MR Imaging Near Orthopedic Implants with Artifact Reduction Using View-Angle Tilting and Off-Resonance Suppression
Clemens Bos¹, Chiel J. den Harder², Gert van Yperen^2
1MR Clinical Science, Philips Healthcare, Best, Netherlands; ²MR CTO, Philips Healthcare, Best, Netherlands

Metal orthopaedic implants are known to cause substantial artifacts in MR imaging of joints, such as slice distortions and displacements of signal in the readout direction. View angle tilting aims to correct for the displacements in readout direction. Off-resonance suppression is proposed as an extension to view angle tilting. Using different slice selection gradients during excitation and refocusing limits the spectral and spatial range from which undesired signal may originate. This combination of techniques has no inherent imaging time penalty and was demonstrated to reduce metal artifacts, both in vitro and in vivo.

SEMAC and MAVRIC for Artifact-Corrected MR Imaging Around Metal in the Knee
Christina A. Chen¹, Weitian Chen², Stuart B. Goodman¹, Brian A. Hargreaves¹, Kevin M. Koch³, Wenmiao Lu¹, Anja C. Brau², Christie E. Draper¹, Scott L. Delp¹, Garry E. Gold^1
1Stanford University, Stanford, CA, United States; ²GE Healthcare Applied Science Lab, Menlo Park, CA, United States; ³GE Healthcare Applied Science Lab, Milwaukee, WI, United States

We have developed 2 three-dimensional MRI prototypes that correct for metal-induced artifacts, Slice Encoding for Metal Artifact Correction (SEMAC) and Multi-Acquisition Variable-Resonance Image Combination (MAVRIC). In 10 knees with metallic total knee replacements (TKR) scanned at 1.5T, SEMAC and MAVRIC both had significantly less artifact than conventional two-dimensional fast spin echo (FSE). In a model of the knee fitted to a TKR of known dimensions, SEMAC and MAVRIC had much smaller percent deviations from actual component dimensions than FSE, indicating their accuracy in measuring geometry in the presence of metal. MAVRIC and SEMAC are promising MR imaging techniques that may allow for improved musculoskeletal follow-up imaging of metallic implants and soft tissue structures surrounding metal in the knee.

Magnetic Resonance Imaging of Periprosthetic Tissues in the Presence of Joint Arthroplasty
Matthew F. Koff¹, Kevin M. Koch², Hollis G. Potter^1
1Department of Radiology and Imaging, Hospital for Special Surgery, New York, United States; ²General Electric Healthcare, Waukesha, WI, United States

Significant in-plane and through-plane susceptibility artifacts occur when performing MRI around orthopedic hardware. This study evaluated standard of care 2D FSE imaging with the multi-acquisition variable-resonance image combination (MAVRIC) technique. Volunteers with joint replacements (hip, shoulder, or knee) were scanned using a 2D FSE sequence optimized for imaging around arthroplasty and a MAVRIC sequence. MAVRIC scans were effective in reducing the metal susceptibility artifact for all joints and also better  highlighted the extent of osteolysis. Higher resolution FSE images were effective for detection of formation of fibrous membrane around arthroplasties. This study further supports the use of MAVRIC for clinical implementation.

Imaging of Metallic Implant Using 3D Ultrashort Echo Time (3D UTE) Pulse Sequence
Jiang Du¹, Kelly Borden¹, Eric Diaz¹, Mark Bydder¹, Won Bae¹, Shantanu Patil², Graeme Bydder¹, Christine Chung^1
1Radiology, University of California, San Diego, CA, United States; ²Shiley Center for Orthopedic Research and Education, La Jolla, CA, United States

Magnetic resonance imaging (MRI) near metal implants suffers from severe artifacts due to large metal-induced field inhomogeneities. The steep field gradients near metal implants result in increased intra-voxel dephasing and a much shortened T2*. Clinical gradient echo (GE) sequences suffer from large signal loss. Spin echo (SE) type sequences only partly refocus the dephased spins, resulting in spatially dependent signal voids and pile-ups. Here we present a 3D ultrashort TE (UTE) sequence which employs short hard pulse excitation and 3D radial sampling with a nominal TE of 8 µs to image metallic implants with markedly reduced artifact.

kf ARC Reconstruction for Improving MRI Around Metal Using MAVRIC
Peng Lai¹, Weitian Chen¹, Christina Chen², Kevin M. Koch³, Anja CS. Brau^1
1Applied Science Laboratory, GE Healthcare, Menlo Park, CA, United States; ²Stanford University, Stanford, CA, United States; ³Applied Science Laboratory, GE Healthcare, Waukesha, WI, United States

This work developed a new method, kf ARC, for highly accelerated MAVRIC imaging around metal implants. The proposed method utilizes both k-space correlation and spectral correlation between adjacent spectral images to improve reconstruction. kf ARC was evaluated on 2 patients with metallic implants in comparison with conventional parallel imaging. Our results show that kf ARC can significantly improve image quality at high acceleration factors and is a promising approach to fast MAVRIC data acquisition.

