Practical, Robust CTDI100 Equivalent Measure Using a Thimble Ion Chamber and a Photodiode
Presentations
TU-D1000-IePD-F8-1 (Tuesday, 7/12/2022) 10:00 AM - 10:30 AM [Eastern Time (GMT-4)]
Exhibit Hall | Forum 8
Purpose: CTDI100 relates to the dose at a central plane of the CTDI phantom for a 100 mm scan. Standard CTDI measurements using the CTDI phantom, a 10cm pencil ionization chamber, and a single axial CT scan are challenging with newer scanning configurations. A majority of CT protocols utilize helical scans and may incorporate dual energy techniques. Not all vendors provide equivalent axial scans and they may not be able to provide required information for dual energy scans. Methods have been proposed to solve this but may only be approximately equivalent. This study developed a practical, yet scientifically robust, method to measure CTDI100 on these systems.
Methods: The instantaneous dose rate of helical scans of a CTDI phantom was measured using a 0.6cc thimble ion chamber over the length of the phantom. The central 10cm of the curve of dose rate vs time was integrated to obtain the dose to the ion chamber over this region. For edge measurements of CT100, the shape and integral of the curve vary depending on the starting angle of the source; this requires that an average of several scans be taken. The variation in shape may also lead to difficulty in determining the central 10 cm of the scan, especially for wider beams. One solution is to use a photodiode for spatial localization to provide a consistent position signal.
Results: This method directly determines the equivalent to CTDI100 by integrating the dose rate over the central 10cm of the phantom. Edge measurements of CTDI100 may require more robust location information for proper averaging.
Conclusion: A direct determination of CTDI100 equivalent for helical scans was developed for robust output measurements in the same geometry patients are imaged. There are not limitations for this method in terms of helical or dual energy scanning.
Keywords
CT, Quality Control, Dual-energy Imaging
Taxonomy
IM- CT: Quality Control and Image Quality Assessment
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