MRI Techniques and Pulse Sequences
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Questions and Answers

What characteristic of T2-weighted images allows them to demonstrate pathology effectively?

  • They utilize only one echo per TR.
  • They are more sensitive to proton density.
  • Diseased tissues are more edematous and/or vascular. (correct)
  • They require longer TR values.

How many echoes does the DUAL SPIN-ECHO sequence obtain per TR?

  • Two echoes (correct)
  • Three echoes
  • Four echoes
  • One echo

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

  • Has no impact on the quality of the image.
  • Increases scan time and decreases T1 weighting. (correct)
  • Decreases effective TE and increases T1 weighting.
  • Increases the number of echoes per TR.

What is the primary purpose of the 180-degree rephasing pulses in the FAST (TURBO) SPIN-ECHO sequence?

<p>To allow for multiple echoes to fill K-Space. (A)</p> Signup and view all the answers

What is the term used to describe the number of 180-degree pulses sent after each 90-degree pulse in the FAST sequence?

<p>Echo train length (A)</p> Signup and view all the answers

In the SINGLE-SHOT FAST SPIN-ECHO Sequence, how are echoes acquired?

<p>In a single TR (A)</p> Signup and view all the answers

How does a short turbo factor affect effective TE in the FAST sequence?

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

What type of imaging do T1-weighted images primarily demonstrate?

<p>Anatomical structures (D)</p> Signup and view all the answers

What characterizes a pulse sequence in MR imaging?

<p>It involves the interaction of RF pulses and magnetic gradients. (A)</p> Signup and view all the answers

Which components are involved in the time chart of a pulse sequence?

<p>Patient's longitudinal magnetization, RF pulses, Gradient activations. (A)</p> Signup and view all the answers

What is the purpose of the 180-degree pulse in the Spin Echo sequence?

<p>To bring protons back into phase after dephasing. (A)</p> Signup and view all the answers

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

<p>Gradient Echo (GRE) sequences. (A)</p> Signup and view all the answers

What is meant by TR in a Spin Echo sequence?

<p>Time elapsed between two successive 90-degree pulses. (D)</p> Signup and view all the answers

Which pulse sequence serves as the foundation for understanding other sequences?

<p>Spin Echo (SE) sequence. (D)</p> Signup and view all the answers

What is FID in the context of MRI?

<p>The initial weak signal produced from transverse magnetization. (B)</p> Signup and view all the answers

In practice, which pulse sequence is primarily applied with inversion recovery?

<p>Spin Echo sequence. (C)</p> Signup and view all the answers

What is a defining characteristic of the Gradient Echo (GRE) sequence compared to the Spin Echo (SE) sequence?

<p>GRE achieves rephasing through gradients instead of pulses. (B)</p> Signup and view all the answers

What characteristic of the flip angle in GRE sequences usually leads to reduced scanning time?

<p>It is usually less than 90 degrees. (A)</p> Signup and view all the answers

What term describes the relaxation caused by dephasing due to magnetic field inhomogeneity in GRE sequences?

<p>T2* relaxation (D)</p> Signup and view all the answers

How do spoiled or incoherent GRE sequences manage the residual transverse magnetization?

<p>They completely destroy it after signal reception. (D)</p> Signup and view all the answers

What is the key aspect of the steady state for Coherent GRE sequences?

<p>The TR remains shorter than both T1 and T2 times. (D)</p> Signup and view all the answers

What major advantage does half-Fourier transformation provide in the SINGLE-SHOT FAST SPIN-ECHO sequence?

<p>It enables reducing scan time by mathematically calculating part of K-Space. (A)</p> Signup and view all the answers

What is the effect of not having a 180-degree pulse in a GRE sequence?

<p>It leads to an inability to compensate for dephasing effects. (C)</p> Signup and view all the answers

Which of the following is NOT a feature of the Gradient Echo (GRE) Sequence?

<p>Requires T1 recovery time to be longer than T2. (C)</p> Signup and view all the answers

What is the primary purpose of diffusion weighted imaging (DWI) in brain stroke assessment?

<p>To differentiate between salvageable and non-salvageable tissue. (A)</p> Signup and view all the answers

Which technique is primarily used to increase the conspicuity of vessels in MRI?

<p>Magnetization Transfer (MT) Contrast (C)</p> Signup and view all the answers

In Time of Flight MRA (TOF-MRA), what combination of techniques is utilized to enhance image quality?

<p>Coherent GRE pulse sequences with TR and flip angle combinations. (C)</p> Signup and view all the answers

What type of MR imaging technique is fMRI categorized as?

<p>Dynamic imaging technique. (B)</p> Signup and view all the answers

Which statement accurately describes the outcome of using Phase Contrast MRA (PC-MRA)?

