Conturo T E, Robinson B H
Department of Radiology & Radiological Science, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
Magn Reson Med. 1992 Jun;25(2):233-47. doi: 10.1002/mrm.1910250203.
The efficiency of balanced versus unbalanced techniques for phase-angle-based velocity magnitude and direction imaging is investigated. Methods having balanced flow-encoding gradients (gradients in positive and negative directions with a zero center of gravity) are compared with unbalanced methods. For three-dimensional imaging, a currently used balanced method is the six-point technique having opposed gradients pairs for each orthogonal direction. A currently used unbalanced method is a four-point null technique which has three orthogonal gradients and an additional acquisition having no specific flow encoding to correct the baseline (null) phase. In the gradient-limited case of slow flow and perfusion, the balanced method is predicted to have higher velocity magnitude-to-noise ratio per time (SNRV) by a factor of 1.63, with similar results for velocity direction. In the wraparound-limited case of faster flows and motions, similar results are found when a null acquisition is added to the balanced method. This results in a seven-point balanced method having an SNRV 1.51 times that of the four-point unbalanced method. If null phases are within the [-pi/2,pi/2] interval, this additional null acquisition is unnecessary. Other four-point methods are also considered. These results indicate that, in general, balanced methods have advantages over unbalanced methods for velocity imaging.
研究了基于相角的速度大小和方向成像中平衡技术与非平衡技术的效率。将具有平衡流动编码梯度（正负方向梯度且重心为零）的方法与非平衡方法进行比较。对于三维成像，当前使用的一种平衡方法是六点技术，在每个正交方向上有相反的梯度对。当前使用的一种非平衡方法是四点零值技术，它有三个正交梯度以及一个没有特定流动编码来校正基线（零值）相位的额外采集。在慢血流和灌注的梯度受限情况下，预计平衡方法的每时间速度大小与噪声比（SNRV）比非平衡方法高1.63倍，速度方向的结果类似。在血流和运动速度较快的环绕受限情况下，当给平衡方法添加零值采集时会得到类似结果。这就产生了一种七点平衡方法，其SNRV是非四点非平衡方法的1.51倍。如果零值相位在[-π/2,π/2]区间内，则不需要这种额外的零值采集。还考虑了其他四点方法。这些结果表明，一般来说，在速度成像方面，平衡方法比非平衡方法具有优势。
