MRI Pulse Sequences Overview
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Questions and Answers

What does the pulse sequence primarily determine in MRI?

  • The length of the imaging session
  • The size of the MRI machine
  • The timing and interaction of RF pulses and gradients (correct)
  • The patient's heart rate

    • Which of the following is NOT a type of pulse sequence?

  • Phase Contrast Sequence (PCS) (correct)
  • Gradient Echo Sequence (GRE)
  • Spin-Echo Sequence (SE)
  • Inversion Recovery Sequence (IR)

    • What is the purpose of the 180-degree RF pulse in a spin echo sequence?

  • To shift the net magnetization along the Z-axis
  • To reduce the amount of free induction decay
  • To rephase dephased protons (correct)
  • To initiate the echo signal

    • What does the term 'Time to Repeat' (TR) refer to in a spin echo pulse sequence?

    <p>The time interval between two 90-degree pulses</p> Signup and view all the answers

    Which gradient is activated during the reception of the echo signal in a spin echo sequence?

    <p>Phase encoding gradient</p> Signup and view all the answers

    Why is the initial signal called free induction decay (FID) considered weak?

    <p>It results from dephasing of protons over time</p> Signup and view all the answers

    Which sequence is primarily used with gradient echo sequences?

    <p>Echo Planar Imaging (EPI)</p> Signup and view all the answers

    What happens to the transverse magnetization (TM) after protons start to dephase?

    <p>It gradually reduces</p> Signup and view all the answers

    What characterizes the steady-state or coherent GRE sequences?

    <p>They allow coexistence of both longitudinal and transverse magnetization.</p> Signup and view all the answers

    Which flip angle range is most commonly associated with steady-state GRE sequences?

    <p>30° to 45°</p> Signup and view all the answers

    What is the primary effect of the Incoherent (Spoiled) Gradient Echo sequences?

    <p>They increase T1 weighting.</p> Signup and view all the answers

    What role does the 180-degree inversion pulse play in an IR sequence?

    <p>It saturates all tissues before recovery.</p> Signup and view all the answers

    How does the time to invert (TI) influence images in an Inversion Recovery sequence?

    <p>It is the primary determinant of contrast in IR sequences.</p> Signup and view all the answers

    Which process most directly follows the application of the 180-degree pulse in an IR sequence?

    <p>Gradual recovery of longitudinal magnetization.</p> Signup and view all the answers

    Which type of GRE sequence is typically used to attain more T2 weighting?

    <p>Steady State Free Precession (SSFP).</p> Signup and view all the answers

    Why are SS GRE sequences advantageous for studying rapid physiological processes?

    <p>They have very short TR and TE, allowing faster imaging.</p> Signup and view all the answers

    What is the primary purpose of the inversion recovery (IR) sequence in MRI?

    <p>To suppress signals from certain tissues</p> Signup and view all the answers

    What happens if a 90-degree excitatory pulse is applied at the halfway stage of recovery after a 180-degree inversion pulse?

    <p>No signal is received from the tissue</p> Signup and view all the answers

    What is the TI value required to null the signal from a tissue in an IR sequence?

    <p>$0.69 imes T1$ relaxation time of that tissue</p> Signup and view all the answers

    Which of the following sequences is used specifically for suppressing fat signals?

    <p>STIR</p> Signup and view all the answers

    In the A-STIR sequence, what is the typical TI value used to suppress fat?

    <p>100-200 ms</p> Signup and view all the answers

    What is the purpose of the B-FLAIR sequence in MRI?

    <p>To suppress the signal from CSF</p> Signup and view all the answers

    What advantage does 4-Echo Planar Imaging (EPI) provide in MRI scanning?

    <p>It significantly decreases scanning time</p> Signup and view all the answers

    What characterizes single shot EPI (SS-EPI) in MRI?

    <p>All lines of K-Space are filled in a single TR</p> Signup and view all the answers

    What is the purpose of the dual spin-echo sequence in MRI?

    <p>To obtain two echoes per TR</p> Signup and view all the answers

    How are T2-weighted images typically characterized in MRI?

