Pulse Sequences in MRI Technology
48 Questions
0 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to lesson

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What function does the 90-degree RF pulse serve in the Spin Echo pulse sequence?

  • It rephases the protons.
  • It activates the phase encoding gradient.
  • It induces free induction decay.
  • It flips the net magnetization vector into the transverse plane. (correct)

    • Which of the following is not a type of pulse sequence classification mentioned?

  • Dual Spin Echo sequence (correct)
  • Echo Planar Imaging (EPI)
  • Spin Echo sequence (SE)
  • Gradient Echo sequence (GRE)

    • What is the name of the signal induced in the receiver coil after the 180-degree pulse in a Spin Echo sequence?

  • Phase Encoding Signal
  • Free Induction Decay (FID)
  • Spin Echo (correct)
  • Transverse Magnetization

    • What does TE stand for in the context of pulse sequences?

    <p>Time to Echo</p> Signup and view all the answers

    In Spin Echo sequences, what is the role of the 180-degree pulse?

    <p>To rephase the dephased protons.</p> Signup and view all the answers

    When is the slice selection gradient activated during a Spin Echo sequence?

    <p>When the 90-degree RF pulse is sent.</p> Signup and view all the answers

    Which of the following statements about the Free Induction Decay (FID) signal is true?

    <p>FID is the first signal received before rephasing.</p> Signup and view all the answers

    Which pulse sequence is primarily associated with Echo Planar Imaging (EPI)?

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

    What is the main advantage of using a single-shot fast spin-echo sequence?

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

    In gradient echo (GRE) sequences, what primarily causes the rephasing of transverse magnetization?

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

    How does T2 relaxation in GRE differ from T2 relaxation in SE sequences?

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

    What is the effect of a smaller flip angle in GRE sequences?

    <p>Allows for an early recovery of longitudinal magnetization.</p> Signup and view all the answers

    Which characteristic distinguishes spoiled or incoherent GRE sequences?

    <p>The destruction of residual transverse magnetization before the next TR.</p> Signup and view all the answers

    What happens to K-Space in a single-shot fast spin-echo sequence?

    <p>More than half is filled instantly while the remainder is mathematically calculated.</p> Signup and view all the answers

    Why is T2* (T2 star) relaxation significant in GRE sequences?

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

    What is one main difference between spin echo (SE) and gradient echo (GRE) sequences?

    <p>GRE sequences eliminate the need for RF pulses for rephasing.</p> Signup and view all the answers

    What sequence forms the basis for understanding all other MRI sequences?

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

    Which type of weighted images are T1-weighted images primarily used for?

    <p>Showing anatomy</p> Signup and view all the answers

    What modification to the conventional SE sequence sends two 180-degree pulses after each 90-degree pulse?

    <p>Dual Spin-Echo</p> Signup and view all the answers

    How does the Turbo factor in a Fast Spin-Echo sequence influence imaging?

    <p>Decreases the scan time</p> Signup and view all the answers

    What term is used for the effective TE at which the center of K-Space is filled in a Turbo Spin-Echo sequence?

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

    In the context of Spin Echo sequences, what does a longer TR in Dual Spin-Echo sequences allow?

    <p>Increased proton density weighting</p> Signup and view all the answers

    What does the term 'Turbo factor' refer to in Turbo Spin-Echo sequences?

    <p>Number of 180-degree pulses per TR</p> Signup and view all the answers

    What relationship exists between turbo factor and effective TE in Fast Spin-Echo sequences?

    <p>Short turbo factor decreases effective TE</p> Signup and view all the answers

    What term is used for echoes generated by multiple 180° pulses?

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

    Which EPI sequence is known to use gradients for rephasing?

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

    What imaging technique is specifically used for studying contrast medium uptake in lesions?

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

    In which scenario is Diffusion Weighted Imaging (DWI) primarily useful?

    <p>To differentiate between salvageable and non-salvageable brain tissue</p> Signup and view all the answers

    What is the main purpose of Magnetization Transfer (MT) contrast?

    <p>Suppress background tissue to enhance visibility of vessels</p> Signup and view all the answers

    Which technique uses coherent GRE pulse sequences to demonstrate arterial and venous flow?

