Exposure System Selection

Transcription

1 Principles of Imaging Science II (RAD120) Exposure Systems Exposure System Selection Radiographic exposure is a very complex process Best technique systems manipulate one variable while holding others constant Goal of any exposure system is consistency Facilities that post and maintain exposure technique charts have lower repeat rates Comparing Exposure Systems Technologists may develop personal preference Should use system employed by imaging facility Students should learn all possible systems before making personal selection 1

2 Anatomically Programmed Radiography (APR) Radiographer can override technique to customize for individual patients If well maintained, only a mas correction factor is necessary Computerized technique charts Radiographer selects body part/projection Phototiming Systems Automatic Exposure Control (AEC) Radiographer selects kvp and possibly backup ma and time Equipment automatically terminates exposure Art of phototiming requires learning when to use it and when not to use it Automatic Exposure Control (AEC, AED) Allows for consistent radiographic images regardless of patient size or pathology Results in lower patient dose, increased dept efficiency, lower film and processing cost, longer tube life 2

3 AEC TYPES - Ion Chamber Located below grid and above the cassette 2 electrodes separated by air encased in thin aluminum shell X-ray photons penetrate ion chamber e- in air are released, ionization occurs. Free e- attracted to positive electrode, flow along electric circuit to a capacitor Stores charge until maximum reached. Additional electricity causes discharge, activates timer for exposure termination. AEC TYPES Ion Chamber Control Panel Activation: AEC, 3 Chambers Selection is based on anatomical part Averaging of chambers AEC kvp, ma, Density Setting Effect on exposure time Backup Timer RT error in setup Correct bucky, tube, or grid centering 600 mas maximum 3

4 AEC: Minimizing Errors Positioning Pathology Prosthetic devices Collimation RSS Calibration AEC TYPES Phototimer Older type of AEC device Consists of: Fluoroescent screen Photomultiplier tube Located below cassette X-ray photons from patient interact with fluorescent screen. PM tube receives light energy from the screen, converts it to electrical energy. Energy passes to capacitor to terminate exposure. Cassettes cannot have lead foil Part Measurement No exposure system can work effectively without accurate measurement of part thickness Failure to measure part correctly will result in increased exposures and patient dose 4

5 Part Measurement Calipers Central ray entrance Thickest part Other Approach must be followed by all department radiographers Designed to produce consistent results Steps in development Produce phantom images Select optimal image Determine chart to utilize Phantom testing Clinical trials Fine tune chart and review periodically Fixed Kilovoltage Most often used Highest optimal kvp is used that is acceptable for the contrast desired mas is doubled for every 5cm of tissue 5 cm ankle = 6 mas 10 cm =? 5

6 Fixed kvp System ADVANTAGES Patient dose is minimized Contrast is uniform Increased exposure latitude Increased X-ray tube life Minimizes motion DISADVANTAGES Long scale contrast Limited variety of mas for pt. sizes Variable Kilovoltage mas is determined by body part thickness 2 kvp/cm + 30, or +40, or +50 Baseline 2 kvp/cm thickness 5 cm ankle = 10 kvp + 30, 40, or 50 If standard is determined to be 60 kvp, then 10 cm ankie = 70 kvp Variable kvp System ADVANTAGES Small kvp changes for body part thickness DISADVANTAGES Variable pt.dose Variable contrast Penetration not assured and/or overpenetration occurs 6

7 Film/Screen, CR, DR Application 7