    <p>They display bright signals for edematous tissues</p> Signup and view all the answers

    What defines the turbo factor in fast spin-echo sequences?

    <p>The number of 180-degree pulses after each 90-degree pulse</p> Signup and view all the answers

    What occurs during the fast (turbo) spin-echo sequence?

    <p>Multiple echoes fill different K-Space lines</p> Signup and view all the answers

    What effect does a short turbo factor have on effective TE in a fast spin-echo sequence?

    <p>It decreases effective TE</p> Signup and view all the answers

    In the context of the dual spin-echo sequence, what type of image does a long TR with short TE yield?

    <p>Proton density weighted image</p> Signup and view all the answers

    Why does a fast spin-echo sequence increase scanning speed?

    <p>It allows multiple echoes to fill K-Space</p> Signup and view all the answers

    Which characteristic is true for T1-weighted images?

    <p>They are useful for anatomical details</p> Signup and view all the answers

    What is the primary characteristic of the single-shot fast spin-echo sequence?

    <p>All echoes needed to form an image are acquired in a single TR.</p> Signup and view all the answers

    How does the T2 relaxation in the gradient echo (GRE) sequence differ from that of the spin echo (SE) sequence?

    <p>T2 relaxation in GRE is referred to as T2* relaxation.</p> Signup and view all the answers

    What is one of the main differences between the spin echo (SE) and gradient echo (GRE) sequences?

    <p>GRE sequences do not use a 180-degree pulse for rephasing.</p> Signup and view all the answers

    What advantage does the use of a smaller flip angle in GRE provide?

    <p>Shorter scanning time due to early recovery of longitudinal magnetization.</p> Signup and view all the answers

    What happens to the residual transverse magnetization (TM) in spoiled or incoherent GRE sequences?

    <p>It is destroyed, preventing interference with the next TR.</p> Signup and view all the answers

    Why does T2* relaxation occur in GRE sequences?

    <p>It results from irreversible dephasing and magnetic field inhomogeneity.</p> Signup and view all the answers

    What is the effect of half-Fourier transformation in the single-shot fast spin-echo sequence?

    <p>It allows for the mathematical calculation of the remaining K-space.</p> Signup and view all the answers

    What is the role of the frequency encoding gradient in a GRE sequence?

    <p>It completes the rephasing of TM.</p> Signup and view all the answers

    What is the primary advantage of GE-EPI over SE-EPI?

    <p>It is faster in capturing images.</p> Signup and view all the answers

    Which imaging technique is primarily used to demonstrate areas of restricted diffusion?

    <p>Diffusion Weighted Imaging (DWI)</p> Signup and view all the answers

    What does MRA allow us to visualize effectively?

    <p>Blood flow in arteries and veins.</p> Signup and view all the answers

    In what scenario would Phase Contrast MRA be preferred over Time of Flight MRA?

    <p>In cases with high background tissue signal.</p> Signup and view all the answers

    What is the key feature of the Perfusion Weighted Imaging (PWI) technique?

    <p>It captures dynamic images during contrast uptake.</p> Signup and view all the answers

    What is the primary purpose of Functional MRI (fMRI)?

    <p>To visualize metabolic changes during activities.</p> Signup and view all the answers

    Which statement accurately describes Magnetization Transfer (MT) Contrast?

    <p>It enhances clarity of blood vessels.</p> Signup and view all the answers

    Why is Diffusion Weighted Imaging (DWI) valuable in stroke evaluation?

    <p>It differentiates between salvageable and non-salvageable tissue.</p> Signup and view all the answers

    Study Notes

    Pulse Sequences and Image Contrast

    • Pulse sequence is an interplay of parameters with RF pulses and gradients to form MRI images.
    • It's a time chart of:
      • Patient's net longitudinal magnetization.
      • Transmission of RF pulses (90, 180 degrees, etc).
      • X, Y, and Z gradient activation for localization.
      • K-Space filling with acquired signals/echoes.
    • Figure 1 shows steps in image acquisition: patient -> pulse sequence -> data acquisition -> stored echoes -> K-Space -> MRI image.

