Velocity Navigator Triggering for Motion Compensated PRF Thermometry - not available
Florian Maier¹, Axel Joachim Krafft¹, Jürgen W. Jenne^2,3, Rüdiger Dillmann⁴, Wolfhard Semmler¹, Michael Bock^1
1Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; ²Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany; ³Mediri GmbH, Heidelberg, Germany; ⁴Institute of Anthropomatics, Karlsruhe Institute of Technology, Karlsruhe, Germany

Proton resonance frequency shift thermometry is sensitive to motion. Artifacts are caused by tissue displacement and susceptibility changes. In this work, a novel navigator technique for triggering MR thermometry image acquisition is presented. Non-velocity and velocity encoded navigator signals were acquired without lengthening of TR. Based on the phase variation of non-encoded values and the estimated velocity, trigger events were generated. The measurements indicate that the proposed triggered segmented EPI pulse sequence allows for motion compensated thermometry of periodically moving tissue.

Fat-Referenced MR Thermometry in Heterogeneous Tissue Using IDEAL
Lorne Hofstetter¹, Desmond Teck Beng Yeo², Cynthia Davis², Thomas K. Foo^2
1GE Global Research, Niskayuna , NY, United States; ²GE Global Research, Niskayuna, NY, United States

Time-varying, non-temperature dependent phase changes affect the accuracy of conventional phase difference proton resonance frequency shift (PRFS) temperature mapping in the breast.  We demonstrate a fat-referenced PRFS technique capable of correcting for this phase variation. This new approach reduced temperature measurement error in the left breast by a factor of 3.6 and in the right breast by a factor of 2.5 when compared to conventional phase difference techniques (n = 1).

The Effects of Spatial Sampling Choices on MR Temperature Measurements
Nick Todd¹, Josh De Bever², Urvi Vyas³, Allison Payne⁴, Dennis L. Parker^5
1Physics, University of Utah, Salt Lake City, UT, United States; ²Robotics, University of Utah, Salt Lake City, UT, United States; ³Bioengineering, University of Utah, Salt Lake City, UT, United States; ⁴Mechanical Engineering, University of Utah, Salt Lake City, UT, United States; ⁵Radiology, University of Utah, Salt Lake City, UT, United States

MR temperature maps are necessarily a discrete representation of a physical quantity that is continuously varying in both space and time. The HIFU focal spot size can be smaller than the imaging voxel dimensions.  Due to averaging effects, it is likely that different choices for the sampling grid location, voxel size, and scan time will lead to variations in the measured temperature distribution. In this abstract we present simulation and experimental results quantifying the effects of the sampling scheme on maximum temperature and thermal dose, and show the effects of zero-filled-interpolation post-processing on the measured maximum temperature and thermal dose.

Reference-Less PRFS MR Thermometry Using a Thin Open Border and the Harmonic Functions Theory: 2D Experimental Validation
R Salomir¹, M Viallon¹, Joerg Roland², Sylvain Terraz¹, Denis Morel³, CD Becker¹, P Gross^2
1Radiologie, Hopital Universitaire de Genève, Geneva, Switzerland; ²Siemens Medical Solutions, Erlangen, Germany; ³Anesthesiology, University Hospitals of Geneva, Geneva, Switzerland

A new method for reference-less MR thermometry is described based on the fundamental theoretical frame of harmonic functions. The method was implemented for a thin open border and validated for 2D situation with HIFU heating in phantoms and in vivo rabbit thigh, and also with baseline acquisition in volunteers liver. Measurement accuracy in liver under free breathing was as good as 0.5°C for 0.3 seconds temporal resolution. The method is insensitive to periodic or accidental motion, tissue expansion or drift, and to external perturbation from interventional device.

Hybrid Multi-Baseline and Referenceless PRF-Shift Thermometry
William A. Grissom^1,2, Andrew B. Holbrook³, Viola A. Rieke², Michael Lustig¹, Juan A. Santos¹, Aravind Swaminathan, Michael V. McConnell, Kim Butts Pauly^2
1Electrical Engineering, Stanford University, Stanford, CA, United States; ²Radiology, Stanford University, Stanford, CA, United States; ³Bioengineering, Stanford University, Stanford, CA, United States

We introduce a new temperature estimation method that is a hybrid of multi-baseline and referenceless methods. From multi-baseline methods the hybrid method inherits the ability to estimate temperature in the presence of rapidly-varying background anatomical phase. From referenceless methods the hybrid method inherits robustness to smooth main field shifts during thermal therapy. The method is demonstrated in the heart and liver.

MRI Monitoring of Skull-Base Heating in Transcranial Focused Ultrasound Ablation
Yuexi Huang¹, Junho Song¹, Kullervo Hynynen^1,2
1Sunnybrook Health Sciences Centre, Toronto, ON, Canada; ²Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada

In transcranial focused ultrasound ablation, the heating of the outer skull surface has been reduced by a hemispherical design of phased-array transducers and active cooling of the skull surface with water circulation. However, the potential heating of the skull base has not been brought into much attention. In this work, experiments were performed with a MR-guided transcranial focused ultrasound system on a full human skull sample to investigate the heating of the skull base. MR thermometry was applied to measure the temperature change of the phantom adjacent to the skull base. The distance of the foci to the bone was varied to measure a safety margin for avoiding significant skull base heating.

