The Dependance of Helically Acquired Ctdi on Beam Width and Pitch
Abstract
Purpose
In 2021, helically acquired CTDI was introduced as an alternative measurement methodology to the traditional, axially acquired CTDI, with a later ACR sponsored study published in 2024. Both publications claim that the accuracy of helical CTDI is independent of beam width and pitch. However, since overranging in CT depends linearly on both pitch and beam width, we hypothesis this statement to be false. The purpose of this study is to quantify the accuracy of helical CTDI with beam width and pitch.
Methods
CTDI was measured using both axial and helical methods over a grid of seven pitches ranging from 0.35 to 1.5, and three nominal beam widths ranging from 19.2 mm to 57.6 mm. All measurements were taken on a Siemens Somatom Force scanner. For helically acquired CTDI, scan length was set to exactly 100 mm. This distance was compared to the “exposed length” as reported by the DICOM RDSR file to determine the overranging distance due to beam width and helical pitch. Accuracy of helical CTDI in reference to axial CTDI was then compared against “true scan length”.
Results
Exposed length was shown to increase linearly with both beam width and pitch, with an R2 > 0.98. When plotted against true scan length, the difference between helically acquired CTDI and axially acquired CTDI increased monotonically.
Conclusion
We confirmed that overranging distance increases linearly with both beam width and pitch causing helical CTDI measurements to monotonically increase with both beam width and pitch. Furthermore, we showed that accuracy of helical CTDI depends linearly on true scan length. This proves the dependence on both beam width and helical pitch. Though measurements were only taken on one scanner, we believe results will generalize to scanners of other vendors and models due to the physical principles governing the relationship remaining the same.