Raising DR tube voltage reduces dose for lumbar studies

By James Brice, AuntMinnie.com contributing writer

August 10, 2011 -- Ignoring manufacturer guidelines for imaging equipment is usually inadvisable, but Kuwaiti researchers have found that doing so for digital radiography (DR) lumbar studies can reduce radiation exposure, according to a poster presentation at last week's American Association of Physicists in Medicine (AAPM) meeting.

The researchers focused on changing the recommended x-ray tube voltage (kVp) settings in a study using an anthropomorphic lumbar spine phantom. Ajit Brindhaban, PhD, an associate professor of radiologic sciences at Kuwait University, and colleagues found entrance surface dose can be cut in half by raising kVp 30% above the manufacturer's recommendations for any of the three systems examined in the study. Lower doses were realized at the cost of modestly lower image quality, but the differences were not statistically significant.

"We can reduce patient dose by experimenting with exposure settings," Brindhaban said. "The message from this poster is not to rely on manufacturers' recommended exposure factors to optimize the balance between image quality and patient dose."

The study demonstrated how higher kVp values and correspondingly lower tube current-time product (mAs) values reduce radiation exposure. Settings for kVp that were 15% and 30% higher than manufacturer recommendations led to a decline in mAs of 25% and 50%, respectively.

The researchers examined DR systems from three vendors: Revolution XR/d from GE Healthcare, DirectView 9000 from Carestream Health, and ddRFormula from Swissray International. Dose reductions were observed for lumbar imaging performed on all three systems, but the absolute amount of dose reduction may be system-specific, according to Brindhaban.

Effect of high kVp and low mA on lumbar anteroposterior radiography
GE Carestream Swissray
Image quality score Effective dose (mSv) Image quality score Effective dose (mSv) Image quality score Effective dose (mSv)
Reference kVp level 4.2 0.34 4.2 0.52 4.8 0.7
Reference + 15% kVp 4.1 0.23 3.9 0.36 4.4 0.43
Reference + 30% kVp 3.9 0.16 3.7 0.25 4.0 0.29
p-value for image quality 0.802 -- 0.39 -- 0.146 --

The entrance surface dose to the phantom and calculated effective dosage fell about 30% when the kVp was raised 15% above the vendor's guidelines. The two radiation measures fell 50% when kVp was raised 30% above the recommended level.

The kVp and mAs modifications had no statistically significant effect on image quality as judged by three experienced radiologists (see chart). Although the average image quality score decreased as kVp increased, scores remained at approximately 3 on a five-point scale. A rating of 3 represents clinically acceptable image quality based on European Union criteria, Brindhaban noted.


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