Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843, USA.
Lab Chip. 2011 Jul 7;11(13):2197-203. doi: 10.1039/c1lc20056a. Epub 2011 May 23.
We present the development of a microfluidically cryo-cooled planar coil for magnetic resonance (MR) microscopy. Cryogenically cooling radiofrequency (RF) coils for magnetic resonance imaging (MRI) can improve the signal to noise ratio (SNR) of the experiment. Conventional cryostats typically use a vacuum gap to keep samples to be imaged, especially biological samples, at or near room temperature during cryo-cooling. This limits how close a cryo-cooled coil can be placed to the sample. At the same time, a small coil-to-sample distance significantly improves the MR imaging capability due to the limited imaging depth of planar MR microcoils. These two conflicting requirements pose challenges to the use of cryo-cooling in MR microcoils. The use of a microfluidic based cryostat for localized cryo-cooling of MR microcoils is a step towards eliminating these constraints. The system presented here consists of planar receive-only coils with integrated cryo-cooling microfluidic channels underneath, and an imaging surface on top of the planar coils separated by a thin nitrogen gas gap. Polymer microfluidic channel structures fabricated through soft lithography processes were used to flow liquid nitrogen under the coils in order to cryo-cool the planar coils to liquid nitrogen temperature (-196 °C). Two unique features of the cryo-cooling system minimize the distance between the coil and the sample: (1) the small dimension of the polymer microfluidic channel enables localized cooling of the planar coils, while minimizing thermal effects on the nearby imaging surface. (2) The imaging surface is separated from the cryo-cooled planar coil by a thin gap through which nitrogen gas flows to thermally insulate the imaging surface, keeping it above 0 °C and preventing potential damage to biological samples. The localized cooling effect was validated by simulations, bench testing, and MR imaging experiments. Using this cryo-cooled planar coil system inside a 4.7 Tesla MR system resulted in an average image SNR enhancement of 1.47 ± 0.11 times relative to similar room-temperature coils.
我们提出了一种用于磁共振（MR）显微镜的微流控冷冻平面线圈的开发。为磁共振成像（MRI）冷却射频（RF）线圈可以提高实验的信噪比（SNR）。传统的低温恒温器通常使用真空间隙来保持待成像的样品，特别是生物样品，在冷冻冷却过程中保持在或接近室温。这限制了冷却线圈可以靠近样品的程度。同时，由于平面 MR 微线圈的有限成像深度，较小的线圈到样品的距离可显著提高 MR 成像能力。这两个相互冲突的要求对 MR 微线圈中的冷却提出了挑战。使用基于微流体的低温恒温器对 MR 微线圈进行局部冷却，是消除这些限制的一种方法。这里提出的系统由带有集成微流控冷却微通道的平面接收线圈组成，在平面线圈的顶部有一个成像表面，两者之间有一个薄的氮气间隙。通过软光刻工艺制造的聚合物微流道结构用于在平面线圈下方流动液氮，以将平面线圈冷却至液氮温度（-196°C）。该冷却系统的两个独特特征将线圈与样品之间的距离最小化：（1）聚合物微流道的小尺寸使平面线圈能够实现局部冷却，同时最小化了对附近成像表面的热影响。（2）成像表面通过薄间隙与冷却的平面线圈隔开，氮气在其中流动以实现对成像表面的热隔离，使其保持在 0°C 以上，防止对生物样品造成潜在损坏。通过模拟、台架测试和磁共振成像实验验证了局部冷却效果。在 4.7T MR 系统内部使用这种冷却的平面线圈系统可使图像 SNR 平均提高 1.47 ± 0.11 倍，与类似的室温线圈相比。
