Radiographic Technique Effects on Image Quality and Exposure

The choice of an imaging technique is a multifactor decision with consideration given to parameters that include contrast, resolution, noise and exposure. OBJECTIVE: This study examined radiographic images taken at two techniques for image quality and exposure. METHODS: A dental General Electric x-ray generator was used to make exposures at 70 kVp, 10 mA, 15/60ths second, and 90 kVp, 10 mA, 15/60ths second. The detector system was Air Techniques #2 photostimuable phosphor plates. Exposure was measured with a Radcal 9010 radiation monitor. The radiographic images were scanned by a ScanX reader connected to a Windows XP Professional computer running MiPACS software v3.1. The scanning parameters were set to 16 bits, no enhancement, and high resolution. The images were exported as DICOM files to a Macintosh computer with OS X v10.4. The image files were displayed and measured using NIH ImageJ v1.34s software. Contrast was determined by percent changes on an aluminum step wedge; resolution by line pairs visible on a line pair phantom; and noise by the signal to noise ratio on the aluminum wedge. The exposure was measured in milliRoentgen units. Eight replications of each technique for each parameter were made. The data were compared using Student's t test with alpha=0.05. RESULTS: Contrast was higher at 70 kVp, mean 84%, than 90 kVp , mean 74%, and significantly different. The resolution was the same with 70 kVp, mean 9 lp/mm, and 90 kVp, mean 9 lp/mm. The signal-to-noise ratio was significantly lower at 70 kVp, mean 40.5, than 90 kVp, mean 53.0. The exposure through air at 18 inches was 46.7 mR at 70 kVp and 79.0 at 90 kVp, which was significantly different. CONCLUSIONS: Contrast was significantly higher at 70 kVP, resolution was the same, and the signal-to-noise ratio and exposure were significantly higher at 90 kVp.