Cutting kVp delivers quality pulmonary CTA

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Lowering the peak kilovoltage setting for pulmonary CT angiography leads to superior vascular enhancement without a deterioration of image quality, according to the results of a retrospective study published in this month’s American Journal of Roentgenology.

Although CT is the first-line imaging modality for patients believed to have a pulmonary embolism, in patients who undergo CT angiography (CTA) for this presentation the prevalence of this finding is less than 10 percent.

“Because of this high percentage of negative results of CT, the risk of unnecessary radiation exposure has become an important issue, especially among young women, who may be exposed to substantial levels of breast radiation or fetal radiation during pregnancy,” the authors wrote.

A 59-year-old woman with chest pain. Pulmonary CT angiogram shows image quality scored 5, excellent, optimal enhancement at least at subsegmental artery level to allow confident diagnosis of presence or absence of clot. Image and caption courtesy of the American Roentgen Ray Society

One of the more effective ways of lowering radiation dose is to decrease the peak kilovoltage for a procedure. Lowering this setting from 120 kVp to 100 kVp during CTA results in an average 44 percent reduction of estimated effective dose, the authors noted.

A total of 400 consecutively registered patients suspected to have pulmonary embolism that presented to the department of radiology at Brigham and Women’s Hospital in Boston were enrolled in the study. A group of 200 patients underwent CTA at the standard peak kilovoltage of either 130 or 120 kVp.

For the other 200 patients, the researchers applied lower kVp settings to both 16- and 64-slice CT systems (Emotion and Somatom Sensation 64, Siemens Healthcare) for CTA exams.

“The peak kilovoltage setting of the 16-MDCT scanner was changed from 130 to 110 kVp, and that of the 64-MDCT scanner was changed from 120 to 100 kVp,” the researchers reported.

A radiologist with 16 years of experience, blinded to the kVp setting, reviewed the CTA exams on a PACS workstation and scored the images on a scale of 1, non-diagnostic, to 5, excellent.

Mean vascular enhancement in the main pulmonary artery had significantly higher attenuation values in the low peak kilovoltage group (376.1 ± 102.9 HU) than in the
standard peak kilovoltage group (309.2 ± 94.8 HU), the authors reported.

Mean attenuation values in all measured segmental and subsegmental arteries were significantly higher in the low peak kilovoltage group than in the standard peak kilovoltage group. As would be expected, image noise in the low peak kilovoltage group was significantly higher than in the standard peak kilovoltage group; and was greater on the 64-slice system compared with the 16-slice system.

However, the most significant finding of the study was that there was no significant difference in the image quality scores of the two groups.

“Even though it was evaluated with the 16- and 64-MDCT scanners separately, vascular enhancement in the low peak kilovoltage group was better than that in standard peak kilovoltage group without deterioration of image quality,” the authors noted.

On the basis of their findings, the researchers recommend a CT protocol of 100 kVp to 110 kVp for evaluation of pulmonary embolism in clinical practice.