Radiation Dose Reduction for Endoscopic Retrograde Cholangiopancreatography: An Initiative for Patient and Endoscopist Radiation Safety

Hong Kong J Radiol 2021 Sep;24(3):192-200

 

 

Department of Radiology, Pamela Youde Nethersole Eastern Hospital, Hong Kong

 

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Ionising radiation is sometimes used during endoscopic procedures, most frequently during endoscopic retrograde cholangiopancreatography (ERCP). With increasing public awareness and concern surrounding radiation risks, it is crucial that procedures be performed according to the “as low as reasonably achievable” (ALARA) principle.

 

Several previous studies have proposed dose reduction measures during ERCP by adding protective lead shields or drapes,[1] [2] [3] but this may involve additional manipulation of patients or changes in equipment positioning. For example, when the shield is used, exposure of endoscopists to radiation was reduced to 17% for diagnostic procedures and to <7% for therapeutic procedures.[1] However, the shield may interfere with the manipulation of catheters and wires or affect fluoroscopic or videoendoscopic visualisation during the procedure.

 

We aimed to evaluate the effectiveness of a dose reduction technique involving the adjustment of acquisition parameters on the fluoroscopy machine. We also aimed to determine if this adjustment affected the length of fluoroscopy due to decreased image quality.

 

To formulate a dose reduction goal, a national dose survey was used as a reference.[4] The means of dose area products (DAPs) for diagnostic and therapeutic ERCPs are 4 and 10 Gycm², respectively. The mean fluoroscopy times for diagnostic and therapeutic ERCP are 2.6 and 4.4 minutes, respectively.

 

A retrospective review of radiation doses received by patients undergoing ERCP examinations from January 2017 to July 2017 in a local tertiary referral centre was performed, before (standard dose) and after (low dose) implementation of radiation dose reduction measures. Two phases of the study have been conducted. The first phase consisted of an audit of radiation dose for ERCP from January to May 2017 (standard dose). These data were used as a baseline reference representing normal practices before implementation of dose reduction measures. Endoscopists were not aware that the study was being undertaken. After implementation of radiation dose reduction measures, an audit of the low-dose cohort from May 2017 to July 2017 was performed. Radiological and clinical records of ERCP performed in these periods were retrospectively reviewed.

 

The major outcomes were radiation dose, ERCP outcome, and image quality. Data including patient demographics, DAP (Gycm²), fluoroscopy time (min), number of spot images taken during examinations, procedure indication, difficulty, complications, technical success, and image quality were collected.

 

The radiation doses were recorded from the fluoroscopic machine display. The clinical variables related to the ERCP were retrieved through dedicated electronic patient records in our institution. Two independent radiologists assessed the image quality.

 

Adult patients (aged >18 years) who underwent ERCP were included. The patients were referred from various departments within the hospital. All ERCPs were clinically indicated.

 

To estimate the sample size, we performed an a priori sample size estimation with the G-Power 3.1.0 (power: 0.8; α: 0.05) by using data from a previous study that examined radiation dose reduction during ERCP,[3] with a calculated effect size of 1 (Cohen’s d). It was estimated that at least 18 subjects in each group would be sufficient.

 

From January 2017 to May 2017 and May 2017 to July 2017, records of 25 consecutive patients who underwent ERCP for the standard and 25 consecutive patients for the low-dose ERCP examination groups were reviewed.

 

All examinations were performed in our fluoroscopic suite (Artis zee multi-purpose system; Siemens, München, Germany).

 

Table 1 shows the detailed acquisition parameters before and after dose reduction for ERCP. Parameters were adjusted based on two aspects: dose reduction and image quality. It is generally thought that the image quality may be compromised in a low-dose setting; we aimed to compensate for this effect.

 

Table 1. Major fluoroscopic acquisition parameters for standard and low-dose ERCP settings.

 

The changes were discussed with the manufacturer and application specialists. In order to achieve ALARA, it seemed unnecessary to use the relatively high-dose settings currently in use to achieve an adequate image quality for biliary tree evaluation. The proposed changes to the protocol to reduce the dose were discussed and agreed by an expert panel in the Working Group on Radiation Safety in our institution. The local Institutional Review Board approved this study.

 

The endoscopists, including hepatobiliary team surgeons and gastrointestinal physicians, performed approximately 1000 ERCP procedures in the year prior to the study at our institution. All endoscopists were unaware that the study, with changes to protocol parameters, was being undertaken.

