Development of a Compact MR Microscope using a 1.0 T Permanent Magnet
Development of a Compact MR Microscope using a 1.0 T Permanent Magnet
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使用 1.0 T 永磁体开发紧凑型 MR 显微镜
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发表时间：
2000
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通讯作者：
Y. Matsuda
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作者：
T. Hahi;T. Uematsu;Y. Akita;Y. Matsuda
引言磁共振显微镜已被广泛用于许多实验室。为此目的，MRM应紧凑且易于使用。为了构造紧凑的MR显微镜，永磁体是首选，因为不需要低温恒温器，并且杂散场面积通常比超导磁体小得多。在以前的研究中，我们已经报道了一个1.0 T永磁体使用钕铁硼磁性材料的MR显微镜 *）。尽管磁场的温度系数很大（0.1%/℃），但我们通过磁体的热绝缘在30分钟内获得了具有（200 μ m）3体素尺寸的3D图像。在本研究中，为了获得更高的空间分辨率，我们实现了内部NMR锁定，这是在一个时间共享模式进行。因此，我们已经成功地获得了3D图像在（50 pm）3（150 pm）3体素大小在一个实际的测量时间（2 - 8 H）。硬件系统图1显示了本研究中开发的MR显微镜的概述。永磁体由住友特殊金属公司（日本大坂）开发。规格如下：磁场：0.968 T（在22“C时），间隙：61 mm，均匀性：12 ppm超过20 mm dsv，尺寸：62 cm（W）x 75 cm（H）x 75 cm（D），重量：1，350 kg。该系统的MRI控制台是最初为MR显微镜开发的“便携式MRI单元”*），利用临床MRI的1.5T磁场。为永磁体开发了两种具有10 mm+和20 mm 4净孔的梯度探头。当永久物被聚苯乙烯泡沫板（30 mm厚）覆盖时，旋进频率的漂移约为500 Hz/小时，即使不调节室温。因为我们通常的3D图像的像素带宽约为200 Hz，所以通过使用下述NMR锁定技术以约几分钟的时间间隔校正NMR参考频率。通过使用样品本身的自旋回波信号测量频率偏差，并使用与MRI单元的PC连接的DDS（直接数字合成器）板改变NMR参考频率来执行NMR锁定。通过使用该方法，我们成功地在采集高分辨率3D图像所需的几个小时内将场-频率关系保持在约20 Hz。结果和讨论图2示出了用3D-GE（TR/TE = 100/6 ms，FA = 90”，Ti = 7.5 H）获得的豌豆和用3D-SE（TR/TE = 100/8 ms，Ti = 7.5 H）获得的沃尔什洋葱根的横截面图像。图3示出了来自固定在保存液中的小鼠的3D图像数据集的横截面图像。成像序列为常规3D-SE：TR/TE = 100/8 ms，NEX = 4，T = 2 H，FOV =（19.2 mm）3，图像矩阵= 128 3，体素大小=（150 p）3。图2和图3表明，即使使用低场MR显微镜，也可以在实际测量时间内获得（50 pm）3（150 Pm）3体素尺寸的3D图像。虽然低场MR显微镜（1 T）的NMR信号的固有SN比远低于使用超导磁体的高场MR显微镜（10 T），但低场MR显微镜具有以下几个优点：T1比在高场中的短得多，T2* 比在高场中的长得多，并且由于局部T2* 效应引起的信号损失与高场中的信号损失相比显著减小。此外，在我们的系统中，使用具有许多匝数的RF线圈（其仅对于低谐振频率（40 MHz）是可能的）具有很大的优点，因为即使线圈非常靠近样品缠绕，这种线圈也可以产生均匀的RF场。现在，我们的合作者刚刚开发出一种更小更轻的环形磁体（30 cm立方体，210 kg，1 T），具有20 mm dsv均匀区域。因此，更紧凑或桌面MRM可以开发与此mamet。我
INTRODUCTION MR microscopes have been expected to be widely used in many laboratories. For such purpose, the MRM should be compact and simple to use. In order to construct a compact MR microscope, permanent magnets are the first choice because no cryostat is required and the stray field area is usually much smaller than that of superconducting magnets. In previous study we have reported an MR microscope with a 1.0 T permanent magnet using NdFeB magnetic materials*). In spite of the large temperature coefficient of the magnetic field (0.1 %/“C), we acquired 3D images with the (200 pm) 3 voxel size within 30 minutes by thermal insulation of the magnet. In present study, to attain much higher spatial resolution, we implemented the internal NMR lock, which was performed in a time sharing mode. As a result we have successfully obtained 3D images at the (50 pm)3 (150 pm) 3 voxel size within a practical measurement time (2 8 H). HARDWARE SYSTEM Figure 1 shows the overview of the MR microscope developed in this study. The permanent magnet was developed in Sumitomo Special Metals Company (Osaka, JPN). The specification is as follows: magnetic field: 0.968 T (at 22 “C), gap: 61 mm, homogeneity: 12 ppm over 20 mm dsv, size: 62 cm (W) x 75 cm (H) x 75 cm (D), weight: 1,350 kg. The MRl console of this system was a “portable MRI unit” developed originally for the MR microscope *) utilizing a 1.5 T magnetic field of a clinical MRI. Two gradient probes with 10 mm+ and 20 mm4 clear bores were developed for the permanent magnet. NMR LOCK When the permanent was covered with polystyrene foam slabs (30 mm thickness), the drift of the precession frequency was around 500 Hz/hour, even if the room temperature was not regulated. Because our usual pixel bandwidth for 3D images was about 200 Hz, the NMR reference frequency was corrected at about several minute time-intervals by using the NMR lock technique described below. The NMR lock was performed by measuring the frequency deviation using the spin-echo signal of the sample itself, and changing the NMR reference frequency using a DDS (direct digital synthesizer) board interfaced with the PC of the MRI unit BY using this method, we successfully kept the field-frequency relation with about 20 Hz over several hours required for acquisition of high-resolution 3D images. RESULTS AND DISCUSSION Figure 2 shows cross-sectional images of a garden peaa) acquired with a 3D-GE (TR/TE = 100/6 ms, FA = 90”, T,, = 7.5 H) and a root of a Walsh onion@ acquired with a 3D-SE (TR/TE = 100/8 ms, T,, = 7.5 H). Figure 3 shows cross-sectional images from 3D image data sets of a mouse fixed in preservation liquid. The imaging sequence was a conventional 3D-SE: TR/TE = 100/8 ms, NEX = 4, T,, = 2 H, FOV = (19.2 mm) 3, image matrix = 128 3, voxel size = (150 p) 3. Figure 2 and 3 demonstrate that 3D images with the (50 pm) 3 (150 Pm) 3 voxel size can be obtained within a practical measurement time even with the low-field MR microscope. Although the intrinsic SN ratio of NMR signal of low-field MR microscopes (1 T) is much lower than that of high-field MR microscopes (10 T) using superconducting magnets, low-field MR microscopes have several advantages: Tl is much shorter than that in high-fields, T2* is much longer than that in high-fields, and the signal loss due to the local T2* effect is drastically reduced as compared with that in high-fields. Furthermore in our system, use of solenoids coils with many turns, which are possible only for the low resonance frequency (40 MHz), has a great advantage because such coil can produce homogeneous rf field even if the coils are wound very close to the samples. Now our collaborator has just developed a smaller and lighter ring-type magnet (30 cm cube, 210 kg, 1 T) having a 20 mm dsv homogeneous region. So much more compact or desktop MRM could be developed with this mamet. I