British Journal of Radiology (2004) 77, 927-930
© 2004 British Institute of Radiology
doi: 10.1259/bjr/32730898
Evaluation of work practices and radiation dose during adult micturating cystourethrography examinations performed using a digital imaging system
R S Livingstone, MSc
C G Koshy, MBBS
and
D V Raj, PhD
Department of Radiodiagnosis, Christian Medical College, Vellore 632004, India
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Abstract
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A micturating cystourethrography (MCU) examination is a specific radiological procedure that is performed under fluoroscopic screening to visualize the bladder by filling it with contrast material and to evaluate the urethral morphology during voiding. It is necessary to evaluate radiation dose during MCU examination since it involves radiosensitive organs such as the gonads. Radiation dose imparted to 109 patients undergoing MCU examination were measured using a dose–area product (DAP) meter. Patients were categorized into two groups based on whether filling of the bladder with contrast medium was done retrogradely (MCU) or by the suprapubic percutaneous route (SP-MCU). The DAP values to Group A (MCU) and Group B (SP-MCU) patients varied from 0.43 Gycm2 to 9.26 Gycm2 and 0.54 Gycm2 to 9.87 Gycm2, respectively. Reduction of radiation dose to patients was possible by the use of optimized exposure factors, precise collimation of X-ray beam, use of 0.2 mm copper filters and by acquiring images digitally.
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Introduction
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Diagnostic radiology contributes a large amount of man-made radiation exposure to the human population. Measurement of radiation doses should therefore be given proper consideration with a view to keeping radiation doses imparted to patients "as low as reasonably achievable" (ALARA) with acceptable image quality. Micturating cystourethrography (MCU) is a widely performed examination for radiological evaluation of the bladder and urethra in adults and children. The usual indications for MCU in adults are to exclude urethral injuries in trauma cases, urethral strictures from prior trauma and infection and for the detection of vesicoureteric reflux [1]. This examination is frequently performed on children and as a consequence, much attention has been given to reducing the radiation dose received during the procedure [2]. Radiation dose to patients during MCU examination is of concern considering the fact that the gonadal region is being irradiated. The need for measuring radiation dose to radiosensitive organs like uterus, ovaries and testes is imperative since stochastic risks are involved, which have no threshold dose. Information about current national normal doses for adult MCU examination was not found in the literature, whereas, numerous publications are available on radiation dose during paediatric MCU examinations. The current study intends to evaluate the radiation dose imparted to adult patients undergoing MCU examinations and also to evaluate work practices by personnel involved in conducting the procedure.
Radiation dose imparted to patients was measured using a dose–area product (DAP) meter. Since exposure parameters are varied throughout the examination and X-ray beams moved over different regions of interest, DAP meters were the preferred method of dose assessment [3]. From measured DAP values, entrance surface doses (ESDs) can also be calculated. ESD was estimated in order to evaluate possible occurrence of deterministic effects on the irradiated skin [4].
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Methods and materials
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This study was performed using a Siemens Iconos R200 Axiom Fluoroscopy machine (Siemens, Erlangen, Germany) equipped with an over-couch X-ray tube. Minimum total filtration of the X-ray beam was 2.5 mm Al and the added filtration was as high as 0.2 mm Cu. The 0.2 mm Cu filtration was used invariably during fluoroscopic screening and image acquisition. The machine had an option of selecting pre-programmed exposure factors based on the type of examinations performed. Personnel involved in operating the machine could also manually change the pre-programmed exposure factors. The machine also had the option to capture the last fluoroscopic image. However, images were acquired separately during MCU examinations since the images obtained by capture of the last fluoroscopic image contained poor radiological details and as a consequence, this option was seldom used. Various image intensifier formats (IIFs) or field sizes such as 36.7 cm, 30 cm, 22 cm and 17 cm, were available in the machine. During the course of the study, the personnel selected an IIF of 36.7 cm for taking scout views and later during the course of the examination, selected 22 cm and 17 cm IIFs. The latter involved image magnification and imparted higher radiation doses to patients compared with the rest of the IIFs. The selection of field sizes was at the discretion of the personnel involved in conducting the procedure. The machine had a charge-coupled device (CCD) camera coupled with the image intensifier. The advantage of using a CCD camera was that it necessitated lower exposure factors and thus imparted a lower radiation dose to patients. Images acquired were sent to a Picture Archival and Communication System (PACS). Fluoroscopy modes such as 3 fs–1, 8 fs–1, 12.5 fs–1 and 25 fs–1 (continuous) were available and personnel performing MCU examination invariably selected 8 fs–1 mode.
