An MRI-compatible platform for one-dimensional motion management studies in MRI

Abstract

Purpose: Abdominal MRI remains challenging because of respiratory motion. Motion compensation strategies are difficult to compare clinically because of the variability across human subjects. The goal of this study was to evaluate a programmable system for one-dimensional motion management MRI research.

Methods: A system comprised of a programmable motorized linear stage and computer was assembled and tested in the MRI environment. Tests of the mutual interference between the platform and a whole-body MRI were performed. Organ trajectories generated from a high-temporal resolution scan of a healthy volunteer were used in phantom tests to evaluate the effects of motion on image quality and quantitative MRI measurements.

Results: No interference between the motion platform and the MRI was observed, and reliable motion could be produced across a wide range of imaging conditions. Motion-related artifacts commensurate with motion amplitude, frequency, and waveform were observed. T2 measurement of a kidney lesion in an abdominal phantom showed that its value decreased by 67% with physiologic motion, but could be partially recovered with navigator-based motion-compensation.

Conclusion: The motion platform can produce reliable linear motion within a whole-body MRI. The system can serve as a foundation for a research platform to investigate and develop motion management approaches for MRI. Magn Reson Med 76:702-712, 2016. © 2015 Wiley Periodicals, Inc.

Keywords: T2; abdominal MRI; breathing; motion; respiratory motion.

FIG. 1.

(A) Rendering of an MRI-compatible motion platform capable of producing linear motion within a clinical MRI scanner. (B) Photograph of the setup used for this study. An abdominal phantom was affixed to the motion platform and a clinical torso array was held above it using a spacer ring. The phantom was free to move in the head–foot direction during imaging to simulate abdominal organ motion.

FIG. 2.

(A) A single frame from a coronal time series (single slice) acquired in the abdomen of a healthy volunteer was used to track the motion of the liver (blue), spleen (orange), right kidney (red), and left kidney (black) over multiple breathing cycles. (B) The displacement in the head–foot direction as a function of time is shown for each organ in the free-breathing volunteer.

FIG. 3.

(A) The displacement versus time of the motion platform during playback of a kidney trajectory. The motion was performed outside the magnet (bench), inside the bore (magnet), during a multislice fast spin echo sequence, and during a single shot diffusion-weighted imaging sequence. The different imaging conditions had negligible impact on the performance of the platform. (B) A noise spectrum analysis was performed to evaluate the impact of the motion platform on MRI. Two additional signals (black arrows) outside the imaging bandwidth (gray region) were observed once the electronics for the system were turned on; however, these were significantly lower in amplitude than the water signal. No additional noise peaks were observed across a wide range of motion trajectories. The large peak within the imaging bandwidth (white arrow) was from a water phantom placed on the motion platform.

FIG. 4.

The mean (left) and coefficient of variance (right) for repeat imaging of the phantom (n = 3) with a sinusoidal through-plane motion trajectory of 20-mm peak-to-peak (15 cycles/min) for two different conditions. The first case (top row) involved three successive imaging scans with the same motion and position. The second case (bottom row) involved three successive scans with the same motion, but in each case the phantom was brought out of the bore and re-landmarked with the system before being scanned. The coefficient of variance was lower for the first case but remained very good for both cases. The repeatability of the location and spacing of ghosts is excellent for both cases.

FIG. 5.

A series of axial 2D T2-weighted images (TR/TE = 1000/100 ms; acquisition matrix = 252 × 201; slice thickness = 4 mm) of the anthropomorphic phantom acquired through the kidneys with varying degrees of sinusoidal displacement in the foot–head direction (0–30 mm) or simulated physiologic kidney motion depict the degradation in image quality with motion. The movement was continuous during the entire image acquisition process and occurred at a frequency of 15 cycles/min in the slice direction (z axis, through-plane). Image acquisition time was approximately 2 min. Different ROIs were placed on the center of the phantom, renal parenchyma, and simulated renal lesion in the right kidney to evaluate the change of signal intensity due to motion. Note the decreased conspicuity of the right renal lesion with increasing degrees of sinusoidal motion (white arrows) and to less extent, with physiologic motion with displacement about 20 mm (black arrow).

FIG. 6.

Rectangular regions of interest (ROIs) displayed in Figure 4 were copied and pasted on all acquisitions (TE = 100 ms) with different degrees of sinusoidal and physiologic motion for quantitative assessment of image quality and lesion detection. (A) Standard deviation of the background signal (large rectangular ROI in the center of the phantom) increases uniformly with sinusoidal motion of increasing amplitude. Physiological motion results in a three-fold increase of the standard deviation of the background. Signal intensities used were obtained from the regions of interest in Figure 5. (B) Renal lesion-to-background ratio of signal intensities decreases uniformly with sinusoidal motion of increasing amplitude making the detection of the lesion more difficult. We also observe a two-fold decrease in the lesion-to-background ratio with physiological motion.

FIG. 7.

A series of coronal images acquired through the phantom with varying degrees of sinusoidal motion (0–30 mm) or simulated physiologic kidney motion in the cranial–caudal direction (z axis, indicated by the arrows). The degradation in image quality with increasing motion amplitude is clear. Movement was continuous during the entire image acquisition process and occurred at a frequency of 15 cycles/min. Image acquisition time was approximately 3 min.

FIG. 8.

The measured and predicted spacing between ghosts for images acquired in the presence of periodic through-plane motion with increasing frequency. A consistent linear increase in ghost spacing was observed, in excellent agreement with the predictions from Haacke and Patrick (35). These results demonstrate the high degree of repeatability and control available with this motion platform.

FIG. 9.

A multiecho coronal T2 mapping acquisition (TR = 1000 ms; TE = 20, 40, 60, 80, 100 ms; acquisition matrix = 252 × 200; slice thickness = 4 mm; voxel size = 1.2 × 1.5 mm²) of the phantom acquired through the kidneys without phantom motion (top), physiologic motion of the phantom in the cranial–caudal (z-axis, arrows) direction (middle), and physiologic motion but using a navigator to trigger acquisition (bottom). The left column shows an image acquired with TE = 100 ms; the right column shows the resulting T2 map calculated from the multiecho series. Large and small dotted-line rectangles were placed on the liver and renal parenchyma, respectively. A dotted-line square was placed on the simulated renal lesion in the right kidney. Note the severe artifacts induced by phantom motion on the T2 map, which are partially corrected with the use of the navigator.

FIG. 10.

T2 values measured in the phantom within the liver, kidney, and simulated renal lesion (regions of interest in Fig. 7). The presence of motion caused a three-fold reduction in the estimate of T2 in the lesion, and an approximately 1.5-fold increase in the T2 of the liver and kidney. When a navigator was used to trigger image acquisition, the T2 estimates in the liver and kidney were much closer to the baseline estimates. This improvement was also present for the simulated renal lesion, although the difference in the calculated T2 compared with the baseline estimate was still substantial.