MRI of weak periodic currents using balanced Steady State Free Precession

描述（由申请人提供）：现有的功能性脑MR成像方法，如血氧水平依赖性（BOLD）成像，是基于血管和代谢对潜在神经元活动变化的反应，因此只能间接反映后者。有人提出（例如Bodurka和Bandettini，2002年），大脑中神经元电流引起的磁场扰动原则上应该可以通过MRI检测到。几个研究小组已经从理论、电流模型和活体实验上证明了这一想法的可行性。然而，迄今为止实现的相对低的信噪比使得现有方法不切实际。平衡稳态自由旋进（Balanced Steady State Free Predeception，bSSFP，TrueFISP，FIESTA）脉冲序列是一种独特的脉冲序列，它可以提供最高的单位时间信噪比，因此将其应用于功能性神经电流成像可以潜在地解决低信噪比的问题。最近的研究表明，某些SSFP序列可能对自旋相位的微小周期性扰动非常敏感。因此，锁定到RF激励脉冲的周期性电流导致可以有效地放大由电流引起的相位偏移的交替平衡稳态（ABSS）。我们的理论计算和使用电流体模在3 T下用bSSFP采集的数据表明，由周期性电流诱导的信号的SNR基本上上级先前使用标准梯度和自旋回波成像实现的SNR（Buracas等人，2007年）。我们建议开发优化的交替稳态成像协议的成像周期电流。首先，我们将优化，分析，在计算机模拟，和电流幻影实验的灵敏度的ABSS方法弱周期性电流和解决各种噪声源的ABSS信号的影响。接下来，我们将应用这种方法来成像神经元电流在体内（使用大鼠胡须刺激和人类视听刺激范例）。我们将对由于呼吸和扫描仪漂移引起的磁场波动进行噪声补偿和运动补偿。总之，目前的建议将解决的可能性，以测量电磁相关的大脑神经元活动的ABSS的手段，并将解决关键的噪声源。所提出的目标的实现有可能引入一种新的成像模式的功能脑成像-高分辨率MRI为基础的脑磁图。公共卫生相关性：已建立的功能性脑MR成像方法易受影响神经血管耦合的因素的影响。最近已经提出并证明MRI可以用于直接成像神经元电流，但是现有方法由于其低SNR而不切实际。拟议的研究将开发超灵敏的MRI方法，用于使用具有最佳SNR效率的平衡稳态自由进动脉冲序列对神经元电流进行成像。

DESCRIPTION (provided by applicant): Existing functional brain MR imaging methods such as blood oxygenation level dependent (BOLD) imaging are based on vascular and metabolic responses to variations in underlying neuronal activity and thus reflect the latter only indirectly. It has been proposed (e.g. Bodurka & Bandettini, 2002) that magnetic field perturbations caused by neuronal currents in the brain in principle ought to be detectable by means of MRI. Several research groups have demonstrated the feasibility of this idea in theory, electric current phantoms and in vivo. However, relatively low signal-to-noise ratio achieved to date renders existing methods impractical. Balanced Steady State Free Precession (bSSFP, also known as TrueFISP, FIESTA) pulse sequence is unique in that it can afford the highest known SNR per unit time and thus application of bSSFP for functional neural current imaging can potentially solve the problem of low SNR. Recent studies suggest that certain SSFP sequences might be exquisitely sensitive to minute periodic perturbations of the spin phase. Thus, periodic currents locked to RF excitation pulse result in an alternating balanced steady state (ABSS) that can effectively amplify the phase offset induced by currents. Our theoretical calculations and the data acquired with bSSFP at 3T using a current phantom indicates SNR of the signal induced by periodic currents that is substantially superior to that achieved previously using standard gradient and spin echo imaging (Buracas et al., 2007). We propose to develop optimized alternating steady-state imaging protocols for imaging of periodic electric currents. First, we will optimize, analytically, in computer simulations, and current phantom experiments the sensitivity of the ABSS method to weak periodic currents and address the impact of various sources of noise on the ABSS signal. Next, we will apply this method to imaging neuronal currents in vivo (using rat whisker stimulation and human audiovisual stimulation paradigms). We will implement noise compensation for magnetic field fluctuation due to respiration and scanner drift and motion compensation. In conclusion, the current proposal will address the possibility to measure electromagnetic correlates of cerebral neuronal activity by means of ABSS and will address critical sources of noise. Accomplishment of the proposed aims has a potential of introducing a new imaging modality for functional brain imaging - high resolution MRI-based magnetoencephalography. PUBLIC HEALTH RELEVANCE: Established functional brain MR imaging methods are susceptible to factors influencing neurovascular coupling. It has been recently proposed and demonstrated that MRI can be used for imaging neuronal currents directly, but existing methods are impractical due to their low SNR. The proposed research will develop ultra- sensitive MRI methods for imaging neuronal currents using balanced Steady State Free Precession pulse sequences that possess best known SNR efficiency.