Feasibility study of surface motion tracking with millimeter wave technology during radiotherapy

Abstract

Purpose Continuous monitoring of patient movement is crucial to administering safe radiation therapy (RT). Conventional optical approaches often cannot be used when the patient’s surface is blocked by immobilization devices. Millimeter waves (mmWaves) are capable of penetrating nonconductive objects. In this study, we investigated using mmWave technology to monitor patient surface displacements, as well as breathing and cardiac phases, through clothing and body fixtures. Methods A mmWave device was mounted inside the bore of a ring‐based radiotherapy linear accelerator and pointed at a reflective surface on top of the couch. Measurements were obtained at displacements of 10, 7.5, 5.0, 2.5, and 1.0 mm at heights 100, 150, and 200 mm below isocenter. Submillimeter displacements were performed at a height of 200 mm. Additionally, millimeter and submillimeter displacements were measured with and without a gown and body mold placed between the surface and the sensor. The device was programmed to transmit chirp signals at 77–81 GHz. The subject’s surface was detected by fast Fourier transform (FFT) of the reflected chirp signal within a rough range bin. Fine displacements within that range bin were calculated through phase extraction and phase demodulation. The displacement data were sent through two separate bandpass filters with passbands of 0.1–0.6 and 0.8–2.0 Hz to obtain the subject’s breathing and cardiac waveforms, respectively. The breathing and cardiac measurements were compared to those of a Vernier Respiration Monitor Belt and an electrocardiogram (EKG), respectively, to assess validity. Results The device was able to detect millimeter and submillimeter displacements as small as 0.1 mm, as well as monitor displacement with an accuracy within 1 mm in the presence of an obstructive object. The device's breathing and cardiac waveforms exhibited a strong phase correlation between the respiration monitor belt (ρ = 0.9156) and EKG (ρ = 0.7895), respectively. Conclusions The mmWave device can monitor surface displacements with an accuracy better than 0.1 mm without obstructions and better than 1 mm with obstructions. It can also provide real‐time monitoring of breathing and cardiac waveforms simultaneously with high correlation with traditional respiratory and cardiac monitoring devices. Overall, mmWave technology demonstrates potential for motion monitoring in the field of radiation oncology.

Millimeter wave‐based patient setup verification and motion tracking during radiotherapy

“…Previously, we performed a study demonstrating the feasibility of using mmWave radar to track patient movement, breathing, and cardiac cycles through common immobilization devices used in radiotherapy. 11 Phase-based relative displacement measurement is very sensitive and can detect submillimeter movement caused by breathing and heartbeats. However, frequency-based distance measurement based on a Fast Fourier Transform (FFT) of the mmWave signal is not accurate enough for SSD measurement due to the bandwidth limitation of mmWave sensors.…”

“…The most appealing feature of these sensors in the medical field is that the RF beams can penetrate nonconductive objects and be insensitive to patients' skin color. Previously, we performed a study demonstrating the feasibility of using mmWave radar to track patient movement, breathing, and cardiac cycles through common immobilization devices used in radiotherapy 11 . Phase‐based relative displacement measurement is very sensitive and can detect submillimeter movement caused by breathing and heartbeats.…”

Feasibility study of fiducial marker localization using microwave radar

“…27 Olick-Gibson et al explored 77-81 GHz frequency-modulated continuous-wave radar for continuous monitoring of patient movement during radiotherapy with less than 1 mm accuracy. 28 In this paper, we present a novel concept of radarbased fiducial monitoring. We used vector network analyzer (VNA) as a signal source generator and receiver to probe gold fiducial markers through multilayered human tissues, including skin, fat, and muscles.…”

“…Olick‐Gibson et al. explored 77–81 GHz frequency‐modulated continuous‐wave radar for continuous monitoring of patient movement during radiotherapy with less than 1 mm accuracy 28 …”