MRI Pulse Sequences and Image Contrast
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MRI Pulse Sequences and Image Contrast

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Questions and Answers

What is the main purpose of the 90-degree RF pulse in a spin echo sequence?

  • To induce free induction decay (FID)
  • To flip the net magnetization vector into the transverse plane (correct)
  • To strengthen the magnetization signal
  • To activate the slice selection gradient

    • What does TR stand for in pulse sequences?

  • Time to Recover
  • Time to Rephase
  • Time to Rotate
  • Time to Repeat (correct)

    • Which pulse sequence is primarily associated with inversion recovery?

  • Gradient Echo
  • Fast Spin Echo
  • Spin Echo (correct)
  • Echo Planar Imaging

    • What is FID in the context of spin echo sequence?

    <p>A small signal induced by transverse magnetization</p> Signup and view all the answers

    Which of the following is NOT a component of a pulse sequence?

    <p>Patient's body temperature monitoring</p> Signup and view all the answers

    What is the role of the 180-degree pulse in the spin echo sequence?

    <p>To enhance the magnitude of transverse magnetization</p> Signup and view all the answers

    In the context of image contrast, which pulse sequences primarily utilize gradient echo?

    <p>Echo Planar Imaging sequences</p> Signup and view all the answers

    What is the effect of dephasing protons in the spin echo sequence?

    <p>It reduces the amount of transverse magnetization</p> Signup and view all the answers

    What is a characteristic of T1-weighted images?

    <p>They are useful for demonstrating anatomy.</p> Signup and view all the answers

    What happens during the DUAL SPIN-ECHO Sequence?

    <p>Two 180-degree pulses are sent after each 90-degree pulse.</p> Signup and view all the answers

    What is the effect of a short turbo factor in the FAST (TURBO) SPIN-ECHO Sequence?

    <p>Decreases effective TE and increases T1 weighting.</p> Signup and view all the answers

    How does the FAST (TURBO) SPIN-ECHO Sequence improve scanning speed?

    <p>By obtaining multiple echoes per TR.</p> Signup and view all the answers

    What is the turbo factor in the context of the FAST (TURBO) SPIN-ECHO Sequence?

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

    What causes T2-weighted images to appear bright?

    <p>Diseased tissues that are often more edematous.</p> Signup and view all the answers

    What is the significance of TE effective in the FAST (TURBO) SPIN-ECHO Sequence?

    <p>It is the TE at which the center of K-Space is filled.</p> Signup and view all the answers

    What is the primary use of conventional Spin Echo (SE) sequences?

    <p>To serve as the basis for understanding all other sequences.</p> Signup and view all the answers

    What characterizes the steady state in Steady State Coherent GRE sequences?

    <p>Coexistence of both longitudinal and transverse magnetization</p> Signup and view all the answers

    What is the typical range of TR values that favor steady state in GRE sequences?

    <p>20 to 50 ms</p> Signup and view all the answers

    What effect does the Incoherent (Spoiled) Gradient Echo pulse sequence have on residual transverse magnetization?

    <p>Minimizes its effect on image contrast</p> Signup and view all the answers

    What is a primary advantage of using SSFP as a type of SS Coherent GRE sequence?

    <p>It achieves more T2 weighting</p> Signup and view all the answers

    What is the purpose of the inverting 180-degree pulse in the Inversion Recovery (IR) sequence?

    <p>To saturate all tissues and allow for varied LM recovery</p> Signup and view all the answers

    Which of the following statements about TI in IR sequences is correct?

    <p>TI is the main determinant of contrast in IR sequences</p> Signup and view all the answers

    Which tissue type recovers faster in the Inversion Recovery (IR) sequence?

    <p>Protons in fat</p> Signup and view all the answers

    What type of imaging process can be effectively studied using SS GRE sequences?

    <p>Rapid physiologic processes such as heart cycles</p> Signup and view all the answers

    What term is used for the imaging sequence generated by multiple 180° pulses?

    <p>Spin Echo Echo Planar Imaging</p> Signup and view all the answers

    Which technique is primarily used to demonstrate areas with restricted diffusion in MR imaging?

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

    What is the primary advantage of using GE-EPI and SS-EPI compared to SE-EPI?

    <p>Increased speed of imaging</p> Signup and view all the answers

    Which MR imaging technique allows visualization of blood vessels with high signal from flowing nuclei?

    <p>Magnetic Resonance Angiography</p> Signup and view all the answers

    Which of the following describes the use of Time of Flight MRA?

    <p>Demonstrates arterial and venous flow</p> Signup and view all the answers

    What is a key characteristic of Functional MRI (fMRI)?

    <p>It can demonstrate brain activity during rest.</p> Signup and view all the answers

    Which EPI technique would be most effective for evaluating lesions in the brain, pancreas, or liver?

