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In this chapter, we review the principles and mechanisms behind various MRI pulse sequences, focusing on their role in manipulating proton spin to produce diagnostic images. It begins with the single-pulse free induction decay (FID) sequence, describing how a radiofrequency (RF) pulse tips the net magnetization vector from the longitudinal (Mz) to the transverse (MT) plane. The FID signal, oscillating at the Larmor frequency and decaying via T2* relaxation, results from proton dephasing due to magnetic field inhomogeneities. This dephasing reduces phase coherence, leading to signal loss, while the transverse magnetization induces a current in the receiver coil according to Faraday’s law. We also review spoiled gradient echo (GRE) sequences-based methods, which nullify residual transverse magnetization through RF spoiling or gradient spoiling, enabling the acquisition of high-contrast T1-weighted, T2*-weighted, or spin density images. Parameter selection for repetition time (TR), echo time (TE), and flip angle (α) determines the image weighting, with specific configurations enhancing diagnostic utility across different tissue types. The chapter also discusses steady-state free precession (SSFP), where transverse magnetization persists between successive RF pulses, producing continuous echoes. Techniques such as GRASS, FISP, and TrueFISP maintain coherence through gradient balancing and phase-alternating RF pulses. Balanced SSFP (b-SSFP) further refines image quality by minimizing gradient-induced dephasing, making it valuable for imaging blood vessels and cerebrospinal fluid. Advanced techniques like ultrashort echo time (UTE) and zero echo time (ZTE) address tissues with very short T2 relaxation times by capturing signals almost immediately after RF excitation, improving imaging of bones and lungs. Lastly, dual- or multi-echo T2 relaxation* explores sequences that capture multiple echoes for T2* quantification, enabling precise tissue characterization and water-fat separation through chemical shift encoding. Collectively, these techniques highlight the adaptability and precision of MRI pulse sequences in addressing diverse imaging challenges across clinical and research applications.

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Spin Manipulations by Pulse Sequences

  • 章节

  • 发表日期：

    2025年

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  • Christiane Sarah Burton,
  • Suraj D. Serai

摘要

In this chapter, we review the principles and mechanisms behind various MRI pulse sequences, focusing on their role in manipulating proton spin to produce diagnostic images. It begins with the single-pulse free induction decay (FID) sequence, describing how a radiofrequency (RF) pulse tips the net magnetization vector from the longitudinal (Mz) to the transverse (MT) plane. The FID signal, oscillating at the Larmor frequency and decaying via T2* relaxation, results from proton dephasing due to magnetic field inhomogeneities. This dephasing reduces phase coherence, leading to signal loss, while the transverse magnetization induces a current in the receiver coil according to Faraday’s law. We also review spoiled gradient echo (GRE) sequences-based methods, which nullify residual transverse magnetization through RF spoiling or gradient spoiling, enabling the acquisition of high-contrast T1-weighted, T2*-weighted, or spin density images. Parameter selection for repetition time (TR), echo time (TE), and flip angle (α) determines the image weighting, with specific configurations enhancing diagnostic utility across different tissue types. The chapter also discusses steady-state free precession (SSFP), where transverse magnetization persists between successive RF pulses, producing continuous echoes. Techniques such as GRASS, FISP, and TrueFISP maintain coherence through gradient balancing and phase-alternating RF pulses. Balanced SSFP (b-SSFP) further refines image quality by minimizing gradient-induced dephasing, making it valuable for imaging blood vessels and cerebrospinal fluid. Advanced techniques like ultrashort echo time (UTE) and zero echo time (ZTE) address tissues with very short T2 relaxation times by capturing signals almost immediately after RF excitation, improving imaging of bones and lungs. Lastly, dual- or multi-echo T2 relaxation* explores sequences that capture multiple echoes for T2* quantification, enabling precise tissue characterization and water-fat separation through chemical shift encoding. Collectively, these techniques highlight the adaptability and precision of MRI pulse sequences in addressing diverse imaging challenges across clinical and research applications.