Simultaneous US/MR imaging

An imaging system that offers the potential to combine the strengths of US and MR imaging is proposed. The purpose of this study was to characterize a prototype system and to develop methods to perform simultaneous ultrasound and MR imaging without significant interference between the two modalities. A computer-controlled device consisting of an MRI-compatible linear positioning system and diagnostic US transducer was designed and built to perform ultrasound imaging inside an MR imager. The ultrasound transducer had a 5 MHz frequency, with a 50-mm focal length. The MR imager was a clinical 1.5T closed-bore scanner. No noise related to the motor movements or the materials was observed. The broadband excitation pulse used to excite the transducer was detected in the MR images. Significant electrical interference from the MRI was also observed in the ultrasound imaging signals. By synchronizing US and MR acquisition, simultaneous imaging could be performed by sending the US pulses between MR RF pulses. A phantom with multiple nylon strings at different spacing (2-12mm) and diameters (0.2, 0.4mm) was imaged with both systems. The strings with a 0.4 mm diameter could be observed in the MR image but the 0.2-mm diameter strings were not visible. Ultrasound imaging during clinical MR imaging was feasible and was more sensitive to structures with a large acoustic impedance mismatch. The setup developed in this study creates an opportunity to perform MRI-guided ultrasound imaging or vice-versa.