Effect of Radiation Dose Reduction and Reconstruction Algorithm on Image Noise, Contrast, Resolution, and Detectability of Subtle Hypoattenuating Liver Lesions at Multidetector CT: Filtered Back Projection versus a Commercial Model–based Iterative Reconstruction Algorithm

Abstract

Purpose To determine the effect of radiation dose and iterative reconstruction (IR) on noise, contrast, resolution, and observer-based detectability of subtle hypoattenuating liver lesions and to estimate the dose reduction potential of the IR algorithm in question. Materials and Methods This prospective, single-center, HIPAA-compliant study was approved by the institutional review board. A dual-source computed tomography (CT) system was used to reconstruct CT projection data from 21 patients into six radiation dose levels (12.5%, 25%, 37.5%, 50%, 75%, and 100%) on the basis of two CT acquisitions. A series of virtual liver lesions (five per patient, 105 total, lesion-to-liver prereconstruction contrast of −15 HU, 12-mm diameter) were inserted into the raw CT projection data and images were reconstructed with filtered back projection (FBP) (B31f kernel) and sinogram-affirmed IR (SAFIRE) (I31f-5 kernel). Image noise (pixel standard deviation), lesion contrast (after reconstruction), lesion boundary sharpness (average normalized gradient at lesion boundary), and contrast-to-noise ratio (CNR) were compared. Next, a two-alternative forced choice perception experiment was performed (16 readers [six radiologists, 10 medical physicists]). A linear mixed-effects statistical model was used to compare detection accuracy between FBP and SAFIRE and to estimate the radiation dose reduction potential of SAFIRE. Results Compared with FBP, SAFIRE reduced noise by a mean of 53% ± 5, lesion contrast by 12% ± 4, and lesion sharpness by 13% ± 10 but increased CNR by 89% ± 19. Detection accuracy was 2% higher on average with SAFIRE than with FBP (P = .03), which translated into an estimated radiation dose reduction potential (±95% confidence interval) of 16% ± 13. Conclusion SAFIRE increases detectability at a given radiation dose (approximately 2% increase in detection accuracy) and allows for imaging at reduced radiation dose (16% ± 13), while maintaining low-contrast detectability of subtle hypoattenuating focal liver lesions. This estimated dose reduction is somewhat smaller than that suggested by past studies.

Radiation Dose Reduction at Pediatric CT: Use of Low Tube Voltage and Iterative Reconstruction

“…However, when CT is performed with considerably reduced doses, diagnostic performance deteriorates for clinical tasks involving low contrast resolution (eg, brain CT, hepatic CT, and pancreatic CT), even if image noise and contrast-to-noise ratio are maintained or improved with IR algorithms (73). The maximum dose reduction potential and imaging texture (IR-related artifacts, such as a pixelated or plastic appearance) differ according to the IR algorithm used (74,75), and there has been controversy over how much the radiation dose can be reduced with each IR algorithm without loss of low contrast detectability (76,77). However, the decision whether to increase the radiation dose substantially to depict subtle lesions in most children, who may have more conspicuous lesions or no lesions at all, should be carefully considered, according to each clinical condition.…”

Observer Performance with Varying Radiation Dose and Reconstruction Methods for Detection of Hepatic Metastases

“…With IR, spatial resolution is dependent on image contrast differences, and whereas it may substantially reduce noise and increase the contrast-to-noise ratio (2), it can also decrease lesion contrast relative to background and lesion sharpness (11), and this combination may potentially result in decreased detection of small lesions with attenuation similar to background liver (5). At the 160 and 120 QRM dose levels, performance and agreement were nearly identical whether or not IR was used despite improvement in image quality.…”

“…Whereas iterative reconstruction (IR) has been shown to improve image quality of lower dose abdominal CT images (2,3), its ability to improve observer performance for low-contrast detection tasks such as detection of hepatic metastases (where similar attenuation can be observed between metastases and background liver) has proven limited in human observer studies (4–11). With IR, spatial resolution is dependent on image contrast differences, so small lesions with ill-defined borders may be obscured at lower doses or in large patients, both of which increase image noise (1,10).…”

Technical Note: Radiation dose reduction from computed tomography localizer radiographs using a tin spectral shaping filter

“…Modern CT scanners are able to automatically select the optimal tube voltage as well as to adjust the tube current according to the individual patients’ anatomy resulting in radiation dose savings of up to 60% . Advanced methods of reconstruction and post‐processing provide additional potential for radiation dose reduction …”

“…4,5 Advanced methods of reconstruction and postprocessing provide additional potential for radiation dose reduction. [6][7][8] The selective photon shield represented by a tin filter mounted in front of the x-ray tube was initially introduced for better separation of the two energy spectra in dual-energy CT. 9 It was recently shown that such spectral shaping can be also used for single-energy CT resulting in more efficient x-ray beam utilization and allowing for further dose reduction. [10][11][12] As a consequence of these dose reduction opportunities leading to CT examinations being at the range of one millisievert for various indications, the contribution from localizer radiographs (LRs) to the overall dose from CT becomes more prominent.…”