    <p>Perfusion Weighted Imaging</p> Signup and view all the answers

    What is one of the main disadvantages of Phase Contrast MRA compared to other techniques?

    <p>It usually requires longer scan times.</p> Signup and view all the answers

    What is the primary benefit of using a single-shot FAST spin-echo sequence?

    <p>It reduces the scan time by acquiring all echoes in a single TR.</p> Signup and view all the answers

    In a Gradient Echo (GRE) sequence, what mechanism is used for rephasing transverse magnetization?

    <p>Gradients, particularly the reversal of the frequency encoding gradient.</p> Signup and view all the answers

    What characterizes T2* (T2 star) relaxation in Gradient Echo sequences?

    <p>It accounts for both reversible and irreversible dephasing.</p> Signup and view all the answers

    How does the flip angle in GRE sequences typically compare to that in spin-echo sequences?

    <p>It is lower, usually less than 90 degrees.</p> Signup and view all the answers

    What is a key difference between spoiled and coherent GRE sequences?

    <p>Spoiled GRE sequences destroy residual transverse magnetization completely.</p> Signup and view all the answers

    What happens to the K-Space when using a single-shot FAST spin-echo sequence?

    <p>More than half of K-Space is filled through half-Fourier transformation.</p> Signup and view all the answers

    Why is T2 relaxation eliminated in the spin-echo sequence compared to GRE?

    <p>Due to the inclusion of the 180-degree pulse in spin-echo sequences.</p> Signup and view all the answers

    Which statement accurately describes the relationship between scan time and TR in GRE sequences?

    <p>Smaller flip angles in GRE lead to reduced TR and shorter scan times.</p> Signup and view all the answers

    What effect does a 180-degree pulse have on longitudinal magnetization (LM)?

    <p>It flips LM along the negative side of the Z-axis.</p> Signup and view all the answers

    In an Inversion Recovery (IR) sequence, what is the significance of applying a 90-degree excitatory pulse after the LM has relaxed?

    <p>It allows for imaging based on T1 recovery differences.</p> Signup and view all the answers

    How does the TI for tissue suppression relate to the T1 relaxation time?

    <p>TI is 0.69 times the T1 relaxation time.</p> Signup and view all the answers

    What is the primary purpose of the STIR pulse sequence in IR imaging?

    <p>To suppress fat signal.</p> Signup and view all the answers

    What TI value is typically used in the STIR sequence to suppress fat?

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

    What does the FLAIR sequence primarily target for suppression during imaging?

    <p>CSF containing areas.</p> Signup and view all the answers

    What characteristic of 4-Echo Planar Imaging (EPI) makes it efficient for scanning?

    <p>It fills multiple lines of K-Space in a single TR.</p> Signup and view all the answers

    What occurs to magnetization when the TI corresponds to the time a particular tissue has recovered to the halfway stage?

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

    Study Notes

    Pulse Sequences and Image Contrast

    • Pulse sequence is the interplay of parameters, leading to a complex cascade of events with RF pulses and gradients that create an MR image
    • It's a time chart of:
      • Patient's net longitudinal magnetization
      • Transmission of RF pulses (90, 180 degrees, or other)
      • X, Y, and Z gradient activation for localization and signal (echo) acquisition
      • K-Space filling with acquired signals or echoes
    • Image acquisition steps:
      • Patient
      • Pulse sequence
      • Data acquisition
      • Acquired echoes stored in K-Space
      • MR image

    Classification of Pulse Sequences

    • Broad categories: spin echo and gradient echo sequences
    • Inversion recovery and echo planar imaging (EPI) can be applied to both, but in practice, inversion recovery is used with spin-echo sequences and EPI with gradient echo sequences
    • Four main sequence types:
      • Spin-echo sequence (SE)
      • Gradient echo sequence (GRE)
      • Inversion Recovery sequences (IR)
      • Echo Planar Imaging (EPI)

    Spin Echo (SE)

    • Consists of 90-degree and 180-degree RF pulses

    • 90-degree pulse flips net magnetization vector to the transverse plane

    • Transverse magnetization precessing at Larmor frequency induces a weak signal (FID)

    • FID is insufficient for image formation; protons start dephasing

    • 180-degree pulse rephases protons, creating a stronger signal (spin echo)

    • TR: time between two 90-degree pulses

    • TE: time between 90-degree pulse and echo signal reception

    • Slice selection gradient is activated when the RF pulse is sent

    • Phase encoding gradient is used between excitation and signal measurement

    • Frequency encoding (read-out gradient) is used during the signal measurement

    • SE sequence is fundamental in understanding other sequences

    • Used in most examinations

    • Diseased tissues (edematous and/or vascular) appear bright on T2-weighted images, making T2-weighted images helpful for pathology demonstration

