Application of the noise power spectrum in modern diagnostic MDCT: part I. Measurement of noise power spectra and noise equivalent quanta

Abstract

Dose reduction efforts in diagnostic CT have brought the tradeoff of dose versus image quality to the forefront. The need for meaningful characterization of image noise beyond that offered by pixel standard deviation is becoming increasingly important. This work aims to study the implementation of the noise power spectrum (NPS) and noise equivalent quanta (NEQ) on modern, multislice diagnostic CT scanners. The details of NPS and NEQ measurement are outlined and special attention is paid to issues unique to multislice CT. Aliasing, filter design and effects of acquisition geometry are investigated. While it was found that both metrics can be implemented in modern CT, it was discovered that NEQ cannot be aptly applied with certain non-traditional reconstruction filters or in helical mode. NPS and NEQ under a variety of conditions are examined. Extensions of NPS and NEQ to uses in protocol standardization are also discussed.

A phantom study investigating the relationship between ground‐glass opacity visibility and physical detectability index in low‐dose chest computed tomography

“…This matched filter SNR (${\text{SNR}}_{\mathrm{M}}$) can be calculated by $SN{R}_M{\hspace{0.17em}}^2=2\pi true{\int }_0^{\infty }u\frac{S^{2}false(ufalse)MT{F}^{2}false(ufalse)}{NPSfalse(ufalse)}du$ where u is the spatial frequency and $Sfalse(ufalse)$ denotes the spatial frequency spectrum of the signal. The $\text{italicMTF}false(ufalse)$ and $\text{italicNPS}false(ufalse)$ were measured using a wire phantom and a water phantom, respectively, 13 , 24 , 25 , 26 , 27 as described in Material & Methods section C.2 and C.3 below. $Sfalse(ufalse)$ was calculated using a numerical simulation, as presented in section C.4 below.…”

“…The MTF of each image set was measured by a wire method that has been previously reported 13 , 24 , 25 . Here, a 50 mm diameter cylindrical water phantom containing a 0.15 mm thin copper wire aligned parallel to the phantom axis was used.…”

“…The NPS for the water phantom image was calculated using a 2D Fourier transform according to a previously published method 13 , 26 , 27 . To generate a 1D NPS representation, radial rebinning of the data points comprising the 2D NPS into 25 frequency bins, was performed, and these bins were then averaged.…”

“…It is well known that noise in a CT image is affected not only by the dose level but also by the slice thickness and reconstruction kernel (13) . Reduced slice thicknesses and sharper reconstruction kernels result in more image noise in the CT image.…”

Practical considerations for noise power spectra estimation for clinical CT scanners

“…The noise power spectrum (NPS), as a more thorough noise descriptor than pixel standard deviation, describes both the magnitude and spatial frequency characteristics of image noise, which plays a critical role in analyzing and optimizing imaging system performance 1 , 2 , 3 , 4 , 5 . It is most often integrated with other metrics to assess image quality for specific tasks, 6 , 7 , 8 , 9 and has been commonly utilized in the development, characterization, optimization, and comparison of many new imaging technologies such as computed radiography, (10) digital mammography, 11 , 12 storage phosphors for dental X‐ray, (13) and other devices in a preclinical 14 , 15 and clinical 16 , 17 environment.…”