Minhas Atul S, Woo Eung Je, Lee Soo Yeol
Department of Biomedical Engineering, College of Electronics and Information, Kyung Hee University, Gyeonggi-do, Korea.
Annu Int Conf IEEE Eng Med Biol Soc. 2009;2009:2276-8. doi: 10.1109/IEMBS.2009.5335084.
Magnetic Resonance Electrical Impedance Tomography (MREIT) utilizes the magnetic flux density B(z), generated due to current injection, to find conductivity distribution inside an object. This B(z) can be measured from MR phase images using spin echo pulse sequence. The SNR of B(z) and the sensitivity of phase produced by B(z) in MR phase image are critical in deciding the resolution of MREIT conductivity images. The conventional spin echo based data acquisition has poor phase sensitivity to current injection. Longer scan time is needed to acquire data with higher SNR. We propose a balanced steady state free precession (b-SSFP) based pulse sequence which is highly sensitive to small off-resonance phase changes. A procedure to reconstruct B(z) from MR signal obtained with b-SSFP sequence is described. Phases for b-SSFP signals for two conductivity phantoms of TX 151 and Gelatin are simulated from the mathematical models of b-SSFP signal. It was observed that the phase changes obtained from b-SSFP pulse sequence are highly sensitive to current injection and hence would produce higher magnetic flux density. However, the b-SSFP signal is dependent on magnetic field inhomogeneity and the signal deteriorated highly for small offset from resonance frequency. The simulation results show that the b-SSFP sequence can be utilized for conductivity imaging of a local region where magnetic field inhomogeneity is small. A proper shimming of magnet is recommended before using the b-SSFP sequence.
磁共振电阻抗断层成像（MREIT）利用注入电流产生的磁通密度B(z)来确定物体内部的电导率分布。该B(z)可通过自旋回波脉冲序列从磁共振相位图像中测量得到。B(z)的信噪比以及其在磁共振相位图像中产生的相位灵敏度对于决定MREIT电导率图像的分辨率至关重要。传统的基于自旋回波的数据采集对电流注入的相位灵敏度较差。需要更长的扫描时间来采集具有更高信噪比的数据。我们提出了一种基于平衡稳态自由进动（b-SSFP）的脉冲序列，它对小的失谐相位变化高度敏感。描述了一种从用b-SSFP序列获得的磁共振信号重建B(z)的方法。根据b-SSFP信号的数学模型模拟了TX 151和明胶两种电导率体模的b-SSFP信号的相位。观察到从b-SSFP脉冲序列获得的相位变化对电流注入高度敏感，因此会产生更高的磁通密度。然而，b-SSFP信号依赖于磁场不均匀性，并且对于偏离共振频率的小偏移，信号会严重恶化。模拟结果表明，b-SSFP序列可用于磁场不均匀性较小的局部区域的电导率成像。在使用b-SSFP序列之前，建议对磁体进行适当的匀场。
