A phantom study on dose efficiency for orthopedic applications: Comparing slot‐scanning radiography using ultra‐small‐angle tomosynthesis to conventional radiography
Authors: Luckner Christoph , Weber Thomas , Herbst Magdalena , Ritschl Ludwig , Kappler Steffen , Maier Andreas 
Published: December 2020

Purpose

This paper studies the abilities of a twin‐robotic x‐ray slot‐scanning system for orthopedic imaging to reduce dose by scatter rejection compared to conventional digital radiography.

Methods

We investigate the dose saving capabilities, especially in terms of the signal‐ and the contrast‐to‐noise ratio, as well as the scatter‐to‐primary ratio of the proposed slot‐scanning method in comparison to the state‐of‐the‐art method for length‐extended imaging. As a baseline, we use x‐ray parameters of two clinically established acquisition protocols that provide the same detector entrance dose but are profoundly different in patient dose. To obtain an estimate of the photon‐related noise directly from an x‐ray image, we implement a Poisson‐Gaussian noise model. This model is used to compare the dose efficiency of two settings and combined with the well‐known to determine the transmission parameters. We present a method with an associated measurement protocol, utilizing the robotic capabilities of the used system to automatically obtain quasi‐scatter‐free ground‐truth data with exact geometric correspondence to full‐field and slot acquisitions. In total, we investigate two body regions (thoracic spine and lumbar spine) in anterior‐posterior view with two patient sizes (BMI = 22 and 30) in two acquisition modes (conventional and slot scan with a flat‐panel detector) with and without anti‐scatter grid using an anthropomorphic upper‐body phantom.

Results

We have shown that it is feasible to combine the proposed approach with the for the determination of scatter rejection parameters. The use of an anti‐scatter grid is indicated for full‐field acquisitions allowing for dose savings up to 46% compared to their gridless counterparts. When changing the acquisition mode to the investigated slot scan, the use of an anti‐scatter grid has no major impact on the image quality in terms of dose efficiency, in particular for patients with a BMI of 22. However, an increased contrast improvement factor was found. For normal‐sized patients, up to 53% of dose can be saved additionally in comparison to full‐field acquisitions with grid. Moreover, we could demonstrate that a slot size of 5 cm and air gap of 10 cm is sufficient to achieve scatter‐to‐primary ratios, which are equal or better compared to those of the full‐field acquisitions with a grid.

Conclusions

We have shown, that the slot‐scanning approach is always superior to the conventional full‐field acquisition in terms of signal‐to‐noise and scatter‐to‐primary ratios. Compared to the state‐of‐the‐art acquisition protocols with a grid, dose savings up to 53% are possible due to the scatter rejection without compromising the SNR. Hence, the use of the slot‐scanning method is indicated, especially when it comes to regularly carried‐out follow‐up acquisitions, for example, in the case of scoliosis monitoring.