Overview of MR-ARFI
where alpha is the absorption coefficient of the tissue. The resulting tissue displacement is proportional to both the ultrasound amplitude and the tissue mechanical properties. Thus, the focal spot can be rapidly localized by imaging the tissue displacement, without a temperature rise. One of the goals of this work is to develop MR-methods based on the acoustic radiation force to aid in the setup and assessment portions of a prostate ablation procedure. In the setup phase, instead of using a test spot of a small temperature rise to determine the location of the focal spot, monitoring the location of the acoustic radiation force displacement could provide the same information, more rapidly and without a temperature rise of even a few degrees.
Nathan McDannold demonstrated MR Acoustic Radiation Force Imaging with unipolar gradients shown below.In this setup, ultrasound is applied from below the phantom. Encoding gradients are applied in the y direction. The gel phantom at the focus experiences the radiation force in the y direction. The sound is triggered on during one of the encoding gradients, shown in the pulse sequence diagram in Figure b. While stationary spins are rephased, those that experienced the radiation force displacement have a net phase shift. Images are acquired in the x-z scan plane with the line scan technique. The sequence is repeated with the displacement encoding gradients reversed and the phase difference between the two images is calculated. After some additional processing, the MR-ARFI image is proportional to displacement, an example of which is shown in Figure c.
Jing Chen modified the technique to use bipolar gradients, which result in a lower b-value and better SNR, for the same encoding sensitivity. Lena Kay has developed a single shot EPI method over a reduced FOV, to improve efficiency over the line scan acquisition.
Chen J, Butts Pauly K. Optimization of Encoding Gradients for MR-ARFI, MRM, in press.
Chen J; Butts Pauly K, Practical Thoughts on Using Repeated Bipolar Gradients for MR-ARFI, ISMRM 2009
Kaye E, Chen J, Butts Pauly K, Rapid FUS Focal Spot Localization Based on MR-ARFI, ISMRM 2009
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