Comparing prediction models for radiographic exposures
W. Chinga*, J. Robinsona, M. F. McEnteeb
aDiscipline of Medical Radiation Sciences, University of Sydney, Lidcombe, New South Wales, Australia; bDiscipline of Medical Radiation Sciences, and Brain and Mind Research Institute,
University of Sydney, Lidcombe, New South Wales, Australia *[email protected]; phone +61 412504308
ABSTRACT
During radiographic exposures the milliampere-seconds (mAs), kilovoltage peak (kVp) and source-to-image distance can be adjusted for variations in patient thicknesses. Several exposure adjustment systems have been developed to assist with this selection. This study compares the accuracy of four systems to predict the required mAs for pelvic radiographs taken on a direct digital radiography system (DDR).
Sixty radiographs were obtained by adjusting mAs to compensate for varying combinations of source-to-image distance (SID), kVp and patient thicknesses. The 25% rule, the DuPont™ Bit System and the DigiBit system were compared to determine which of these three most accurately predicted the mAs required for an increase in patient thickness. Similarly, the 15% rule, the DuPont™ Bit System and the DigiBit system were compared for an increase in kVp. The exposure index (EI) was used as an indication of exposure to the DDR. For each exposure combination the mAs was adjusted until an EI of 1500+/-2% was achieved.
The 25% rule was the most accurate at predicting the mAs required for an increase in patient thickness, with 53% of the mAs predictions correct. The DigiBit system was the most accurate at predicting mAs needed for changes in kVp, with 33% of predictions correct.
This study demonstrated that the 25% rule and DigiBit system were the most accurate predictors of mAs required for an increase in patient thickness and kVp respectively. The DigiBit system worked well in both scenarios as it is a single exposure adjustment system that considers a variety of exposure factors.
Keywords: Exposure, Radiography, DigiBit, Bit System, 25% rule, 15% rule.
SUMMARY Inaccurate exposure adjustment to variations between patients can lead to under or over exposure to patients which can lead to misdiagnosis due to noise or increased patient dose respectively. To ensure the accuracy of exposure selection several exposure adjustments systems have been developed. This study compared the accuracy of four exposure adjustment systems, 25% rule, 10% rule, DuPont™ Bit System and the authors own DigiBit system, at predicting the mAs required to compensate for changes in patient thickness and kVp.
INTRODUCTION There has been a transition in radiography from film/screen to digital technology. The emergence of digital radiography systems has come with several advantages including, but not limited to, digital storage and transfer of images, elimination of processing chemicals, wider exposure latitudes and post processing algorithms [1]. The wider exposure latitudes and post processing algorithms have minimized the visibility of errors associated with under and over exposure. This has resulted in a reduction in patient dose due to the reduced number of repeat radiographs [2, 3]. However, it also increases the risk of frequent under and overexposure which can be detrimental to the patient [4].
The automatic exposure control (AEC) is an exposure adjustment system developed to produce consistent image quality. The AEC uses a radiation detection device placed behind or in front of the imaging detector. The radiation detection device measures the amount of radiation reaching the imaging detector. Once a preset amount of radiation is detected the exposure is terminated. The AEC allows the production of radiographs with consistent image quality regardless of variations in patient thicknesses, kilovoltage peak (kVp) or source to image distance (SID) [3].
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However, an AEC is not always available, particularly on the ward or in the emergency setting. Gibson and Davidson’s (2012) study demonstrated that there is a tendency to over expose patients when performing radiographic examinations in the intensive and critical care unit (ICCU), and emergency departments [4]. This is primarily due to the fact that there is no AEC in place. Their study highlights the need for an alternative method of exposure adjustment for situations when AEC is not available.
In the past, to assist with exposure selection, and combat the narrow exposure latitudes and lack of post processing algorithms, numerous exposure adjustment systems have been developed to ensure diagnostic radiographs are produced. Some exposure adjustment systems that have been developed are the 25% rule, 15% rule and the DuPont™ Bit System. The 25% rule states that every centimeter (cm) increase in patient thickness requires an increase in milliampere-seconds (mAs) by 25% [5, 6]. The 15% rule states that when the kVp is increased by 15% the mAs needs to be halved to maintain the same optical density [5, 7]. The DuPont™ Bit System is a chart that has assigned arbitrary ‘bit’ values to different exposure factors. A change in a certain number of ‘bits’ of one factor can be compensated by a change in the same number of ‘bits’ in another factor. This compensation will produce a radiograph of equivalent optical density [6, 8].
Optical density (OD) was used to measure the density of radiographs produced with film/screen technology. Previous studies examining the accuracy of exposure adjustment systems had used OD to assess radiographs. Radiographs were considered appropriately exposed if the OD was 1.3-1.5 [9, 10]. The post processing abilities of digital systems, which allow manipulation of window level and window width, means OD is no longer a valid measure of exposure. The introduction of digital radiography systems has also resulted in the introduction of detector dose indices (DDI). DDI provides an indication of image quality as it measures the radiation exposure to the detector [11]. As a result, DDI provides a suitable substitute for OD. Each manufacture has produced their own DDI, with their own scale and values that indicate appropriate exposure. For example, Carestream uses exposure indices (EI) while AGFA uses log median exposure (lgM) [11].
