Ultra low-dose helical CT of the chest: evaluation in clinical cases

To compare the visibility of normal lung structures and pulmonary abnormalities between ultra low-dose helical CT (ULHCT: 6 mA) and low-dose helical CT (LHCT: 50 mA), and to assess the feasibility of ULHCT for lung cancer screening.

The reduction of tube current to 6 mA was achieved by using an alminum filter installed in an X-ray tube. After obtaining informed consent, both ULHCT and LHCT of the whole lung were performed in five volunteers and 51 patients, with scanning parameters of 120 kV, 1 rotation/second, 10 mm collimation, and 20 mm/second table speed, during a single breath hold. Images were reconstructed every 5 mm with a 180-degree interpolation algorithm. Three chest radiologists were independently asked to compare the visibilities of normal lung structures using a four-point grading scale, and the scores were compared between ULHCT and LHCT. Pulmonary abnormalities including nodules (less than 20 mm) and other abnormalities (mass, consolidation) were evaluated using four decision levels (A: ULHCT equal to LHCT, B: inferior to LHCT but acceptable, C: much inferior to LHCT and not acceptable, D: not visible) and compared between ULHCT and LHCT. Visibility was also compared between the apical and non-apical regions.

99% of the normal lung structures were judged to be visible in ULHCT, and 236 of 345 (68%) of the nodules were judged as "A" and 92 (27%) as "B". Even with nodules of less than 5 mm, judgements of "A" and "B" were made in 74% and 23%, respectively. The visibility of nodules on the ULHCT was worse in the apical region than in the non-apical region (p<0.01). Other lung abnormalities were also graded as "A" (61%) or "B" (34%). No nodules or abnormalities were graded as "D".

ULHCT has the potential to be utilized in lung cancer screening.