150-μm Spatial Resolution Using Photon-Counting Detector Computed Tomography Technology: Technical Performance and First Patient Images

Abstract

Objective: The aims of this study were to quantitatively assess two new scan modes on a photon-counting detector computed tomography system, each designed to maximize spatial resolution, and to qualitatively demonstrate potential clinical impact using patient data.

Materials and methods: This Health Insurance Portability Act-compliant study was approved by our institutional review board. Two high-spatial-resolution scan modes (Sharp and UHR) were evaluated using phantoms to quantify spatial resolution and image noise, and results were compared with the standard mode (Macro). Patients were scanned using a conventional energy-integrating detector scanner and the photon-counting detector scanner using the same radiation dose. In first patient images, anatomic details were qualitatively evaluated to demonstrate potential clinical impact.

Results: Sharp and UHR modes had a 69% and 87% improvement in in-plane spatial resolution, respectively, compared with Macro mode (10% modulation-translation-function values of 16.05, 17.69, and 9.48 lp/cm, respectively). The cutoff spatial frequency of the UHR mode (32.4 lp/cm) corresponded to a limiting spatial resolution of 150 μm. The full-width-at-half-maximum values of the section sensitivity profiles were 0.41, 0.44, and 0.67 mm for the thinnest image thickness for each mode (0.25, 0.25, and 0.5 mm, respectively). At the same in-plane spatial resolution, Sharp and UHR images had up to 15% lower noise than Macro images. Patient images acquired in Sharp mode demonstrated better delineation of fine anatomic structures compared with Macro mode images.

Conclusions: Phantom studies demonstrated superior resolution and noise properties for the Sharp and UHR modes relative to the standard Macro mode and patient images demonstrated the potential benefit of these scan modes for clinical practice.

Figure 1

Detector pixel configuration and z-axis collimation for the 4 scans modes on the PCD-CT system: Macro, Chess, Sharp and UHR. Native detector pixels are delineated by the dashed lines while detector readout units are delineated by solid lines for the low energy (green) and high energy (red) data.

Figure 2

MTF curves and corresponding 10% MTF values for Macro, Sharp and UHR modes with images reconstructed with B70 kernel (a) and S80 kernel (b).

Figure 3

SSPs of Macro, Sharp and UHR images reconstructed at 0.5 and 0.25 mm slice thickness, with corresponding FWHM of each SSP labeled.

Figure 4

Images of an anthropomorphic head phantom scanned with Macro (a) and Sharp (b) modes and reconstructed with the same head kernel. Noise measurements showed 15% noise reduction (from 94 to 80 HU) using Sharp mode compared to Macro mode.

Figure 5

Lung images from the same patient scanned on an EID-CT (a) and the PCD-CT (b). Image locations were selected to be as similar as possible. Compared to EID-CT, the PCD-CT image was much sharper, with excellent airway wall delineation and more small vessels observed.

Figure 6

Shoulder images from the same patient scanned on an EID-CT (a) and the PCD-CT (b). Image locations were selected to be as similar as possible. The PCD image shows sharper cortex and trabecular bone, and subchondral cysts and sclerosis compared to the EID image.

Figure 7

Temporal bone images show high spatial resolution and clear delineation of the incudomallear joint (arrow) for both EID-CT (a) and PCD-CT (b). However, measurements showed a 21% noise reduction (141 to 112 HU) for PCD-CT compared to EID-CT, which uses a dose-inefficient comb filter to decrease the effective detector pixel sizes.

Figure 8

Images from a single head CTA exam using PCD-CT can provide both a high spatial resolution single energy image (a), and a dual-energy processed image before (b) and after bone removal (c). Display settings: W/L = 1800/400 HU bone window for (a), W/L = 40/300 HU soft tissue window for (b) and (c).

Figure 9

The low-energy threshold image demonstrates sharp boundaries of the stone (a). The dual-energy post-processed image (b) demonstrates a pure uric acid stone (red) and a mixed stone with both uric acid (red) and non-uric-acid (blue) components.