 

Among the available metrics for radiation exposure, DAP was appropriate for monitoring patient radiation dose.[5] DAP is defined as the absorbed dose multiplied by the X-ray beam cross-sectional area at the point of measurement. Our fluoroscopic machine was equipped with a DAP meter. The DAP was shown on the live screen in the examination room, on the data display in the examination room, and on the console monitor in the control room. The DAP meter was calibrated at regular intervals.

 

Although related, procedure indication and intent were distinct issues. The indication related to the reason for the procedure. The intent was the goal for a specific procedure. For example, an ERCP might be indicated in a patient with obstructive jaundice, whereas the intent could be either to find the cause of jaundice (diagnostic) or to relieve the jaundice (therapeutic). It was necessary to assess indication and intent because both were integral components of determining procedure success.

 

ERCP difficulty was determined by a published grading system that was developed for the assessment of ERCP outcomes.[6] All ERCPs are classified into three levels based on the technical difficulty of each manoeuvre (Table 2). This classification provides a simple and objective measure of the clinical complexity of an ERCP and may also provide a measure of procedure risk. The difficulty of a procedure was also important for assessing procedure success.

 

Table 2. Grading system for technical difficulty in endoscopic retrograde cholangiopancreatography.

 

The specific method of identifying and collecting unplanned events/complications remains controversial for ERCP.[6] Complications such as pancreatitis or perforation are obvious and should be tracked. The significance of other events associated with the procedure remains unclear. Further research is required before the optimal method to record delayed complications can be firmly established. Nevertheless, unplanned events were tracked in our study.

 

Technical success was based on the technical difficulty of ERCP. Thus, technical success was stratified based on technique and described as complete success (diagnostic and therapeutic), partial success (access to desired duct with incomplete or partial therapy) or failed (failure to access or drain the desired duct). For example, a simple cannulation was not sufficient to represent success in cases where basic therapeutic measures were necessary.

 

To the best of our knowledge, there are no objective quality criteria for ERCP radiographic images to date. In order to better assess the quality of ERCP radiographic images, we adopted the quality criteria for urinary tract radiographic images before and after administration of contrast medium according to a published guideline,[7] with some modifications for the biliary tract (Table 3). Subjective evaluation of image quality was also performed using a scoring system, with particular attention to image contrast and noise, ranking from 0 to 4 (Table 3).

 

Table 3. Modified quality criteria for ERCP radiographic images and subjective image quality criteria.

 

Two independent radiologists were aware of the clinical information and assessed the image quality of the spot ERCP radiographic images of different patients in randomised order. They were blinded to each other’s results and the parameters used to acquire the images (both the low-dose and standard-dose protocols). They were asked to record the image quality score according to the above proposed criteria. All images were reviewed on a dedicated workstation (Carestream PACS Workstation, Carestream, Genova, Italy).

 

Statistical analysis was performed with commercially available statistical software SPSS (Windows version 22.0; IBM Corp, Armonk [NY], United States). Comparison of demographics, radiation dose, ERCP outcome, and image quality were done with t test, Mann-Whitney U test, or Chi-square test as appropriate.

 

Standard multiple regression was used to identify any possible predictors/confounding factors for DAP. Relevant variables known to have an association with DAP were included in the model. Age, body mass index (BMI), fluoroscopy time, number of spot images taken, and ERCP difficulty grade were added into the regression model as predictors. A general linear model was performed to mitigate any potential confounding effects of the confounders on DAP. Statistical significance for all of the tests was set at p < 0.05.

 

Inter-observer agreement of the image quality scores were assessed using the kappa statistic. The κ strengths were categorised as follows: <0.20, poor; 0.21 to 0.40, fair; 0.41 to 0.60, moderate; 0.61 to 0.80, good; and 0.81 to 1.00, very good.

 

The median DAP was 13.0 Gycm² for standard ERCP settings and 6.1 Gycm² for low-dose ERCP settings. The difference in median DAP between the two groups was statistically significant (Mann-Whitney U test, p < 0.001). The median DAP for the low-dose examination was 53.4% lower than the standard-dose examination (Table 4).

 

Table 4. Comparison of patients’ demographic and fluoroscopic examination variables.

 

There were no statistically significant differences in age, gender, height, weight, or BMI between the two groups (Chi-square test or t test, p > 0.05). There were also no statistically significant differences in fluoroscopy time and number of spot images taken between the two groups (t test and Mann-Whitney U test, respectively; Table 4).

