Saint-Jalmes Hervé, Bordelois Alejandro, Gambarota Giulio
INSERM, UMR 1099, Rennes, France.
Université de Rennes 1, LTSI, Rennes, France.
Med Phys. 2018 Jan;45(1):250-257. doi: 10.1002/mp.12658. Epub 2017 Nov 27.
The purpose of this study was to implement Virtual Phantom Magnetic Resonance Imaging (ViP MRI), a technique that allows for generating reference signals in MR images using radiofrequency (RF) signals, on a clinical MR system and to test newly designed virtual phantoms.
MRI experiments were conducted on a 1.5 T MRI scanner. Electromagnetic modelling of the ViP system was done using the principle of reciprocity. The ViP RF signals were generated using a compact waveform generator (dimensions of 26 cm × 18 cm × 16 cm), connected to a homebuilt 25 mm-diameter RF coil. The ViP RF signals were transmitted to the MRI scanner bore, simultaneously with the acquisition of the signal from the object of interest. Different types of MRI data acquisition (2D and 3D gradient-echo) as well as different phantoms, including the Shepp-Logan phantom, were tested. Furthermore, a uniquely designed virtual phantom - in the shape of a grid - was generated; this newly proposed phantom allows for the investigations of the vendor distortion correction field.
High quality MR images of virtual phantoms were obtained. An excellent agreement was found between the experimental data and the inverse cube law, which was the expected functional dependence obtained from the electromagnetic modelling of the ViP system. Short-term time stability measurements yielded a coefficient of variation in the signal intensity over time equal to 0.23% and 0.13% for virtual and physical phantom, respectively. MR images of the virtual grid-shaped phantom were reconstructed with the vendor distortion correction; this allowed for a direct visualization of the vendor distortion correction field. Furthermore, as expected from the electromagnetic modelling of the ViP system, a very compact coil (diameter ~ cm) and very small currents (intensity ~ mA) were sufficient to generate a signal comparable to that of physical phantoms in MRI experiments.
The ViP MRI technique was successfully implemented on a clinical MR system. One of the major advantages of ViP MRI over previous approaches is that the generation and transmission of RF signals can be achieved with a self-contained apparatus. As such, the ViP MRI technique is transposable to different platforms (preclinical and clinical) of different vendors. It is also shown here that ViP MRI could be used to generate signals whose characteristics cannot be reproduced by physical objects. This could be exploited to assess MRI system properties, such as the vendor distortion correction field.
本研究的目的是在临床磁共振（MR）系统上实施虚拟体模磁共振成像（ViP MRI）技术，该技术可利用射频（RF）信号在MR图像中生成参考信号，并测试新设计的虚拟体模。
在1.5T MR扫描仪上进行MRI实验。利用互易原理对ViP系统进行电磁建模。ViP RF信号由一个紧凑型波形发生器（尺寸为26cm×18cm×16cm）产生，该波形发生器连接到一个自制的直径25mm的RF线圈。在采集感兴趣对象信号的同时，将ViP RF信号传输到MR扫描仪孔内。测试了不同类型的MRI数据采集（二维和三维梯度回波）以及不同的体模，包括Shepp-Logan体模。此外，还生成了一种独特设计的网格形状的虚拟体模；这种新提出的体模可用于研究供应商失真校正场。
获得了高质量的虚拟体模MR图像。实验数据与反立方定律之间取得了极好的一致性，反立方定律是从ViP系统电磁建模中获得的预期函数依赖关系。短期时间稳定性测量得出，虚拟体模和物理体模的信号强度随时间的变异系数分别为0.23%和0.13%。利用供应商失真校正重建了虚拟网格状体模的MR图像；这使得能够直接可视化供应商失真校正场。此外，正如ViP系统电磁建模所预期的那样，一个非常紧凑的线圈（直径约为厘米）和非常小的电流（强度约为毫安）足以在MRI实验中产生与物理体模相当的信号。
ViP MRI技术在临床MR系统上成功实施。ViP MRI相对于以前方法的主要优点之一是，RF信号的产生和传输可以通过一个独立的设备实现。因此，ViP MRI技术可移植到不同供应商的不同平台（临床前和临床）。本文还表明，ViP MRI可用于生成物理对象无法再现其特征的信号。这可用于评估MRI系统特性，如供应商失真校正场。
