Abstract
ObjectivesTo utilize a novel objective approach combining a software phantom and an image quality metric to systematically evaluate the influence of sinogram affirmed iterative reconstruction (SAFIRE) of multidetector computed tomography (MDCT) data on image noise characteristics and low-contrast detectability (LCD).Materials and MethodsA low-contrast and a high-contrast phantom were examined on a 128-slice scanner at different dose levels. The datasets were reconstructed using filtered back projection (FBP) and SAFIRE and virtual low-contrast lesions (-20HU) were inserted. LCD was evaluated using the multiscale structural similarity index (MS-SIM*). Image noise texture and spatial resolution were objectively evaluated.ResultsThe use of SAFIRE led to an improvement of LCD for all dose levels and lesions sizes. The relative improvement of LCD was inversely related to the dose level, declining from 208%(±37%), 259%(±30%) and 309%(±35%) at 25mAs to 106%(±6%), 119%(±9%) and 123%(±8%) at 200mAs for SAFIRE filter strengths of 1, 3 and 5 (p<0.05). SAFIRE reached at least the LCD of FBP at a relative dose of 50%. There was no statistically significant difference in spatial resolution. The use of SAFIRE led to coarser image noise granularity.ConclusionA novel objective approach combining a software phantom and the MS-SSIM* image quality metric was used to analyze the detectability of virtual low-contrast lesions against the background of image noise as created using SAFIRE in comparison to filtered back-projection. We found, that image noise characteristics using SAFIRE at 50% dose were comparable to the use of FBP at 100% dose with respect to lesion detectability. The unfamiliar imaging appearance of iteratively reconstructed datasets may in part be explained by a different, coarser noise characteristic as demonstrated by a granulometric analysis.
Highlights
The use of computed tomography (CT) has increased dramatically during the last two decades, accelerated by the rapid technical improvement with milestones such as the introduction of spiral scanning and multidetector CT (MDCT)
sinogram affirmed iterative reconstruction (SAFIRE) reached at least the low-contrast detectability (LCD) of filtered back projection (FBP) at a relative dose of 50%
A novel objective approach combining a software phantom and the multiscale structural similarity index (MS-SSIM)* image quality metric was used to analyze the detectability of virtual low-contrast lesions against the background of image noise as created using SAFIRE in comparison to filtered back-projection
Summary
The use of computed tomography (CT) has increased dramatically during the last two decades, accelerated by the rapid technical improvement with milestones such as the introduction of spiral scanning and multidetector CT (MDCT). Iterative reconstruction algorithms are well established in the field of nuclear medicine as they allow the generation of diagnostic images even from statistically poor raw data. Their routine use for CT reconstruction had been limited by the availability of sufficient computing power that enables a reconstruction time suitable for clinical use [4]. Several manufacturers have introduced proprietary iterative reconstruction algorithms in their commercially available scanners and suggest a significant improvement in the signal-to-noise ratio, allowing for dose-wise scanning protocols [5] while retaining the detectability of lowcontrast objects
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