High-Pitch Photon-Counting Detector Computed Tomography Angiography of the Aorta
Authors: Euler André , Higashigaito Kai , Mergen Victor , Sartoretti Thomas , Zanini Bettina , Schmidt Bernhard , Flohr Thomas G. , Ulzheimer Stefan , Eberhard Matthias , Alkadhi Hatem 
Published: August 2021

Purpose
The aims of this study were to determine the objective and subjective image quality of high-pitch computed tomography (CT) angiography of the aorta in clinical dual-source photon-counting detector CT (PCD-CT) and to compare the image quality to conventional dual-source energy-integrating detector CT (EID-CT) in the same patients at equal radiation dose.

Materials and Methods
Patients with prior CT angiography of the thoracoabdominal aorta acquired on third-generation dual-source EID-CT in the high-pitch mode and with automatic tube voltage selection (ATVS, reference tube voltage 100 kV) were included. Follow-up imaging was performed on a first-generation, clinical dual-source PCD-CT scanner in the high-pitch and multienergy (QuantumPlus) mode at 120 kV using the same contrast media protocol as with EID-CT. Radiation doses between scans were matched by adapting the tube current of PCD-CT. Polychromatic images for both EID-CT and PCD-CT (called T3D) and virtual monoenergetic images at 40, 45, 50, and 55 keV for PCD-CT were reconstructed. Computed tomography attenuation was measured in the aorta; noise was defined as the standard deviation of attenuation; contrast-to-noise ratio (CNR) was calculated. Subjective image quality (noise, vessel attenuation, vessel sharpness, and overall quality) was rated by 2 blinded, independent radiologists.

Results
Forty patients were included (mean age, 63 years; 8 women; mean body mass index [BMI], 26 kg/m2). There was no significant difference in BMI, effective diameter, or radiation dose between scans (all P's > 0.05). The ATVS in EID-CT selected 70, 80, 90, 100, 110, and 120 kV in 2, 14, 14, 7, 2, and 1 patients, respectively. Mean CNR was 17 ± 8 for EID-CT and 22 ± 7, 20 ± 6, 18 ± 5, 16 ± 5, and 12 ± 4 for PCD-CT at 40, 45, 50, 55 keV, and T3D, respectively. Contrast-to-noise ratio was significantly higher for 40 and 45 keV of PCD-CT as compared with EID-CT (both P's < 0.05). The linear regression model (adjusted R
2, 0.38; P < 0.001) revealed that PCD-CT reconstruction (P < 0.001), BMI group (P = 0.007), and kV of the EID-CT scan (P = 0.01) were significantly associated with CNR difference, with an increase by 34% with PCD-CT for overweight as compared with normal weight patients. Subjective image quality reading revealed slight differences between readers for subjective vessel attenuation and sharpness, whereas subjective noise was rated significantly higher for 40 and 45 keV (P < 0.001) and overall quality similar (P > 0.05) between scans.

Conclusions
High-pitch PCD-CT angiography of the aorta with VMI at 40 and 45 keV resulted in significantly increased CNR compared with EID-CT with ATVS at matched radiation dose. The CNR gain of PCD-CT increased in overweight patients. Taking into account the subjective analysis, VMI at 45 to 50 keV is proposed as the best trade-off between objective and subjective image quality.