MR Physics 2: Generating an Image

Question 1

At a basic level how to we generate an image?

Answer 1

We need to measure the 1H2O signal in 3D

Distinguish signal from 3 orthogonal axis

Use 3 magnetic field gradients: Z, X, Y

Question 2

Why do MRI scanners need magnetic field gradients?

Answer 2

Magnetic field gradients are needed to encode the signal spatially

They produce a linear variation in magnetic field intensity in a direction in space

This variation in magnetic field intensity is added to the main magnetic field, which is far more powerful

Question 3

How are different magnetic fields generated?

Answer 3

The variation is produced by pairs of coils, placed in each spatial direction (X, Y & Z)

Question 4

How is the frequency measured?

Answer 4

By adding variation in field intensity to the main magnetic field (the field that is kept homogenous through the shim coils and ferromagnetic blocks in the bore)

Question 5

Frequency is a function of what?

Answer 5

The gradient and the position of the nuclei

Question 6

What is the MRI pulse sequence?

Answer 6

A programmed set of changing magnetic gradients - a seqeunce of events that control the scanner to control magnetisation

Question 7

What axis does slice selection use?

Answer 7

The Z-axis gradient

Question 8

Describe how we do slice selection.

Answer 8

Turn on a z-axis gradient to define frequency in space​

Apply a one-dimensional, linear magnetic field gradient during the period that the RF pulse is applied

A 90° pulse applied in conjunction with a magnetic field gradient will rotate spins which are located in a slice or plane through the object.

Water magnetisation with Larmor frequency matching rf pulse are excited​

Question 9

What axis does frequency encoding use?

Answer 9

The X-axis gradient

Question 10

Describe frequency encoding.

Answer 10

Frequency-encoding may be used to define location within a slice or between slices.

Done once slice selection has been done in the Z-axis

Apply another magnetic gradient but this time in the X-axis - this changes the Larmor frequencies of the nuclei in a gradient along the x-axis.

Each segment now returns a signal of a different frequency depending on its location along the slice – as they are of different frequencies, they eventually become of different phases.

Adding the signals together gives a large signal at the start, when they are still all in phase, but this signal drops off as the phases diverge.

This gradient is called the “read out” or “frequency encoding” gradient.

Question 11

What axis does phase encoding use?

Answer 11

The Y-axis gradient

Question 12

Describe phase encoding.

Answer 12

Applied in the Y-axis gradient & while it is applied it modifies the spin resonance frequencies, inducing dephasing of the spins, which persists after the gradient is interrupted

This results in all the protons precessing in the same frequency but in different phases

On receiving this signal, each row of protons will be slightly out of phase- their signals are more or less out of phase

Question 13

What is Fourier transform in the process of generating an image?

Answer 13

The Fourier transform is a mathematical technique that allows an MR signal to be decomposed into a sum of sine waves of different frequencies, phases, and amplitudes.

It resolves the frequency- and phase-encoded MR signals that compose k-space

Converts amplitude as a function of time to amplitude as a function of frequency

Question 14

What is a 2D spin warp sequence?

Answer 14

Basic method of image generation

Question 15

Describe the stages of 2D spin warp sequence.

Answer 15

1. Slice is selected (Z axis gradient)
2. Readout gradient (X axis gradient) data is acquired for each repetition of the sequence
3. Phase encode gradient (Y axis gradient) is applied at different strengths before data acquisition
4. Data is stored in a matrix called k-space

Question 16

What is K space?

Answer 16

The raw data matrix - signals acquired for x and y dimensions are stored in a 2D matrix called k-space

It is a matrix of FIDs that sample spatial frequency

Question 17

Describe information stored in the centre of K space.

Answer 17

Center of k-space has maximum signal which contributes mostly to intensity

It contains low spatial frequency information, determining overall image contrast, brightness, and general shapes

High amplitude

Question 18

Describe the information stored at the edge of K space.

Answer 18

Edge of k-space ghas the smallest signal but contributes to the detail

It contains high spatial frequency information (edges, details, sharp transitions)

Low amplitude

Question 19

How can resolution of K-space be increased?

Answer 19

By acquiring more data points

Question 20

What does increasing the size of the K-space matrix (number of pixels in the matrix) do?

Answer 20

Increases spatial resolution – smaller pixels/voxels means better detail

Decreases signal – there are fewer photons per voxel so the signal is less

Increases scan time – more voxels need to be acquired becuase more signals need to be created (only in the phase encoding direction as each voxel requires a new signal)

Question 21

How does increasing the field of view affect MRI image quality?

Answer 21

larger field of view =larger area imaged but the matrix size remains the same and so, to fill up a larger area, the voxel becomes larger

Increases the signal – a larger voxel means more signal received per voxel

Lower resolution – the voxels become larger

Increased viewing area

Question 22

How does slice thickness affect the quality of the MRI image?

Answer 22

Want to reduce the amount of space between each slice to prevent sections being missed but when slices overlap an area of cross-talk results which causes artefacts

Increased slice gap =
Less cross-talk
Increased coverage – slices placed further apart and, therefore, cover a larger area.

Question 23

What are gradient echo sequences?

Answer 23

A gradient echo is a manipulation of the FID signal that begins by applying an external dephasing gradient field across the specimen or tissue.

This gradient causes a calibrated change in local magnetic fields and alters the resonance frequencies slightly across the specimen.

This results in accelerated dephasing and ‘squelching/ scrambling’ of the FID.

In step 2, the process is reversed.

A rephasing gradient is applied with the same strength but opposite polarity to the dephasing gradient, reversing/ undoing the phase scramble.

Question 24

What is a benefit of gradient echo sequences?

Answer 24

They permit the use of shorter TR values and this produces faster image acquisition.

Question 25

Why do we sample K-space?

Answer 25

Understanding k-space sampling offers important insight into the properties of a sequence e.g. info about signal-to-noise ratio, image distortion, resolution and contrast

K-space is sampled at regular points to capture array of spatial frequencies

Question 26

What is the most common scheme to sample K-space?

Answer 26

Cartesian

Question 27

What are other schemes used to sample K-space?

Answer 27

Radial
Spiral
Zigzag

Question 28

What is compressed sensing of K-space?

Answer 28

The symmetry of k-space can be exploited to reduce imaging time

k-space lines are sampled semi-randomly. The centre of k-space is sampled at higher density

An algorithm then reconstructs the missing data