Abstract
IntroductionWhile tin prefiltration is established in various CT applications, its value in extremity cone-beam CT relative to optimized spectra has not been thoroughly assessed thus far. This study aims to investigate the effect of tin filters in extremity cone-beam CT with a twin-robotic X-ray system. MethodsWrist, elbow and ankle joints of two cadaveric specimens were examined in a laboratory setup with different combinations of prefiltration (copper, tin), tube voltage and current–time product. Image quality was assessed subjectively by five radiologists with Fleiss’ kappa being computed to measure interrater agreement. To provide a semiquantitative criterion for image quality, contrast-to-noise ratios (CNR) were compared for standardized regions of interest. Volume CT dose indices were calculated for a 16 cm polymethylmethacrylate phantom. ResultsRadiation dose ranged from 17.4 mGy in the clinical standard protocol without tin filter to as low as 0.7 mGy with tin prefiltration. Image quality ratings and CNR for tin-filtered scans with 100 kV were lower than for 80 kV studies with copper prefiltration despite higher dose (11.2 and 5.6 vs. 4.5 mGy; p < 0.001). No difference was ascertained between 100 kV scans with tin filtration and 60 kV copper-filtered scans with 75% dose reduction (subjective: p = 0.101; CNR: p = 0.706). Fleiss’ kappa of 0.597 (95% confidence interval 0.567–0.626; p < 0.001) indicated moderate interrater agreement. ConclusionConsiderable dose reduction is feasible with tin prefiltration, however, the twin-robotic X-ray system's low-dose potential for extremity 3D imaging is maximized with a dedicated low-kilovolt scan protocol in situations without extensive beam-hardening artifacts. Implications for practiceLow-kilovolt imaging with copper prefiltration provides a superior trade-off between dose reduction and image quality compared to tin-filtered cone-beam CT scan protocols with higher tube voltage.
Highlights
While tin prefiltration is established in various CT applications, its value in extremity conebeam CT relative to optimized spectra has not been thoroughly assessed far
The other authors of this manuscript declare no relationships with any companies, whose products or services may be related to the subject matter of the article
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Summary
While tin prefiltration is established in various CT applications, its value in extremity conebeam CT relative to optimized spectra has not been thoroughly assessed far. Cone-beam computed tomography (CBCT), which combines pyramid-shaped X-ray beams and flat-panel detector technology, represents a dose-effective 3D imaging alternative with superior spatial resolution for certain applications.[8] With a long pedigree in dental radiology since the 1990s,9 the emergence of dedicated extremity scanners in the last decade has allowed CBCT to transfer its niche between radiography and CT to musculoskeletal applications.10e13 Regarding filter material, copper is widely established in plain radiography, and since CBCT scanners are directly derived from traditional X-ray systems, copper prefiltration is the clinical standard in most CBCT scanners Another promising approach to radiation exposure minimization, that was originally introduced in MDCT imaging, is the use of tin filters.14e17 Capable of absorbing low-energy photons that do not contribute substantially to the image information in high-contrast scans (e.g., in bone examinations), they have the potential to effectively reduce the radiation dose in various imaging tasks.[18] For CBCT scans of the appendicular skeleton, spectral shaping through tin prefiltration has not been evaluated far, and, to the authors’ best knowledge, tin filters are not implemented in any commercially available CBCT system at the moment
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