Comparison of Low-tube-voltage Images and Virtual Monoenergetic Images in CTA Imaging of the Head with Photon-counting Detector CT

PURPOSE: A head computed tomography angiography (CTA) is widely used for diagnosing cerebrovascular diseases and preoperative vascular assessment. A photon-counting detector CT (PCD-CT)enables virtual monochromatic image (VMI)reconstruction regardless of tube voltage. In PCD-CT, both a low-voltage polyenergetic reconstruction (Poly) and a VMI reconstruction (Mono) can be selected to enhance contrast and reduce contrast medium usage. However, Mono increases noise at lower energy levels, necessitating noise reduction techniques that may degrade spatial resolution. This study aimed to compare the image quality between 70 kVp-Poly and 140 kVp-50 keV in PCD-CT head CTA to determine the optimal scanning method. METHODS: The task transfer function (TTF) and the noise power spectrum were evaluated using a cylindrical water phantom with an attached bone ring. A vascular phantom with 1 mm branches was scanned for full-width at half-maximum (FWHM) and contrast-to-noise ratio (CNR) measurements. Visual assessment was performed by 10 radiological technologists using a five-point scale. RESULTS: No significant difference was observed in TTF or FWHM. However, 70 kVp-Poly had a significantly higher CNR than 140 kVp-50 keV. Visual scores were also significantly higher for 70 kVp-Poly. CONCLUSION: In PCD-CT head CTA, 70 kVp-Poly provides superior image quality over 140 kVp-50 keV, making it a preferable choice for enhanced vascular depiction while minimizing image degradation.