    Classification

    • Pulse sequences are broadly divided into spin-echo and gradient-echo sequences.
    • Inversion recovery and echo planar imaging (EPI) are applicable to both.
    • Practically, four sequences are considered:
      • Spin-echo (SE)
      • Gradient-echo (GRE)
      • Inversion recovery (IR)
      • Echo planar imaging (EPI)

    Spin Echo (SE)

    • SE sequence uses 90° and 180° RF pulses.
    • 90° pulse flips magnetization into the transverse plane.
    • Precessing magnetization induces a small signal (FID).
    • 180° pulse rephases protons to increase signal strength (spin echo).
    • TR (Time to Repeat) is the time between 90° pulses.
    • TE (Time to Echo) is the time between the 90° pulse and echo reception.
    • Slice selection, phase encoding, and frequency encoding gradients are used for signal localization.

    Modifications of SE Sequences

    • Conventional SE fills one line of K-Space per TR.
    • Dual Spin-echo sequences send two 180° pulses after a 90° pulse, creating two echoes per TR.
    • This can provide proton density (PD) and T2-weighted images.

    FAST (Turbo) Spin-Echo

    • Multiple 180° pulses are sent after the 90° pulse to obtain more echoes per TR.
    • Filling K-Space is faster.
    • Turbo factor is the number of 180° pulses.
    • The amplitude of the echo and TE increase with higher turbo factor.

    Single-Shot Fast Spin-Echo

    • All echoes are acquired in a single TR.
    • Only half of the K-Space is filled, and the other half is calculated mathematically.
    • This significantly reduces scanning time.

    Gradient Echo (GRE)

    • No 180° pulse is used.
    • Rephasing is achieved using gradients, especially frequency encoding.
    • Flip angles are typically less than 90°.
      • This reduces the scanning time due to faster longitudinal magnetization recovery.
    • Two types of GRE sequences:
      • Spoiled (incoherent): residual TM is destroyed for each TR.
      • Steady-state (coherent): residual TM is refocused.

    Inversion Recovery (IR)

    • An inverting 180° pulse is used before a typical spin-echo pulse.
    • This completely saturates tissues initially.
    • Longitudinal magnetization slowly recovers.
    • Tissues with different T1 values recover at different rates.
    • TI (time to invert) is crucial for contrast in IR sequences.
    • Types of IR Sequences:
      • STIR (short inversion recovery): To suppress fat signal.
      • FLAIR (fluid attenuated inversion recovery): To suppress CSF signal.

    Echo Planar Imaging (EPI)

    • Scanning time is reduced by filling multiple lines of K-Space in a single TR.
    • Single-shot EPI (SS-EPI) fills all lines of K-Space in a single TR.
    • Spin-echo EPI (SE-EPI) uses multiple 180° pulses to generate echoes.
    • Gradient-echo EPI (GE-EPI) uses gradients for rephasing.
    • EPI is faster than SE-EPI.

    Some Examples for EPI Sequences

    • Perfusion-weighted imaging (PWI): studies contrast uptake to analyze brain, pancreas, liver, and prostate abnormalities.
    • Diffusion-weighted imaging (DWI): uses GRE or EPI to visualize areas with restricted diffusion (e.g., stroke).
    • Functional MRI (fMRI): measures brain activity by comparing images during stimulus and rest.
    • Magnetization transfer (MT) contrast: enhances conspicuity of vessels by suppressing background tissue.
    • Magnetic Resonance Angiography (MRA): assesses blood vessels with higher signal from flowing nuclei.
    • Time-of-Flight (TOF-MRA) and Phase Contrast (PC-MRA): specific MRA techniques.

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    Pulse Sequences and Image Contrast (MRI) PDF

    Description

    Explore the fundamentals of pulse sequences in MRI, including their classification and image contrast. This quiz covers key concepts such as spin-echo, gradient-echo, inversion recovery, and echo planar imaging. Test your understanding of how RF pulses and gradients work together to create MRI images.

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