    <p>Time of Flight MRA</p> Signup and view all the answers

    How do scan times of Phase Contrast MRA compare to GE pulse sequences?

    <p>They are longer</p> Signup and view all the answers

    What defines Functional MRI (fMRI)?

    <p>It involves subtraction of images acquired during rest and stimulation</p> Signup and view all the answers

    What is a characteristic feature of steady-state or coherent GRE sequences?

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

    Which flip angle range is most commonly used to achieve a steady state in GRE sequences?

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

    What is the effect of residual transverse magnetization in incoherent (spoiled) gradient echo sequences?

    <p>It is minimized to reduce its effect on contrast.</p> Signup and view all the answers

    What distinguishes Inversion Recovery (IR) sequences from standard GRE sequences?

    <p>They utilize a 180-degree inverting pulse prior to imaging.</p> Signup and view all the answers

    Which factor is the main determinant of contrast in Inversion Recovery (IR) sequences?

    <p>Time to invert (TI)</p> Signup and view all the answers

    Why do tissues with longer T2 values appear with high signal intensity in steady-state GRE sequences?

    <p>Because of minimal spoilage of residual magnetization.</p> Signup and view all the answers

    What is the primary recovery difference between protons in fat and water during an IR sequence?

    <p>Protons in fat recover faster than protons in water.</p> Signup and view all the answers

    What role does a shorter TR and TE play in steady-state GRE sequences?

    <p>They enable the sequences to be fast and acquired with breath-hold.</p> Signup and view all the answers

    What is the primary purpose of the 180-degree pulse in the Inversion Recovery sequence?

    <p>To completely saturate fat and water</p> Signup and view all the answers

    How does the TI value used in Inversion Recovery affect tissue suppression?

    <p>It corresponds to the relaxation time of tissues</p> Signup and view all the answers

    What is the typical TI value used in the A-STIR pulse sequence?

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

    What does B-FLAIR sequence target for suppression?

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

    What happens to the signal from a tissue if the TI corresponds to its recovery time at the halfway stage?

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

    What is a characteristic of 4-Echo Planar Imaging (EPI)?

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

    What is a key effect of using an inversion recovery image in MRI?

    <p>It highlights differences in T1 relaxation times</p> Signup and view all the answers

    In the context of IR sequences, what does TI stand for?

    <p>Inversion Time</p> Signup and view all the answers

    Study Notes

    Pulse Sequences and Image Contrast

    • Pulse sequences are a series of parameters that create a complex cascade of events using RF pulses and gradients to form a magnetic resonance (MR) image.
    • Pulse sequences are essentially a timetable of these interactions.
    • The timetable includes:
      • Patient's longitudinal magnetization
      • Transmission of RF pulses (adjustable degrees)
      • Gradient activation for localization and signal (echo) acquisition
      • K-Space filling with acquired signals or echoes

    Outline of Presentation

    • What is a pulse sequence?
    • Classification of pulse sequences
    • Spin Echo (SE) sequences
    • Modifications of SE sequences:
      • Dual Spin-Echo
      • Fast (Turbo) Spin-Echo
      • Single-Shot Fast Spin-Echo

    Spin Echo (SE)

    • SE sequences use 90° and 180° RF pulses.
    • The 90° pulse flips net magnetization from the Z-axis into the transverse (X-Y) plane.
    • Transverse magnetization precesses at Larmor frequency, creating a signal (FID).
    • FID is weak and prone to decay (dephasing), requiring a 180° pulse to rephas the protons.
    • Rephasing increases magnetization and creates a stronger signal (spin echo).
    • TR (Time to Repeat): Time between two 90° pulses
    • TE (Time to Echo): Time between the 90° pulse and echo signal reception
    • Slice selection gradient is turned on during RF pulse.
    • Phase encoding occurs between the excitation (90°) pulse and signal measurement
    • Frequency encoding takes place during signal measurement
    • SE sequences form the basis for understanding other sequences (common in all exams).
    • T1-weighted images are helpful for anatomical depiction.
    • T2-weighted images highlight edema and vascularity, showing pathology well.

