Wide Dynamic Range MR Elastography of Liver
Dieter Klatt¹, Detlef Stiller², Thomas Kaulisch², Heiko Nießen², Kerstin Riek¹, Sebastian Papazoglou¹, Thomas Elgeti¹, Ingolf Sack¹, Jürgen Braun^3
1Institute of Radiology, Charité - University Medicine Berlin, Berlin, Germany; ²Boehringer Ingelheim Pharma GmbH & Co. KG, Biberach, Germany; ³Institute of Medical Informatics, Charité - University Medicine Berlin, Berlin, Germany

MR elastography (MRE) enables the measurement of the complex shear modulus G* of biological tissue. Using MRE, the frequency dependency of G* has been examined in the past within a limited dynamic range due to inherent technical restrictions. In this study, G* of liver in a wide dynamic range of more than 4.5 octaves was measured by combining MRE at a 1.5T human scanner system with MRE at a 7T animal scanner. The results of both systems agreed excellently and revealed a power-law behavior of G* between 25Hz and 600Hz vibration frequency. The springpot-model was used for calculating viscoelastic parameters.

Frequency Dependence of Mouse Brain Tissue Stiffness Measured in Vivo with MR Elastography
Erik Holt Clayton¹, Joel R. Garbow², Philip V. Bayly^1,3
1Mechanical Aerospace & Structural Engineering, Washington University in St. Louis, Saint Louis, MO, United States; ²Biomedical MR Laboratory, Mallinckrodt Institute of Radiology, Washington University School of Medicine, Saint Louis, MO, United States; ³Biomedical Engineering, Washington University in St. Louis, Saint Louis, MO, United States

Multifrequency MR elastography (MRE) has been used to measure mechanical stiffness of human brain tissue. The development of cancer treatment protocols may benefit from similar studies in rodent models. Here the viscoelastic material properties of mouse brain were determined by MRE over a range of driving frequencies (600 - 1800 Hz). A novel non-invasive brain actuator was devised to introduce propagating shear waves. Wave motion was imaged with a phase-locked spin echo pulse sequence. Displacement data were inverted in a least-squares manner to obtain complex modulus estimates. Results suggest the frequency response of brain tissue may provide diagnostic value.

Improving Spatial Resolution of Strain-Encoded (SENC) Magnetic Resonance Elastography (MRE) for Enhancing Stiff-Mass Detection
Ahmed Amr Harouni¹, Jakir Hossain¹, Michael A. Jacobs², Nael Fakhry Osman^1,2
1Electrical and Computer Engineering, Johns Hopkins University, Baltimore, MD, United States; ²Department of Radiology, Johns Hopkins University, Baltimore, MD, United States

Early detection through periodic screening is the key to decrease beast cancer mortality. Fast Strain-encoded (FSENC) MR with a limited hardware was previously introduced to detect different stiffness by measuring the strain. In this work, we introduce a new hardware capable of periodically compressing the breast, which allows us to achieve higher resolution while maintaining same SNR by prolonging scan time. Simple controls and redundant safety measures were added to ensure accurate, repeatable and safe in-vivo experiments. Results show that high-resolution SENC images have four-fold CNR increase relative to low-resolution FSENC images, which leads to better tumor detection.

Focused Acoustic Driver to Generate High-Frequency Shear Waves in Deep Regions for Magnetic Resonance Elastography
Mikio Suga^1,2, Takayuki Obata², Masashi Sekine³, Masaya Hirano⁴, Hisayuki Miura⁵, Ken Arai⁵, Shinya Ozawa⁵, Hiroo Ikehira^2
1Graduate School of Technology, Chiba University, Chiba , Japan; ²Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan; ³Research Center for Frontier Medical Engineering, Chiba University, Japan; ⁴GE Healthcare Japan, Tokyo, Japan; ⁵Graduate School of Technology, Chiba University, Chiba, Japan

Magnetic resonance elastography (MRE) can noninvasively visualize shear waves patterns within tissue. To acquire an accurate shear modulus map in high spatial resolution in deep regions, external drivers must generate a precisely controlled high frequency and a large amplitude vibration. In this study, we develop a simple and robustly designed focused acoustic driver to enhance shear wave amplitude in deep regions by high frequency using a piezoelectric actuator. From the results of the experimental studies, it was shown that the focused acoustic driver increases the SNR of the shear wave image in the deep region and improves shear modulus quantitatively.

