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DEI – Detector Exposure Indicator DETECTOR EXPOSURE INDICATOR IN GE X-RAY SYSTEMS Prepared by Sung Jeon, GE Healthcare Abstract: 1.1.Introduction The direct connection between the level of detector exposure and optical density is well established in film-screen radiography. However, that is not the case in processed digital images because of dynamic detector range and automatic image processing that optimizes image display (Figure 1.) Over- or under-exposure is not easily detectable in digital imaging. Lack of visual cue for over-exposure is more problematic: the radiographer tends to attempt to achieve noiseless images in order to satisfy the radiologist, and this tendency could lead to unnecessary increases in patient exposure. This is well known as “Dose Creep.” To help address such behaviors, system manufacturers have provided ways for the users to read and understand detector exposure information. Underexposure Optimal Overexposure Digital 1.Background Film Detector exposure indicator (DEI) is a feature that provides visual information for detector exposure in digital X-ray images. The lack of visual/quantitative information in digital X-ray images led several X-ray system manufacturers to develop DEI feature on their systems over the last few years. However, different DEI calculation methods and reports by different vendors often caused confusion to the users. In recent years, efforts for DEI standardization have been driven by the medical physics and radiology communities. In response to standardization efforts, the IEC developed and published the standard IEC 62494-1. In this whitepaper, we discuss the DEI feature in GE X-ray systems using the terminologies defined in IEC 62494-1. This whitepaper is intended to provide essential information of the DEI feature in GE X-ray systems for typical users as well as DEI design details for advanced users/physicists. Figure 1: Exposure level is easy to detect in Film Radiography (top row) while it is difficult to detect in Digital Radiography (bottom row.) 1.2. DEI standardization Over the past few years, different manufacturers, including GE Healthcare, have come up with their own methods for calculating and displaying detector exposure indicator/index (DEI). This has been a source of confusion among the end users. With facilities having multi-vendor equipment, it became a challenge to interpret among the DEI values of different vendors and to have effective Quality Control (QC) or Quality Assurance (QA) in place. To address such diversities, the radiology community acted on harmonizing detector exposure indicator in the past few years: The International Electrotechnical Commission (IEC) published the standard in 2008 [1.] The American Association of Physicist in Medicine (AAPM) published the report based on the efforts driven by Task Group 116 in 2009 [2.] ImageGently stakeholders agreed to adopt the IEC standard in 2010 [3.] GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 1 DEI – Detector Exposure Indicator In response to standardization efforts, GE Healthcare introduced a new design for DEI starting from the products released in year 2011 (e.g. Precision* 500D/ Proteus* WDR1 Upgrade, Optima* XR220amx, and Discovery* XR656). The new design is intended to provide detector entrance exposure information as defined in IEC 62494-1. This whitepaper describes the DEI feature in GE Healthcare x-ray systems. Section 2 contains basic information about the DEI feature, including the user interface, available operation options and guidelines for the users. Section 3 includes details about DEI design and algorithm, which may be of interest to advanced users or physicists. Note that all references to DEI in the current paper refer to the new DEI design observed in GE Healthcare x-ray systems introduced since 2011, unless stated otherwise. DI is a relative indicator showing how far EI deviates from EIT with the base 10 logarithmic scale. A scaling factor of 10 is applied to calculate DI number. When EI is close to EIT, DI becomes close to 0. When EI is 25% higher than EIT, DI becomes 1. In other words, DI = 1 means that detector entrance exposure for the image is 25% larger than the expected exposure index for the selected anatomy and view. DI becomes -1 when EI is 20% lower than EIT. Similarly, DI becomes 3 when EI is 100% higher than EIT, and it becomes -3 when EI is 50% lower than EIT. 2.3. Displaying DI values on GE Healthcare X-Ray Systems The DI result for each exposure is displayed with a graphical bar indicator as shown in Figure 2, and both EI and DI are displayed in the annotation as well. By default, DI ranges and limits for GE X-ray systems are set as following: • If -3 <= DI <=2: Display ‘Optimal Range’ text message with green color in the bar indicator. 