    Modifications of SE Sequences

    • Conventional SE sequences fill one line in K-Space per TR. More than one echo can be obtained per TR (180° pulses).
    • Dual Spin-Echo Sequence: Two 180° pulses create two echoes per TR (PD+T2 double echo sequence).
    • Long TR, Short TE sequence produces a proton density weighted image after the first 180° pulse.
    • Long TR, Long TE sequence produces T2-weighted images after the second 180° pulse.

    Fast (Turbo) Spin-Echo Sequence

    • Turbo sequences obtain multiple echoes per TR with each 180° pulse filling a single K-Space line (rapid scan).
    • Turbo factor determines the number of 180° pulses after a 90° pulse.
    • TE increases as the number of 180° pulses increases.
    • Higher turbo factors maximize signal at TE effective, improving T1-weighting while reducing scanning time. Lower turbo factors increase T2-weighting, improving T2-weighting while reducing scanning time.

    Single-Shot Fast Spin-Echo Sequence

    • Acquires all echoes in a single TR, significantly reducing scan time.
    • Fills roughly half a K-Space line resulting in halved scan time, with the other half calculated from the acquired information.

    Gradient Echo (GRE) Sequence

    • No 180° pulses in GRE sequences; rephasing by gradients instead.
    • Flip angles are usually less than 90°.
    • Shorter TR values enhance scanning speed.
    • GRE sequences are susceptible to magnetic field inhomogeneity which results in T2* (T2 star) relaxation.

    Types of GRE Sequences

    • Spoiled/Incoherent GRE sequences: Residual TM is nullified after each TR. Increased T1 weighting.
    • Steady-State/Coherent GRE sequences: Residual TM is refocused after a few TRs resulting in a steady-state magnitude of LM and TM. (favors T2 weighting).

    Inversion Recovery (IR) Sequence

    • 180° pulse is applied before the usual spin-echo/gradient echo sequence (inverting longitudinal magnetization).
    • This sequence saturates tissue, and recovery is dependent on T1 relaxation times, leading to differing contrast depending on tissue type.
    • TI (Time to Invert): time between the 180° inversion pulse and the 90° excitation pulse The inversion 180° pulse flips longitudinal magnetization (LM) along the negative Z-axis.
    • Tissue with shorter T1 values will recover faster and produce a brighter signal on the image.
    • Can be used to suppress specific tissues.

    Types of IR Sequences

    • Short Inversion Recovery (STIR): Suppresses fat signal in images, often for better visualization of edema (tissues)
    • Fluid-Attenuated Inversion Recovery (FLAIR): Suppresses CSF (cerebrospinal fluid) signal, allowing better visualization of brain lesions.

    4-Echo Planar Imaging (EPI)

    • EPI sequences fill K-space in a single TR by generating echoes with multiple 180° pulses thus increasing speed.
    • Spin-echo EPI(SE-EPI): involves multiple 180° pulses to generate echoes..
    • Gradient-echo EPI (GE-EPI) : uses gradients to rephrase, making it faster,
    • Single-shot EPI(SS-EPI) fills complete k-space in a single TR , further improving speed.
    • EPI sequences are faster than conventional SE sequences.

    Examples of EPI Sequences

    • Perfusion-weighted imaging (PWI): Used with contrast to study tissue uptake. Helpful in abnormalities of brain, pancreas, liver and prostate.
    • Diffusion-weighted imaging (DWI): Show areas with restricted diffusion (often indicative of infarcted tissue)
    • Functional magnetic resonance imaging (fMRI): Demonstrates brain activity by detecting changes in blood flow during tasks or stimulation.
    • Magnetization transfer (MT) contrast:Suppresses background tissue, improving visibility of vessels and certain disease processes
    • Magnetic Resonance Angiography (MRA):-Shows blood vessels -Time of Flight (TOF) MRA: Uses gre sequences in demonstrating arterial and venous flow
      -Phase Contrast (PC) MRA: uses gre sequences, offering excellent background suppression but with longer scan times.

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    Related Documents

    Pulse Sequences and Image Contrast (MRI) PDF

    Description

    Explore the intricate world of pulse sequences used in magnetic resonance imaging (MRI). This quiz covers the various types of pulse sequences including Spin Echo and its modifications, along with the elements that contribute to MR image contrast. Test your knowledge on how these parameters interact to form detailed images.

    More Like This

    Use Quizgecko on...
    Browser
    Open
    Browser
    Browser