Dosimetry and calculations
Radiation dose imparted to patients was measured using a DAP meter (diamentor PTW; Freiburg, Germany) which was fitted on top of the collimator assembly. The readings from this were used to estimate radiation dose imparted to patients during MCU examinations. Calibration of the DAP meter was performed using an ionization chamber (Victoreen X-ray exposure meter capable of making measurements from 0.001 to 2 R with reproducibility within ±3%; Nuclear Associates, USA). The DAP meter measured radiation dose contributed from fluoroscopy screening and image acquisition. During the course of the examination, personnel involved in the data collection continuously monitored DAP values and this facilitated acquisition of separate values pertaining to scout imaging, spot imaging and screening. It was also possible to obtain a cumulative value of DAP by taking readings at the beginning and at the end of every image acquisition and fluoroscopy screening period [5]. The area of the irradiated field was displayed on the collimator assembly and was monitored continuously during MCU examinations. The area of irradiation, thickness of patient, focus to skin distance (FSD) and focus to film distance (FFD) were useful parameters in estimating ESD. The calculation of ESD from the measured values of DAP requires the field dimension and FSD [6]. The FFD was maintained at 115 cm during MCU examinations. One difficulty in assessing direct skin entrance dose using a DAP meter was that the DAP values obtained from it did not include a back scatter factor (BSF). Hence it was necessary to determine BSF separately and include it in the calculation of ESD [7]. Exclusion of BSF in the calculation of ESD from DAP values and area irradiated at the entrance plane would result in an underestimation of the ESD by 40% [6]. MCU examinations were performed by one senior radiologist and two resident junior radiologists during the course of the study.
MCU examination
This examination demonstrated the bladder by retrograde filling with contrast medium and visualizing the urethra during voiding. At the start of the procedure, in order to detect any radio-opaque calculi, a single scout image (anteroposterior radiograph of the kidney-urethra-bladder region) was acquired by capturing it on a storage phosphor cassette of size 17'' x 14'' (43.18 cm x 35.56 cm). The irradiated field area in this context was larger than other spot images taken during the course of the examination. Subsequently, a retrograde urethrogram (RUG) was performed. This involved the insertion of a small Foley balloon catheter into the distal urethra and instillation of contrast medium under fluoroscopic screening for demonstration of strictures or anterior urethral disease [1]. A few spot images were also acquired during this course. Additional spot images were acquired of the contrast-filled urinary bladder in order to look for any bladder wall abnormalities, lesions in the posterior urethra and to detect vesicoureteric reflux. During and after voiding, images of the suprapubic region were acquired in order to detect and assess any post-void residue. The suprapubic-MCU (SP-MCU) was performed if there was any difficulty in inserting the catheter into the urethra, wherein the bladder was punctured percutaneously and filled with contrast [1]. Both these approaches (MCU and SP-MCU) were preceded by a RUG. The patients were grouped into two categories as follows:
- Group A – patients who underwent the MCU examination
- Group B – patients who underwent SP-MCU examination
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Results
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Out of the 109 patients who underwent MCU examinations, 101 were male patients and 8 were female patients. The average age of patients undergoing the MCU examination was 41.3 years and 40.9 years for Groups A and B, respectively. Table 1
shows patient related parameters such as age and thickness of patients along with exposure factors used during fluoroscopic screening and image acquisition. The mean exposure factors for Groups A and B were similar during fluoroscopic screening and image acquisition. The mean value of mAs during image acquisition reported in Table 1 is for the entire examination. The duration of fluoroscopic screening depended upon the personnel performing the procedure and the clinical condition of the patient. The number of images taken varied according to the amount of information required for the study.
Table 2 shows DAP and ESD values during MCU examinations. For estimating ESD from DAP values, it was necessary to know the irradiated field area at the FSD concerned. The average irradiated field area for Group A and Group B patients was found to be 510.6 cm2 (274–794 cm2) and 530.7 cm2 (307–973 cm2), respectively. The reported DAP values for fluoroscopic screening and image acquisition did not include BSF correction. The radiation dose from image acquisition (Table 2) included scout images recorded on the storage phosphor cassette and spot images acquired digitally. For both the groups, the contribution of radiation dose to patients from image acquisition was greater than from fluoroscopic screening.
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Table 2. Dose–area product (DAP) and entrance surface dose (ESD) during micturating cystourethrography examinations
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Discussion
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The reported study was intended to estimate radiation dose imparted to patients and gain an insight into the work practices involved. The MCU examination being a dynamic study, its duration depended upon the nature of pathology. During the RUG study, if there was a problem in the insertion of the catheter into the urethra causing spillage, additional images were acquired. Repeat images were needed for a few patients due to non co-operation of those patients, erroneous selection of the catheter for cannulation and improper patient positioning. The mean number of radiographic exposures taken for patients of Groups A and B was found to be 8.6 (range 4–13) and 7.8 (range 3–11), respectively. On the other hand, in the study reported by Merkle et al [8], an average of 4.6 radiographic exposures (range 3–8) was taken during the RUG procedure for 35 patients and the percentage contribution of radiation dose imparted to patients from fluoroscopic screening was higher than from radiography.