2. DEI – Typical Usage 2.1. Basic components DEI design in GE X-ray systems provides detector entrance exposure values in terminologies as defined in IEC 62494-1 [1]. The following definitions of main DEI components are excerpted from IEC 62494-1. • Exposure Index (EI) – Measure of the detector response to radiation in the relevant image region of an image acquired with a digital X-ray imaging system. • Target Exposure Index (EIT) – Expected value of the exposure index when exposing the X-ray image receptor properly. • Deviation Index (DI) – Number quantifying the deviation of the actual exposure index from a target exposure index. 2.2. Interpretation of DEI values EI allows the operator to judge if an image was taken at a detector exposure level suitable for the intended level of image quality. EI is proportional to the detector entrance exposure: If the entrance exposure is increased, EI value is increased. The value of EI divided by 100 is comparable to detector entrance dose measurement in the unit of µGy at calibration conditions. Figure 2: Color Coded DI calculation result, DI limits, and text messages are displayed for each acquired image and reprocessed image. The user has an option for not displaying DI information at all or displaying numerical DI result only. • If -3 > DI >=-5 or 2 < DI<= 4: Display ‘Acceptable Range’ text message with yellow color in the bar indicator. • If DI <-5 or DI> 4: Display ‘Out of desired range’ text message with orange color in the bar indicator. The same DI ranges and limits are applied to all digital images. IEC 62494-1 does not specify requirements for displaying DI range and limit. AAPM Task Group 116 recommends 5 ranges with 4 limits in their report, which is similar to the values used in GE X-ray systems, but their initial recommendation was tighter than GE’s default range and limit setting [2.] GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 2 DEI – Detector Exposure Indicator 2.5. Adjusting EIT DEI feature on GE X-ray systems is designed to allow the user to set EIT values independently for different combinations of anatomy, view, and patient size (Adult/Pediatric). For each anatomy, view, and patient size, there are two EIT values: Factory EIT and Custom EIT as shown in Figure 4. The Custom EIT is the used for DI calculation and is the adjusted value based upon Factory EIT and EIT Adjustment factor. At installation, Custom EIT value equals the Factory EIT value. Changing the EIT Adjustment by 1.0 adjusts the Custom EIT by 1 Renard step. For a fixed exposure, mAs increase of 1 Renard step increases the mAs by approximately 25%. Figure 3: The limits and text messages are configurable in the user preference (Preference 6 DI 6 DI Control Limits.) GE X-ray systems also provide an option to change the limits and the corresponding messages in the user preference (Figure 3.) 2.4. DEI export/DICOM The DI export option, which can be found in Preferences 6 DI 6 DI Export Logs, allows the user with the ability to retrieve DEI information onto a CD or USB flash drive in the form of a CSV file. The exported CSV file lists DEI information with acquisition time and date, relevant X-ray techniques, selected anatomy/view/patient size for past exposures. It can be useful to analyze trends of X-ray techniques and the corresponding DEI values for different patient anatomy/view/size. The information in this file also may be useful to adjust EIT values. For each acquired image and processed image, the different DEI values are also stored in the DICOM tags. The table below lists the details. DICOM Tags Description 0011,1036 Median (Anatomy) Count 0011,1037 DEI limit values 0018,1411 Exposure Index 0018,1412 (Compensated) Target Exposure Index 0018,1413 Deviation Index 0011, 106D Uncompensated Detector Sensitivity 0011,1034 Compensated Detector Exposure 0018,6000 Compensated Detector Sensitivity Figure 4: EIT values are listed in Preferences 6 DI 6 DI-Preferences. For each anatomy/ view/patient size, ‘Factory Target EI’ is a reference, and ‘Custom Target EI,’ which is equal to ‘Factory Target EI’ by default, can be modified by changing the ‘Target EI Adjustment’ factor. In the example above, indicated by the square box, the EIT Adjustment was set to (-2) for Abdomen/antero posterior/pediatric, making Custom EIT approximately 2 Renard steps lower than the Factory EIT for Abdomen/antero posterior/pediatric exposures. 2.6. Steps for customizing DEI Users should not rely on the DI result alone in order to determine appropriate X-ray techniques. It is also suggested not to modify Custom EIT as a first mitigation step when DI value deviates from the desired range based on one or two exams. The tradeoff of IQ and dose should be the dominant factor to determine appropriate EIT and proper X-ray technique based on each site’s preference. Advanced users and physicists who understand DEI and the IQ/ Dose tradeoff should be involved in making Custom EIT modification decisions. Below are general guidelines for adjusting X-ray techniques and/or EIT when necessary. Table 1: Relevant DICOM tags for the DEI feature. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 3 DEI – Detector Exposure Indicator Step 1: Confirm that detector calibrations were recently completed by the local field engineer. Poor or out of date calibration results can adversely impact EI and DI accuracy. Step 2: Make sure that imaging conditions are appropriate and do not deviate from the nominal. For example, check following factors when EI/DI values are unusual. • Collimation and shuttering. • Unusual body habitus. • Presence of prosthetics. • Shielding material in the imaging region of interest. 2.7. Impact of re-shuttering on EI/DI EI/DI are calculated immediately after image acquisition and after every instance of image reprocessing. After a reshuttering operation, EI/DI recalculation is essential to account for any change in the relevant image region (Figure 5). If re-shuttering the image changes the extent of anatomical content in shuttered region of image, then the value of interest used in the DEI algorithm could also change, thus affecting the EI/DI values. Users should note that the new EI/DI values are generated only after the image reprocessing operation is fully complete, which happens only when the user selects the image reprocess button after adjusting the shutter position. • AEC functionality and patient positioning relative to enabled ion chamber cells. Step 3: If image quality and/or patient entrance dose are within user’s expectation, but DI is beyond desired range for the selected exposure techniques, consider the following: • When DI is higher than 0, increase EIT Adjustment in order to set the Custom EIT higher than the Factory EIT. • When DI is lower than 0, decrease EIT Adjustment in order to set the Custom EIT lower than Factory EIT. • It’s not advisable to make a change based on only a few results. When DI consistently deviates from default EIT, the users may consider modifying Custom EIT. Step 4: If image quality and/or patient entrance dose do not meet the expectation: • When assessing image quality, always ensure that Window Width/Window Level (WW/WL) is set correctly. • Adjust kVp, mAs, and/or speed in AEC mode if necessary to achieve desired image quality and patient entrance dose. Figure 5: (1) Processed image after data acquisition and (2) Re-processed image after manual shuttering. Both of them share identical raw data. EI and DI values were changed because manual shuttering changed the median anatomy count. Note: the date and time and institution name on these two images are blocked. 2.8. Benefits of the DEI feature Some benefits and practical usages of DEI information are listed below. • If DI is still out of range although image quality is acceptable, customize EIT as described in Step 3. • DEI features provides appropriate detector exposure level in clinical imaging, which is not easily detectable in digital radiographic images. Depending on the system type, patient entrance dose or DAP is available from direct DAP meter reading or Dose/DAP calculation based on the x-ray techniques. • In Fixed mode, EI/DI values can be monitored to minimize the tendency to increase exposure over time (prevent ‘dose creep’.) • In AEC mode, variation in EI/DI values can be monitored as indicators of patient and ion chamber positioning or collimation problems. • DEI feature enables technologists to monitor patient-specific detector exposure levels and to maintain consistent IQ with minimum dose. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 4 DEI – Detector Exposure Indicator Recent studies and investigations performed by different research groups [4-8,] have strengthened the case that DEI can be a useful tool for QA/QC in X-ray digital images. 3.DEI design details – for advanced users/physicists 2.9.Summary Following factors impacts on DEI calculation: DEI is designed to provide detector entrance exposure information in digital radiography where under- or over-exposure is not obvious from the digital image due to the wide dynamic range of the digital detector and automatic image processing. Harmonized DEI terminologies found in recent GE X-ray systems would help reduce the potential confusion for utilizing DEI information. • X-ray techniques including filter/grid status DEI is different from and cannot replace patient entrance dose, which is provided by the DAP meter or Dose/DAP calculation in GE Healthcare systems. The relationship between patient entrance dose and detector entrance dose depends on many variables including patient thickness, kV, collimation, etc. X-ray beam quality is different before and after passing through the patient and thereby DEI doesn’t directly convey dose to the patient. EI is proportional to detector entrance exposure, and EI divided by 100 reflects detector entrance dose in the unit of µGy at calibration conditions. DI is to calculate the deviation of EI from EIT. DI is a metric for understanding appropriateness of the exposure for each anatomy/view/patient size. It’s important to understand that EI or DI is not the indicator for image quality. Appropriateness of the exposure based on DI number comes with the assumption that EIT is properly set for each anatomy/view/patient size although the default EIT may not be optimized. Each site may consider adjusting EIT if DI is consistently not in desired ranges despite acceptable image quality. Recent studies by different research groups have also discussed the challenges and the importance of EIT optimization [3-5.] 3.1. Algorithm overview • Collimation and Electronic Shuttering • Raw data (image) • Detector calibration results • Anatomy segmentation performance • Pre-defined scaling factors addressing X-ray technique variation compared to the calibration (near-RQA5) condition The figure below illustrates DEI calculation workflow, which is executed for each exposure. Figure 6: DEI calculation workflow diagram. Values in bold are recorded in the DICOM tags as listed in Table 1. 3.2. Raw Image/Raw data in GE X-ray systems DEI calculation is always done using the original data, not the processed data. Per IEC 62494-1, data type definitions are as following [1]: • Raw data – pixel values read directly after the analogue-digital-conversion from the digital X-ray imaging device without any software corrections. • Original data – Raw data to which the allowed corrections are applied. Allowed corrections include bad pixel corrections, flat-field correction (gain and offset corrections), and geometrical distortion correction. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 5 DEI – Detector Exposure Indicator In GE documents (e.g. service manual, operator manual), the terminology ‘raw data’ is in fact referring to the original data defined in IEC 62494-1. In this document, raw and original data are not differentiated, and both of them refer to original data defined in IEC 62494-1. 3.3. Details for Exposure Index (EI) (1) According to IEC 62494-1, EI is related to the value of interest V: (2) Figure 7: (1) Anthropomorphic phantom image and (2) Output of anatomy segmentation algorithm. The black area in the subsampled image is considered as the relevant anatomic image region for DEI calculation. EI=C0∙g(V) where C0=100µGy-1 and g(V) is an equipment-specific inverse calibration function. The value of interest is defined as the central tendency of the original data in the relevant image region [1.] The inverse calibration function in GE Healthcare systems is proportional to the median image count in the raw data, the value of interest V, and inversely proportional to the uncompensated detector sensitivity (counts/µGy.) Hence, the above equation is implemented as following on the GE Healthcare system. EI=100 [µGy-1]∙ Median Count [counts] Detector sensitivity counts µGy The original data comes from the image read from the digital detector after bad pixel, offset, and gain correction and is available to the user. The relevant image region in GE Healthcare systems is defined as the anatomic region under all clinical conditions. The anatomic region is identified from image processing by removing the collimator blades and the areas of raw radiation. An example of anatomic region identification is shown in Figure 7. The detector calibration procedure determines the value of conversion factor (CF, electrons/X-ray photon,) which is used to calculate the detector sensitivity value. Hence, proper detector CF calibration is vital in order to achieve accurate EI values. As the count in the raw data is linear to the amount of exposure, EI shows direct linear relationship with Air Kerma measurement. At calibration conditions, the relationship between EI/100 and measured dose in the unit of µGy is one-to-one. Figure 8 demonstrates the relationship between Air Kerma measurement and EI. In the test, EI accuracy was below the specified accuracy limit (20%) in IEC 62494-1 at calibration conditions. Figure 8: Calculated EI vs Direct Detector Air Kerma measurement. Detector Air Kerma was measured at the center of the detector position using a dosimeter (RadCal 9010 with 6 cc probe.) (1) Linear relationship is apparent between EI and Detector Air Kerma measurement. (2) The error is calculated using the formula 100*(Air Kerma –EI/100)/ Air Kerma, and it is done for different mAs. The errors are below the accuracy limit (20%) specified in IEC 62494-1. The measurement was done in Discovery XR656 Digital Cassette mode with 20 mmAl phantom, SID = 100 cm, No Copper filter, No Grid, 80 kVp, 41cmx41cm collimator opening, and DI technical mode. Note that the result in Figure 8 was obtained with ‘DI Technical Mode’ with 20 mmAl flat field phantom in the X-ray beam and fully open collimation FOV. EI in clinical conditions does not necessarily equal the detector entrance dose due to differences in beam hardening and scattering. It is important to set the detector Air Kerma measurement condition equal to the detector EI measurement condition when evaluating EI performance – e.g. (1) Position of the dosimeter probe should be at the center of the image plane, (2) The dosimeter probe should be located at GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 6 DEI – Detector Exposure Indicator the same source-to-image distance (SID) compared to the detector: otherwise, the distance ratio adjustment (R-squared correction) may also be considered if the probe cannot be placed at the detector position, (3) Attenuation, scattering and backscattering condition for dose measurement should be set equivalent to those for the detector, and (4) The acceptance angle of the dosimeter probe should be similar to the one for the detector. Proper collimation is an essential step to help minimize - dose to patient, and it also helps identifying the relevant image region properly. Incorrect collimation may lead to improper anatomy segmentation, and it may result in biased EI and DI. 3.4. Details for Target Exposure Index At system installation, Custom EIT is equal to Factory EIT. These default EIT values were determined empirically based on statistical data analysis of median count values for different anatomy and view combinations in a fixed radiographic system. During the design phase, the data used for setting EIT in most cases were acquired with in AEC mode. Each anatomy has different EIT values, because expected median count values and contrast are different for each anatomy. For example, EIT for chest AP/PA is different from that for abdomen because the radiographic contrast in the anatomical area of the chest image is different from that of abdomen. The contrast is important because the location of the ion chambers used for AEC is in the lung area, but a large fraction of the image has lower signal (Spine, heart, diaphragm.) The Factory EIT should not be considered as the gold standard target value in all circumstance for following reasons: (1) Factory EIT does not address variability in system usage or patient size/position because limited data were used for the analysis. (2) Although the system lets the user set different EIT depending on patient size, Factory EIT values for pediatric mode are currently equal to those for adult mode. Since the exposure level for pediatric is generally lower than the one for the adult, EIT for pediatric should ideally be lower than EIT for adult. (3) Factory EIT values for the mobile system (Optima XR220amx) were derived from those for the fixed system while the factory X-ray technique setting for each anatomy and view are not same between the mobile and the fixed system. GE Healthcare continues to collect clinical feedback and perform ongoing experimental analysis to monitor and to evaluate the performance of the DEI feature. This ongoing effort will be continually used to provide improved Factory EIT for the users. 3.5. Details for Deviation Index DI in GE X-ray systems is defined as the following: where EIT’ (Compensated EIT) is based on Custom EIT shown in Figure 4 and accounts for kVp, Cu filter, grid, and receptor difference between actual exposure and the condition when setting the default EIT. EIT’ and Custom EIT can be different depending on X-ray techniques being used. EIT’ is saved in DICOM header as described in Table 1 and the DI Export log file. The relationship between EIT and EIT’ can be expressed as the following. Film equivalent speeds other than 400 will impact EIT’. Details for the speed factor are discussed in the next two sections. kVp, Grid, Cu filter, and receptor changes compared to the condition for default EIT also impact EIT’. Their contributions were predetermined and stored in look-up tables based on laboratory tests with different thickness of a PMMA (Poly Methyl MethAcrylate) phantom, also known as Plexiglas, at different kVp. 3.6. Speed setting for X-ray systems ‘Speed’ should be used and understood differently between Film and CR/DR. This section provides clarification on the use of ‘Speed’ on digital radiography systems. ‘Speed’ for Film • Speed is a characteristic of the film/screen system and is inversely proportional to the exposure to the film/screen required to reach a specific optical density. • In AEC mode, exposure to the film/screen is controlled by the system based on clinical protocols. • In non-AEC mode, a clinically relevant exposure for each application (based on anatomy/view) requires the correct film/screen combination to be selected. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 7 DEI – Detector Exposure Indicator ‘Speed’ for Digital Systems In “Fixed” exposure mode: • On a digital system, equivalent film speed is NOT related to specific image brightness. Post image processing algorithms determine brightness and contrast values for display. • When the speed is increased, EI doesn’t change, but EIT’ is scaled down inversely proportional to speed. As a result, the calculated DI increases for the same fixed exposure. • Speed in digital X-Ray is not determined by the sensitivity of the digital detector. • When the speed is decreased, EI doesn’t change, but EIT’ is scaled up inversely proportional to speed. As a result, the calculated DI decreases for the same fixed exposure. • In AEC mode, exposure to the detector is controlled by the system based on clinical protocols. –– Different clinical protocols are required to meet different imaging needs. Speed is considered a metric for the exposure and is included in each clinical protocol. –– High Speed: pediatric application requiring low dose. –– Low Speed: higher signal-to-noise ratio, higher dose imaging applications. • In non-AEC mode, a clinically relevant exposure for each application (based on anatomy and view) is determined by the specific X-ray techniques in protocols not by the speed. 