MRI of Short and Ultrashort-T_2 Tissues: Making the Invisible Visible

This book comprehensively covers ultrashort echo time (UTE), zero echo time (ZTE), and other magnetic resonance imaging (MRI) acquisition techniques for imaging of short and ultrashort-T2 tissues. MRI uses a large magnet and radio waves to generate images of tissues in the body. The MRI signal is characterized by two time constants, spin-lattice relaxation time (T1) which describes how fast the longitudinal magnetization recovers to its initial value after tipping to the transverse plane, and spin-spin relaxation time (T2) which describes how fast the transverse magnetization decays. Conventional MRI techniques have been developed to image and quantify tissues with relatively long T2s. However, the body also contains many tissues and tissue components such as cortical bone, menisci, ligaments, tendons, the osteochondral junction, calcified tissues, lung parenchyma, iron containing tissues, and myelin, which have short or ultrashort-T2s. These tissues are “invisible” with conventional MRI, and their MR and tissue properties are not measurable. UTE and ZTE type sequences resolve these challenges and make these tissues visible and quantifiable. This book first introduces the basic physics of conventional MRI as well as UTE and ZTE type MRI, including radiofrequency excitation, data acquisition, and image reconstruction. A series of contrast mechanisms are then introduced and these provide high resolution, high contrast imaging of short and ultrashort-T2 tissues. A series of quantitative UTE imaging techniques are described for measurement of MR tissue properties (proton density, T1, T2, T2*, T1p,magnetization transfer, susceptibility, perfusion and diffusion). Finally, clinical applications in the musculoskeletal, neurological, pulmonary and cardiovascular systems are described. This is an ideal guide for physicists and radiologists interested in learning more about the use of UTE and ZTE type techniques for MRI of short and ultrashort-T2 tissues. Foreword Preface Contents Contributors Abbreviations Part I: Data Acquisition 1: Introduction to MRI of Short- and Ultrashort-T2 Tissues History Tissue Properties (TPs): Normal Values, Changes in Disease, and Changes with Contrast Agents Normal Values of T1, T2, and Mobile Proton Density (ρm) Change in TPs in Disease Changes in TPs with Contrast Agents Approaches to Imaging and Quantitation of Short-, Ultrashort- and Supershort-T2 Tissues Data Acquisitions Contrast Mechanisms Quantitation Clinical Applications Key Concepts Conclusion References 2: Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Introduction Pulse Sequence K-Space Trajectory Multipoint Acquisition Flip Angle and RF Power Hybrid-SPRITE Magnetization Preparation and Motion Encoding Conclusion References 3: Two-Dimensional Ultrashort Echo Time (2D UTE) Imaging Introduction The 2D UTE Sequence Half-Pulse Excitation Half-Pulse Excitation with Varying Slice-Select Gradient Half-Pulse Design Exciting an Off-Isocenter Slice K-Space Sampling and Imaging Reconstruction Practical Considerations Excitation The Impact of Gradient Timing Error The Impact of Eddy Currents Pre-compensating Slice-Select Gradient Pre-compensating Both the RF Pulse and the Slice-Select Gradient Other Strategies to Mitigate the Impact of Eddy Currents The Impact of Off-Resonance The Impact of T2 Decay Data Acquisition and Reconstruction Gradient Timing Errors and Eddy Currents Off-Resonance Effects T2 Decay Imaging Multiple Slices Conclusion References 4: Three-Dimensional Ultrashort Echo Time (3D UTE) Imaging Introduction Data Acquisition Reconstruction RF Excitation