High-power diffusion probe for human breast MRI

现代磁共振成像（MRI）扫描仪产生临床相关的图像，但对体内单个细胞的直接成像超出了目前的灵敏度极限。尽管如此，测量体内与组织微观结构相关的扩散水运动的度量是可能的。在临床诊断中，每天使用常规MRI扫描仪进行弥散加权成像。不幸的是，目前的临床磁共振扫描仪无法提供直接评估组织微观结构所需的磁场梯度振幅。相反，人们受限于“表观”指标，如广泛使用的表观扩散系数。本提案的目的是开发一种新设备-“高功率扩散探针”-局部提供比目前临床磁共振成像仪强几个数量级的梯度振幅，以便更直接地表征组织微观结构。重点将放在乳房成像上，因为预备模拟显示了良好的技术可行性，而且由于申请人已经获得了正在进行的乳腺癌成像研究的直接经验，从而意识到改进这一领域诊断的临床需要。前期研究进一步表明，如果用一种新的非线性梯度场代替传统的线性场分布，可以对目标几何结构产生明显更强的梯度场。所提出的非线性场分布有可能进一步抵消与神经刺激和受试者安全有关的一些问题。该项目的第一阶段将重点开发具有低占空比的设备的可行性版本。该结构将基于单层和多层设计的模拟中确定的最佳载流表面，同时考虑到良好的梯度效率以及力和扭矩平衡等机械性能的需求。一个重要的方面将是受试者的安全性，这将通过调整设备本身的特性（热和机电特性）以及通过评估产生的时间依赖性梯度场对周围神经刺激和心脏功能的影响来解决。将开发适当的扩散加权序列，例如补偿涡流可能增加的影响，以便在初始可行性实验中成功应用。在第二阶段，计划开发一个全功率原型，能够根据临床和科学相关的体内应用的要求进行高占空比操作，这也将在该阶段进行开发和演示。

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.