Iterative reconstruction methods in two different MDCT scanners: Physical metrics and 4-alternative forced-choice detectability experiments – A phantom approach

Abstract

This paper characterizes and evaluates the potential of three commercial CT iterative reconstruction methods (ASIR™, VEO™ and iDose⁴(™)) for dose reduction and image quality improvement. We measured CT number accuracy, standard deviation (SD), noise power spectrum (NPS) and modulation transfer function (MTF) metrics on Catphan phantom images while five human observers performed four-alternative forced-choice (4AFC) experiments to assess the detectability of low- and high-contrast objects embedded in two pediatric phantoms. Results show that 40% and 100% ASIR as well as iDose⁴ levels 3 and 6 do not affect CT number and strongly decrease image noise with relative SD constant in a large range of dose. However, while ASIR produces a shift of the NPS curve apex, less change is observed with iDose⁴ with respect to FBP methods. With second-generation iterative reconstruction VEO, physical metrics are even further improved: SD decreased to 70.4% at 0.5 mGy and spatial resolution improved to 37% (MTF(50%)). 4AFC experiments show that few improvements in detection task performance are obtained with ASIR and iDose⁴, whereas VEO makes excellent detections possible even at an ultra-low-dose (0.3 mGy), leading to a potential dose reduction of a factor 3 to 7 (67%-86%). In spite of its longer reconstruction time and the fact that clinical studies are still required to complete these results, VEO clearly confirms the tremendous potential of iterative reconstructions for dose reduction in CT and appears to be an important tool for patient follow-up, especially for pediatric patients where cumulative lifetime dose still remains high.

Comparison of the low‐contrast detectability of two ultrasound systems using a grayscale phantom

“…In these cases it is impossible to numerically transform PC to d’ since the value of d’ that corresponds to $\text{PC}=1$ is plus infinity. For this reason, only PC is presented in the results, as in the study of Miéville et al (20) and no attempt to convert the FOM to d’ was made. A similar bootstrap technique was used to determine the uncertainty of the difference in the FOM between the two scanners.…”

Practical considerations for noise power spectra estimation for clinical CT scanners

“…NPS calculation methods should therefore consider this limitation and attempt to acquire image data samples with more stationary noise properties by varying experimental conditions. One such approach is to employ the concept of a local NPS, 4 , 20 , 21 , 22 only analyzing the noise in a localized region of interest (ROI) where the noise is approximately wide‐sense stationary.…”

“…Previous studies have utilized various square ROI sizes, with side lengths of 128 pixels, 4 , 21 , 22 64 pixels, (23) and 32 pixels (19) . Similarly, multiple background removal methods have been proposed and implemented.…”

“…Similarly, multiple background removal methods have been proposed and implemented. Subtraction of two consecutively acquired image sets is the most common method for background removal, 4 , 20 , 21 but various other background removal methods have also been proposed and implemented, such as the subtraction of a first‐order or second‐order polynomial fit 14 , 22 , 24 to the image data. Little has been done in previous literature either to explain the details of background removal methods or to analyze the effect of different background removal methods on the calculated NPS.…”

“…Little has been done in previous literature either to explain the details of background removal methods or to analyze the effect of different background removal methods on the calculated NPS. Additionally, some studies have opted not to apply a window function prior to Fourier transformation, 21 , 22 while others have applied Hann and rectangular window functions to the data (20) . Moreover, the effect of calculation parameters on the local NPS can vary depending on the CT acquisition and reconstruction parameters.…”