Hard RF Excitation Slab-Selective Excitation Water Excitation Sampling Window Eddy Currents Contrast Mechanisms UTE Imaging with Fat Saturation UTE Imaging with Multi-echo Acquisition and Subtraction Adiabatic Inversion Recovery UTE (IR-UTE) Quantitative UTE Imaging T2* Quantification T1 Quantification T1ρ Quantification Magnetization Transfer Quantification Conclusion References 5: Zero Echo Time (ZTE) MRI Introduction The Dead-Time Gap Minimizing Dead-Time Providing Missing Data Simultaneous RF Operation Excitation Hard Pulse Sweep Pulse Reducing Gradient Strength Image Reconstruction Algebraic ZTE ZTE with Gap-Filling RF Pulses Further Sequence Aspects Gradients Repetition Time Geometry Contrast Hardware RF Chain Console Gradient System Applications Moderate Bandwidth High Bandwidth Conclusion References 6: Pointwise Encoding Time Reduction with Radial Acquisition (PETRA) MRI Introduction The Petra Acquisition Image Reconstruction Correction of Unintended Slice Selectivity [1] Discussion and Outlook References 7: Ramped Hybrid Encoding Introduction Single-Point Imaging (SPI) Slice Selectivity with ZTE Sequences Ramped Hybrid Encoding (RHE) Encoding Speed and T2* Blurring with RHE The Efficacy of SPI in RHE Applications and Variants of RHE MR-Based Attenuation Correction Single-Scan Bicomponent T2* Mapping in the Knee Direct Myelin Imaging with Interleaved Hybrid Encoding RHE-Based Sodium MRI Conclusion References 8: Acquisition-Weighted Stack of Spirals (AWSOS) MRI Introduction Pulse Sequence Mathematics Slab-Select Gradient and Minimum TE Slice Encoding Gradient Spiral Encoding Gradients Data Acquisition MRI Systems Optimization of RF Excitation Optimal Parameters for Spiral Trajectories Typical MRI Scans of the Head and Knee Sampling Efficiency and SNR Technical Tips Positioning of Study Subjects B0 Field Shimming Image Blurring Due to Off-Resonance Effects Image Blurring Due to T2* Decay Examples: Brain Imaging Examples: Knee Imaging Patellar Cartilage and Tendon Images Meniscus, ACL, and PCL Examples: Imaging at 7 T Discussion Summary References 9: The Variable Echo Time (vTE) Sequence Introduction Sequence Description RF Excitation Multiple Contrasts and Reconstruction Comparison with Other Short TE Sequences Outlook MR Microscopy Using a vTE Sequence Clinical Applications of the vTE Sequence The vTE Sequence for Sodium (23Na) MRI References 10: Looping Star: Time-Multiplexed, Gradient Echo Zero TE MR Imaging Introduction Recap of ZTE Gradient Refocusing Along Polygonal k-Space Trajectories Looping Star Echo IN/OUT Overlap Coherence-Resolved Looping Star Image Reconstruction Further Technical Considerations Initial Applications and Future Prospects References Part II: Contrast Mechanisms 11: UTE with Subtraction for High Contrast Imaging of Short-T2 Tissues Introduction Dual-Echo UTE with Echo Subtraction Dual-Echo UTE with Rescaled Echo Subtraction Dual-Echo UTE with Fat Saturation and Echo Subtraction Dual-Echo UTE with Long-T2 Saturation and Echo Subtraction UTE with Off-Resonance Saturation Contrast UTE with Adiabatic Inversion and Echo Subtraction UTE with Adiabatic Inversion and Complex Echo Subtraction UTE with Interleaved Adiabatic Inversion and Subtraction UTE with Relaxation-Parameter Contrast and Subtraction Dual-Radiofrequency and Dual-Echo (DURANDE) UTE with Subtraction Conclusion References 12: T2 Relaxation During Radiofrequency (RF) Pulses Introduction Theory Excitation and T2 Relaxation Saturation and T2 Relaxation Inversion and T2 Relaxation Conclusion References 13: Adiabatic Inversion Recovery: Creating High Contrast for UTE Imaging of Short-T2 Species Introduction Theory Single Adiabatic