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Session: Cone-Beam CT [Return to Session]

Dosimetry Measurements for CBCT O-Arms Using Phantom to Air Ratios

N Busse*, M Heard, Colorado Associates in Medical Physics, Denver, CO


SU-H300-IePD-F8-1 (Sunday, 7/10/2022) 3:00 PM - 3:30 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 8

Purpose: O-arm imaging systems have become common in ORs for multiple applications. These systems offer fluoroscopic and CBCT acquisition using a flat panel detector. They present dosimetric challenges - CTDI is reported by the system, but not well-defined for CBCT. Our work assessed a measurement technique appropriate for CBCT and evaluated the use of phantom to in-air ratios (PTIAR) for ease of measurement.

Methods: Measurements were made in the center and periphery of a 32 cm PMMA phantom using two pencil ion chambers with effective lengths of 100 and 121 mm on two different O-arm models. The 15 cm long PMMA phantom was fully contained within the 16.7 cm beam at isocenter. Cone beam dose index (CBDI) was calculated in a manner analogous to CTDI, but without dividing by total beam collimation. In-air measurements were also made at isocenter using the same two chambers. Next, five different pencil ion chambers were used to measure dose in air at isocenter on 17 different O-arms. PTIARs were used to calculate dose at the center and periphery and CBDI. CBDI was compared to system-reported CTDI.

Results: Peripheral measurements in the phantom were within 3% at 3, 6, and 9 o’clock locations. Measurements at 12 o’clock were 20-40% higher, most likely due to angular sampling greater than 360 degrees centered around 12 o’clock. Using only values for the 20 cm FOV scans, PTIARs of 0.206 at the center and 0.479 at the periphery were calculated. CBDI was within +/- 15% for all 68 measurements, well within Medtronic’s specified tolerance of 40%.

Conclusion: This method provides a simple technique for assessing CBCT dose and comparing the result to system-reported values. Using PTIARs saves time and reduces the amount of equipment that must be taken into the OR, where these systems are generally used.


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