There have been a limited number of papers that examine exposure adjustment systems for digital radiography. Previous work by our team has developed a preliminary exposure adjustment system to suit direct digital radiography (DDR), entitled the DigiBit system [12]. It is based on the DuPont™ Bit System but modified to suit the materials that compose the direct digital imaging detector. This study aims to compare the DigiBit system to the 25% rule, 15% rule and the DuPont™ Bit System to further assess its suitability as an exposure adjustment system for the digital era.
METHOD 1.1 Equipment
Radiographs were taken with a Carestream DDR system. It is comprised of an x-ray tube with a total filtration of 3.2 millimeters (mm) aluminum, focal spot sizes of 1.2x1.2mm and 0.6x0.6mm (Varian Medical System; UT, USA) and a 35x43cm wireless gadolinium oxysulphide (Gd2O2S) detector (Carestream DRX-1; Rochester, New York). Radiographs were taken of a Kyoto abdomen-pelvis anthropomorphic phantom (Kyoto Kagaku Co. LTD; Kyoto, Japan) with a tissue equivalence of a 70kg person to simulate an average patient. A 180x170x30mm section of pork belly from the sus scrofa domesticus (domestic farm pig) was placed on the anterior surface of the phantom to simulate an increase in patient thickness.
1.2 Quality Control
The x-ray tube’s performance was assessed through quality control test. Tests for kVp accuracy and reproducibility, timer accuracy and reproducibility and exposure output testing were performed. The testing revealed that the x-ray tube was performing within acceptable ranges [13, 14]. The kVp and timer accuracy was within 5% and the kVp and timer reproducibility had a coefficient of variance less than 2%. The exposure output had a reproducibility value less than 0.1. Tube warm up and detector calibration was performed prior to each testing session.
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1.3 Experimental Design
The DDR system was used to produce 60 radiographs of the phantom with 60 different exposure combinations. The kVp ranged from 60-100 in 10kVp intervals, the SID ranged from 80-130cm in 10cm intervals, and the thicknesses were 18.5cm (phantom only) and 21.5cm (phantom plus pork belly). For each exposure combination the mAs was adjusted to produce a radiograph with an EI of 1500+/-2%.
To select the target EI of 1500+/-2% three exposures were made of the phantom using the exposure parameters suggested in Grodin et. al. (2004) combined with the AEC to provide the initial mAs of 25 [15]. The phantom was exposed another three times using Grodin et. al. (2004) exposure parameters with a mAs of 25. The resultant EI of these radiographs was averaged out giving an EI of 1500. The range of +/-2% was chosen as it provided sufficient latitude for all exposure combinations to achieve the target value. This EI was chosen as an indication of an appropriate exposure to the detector.
.
Figure 1. The equipment was set up for a sacral radiograph where the x-ray tube was centered to the cassette in the table bucky. The phantom was positioned so that the central beam passed through the center of the sacrum and the longitudinal axis of the table. A shows the phantom alone being imaged and B shows the phantom with the pork belly being imaged.
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1.4 Thickness
Thirty exposure combinations involved an increase in patient thickness from 18.5cm to 21.5cm. For each combination the exposure adjustment systems were used to predict the mAs required to compensate for the change. The value predicted by each system was compared with the mAs required to achieve an EI of 1500+/-2%.
1.5 kVp
Forty-eight exposure combinations involved increasing kVp by approximately 15%. For each combination the exposure adjustment systems were used to predict the mAs required to compensate for the change in kVp. The value predicted by each system was compared with the mAs required to achieve an EI of 1500+/-2%.
RESULTS
The predicted mAs required for a change in thickness or kVp was calculated and compared to the mAs needed to achieve an EI of 1500+/-2%. If the exposure adjustment system predicted the mAs within 5% of the mAs needed to achieve an EI of 1500+/-2% it was considered accurate.
1.6 Thickness
The accuracy of the 25% rule, the DuPont™ Bit System and the DigiBit system was assessed. It revealed that the 25% rule was able to predict the appropriate mAs for 53% of the 30 exposure combinations; the DigiBit system accurately predicted mAs for 33% and the DuPont™ Bit System accurately predicted 0% (table 1).
1.7 kVp
The accuracy of the 15% rule, the DuPont™ Bit System and the DigiBit system was assessed for changes in kVp. This revealed that the DigiBit system was able to predict the appropriate mAs for 33% of the 48 exposure combinations, the DuPont™ Bit System accurately predicted mAs for 31% of the 48 exposure combinations and the 15% rule accurately predicted 10% (table 2).