Inversion Recovery UTE (IR-UTE) Single Adiabatic Inversion Recovery UTE with Echo Subtraction (IR-UTE-ES) Adiabatic Inversion Recovery Fat Saturation UTE (IR-FS-UTE) Dual Adiabatic Inversion Recovery UTE (Dual-IR-UTE) Double Adiabatic Inversion Recovery UTE (Double-IR-UTE) Double Echo Sliding Inversion Recovery UTE (DESIRE-UTE) Short TR Adiabatic Inversion Recovery UTE (STAIR-UTE) Adiabatic Inversion Recovery UTE (AIR-UTE) and Double Adiabatic Full Passage UTE (DAFP-UTE) Conclusion References 14: Ultrashort Echo Time Magnetic Resonance Imaging with Water Excitation Introduction Binomial Water Excitation Pulses Soft-Hard Water Excitation Pulses Conclusion References 15: UTE–Dixon Fat–Water Imaging Introduction Dixon Imaging for Fat Suppression with UTE Acquisitions Data Acquisition Techniques for UTE–Dixon Imaging Water–Fat Separation Methods Theory of Signal Models for Water–Fat Separation Water–Fat Separation in Multi-TE Dixon Imaging: The Methodology Water–Fat Separation in Multi-TE UTE–Dixon Imaging of Tissues with Short T2s Water–Fat Separation in Single-TE Dixon Imaging: The Methodology Water–Fat Separation in Single-TE UTE–Dixon Imaging of Tissues with Short T2s Clinical Applications PET Attenuation Correction Improved Tissue Delineation of Short-T2 Tissues in Musculoskeletal Imaging Water-Separated Images for Assessment of Edema in Musculoskeletal Imaging Proton Density Fat Fraction (PDFF) Quantification Conclusion References 16: Ultrashort Echo Time Spectroscopic Imaging (UTESI) of Short-T2 Tissues Introduction The UTESI Pulse Sequence UTESI Image Reconstruction UTESI Signal Processing UTESI Image Quantification T2* Quantification Chemical Shift Proton Density Bulk Susceptibility Fiber-Dependent Frequency Shift Bicomponent T2* Analysis Conclusions References 17: UTE Phase Imaging Introduction Theory Experimental Verification Specimen Study UTE Phase Imaging of the Lungs UTE Phase Imaging in the Brain UTE Phase Imaging in the Vasculature Conclusions References 18: Chemical Shift Artifacts Produced by Center-out Radial Sampling: A Potential Pitfall in Clinical Diagnosis Introduction Phantom Studies Human Volunteer Studies Conclusions References 19: Pulse Sequences as Tissue Property Filters and the Central Contrast Theorem: A Way of Understanding the Signal, Contrast, and Weighting of Magnetic Resonance Images Introduction Normal Tissue Properties (TPs), Changes of TPs in Disease, and the Effects of Contrast Agents The Central Contrast Problem Problems with Qualitative Weighting Pulse Sequences as Tissue Property (TP) Filters The Spin Echo (SE) Sequence (Univariate Model) The Spin Echo (SE) Sequence (Multivariate Model) and the Central Contrast Theorem (CCT) The Inversion Recovery (IR) Sequence The Pulsed Gradient Spin Echo (PGSE) Sequence The Spoiled Gradient Echo (SGE) Sequence Ultrashort Echo Time (UTE) and Zero Echo Time (ZTE) Sequences Features of TP-Filters Features of the Central Contrast Theorem (CCT) and Its Corollaries Conclusions References 20: MASDIR (Multiplied, Added, Subtracted, and/or Divided Inversion Recovery) Pulse Sequences and Synergistic Contrast MRI (scMRI) Introduction Development of MASDIR Sequences Classification of MASDIR Sequences AIR, SIR, and dSIR Sequences Synergistic Contrast MRI (scMRI) Image Processing to Achieve Synergistic Contrast The Subtracted IR Echo Subtraction (SIRES) Sequence The Subtracted IR Echo and Diffusion Subtracted (SIREDS) Sequence Contrast at Tissue Boundaries Examples Conclusions Appendix: Features of the dSIR and drSIR Sequences, Including Their Use As T1 Maps References Part III: Quantitation 21: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: