Moving stereotactic fiducial system to obtain a respiratory signal: proof of principle

The purpose of this study was to obtain a respiratory signal with the use of an add-on device to a specific stereotactic body frame and evaluate precision and accuracy of the method, with the use of a dynamic phantom. The authors designed and constructed a simple add-on device which, attached to a stereotactic body frame, provides information of the patient's respiratory signal in every CT axial image acquired. To assess the approach, 12 CT studies were acquired, on a phantom that simulates respiratory motion, which was placed inside the frame with the add-on device. Images of the phantom with sinusoidal and shark-fin motion patterns were acquired, with different amplitude in the movement of the external surrogate and the target. Cycle time was 6 s. Images were retrospectively processed to obtain a respiratory signal from the vertical movement of the "abdomen." The obtained signal was adjusted to a sinusoidal function; the resultant amplitude and cycle time were compared with the preset function in the phantom. The cycle amplitude and time obtained with the method agreed with the preset values within 0.4 mm and 0.29 s, respectively. In the cases of sinusoidal movements the maximal discrepancy was less than 1 mm. A respiratory signal was obtained in all cine CT sequence studies with this method that consistently coincides with the preset motion of the phantom. The authors proposed a tool to obtain a respiratory signal based on information contained into the CT axial images.