Optimal slice thickness for object detection with longitudinal partial volume effects in computed tomography

Abstract

Longitudinal partial volume effects (z‐axial PVE), which occur when an object partly occupies a slice, degrade image resolution and contrast in computed tomography (CT). Z‐axial PVE is unavoidable for subslice objects and reduces their contrast according to their fraction contained within the slice. This effect can be countered using a smaller slice thickness, but at the cost of an increased image noise or radiation dose. The aim of this study is to offer a tool for optimizing the reconstruction parameters (slice thickness and slice spacing) in CT protocols in the case of partial volume effects. This optimization is based on the tradeoff between axial resolution and noise. For that purpose, we developed a simplified analytical model investigating the average statistical effect of z‐axial PVE on contrast and contrast‐to‐noise ratio (CNR). A Catphan 500 phantom was scanned with various pitches and CTDI and reconstructed with different slice thicknesses to assess the visibility of subslice targets that simulate low contrast anatomical features present in CT exams. The detectability score of human observers was used to rank the perceptual image quality against the CNR. Contrast and CNR reduction due to z‐axial PVE measured on experimental data were first compared to numerical calculations and then to the analytical model. Compared to numerical calculations, the simplified algebraic model slightly overestimated the contrast but the differences remained below 5%. It could determine the optimal reconstruction parameters that maximize the objects visibility for a given dose in the case of z‐axial PVE. An optimal slice thickness equal to three‐fourth of the object width was correctly proposed by the model for nonoverlapping slices. The tradeoff between detectability and dose is maximized for a slice spacing of half the slice thickness associated with a slice width equal to the characteristic object width.

Impact of reconstruction parameters on the accuracy of myocardial extracellular volume quantification on a first-generation, photon-counting detector CT

“…PCD-CT offers the unique possibility to overcome the limitation of partial volume averaging without reducing image quality due to its intrinsic increased spatial resolution and reduced image noise compared to EID-CT [ 24 , 25 ]. The highest concordance that thinner slices have with MRI quantification of ECV is bound to the higher contrast resolution of myocardial tissue that is achieved with reduced slice thickness and hence reduced z -axial partial volume effect [ 26 ]. The increased spatial resolution and consequent increase in contrast resolution are fundamental in CT compared to MRI, where the presence of partial volume effect (MOLLI sequences conventionally use 8-mm slice thickness [ 27 ]) is balanced by the strength of pixel-wise mapping and the ability of MRI to detect small abnormalities and discriminate low-contrast structures [ 28 ].…”

A challenge and solution for automatic thin slice thickness measurements on images of the Catphan phantom

“…This has enhanced the broad utilization of CT in clinical practice worldwide, especially in cancer and tumour diagnosis and treatment [7][8][9][10][11][12][13][14][15]. However, thin slice thickness leads to a higher level of image noise [16][17][18][19][20]. Hence, optimization of the slice thickness in each specific case is essential [20,21].…”

“…However, thin slice thickness leads to a higher level of image noise [16][17][18][19][20]. Hence, optimization of the slice thickness in each specific case is essential [20,21].…”