    Modifications of SE Sequences

    • Conventional SE sequences fill one line of K-Space per TR
    • Modified SE sequences can have more than one echo per TR, achieved by sending multiple 180-degree pulses after the excitation pulse
    • A-Dual Spin-Echo sequence: Two 180-degree pulses after each 90-degree pulse create two echoes per TR
    • PD + T2 double echo sequence is an example of this
    • First 180 pulse with short TE results in a proton density-weighted image.
    • Second 180 pulse with long TE creates a T2-weighted image
    • The two echoes contribute to distinct K-Space lines

    FAST (TURBO) Spin-Echo Sequence

    • Multiple 180-degree rephasing pulses are sent after each 90-degree pulse
    • The echoes from these pulses are used to fill a single K-Space, hence considerably faster scanning
    • Turbo factor: the number of 180 pulses sent
    • Signal amplitude varies as the TE increases and is maximized at the TE effective
    • Short turbo factor decreases effective TE and increases T1 weighting.
    • Long turbo factor increases effective TE, increases T2 weighting, and decreases scan time.

    Single-Shot Fast Spin-Echo Sequence

    • All echoes required to form an image are acquired in a single TR, hence "single-shot"
    • K-Space is only partially filled reducing scan time by half (using half-Fourier transformation)
    • This sequence is significantly faster than other sequences due to a single TR and partially filled k-space

    Gradient Echo (GRE) Sequence

    • No 180-degree pulse; rephasing of TM is done by gradients
    • Has a smaller flip angle (typically less than 90 degrees); leading to faster recovery of longitudinal magnetization and minimized imaging time
    • Causes of transverse relaxation:
      • Irreversible dephasing of TM due to nuclear magnetic interactions with protons
    • Two types of GRE sequences:
      • Spoiled/incoherent GRE: when residual TM is destroyed, not interfering with the next TR
      • Steady-state/coherent GRE: when residual TM is refocused, leading to consistent LM and TM after several TRs

    A -Steady State (SS) or Coherent GRE sequences

    • TR is shorter than T1 and T2 times of the tissues
    • Coexistence of longitudinal and transverse magnetization
    • Flip angles usually range from 30° to 45° with TR values of 20-50ms
    • High signal intensity in tissues with long T2 values
    • Short TR and TE, leading to fast image acquisition suitable for breath-hold

    B -Incoherent (Spoiled) Gradient Echo pulse sequence

    • Begins with variable flip angle excitation pulse and uses frequency encoding gradient rephasing to give a gradient echo
    • Spoils (or dephases) residual transverse magnetization so its effect is minimal on image contrast; leads to increased T1 weighting

    Inversion Recovery (IR) Sequence

    • An inverting 180-degree pulse comes before the standard spin-echo or gradient echo sequence, usually with SE sequences.
    • The inverting 180-degree pulse flips LM from the positive side of the Z-axis to the negative side, saturating all tissues
    • LM gradually recovers back along the positive Z-axis, different for different tissues based on T1 values
    • Fat recovers faster than water
    • Sequences use TI (time to invert), main determinant of contrast in IR sequences
    • Different types of tissue require different Tl values to suppress particular tissues

    Types of IR Sequences

    • A-STIR: used to suppress fat signal in the anatomy of interest; Tl value determines how long fat takes to recover to the transverse plane (no fat signal)
    • B-FLAIR: used to suppress CSF signal; high Tl value around 2000ms

    4-Echo Planar Imaging (EPI)

    • Scans reduce time by filling multiple lines of K-Space simultaneously in a single TR
    • Single-shot EPI (SS-EPI) fills all K-Space lines in a single TR
    • Spin-echo EPI (SE-EPI) uses multiple 180-degree pulses to create echoes
    • Gradient-echo EPI uses gradients for rephasing
    • GE-EPI and SS-EPI are significantly faster than SE-EPI

    Examples for EPI Sequences

    • Perfusion-weighted imaging (PWI): dynamic MR imaging using GRE or EPI sequences with contrast enhancements, to study contrast uptake by the lesion
    • Diffusion-weighted imaging (DWI): detects restricted diffusion of extracellular water, useful in differentiating salvageable and non-salvageable brain tissue after a stroke
    • Functional MRI (fMRI): dynamic MR imaging to acquire brain images during stimulus and at rest to show functional activity differences
    • Magnetization transfer (MT) contrast: suppress background tissue, increasing vessel conspicuity
    • Magnetic resonance angiography (MRA): images with high signal from flowing nuclei and low signal from stationary nuclei (higher clarity of blood vessels)
      • Time of Flight MRA (TOF-MRA) uses coherent GRE sequences
      • Phase Contrast MRA (PC-MRA) uses coherent GRE sequences as well, but PC-MRA scan times are longer

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    Pulse Sequences and Image Contrast (MRI) PDF

    Description

    Explore the intricacies of MRI pulse sequences and their effect on image contrast. This quiz covers the anatomy of pulse sequences, including RF pulses, gradient activation, and K-Space filling. Understand the classifications and applications of various MRI sequence types.

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