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Using Exposure Index to Develop Thickness-Based Technique Chart for Cross-Table Lateral Hip Radiography

C Topbas*, G Fong, V Singh, K Hulme, X Li, Cleveland Clinic, Cleveland, OH


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Cross-table lateral hip radiography is commonly performed to assess hip fracture or dislocation in trauma or surgery patients. Technique optimization for this exam is complex due to the absence of automated exposure control (AEC), atypical patient positioning, ineffective scatter reduction, and the possible use of additional beam filtration. The goal of this study was to establish a thickness-based technique chart by using the exposure index (EI) as a surrogate for AEC to achieve consistent image receptor air kerma across patient sizes.

Methods: Cross-table lateral hip thickness and the corresponding image field-of-view size were measured from 68 clinical exams to establish a mathematical relationship between the two. Lucite blocks were then used to simulate patient attenuation and were arranged on the tabletop to mimic the typical exam set-up with a source-to-image distance of 50 inches and a clip-on grid. Exposures were made at 85 kV for five lucite thicknesses, representing 10??, 25??, 50??, 75?? and 90?? percentile cross-table lateral hip thicknesses. Field-of-view size appropriate for each hip thickness was used. For each exposure, mAs, incident air kerma, and the IEC EI were recorded. The measurements were then repeated with an additional 0.2-mm copper filtration.

Results: To achieve a constant EI of 350, the required mAs increased exponentially with the cross-table lateral hip thicknesses (r²=0.99). For 10??, 25??, 50??, 75?? and 90?? percentile hip thicknesses, the required mAs values were 24, 35, 52, 80, and 122, respectively without copper filtration and 32, 46, 69, 105 and 159, respectively, with copper filtration. Incident air kerma was approximately 50% lower when copper filtration was used.

Conclusion: By creating a realistic clinical setup and adjusting manual technique to achieve a constant EI across patient sizes, we successfully developed thickness-based technique charts for cross-table lateral hip radiography.


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