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MRI Questions#

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MRI System#

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MR Physics#

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Magnetic Fields and RF Coils#

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Contrast#

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  1. What flip angle gives the highest SNR for a spoiled gradient echo pulse sequence?

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    • 45-degrees

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    • 90-degrees

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    • 180-degrees

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    • \(\cos^{-1} ( \exp(-TR/T_1) )\)

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  2. What magnetic resonance property is used to perform fat/water (Dixon) imaging?

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    • Proton density

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    • T1

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    • T2

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    • T2*

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    • Chemical Shift

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  3. What are the minimum measurements required to create separate fat and water images?

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    • In-phase TE

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    • Out-of-phase TE

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    • In-phase TE & Out-of-phase TE

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  4. What is “magnetization preparation” used for?

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    • polarization

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    • data acquisition

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    • create additoinal contrast

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    • tissue suppression

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  5. (T/F) Multiple readouts can be used following a magnetization preparation pulse to improve efficiency

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Pulse Sequence#

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In a typical pulse sequence, identify the gradients that serve the following functions:

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  1. spoil transverse magnetization

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  2. refocus Mxy phase across the slice

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  3. move to the edge of k-space

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Which of the following statement is true for the slice select refocusing gradient?

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  • The slice select refocusing gradient must have the same gradient amplitude with the slice selective excitation pulse.

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  • The slice select refocusing gradient must have the same gradient area with the slice selective excitation pulse.

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  • The slice select refocusing gradient can overlap with the prewinder of the frequency encoding gradient.

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  • The slice select refocusing gradient can overlap with the gradient echo.

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(T/F) For 2D FT imaging, gradient spoiling is usually applied in the slice select direction because the voxel size is larger thus more dephasing can happen within a voxel in that direction.

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(T/F) For a GRE or SE with no phase encoding gradient, the k-space position at TE equals 0 (i.e, \(\vec{k}(TE) = 0\)).

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RF Pulses#

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Spatial Encoding#

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Which k-space line is acquired by the following magnetic field gradients?

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In 3DFT imaging, the gradient added to the slice encoding axis (compared to a 2DFT) is a …

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  • frequency encoding gradient

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  • phase encoding gradient

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  • either frequency or phase encoding gradient

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Image Reconstruction#

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For a real-valued image \(m(x,y)\), which of the following equation holds for its k-space data \(M(k_x,k_y)\)?

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  • \(M(k_x,k_y) = M(-k_x,-k_y)\)

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  • \(M(k_x,k_y) = -M(-k_x,-k_y)\)

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  • \(\mathcal{Real}\{M(k_x,k_y)\} = \mathcal{Real}\{ M(-k_x,-k_y) \}\)

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  • \(\mathcal{Imag}\{M(k_x,k_y) \} = \mathcal{Imag}\{M(-k_x,-k_y) \}\)

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(T/F) In MRI we only look at the magnitude images.

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(T/F) The center of k-space always contains the maximum signal.

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Image Characteristics (FOV and Resolution)#

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The field of view is inversely proportional to…

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  • receiver BW (RBW)

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  • readout gradient strength (Gxr)

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  • TR

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  • TE

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The field of view is directly proportional to…

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  • receiver BW (RBW)

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  • readout gradient strength (Gxr)

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  • TR

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  • TE

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(T/F) An anti-aliasing filter can be applied in the phase encoding direction.

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The resolution in frequency encoding direction ( \(\delta_x\) ) is equal to …

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  • \( \frac{1}{W_{kx}} \)

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  • \( \frac{1}{\frac{\gamma}{2\pi} G_{xr} t_{read}} \)

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  • \( \frac{FOV_x}{N_{FE}} \)

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  • None of the above

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SNR#

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SNR can be increased by …

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  • decreasing voxel size

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  • increasing total time

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  • increasing NEX

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Artifacts#

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Chemical shift displacement artifact is characterized by …

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  • signal stretch and pile-up

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  • bright and dark bands

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  • Gibbs ringing

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  • ghosting

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Susceptibility displacement artifact is characterized by …

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  • signal stretch and pile-up

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  • bright and dark bands

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  • Gibbs ringing

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  • ghosting

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Truncation artifacts can be reduced by …

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  • improving resolution

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  • filtering in k-space

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  • increasing FOV

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  • increasing NEX

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(T/F) Motion artifacts occur only along the phase-encoding direction.

