Understanding the physical relations governing the noise navigator

Abstract

Purpose The noise navigator is a passive way to detect physiological motion occurring in a patient through thermal noise modulations measured by standard clinical radiofrequency receive coils. The aim is to gain a deeper understanding of the potential and applications of physiologically induced thermal noise modulations. Methods Numerical electromagnetic simulations and MR measurements were performed to investigate the relative contribution of tissue displacement versus modulation of the dielectric lung properties over the respiratory cycle, the impact of coil diameter and position with respect to the body. Furthermore, the spatial motion sensitivity of specific noise covariance matrix elements of a receive array was investigated. Results The influence of dielectric lung property variations on the noise variance is negligible compared to tissue displacement. Coil size affected the thermal noise variance modulation, but the location of the coil with respect to the body had a larger impact. The modulation depth of a 15 cm diameter stationary coil approximately 3 cm away from the chest (i.e. radiotherapy setup) was 39.7% compared to 4.2% for a coil of the same size on the chest, moving along with respiratory motion. A combination of particular noise covariance matrix elements creates a specific spatial sensitivity for motion. Conclusions The insight gained on the physical relations governing the noise navigator will allow for optimized use and development of new applications. An optimized combination of elements from the noise covariance matrix offer new ways of performing, e.g. motion tracking.

Pilot tone–based motion correction for prospective respiratory compensated cardiac cine MRI

“…Patient motion (ie, due to breathing or the heartbeat) leads to a coil‐dependent variation of the signal amplitude. The underlying principle is based on the different wave impedances due to different loading conditions of the receiver coils 23,24 . Motion causes changes in local coil load and in the coupling between transmitter and receiver.…”

“…The underlying principle is based on the different wave impedances due to different loading conditions of the receiver coils. 23,24 Motion causes changes in local coil load and in the coupling between transmitter and receiver. The PT signal amplitude is approximately proportional to the motion amplitude, yet unitless; therefore, the PT is a useful tool to monitor patient motion.…”

The noise navigator for MRI-guided radiotherapy: an independent method to detect physiological motion

“…Moreover, these ECG skin electrodes and cables could interfere with radiation dose delivery and thus cannot be used during MRIgRT. The noise navigator, on the other hand, does not require additional hardware and should in principle be sensitive to cardiac activity as shown in our previous electromagnetic simulation study (Navest et al 2019). The feasibility of cardiac activity detection by means of the noise navigator was investigated in the context of MRIgRT.…”

“…With the application of MRI-guided radiotherapy in mind, it would be desirable to detect any occurrence of motion. From our previous work that investigated the physical origins of the noise navigator (Navest et al 2019), we know that the noise navigator in principle can detect (i.e. qualitatively measure) the occurrence of tissue displacement and thus is not limited to the detection of respiratory motion.…”

Technical developments for quantitative and motion resolved MR-guided radiotherapy

“…This continuation enables the MRF reconstruction to include the data acquired prior to the MRF scan as additional measurements, which provides a lot of extra information for the parameter quantification. Advantage 2: The spatial basis for the motion fields reconstructed by MR-MOTUS could be correlated to a motion surrogate that is not dependent on the flip angle [294,295], which could enable highly efficient motion compensated 3D MRF reconstructions [264].…”