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Initial Evaluation of a Low-Focus Grid Versus Air Gap for Lateral Hip Digital Radiography Using a Phantom

K Kirby*, B Schueler, S Broski, T Daly, Z Long, Mayo Clinic, Rochester, MN

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PO-GePV-I-92 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Lateral hip digital radiography (DR) presents technical and positioning challenges due to cross-table orientation and high scatter. In our practice, lateral hip DR is acquired with a low-focus antiscatter grid to facilitate proper alignment. The purpose of this study was to evaluate lateral hip DR image quality with low-focus grid versus air gap techniques.

Methods: A clinical survey of lateral hip thicknesses determined appropriate phantom thicknesses. A contrast-detail phantom (CDRAD 2.0) was placed in the middle of polymethylmethacrylate (PMMA)/aluminum slabs of thicknesses 20cmPMMA/4mmAl, 25cmPMMA/5mmAl, and 30cmPMMA/6mmAl. mAs values for each thickness were obtained for the gridded setup using a clinical protocol of 85kV, source-to-image distance 130cm, and target exposure index 500 on a GE Discovery XR656 HD scanner. The same technique was used for gridded and air gap (0cm, 10cm, 20cm, 30cm) acquisitions of the same phantom thickness thus maintaining patient entrance air kerma. Grid specifications included 70lp/mm and grid ratio 8:1. The inverse image quality factor (IQFinv), corresponding to contrast-detail detectability, was calculated for each image after flat processing. Group IQFinv was generated based on four repeated exposures and compared within each phantom thickness.

Results: IQFinv decreased with increasing phantom thickness. The 20cm air gap yielded the highest IQFinv compared to other air gaps for all thicknesses. For small phantom, IQFinv was 7.04 for the low-focus grid and 7.43 for the 20cm air gap. For medium phantom, the grid’s IQFinv was 6.50, and 7.11 for 20cm air gap. For large phantom, the grid’s IQFinv was 6.11, and the 20cm air gap was 6.07.

Conclusion: IQFinv differences were demonstrated between the low-focus grid and the air gap technique for cross table lateral hip DR using a phantom study. 20cm air gap appears to provide the best IQFinv overall as well as dose reduction potential for small to medium patients.

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