3.7. Impact of speed change on DI calculation The amount of exposure is controlled by the equivalent film speed setting in AEC mode while it is manually controlled by mA and exposure time in Fixed exposure technique. In both AEC and Fixed mode, equivalent film speed is used as a factor adjusting the expected exposure level. The DI calculation result will be different between AEC and Fixed mode depending on the speed setting because speed change does not contribute to exposure change in Fixed mode. In AEC mode: • When the speed is increased, EI becomes lower as the amount of exposure decreases, and EIT’ is scaled down inversely proportional to speed. As a result, the calculated DI is constant. • When the speed is decreased, EI becomes higher as the amount of exposure increases, and EIT’ is scaled up inversely proportional to speed. As a result, the calculated DI is constant. 3.8. DEI (DI) Technical Mode DEI (DI) Technical Mode option is provided in DI preferences (a checkbox shown in Figure 4.) Once it is enabled, the anatomic segmentation step is disabled, and the region of interest (ROI) used is the central rectangular region (512x512 pixels. Median count is obtained from the fixed ROI. A central ROI is also used for the CF calibration. CF calibration impacts the detector sensitivity and thereby EI/DI calculation result. The technical mode can be useful for evaluating DEI performance since it will help eliminate the potential variability introduced by the anatomy segmentation, but it shouldn’t be used for regular clinical exposures for the same reason. 3.9. Compensated Detector Exposure (CDExp) CDExp is not defined in IEC 62494-1. It has been used to display detector exposure information in the previous DEI feature and is carried forward into the new DEI feature. To calculate CDExp, the median count in the identified anatomy is converted to exposure in μGy based on the compensated detector sensitivity, which is the adjusted value of uncompensated detector sensitivity based on receptor type, kVp, Grid, and Cu filtration changes. Unlike EI, CDExp reports detector entrance exposure in simulated clinical conditions. In order to simulate patient attenuation, a set of Plexiglas has been used as attenuation material resulting in a different X-ray spectral attenuation than RQA5 beam condition. However, a set of Plexiglas still cannot account for expected variations in actual patient thicknesses and body compositions. Depending on patient size and type, CDExp may over- or underestimate detector entrance exposure. CDExp, Although the units of CDExp are Gray, it should not be considered as an absolute detector entrance exposure but as another indicator for the detector exposure. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 8 DEI – Detector Exposure Indicator To demonstrate the difference between EI and CDExp, test results are presented in Table 2 EI represents Detector Air Kerma at the calibration condition whereas CDExp closely represents detector Air Kerma at the ‘simulated’ clinical imaging condition. In the table below, actual detector Air Kerma is close to reported EI/100 with 20 mm Al plate (GE flat-field phantom.) Air Kerma is close to CDExp with Plexiglas. Depending on the systems’ calibration status and measurement setup, the test results will vary. It’s important to understand that EI relates to Air Kerma at a hard beam (near RQA5) condition realized with a 20 mm Al plate, and CDExp relates to Air Kerma at a soft beam condition realized with a set of Plexiglas. X-ray technique 20 mm Al Plate 4.75 inches Plexiglas kV mAs EI CDExp [µGy] Air Kerma [µGy] EI CDExp [µGy] Air Kerma [µGy] 80 1 377 7.7 3.64 466 9.52 9.44 2 769 15.69 7.4 948 19.36 19.15 Table 2: Comparison among Air Kerma, EI, and CDExp. The test was done in Digital Cassette mode without grid. Note: As more manufacturers follow the IEC 62494-1 standard for expressing detector entrance exposure and as more customers interpret detector entrance exposure using EI/DI from the IEC 62494-1 standard, CDExp won’t play a significant role for delivering detector exposure information. In some instances, the customers are confused for understanding detector exposure information in the context of CDExp along with EI/DI. Therefore, to minimize such confusion and to simplify DEI information, DEI feature on future systems may not include CDExp on the