Retrospective 4D-CT binning based on concurrent non-contact respiratory and cardiac phase tracking using millimeter wave (mmWave) radar

Background: Concurrent measurement of respiratory and cardiac phases during 4D-CT scans is desired for the treatment of targets in proximity to the heart. Traditional methods require two separate contact-based sensors to monitor breathing and heart beats, which can be inconvenient and intrusive. The mmWave radar can provide non-contact monitoring of patient's motion and has the potential for concurrent monitoring of respiratory and cardiac cycles, offering a promising alternative for phase tracking in 4D-CT. Purpose: The aim of this study is to validate mmWave radars for non-contact phase tracking of respiratory and cardiac waveforms and investigate its application in 4D-CT imaging. Methods: A 60–64 GHz frequency modulated continuous wave (FMCW) radar was used to track the chest wall displacement of healthy volunteers. Respiratory and cardiac signals were extracted from the reflected signals by Fast Fourier Transform (FFT) algorithms and phase unwrapping method. Meanwhile, a respiration bellow and an ECG sensor were used to monitor respiratory and cardiac cycles, respectively, as references. The phase correlations between respiratory and cardiac cycles from the mmWave radar and reference devices were studied, and 4D-CT phase binning was conducted using mmWave signals. Results: The mmWave radar measurement demonstrated strong correlation with the reference measurements, showing high Pearson correlation coefficients ranging from 0.9437 to 0.9533 for respiratory waveforms and 0.9632–0.9896 for cardiac waveforms. Using mmWave signals, phase binning for 4D respiratory and cardiac CT achieved high accuracy, with relative errors of 0.1% and 2.3%, respectively. The system was able to track both signals accurately, with minimal time offsets, highlighting its capability for simultaneous respiratory and cardiac monitoring and precise phase binning in 4D-CT. Conclusions: The mmWave FMCW radar presents a convenient and effective solution for non-contact, concurrent respiratory and cardiac phase tracking, enabling precise phase binning in 4D-CT imaging. Its ability to detect subtle chest wall movements associated with breathing and heartbeats eliminates the need for physical contact sensors, offering a potential alternative to traditional contact-based methods in 4D-CT.