Impact of Different Scanning Modes and Noise Index on Inner Ear CT Image Quality and Radiation Dose

Objective: This study investigates the effects of different scanning modes and noise index (NI) on inner ear computed tomography (CT) image quality and radiation dose. Methods: (1) Head anthropomorphs phantoms were scanned using a 64-slice helical CT scanner. Both axial and helical scanning modes were employed, with each mode using progressively increasing NI values (3–6, totaling 21 values). In the helical mode, each NI value was used to reconstruct images in two modes: conventional helical reconstruction and helical reconstruction with spiral artifact correction (IQE). Three groups of images were obtained, each comprising 63 sequential images collected and reconstructed. Axial scanning was performed with a rotational speed of 1 second, a helical scanning pitch of 0.531:1, and a speed of 1 second, whereas the other scanning parameters remained constant. (2) Volume CT dose index (CTDIvol) and dose-length product (DLP) were recorded under different scanning conditions to analyze radiation dose. (3) Both objective and subjective image quality assessments were performed. Objective evaluations were conducted on three image groups, calculating the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and figure of merit (FOM) for the region of interest (ROI). Subjective evaluations included assessments of image edge contrast, image intensity, axial noise and artifacts, and multiplanar reconstruction (MPR) quality; a double-blinded method with a 5-point scale was used. The optimal NI value was determined based on the FOM values and subjective image quality assessments. Results: (1) In both axial and helical scanning modes, CTDIvol and DLP decreased as NI values increased. No significant difference was observed in CTDIvol (F=0.03, P > 0.05), but a significant difference was noted in DLP (F=19.78, P < 0.05). (2) Statistically significant differences were observed in soft tissue SNR, as well as SNR, CNR, and FOM for bone tissue, between axial and helical conventional modes and IQE mode (F=27.92, 20.14, 15.71, 1146.98, respectively; P < 0.05), Axial imaging with an NI of 4.7 yielded optimal FOM and subjective evaluation scores. Conclusion: For inner ear CT scanning (range ≤4 mm) using a 64-row CT scanner, MPR reconstruction without fault artifacts can be achieved by obtaining thin-layer images through single-layer scanning without bed movement. Considering that the radiation dose and image quality are better than those of conventional spiral scans or even IQE scans, MPR reconstruction without fault artifacts can be performed using 64-row CT. The recommended NI setting for inner ear imaging is 4.7, as it provides satisfactory image quality while minimizing radiation exposure.