Physics 2 Final study guide

What causes a flow-related signal void in MRI (time-of-flight effect)?

Flowing protons move out of the excited slice before the 180° refocusing pulse hits, missing refocusing and producing no signal (appearing dark).

Which imaging parameters increase flow voids due to time-of-flight?

Thin slices, long TE, and fast-flowing blood.

How can technical parameters be adjusted to reduce time-of-flight flow voids?

Use the shortest TE possible and increase slice thickness

Which sequence family is least likely to produce a flow-related time-of-flight signal void?

Gradient echo (GRE).

Why is gradient echo least likely to produce dark-blood signal voids from time-of-flight?

Refocusing is performed by non-slice-selective gradients across the whole bore, so protons rephase even if they leave the excited slice.

In which pulse-sequence family are time-of-flight flow voids most prominent, and why?

Spin echo / fast spin echo

Which technique is NOT used to compensate for flow-related artifacts: even echo rephasing, fast spin echo, spatial presaturation, or gradient moment nulling?

Fast spin echo

What diagnostic technique intentionally uses flow voids created by fast spin echo?

Dark blood (black-blood) imaging, used to visualize myocardium and vessel walls without blood signal.

In Time-of-Flight (TOF) MRA, why does blood appear bright while stationary tissue appears dark without contrast?

Inflowing blood contains fresh, unsaturated spins (strong signal), whereas stationary tissue experiences repeated RF pulses and becomes saturated (dark signal).

What is the fundamental signal difference between flowing blood and stationary tissue in TOF MRA?

Inflowing blood has fresh, unsaturated longitudinal magnetization, while stationary tissue is saturated by repeated excitation.

In Phase Contrast (PC) MRA, what is used to encode flow information?

Bipolar gradients

What unique information does Phase Contrast (PC) MRA provide compared to TOF MRA?

Quantitative flow velocity and flow direction.

What key parameter is adjusted in PC MRA to optimize imaging for different blood flow speeds?

Velocity Encoding (VENC)

For slow blood flow in PC MRA, is the amplitude of the bipolar gradient set higher or lower?

Higher amplitude

Which pulse-sequence family is most resistant to high-velocity signal loss and time-of-flight effects?

Gradient echo (GRE) sequences

Why does inflow enhancement increase vascular signal in TOF MRA?

Inflowing blood delivers unsaturated (fresh) spins into the slice compared to saturated background tissue.

What is the primary purpose of Gradient Moment Nulling (flow compensation)?

To reduce phase-encoding errors and intravoxel dephasing caused by steady (laminar) flow.

What is the Entry Slice Phenomenon (ESP)?

An artifact where fresh, unsaturated spins entering the first slice of a multi-slice set produce an artificially bright signal compared to inner slices.

Which parameter does NOT influence entry slice phenomenon: TR, TE, flow direction, or slice thickness?

TE does not influence entry slice phenomenon.

How does co-current flow (blood flowing in the same direction as slice acquisition) affect entry slice phenomenon?

It decreases entry slice phenomenon because spins become quickly saturated as they travel through consecutive newly excited slices.

What is retrospective cardiac gating?

Continuous data collection throughout the cardiac cycle that is sorted into dynamic cardiac phases post-acquisition based on ECG R-R intervals.

What is cardiac triggering?

Using the R wave of an ECG signal to initiate the TR period for each data acquisition line.

What is the difference between triggering and gating in respiratory monitoring?

Triggering initiates data acquisition at a specific point in the cycle (e.g., end-expiration); gating defines the time window duration during which data is collected.

What does cardiac cine imaging display?

A dynamic movie-like loop showing multiple phases of heart motion across the cardiac cycle.

What is pseudo cardiac gating?

Manually setting the TR to approximate the patient's measured R-R interval without using active ECG lead triggering.

What causes intravoxel dephasing?

Spins moving at different velocities or directions within a single voxel acquire different phases, causing destructive interference and signal loss.

For which type of flow is gradient moment nulling (flow compensation) LEAST effective?

Turbulent or accelerating flow

True or False: If a proton receives an excitation pulse but is never rephased, it will produce a measurable MR signal.

False

How does Gadolinium contrast enhance vascular signal on T1-weighted MRA sequences?

It dramatically shortens the T1 recovery time of blood, causing rapid longitudinal recovery and bright signal.

Which parameter does NOT affect time-of-flight signal voids: slice thickness, flow velocity, TR, or TE?

TR

What post-processing technique displays the brightest voxels to form 3D angiographic images, and what does it stand for?

MIP (Maximum Intensity Projection).

Why do gradient echo sequences inherently display bright blood?

Gradient rephasing works across the whole bore and combined with short TE, moving spins remain rephased and produce bright signal.

