MRI in practice

Description

MRI in Practice Contents Preface to the fifth edition Acknowledgments Acronyms Nomenclature About the Companion Website 1 Basic principles Introduction Atomic structure Motion in the atom MR-active nuclei The hydrogen nucleus Alignment Net magnetic vector (NMV) Precession and precessional (Larmor) frequency Precessional phase Resonance The results of resonance – classical theory The results of resonance – quantum theory MR signal The free induction decay (FID) signal Pulse timing parameters References 2 Image weighting and contrast Introduction Image contrast Relaxation T1 recovery T2 decay Contrast mechanisms Relaxation in different tissues Fat and water T1 recovery in fat T1 recovery in water T2 decay in fat T2 decay in water T1 contrast T2 contrast Proton density contrast Weighting T1 weighting T2 weighting Other contrast mechanisms Diffusion-weighted imaging (DWI) Functional MRI Magnetization transfer contrast Susceptibility weighting (SWI) Contrast agents References 3 Spin-echo pulse sequences Introduction RF rephasing Conventional spin-echo Mechanism Spin-echo using one echo Spin-echo using two echoes Uses Suggested parameters Fast or turbo spin-echo (FSE/TSE) Mechanism Weighting in TSE Uses Suggested parameters Single-shot turbo spin-echo (SS-TSE) Driven equilibrium Inversion recovery (IR) Mechanism Uses Suggested parameters Fast inversion recovery Short tau inversion recovery (STIR) Mechanism Uses Suggested parameters Fluid attenuated inversion recovery (FLAIR) Mechanism Uses Suggested parameters IR prep sequences References 4 Gradient-echo pulse sequences Introduction Variable flip angle Gradient rephasing How gradients dephase How gradients rephase Weighting in gradient-echo pulse sequences Weighting mechanism 1 – extrinsic contrast parameters Weighting mechanism 2 – the steady state Weighting mechanism 3 – residual transverse magnetization Coherent or rewound gradient-echo Mechanism Uses Suggested parameters Incoherent or spoiled gradient-echo Mechanism Uses Suggested parameters Reverse-echo gradient-echo Mechanism Uses Suggested parameters Balanced gradient-echo Mechanism Uses Suggested parameters Fast gradient-echo Echo planar imaging Uses and limitations References 5 Spatial encoding Introduction Mechanism of gradients Gradient axes Slice-selection How does it work? When does slice-selection occur? Slice thickness and slice-selection Frequency encoding How does it work? When does frequency encoding occur? FOV and frequency encoding Phase encoding How does it work? When does phase encoding occur? Phase matrix, phase resolution, and phase encoding Bringing it all together – pulse sequence timing References 6 k-Space Introduction Part 1: What is k-space? Part 2: How are data acquired and how are images created from these data? Sampling Receive bandwidth Sampling window (sampling time) Fast Fourier transform (FFT) Part 3: Some important facts about k-space! Fact 1: k-Space is not the image Fact 2: Data are symmetrical in k-space Fact 3: Data acquired in the central lines contribute signal and contrast, while data acquired in the outer lines contribute resolution Fact 4: The scan time is the time to fill k-space Fact 5: The incremental step between each data point in k-space determines the FOV Fact 6: The dimensions of k-space determine pixel size Part 4: How do pulse sequences fill k-space? Part 5: Options that fill k-space Partial, fractional averaging, or half Fourier Partial echo Parallel imaging Single-shot Spiral k-space filling Propeller or radial k-space filling Sequential and 3D (volume) acquisition References 7 Protocol optimization Introduction Signal-to-noise ratio (SNR) Magnetic field strength Proton density Type of coil TR, TE, and flip angle Number of signal averages (NSA or NEX) Receive bandwidth Voxel volume Contrast-to-noise ratio (CNR) Presaturation Spatial resolution Rectangular FOV Scan time Trade-offs Protocol development and modification References 8 Artifacts Introduction Phase mismapping Appearance Cause Remedy Aliasing Appearance Cause Remedy Chemical shift artifact Appearance Cause Remedy Out-of-phase signal cancellation Appearance Cause Remedy Magnetic susceptibility artifact Appearance Cause Remedy Truncation artifact Appearance Cause Remedy Cross-excitation/cross-talk Appearance Cause Remedy Zipper artifact Appearance Cause Remedy Shading artifact Appearance Cause Remedy Moiré artifact Appearance Magic angle Appearance Cause Remedy Equipment faults Flow artifacts Entry-slice phenomenon (ESP) Time-of-flight (TOF) phenomenon Flow artifact remedies Flow-dependent (non-contrast-enhanced) angiography Inflow MRA 2D sequential inflow angiograms 3D volumetric inflow angiograms Black-blood imaging Phase-contrast MRA ECG-triggered 3D FSE (flow-spoiled fresh blood imaging) MRA summary References 9 Instrumentation Introduction Magnetism Diamagnetism Paramagnetism Ferromagnetism Scanner configurations Closed-bore systems Open systems Extremity systems Magnet system Permanent magnets Resistive electromagnets Superconducting electromagnets Solenoid magnets Ramping a magnet Field strength (flux density) Magnet shielding Passive shielding Active shielding Shim system Homogeneity Passive shimming Active shimming Gradient offset (dynamic) shimming Gradient system Gradient coil Gradient amplifiers Gradient characteristics Gradient amplitude Gradient rise time Gradient slew rate Gradient power duty cycle Acoustic gradient noise Balanced gradients RF system RF shielding RF transmit system RF transmit coils RF receive system RF receive coils Patient transport system Computer system and graphical user interface References 10 MRI safety Introduction (and disclaimer) Definitions used in MRI safety Safety zones Personnel Device safety Psychological effects The spatially varying static field Transient biological effects Projectile hazards Torque on implanted devices Foreign bodies in the static field Electromagnetic (radiofrequency) fields Heating The antenna effect Induced currents in implanted devices Time-varying gradient magnetic fields Nerve stimulation Effect of time-varying gradients on implanted devices Acoustic noise Cryogens Thermal sensitivity Asphyxia Quench Explosion Safety tips Additional resources References Glossary Index EULA