High-power diffusion probe for human breast MRI

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Modern magnetic resonance imaging (MRI) scanners yield clinically relevant images, but direct imaging of individual cells in vivo lies beyond the current sensitivity limit. Nonetheless, it is possible to measure metrics of the diffusive water motion in vivo that are linked to the tissue microstructure. Diffusion-weighted imaging is performed with conventional MRI scanners on a daily basis in clinical diagnostics. Unfortunately current clinical MR scanners cannot deliver the magnetic field gradient amplitudes that would be needed to directly assess the tissue microstructure. Instead, one is limited to "apparent" metrics such as the widely used apparent diffusion coefficient. The aim of the present proposal is to develop a new device - a "High-Power Diffusion Probe" - that locally provides orders of magnitude stronger gradient amplitudes than presently available in clinical MR imagers, in order to approach a more direct characterization of tissue microstructure. The focus will be on breast imaging, since preparatory simulations have revealed good technical feasibility and since the applicants have acquired direct experience in ongoing breast cancer imaging studies thus becoming aware of the clinical need for improved diagnostics in this field. The preparatory investigations have further shown that a significantly stronger gradient field can be generated for the target geometry if the traditional linear field distribution is replaced by a novel nonlinear gradient field. The proposed nonlinear field distribution has a further potential of offsetting some of the concerns related to the neural stimulation and subject safety. Phase 1 of the project will focus on developing a feasibility version of the device with a low duty cycle. The construction will be based on optimal current carrying surfaces that will be determined in simulations for single and multilayer designs, respecting the needs for good gradient efficiency as well as mechanical properties such as force and torque balancing. An important aspect will be subject safety, which will be addressed by adjusting the properties of the device itself (thermal and electro-mechanical properties) and by evaluating the effect of the generated time-dependent gradient fields on peripheral nerve stimulations and on cardiac function. Appropriate diffusion-weighted sequences that e.g. compensate for the potentially increased effect of eddy currents will be developed to enable successful application in initial feasibility experiments. In Phase 2 it is planned to develop a full-power prototype capable of a high duty cycle operation as required by the target clinically and scientifically relevant in vivo applications, which will also be developed and demonstrated in that phase.