image display. CDExp value will continue to be available in DICOM headers and the DI log export file. 3.10.Difference with previous DEI design – for the users who are familiar with previous DEI design Prior to introducing new DEI design, GEHC X-ray systems released earlier than year 2010 (e.g. Definium 5000, Definium 6000, Definium 8000, Discovery XR650, Optima XR640, Definium AMX 700) displayed three DEI values characterizing detector entrance exposure. • Uncompensated Detector Exposure (UDExp): Median count value in the anatomy converted to exposure in μGy based on the detector sensitivity in counts/μGy at 80 kVp, no Cu filtration. And no grid. It includes a factor to correct to a Soft Beam condition. • Detector Exposure Index (DEI): The raw count value in the segmented anatomy divided by the expected count value for the given acquisition parameters (Speed setting, kVp, Cu, Grid). The expected count value is specific for certain thickness of Plexiglas (e.g. 5 inches for 80 kVp.) Ideal DEI (index) ranges vary depending on anatomy/view. • Compensated Detector Exposure (CDExp): Same definition with new DEI design. UDExp in prior DEI design is replaced by EI in new DEI design. The difference is that UDExp characterizes detector entrance exposure in soft beam condition (5 inches of Plexiglas) whereas EI characterizes detector entrance exposure in hard beam condition (calibration condition.) In addition, the unit for UDExp is μGy while EI is unitless and equal to 100 times μGy. Detector exposure index (displayed as ‘DEI’ on the monitor) in previous DEI design is replaced by DI in the new DEI design. Detector exposure index is the ratio of measured count divided by the expected median count. Expected median count is derived at specific conditions (different thickness of Plexiglas for different kVp) while raw median count is calculated for each clinical exposure. Therefore, optimal detector exposure index for each anatomy is different while the optimal DI is fixed. To understand the optimal detector exposure in the context of detector exposure index, the user must know the reference ranges for detector exposure index, which were predetermined based on empirical data and can be configured by users. For example, the factory default detector exposure index range for chest PA is set to 0.2-0.6 and the one for Abdomen is set to 0.56-1.68. The chart below summarizes the difference between Detector exposure index in the previous design and DI in new design. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 9 DEI – Detector Exposure Indicator Property Detector exposure index in old design DI in new design Expected Count or EIT (denominator in index or DI calculation) per Anatomy/View Fixed Varies and configurable Optimal outcome (score) per Anatomy/View Dependent Independent Limit (range) per Anatomy/View Varies and configurable Constant Number of ranges 3 5 Linearity with dose Linear Log with the base 10 Table 3: Comparison between DEI (Index) in previous design and DI in the new design. GE Healthcare Proprietary – Do not redistribute the document Data subject to change without notice 10 4.References: 1. ‘Medical electrical equipment – Exposure Index of digital X-ray imaging systems – Part 1: Definitions and requirements for general radiography,’ IEC 62494-1 Ed 1.0 2008-08 2. 5. S. Jeff Shepard et al, ‘An Exposure Indicator for digital radiography: AAPM Task Group 116 (Executive Summary),’ Med. Phys. 36(7), July 2009, p2898-2914 J. Anthony Seibert et al, ‘The standardized exposure index for digital radiography: an opportunity for optimization of radiation dose to the pediatric population,’ Pediatr Radiol, 841, 2011, p573581 6. 3. Quentin Moore et al, ‘Image Gently: using Exposure Indicators To Improve Pediatric Digital Radiography,’ Radio Tech, 84(1), Sep/ Oct 2012, p93-99 Steven Don et al, ‘New Exposure Indicators for Digital Radiography Simplified for Radiologists and Technologists,’ AJR, 199, Dec 2012, p1337-1341 7. 4. Menglong Zhang et al, ‘Dose optimization for different medical imaging tasks from exposure index, exposure control factor, and mAs in Digital Radiography,’ Health Physics, 103(3), Sep 2012, p235-240 H Warren-Forward et al, ‘An assessment of exposure indices in computed radiography for the posterior-anterior chest and the lateral lumbar spine,’ The British Journal of Radiology, 80, Jan 2007, p26-31 8. M.L. Butler et al, ‘Are exposure index values consistent in clinical practice? A multi-manufacturer investigation,’ Radiation Protection Dosimetry, 2010, p1-4 General Electric Company reserves the right to make changes in specifications and features shown herein, or discontinue any products described at any time without notice or obligation. Please contact your GE representative for the most current information. © 2014 General Electric Company-All rights reserved. GE and GE Monogram are trademarks of General Electric Company. *Trademarks of General Electric Company. 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