MRI Sequences

By MAAJID MOHI UD DIN MALIK Radiology Hub -October 15, 2024

0

Comprehensive Guide to MRI Sequences

1. Introduction to MRI Sequences

MRI sequences are specific combinations of radiofrequency (RF) pulses and magnetic field gradients designed to generate specific tissue contrasts and highlight particular anatomical or functional features. The choice of sequence profoundly impacts image quality, acquisition time, and diagnostic information.

1.1 Basic Principles

TR (Repetition Time): Time between consecutive excitation pulses.
TE (Echo Time): Time between excitation pulse and signal readout.
Flip Angle: Angle through which the net magnetization is rotated by the RF pulse.
k-space: The raw data space in which MRI signals are acquired before image reconstruction.

1.2 Sequence Components

RF Pulses: Excitation, refocusing, and inversion pulses.
Gradients: Slice selection, phase encoding, and frequency encoding.
Signal Readout: Free Induction Decay (FID) or echo formation.

2. Spin Echo Sequences

Spin Echo (SE) sequences are fundamental in MRI, known for their robustness against field inhomogeneities and ability to produce high-quality T1, T2, and proton density-weighted images.

2.1 Basic Spin Echo

Consists of a 90° excitation pulse followed by one or more 180° refocusing pulses.

Sequence: 90° - TE/2 - 180° - TE/2 - Echo
Contrast: Determined by TR and TE
T1-weighted: Short TR, short TE
T2-weighted: Long TR, long TE
Proton Density-weighted: Long TR, short TE

2.2 Fast Spin Echo (FSE) / Turbo Spin Echo (TSE)

Multiple 180° pulses follow each 90° pulse, acquiring multiple echoes per TR.

Advantages: Faster acquisition, reduced motion artifacts
Considerations: Increased SAR, potential blurring
Applications: T2-weighted imaging of brain, spine, joints

2.3 Single-Shot Fast Spin Echo (SSFSE)

Acquires all k-space lines following a single excitation.

Advantages: Very fast acquisition, motion-insensitive
Limitations: Lower spatial resolution, T2 blurring
Applications: Fetal imaging, MRCP

3. Gradient Echo Sequences

Gradient Echo (GRE) sequences use gradient reversal to form echoes, allowing for faster imaging and unique contrast mechanisms.

3.1 Spoiled Gradient Echo

Residual transverse magnetization is "spoiled" before each RF pulse.

Examples: FLASH, SPGR, T1-FFE
Contrast: T1-weighted with some T2* influence
Applications: 3D imaging, contrast-enhanced MRA

3.2 Balanced Steady-State Free Precession (bSSFP)

Maintains both longitudinal and transverse steady-state.

Examples: TrueFISP, FIESTA, balanced-FFE
Contrast: T2/T1 ratio
Advantages: High SNR, fast acquisition
Limitations: Banding artifacts, sensitivity to off-resonance
Applications: Cardiac imaging, CSF flow studies

3.3 Echo Planar Imaging (EPI)

Ultra-fast imaging technique acquiring multiple k-space lines per excitation.

Types: Single-shot EPI, multi-shot EPI
Advantages: Very fast acquisition (ms range)
Limitations: Susceptibility to artifacts, geometric distortions
Applications: Diffusion-weighted imaging, fMRI, perfusion imaging

4. Inversion Recovery Sequences

Inversion Recovery (IR) sequences use an initial 180° inversion pulse to enhance contrast and suppress specific tissues.

4.1 Short TI Inversion Recovery (STIR)

Purpose: Fat suppression
Principle: TI chosen to null fat signal (approx. 150-180ms at 1.5T)
Applications: Musculoskeletal imaging, oncology

4.2 Fluid-Attenuated Inversion Recovery (FLAIR)

Purpose: CSF signal suppression
Principle: Long TI to null CSF (approx. 2000-2500ms at 1.5T)
Applications: Brain imaging, multiple sclerosis lesion detection

4.3 Double Inversion Recovery (DIR)

Purpose: Simultaneous suppression of two tissues (e.g., CSF and white matter)
Applications: Cortical lesion detection in MS, epilepsy

5. Advanced and Specialized Sequences

5.1 Diffusion-Weighted Imaging (DWI)

Principle: Measures water molecule diffusion
Key parameter: b-value (determines diffusion weighting)
Applications: Stroke diagnosis, tumor characterization
Advanced techniques: DTI, HARDI, q-space imaging

5.2 Perfusion Imaging

Dynamic Susceptibility Contrast (DSC): T2*-weighted imaging of first-pass contrast
Dynamic Contrast Enhanced (DCE): T1-weighted imaging of contrast kinetics
Arterial Spin Labeling (ASL): Non-contrast perfusion using magnetically labeled blood
Applications: Tumor perfusion, stroke, neurodegenerative diseases

5.3 Magnetic Resonance Spectroscopy (MRS)

Principle: Measures metabolite concentrations based on chemical shift
Types: Single voxel, multi-voxel (CSI)
Key metabolites: NAA, Choline, Creatine, Lactate
Applications: Brain tumors, metabolic disorders, prostate cancer

5.4 Functional MRI (fMRI)

Principle: Detects BOLD (Blood Oxygen Level Dependent) signal changes
Sequence: Typically T2*-weighted EPI
Applications: Brain activation studies, pre-surgical planning

5.5 Non-Cartesian Sequences

Radial: Center-out k-space trajectories, motion-resistant
Spiral: Efficient k-space coverage, reduced flow artifacts
PROPELLER/BLADE: Rotating k-space strips, motion correction

6. Contrast Mechanisms and Tissue Characterization

6.1 Intrinsic Contrast Mechanisms

T1 contrast: Based on longitudinal relaxation times
T2 contrast: Based on transverse relaxation times
T2* contrast: Includes effects of local field inhomogeneities
Proton density: Based on concentration of MR-visible protons

6.2 Extrinsic Contrast Mechanisms

Gadolinium-based contrast agents: Shortens T1 relaxation time
Superparamagnetic iron oxide (SPIO): Creates local field inhomogeneities, T2* effects
Chemical Exchange Saturation Transfer (CEST): Detects specific metabolites through saturation transfer

6.3 Quantitative MRI

T1 mapping: Inversion recovery, variable flip angle methods
T2 mapping: Multi-echo spin echo sequences
T2* mapping: Multi-echo gradient echo sequences
Diffusion mapping: ADC maps, tensor fitting for DTI

7. Artifacts and Their Mitigation

7.1 Motion Artifacts

Causes: Patient movement, respiratory motion, cardiac pulsation
Mitigation: Motion correction techniques, gating, fast sequences

7.2 Susceptibility Artifacts

Causes: Local magnetic field inhomogeneities
Affected sequences: Gradient echo, EPI
Mitigation: Shorter TE, higher bandwidth, advanced shimming

7.3 Chemical Shift Artifacts

Causes: Difference in resonance frequency between fat and water
Mitigation: Fat suppression techniques, in/opposed phase imaging

7.4 Aliasing (Wrap-around) Artifacts

Causes: Field of view smaller than imaged object
Mitigation: Larger FOV, phase oversampling, saturation bands

Mitigation: Larger FOV, phase oversampling, saturation bands

7.5 Gibbs Ringing Artifacts

Causes: Truncation of k-space data
Appearance: Parallel lines near sharp intensity transitions
Mitigation: Higher spatial resolution, post-processing filters

7.6 Zipper Artifacts

Causes: RF interference from external sources
Mitigation: Improved RF shielding, identifying and removing interference sources

8. Sequence Optimization and Protocol Design

8.1 Signal-to-Noise Ratio (SNR) Optimization

Factors affecting SNR: Field strength, coil selection, voxel size, bandwidth
Trade-offs: SNR vs. spatial resolution, SNR vs. scan time
Techniques: Signal averaging, optimized RF coil design

