Low pitch significantly reduces helical artifacts in abdominal CT

Abstract

Purpose: High helical pitch scanning minimizes scan times in CT imaging, and thus also minimizes motion artifact and mis-synchronization with contrast bolus. However, high pitch produces helical artifacts that may adversely affect diagnostic image quality. This study aims to determine the severity and incidence of helical artifacts in abdominal CT imaging and their relation to the helical pitch scan parameter.

Methods: To obtain a dataset with varying pitch values, we used CT exam data both internal and external to our center. A cohort of 59 consecutive adult patients receiving an abdomen CT examination at our center with an accompanying prior examination from an external center was selected for retrospective review. Two expert observers performed a blinded rating of helical artifact in each examination using a five-point Likert scale. The incidence of artifacts with respect to the helical pitch was assessed. A generalized linear mixed-effects regression (GLMER) model, with study arm (Internal or External to our center) and helical pitch as the fixed-effect predictor variables, was fit to the artifact ratings, and significance of the predictor variables was tested.

Results: For a pitch of <0.75, the proportion of exams with mild or worse helical artifacts (Likert scores of 1-3) was <1%. The proportion increased to 16% for exams with pitch between 0.75 and 1.2, and further increased to 78% for exams with a pitch greater than 1.2. Pitch was significantly associated with helical artifact in the GLMER model (p = 2.8 × 10^-9), while study arm was not a significant factor (p = 0.76).

Conclusion: The incidence and severity of helical artifact increased with helical pitch. This difference persisted even after accounting for the potential confounding factor of the center where the study was performed.

Keywords: Abdomen; Artifacts; CT; Pitch; Technique.

Figure 1 –

Reconstructions of the tilted ACR phantom with scans of varying helical pitch. Top row: axial plane, from left to right: pitch of 0.52, 0.98, 1.38. Bottom row: coronal plane, from left to right: pitch of 0.52, 0.98, 1.38.

Figure 2 –

Example cases with helical artifacts. A: exam performed with pitch of 0.52 with artifact scores of 5 and 5. B: exam of the same patient performed with pitch of 1.38 with artifact scores of 1 and 2. Helical artifacts are prominent throughout the abdominal visceral. C: exam performed with pitch of 0.52 with artifact scores of 5 and 5. D: exam of the same patient performed with pitch of 1.38 with artifact scores of 2 and 3. Helical artifacts are annotated with solid arrowheads.

Figure 3–

Distribution of scanner models used in the Internal and External study arms.

Figure 4–

Distribution of helical pitch parameter values in the Internal and External study arms

Figure 5–

Plot of artifact scores versus helical pitch scan parameter. Scores across multiple observers were averaged, and a small plot jitter is added to avoid display of overlapping data points. A linear regression fit line and 95% confidence interval band are shown.