Technical Note: Efficacy of CT spatial resolution metrics in clinical settings

The task-based transfer function (TTF) and modulation transfer function (MTF) are spatial resolution metrics in CT; with TTF understood as being more relevant for use on modern nonlinear image reconstruction algorithms.

In clinical practice, images are "magnified" and created at varying reconstruction fields of view. The impact of PACS workstation interpolation and zoomed reconstruction on the TTF and MTF may affect their clinically perceived spatial resolution. This paper studies this potential issue.

A wire and an iodine rod were imaged to calculate MTF and TTF respectively. MTF values were measured from instances of a wire centered on a pixel and shifted to lay over multiple pixels; analogous to how a clinical detail may either be centered or shifted with respect to the pixel matrix. Images were reconstructed using both low- and high-resolution kernels at reconstruction fields of view (RFOV) 60 to 500 mm. A cadaveric temporal bone was similarly imaged. The effect of PACS interpolation was duplicated by applying bi-cubic interpolation.

For the low-resolution kernel, with and without PACS interpolation, the MTF and TTF provided almost consistent spatial resolution measurements only below 300 mm RFOV. For the high-resolution kernel without PACS interpolation, the MTF and TTF measurements were consistent up to approximately 220 mm. With PACS interpolation applied, the centered and shifted MTF diverged, which signaled the images were suffering from pixelation which was confirmed visually in the cadaveric images.

This paper demonstrates a limitation of using the TTF related to RFOV. The TTF does not account for degradations in image quality easily observed by the human eye (predicted by divergent centered and shifted MTF) due to pixel sampling common to RFOVs used in the clinic.