. The films were sorted randomly and all markings and legends obscured so the assessors could not distinguish between tomograms and zonograms. The films were presented individually to the members of the panel who scored each film in the three categories already outlined. The films were viewed under the same conditions: against a single light box in a darkened room. Inter-observer variation was checked statistically.
Daniels and Brennan
104
y
7-
2
©
o
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T Tomography
• Zonography
Figure 5. Entrance and exit surface doses for tomography and zonography. D0 Entrance surface dose; m, exit surface dose.
Statistical analysis The data from the dose measurements, the number of slices, and the image quality scoring were all analysed using a one-way analysis of variance (ANOVA) and comparisons between pairs of means using the Scheffe-F method [15]. A probability level of P<0.05 was regarded as significant.
Equipment calibration tests To ensure reproducibility of the results the tomographic equipment was tested each morning for consistency before any experiments were carried out. The angle of exposure, fulcrum height, depth of focal plane and tube output were measured using the fan, the pin tool and focal plane thickness tests [16]. For all aspects of this study the same tomographic unit a Philips BTS4 (Philips Medical systems, Hammersmith, London, U.K.), the same film/screen combination (Quanta Super Rapid screens with H R - L X R F film) and the same X-ray processing unit a Du Pont daylight system (Du Pont [U.K.] Ltd., Stevenage, Hertfordshire, U.K.) were employed.
RESULTS
Phantom tests The entrance and exit doses for the phantom using both the tomographic and zonographic techniques are illustrated in Fig. 5. No statistically significant difference was observed between the two techniques. In addition, no subjective differences between the two techniques were evident following comparison of the resultant images. In the wake of the satisfactory outcome of these results, the patient trial was considered to be feasible and ethical approval was granted.
Tomography vs Zonography During IVUs
105
2
©
¢5
Tomography Figure 6.
c~
4
Zonography
Slices required per examination for tomography and zonography.
q-
3 09
2 0~
1
0
Tomography
Zonography
Figure 7. Qualitycriteria ibr tomography and zonography. [], Contrast; [], sharpness; l , bowel blurring.
Patient trials Following statistical analysis of the data the average number of exposures per patient was Shown to be significantly higher with t o m o g r a p h y P = 0.002 (Fig. 6). Analysis of the image quality scores for each film, both for the individual categories (Fig. 7) and for the total summation, demonstrated no statistical difference between the two techniques (Fig. 8). No significant inter-observer error was noted.
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Zonography
Figure 8. Summation of quality criteria scores for tomography and zonography. Table 1. Raw data from the phantom experiments Radiation entrance dose (mSv)
Radiation exit dose (mSv)
2.08 2.13
0.41 0.39
Tomography Zonography
Table 2. Raw data from the patient experiments Image sharpness
Image contrast
Image blurring
Summation of image scores
No. of slices per patient*
3"09 3'39
2.98 3'20
3.62 3.39
9.69 9.98
2.53 1.6
Tomography Zonography *Significant difference
The raw data for both p h a n t o m and patient experiments are summarized in Table 1. DISCUSSION The overall aim of this work was to establish the possibility of employing a zonographic technique for intravenous urography instead of tomography as currently practised in m a n y departments in the U.K. and Ireland. The advantage of reduced slices per examination with zonography due to an increased focal depth must be balanced against
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any possible reduction in image quality. It is also essential to consider the implications for radiation dose. Initially it was necessary to ensure that dose to the patient would not increase, or image quality substantially decrease, as a result of a change in technique. Preliminary work was therefore carried out using a phantom designed and built by the authors. Although not ideal, the resultant images were quite realistic, and the exposure used and dose administered suggested that the degree of attenuation within the phantom was similar to that within a live individual. "The quality of the resultant phantom images produced by both techniques was considered to be comparable. The doses recorded using the two techniques were very similar and no significant differences were seen. In the patient trial, the number of exposures required per examination was reduced significantly using zonography, therefore, it is safe to assume that patients undergoing IVUs with zonography will receive a substantially lower radiation dose. Following an estimation of the effective dose equivalent using the data provided in the National Radiological Protection Board Report 262 [17] and the entrance surface measurements recorded with this investigation, it is proposed that for a complete, average IVU examination there will be approximately a 10 per cent reduction in radiation dose using zonography. It is reassuring to note that the dose delivered for both zonographic and tomographic projections has reduced significantly from that recorded in the late 1970s [7]. A larger dose technique may be considered acceptable if the images resulting from this are more informative. However in this study the objective assessment of the evaluation panel found no such advantage. This appears to be in direct contrast to the work of Dure-Smith and McArdle in 1972 [18], which states that zonography does not dispense with bowel shadowing adequately. Factors that may contribute to the acceptability of zonography in the current work may be the emergence of higher quality receptors over the past two decades, along with more consistent and accurate tomographic equipment and shorter exposure times. It is interesting to note that some of the earlier workers did advocate zonography, which in an attempt to reduce the number of tomographic cuts was laudable, however the benefits were merely linked to time and expense with no emphasis placed on radiation dose and no objective assessment made to the resultant image quality [19 21]. The use of a thicker slice in renal zonography also maximizes subject contrast and information in an area where inherent contrast is low [1 l]. These factors have led to the adoption of small exposure angles for other regions of the body [22]. It must be acknowledged that this study did not examine the ability to detect specific pathologies as a discriminant between the two techniques. The authors, although proposing zonography as the optimal technique, cannot therefore eliminate the need to employ wide angle tomography as an accessory or even alternative technique for some patients. Dure-Smith and McArdle [18] concluded that the optimum method of imaging the kidneys required a minimum of three radiographs and rejected the use of zonography because bowel shadowing was not totally eliminated. Some authors, notably Chapman and Nakielny [23] and Whitehouse [24], base their descriptions of IVU tomography on the 20-year-old work of Dure-Smith and McArdle [18]. Consequently many practitioners employ large angles for renal examination. Recent advances in technology, the need for reduction in radiation dose and the financial and time savings achieved by taking fewer radiographs support the use of zonography as described in this paper rather than the earlier conclusions of Dure-Smith and McArdle.
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Daniels and Brennan CONCLUSIONS
(a) Zonography required fewer slices than tomography to demonstrate the kidneys during excretion urography. (b) The radiation dose per slice for each technique demonstrated no significant difference. (c) N o loss of quality was noted using zonography. (d) Zonography should be employed for routine excretion urography in preference to wide angle tomography.
Acknowledgements The authors are grateful to Ms Kate Matthews for discussing the results and helping to prepare the manuscript. Thanks are also due to Mr Stephen O'Connor who helped with the illustrations.
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