8.2 Contrast-to-Noise Ratio (CNR) Optimization

Adjusting sequence parameters: TR, TE, TI, flip angle
Contrast enhancement: Use of exogenous contrast agents
Advanced techniques: Magnetization transfer, dixon techniques

8.3 Spatial Resolution Considerations

Factors: Matrix size, field of view (FOV), slice thickness
Trade-offs: Resolution vs. scan time, resolution vs. SNR
Techniques: Partial Fourier, parallel imaging

8.4 Temporal Resolution and Dynamic Imaging

Fast imaging techniques: EPI, FLASH, balanced SSFP
Acceleration methods: Parallel imaging, compressed sensing
Applications: Cardiac imaging, fMRI, perfusion studies

8.5 SAR Management

Factors affecting SAR: RF power, duty cycle, patient size
Strategies: Reduced flip angles, longer TR, parallel transmission
Considerations: High field imaging, pediatric protocols

8.6 Protocol Design Principles

Clinical question: Tailoring sequences to specific diagnostic needs
Patient factors: Age, compliance, implants
Time constraints: Balancing comprehensiveness with practicality
Standardization: Ensuring consistency across examinations and centers

9. Emerging Trends in MRI Sequences

9.1 Artificial Intelligence in Sequence Optimization

AI-driven protocol selection: Automated choice of optimal sequences based on clinical indication
Real-time sequence adaptation: Dynamic adjustment of parameters during scanning
Deep learning reconstruction: Improving image quality and reducing scan times

9.2 Multi-parametric Imaging

Simultaneous multi-contrast acquisition: e.g., MR Fingerprinting
Synthetic contrasts: Generating multiple contrasts from a single acquisition
Integrated quantitative mapping: Combining relaxometry, diffusion, and perfusion in a single scan

9.3 Ultra-fast Imaging

Advanced k-space trajectories: Spiral, radial, wave-CAIPI
Simultaneous multi-slice imaging: Accelerated 2D and 3D acquisitions
Sub-second 3D imaging: Whole-brain coverage in cardiac and respiratory cycles

9.4 Motion-robust Sequences

Self-navigated sequences: Intrinsic motion tracking without external devices
Prospective motion correction: Real-time adjustment of gradients and RF
Free-breathing techniques: Respiratory-resolved 3D imaging

9.5 Advanced Diffusion Techniques

Multi-shell diffusion: Improved characterization of tissue microstructure
Oscillating gradient spin echo (OGSE): Probing short diffusion time scales
Diffusion-relaxation correlation spectroscopy: Joint analysis of diffusion and relaxation properties

9.6 Novel Contrast Mechanisms

Susceptibility-based imaging: QSM, SWI advancements
Exchange-sensitive contrasts: CEST, MT imaging developments
Functional contrast beyond BOLD: Diffusion fMRI, molecular fMRI

9.7 MRI-guided Interventions

Real-time sequence adaptation: Dynamic updating of imaging plane and contrast
Hybrid systems: Integration of MRI with therapeutic modalities (e.g., HIFU, radiotherapy)
Interventional-friendly sequences: Balancing speed, contrast, and artifacts for procedural guidance

Conclusion

MRI sequences form the backbone of magnetic resonance imaging, enabling a vast array of diagnostic and research applications. From the fundamental spin echo to cutting-edge artificial intelligence-driven techniques, the field of MRI sequences continues to evolve, pushing the boundaries of spatial resolution, temporal resolution, and tissue characterization. As we look to the future, emerging trends promise even greater capabilities, with potential for more personalized, efficient, and informative MRI examinations. Understanding the principles, optimizations, and emerging trends in MRI sequences is crucial for radiologists, medical physicists, and researchers to fully leverage the power of this remarkable imaging modality.

Tags: Physics of Medical Imaging