Temperature Measurement Nearby an Iceball Using the Proton Resonance Frequency Method: Recalculation of Susceptibility Artifacts
Antje Kickhefel¹, Rares Salomir^2,3, Jörg Roland⁴, Patrick Gross⁴, Fritz Schick⁵, Clifford R. Weiss^6
1Eberhard-Karls-University Tübingen , Tübingen,  Baden-Württemberg, Germany; ²University Hospitals of Geneva, Switzerland; ³ University Hospitals of Geneva; ⁴Siemens Healthcare, Erlangen, Germany; ⁵Eberhard-Karls-University Tübingen, Tübingen,  Baden-Württemberg, Germany; ⁶Department of Radiology, The Johns Hopkins University School of Medicine, Baltimore, United States

The study demonstrates that susceptibility artifacts in GRE phase image induced by ice ball can be corrected allowing the PRF method to be used to monitor the near zero temperature during cryoablation. Susceptibility artifacts were corrected in post-processing. First the susceptibility contrast between frozen and melted meat was determined and second the magnetic perturbation was calculated using a convolution filter in the k-space. The susceptibility artifacts were fully corrected. In conclusion, using an in-line post processing system, this method could be applied during clinical MR-guided cryotherapy, and allow for the non-invasive monitoring of near zero temperatures.

PRF Based MR-Thermometry on Abdominal Organs: A Pragmatic Comparison of Referenceless Vs Multi-Baseline
Baudouin Denis de Senneville¹, Sébastien Roujol^1,2, Chrit Moonen¹, Mario Ries^1
1Laboratory for Molecular and Functional Imaging: From Physiology to Therapy, CNRS/ University Bordeaux 2, Bordeaux, Aquitaine, France; ²LaBRI, CNRS/ University Bordeaux 1, Talence, Aquitaine, France

Reliable temperature and thermal-dose measurements using PRF based MR-thermometry for MR-guided ablation therapy on abdominal organs require a robust correction of artefacts induced by the target displacement through an inhomogeneous and time-variant magnetic field. The presented study combines the two most promising candidates for this role, the multi-baseline and the referenceless method, with a real-time in-plane motion correction which permits thermal-dose calculations and evaluates the practical aspects of both methods in an ex-vivo RF-ablation and an in-vivo high-intensity focused ultrasound ablation of a porcine kidney.

Quantitative Perfusion Analysis for Transcatheter Intraarterial Perfusion MR Imaging
Dingxin Wang¹, Johnathan Chung², Robert Lewandowski², Richard Tang², Rachel Klein², Reed Omary^1,3, Andrew Larson^1,3
1Departments of Radiology and Biomedical Engineering, Northwestern University, Chicago, IL, United States; ²Department of Radiology, Northwestern University, Chicago, IL, United States; ³Robert H. Lurie Comprehensive Cancer Center, Northwestern University, Chicago, IL, United States

In this study, we presented a new quantitative TRIP-MRI perfusion analysis approach, and evaluated its efficacy in a gel perfusion phantom and in rabbits with VX2 liver tumors during TAE. Our results successfully evaluated the efficacy of this proposed perfusion analysis method for TRIP-MRI datasets in the perfusion phantom, and demonstrated the use of quantitative TRIP-MRI to monitor reductions in liver tumor perfusion during TAE.

MR-Based Dosimetry of  ¹⁶⁶holmium-Loaded Microspheres for Internal Radiation Therapy Treatment Planning
Peter Roland Seevinck¹, Tim C. de Wit², Gerrit Hendrik van de Maat¹, Maarten A.D. Vente³, Mattijs Elschot³, Mark Konijnenberg⁴, Johannes F.W. Nijsen³, Chris JG Bakker^1,2
1Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands; ²Dept. of Radiology, University Medical Center Utrecht, Utrecht, Netherlands; ³Dept. of Nucleair Medicine, University Medical Center Utrecht, Utrecht, Netherlands; ⁴Research and Development, Mallinckrodt Medical BV, Covidien, Petten, Netherlands

The potential of MRI for dose calculations of Holmium-166 loaded microspheres to enable MR-based treatment planning of transcatheter radioembolization of hepatic malignancies was investigated. MRI and SPECT experiments were conducted using an anthropomorphic agarose gel phantom containing tumor-simulating gel samples with known amounts of ¹⁶⁶Ho-PLLA-MS. Excellent agreement was observed both qualitatively and quantitatively when comparing MR-based to SPECT-based dose maps to reference data obtained with a dose calibrator. In conclusion, MR-based dosimetry of ¹⁶⁶Ho-PLLA-MS was demonstrated to be feasible, indicating the potential of MR-based dosimetry for planning, guidance and evaluation of transcatheter radioembolization treatment of hepatic malignancies with ¹⁶⁶Ho-PLLA-MS.