The tube potential of 77 kV was manually selected for the image acquisition, but the machine varied the tube potentials from 62 kV to 110 kV depending on the thickness of the patients. The variation of exposure factors were also due to the selection of IIFs available in the machine. During image acquisition, the mAs varied from 1.3 to 62 and this variation was due to the recruitment of a wide range of tube potentials based on the thickness of the patients. During the first exposure, i.e when a scout image was taken, the machine employed high mAs values since the images had to be captured using a phosphor screen cassette. The lowest mAs of 1.3 was employed for a tube potential of 83 kV for a patient of thickness 12 cm and highest mAs of 62 for a tube potential of 110 kV and 24 cm patient thickness. It has already been reported in the literature that the advent of sophisticated radiological equipment provides an option of selecting high tube potentials and low current and has made a significant reduction in patient dose [9]. Prolonged fluoroscopic screening was necessary for patients who had strictures of the proximal urethra since it was necessary to follow the micturating pattern and instantly acquire the image.
The mean DAP values during fluoroscopic screening and image acquisition for both groups, given in Table 2
, were higher than those reported by Merkle et al [8] where the mean DAP value was 316 cGycm2 for 40 patients and the maximum DAP recorded was 768 cGycm2. The total DAP values ranged from 0.43 Gycm2 to 9.26 Gycm2 for Group A patients and 0.54 Gycm2 to 9.87 Gycm2 for Group B patients in the current study. The study reported by Merkle et al [8] included RUG alone. The ESD values have been calculated from DAP values with a view to evaluating the possible occurrence of deterministic effects on the irradiated skin. In the calculation of ESD, the average area included the irradiated area from scout image, spot images and fluoroscopic screening. This involved a certain amount of error by virtue of the fact that the irradiated area for the scout film was more than the area involved during spot imaging and fluoroscopic screening. The maximum ESD of 32.5 mGy from Group B category was less than the dose of 2 Gy that can cause early transient erythema [10]. The mean ESD of 20.0 mGy for 6–18-year-old patients during MCU for paediatric patients reported by Chapple et al [11] was higher than the mean ESD values of 11.4 mGy and 10.8 mGy for Groups A and B, respectively. The difference can be attributed to the duration of fluoroscopy screening and the number of images acquired.
During the course of the study, it was possible to monitor radiation dose imparted to patients during fluoroscopic screening and image acquisition separately. However, it was not possible to monitor the radiation dose corresponding to use of various IIFs separately since personnel operating the machine switched from one IIF to another rapidly according to the necessity of information to be elicited in that context. The personnel invariably selected 22 cm and 17 cm IIFs, and this imparted higher radiation doses and produced better image resolution than 30 cm and 36.7 cm IIFs. Judicious choice of IIFs such as a 36.7 cm field size and 30 cm in combination with fluoroscopy pulse modes without adversely affecting the diagnostic information sought for can make a significant contribution to dose reduction. However, this depended very much on the skill of the radiologist and others who perform these procedures.
Dose optimization is possible by choosing optimal tube potentials based on the thickness of patient, without sacrificing the image quality. It is important to note that selection of higher tube potentials has an associated disadvantage of reduced contrast and consequent reduction in diagnostic value. Application of the well known concept that the irradiated area should be restricted to the region of interest also plays a significant role in optimization. It is possible to further optimize radiation dose imparted to patients by developing specific protocols to limit the number of images acquired and duration of screening [12]. Significant dose reduction is possible by the use of digital systems equipped with the additional facility of last image capture during fluoroscopy [13]. Use of additional filtration of at least 0.1–0.2 mm copper is recommended, since there was a reduction of 40% of absorbed dose with only minimal loss of image quality [14, 15]. Acquiring images digitally rather than using a spot imaging technique can also lead to a significant reduction in radiation dose.
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Conclusion
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The reported study throws light onto the hitherto unreported area of radiation dose imparted to adult patients during MCU examination. There was no significant difference in radiation dose imparted to patients of either Group A or B during MCU examination. The estimated ESD involved during the MCU examinations was below the threshold for deterministic skin damage. Reduction of radiation dose to patients and consequent minimization of overall radiation risk are obtainable by adopting optimal exposure techniques, precise collimation of X-ray beam, use of 0.2 mm copper filters, and by developing specific protocols to limit the number of image acquisitions and duration of screening. A periodic assessment of various techniques involved in dose auditing will help to improve work practices during radiological procedures.
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Acknowledgments
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The authors would like to express their gratitude to Atomic Energy Regulatory Board of India for having provided financial support for this work.
Received for publication November 25, 2003.
Revision received March 22, 2004.
Accepted for publication June 9, 2004.
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Abstract
Introduction