<p>It typically incurs longer scan times than other MRA techniques. (B)</p> Signup and view all the answers

What happens to the magnetization at the halfway stage during recovery after a 180-degree inversion pulse?

<p>Magnetization reaches zero level. (B)</p> Signup and view all the answers

What is the primary purpose of the STIR pulse sequence?

<p>To suppress the signal from fat tissues. (C)</p> Signup and view all the answers

What TI value is generally used for the STIR sequence?

<p>Around 100-200 ms. (C)</p> Signup and view all the answers

What is the main function of the FLAIR sequence?

<p>To suppress fluid signals in the imaging data. (D)</p> Signup and view all the answers

Which EPI sequence is specifically faster due to its use of gradients for rephasing?

<p>GE-EPI. (B)</p> Signup and view all the answers

What characterizes Single Shot EPI (SS-EPI)?

<p>Filling all lines of K-Space in a single TR. (C)</p> Signup and view all the answers

What purpose does Perfusion Weighted Imaging (PWI) serve?

<p>To analyze perfusion dynamics in specific organs. (B)</p> Signup and view all the answers

Which statement about EPI sequences is correct?

<p>GE-EPI and SS-EPI are generally faster than SE-EPI. (C)</p> Signup and view all the answers

What flip angle range is generally favored for achieving a steady state in SS sequences?

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

What is a characteristic feature of SS sequences?

<p>Very short TR and TE times (B)</p> Signup and view all the answers

Which sequence utilizes a variable flip angle excitation pulse to create images?

<p>Incoherent (Spoiled) Gradient Echo (D)</p> Signup and view all the answers

What is the primary purpose of using an inversion 180-degree pulse in IR sequences?

<p>To saturate all the tissues (B)</p> Signup and view all the answers

What does the term T1 weighting in imaging refer to?

<p>The contrast difference created by longitudinal relaxation (B)</p> Signup and view all the answers

Which of the following factors contributes to different recovery rates of longitudinal magnetization in IR sequences?

<p>T1 values of the tissues (D)</p> Signup and view all the answers

Which statement about IR images is accurate?

<p>They are heavily T1-weighted with significant contrast differences. (B)</p> Signup and view all the answers

In Gradient Echo (GRE) sequences, what is the effect of spoiling residual transverse magnetization?

<p>It minimizes its effect on image contrast. (A)</p> Signup and view all the answers

Flashcards

What is a pulse sequence?

A series of precisely timed radiofrequency (RF) pulses and magnetic field gradients that manipulate nuclear spins to form a magnetic resonance (MR) image.

What is TR (Time to Repeat)?

The time between two 90-degree RF pulses in a spin-echo sequence. It determines how much time is allowed for the longitudinal magnetization to recover.

What is TE (Time to Echo)?

The time between the 90-degree RF pulse and the reception of the spin-echo signal. It controls the amount of signal from tissues with different T2 relaxation times.

What is a Spin-Echo (SE) sequence?

A pulse sequence that uses 90- and 180-degree RF pulses to manipulate nuclear spins. This sequence produces a spin echo signal which is used to create MR images.

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What is slice selection?

The process of applying magnetic field gradients to create a slice of tissue that is selectively excited.

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What is phase encoding?

The process of applying magnetic field gradients to encode spatial information along one axis. This information is used to reconstruct the image.

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What is frequency encoding?

The process of acquiring signals from all the different spatial locations within a slice. This information is used to reconstruct the image.

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What is a Gradient-Echo (GRE) sequence?

Gradient-echo imaging is a pulse sequence that uses gradient magnetic fields to manipulate spin echoes.

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Fast (Turbo) Spin-Echo

A type of MRI sequence that uses multiple echoes to obtain images. This technique speeds up the scanning process by filling the K-Space faster.

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Dual Spin-Echo

A type of MRI sequence that uses two 180-degree pulses after each 90-degree pulse to obtain two echoes per TR. This allows for proton density-weighted images (PD) and T2-weighted images.

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Turbo Factor

The number of 180-degree pulses sent after each 90-degree pulse in a Turbo Spin-Echo sequence. It determines the length of the echo train and affects the scan time and image weighting.

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TE Effective

A measure of the time between the excitation pulse and the center of the K-Space. It affects image contrast and determines the type of weighting (T1, T2, or PD).

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Single-Shot Fast Spin-Echo

A type of MRI sequence that acquires all the echoes needed for an image in a single TR, making it extremely fast. It is also known as a single-shot sequence.

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Conventional Spin-Echo (SE)

A type of MRI sequence that uses a single 180-degree pulse after each 90-degree pulse. This is the simplest and most common type of spin echo sequence.

