Wansapura J P, Daniel B L, Pauly J, Butts K
Department of Radiology, Stanford University, Stanford, California 94305, USA.
Magn Reson Med. 2001 Nov;46(5):985-92. doi: 10.1002/mrm.1285.
Cryosurgery has been shown to be an effective therapy for prostate cancer. Temperature monitoring throughout the cryosurgical iceball could dramatically improve efficacy, since end temperatures of at least -40 degrees C are required. The results of this study indicate that MR thermometry based on tissue R()(2) has the potential to provide this information. Frozen tissue appears as a complete signal void on conventional MRI. Ultrashort echo times (TEs), achievable with half pulse excitation and a short spiral readout, allow frozen tissue to be imaged and MR characteristics to be measured. However, half pulse excitation is highly sensitive to eddy current distortions of the slice-select gradient. In this work, the effects of eddy currents on the half pulse technique are characterized and methods to overcome these effects are developed. The methods include: 1) eddy current compensated slice-select gradients, and 2) a correction for the phase shift between the first and second half excitations at the center of the slice. The effectiveness of these methods is demonstrated in R()(2) maps calculated within the frozen region during cryoablation.
冷冻手术已被证明是治疗前列腺癌的一种有效疗法。在整个冷冻手术冰球区域进行温度监测可显著提高疗效，因为需要至少-40摄氏度的最终温度。本研究结果表明，基于组织R()(2)的磁共振温度测量法有潜力提供这一信息。在传统磁共振成像中，冷冻组织表现为完全信号缺失。通过半脉冲激发和短螺旋读出可实现超短回波时间（TE），从而能够对冷冻组织进行成像并测量其磁共振特性。然而，半脉冲激发对切片选择梯度的涡流畸变高度敏感。在这项工作中，对涡流对半脉冲技术的影响进行了表征，并开发了克服这些影响的方法。这些方法包括：1）涡流补偿切片选择梯度，以及2）对切片中心处第一个和第二个半激发之间的相移进行校正。这些方法的有效性在冷冻消融过程中冷冻区域内计算出的R()(2)图中得到了证明。