MRI spatial localization is achieved through the controlled application of which elements?

Magnetic field gradients.

Slice selection is accomplished by applying a gradient magnetic field simultaneously with what?

A slice-selective RF transmit pulse

The RF pulse used for slice selection must be transmitted at what?

A specific range of frequencies that matches the tissues within the desired slice.

What process distinguishes spins according to how rapidly they precess in the presence of a gradient field?

Frequency encoding.

What process induces a position-dependent phase shift along the gradient axis?

Phase encoding.

The Fast Fourier Transform (FFT) converts MRI signal data from the frequency domain into what domain?

The spatial domain (spatial locations in the image).

True or False: During phase encoding, the system applies a bipolar gradient to ensure phase coherence.

False.

Decreasing the phase field of view (FOV) while maintaining matrix size results in what?

Better spatial resolution

Lower SNR.

Thicker slices produce what image changes?

Higher SNR,

Lower through-plane spatial resolution.

Decreasing (narrowing) the receive bandwidth produces what effects?

Increased SNR

Increased chemical shift artifact,

increased minimum TE.

Why does the minimum TE increase when you decrease receive bandwidth?

Because sampling takes longer so the echo peak must be placed later in time.

Which parameters directly affect total scan time in 2D MRI imaging?

TR,

Phase matrix,

NEX

ETL (echo train length).

How do parameters like bandwidth, frequency matrix, and TE indirectly affect scan time?

They alter the maximum number of slices allowed per TR, which may force an increase in TR to cover the required anatomy.

If you reduce the phase matrix while keeping TR constant, what happens?

Faster scan time,

Lower SNR

Lower spatial resolution in the phase direction.

What is the sample window in MRI?

The time duration during which the echo is sampled

What two parameters determine the duration of the sample window?

Frequency matrix size

Receive bandwidth.

What parameter primarily determines the sampling rate (how fast individual data points are collected)?

Receive bandwidth

Why does chemical shift misregistration occur between fat and water?

Fat and water precess at slightly different frequencies. Since FFT maps position based on frequency, this frequency difference causes spatial misregistration along the readout axis.

How does parallel imaging reduce total scan time?

By acquiring fewer phase-encoding lines and using multi-channel coil sensitivity maps to mathematically unwrap spatial aliasing.

What is the primary SNR drawback of using parallel imaging?

Decreased SNR

What artifact risk is associated with parallel imaging?

Increased sensitivity to reconstruction artifacts,

Why do multi-channel phased-array coils provide higher SNR than a single large volume coil?

Multiple small coil elements placed close to anatomy detect signal more efficiently with less noise, yielding higher overall combined SNR.

What image characteristics are primarily determined by the central region of k-space?

Image contrast and overall signal (SNR).

What image characteristics are primarily determined by the peripheral regions of k-space?

Fine detail and spatial resolution.

How does Partial Fourier imaging shorten total scan time?

By filling slightly more than half of k-space and mathematically estimating the remaining data using k-space symmetry.

What condition must be met before an image can be reconstructed via FFT?

k-space must be completely filled

In which domain do raw digital signal intensity values reside prior to FFT processing?

The k-space (spatial frequency) domain.

In a Fast Spin Echo (FSE) sequence with an Echo Train Length (ETL) of 2, how many k-space lines does each individual echo fill?

Each echo fills exactly one line of k-space per TR.

What is the primary purpose of phase or frequency oversampling in MRI?

To prevent aliasing (phase wrap / wrap-around artifact).

How are individual slices generated within the volume during 3D imaging?

Through slice encoding

Why does 3D volume imaging take longer to acquire than 2D slice imaging?

Because k-space becomes 3D; a full 2D k-space matrix must be filled for every individual slice-encoding step.

What is slice wrap in 3D imaging and how is it prevented?

Slice wrap is aliasing along the slice axis where outside anatomy folds into outer slices. It is prevented by applying slice oversampling.

Increasing receive bandwidth requires what change to the readout (frequency-encoding) gradient?

An increase in readout gradient amplitude

What are the two main disadvantages of parallel imaging?

Reduced SNR

Increased susceptibility to reconstruction artifacts

What are the main differences between Fat Saturation (fat sat) and STIR?

Fat sat is frequency-selective STIR uses T1 recovery differences to null fat at a specific TI,

How does STIR work, and what property allows fat suppression?

STIR applies a 180° inversion pulse and waits a short inversion time (TI) until fat magnetization reaches the null point (M=0).

Why do different tissues reach the null point (M=0) at different times during inversion recovery?

Because different tissues have different T1 recovery times

What is the Dixon technique for fat suppression and how is a water-only image calculated?

Dixon uses in-phase and out-of-phase echoes to separate fat and water. Water Image = (In-phase + Out-of-phase) divided by 2.

