Multi-echo Pseudo-Golden Angle Stack of Stars Thermometry with High Spatial and Temporal Resolution
Bryant Svedin1 and Dennis L. Parker1

1University of Utah, Salt Lake City, UT, United States

Synopsis

A multi-echo pseudo-golden angle stack of stars sequence is investigated for use in MR thermometry. High spatial and temporal resolution is achieved through k-space filtering. PRF temperature, T2*, ρ (signal magnituade at TE = 0), respiration correction and fat/water separation are simultaneously measured. Use of a pseudo-golden angle increment allows for the removal of phase (and therefore PRF temperature) artifacts due to changing k-space sampling between reconstructed time points. k-Space sampling based phase reference greatly improves temperature standard deviation. FUS heating experiments are performed while simulating respiration artifacts.

Purpose

Radial acquisitions offer several unique advantages for proton resonance frequency (PRF) shift thermometry. Frequently sampling the k-space center provides motion robust images, as well as the ability to correct for respiration induced off resonance1. It has been shown that arbitrary high spatial and temporal resolution can be achieved in dynamic MRI by acquiring successive radial spokes separated by the golden angle and applying a sliding k-space filter, similar to the k-space weighted image contrast (KWIC) filter2, to the reconstruction3. This work investigates a pseudo-golden angle 3D multi-echo stack of stars acquisition to simultaneously measure PRF shift temperature, T2* and ρ (signal magnitude at TE = 0), correct respiration induced off resonance, and provide water/fat separation with high spatial and temporal resolution.

Methods

Experiment: A 3D stack of stars spoiled GRE sequence was modified to acquire multiple echo contrasts using a bipolar readout and a pseudo-golden angle increment. The angle used, based on the ratio of two Fibanacci numbers α = (1 – 233/377)*360 ≈ 137.5066, will repeat the k-space trajectory after 377 views. Experiments were performed in an ex vivo pork phantom on a Siemens 3T Trio scanner to assess the effectiveness of this sequence and reconstruction technique (1.3x1.3x3 mm, FOV = 166 mm, Matrix Size = 128x128x8, Flip Angle = 10, TR = 20 ms, 13 Echoes, TE = 2.46/3.69/4.92/6.15/7.38/8.61/8.84/11.07/12.3/ 13.53/14.76/15.99/17.22 ms). An ambu-bag with two 1 liter saline bags, placed above the phantom, was manually inflated periodically to simulate respiration artifacts. The phantom was sonicated with focused ultrasound (FUS) with 125 electric Watts for 30 seconds while imaging. Four sets of data were collected. Two image sets were acquired without manually simulated respiration or FUS. The first set served as a control. The second provided a baseline of the FUS heating without the respiration artifact. The third and fourth image sets repeated sets one and two while manually simulating respiration.

Reconstruction: The center of k-space was corrected with the method described by (4) using the first and second echoes of each view to calculate the pixel shift needed. The slope of the phase at the center of k-space was used for respiration correction1. Data was then reconstructed using a sliding filter with 13 innermost lines, with each successive ring using the minimum number of lines to meet the Nyquist criteria, using 377 lines in the outermost ring. The sliding window was advanced 13 views between each reconstruction time point providing a temporal resolution of 1.56 seconds. The k-space sampling pattern was repeated after 29 reconstructed time points. PRF temperatures were calculated using both the first time point and the time point with the same k-space sampling pattern as the reference phase. T2*/ρ maps were calculated using linear regression of the log of the magnitude images along the echo dimension. Water and fat images were produced using the three point Dixon method with the second, third and fourth echoes.

Results

Figure 1 shows the water and fat images produced from the sequence. Figure 2 shows the slope of the phase change through the center of k-space induced by simulated respiration. Figure 3 shows the measured PRF temperature change in an example aqueous tissue voxel for the respiration non-heating case. Data is displayed with and without respiration correction and using the 1st image as the reference phase as well as a sampling pattern based phase reference. Figure 4 shows the standard deviation through time images of the PRF temperature measurements for the same cases as Figure 3. Figures 5a, b and c show the PRF temperature, ρ and T2* values vs time of the hottest voxel while heating with FUS. PRF temperature is shown using both the first image as the phase reference and a sampling pattern based phase reference to calculate the temperature difference.

Discussion, and Conclusions

All measurements display a structured artifact based on the k-space sampling pattern used for reconstruction. For this reason, a pseudo and not pure golden angle increment was chosen to cause the artifact to repeat and thus be removable in temperature difference measurements. The method shown here provides promising results for this sequence and reconstruction method for use in free breathing interventional treatments. The respiration correction method also corrects for main field drift. The high bandwidth multi-echo readout removes the need for fat saturation as well as provides T2*/ρ measurements, which could be used as another possible measure of temperature change, especially in adipose tissue which does not exhibit a PRF shift with temperature5.

Acknowledgements

Funding Sources: NIH R01 EB013433, CA 172787

References

1. Svedin BT, Payne A, & Parker DL. Respiration artifact correction in three-dimensional proton resonance frequency MR thermometry using phase navigators. Magn Reson Med. 2015;doi:10.1002/mrm.25860

2. Song HK, & Dougherty L. k-space weighted image contrast (KWIC) for contrast manipulation in projection reconstruction MRI. Magn Reson Med, 2000;44(6), 825-832.

3. Winkelmann S, Schaeffter T, Koehler T, Eggers H, & Doessel O. An optimal radial profile order based on the Golden Ratio for time-resolved MRI. IEEE Trans Med Imaging, 2007;26(1), 68-76.

4. Block KT, Uecker M. Simple Method for Adaptive Gradient Delay Compensation in Radial MRI. Proceedings of ISMRM, Montreal, 2011. Abstract # 2816.

5. Baron P, Ries M, Deckers R, de Greef M, et al. In vivo T2 -based MR thermometry in adipose tissue layers for high-intensity focused ultrasound near-field monitoring. Magn Reson Med, 2014;72(4), 1057-1064.

Figures

Figure 1. Water (left) and fat (right) images.

Figure 2. Slope of phase change through the center of k-Space due to simulated respiration artifact.

Figure 3. Measured PRF temperature change in an example voxel while manually simulating respiration in a non-heating situation. See legend for description.

Figure 4. PRF temperature standard deviation through time maps for the four methods of reconstruction for the respiration case.

Figure 5. a) PRF temperature (See legend for description), b) Initial signal magnitude ρ, and c) T2* vs time of the hottest voxel while heating with FUS.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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