T1 Introduction Saturation Recovery UTE (SR-UTE) UTE with Variable TR (UTE-VTR) UTE Actual Flip Angle Imaging (UTE-AFI) UTE with Variable FA (UTE-VFA) Inversion Recovery UTE (IR-UTE) Conclusions References 22: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: T2* Introduction UTE T2* Single-Component Analysis UTE T2* Bicomponent Analysis UTE T2* Tri-Component Analysis IR-UTE T2* Measurement STAIR-UTE T2* Measurement Dual-IR-UTE T2* Measurement Conclusion References 23: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: T1ρ Introduction UTE T1ρ Imaging UTE-T1ρ Dispersion Dual Adiabatic Inversion Recovery UTE T1ρ Imaging Magic Angle Effects with T1ρ and UTE-T1ρ Imaging UTE Adiabatic T1ρ Magic Angle Effect in UTE-AdiabT1ρ UTE-AdiabT1ρ Imaging in Knee Joint Degeneration UTE-AdiabT1ρ Imaging in Spine Degeneration Conclusion References 24: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: Proton Density Introduction Total Bone Water PD Mapping in Cortical Bone Bound Water, Pore Water, and Total Water PD Mapping in Cortical Bone Water and Collagen PD Mapping in Cortical Bone Water- and Fat-Suppressed Proton Projection MRI (WASPI) Bound Water PD Mapping in Trabecular Bone Myelin PD Mapping in Brain Conclusion References 25: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: Magnetization Transfer Introduction UTE-Magnetization Transfer Ratio (UTE-MTR) 2D UTE-Magnetization Transfer (UTE-MT) Modeling 3D UTE-Magnetization Transfer (UTE-MT) Modeling Magic Angle Effect Sensitivity Conclusion References 26: Quantitative Susceptibility Mapping Introduction Quantitative Susceptibility Mapping (QSM) UTE-QSM Chemical Shift in UTE-QSM UTE-QSM for Quantitative Imaging of Bone Mineral Density UTE-QSM for Detection of Iron Overload UTE-QSM for Stem Cell Tracking Conclusion References 27: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging: Perfusion Introduction Contrast-Enhanced MRI of Menisci Contrast-Enhanced MRI of Tendons Contrast-Enhanced MRI of Bone Conclusion References 28: UTE Diffusion-Weighted Imaging (UTE-DWI) Introduction Challenges with UTE Diffusion-Weighted Imaging (UTE-DWI) Stimulated Echo Acquisition Mode (STEAM)-UTE-DWI Quantitative UTE-Based Double Echo Steady State (qUTE-DESS) Double Echo Steady State (DESS) UTE-Based Double Echo Steady State (UTE-DESS) Fat Suppression with UTE-DESS Eddy Current Correction for UTE-DESS ADC Mapping with qUTE-DESS Conclusion References 29: Deep Learning for Automated Segmentation and Quantitative Mapping with UTE MRI Introduction Deep Learning in MRI Reconstruction Registration Segmentation Quantification Deep Learning in Knee Osteoarthritis with MRI Deep Learning in Knee Osteoarthritis with UTE MRI Segmentation with UTE MRI Attention U-Net-Based Meniscus Segmentation Attention U-Net-Based Cartilage Segmentation Quantification with UTE MRI Simultaneous Segmentation and Quantification with UTE MRI MSQ-Net and pcMSQ-Net MSMQ-Net Conclusion References Part IV: Applications 30: MR Imaging and Spectroscopy of Collagen Introduction Chemical Structure and Synthesis of Collagen The MR Visibility of Collagen MR Studies of Collagen In Vitro Collagen-Related Signals in Tissues Without Long-T2 Signal Components MR Studies in Collagen in the Parenchymal Tissue and Organ Fibrosis The Skeletal Muscles The Myocardium The Liver The Lungs Challenges and New Methodological Approaches References 31: Quantitative Ultrashort Echo Time Magnetic Resonance Imaging of the Knee in Osteoarthritis Introduction UTE Versus Conventional Clinical MRI in the Knee Joint Quantitative UTE Imaging of the Knee Joint: Ex Vivo Evaluation Ex Vivo UTE-Based T2* (UTE-T2*) Imaging Ex Vivo UTE-Based T1ρ (UTE-T1ρ) Imaging Ex Vivo UTE Adiabatic T1ρ (UTE-AdiabT1ρ) Imaging Ex Vivo UTE-Magnetization Transfer (UTE-MT) Imaging Ex Vivo Evaluation of OA with a Panel of Quantitative UTE Biomarkers Quantitative UTE Imaging of the Knee Joint: In Vivo Evaluation In Vivo UTE-T2* Quantification of Knee Joint Degeneration In Vivo UTE-AdiabT1ρ Quantification of Knee Joint Degeneration In Vivo UTE-MT Imaging in Knee Joint Degeneration Morphological and Quantitative UTE Imaging: New Directions UTE Assessment of the Osteochondral Junction (OCJ) UTE Imaging for Monitoring Therapy and Recovery Conclusions Summary References 32: Bound Water and Pore Water in Osteoporosis Introduction Beyond Bone Mineral Density (BMD) 1H Nuclear Magnetic Resonance (NMR) Signals of the Cortical Bone UTE-MRI Imaging of BW and PW AIR and DAFP Imaging of BW and PW Technical Challenges and Solutions BW and PW as Surrogates of Cortical Bone Strength Determinants of BW and PW in Cortical Bone References 33: Porosity Index and Suppression Ratio in Osteoporosis Introduction Cortical T2 Species The Suppression Ratio The Porosity Index (PI) Conclusions References 34: A UTE-Based Biomarker Panel in Osteoporosis Introduction Cortical Bone Water Content Assessment Total Water Assessment with Basic UTE Bound Water Assessment with Inversion Recovery UTE Pore Water Assessment with Double Adiabatic Full-Passage (DAFP) UTE BW and PW Estimation Using UTE Signal Fractional Indexes BW and PW Assessment with Bicomponent Signal Modeling BW, PW, and Fat Content Assessment with Tri-component Signal Modeling Cortical Bone Organic Matrix Assessment Cortical Bone Mineral Assessment Quantitative Susceptibility Mapping (UTE-QSM) for Mineral Assessment UTE 31P Imaging for Assessment of Bone Minerals Trabecular Bone Quantification Conclusions References 35: UTE-MRI for Spinal Applications Introduction The Vertebral Cartilage Endplate (CEP) Endplate Assessment with UTE-MRI T2* Mapping Ligaments, Tendons, and Entheses The Facet Joints Bone The Sacroiliac Joints (SIJs) Technical Considerations Conclusions References 36: MRI of Tendinopathy Using Ultrashort TE (UTE) Sequences Introduction Imaging of Tendons Quantitative MRI in Tendinopathy An Example of UTE-MRI Implementation in a Clinical Trial of Patellar Tendinopathy Image Acquisition Image Preparation Image Analysis Results Limitations Future Perspectives References 37: Hemophilic Arthropathy Introduction Current MRI Techniques in HA Potential of UTE-MRI in HA UTE-T2* Mapping in HA UTE Quantitative Susceptibility Mapping (UTE-QSM) in HA Conclusions References 38: Rotator Cuff Injury Introduction Anatomy Rotator Cuff Tendinopathy MRI Challenges Recent MRI Advances Future Studies References 39: Ultrashort Echo Time Magnetic Resonance Imaging of the Temporomandibular Joint (TMJ) Introduction TMJ Anatomy TMJ MRI Morphological UTE Imaging Quantitative UTE Imaging UTE-T2* Imaging UTE-T1 Imaging UTE-T1ρ Imaging UTE-MT Imaging UTE-DESS Imaging Conclusions References 40: Ultrashort Echo Time Magnetic Resonance Imaging of Myelin in Multiple Sclerosis Introduction UTE Imaging of Myelin: Phantom Studies UTE Imaging of Myelin: Contrast Mechanisms Long-T2-Saturated UTE Off-Resonance Saturated UTE Inversion Recovery UTE (IR-UTE) Double Echo Sliding Inversion Recovery UTE (DESIRE-UTE) Short TR Adiabatic Inversion Recovery UTE (STAIR-UTE) Hybrid Filling Zero Echo Time (HYFI-ZTE) Inversion Recovery Interleaved Hybrid Encoding (IR-IHE) UTE Imaging of Myelin: Further Validation Animal Validation Study Cadaveric Human Brain Validation Study UTE Imaging of Myelin in Multiple Sclerosis Morphological UTE Imaging of Myelin in Multiple Sclerosis Quantitative UTE Imaging