 

In the regression model with DAP as the dependent variable (adjusted R square = 0.561, p < 0.001), BMI (standardised beta = 0.233, p = 0.031), number of spot images taken (standardised beta = 0.334, p = 0.004), and fluoroscopy time (standardised beta = 0.481, p < 0.001) made statistically significant contributions to the prediction of the dependent variable.

 

A general linear model was conducted to further explore differences in DAP between the two groups while controlling for possible covariates/confounders. BMI, number of spot images taken, and fluoroscopy time were used as covariates in the analysis of DAP. After adjusting for the above-named covariates/confounders, there was a statistically significant difference in DAP between the two groups (adjusted R squared = 0.652, p < 0.001).

 

All ERCPs were performed for therapeutic purposes. Nearly half of the ERCPs were performed for follow-up (e.g., after removal of a stent). There were no statistically significant differences in ERCP degree of difficulty, complication rate, or technical success rate between the standard and low-dose examination groups (Chi-square test, p > 0.05) [Table 5]. Median follow-up time for post-ERCP complication was 3 months (range, 2-6 months). For patients with complications following ERCP, none of them was definitely attributed to the low-dose setting.

 

Table 5. Comparison of ERCP procedural variables.

 

Radiographic and subjective image quality evaluation (Figures 1 2 3) by the two independent radiologists showed no significant difference in the scores among the two patient groups (Chi-square test, p > 0.05). The inter-observer agreement on modified image quality criteria and subjective image quality were good (kappa = 0.669 and 0.722, respectively) [Table 6].

 

Figure 1. Radiographic image quality evaluation by modified quality criteria. (a) Whole biliary tract including the gallbladder, intrahepatic ducts, common bile duct, and duodenal region (star). (a, b) Sharp reproduction of the bones (crosses). (b) After administration of contrast medium, the following should be assessed: sharp reproduction of the biliary tract, tailored to the individual patient, (e.g., sharp unobstructed intrahepatic ducts or common bile ducts) [arrow].

 

Figure 2. Subjective image quality evaluation. Excellent image quality: excellent opacification of biliary tree, excellent guidewire visualisation, absent or minimal image noise.

 

Figure 3. (a-c) A 69-year-old man, body weight 63 kg, with history of cholecystectomy and recurrent common bile duct stones, underwent follow-up ERCP in standard-dose setting. Common duct stone removal (arrows in a) was performed with stone extraction balloon (arrow in b). Fluoroscopy time: 7.5 minutes; DAP: 13.5 Gycm2. (d-f) A 85-year-old man, body weight 64 kg, with history of recurrent cholangitis, had follow-up ERCP with the low-dose settings. Common duct stone removal (arrow in e) was performed with a mechanical lithotripter basket. Fluoroscopy time: 9.5 minutes; DAP: 10.4 Gycm2. Note the comparable radiographic image quality between standard (a-c) and low-dose settings (d-f).

 

Table 6. Comparison of image quality variables.

 

The two basic principles of radiation protection of the patient as recommended by the International Commission on Radiological Protection are justification of practice and optimisation of protection, including the consideration of dose reference levels. Justification is the first step in radiation protection. It is accepted that no diagnostic exposure is justifiable without a valid clinical indication, no matter how good the imaging performance may be. Every examination must result in a net benefit for the patient. This only applies when it can be anticipated that the examination will influence the clinical decision with respect to the following: diagnosis, patient management and therapy, and final outcome for the patient.

 

With respect to diagnostic examinations, the International Commission on Radiological Protection does not recommend the application of dose limits to patient irradiation but draws attention to the use of dose reference levels as an aid to optimisation of protection in medical exposure. Once a diagnostic examination has been clinically justified, the subsequent imaging process must be optimised. The optimal use of ionising radiation involves the interplay of three important aspects of the imaging process: the diagnostic quality of the radiographic image, the radiation dose to the patient, and the choice of radiographic technique.

 

Radiation dose to patients during ERCP depends on many factors.[5] Some factors are related to the endoscopist, including fluoroscopy time, number of digital spot images taken, and use of collimation. The endoscopist cannot control some variables, such as patient size or procedure type (diagnostic vs. interventional). Other factors are intrinsic to the equipment. For instance, pulsed fluoroscopy, whereby the X-ray beam is turned on and off at a fixed rate, can significantly reduce exposure without having the X-ray beam on continuously[8]; copper X-ray beam filtration, which limits patient dose from low-energy X-rays; fluoroscopic loop review, and last-image hold options that allow review of images without additional X-ray exposure. There is often a trade-off between image quality and radiation exposure. For example, choosing a low dose may result in a noisy image, especially for an obese patient, so for such patients, the endoscopist will often choose a medium- or high-dose setting.

