Dual kV CT to detect calcification in solitary pulmonary nodule

At the high kVp values used in CT scanning, almost all interactions in soft tissues occur by Compton scattering. By lowering the kVp close to the k-edge of calcium, beam attenuation by calcium will be greater and will be reflected as an increase in density reading (DR). The presence of calcification in solitary pulmonary nodules (SPNs), and thus an implied benign etiology, is currently being diagnosed on CT by using reference phantoms. We explored the possibility of utilizing low kVp (i.e., 80 kVp) to detect the presence of such calcification in SPNs, thus obviating the need for expensive and cumbersome reference phantoms.

We first tested this phenomenon of photoelectric interaction by CT and its effect on DRs by scanning various dilutions of calcium bicarbonate solution at the standard 140 kVp and at a lower, 80 kVp, setting. After confirming the hypothesis, we conducted a prospective clinical study of 27 consecutive SPNs and scanned them at 140 and 80 kVp to detect the presence of calcification by measuring their DRs at both kVp values.

All calcium solutions showed an increase in DR on the 80 kVp scan. Of the 27 nodules, 11 (41%) showed an increase in DR, suggesting the presence of calcification: 10 (91%) were benign, and 1 (9%) was malignant.

Our study demonstrates that dual kVp CT could be reliably used to identify calcifications in SPNs, very similar to the use of the reference phantoms currently being applied for the purpose.