Morphological and Quantitative Evaluation of Meniscal Calcifications by Novel 2D IR and 3D UTE MR Techniques
Patrick Omoumi^1,2, Eric S. Diaz¹, Jiang Du¹, Sheronda S. Statum¹, Won C. Bae¹, Graeme Bydder¹, Christine B. Chung^1
1University of California, San Diego, San Diego, CA, United States; ²Cliniques Universitaire St Luc, Brussels, Belgium

Meniscal calcifications are frequent and likely alter the normal biomechanics of the meniscus. Although MR imaging is the non-invasive technique of choice for the evaluation of meniscal pathology, it does not allow the facile visualization of meniscal calcifications. This is due to a lack of contrast (both calcifications and menisci have relatively short T2 relaxation times), and a lack of saptial résolution with standard clinical sequences. We describe novel MR imaging techniques based on 2D-UTE inversion recovery and 3D-UTE data acquisition to address these factors. We assessed the ability of these sequences to allow the visualization, characterization and quantitative evaluation of meniscal calcifications.

Fiber Tracking of Dipolar Directions in the Meniscus
Nikolaus M. Szeverenyi¹, Graeme M. Bydder^1
1Radiology, University of California, San Diego, San Diego, CA, United States

This study examines a method to extract and use dipolar information to characterize an ex-vivo meniscus sample.  A goat meniscus was embedded in a spherical epoxy ball and the MR signal intensity examined as a function of orientation to a 3T static field.  Unaveraged dipolar interactions caused dramatic signal variations in sub-structures.  After correcting for coil sensitivity and co-registering all images, a principle dipolar direction was extracted for each voxel.  This directional data could be analyzed and viewed as a direction map, similar to DTI brain data.  The intensity fluctuations provided a FA map.  Fiber tracks were generated.

Ultrashort Echo Imaging (UTE) of Rotator Cuff Repair in an Ovine Model
Matthew F. Koff¹, Hollis G. Potter^1
1Department of Radiology and Imaging, Hospital for Special Surgery, New York, United States

The rotator cuff tendons typically display low signal on standard clinical images due to the highly ordered collagen within the tissue. Ultrashort echo (UTE) imaging creates contrast for visualization and for T2* quantitation. This study used T2* mapping to evaluate rotator cuff repair in an ovine model. Reparative surgery was performed to the supraspinatus tendon in sheep. Shoulders were scanned ex-vivo 8 weeks post-operatively. T2* values of repaired tendon were significantly longer than normal tendon. The T2* values decreased in magnitude along the length of the repair, but not significantly. This pilot study highlights the use of UTE for quantitative evaluation of soft tissue repair.

Detection of Dipolar Splitting in Rodent Tendons as a Function Axial Position with Double-Quantum Filtered Spectroscopic Imaging
Henry H. Ong¹, Joseph J. Sarver², Jason E. Hsu², Louis J. Soslowsky², Felix W. Wehrli^1
1Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania School of Medicine, Philadelphia, PA, United States; ²McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, United States

Tendons are comprised of parallel collagen fibers that connect muscles to bone. Collagen-associated water has anisotropic rotational motion, which gives rise to residual dipolar splitting in 1H NMR. Double-quantum filtered (DQF) NMR and MRI can be used to observe the splitting and study the biophysical and structural properties of tendon. Here, we modified a DQF 1D spectroscopic imaging sequence to obtain 1H DQF spectra along the axis of the flexor digitorum profundus (FDP) tendons from rat hind limbs and show spectral differences in the region that wraps under the calcaneus, which experiences compressive forces.

Magnetization Transfer (MT) Segmentation of Foot Peripheral Nerves at 3 T
Giulio Gambarota¹, Bénédicte Mortamet², Nicolas Chevrey³, Cristina Granziera⁴, Gunnar Krueger², Nicolas Theumann³, Ralf Mekle^3
1GlaxoSmithKline Clinical Imaging Center, London, United Kingdom; ²Healthcare Sector IM&WS S, Siemens Schweiz AG, Renens, Switzerland; ³Radiology, University of Lausanne, Lausanne, Switzerland; ⁴Neurology, Geneva University Hospital, Geneva, Switzerland

The ability of tracking peripheral nerves in foot could be of great benefit for a number of investigations, which include traumas, diabetes and infections. Previous approaches to nerve tracking have employed diffusion tensor imaging DTI. One limitation of DTI is the low signal-to-noise ratio due to short T2 (~30ms at 3T) of water protons in nerves.  Here, we propose a novel approach to nerve tracking, which exploits the difference in MT ratio between muscle and foot nerves.