Table 1: The number of mAs predictions within 5% of correct mAs for changes in patient thickness. Total of 30 Exposures combinations. 25% Rule DuPont™ Bit System DigiBit System Number of Correct Predictions
16/30 (53%) 0/30 (0%) 10/30 (33%)
Table 2: The number of mAs predictions within 5% of correct mAs for changes in kVp. Total of 48 Exposures combinations 15% Rule DuPont™ Bit System DigiBit System Number of Correct Predictions
4/48 (10%) 15/48 (31%) 16/48 (33%)
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DISCUSSION A preliminary version of the DigiBit system has been developed to guide exposure adjustment in the digital era. The DigiBit system was developed as a tool for radiographers to adjust exposure parameters to suit the variation in patient thicknesses in the clinical setting, particular for mobile radiography where an AEC is not present. Gibson and Davidson (2012) showed that a large number of exposures were either over or under exposed indicating a need for a system to accuracy select exposure parameters [4]. Optimal exposure parameter selection is necessary for accurate diagnosis as well as adherence to the ALARA principle. When the image plate is underexposed, the resultant images are noisier than the optimally exposed images. The noise artefact can obscure pathology, which may lead to misdiagnosis. Digital radiography possesses post processing algorithms and wider exposure latitudes, which means overexposing radiographs would lead to more aesthetically pleasing images due to reduced noise. As a result there is a tendency to overexpose patients in order to reduce the need to repeat exposures as demonstrated by Gibson and Davidson’s (2012) study [4]. However, overexposure means that patients are receiving a greater dose than required, which conflicts with the ALARA principle stating we should keep dose “as low as reasonably achievable.” The DigiBit system will help guide radiographers in selecting the appropriate exposure parameter for variations in patient size. In order for the exposure adjustment systems to work effectively in the clinical setting an accurate value of patient thickness must be known. This will allow an accurate adjustment of exposure factors. Patient thickness is considered as the distance the central ray of the beam has to travel from the anterior surface to the posterior surface of the patient. In order to obtain an accurate value of patient thickness, each patient must be measured. Two methods of measuring patients were suggested for film/screen radiography. One was the use of a caliper tool [16] and the other was the use of the x-ray tube itself [17]. The study examined the preliminary version of the DigiBit system, which has ‘bit’ values for a limited number of exposure factors. As a result the comparison of the 4 systems was limited to the exposure factors present on the DigiBit system. Once the DigiBit has been developed further to include the other factors that affect radiographic exposure such as beam collimation, tube filtration, grid and pathology; a reassessment of the exposure systems would be worthwhile. This study used the DDI as a measure of image quality. The Carestream system provided a DDI known as an exposure index (EI). The mAs selected by the AEC was set as the initial exposure and radiographs of the phantoms were made. The resultant images had an average EI of 1500, which then was set as the target for subsequent exposures. Zhang et. al. (2012) conducted a study comparing normal radiographic technique with a formula that selects the mAs need to achieve a particular EI. It was found that the intervention was able to reduce patient dose by 20-80% [18]. Therefore, setting a certain EI as a target in order to maintain image quality is a suitable approach to reducing patient dose in the digital era. It is unclear whether selecting an EI of 1500 as the target value is appropriate. This is because the manufactures do not provide any documentation regarding appropriate target EIs. They do verbally suggest appropriate ranges of EI for particular regions of interest, such as 1500-1800 for pelvic examinations. However, they do leave the selection of target values up to the consumer’s preference. Furthermore, different x-ray systems use different types of DDI values. The Carestream DRX-1 system used in this study uses EI as the DDI while AGFA uses log of median histogram (lgM) as its DDI [11]. Therefore a target EI of 1500 is not be suitable for use with other systems. The AEC allows the selection of appropriate exposure to suit variations in patient thickness. The AEC was used to select the mAs required to adequately expose the phantom. The resultant images required 25mAs. However, this may not be replicated by other systems as the calibration of the AEC of different systems can vary. The change from film/screen to digital radiography led to the change from using OD to detector air kerma (DAK) or DDI for calibration [19]. The target DAK will vary depending on the clinical conditions. The target DAK also varies depending on the x-ray system, as CR requires a greater dose to produce an acceptable image [20].
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DDI needs to be calibrated to DAK if the AEC is going to be calibrated to the DDI. Since DDI follows a similar trend to signal to noise ratio (SNR) it is suitable for calibration [19]. However, the selection of DDI as the target for calibration is difficult as it is dependent on the clinical situation. If the purpose of a chest radiograph was to identify lung metastasis then minimal noise would be desired. This could mean a higher exposure is needed. When assessing prosthesis alignment then a lower exposure is acceptable. Therefore selecting an EI of 1500, for the Carestream system, may be suitable for identifying lung metastasis but not for prosthesis alignment.
CONCLUSIONS A comparison of 4 exposure adjustment systems demonstrated that the 25% rule was the most accurate predictor of the mAs required when there is an increase in patient thickness and that the DigiBit system was the most accurate predictor of mAs required for an increase in kVp. The DigiBit system worked well to predict mAs both with increasing thickness and kVp as it can adjust for a variety of exposure factors. This may make it a valuable tool for radiographers in the absence of an AEC.
PRIOR SUBMISSIONS The authors have submitted a paper to Radiologic Technology which described the development of the DigiBit system [12]. This work complements the previous work by further assessing the DigiBit system in comparison to other exposure adjustment systems.
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