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Fast Imaging Pulse Sequences#

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What does it mean to use a multiple spin echo pulse sequence?

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  • multiple spin echoes are created following a single excitation pulse

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  • multiple k-space lines acquired sequentially

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  • multiple gradient-echo repetitions after a magnetization preparation pulse

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  • fully refocused gradients and no spoiling in every TR

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What types of contrast can be created with a multiple spin echo pulse sequence?

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  • proton density weighted

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  • T1 weighted

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  • T2 weighted

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  • T2* weighted

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What are the limitations of multiple spin echo pulse sequences?

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  • Chemical shift and susceptibility displacement artifacts

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  • T2 blurring artifacts

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  • ghosting

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  • SAR

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What does it mean to use echo planar imaging (EPI)?

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  • multiple spin echoes are created following a single excitation pulse

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  • multiple k-space lines acquired sequentially

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  • multiple gradient-echo repetitions after a magnetization preparation pulse

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  • fully refocused gradients and no spoiling in every TR

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What are the advantages of EPI?

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  • Create additional T1, T2, and/or T2* contrast

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  • Rapidly acquire k-space

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  • Robust to motion

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  • Repeated refocusing of intravoxel dephasing

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What are the artifacts associated with EPI?

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  • Chemical shift displacement

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  • Distortion due to magnetic susceptibility differences

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  • T2* blurring

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  • ghosting

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What does it mean to use balanced steady-state free-precession (bSSFP)?

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  • multiple spin echoes are created following a single excitation pulse

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  • multiple k-space lines acquired sequentially

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  • multiple gradient-echo repetitions after a magnetization preparation pulse

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  • fully refocused gradients and no spoiling in every TR

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Accelerated Imaging Methods#

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Match the acceleration methods

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  • Partial Fourier

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  • Parallel Imaging

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  • Compressed Sensing

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  • Deep Learning

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with the following concept they rely on:

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  • conjugate symmetry in k-space

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  • spatial encoding from receive coil arrays

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  • a sparse representation of the image

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  • training on prior images to learn expected patterns

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(T/R) Partial Fourier, parallel imaging, and compressed sensing or deep learning reconstructions can be used simultaneously.

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How is coil sensitivity information gathered for parallel imaging?

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  • It is stored in a database on the scanner

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  • A separate scan to measure coil sensitivity maps

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  • Using fully-sampled data from the center of k-space

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  • Using fully-sampled data from outer k-space

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What is the “g-factor” in parallel imaging?

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  • How much faster scan can be performed

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  • Describes noise amplification

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  • Describes SNR loss

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  • Describes magnetic field gradients

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Compared to the SNR a fully sampled acquistion (\(SNR_{full}\)), the SNR Of a parallel imaging acquisition (\(SNR_{PI}\)) with an acceleration factor, \(R\), and \(g\)-factor is

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  • \(SNR_{PI} = SNR_{full}\)

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  • \(SNR_{PI} = SNR_{full} / \sqrt{R}\)

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  • \(SNR_{PI} = SNR_{full} / g(\vec{r}) \)

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  • \(SNR_{PI} = SNR_{full} / (g(\vec{r}) \sqrt{R})\)

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(T/F) Parallel imaging undersampling can be performed in any direction, regardless of the RF coil configuration.

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Simultaneous multi-slice parallel imaging

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  • enables acceleration in the slice direction

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  • requires no modifications to the pulse sequence

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  • requires RF pulses that excite multiple slices

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  • requires coil sensitivity information

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What type of k-space sampling is required for compressed sensing?

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  • full sampling

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  • equally spaced undersampling

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  • equally spaced undersampling with fully-sampled center of k-space

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  • pseudo-random undersampling

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At least how many training datasets are typically required to develop deep learning MRI reconstruction methods?

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  • 1

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  • 10-100

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  • 1000-10,000

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  • 100,000-1,000,000

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Generalization problems can arise in deep learning MRI reconstruction methods when applied to situations that area different from the training data in which of the following ways:

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  • different anatomy

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  • different contrasts

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  • different sampling patterns

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  • different B0

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Which type of architecture is commonly used for physics-based deep learning MRI reconstruction networks?

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  • Encoder-decoder

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  • Unet

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  • Unrolled

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  • Recurrant

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