Effect of Off-Frequency Encoding in Magnetic Resonance Elastography
Curtis L. Johnson¹, Danchin Chen¹, Harish Sharma², Bradley P. Sutton^2,3, William C. Olivero^2,4, John G. Georgiadis^1,2
1Mechanical Science and Engineering Department, University of Illinois at Urbana-Champaign, Urbana, IL, United States; ²Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, United States; ³Bioengineering Department, University of Illinois at Urbana-Champaign, Urbana, IL, United States; ⁴Department of Neurosurgery, University of Illinois at Urbana-Champaign, Urbana, IL, United States

The effects of encoding displacement at a frequency other than the driving frequency with Magnetic Resonance Elastography (MRE) were investigated.  Off-frequency responses can occur due to possible nonlinearities in the overall dynamic system being actuated.  Results demonstrated that undesired off-frequency encoding could result in errors in mean estimated stiffness of tissue, as well as local fluctuations in estimated stiffness, which will have implications for MRE with nonlinear dynamic systems.

SSFSE Sequence for Fast Elastography in the Presence of Susceptibility
Ken-Pin Hwang^1,2, Zhenghui Zhang³, Brandy J. Reed⁴, Michelle L. Underwood⁴, Roger Jason Stafford⁴, Peggy T. Tinkey⁵, David C. Alsop^6,7, Rajesh Uthamanthil^5
1Applied Science Laboratory, General Electric Healthcare, Houston, TX, United States; ²Department of Imaging Physics, UT MD Anderson Cancer Center, Houston, TX, United States; ³GE Healthcare, Waueksha, WI, United States; ⁴Department of Imaging Physics, University of Texas MD Anderson Cancer Center, Houston, TX, United States; ⁵Department of Veterinary Medicine and Surgery, University of Texas MD Anderson Cancer Center, Houston, TX, United States; ⁶Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA, United States; ⁷Department of Radiology, Harvard Medical School, Boston, MA, United States

The use of a modified phase contrast gradient echo sequence has been shown to be a robust technique for MR elastography of the liver.  However, each phase encoded view requires long motion encoding gradients that extended the echo time, making the sequence sensitive to susceptibility and lengthening overall acquisition time.  In this work we combine a motion encoding preparation sequence with an SSFSE sequence originally designed for diffusion weighted imaging.  Phase information from a single set of motion encoding gradients is preserved for each echo in the echo train, thus accelerating acquisition in a spin echo based sequence.

Improvements in Shear Modulus Reconstruction In-Vivo Breast Data Using a Viscoelasitc Material Model in Optimization Driven Mr Elastography
Matthew Mcgarry¹, Irina Perreard², Adam Jeffry Pattison¹, Elijah van Houten³, John Weaver², Keith Paulsen^1
1Thayer School of Engineering, Dartmouth College, Hanover, NH, United States; ²Department of Radiology, Dartmouth-Hitchcock Medical Center, Lebanon, NH, United States; ³Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand

This work demonstrates the improvements in in-vivo breast shear modulus reconstruction gained through considering the effects of viscoelasticity in a model-based, optimization driven MR elastography algorithm. Three cases with 12 reconstructions are presented where increased shear modulus in the region of a malignant tumor is apparent using a viscoelastic material model. It is shown that using an undamped linear elastic model produces inconclusive results. The improvements are due to a reduction in the model-data mismatch by using a viscoelastic model to fit tissue, which is known to have a significant viscous component.

Validity Study of Spin Echo EPI Based Hepatic MR Elastography at 3.0T
David W. Stanley¹, Kevin J. Glaser², Meng Yin², Jun Chen², Richard L. Ehman^2
1MR, GE Healthcare, Proctor, MN, United States; ²Department of Radiology, Mayo Clinic, Rochester, MN, United States

The purpose of this study was to evaluate a SE-EPI MRE protocol and compare it to a standard GRE MRE protocol at both 1.5T and 3.0T in healthy volunteers with no known liver disease to determine if the signal variations characteristic of the different imaging sequences and field strengths cause a significant change in the SNR of the data or adversely affect the estimates of tissue stiffness.

Measuring the Effect of Formalin Fixation on Ex Vivo Tissue Material Properties Using High Resolution 3D Quasi-Static MR Elastography at 7 Tesla for Improved Biomechanical Registration of Histopathology, and Correlation with the Effect of Fixation on T<su
Deirdre Maria McGrath¹, Warren D. Foltz¹, Kristy K. Brock^1,2
1Radiation Medicine Program, Princess Margaret Hospital, Toronto, Ontario, Canada; ²Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada

Correlation of 3D histopathology with in vivo images improves the understanding of disease representation in imaging. The pathology fixation process changes the material properties of tissue non-uniformly and if biomechanical registration is used, measures of these effects are required. A high resolution 3D quasi-static MR elastography (MRE) method at 7 T is presented for voxel-wise mapping of Young’s modulus across tissue volumes, and is applied to ex vivo canine prostate samples, pre- and post-fixation. The measures are validated using indentation testing. The effect of fixation on T1, T2 and ADC is also measured, to determine the relationship with material property changes.