 

In our study, radiation and image acquisition parameters were adjusted. For radiation parameters, the fluoroscopic tube voltage was increased from 73 kVp to 81 kVp. The fluoroscopic tube voltage in kV was maintained automatically for as long as possible. For extremely thin or small patients, the kV value was reduced. In case of lower transparency (thicker objects), the kV value would be increased to ensure correct image brightness. It was recommended by the manufacturer to set the X-ray tube voltage as high as possible (not forgetting the image quality and image contrast). It is reported that a high kV technique (80-100 kV) could reduce the dose to a patient up to 50%, compared to the conventional technique (75-96 kV).[8] The dose in the scanning protocol parameters refers to dose per spot image. It was a nominal value which applied to the measuring conditions at 70 kV, 2.1-mm Cu pre-filtering, 16-cm flat detector input field. It was decreased from 55 nGy per pulse to 36 nGy per pulse, which decreased the dose from every single fluoroscopic image. According to manufacturer instructions, ‘kV dose’ was the value at which the dose was switched over for dose reduction. Dose reduction would be performed at a preselected kV.

 

For image parameters, edge enhancement was increased from 15% to 20%. It resulted in a clearer display of contrast differences (e.g., outline of biliary tree). However, this also caused more noise. The iNoise reduction setting (Artis zee multi-purpose system) compensated for increased noise. If the dose is reduced, additional noise will be perceived as poor image quality. To compensate for the increased image noise, the acquisition program can be configured to adjust the edge enhancement value. DDO-Kernel refers to digital density optimisation Kernel. DDO (harmonisation) reduces the dynamic range of an image (bright areas are less bright, dark areas are less dark). By reducing the dynamic range, this allowed us to increase the contrast without saturation of the image in bright or dark areas. According to manufacturer instructions, K-factor was based on averaging over time. Reduction of image noise could be achieved by weighted averaging by the factor √(2k-1). However, noise reduction by averaging may result in reduced contrast and ‘ghost images’ of fast-moving objects. The K-factor influences the noise impression of the image. A high K-factor means less, and a low K-factor, more image noise. If high K-factors were used, lag effects (in which the image becomes blurred due to patient’s movement) appeared in the image.

 

The fluoroscopic machine uses various copper filters. These filter out the low-energy components of the X-ray spectrum that are not needed to create the image. This causes hardening of the beam, reducing not only the skin dose to the patient but also the scattered radiation to not only endoscopist, but also other staff in the room. The copper filters were from 0.2 to 0.6 mm thickness in our setting.

 

A reduced pulse rate would help reduce the radiation load on the patient and other personnel, but it would require endoscopists’ adjustment. It was kept constant at 7.5 fps in our study. Perhaps the most readily reliable method of reducing radiation exposure is the reduction of the fluoroscopy time. However, a very complex combination of patient-, endoscopist-, and procedure-related factors contribute to determine the final fluoroscopy time.

 

Several limitations have to be addressed in this study. First, there are no well-validated objective quality criteria for ERCP radiographic images, rendering the assessment of image quality not standardised. The proposed modified quality criteria adopted from European guidelines[6] may not necessarily be truly representative of objective image quality. Further study is warranted to establish quality criteria for ERCP radiographic images so that the diagnostic quality can be better assessed under a standardised approach. Second, the use of a dedicated fluoroscopic machine (Artis zee multi-purpose system) may render the results not generalisable. The radiographic techniques and acquisition parameters in models from various manufacturers differ. Every effort should be made to ensure good radiographic techniques and to tailor specific examinations to the indication, patient and user, in an attempt to achieve the ALARA principle. Third, use of thermoluminescent dosimeters may better reflect the effective dose to patients, endoscopists, and assistants. Fourth, several potential confounding factors were not recorded and controlled for, including endoscopists’ experience, trainee involvement, specific ERCP procedures (such as pre-cut sphincterotomy or hilar stent placement), and patient positioning.[9] However, we believed there was a high probability that the low-dose setting in our machine could still offer substantial radiation dose reduction, after the above-named covariates being adjusted for.

 

Our study devised a simple practical dose reduction method by adjusting the acquisition parameters on the fluoroscopy machine during ERCP, without lengthening the fluoroscopy time or significant compromising image quality and outcome of ERCP.