Which gradient echo method generates multiple echoes per TR by oscillating the readout gradient?

Echo Planar Imaging (EPI).

What is the primary purpose of Inversion Recovery (IR) sequences?

To suppress signal from a specific tissue based on T1 recovery differences.

What sequence consists of a 180° inversion pulse, a 90° excitation pulse, and multiple 180° refocusing pulses?

Inversion Recovery Fast Spin Echo (IR-FSE)

How does TE determine image contrast in Fast Spin Echo (FSE), and how is central k-space filled?

The echo placed in central k-space determines overall image contrast. central k-space is filled

central k-space is filled using raw MRI data collected when the phase-encoding gradient strength is zero or near zero

What scan parameter determines whether fat or fluid is suppressed in an Inversion Recovery sequence?

The Inversion Time (TI).

What components make up the steady-state signal in gradient echo sequences?

Free Induction Decay (FID) and stimulated/Hahn echoes

What scan conditions make a Gradient Echo (GRE) image more T1-weighted vs. more T2*/steady-state weighted?

More T1-weighted: Spoiled (destroyed residual transverse magnetization), large flip angle, short TE.

More T2*-weighted: Coherent steady state maintained, low flip angle, longer TE.

What is flow compensation (gradient moment nulling), and along which gradient axis is it applied?

A technique to correct phase encoding errors from flowing blood or CSF. It is applied along the direction of flow:

What is a Spoiled Gradient Echo (SPGR) sequence and when is it used?

A GRE sequence that actively destroys residual transverse magnetization between TRs. It is used to obtain pure T1 weighting and remove T2*/steady-state contributions.

In an inversion recovery sequence, what is the flip angle of the first RF pulse?

180° inversion pulse.

What is the typical Inversion Time (TI) for FLAIR at 1.5 Tesla?

Approximately 2200 ms (2.2 seconds).

In steady-state gradient echo, how is residual transverse magnetization rephased by later RF pulses?

It is rephased into overlapping Hahn echoes or stimulated echoes.

What can happen if you add more slices to a scan sequence?

It may require increasing TR, which increases overall scan time and can alter image contrast. (It does NOT directly alter minimum TE).

What factor directly determines the minimum allowable TE?

Receive bandwidth

What is an Inversion Recovery sequence that uses a short TI to null fat signal called?

STIR (Short TI Inversion Recovery).

What is Inversion Time (TI)?

The time interval between the initial 180° inversion RF pulse and the subsequent 90° excitation RF pulse.

How does a spoiled gradient echo differ from a coherent gradient echo?

Spoiled GRE actively destroys residual transverse magnetization to eliminate T2* effects for pure T1 weighting.

Coherent GRE preserves residual transverse magnetization to sample the steady state for T2* weighting.

What are the trade-offs of decreasing the phase matrix size?

Decreases scan time and increases SNR but worsens spatial resolution in the phase direction.

Which gradient echo technique produces the most true T2 weighting?

Reverse (rephased) steady-state gradient echo.

At 1.5 Tesla, what TE produces an out-of-phase relationship between fat and water in GRE?

Approximately 2.2 ms (and 6.6 ms).

What are the two methods of spoiling residual transverse magnetization, and which is more effective?

RF spoiling and gradient spoiling. RF spoiling is generally more effective.

What are the consequences of reducing receive bandwidth?

Increases SNR,

worsens chemical shift artifact

lengthens minimum TE,

reduces maximum available slices per TR.

Which technical parameters define physical slice thickness in MRI?

Transmit RF bandwidth and slice-select gradient slope (amplitude).

How can scan time be shortened if a patient cannot remain still?

Decrease phase matrix,

decrease TR

, decrease NEX,

increase ETL

What is the scan time equation for a 2D spin echo or FSE pulse sequence?

Scan time = (TR × Phase Matrix × NEX) / ETL

Why are Inversion Recovery sequences typically longer than standard Spin Echo sequences?

Because IR includes an extra TI delay period between the 180° inversion and 90° excitation pulses, lengthening total scan time.

What pulse sequence begins with a small flip angle and uses the frequency-encoding gradient to rephase signal?

A standard Gradient Echo (GRE) sequence.

What primary factors determine image contrast in a Gradient Echo sequence?

Flip angle, TE, and whether residual transverse magnetization is spoiled or preserved (coherent/steady-state).

Why don’t we have to sample every single line of k-space to reconstruct an image?

Because k-space is symmetrical. The top and bottom (and opposite sides) contain mirrored information, so the system can use partial Fourier:

What is Partial Fourier?

A technique where the system acquires slightly more than half of k-space and then estimates the remaining data using the symmetry of k-space.