Understanding Exposure and Deviation Indices

Transcription

1 Imaging Educational Course - Radiography III Understanding Exposure and Deviation Indices David L. Leong 1,2 Marie-Louise Butler 2 1 Analogic Corporation, Peabody, MA, USA 2 UCD School of Medicine and Medical Sciences, University College Dublin, Dublin, Ireland AAPM 2011 WE-A Learning Objectives 1. Understand AAPM recommendations, 2. Understand the IEC standard, 3. Understand factors that affect Exposure and Deviation Index values. 4. Understand DICOM support for Exposure and Deviation Index values. 2 1

2 State of Confusion Manufacturer Specific Indices Rex S lgm EI DEI EXI EVI f# A range of units and relationships to exposure. Calibration conditions ranged from 70 to 80 kvp with different filtration selections 3 AAPM Task Group 116 TG116 was started in July 25, 2004 Report published June individuals participated in the Task Group Full report available on AAPM Website Executive summary published in Medical Physics S. J. Shepard et al. "An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary)," Med. Phys. 36, (2009)

3 IEC Working Group 43/62b Preparatory work in the German (DIN) standardization organization Proposal for standard by the German National Committee June 2006 WG43 was started in September 2006 Experts from Germany, Belgium, Canada, China, Japan, Netherlands, United States 16 members, 6 people overlapped with TG116 First meeting October 2006 in Hamburg, Germany IEC published August 2008 Stability date: Keys Parts Exposure Index Target Exposure Index Deviation Index Calibration Condition 6 3

4 Exposure Index Name IEC Exposure Index (EI) AAPM TG116 Indicated Equivalent Air Kerma (K ind ) Units Unitless μgy Precision Not specified 3 significant Different definitions, but substantially the same. K ind typically less than 1 as it is μgy EI is designed to be greater than 1 7 Exposure Index Formulas IEC: CAL 1 1 K f V g V f V V f K CAL CAL CAL -1 1 EI c0 g V 100μGy f V TG116: K STD K STD Q K 1000 log log10 K Q 1000 IND 0 0 K K 10, where K Gy 8 4

5 Target Exposure Index Target values: K TGT (TG116) and EI T (IEC) EI T expected value of EI when the detector is properly exposed Manufacturer / Site Specified Needs to be adjustable by user Can be established by professional societies Body part and view specific targets (recommended by TG116) 9 Deviation Index IEC AAPM TG116 Name Deviation Index (EI) Deviation Index (EI) Units Unitless Unitless Precision Not specified 1 decimal place TG116 and IEC are the same: DI EI 10 log EI T More important clinically than EI 10 5

6 Action Levels (TG116) DI Table 2. Exposure Indicator DI Control Limits for Clinical Images Range Action > +3.0 Excessive patient radiation exposure Repeat only if relevant anatomy is clipped or burned out Require immediate management followup. +1 to +3.0 Overexposure: Repeat only if relevant anatomy is clipped or burned out 0.5 to +0.5 Target range Less than 1.0 Underexposed: Consult radiologist for repeat Less than 3.0 Repeat 11 Calibration IEC kvp: Based on RQA5 from IEC HVL: 6.8 ± 0.3 mm Al Added filter of either 21 mm aluminum or 0.5 mm copper and 2 mm aluminum Adjustment of the kvp from 66 to 74 is permitted to achieve the target HVL AAPM kvp: 70 ± 4 HVL: 6.8 ± 0.25 mm Al Added filter of either 21 mm aluminum or 0.5 mm copper and 0 to 4 mm aluminum*, Adjustment of the kvp and filter permitted to achieve the target HVL *Type 1100 Document the added filter and X-ray tube voltage used for calibration 12 6

7 Calibration - Recommendation Start as close to 70 kvp as possible Measure non-invasively without added filtration If your meter support Practical Peak Voltage (PPV), use it Use at least 1 meter Source to Detector distance Adjust added filtration to achieve 6.8 mm HVL No less than 0.5 mm Cu or 18 mm Al No more than 0.5 mm Cu + 4 mm Al or 23 mm Al Raise or lower kv as a last resort Record the final filter thickness and generator kvp On repeat calibrations, start with the previous filter thickness 13 Summary Exposure Indicator Name IEC Exposure Index (EI) AAPM TG116 Indicated Equivalent Air Kerma (K ind ) Exposure Indicator Units Unitless μgy Exposure Indicator Precision Not specified 3 significant EI EI T K KTGT IND Deviation Index formula DI 10 log DI 10*log Deviation Index Units Unitless Unitless Deviation Index Precision Not specified 1 decimal place Beam Conditions 6.8 ± 0.3 mm Al 6.8 ± 0.25 mm Al 14 7

8 Factors that affect EI Calculation of EI based on anatomical region of interest. (ROI) - Relevant Image Region Method to determine ROI and area of ROI affect EI value Either define a specific procedure or method 15 ROI and Exposure Variation 16 8

9 30 Exposures No Changes Bone and Soft Tissue Mostly Bone Bone Only Mean Std Chest Exposures All Image Anatomy Mean Std

10 ROI Effects on DI Bone and Soft Tissue Mostly Bone Bone Only 4 mas mas DICOM Changes CP1024 written to support new Exposure Indices Became Final Text June 21, 2010 Use is not mandatory 20 10

11 DICOM Changes Legacy Exposure Indices Attribute Name Tag Type Attribute Description Relative X-Ray Exposure (0018,1405) 3 Relative x-ray exposure on the plate. Meaning of values is implementation specific. May be used to adjust the dynamic range of the plate digitizer (scanner). Note: This value is manufacturerspecific. DICOM specifies standard Attributes in Table 10.x-1 Exposure Index Macro which are recommended. Sensitivity (0018,6000) 3 Read out sensitivity. Note: This value is manufacturerspecific. DICOM specifies standard Attributes in Table 10.x-1 Exposure Index Macro which are recommended. 21 DICOM Changes New Exposure Indices Attribute Name Tag Type Attribute Description Exposure Index (0018,1411) 3 Measure of the detector response to radiation in the relevant image region of an image acquired with a digital x-ray imaging system as defined in IEC Notes: 1. A string rather than binary Value Representation is used for this Attribute, in order to allow the sender to control the precision of the value as suggested in the report of AAPM Task Group This index value is scaled as defined by IEC Target Exposure Index (0018,1412) 3 The target value used to calculate the Deviation Index (0018,1413) as defined in IEC Deviation Index (0018,1413) 3 A scaled representation of the difference of the Exposure Index compared to the Target Exposure Index as defined in IEC and the report of AAPM TG

12 Recommendations Insist that all new CR and DR equipment support IEC Both EI and DI Push your PACS vendor to support and display DI on all workstations (CP1024) Attempt to upgrade older CR and DR systems to add support for IEC Develop a set of Target Exposure Values Setup a training program on the use of EI, DI and action limits 23 Learning Objectives 1. Understand AAPM recommendations, 2. Understand the IEC standard, 3. Understand factors that affect Exposure and Deviation Index values. 4. Understand DICOM support for Exposure and Deviation Index values

13 Final Comment It is essential that t all aspects of good clinical technique be adhered to and an appropriate DI value should not be interpreted as proof of good work. TG116 Report Section X 25 Thank You UCD School of Medicine & Medical Science Scoil an Leighis agus Eolaíocht An Leighis UCD 13