Background. Magnetic Resonance Spectroscopy (MRS) non invasively measures metabolite concentrations

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1 Applying FID navigators to high field MRS in the body AAPM North Central Chapter Spring Meeting April 7th, 2017 Ryan M. Kalmoe Department of Radiation Oncology, University of Minnesota Advisor: Dr. Gregory Metzger Department of Radiology, University of Minnesota Center for Magnetic Resonance Research

2 Background Magnetic Resonance Spectroscopy (MRS) non invasively measures metabolite concentrations PCho Cho Spm Cr Cit 3T 180 L Advantages of high field strengths in MRS (i.e. 7 tesla): Increased spatial and spectral resolution, signal to noise ratio (SNR) Disadvantages: Motion and B 0 inhomogeneity significantly degrade spectral quality 7T 80 L ppm

3 Purpose and Significance Develop the means to detect significant fluctuations in B 0 during spectroscopic sequences thereby improve spectral quality and quantification Metabolites present differently in cancerous tissue [1] Total Choline / Citrate ratios increase significantly Spermine concentration decreases; marker of aggressiveness and grade Incorporate spectroscopic data into a multi parametric model for prostate cancer

4 Theory and Methodology Free Induction Decay (FID) navigators are short, small tip angle pulses with readout incorporated into a MRI pulse sequence Previously demonstrated in brain imaging Advantages: [2] [3] Inexpensive No additional hardware required Minimal time cost ~7s total addition to 5 minute scan Computationally inexpensive Disadvantages: Only detects motion; does not involve correction

5 Theory and Methodology FID navigators exploit signal phase and magnitude dependence on changes to B 0 and positioning relative to the detector: sin SVS Signal Equation:

6 Theory and Methodology Result is a non localized signal characterizing B 0 fluctuations Due to receive profile and close proximity of endorectal coil to volume of interest (VOI), FID navigator is semi localized

7 Theory and Methodology Can determine an initial baseline average and use a complex difference to track a percent change throughout scan Set a threshold (3 standard deviations from all accepted points) to classify acquisitions as corrupt Correct for system drift

8 Theory and Methodology FID Navigator Data Water Reference Scan Channel Combination Remove Data Point Linear Regression & Extrapolate Within Threshold? Calculate 3*STD FID Navigator Workflow No Yes Keep Data Point & Recalculate

9 Experimental Setup Semi LASER [4] sequence (TR=1900ms, TE=70ms) with VAPOR [5] water suppression, MEGA [6] lipid suppression, and GOIA [7] pulses for adiabatic refocusing Modified with nonselective excitation pulse (flip angle = 9 o ) after readout and rewinding gradients followed by short readout (~50ms)

10 Experimental Setup ~18 L, 30x45x19cm (length x width x height) custom made body phantom fitted with 9cm long, 3cm diameter cylinder to represent human rectum 10mL Foley catheter positioned next to prostate phantom was inflated to induce B 0 fluctuations 16 channel loop dipole surface array [8] used in conjunction with a 2 channel actively tuned endorectal coil [9]

11 Results Baseline FIDnav for in vitro phantom imaging

12 Results Motion detected by FIDnav during in vitro phantom imaging

13 Results Motion detected by FIDnav during in vivo scan

14 Results Motion detected by FIDnav during in vivo scan

15 Results Full dataset (left) vs. filtered dataset using FIDnav to identify corrupted acquisitions (right). Spectra fitted using LCModel. Citrate linewidth for full dataset was 20 Hz. Linewidth improved to 15 Hz with removal of corrupted acquisitions.

16 Conclusions FID navigators, with appropriate signal combination and threshold, are capable of detecting significant motion events both in vitro and in vivo at 7T. The removal of corrupted data leads to improved spectral quality at the cost of SNR. Single voxel acquisitions are demonstrated here and are being applied to 2D and 3D CSI sequences Identification of corrupted data is not trivial Reacquisition of data based on real time FIDnav evaluation is ideal

17 Future Work Optimize application for 2D and 3D sequences Ideal k space sampling? Reject, reacquire, or modify readout? Which is best? Tradeoffs? In retrospective applications, utilizing parallel imaging reconstruction to interpolate corrupted data points.

18 Acknowledgements Clinical Collaborators Diane Hutter University of Minnesota Greg Metzger Edward Auerbach Gosia Marjanska Patrick Bolan Ivan Tkac ArcanErtük Siemens Healthcare Tobias Kober Funding NCI R01 CA NIBIB P41 EB015894

19 References [1] Swanson MG, Vigneron DB, Tabatabai ZL, Males RG, Schmitt L, et al. (2003) Proton HR MAS spectroscopy and quantitative pathologic analysis of MRI/3D MRSI targeted postsurgical prostate tissues. Magnetic Resonance in Medicine 50: [2] Kober, T., Marques, J. P., Gruetter, R. and Krueger, G. Head motion detection using FID navigators. Magn. Reson. Med., 2012, 66: [3] Kober, T., J. P., Gruetter, R. and Krueger, G. Prospective and retrospective motion correction in diffusion magnetic resonance imaging of the human brain, NeuroImage, 2012, 59, 1, 389. [4] Scheenen, T.W., A. Heerschap, and D.W. Klomp, Towards 1H MRSI of the human brain at 7T with slice selective adiabatic refocusing pulses. Magma, (1 2): p [5] Tkac, I., et al., In vivo 1H NMR spectroscopy of rat brain at 1 ms echo time. Magn Reson Med, (4): p [6] Mescher, M., et al., Simultaneous in vivo spectral editing and water suppression. NMR Biomed, (6): p [7] Tannus, A. & Garwood, M. Adiabatic pulses. (1997) NMR Biomed 10, [8] Ertürk, M. A., Raaijmakers, A. J. E., Adriany, G., Uğurbil, K. and Metzger, G. J., A 16 channel combined loop dipole transceiver array for 7 Tesla body MRI. Magn. Reson. Med.., 2016, doi: /mrm [9] Ertürk, M. A., Tian, J., Van de Moortele, P. F., Adriany, G. and Metzger, G. J. (2016), Development and evaluation of a multichannel endorectal RF coil for prostate MRI at 7T in combination with an external surface array. J. Magn. Reson. Imaging, 2016, 43: