B1 Shimming Performance Versus Channel/Mode Count

Introduction. Clinical 3T MRI systems capable of parallel RF transmission have recently been introduced [1]. With such functionality it has become possible to significantly minimize and even eliminate the RF uniformity issues encountered at these higher field strengths while simultaneously reducing SAR [2]. With the development of any parallel imaging technology the dominant question is “how does performance scale with the number of channels?”. The answer is rarely straightforward and often depends on the application, the definition of what is a channel, and the definition of performance. In the case of parallel transmission, the primary application is to improve B1 field uniformity. This is different to parallel reception in which the primary application is generally scan time reduction. For parallel reception, the SNR is often the limiting constraint. For parallel transmission, the maximum available RF power, and its distribution, becomes a dominant constraint. This study reports on abdominal B1 shimming performance, using a multi-element volume body coil, as a function of transmit channel/mode count and available RF power.