Noninvasive pulmonary nodule elastometry by CT and deformable image registration

Background and purpose: To develop a noninvasive method for determining malignant pulmonary nodule (MPN) elasticity, and compare it against expert dual-observer manual contouring.

Methods and materials: We analyzed breath-hold images at extreme tidal volumes of 23 patients with 30 MPN treated with stereotactic ablative radiotherapy. Deformable image registration (DIR) was applied to the breath-hold images to determine the volumes of the MPNs and a ring of surrounding lung tissue (ring) in each state. MPNs were also manually delineated on deep inhale and exhale images by two observers. Volumes were compared between observers and DIR by Dice similarity. Elasticity was defined as the absolute value of the volume ratio of the MPN minus one normalized to that of the ring.

Results: For all 30 tumors the Dice coefficient was 0.79±0.07 and 0.79±0.06 between DIR with observers 1 and 2, respectively, close to the inter-observer Dice value, 0.81±0.1. The elasticity of MPNs was 1.24±0.26, demonstrating that volume change of the MPN was less than that of the surrounding lung.

Conclusion: We developed a noninvasive CT elastometry method based on DIR that measures the elasticity of biopsy-proven MPN. Our future direction would be to develop this method to distinguish malignant from benign nodules.