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TE (Echo Time)

The time between the excitation pulse and the first echo. It affects the image contrast and determines the type of weighting (T1, T2, or PD).

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TR (Repetition Time)

The time between each 90-degree pulse in an MRI sequence. Longer TRs result in more T1-weighted images, while shorter TRs result in more T2-weighted images.

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What is Single-Shot Fast Spin Echo?

In this sequence, all K-Space lines are acquired in a single excitation, and only half of the K-Space is filled, reducing scan time by half. The remaining half of the K-Space is calculated mathematically using half-Fourier transformation.

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What is T2*?

It refers to the relaxation time in GRE sequences where dephasing caused by magnetic field inhomogeneities is not compensated for.

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What are spoiled or incoherent GRE sequences?

These are GRE sequences where the residual transverse magnetization is destroyed after each repetition, preventing interference with the next pulse.

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What are steady-state or coherent GRE sequences?

These are GRE sequences where the residual transverse magnetization is not destroyed but refocused, leading to a steady-state equilibrium.

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What is a steady state?

In this state, TR is shorter than T1 and T2 times, leading to a balance between longitudinal and transverse magnetization. This is commonly used in GRE sequences.

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Steady State Free Precession (SSFP)

A type of Gradient-Echo (GRE) sequence that favors T2 weighting and produces a more intense signal for tissues with longer T2 relaxation times.

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Incoherent (Spoiled) Gradient Echo

This pulse sequence uses a variable flip angle and frequency encoding to create images. It helps reduce the effect of residual magnetization, resulting in clearer images.

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Inversion Recovery (IR)

This sequence uses a 180-degree pulse to flip the magnetization along the negative Z-axis, saturating all tissues. This allows for better visualization of tissues based on their T1 recovery times.

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Time to Repeat (TR)

The time between two 90-degree RF pulses in a sequence, determining how much time is allowed for the longitudinal magnetization to recover.

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Time to Echo (TE)

The time between the 90-degree RF pulse and the reception of the echo signal, controlling how much signal comes from tissues with different T2 relaxation times.

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Spin-Echo (SE) Sequence

A pulse sequence that uses 90- and 180-degree RF pulses to manipulate nuclear spins, resulting in a spin echo signal used to create MR images.

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Phase Encoding

The process of applying magnetic field gradients to encode spatial information along one axis, used to reconstruct the image.

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Frequency Encoding

The process of acquiring signals from all the different spatial locations within a slice, used to reconstruct the image.

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What is Diffusion Weighted Imaging (DWI)?

A type of MRI imaging used to identify areas of restricted diffusion, often seen in damaged tissue like after a stroke. High signal intensity on DWI images indicates restricted diffusion.

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What is Functional MRI (fMRI)?

A dynamic MRI technique that captures images of the brain during activity (stimulus) and at rest, allowing us to understand brain function.

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What is Magnetization Transfer (MT) Contrast?

A technique that suppresses background tissue signal to make specific structures (like blood vessels) more prominent. This enhances contrast in images.

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What is Magnetic Resonance Angiography (MRA)?

A technique used to visualize blood vessels by taking advantage of the different magnetic properties of moving blood compared to stationary tissue.

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What is Time of Flight MRA (TOF-MRA)?

A type of MRA relying on a special pulse sequence that saturates background tissue, allowing only flowing blood to show up prominently. It's useful for arteries and veins in the head, neck, and limbs.

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Inversion Recovery (IR) Sequence

An MRI sequence that uses an inversion pulse to suppress the signal from certain tissues by manipulating the time it takes for their magnetization to recover.

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Short Inversion Recovery (STIR)

A type of IR sequence that uses a short inversion time (TI) to suppress fat signals, allowing clearer visualization of other tissues.

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Fluid Attenuated Inversion Recovery (FLAIR)

A variation of the IR sequence that employs a longer inversion time (TI) to suppress the signal from fluids, particularly cerebrospinal fluid (CSF), enabling better visualization of other structures.

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Echo Planar Imaging (EPI)

A technique that accelerates MRI acquisition by filling multiple lines of k-space within a single repetition time (TR).

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Single Shot EPI (SS-EPI)

A type of EPI sequence that acquires all lines of k-space within a single TR, enabling very fast image acquisition.

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Spin Echo Echo Planar Imaging (SE-EPI)

A type of EPI sequence that uses multiple 180-degree pulses to generate echoes, resulting in a spin-echo EPI sequence.

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Gradient Echo EPI (GE-EPI)

A type of EPI sequence that utilizes gradient magnetic fields to re-phase spins instead of 180° pulses, leading to a faster acquisition than SE-EPI.

