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Measurement of extrafocal radiation by computed radiography

Department of Diagnostic Radiology, Helsinki University Central Hospital, Meilahti Clinics, 00290 Helsinki, Finland

Measurement of radiation differences from image data is made possible by computed radiography (CR) based on storage phosphor imaging plates. The photostimulable luminescence (PSL) response is linear in a range greater than four orders of magnitude. Pinhole radiographs of focal and extrafocal radiation obtained by CR were quantitatively analysed using the technique originally introduced in recent scatter studies. No films or hard copies were used in the analysis. The material consisted of 54 images for measurement of the focal peak intensity, and a corresponding set of 54 images obtained at a 250 times greater operating sensitivity for the imaging of the extrafocal emission. The extrafocal-to-focal emission ratio was in the range of 0.001–0.006 depending on the off-focus distance. Because of the relatively large area of origin, exposure resulting from extrafocal radiation is estimated to reach up to 20% of the total radiation dosage. The results are in agreement with previous reports using other techniques and show that CR can also be used in a quantitative manner in this kind of comparative measurement.

Key Words: Radiography • Digital • Technique • Instrumentation

Received for publication November 28, 1990. Revision received May 15, 1991. Accepted for publication October 3, 1991.