Radiation Awareness for Endovascular AAA Repair in the Hybrid OR

February 16, 2018

Radiation Awareness for Endovascular AAA Repair in the Hybrid OR
Complex endovascular procedures can result in higher radiation doses for both the patient and the caregiver, with variation based on several different parameters. De Ruiter, et al reported in a retrospective study analysis dated last year, that there are many different characteristics that can predict patient radiation dose rates, not just Cumulative Air Kerma (CAK). Using data from 74 EVAR procedures, including 16,889 X-ray runs using fixed C-arm imaging equipment, a patient risk chart was created using several parameters such as body mass index (BMI), protocols used, C-arm position, and the image acquisitions used. Technologies such as Centerline Biomedical’s Intra-Operative Position System (IOPS™) can provide enhanced visualization, reducing time under radiation since surgeons can see detailed 3D visualization, leading to more efficient procedures.

Often, surgeons are tasked with performing complex endovascular procedures that increase time under radiation, increasing dose levels for both the caregiver and the patient. This results in having to reduce X‑ray flux, which then results in lower X-ray dose, but causes lower image quality. High-contrast X-ray images from fixed C-arms increase the radiation dose rates (DR).

Final DR is multifactorial and should incorporate various DR predictors, such as the radiation protocol, BMI, C-arm rotation and angulation, the air gap, and the field size. This leads to a better understanding of the actual clinical DRs to eventually steer decision making for DR reduction during intervention.

The study encompassed two imaging modalities, fluoroscopy and Digital Subtraction Angiography (DSA). Fluoroscopy is used for the majority of the procedure, with ‘low’, ‘medium’, and ‘normal’ settings; DSA is used to check vessel/device patency, existence of endoleaks, and outflow using 2 frames per second (fps) or 3 fps acquisition.

The effects estimates derived from the regression analysis using the various DR predictors were used to create a color-coded risk chart (below) that shows the maximum allowed radiation duration until the cumulative threshold of 2-Gy would be reached. 

The final analysis of 16,889 X-ray runs consisted of 16,031 (94.9%) fluoroscopy runs, with a mean DRAK (Air Kerma) of 0.35±0.31 mGy/s and a mean DRDAP (Dose Area Product) of 74±67 mGycm2/s, and 858 (5.1%) DSA runs (DRAK of 6.8±5.5 mGy/s and DRDAP 1392±1071 mGycm2/s).

The time taken before the 2-Gy skin threshold was reached and decreased with increasing radiation dose. The low, normal, and medium fluoroscopy protocols corresponded with means of 218, 79, and 40 minutes, respectively. For DSA imaging, the 2-fps, and 3-fps protocols produced means of 5.8 and 3.6 minutes, respectively. This correlates with the fact that longer procedures require lower resolution for a bulk of the procedure. The mean DRs increased approximately 3 to 4 times when the C-arm position was rotated to 75 to 90°. The mean DRAK also increased an average of 9 times for fluoroscopic imaging with increasing BMI and body thickness and approximately 12 times for DSA imaging. Reducing the field size also increased the fluoroscopy DRAK.

For fluoroscopy, the chosen protocol was the most substantial predictor of DR. Therefore, higher resolution imaging results in higher DR, reducing the amount of time fluoroscopy can be used until the skin threshold is reached. Using the above characteristics – for example, a BMI of 30 kg/m2 combined with 45° of rotation and a field size of 800 cm2 in the medium fluoroscopy protocol – predicts a DRAK of 0.39 mGy/s (or 85.5 minutes until the 2-Gy skin threshold is reached). The below risk chart for radiation dose rates demonstrates that the intraoperative DRs are highly dependent on not one, but a combination of factors: the X-Ray mode (fluoroscopy or DSA) with the chosen protocol, the C-arm rotation or angulation, patient BMI, field size and the Source-to-Image distance (SID). 


The study shows that because of the concerns of higher radiation exposure, lower resolution and fps rates need to be used, which can interfere with the smoothness of a moving object, such as a guidewire, and a fps rate that is too low can hinder the radiologist’s eye-hand coordination.
Technologies such as Centerline’s IOPS can reduce the fluoroscopy time and radiation dose by improving visualization. Improved visualization will enable high-resolution view of the patient’s vasculature, something that is not possible for a full procedure with current standard of care.