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Perfusion Weighted Imaging (PWI)

A dynamic MRI technique that uses GRE or EPI sequences with contrast enhancement to study how quickly a contrast agent is taken up by a tissue, often used to detect abnormalities.

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Study Notes

Pulse Sequences and Image Contrast

  • Pulse sequences are a series of parameters that create an MRI image.
  • Pulse sequences comprise RF pulses and gradients.
  • A pulse sequence is a time chart reflecting:
    • Patient's net longitudinal magnetization.
    • RF pulse transmission (e.g., 90°, 180°).
    • Gradient activation (X, Y, Z) for localization.
    • Signal (echo) acquisition.
    • K-Space filling.

Steps in Image Acquisition

  • The patient enters the MRI machine.
  • A pulse sequence is applied.
  • Data is acquired.
  • Acquired echoes are stored in K-Space.
  • The MRI image is generated.

Pulse Sequence Classifications

  • Pulse sequences are broadly categorized into spin-echo and gradient-echo sequences.
  • Inversion recovery and echo planar imaging (EPI) can be applied to both types theoretically.
  • Practically, inversion recovery is used with spin-echo sequences, and EPI is used with gradient-echo.
  • Four common pulse sequence types are:
    • Spin-echo (SE).
    • Gradient echo (GRE).
    • Inversion recovery (IR).
    • Echo planar imaging (EPI).

Spin Echo (SE)

  • Consists of 90° and 180° RF pulses.
  • The 90° pulse flips net magnetization into the transverse plane.
  • Free induction decay (FID) is a weak initial signal.
  • The 180° pulse rephases the dephasing magnetization, creating a spin echo.
  • TR: Time to repeat (time between 90° pulses).
  • TE: Time to echo (time between 90° pulse and echo signal).
  • Slice selection gradient, phase encoding gradient, and frequency encoding gradient are used for localization.

Spin Echo (SE) Modifications

  • A-Dual Spin Echo: Two 180° pulses are used, obtaining two echoes per TR.
  • Produces both proton density (PD) and T2-weighted images.
  • B-FAST (Turbo) Spin Echo: Multiple 180° pulses are used.
    • More echoes are created per TR.
    • Reduces scan time.
    • Turbo factor refers to number of 180° pulses.
    • Higher turbo factor means shorter scan time, with T1 weighting potentially increasing.
  • C-Single-Shot Fast Spin Echo:
    • Fills K-Space in a single TR.
    • Reduces scan time drastically.
    • Calculates half of K-Space data, using half-Fourier transformation.

Gradient Echo (GRE)

  • No 180° pulse.
  • Rephasing of transverse magnetization (TM) achieved using gradients.
  • Flip angle is typically smaller than 90°.
  • Scan time is shorter compared to spin echo, due to reduced TR.
  • Possible variants include:
    • Steady-state (SS) or Coherent.
    • Incoherent (Spoiled).

Inversion Recovery (IR)

  • An 180° pulse is applied before the usual spin-echo or gradient-echo sequence.
  • This pulse flips the longitudinal magnetization to the negative side of the z-axis.
  • The time between the 180° and 90° pulses is called TI (Time to invert)
  • Tissues with different T1 values recover at different rates.
  • Different types of IR sequences vary based on their TI value to perform specific tissue suppression.
    • STIR (Short Inversion Recovery).
    • FLAIR (Fluid-Attenuated Inversion Recovery)

Echo Planar Imaging (EPI)

  • Fills multiple K-space lines in a single TR.
  • Results in a significant reduction in scan time.
  • Can be spin echo (SE) based (using multiple 180° pulses) or Gradient echo (GE) based using gradients for rephasing.

Applications of EPI Sequences

  • Perfusion Weighted Imaging (PWI): Studies contrast uptake in lesions (e.g., brain, liver, pancreas).
  • Diffusion Weighted Imaging (DWI): Measures water diffusion to identify restricted diffusion, usefule in stroke.
  • Functional MRI (fMRI): Tracks brain activity during stimulus.
  • Magnetization Transfer (MT): Suppresses background tissue, increasing contrast of vessels.
  • Magnetic Resonance Angiography (MRA): Visualizes blood vessels with high signal, differing from surrounding tissue based on flow.
    • Time of Flight (TOF): Detects moving blood with GRE.
    • Phase Contrast (PC): Measures blood flow using coherent GRE sequences.

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MRI Pulse Sequences PDF

Description

This quiz explores various aspects of MRI techniques, focusing on T2-weighted images, pulse sequences, and echo acquisition methods. Test your knowledge on the principles behind FAST (TURBO) SPIN-ECHO sequences and the significance of different parameters in MRI imaging. Ideal for students and professionals in radiology and medical imaging fields.

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