MRI physics 2 summary

What parameter affects T1 weighting

TR (Repetition Time).

how does TR influence T1 signal?

Short TR = Strong T1 weighting (higher signal from tissues with short T1).

What parameter affects T2 weighting

TE (Echo Time).

how does TE influence T2 signal?

Long TE = Strong T2 weighting (higher signal from tissues with long T2).

What is the role of Proton Density in MRI signal?

affects the overall signal strength but is always present.

What is the purpose of the slice select gradient in MRI?

to select the slice by applying a gradient along the slice direction.

What is the function of phase encode gradients in MRI?

assign spatial positions along the phase encoding direction (y axis) through a series of steps.

What is the function of the frequency encode gradient?

to localize the signal in the frequency direction (x axis).

When is the frequency encode gradient applied?

applied during the readout phase

Where is raw MRI data stored, and what does it represent?

Raw data is stored in a matrix called k-space, which represents spatial frequency rather than the final image.

What information is found at the center of k-space?

Contains most of the image's contrast

What information is found at the edges of k-space?

Contain sharp details

How is k-space data converted into an image?

A 2D inverse Fourier transform is applied to convert k-space data into an image by transforming spatial frequency information into spatial position.

How does Fast Spin Echo accelerate imaging?

by applying multiple 180° echo pulses within a single TR. Each echo fills one line of k-space for a slice.

What happens after each 180° echo pulse in FSE?

A rewinding phase encoding gradient resets the phase, allowing a new phase encoding gradient to be applied for the next line of k-space.

How does FSE handle signal amplitude decay?

Signal amplitude decreases due to T2 decay, but each 180° pulse eliminates T2* effects (free induction decay).

Why does FSE use a relatively long TR?

A long TR allows for full relaxation of longitudinal magnetization, maximizing signal amplitude for the next 90° pulse.

What are the advantages of FSE?

1. Significant time savings compared to conventional spin echo.

2. Echo Train Length (ETL): Multiple 180° pulses produce multiple echoes per TR.

3. Efficient k-space filling: Multiple lines of k-space are filled per TR, equal to the ETL.

What determines the time savings in FSE?

The Echo Train Length (ETL)

What is the primary principle of gradient echo imaging?

uses a small flip angle to excite magnetization into the transverse plane, with gradients inducing and correcting free induction decay (FID) to create a gradient echo.

What steps occur in GRE imaging to produce a gradient echo?

• A small flip angle (α) excites part of Mz into the transverse plane.

• A phase encoding gradient spatially encodes the signal.

• A dephasing frequency encoding gradient creates an FID.

• An equal and opposite rephasing gradient corrects the FID, producing the gradient echo at TE.

What are the advantages of GRE imaging?

1. Short TR and TE: Small flip angles allow for rapid recovery of Mz, reducing TR (typically under 50 ms) for faster image acquisition.

2. Efficient magnetization use: Retains most of Mz, ready for the next pulse quickly.

What are the main limitations of GRE imaging?

1. No 180° refocusing pulse: T2* effects remain uncorrected.

2. Increased sensitivity to magnetic field inhomogeneities, causing signal degradation.

What is the tradeoff in GRE imaging?

it achieves faster imaging by using small flip angles and gradient-induced echoes but is more sensitive to T2* effects.

How does Echo Planar Imaging fill an entire k-space matrix quickly?

rapidly generates multiple gradient echoes within a single sequence, filling the entire k-space matrix in a very short time.

How are gradient echoes generated in EPI?

generates multiple gradient echoes during the Free Induction Decay (FID) by alternating dephase and rephase gradients, instead of using a single pair.

How is phase encoding achieved in EPI?

applied after each oscillation, filling one line of k-space with each oscillation.

How does T2* decay affect EPI?

T2* effects cause the FID signal to decay, limiting the number of k-space lines that can be accurately filled.

How can T2* decay limitations in EPI be addressed?

A 180° RF pulse can be applied to create a spin echo, restoring signal coherence and producing a symmetric signal decay at TE.

What are the main advantages of Echo Planar Imaging?

• Extremely fast acquisition: An entire image can be acquired in 50–100 ms, ideal for eliminating motion artifacts.

• Efficient k-space filling: A single echo fills the entire k-space matrix, reducing scan time.

What are the primary disadvantages of EPI?

1. Edge of k-space issues: Signal amplitude diminishes at the k-space periphery, reducing spatial resolution.

2. Artifacts: Increased risk of artifacts due to signal amplitude decay.

What is the tradeoff when using EPI?

rapid, motion-resistant imaging but sacrifices spatial resolution and introduces artifacts due to signal amplitude decay at the edges of k-space.