Remote sensing, tracking and imaging inside MRI systems

In addition to diagnostic and therapeutic uses, clinical magnetic resonance imaging (MRI) systems have also been used for interventional procedures. However, safety risks associated with increased specific absorption rate and tissue overheating should be measured in-situ and controlled during such procedures. Here, we introduce a self- resonating radio frequency (RF) sensor capable of remote temperature sensing to serve as a visual indicator of in-situ temperature changes during real-time MRI interventional operations. We propose a new sensor design that uses dielectric properties as the tuning mechanism for the sensor resonant frequency and the temperature dependence of permittivity. Using a 7 Tesla (7T) preclinical MRI, we demonstrate ex vivo feasibility and remote temperature sensing capabilities in the clinically relevant temperature range of 36- 42°C. The miniature design of the sensor allows its placement on the tip of a catheter, where it can be used for both catheter tracking and temperature sensing. The RF sensor is tuned to match the resonant frequency of 7T MRI (298 MHz), enabling a hyperintense signal on the sensor in the MR images. Hence, it can be used for three-dimensional position tracking during the steering of a given interventional medical device, such as a catheter. As temperature increases, the sensor detunes due to the change in the relative permittivity, and the hyperintense signal disappears in the MR image, serving as a direct visual indicator of the temperature change in real-time without a need for post-processing. Since this technique is based on common MR imaging sequences, the same image can be used for both device localization and temperature measurement. 0.6°C accuracy is achieved in the physiological range between 36°C and 42°C. Such RF sensors could provide safer operations in future MRI interventional procedures with potential local increased temperatures. MRI is also a heavily utilized medical diagnostic tool that usually employs a contrast agent for enhanced image sensitivity and selectivity. Superparamagnetic iron oxide nanoparticles (SPIONs) have been shown as strong MRI contrast agents in the literature but usually with a dark (T2) contrast. However, bright contrast (T1 contrast agent) is preferred by the radiologists in the clinic, yet difficult to achieve. Here, we show T1 contrast generation of polyacrylic acid-coated superparamagnetic iron oxide nanoparticles (SPION-PAA) in our 7 Tesla MRI. We showed that such T1 contrast is achievable in ex vivo mouse experiments, as well. Interaction of SPION with UV or visible light is also an exciting phenomenon that may be exploited in photopolymerization to produce polymer/SPION nanocomposites for both enhancing MRI imaging contrast and 3D printing small-scale MRI robots. Using biocompatible SPIONs of nanoscale size and high stability eliminates the need for an initiator that might be undesirable in biological applications. We show that SPION-PAA can also be used as an initiator in the photopolymerization of vinyl monomers. Differential scanning calorimeter experiments were conducted to find optimal parameters for the highest and fastest conversion conditions. Furthermore, synthesized SPION/polymer hybrids/gels were demonstrated as efficient sensitizers for hyperthermia in an alternating magnetic field.