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Science to Practice |
Department of Diagnostic Imaging,
Vanderbilt Children's Hospital,
2200 Children's Way,
Nashville, TN 37232-9700,
marta.schulman@vanderbilt.edu
SUMMARY
Through the use of an animal model with controlled conditions and direct measurement of skin doses, Ward et al have demonstrated the feasibility of achieving significant dose reduction while maintaining diagnostic quality during the performance of routine fluoroscopic procedures in pediatric patients.
THE SETTING
As our knowledge and awareness of the deleterious effects of radiation have increased, our focus has shifted from deterministic to stochastic effects and has led to the ALARA concept—that diagnostic radiation exposure should be as low as reasonably achievable, considering the capability of the equipment and diagnostic value of the examination. Ward et al (1) note that the ALARA concept has been a subject of much research and concern as applied to computed tomographic (CT) scans, and rightly so, since CT accounts for approximately 40% of medical radiation exposure. However, radiographic and fluoroscopic studies (eg, voiding cystourethrography, upper gastrointestinal examinations) represent approximately 90% of pediatric diagnostic examinations (2). It is therefore important that radiation exposure to this population be addressed. In this issue of Radiology, Ward et al (1) investigate the direct dose reduction achievable with a pulsed fluoroscopy system in an in vivo animal model, as well as its translation into images of diagnostic quality.
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Pulsed fluoroscopy was initially developed as an attempt to decrease fluoroscopic radiation dose by limiting the time during which the patient was exposed to the x-ray beam, by using reduction in the number of exposures per second. However, early units produced radiation pulses at the generator. The result was a ramp and trail effect, dependent on the length of cable from generator to x-ray tube, with a bell-shaped curve of increasing and decreasing current (3). The attendant low energy x-rays therefore increased the radiation dose per pulse and did not result in a meaningful decrease in radiation dose.
Current grid-controlled pulsed fluoroscopy units, such as the one used in the study by Ward et al (1), use a negatively charged grid interposed between the cathode and the anode of the x-ray tube. The grid can be rapidly switched on and off, which thereby allows appropriate energy electrons generated to be intermittently passed through the grid to produce x-rays. Optimization of fluoroscopy pulse widths and careful choice of entrance exposure per pulse during calibration of the unit permit additional dose savings outlined in the article. Previous researchers have investigated results of dose reduction versus image quality with grid-controlled pulsed fluoroscopy units. By using phantoms with grid-controlled pulsed fluoroscopy at 3.75 pulses per second, Lederman et al (4) found that dose could be reduced 10-fold without significant reduction of contrast or spatial resolution. Boland et al (3) found no significant difference in diagnostic acceptability or image quality between continuous fluoroscopy and pulsed fluoroscopy (at 15.0, 7.5, and 3.75 frames per second) in patients undergoing a variety of fluoroscopic procedures. Ward et al (1) used an in vivo animal model of reflux to simulate infant, toddler, and child sizes, with direct measurement of entrance radiation exposure. They found that the use of intermittent (pulsed) fluoroscopy decreased radiation exposure by a factor of 4.6–7.5 compared with the conventional unit, with no significant loss of diagnostic quality.
THE PRACTICE
Clinical use: The data from investigations of grid-controlledpulsed fluoroscopy units indicate that thoughtful use of this equipment permits substantial reduction in exposure without loss of diagnostic information and validate its use in routine clinical practice. The benefits to patients, particularly to children, are obvious. The benefits during fluoroscopic procedures that require repeated fluoroscopic observation over long periods of time, such as intussusception reduction, are likely multiplicative.
Future opportunities and challenges: Other methods of application of the ALARA principle remain to be investigated. These include the lower limit of pulsed fluoroscopy frames per second in moving structures or patients, advanced digital imaging processing methods, newer types of image receptors, optimization of filters, peak voltage and amperage, source-to-skin distance, field of view, and matrix size. However, the most important factor, awareness of the potentially harmful stochastic effects of radiation exposure, has been overcome, as the radiologist reaches beyond the traditional role of diagnostician to that of an active guardian and protector of the present and future health of our patients.
References
Related Article
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  P P Dendy Radiation risks in interventional radiology Br. J. Radiol., January 1, 2008; 81(961): 1 - 7. [Abstract] [Full Text] [PDF]
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