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Design Considerations for a Dose-Reducing Region of Interest (ROI) Attenuator Built in the Collimator Assembly of a Fluoroscopic Interventional C-Arm

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S Setlur Nagesh

S Setlur Nagesh*, A Jain, C Ionita, A Titus, D Bednarek, S Rudin, Univ. at Buffalo (SUNY) ,Buffalo, NY

SU-D-134-3 Sunday 2:05PM - 3:00PM Room: 134

Purpose:
ROI fluoroscopy involves the use of an x-ray beam attenuator with higher attenuation in the periphery than the center thus allowing for dose reduction to the patient. This study presents the design considerations for placing an x-ray ROI attenuator made of copper inside the collimator assembly of an angiographic c-arm.

Methods:
The two important considerations for the design of the attenuator are the size of the ROI and the attenuation (and hence thickness of the material) needed outside the ROI. An attenuation of 80% outside the ROI, and none inside the ROI was assumed. To calculate the thickness, exposures were measured for different thicknesses of copper at various kVp's and different inherent filtration of the system. Attenuation percentage was calculated from these readings and the thickness of copper was determined. The field-of-view (FOV) requirement depends on the type of procedure: smaller for a neurovascular intervention and larger for a cardiac procedure. An average FOV of 33% of 21cm x 21cm at 100cm SID with a circular ROI was assumed to calculate the diameter of the ROI in the attenuator.

Results:
For kVp's ranging from 80 to 90, with an added filtration of 0.2mm copper, to get an average attenuation of 80%, 0.7mm of copper was needed for the thickness of the attenuator. The attenuator was placed 13cm from the focal spot and the diameter of the ROI at this distance was calculated to be 10mm.

Conclusion:
The ROI attenuator can be mounted inside the beam limiting mechanism of the c-arm. This allows for the flexibility in the usage of this technique during fluoroscopic interventions, thus achieving patient-dose reduction. Since the attenuation for copper varies with varying kVp, different masks for different kVp's are to be used for brightness equalization.

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