In vivo MR acoustic radiation force imaging in the porcine liver
MR-ARFI RS-EPI Pulse Sequence. A small FOV is achieved by exciting perpendicular slices with the 90° and 180° RF pulses. Ultrasound is on during the second shaded period. Optional out of slice saturation slices at the beginning could be applied at the beginning to saturate signal from out of slice flow.
MR-ARFI (μm) vs thermal ablation (temperature rise) in a phantom. The horizontal lines highlight the alignment along the L/R and S/I directions, respectively, for these foci. For the images perpendicular to the ultrasound beam, the differences between foci were 0.37 and 0.15 mm in the L/R and S/I directions, respectively. For the images parallel to the beam, the difference were 1.99 and 0.02 mm in the A/P and S/I directions, respectively.
Displacement images at various applied transducer powers. These images are a subset of the power to displacement curve presented in Fig. 4. These measurements were recorded at the same focus location throughout one breath-hold. The units are acoustic watts for the applied power. As the power increases, the displacement at the focus also increases.
Plot of power versus displacement, including displacement information from images from Fig. 3. The displacements for each image were the maximum values at the focus. The error bars represent the displacement uncertainty calculated from the magnitude SNR of each image. The uncertainty ranged from 0.15 to 0.20 μm. A linear best-line fitted to the data. As power increases, the focal displacement increases linearly: y = [0.0403 ± 0.00321]x + [0.8272 ± 0.327] μm (R2 = 0.96).
Reduced field of view displacement maps demonstrating focal spot steering. The focal beam is focused to various locations within the liver. Each image was acquired in separate breath-holds.
Representative images from breath-hold ablation experiments. During a single breath-hold, MR-ARFI was performed at a focus, immediately followed by a thermal ablation. The MR-ARFI image of the focus (left, in micrometer) was compared with the subsequent thermal ablation (right, measured as °C temperature rise). The horizontal line highlights that the foci align. The standard deviation of the discrepancy between displacement focus and thermal focus in the L/R and S/I directions for all seven breath-hold ARFI/ablation comparisons was 0.83 and 0.99 mm, respectively (see Table I).
MR-ARFI for focal calibration. The initial precalibration image is acquired (left) during a gated scan. While the beam was intended to focus at the crosshairs, it instead was evident at the location of the circle. With a single click, the focus is corrected, and a verification image (right) shows the focus in the correct location. In both images, the ARFI image is outlined by the rectangular tile. The outer image is a large FOV anatomical reference image.
Representative free breathing sagittal and coronal MR-ARFI gated verification images (prior to ablation) compared with an image from a free breathing thermal ablation. In all images, the ARFI image or thermometry image is only the part contained in the rectangular tile. The outer image is a large FOV anatomical reference image. In this example, the horizontal line highlights that the spots are aligned in the inferior/superior direction. As discussed in Table II, the mean error during the six ablations, measured in the non respiration direction, was 1.03 mm.
Displacement and thermal foci locations in each breath-hold ablation along the left–right (L/R) direction and superior–inferior (head–feet) (S/I) direction. The values are positions relative to isocenter of the magnet. The error was less than a millimeter in either direction.
Distances from the left of an image to the center of steered, gated ARFI and corresponding steered, breathing ablation foci. The left/right (L/R) direction was orthogonal to the main axis of motion. The error in the nonmotion direction was approximately 1 mm, suggesting good alignment between the MR-ARFI and the thermal foci.
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