of Myelin in Multiple Sclerosis Conclusions References 41: Myelin Bilayer Imaging Introduction Background and Fundamental Concepts Myelin Function and Overview Role in CNS Disorders Composition Myelin MRI Indirect Myelin Imaging Techniques Myelin Bilayer Signal Properties The Question of Myelin Specificity Short-T2 Imaging Signal Capture Sequence RF Dead-Time Spatial Encoding and Data Acquisition Hardware Considerations The Gradient System Unwanted Signals RF Excitation BW Handling the RF Dead-Time Gap Myelin Bilayer Imaging Prospects Current State of Myelin Bilayer Imaging SPI: Signal Analysis and Quantitative Mapping ZTE: Myelin-Weighted Imaging Compromising Between Myelin Specificity and Technique Complexity Future Directions Early Stages Later Stages Possible Uses References 42: Lung Imaging with UTE-MRI Introduction Challenges and Opportunities for Magnetic Resonance Imaging (MRI) of the Lungs Why Use UTE for MRI of the Lungs? Methods for UTE Lung MRI Pulse Sequences The k-Space Trajectory Motion Management Image Reconstruction Contrast Mechanisms with UTE-MRI T1-Weighting Non-contrast Ventilation Oxygen-Enhanced Ventilation New Frontiers Conclusions Resources References 43: Quantification of Liver Iron Overload with UTE Imaging Introduction Assessment of Hepatic Iron Overload Emerging Role of Magnetic Resonance Imaging (MRI) in Noninvasive Iron Quantification Limitations of Current Biopsy Calibrations and MR Methods UTE Imaging for Iron Overload Assessment 2D Vs. 3D UTE 2D UTE 3D UTE R2* Quantification and Confounding Effects Limitations Summary and Conclusions References 44: CT-like Contrast for Bone Imaging with ZTE-MRI Introduction ZTE Image Acquisition and Post-processing Image Acquisition Coil Selection Sequence Parameters Slab Prescription Considerations and Field of View (FOV) Post-processing of ZTE Images Intensity Correction Contrast Inversion Background Segmentation Orthopedic Applications of ZTE-MRI The Cranium The Shoulder The Spine The Hip/Pelvis The Knee The Foot and Ankle Limitations of ZTE-MRI for the Bone Conclusions References 45: MR-Based Attenuation Correction in PET–MRI Introduction Attenuation Correction in PET MR Methods for Deriving Attenuation Correction Maps Segmentation of Tissues Segmentation of Tissues with Short T2*s Segmentation of Tissues with Short T2* from Water and Fat Soft Tissues System Imperfections Applications Discussion Conclusions References 46: Zero Acoustic Noise with Zero TE MRI Introduction Recap of ZTE: Quick and Quiet MRI with TE = 0 Native ZTE: Illuminating the MR Invisible ZTE Bone Imaging ZTE Lung Imaging Magnetization-Prepared ZTE: Beyond SPGR Contrast Inversion Recovery (IR)-Prepared ZTE Arterial Spin Labeling (ASL)-Prepared ZTE Other Preparations: T2, Diffusion, Magnetization Transfer (MT), and Fat Saturation Quantitative Parameter Mapping Looping Star: Silent, Time-Multiplexed, Gradient Echo ZTE Acoustic Noise Reduction via Gradient Attenuation Outlook and Future Prospects References 47: Current Status, Comparisons of Techniques, Challenges, and Future Directions for MRI of Short- and Ultrashort-T2 Tissues Introduction Other Contributions to MRI of Short- and Ultrashort-T2 Tissues Comparative Assessment of Techniques Data Acquisition Techniques Contrast Mechanisms and Their Clinical Applications Quantification Techniques and Their Clinical Applications Clinical Applications Challenges in MRI of Short- and Ultrashort-T2 Tissues The Future of MRI of Short- and Ultrashort-T2 Tissues General Aspects of Imaging General Aspects of MRI of Short- and Ultrashort-T2 Tissues Specific Future Directions for MRI of Short- and Ultrashort-T2 Tissues References Index