Ultra-High-Resolution Photon-Counting Detector CT Arthrography of the Ankle: A Feasibility Study

Abstract

This study was designed to investigate the image quality of ultra-high-resolution ankle arthrography employing a photon-counting detector CT. Bilateral arthrograms were acquired in four cadaveric specimens with full-dose (10 mGy) and low-dose (3 mGy) scan protocols. Three convolution kernels with different spatial frequencies were utilized for image reconstruction (ρ₅₀; Br98: 39.0, Br84: 22.6, Br76: 16.5 lp/cm). Seven radiologists subjectively assessed the image quality regarding the depiction of bone, hyaline cartilage, and ligaments. An additional quantitative assessment comprised the measurement of noise and the computation of contrast-to-noise ratios (CNR). While an optimal depiction of bone tissue was achieved with the ultra-sharp Br98 kernel (S ≤ 0.043), the visualization of cartilage improved with lower modulation transfer functions at each dose level (p ≤ 0.014). The interrater reliability ranged from good to excellent for all assessed tissues (intraclass correlation coefficient ≥ 0.805). The noise levels in subcutaneous fat decreased with reduced spatial frequency (p < 0.001). Notably, the low-dose Br76 matched the CNR of the full-dose Br84 (p > 0.999) and superseded Br98 (p < 0.001) in all tissues. Based on the reported results, a photon-counting detector CT arthrography of the ankle with an ultra-high-resolution collimation offers stellar image quality and tissue assessability, improving the evaluation of miniscule anatomical structures. While bone depiction was superior in combination with an ultra-sharp convolution kernel, soft tissue evaluation benefited from employing a lower spatial frequency.

Keywords: ankle; arthrography; cartilage; photon-counting CT; radiation dosage.

Figure 1

Photon-counting arthrographies in a cadaveric specimen performed with the full-dose (10 mGy; A–C) and low-dose scan protocol (3 mGy; D–F). Reconstructions were performed with the ultra-sharp kernel Br98 (A/D), medium UHR kernel Br84 (B/E), and non-UHR kernel Br76 (C/F). Please note the full thickness cartilage defect at the lateral aspect of the talus (red circle) versus the pseudodefect of the tibial plafond known as the “Notch of Harty” (blue circle). A superficial cartilage lesion of the central tibia is better visualized using means of full-dose arthrography (red arrow).

Figure 2

Depiction of an intact posterior tibiofibular ligament (blue asterisk) in axial (A) and coronal orientation (B). Photon-counting CT arthrography was performed with a CTDI_vol of 10 mGy. The acquired dataset was reconstructed with the ultra-sharp Br98 kernel, which possesses the highest spatial frequency of all convolution kernels available for ultra-high-resolution imaging on a photon-counting detector.

Figure 3

Tri-planar reformatting of a full-dose photon-counting CT arthrogram in axial (A), coronal (B), and sagittal orientation (C) using the medium BR84 kernel displays an osteochondral lesion of the medial talus shoulder (red circle). Additionally, a partial thickness cartilage injury of the lateral talus shoulder can be diagnosed (red arrow).