What s the Issue? Radiation Dose in the OR. Exposure to OR Personnel. Effects of Proper Collimation. Image-guided procedures are increasingly complex

Transcription

1 Radiation Dose in the OR Robert G. Gould, Sc.D. Professor and Vice Chair Department of Radiology and Biomedical Imaging University of California San Francisco What s the Issue? Image-guided procedures are increasingly complex Enabled by better imaging equipment Longer procedure times with more x-ray use Potentially high patient skin dose Acute skin effects (largely preventable) Higher doses to OR personnel Exposure to OR Personnel Scattered radiation from the patient Compton interactions within the patient Rule of thumb: intensity ~ 1/1000 the primary beam entering the patient Reduced by: Collimation Moving the detector closer to the patient Effects of Proper Collimation Reduces the amount of scatter Lower levels in the room, lower personnel exposure Improved image quality Reduction in patient radiation burden 1

2 Scatter: PA vs Lateral Radiation Protection for Personnel Detector Table top Scatter Scatter X-ray tube Detector Table top 1. TIME Minimize x-ray tube on time (consistent with good patient care). 2. DISTANCE Stand as far from the x-ray beam as possible (consistent with good patient care). 3. SHIELDING Wear protective 'wrap-around' apron. Use floor or ceiling suspended shields. Image Intensifier Fluoroscopy Image Intensifier: Factors Effecting Patient Dose Used in mobile C-arms and older angio equipment Digitize video signal Image processing Flexible frame rates Electronic subtraction Established technology Comparatively simple Video camera Image Intensifier Digital Video Processor Collimator Video Monitor Automatic exposure control (AEC) Adjusts the KV and ma during fluoro Limits the maximum patient dose rate Magnification mode Smaller fields (more mag) increase resolution at a cost of a higher dose rate Collimation Geometry 2

3 Automatic Exposure Control (AEC) Feedback mechanism that maintains a constant brightness level from the center portion of the output screen Adjusts the X-ray technique factors (ma and KV) + KV + ma Fluctuating patient entrance dose Patient Feedback loop Constant input exposure Image intensifier Constant light level Flat Panel Angiographic Systems Pixilated detector Digitization within the detector Dynamic range >> analog systems AEC not the same as in II based systems Complex Automatic Exposure Control Considerations (Vendor Spin) Purposes Provide adequate image quality Maintain appropriate patient dose Stay within regulations at all times Eliminate all manual adjustment of technique factors Fast and easy regulation Provide long x-ray tube life Improve cost effectiveness Automatic Exposure Control Considerations NOT a dose saving system Design factors are to maintain a given image quality (contrast-to-noise factor) while staying within equipment capabilities and patient dose regulations limits User preference Can have low dose settings Limit patient dose to fraction of regulatory limits 3

4 Automatic Exposure Control Variables Pulse Rate Tube potential Beam filtration Use both Cu and Al mas per pulse Both ma and pulse width vary Patient dose rate 15 fps normal 7.5 fps normal Variable slope Patient thickness 15 fps low dose 7.5 fps low dose 4.4 cgy/min 2.2 cgy/min Flat panel systems always operate in pulsed fluoro Effect on patient dose depends on other factors Patient dose does not always scale with rate mas per pulse, filtration, and kvp can all change Patient Dose Rate: Pulsed Fluoro Patient Dose Indicators (post 2006) ~ 2x reduction < 2x reduction No difference! 1. on time (min) Weak relationship with pt skin dose Patient dose rate Patient thickness 4.4 cgy/min 15 fps normal 7.5 fps normal 2. Dose-Area Product (mgy-cm 2 ) Energy imparted to patient Weak relationship with pt skin dose Does relate to OR staff dose Unintelligible 3. Air kerma (mgy) Air dose at a reference point Most closely associated with skin dose 4

5 K a at Interventional Reference Point 15 cm from the isocenter towards the x-ray tube ~ 30 cm from the image detector Cumulative kerma correlates with peak skin dose Does not account for panning of the beam Does not include scatter Both air kerma rate (dose rate) and the cumulative air kerma shown 15 cm Central Ray Isocenter Reference point Methods to Reduce Patient Dose Minimize x-ray beam on time Use last image hold Use virtual (electronic) collimation Position the x-ray source and the detector optimally Inverse square law Collimate the beam Understand and use pulsed fluoroscopy When possible use store fluoro loop instead of an angiographic (cine) run 8-10x reduction Vary the entrance port site Use the least amount of magnification possible Avoid steeply angled projections if possible Conclusions Spend some time to understand the equipment settings Be wary of the out-of-the-box set up conditions Be aware of the patient dose indicators React before K a reaches 3000 mgy Get to know your medical physicist 5