Performance optimization of capacitive motion sensing (CMS) system for intra-fraction motion detection during stereotactic radiosurgery

Abstract

The purpose of this investigation is to improve intra-fractional motion detection during cranial stereotactic radiosurgery with a novel capacitive motion sensing (CMS) system. Previous work showed that a capacitive detection system, based on a MPR121 capacitance-to-digital converter, provided a number of advantages over existing patient imaging systems used in the clinic, by uniquely offering ionizing-radiation-free and continuous monitoring without modification to the immobilization mask or treatment room. However, in order to provide submillimeter detection accuracy, the MPR121based CMS system required relatively large sensors in close proximity to the patient. Therefore, the aim of this investigation was to improve sensitivity of the system, allowing reduction in sensor size and preserving its stable operation in the linear accelerator environment. For this, we developed, characterized and compared motion detection capabilities of four CMS systems based on different capacitance-to-digital converters: MPR121, CPT212B, FDC1004 and FDC2214. Among all candidates, the FDC2214-based system was found to uniquely combine accurate 3D motion detection in real time, with stable performance under ionizing radiation. It exhibited an order of magnitude improvement in sensitivity in comparison with the proof-of-study system, allowing a spatial precision as low as 0.3 mm, and its overall performance was found to satisfy the AAPM practice guidelines of positioning tolerance within 1 mm. Furthermore, the high sensitivity of the system allows both reduction of the sensor area and location more distant from the patient surface, which are key improvements with regard to development of a clinical device.

Six degrees of freedom intrafraction cranial motion detection using a novel capacitive monitoring technique: evaluation with human subjects

“…The purpose of this investigation was to study the performance of a capacitive monitoring array for 6D cranial motion detection on a volunteer cohort with a range of cranium size and shape. The capabilities of the capacitive monitoring system were investigated in our previous publication using a cranial phantom [16]. The detection precision (k-value) was shown to be as low as 0.3 mm without noise filtration, thus enabling detection of sub-millimeter motion even in the absence of any post-processing measures.…”

“…Detection precision is a measure of smallest step of motion in each dimension, starting at the origin, that can be detected by the capacitive monitoring array. Detection precision within the ±1 mm and ±1 deg range was calculated for each dimension of motion using equation (9), where k, ΔN, and ΔS represent detection precision, noise amplitude at the origin (neutral position), and absolute change in the baseline signal, respectively [16].…”

“…A capacitive motion monitoring technology has been recently developed to provide non-contact, continuous motion information [15][16][17] without the use of ionizing radiation. This technique employs capacitive sensors, which have the ability to sense the cranial position inside a thermoplastic mask due to the high conductivity of the cranium relative to that of thermoplastic [15].…”

“…We have previously reported on the stability of the capacitive system in linac environment, with and without radiation present, as well as the capabilities of this method in detecting translational motion in three dimensions through the thermoplastic mask and under the therapeutic radiation beam [15,16]. A thorough Finite Elements Analysis (FEA) was carried out to determine the response and sensitivity of different capacitive array designs [18].…”