Comparing 10 kVp and 15% Rules in Extremity Radiography.

PURPOSE

To investigate the viability of the 10 kVp rule as a substitute for the 15% rule in obtaining extremity radiographs of high diagnostic quality while lowering patient radiation dose. The intention of this substitution is to allow technologists a simpler and quicker calculation for use in clinical settings to benefit patients without disrupting workflow.

METHODS

Three anteroposterior or posteroanterior exposures were made on 3 anthropomorphic phantom extremities: hand, knee, and shoulder. Thermoluminescent dosimeters (TLDs) were placed at the center of each phantom; a new TLD was used for each exposure. Exposures were made on a Defininium 8000 digital radiography system (General Electric) using the unaltered, system-preset techniques used by the authors' imaging department, techniques adapted from the 10 kVp rule, and techniques adapted from the 15% rule. The detector exposure indicator from each image was used to compare image quality among the 3 exposures in each group. Entrance skin dose measured from the TLDs also were compared among the 3 exposures for each extremity. One TLD was used as a control to determine the amount of background radiation to which the TLDs were exposed.

RESULTS

The detector exposure indicator remained in optimal range for every exposure with both applied rules as well as the system-preset techniques. Entrance skin dose was lower for both rules in the shoulder and hand trials, with the possibility of error during exposure and reading of the system-preset technique used on the knee phantom. Entrance skin dose also was lower for the shoulder and hand exposures when using the 10 kVp techniques compared with the 15% rule techniques.

DISCUSSION

Aside from unknown entrance skin dose errors during knee imaging, no failures of the tested rules were found. Using manual technique selection rather than automatic exposure control allowed for consistent manipulation of technical factors. TLDs allowed for relatively reliable and quick readings from each exposure, although they could have been the cause of an error in the exposure reading of the preset technique of the knee. Environment and equipment were unchanged to maintain consistency.

CONCLUSION

Results demonstrated that both the 15% rule and the 10 kVp rule successfully maintained or slightly improved image quality according to the detector exposure indicators. The results also demonstrated lowered patient dose according to the entrance skin doses. Based on these results, the 10 kVp rule might be a reasonable substitute for the 15% rule. Further research should be conducted with more repetitions that include digital radiography equipment for a wider range of examinations.