MR Physics 2

Question 1

What are the 3 main steps that need to be completed to generate an image?

Answer 1

1. Measure the H20 signal in 3D
2. Distinguish signal from 3 orthogonal axis
3. Use 3 magnetic field gradients: x, y, z

Question 2

Why are magnetic field gradients needed and what do they do?

Answer 2

By passing current through gradients created by coils of wire (gradient coils), the magnetic field strength is altered in a controlled and predictable way. Gradients add or subtract from the existing field in a linear fashion, so that the magnetic field strength at any point along the gradient is known

The strength/amplitude of the gradient is determined by the amount of current applied to the gradient coil (maximum amplitude determines maximum achievable resolution)

Question 3

How is variation in magnetic field intensity produced?

Answer 3

Produced by a pair of coils placed in each spatial direction

x-axis gradient
y-axis gradient
z-axis gradient

Question 4

How do gradient coils work?

Answer 4

When current is passed through these coils a secondary magnetic field is created

This gradient field slightly distorts the main magnetic field in a predictable pattern, causing the resonance frequency of protons to vary in as a function of position

Question 5

How do we generate an image?

Answer 5

1. Measure the H20 signal in 3D
2. Define change in signal with respect to location in 3 axis
3. Use 3 magnetic field gradients applied at different times

Question 6

What are the 3 magnetic field gradients applied at different times?

Answer 6

Slice direction
Read direction
Phase direction

Question 7

Explain slice selection, what axis does slice selection utilise?

Answer 7

z-axis gradient

Slice selection is the selection of spins in a plane through the object
Achieved by applying a one-dimensional, linear magnetic field gradient during the period that the RF pulse is applied
A 90 degree 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 8

Explain frequency encoding and what plane?

Answer 8

x-axis gradient
Once you have selected a slice by applying a gradient in the z-axis, we want to select a section of the slice in the x-axis
Do this by applying another magnetic gradient in the x-axis
Each segment will now return a signal of a different frequency depending on the location along the slice
As they are of different frequencies, they will eventually become of different phases
Adding the signals together gives a large signal which eventually drops off as the phases diverge - this is the frequency encoding gradient

Question 9

What is phase encoding?

Answer 9

Localises the signal along the single column in the single slice by applying a gradient in the y-axis

1. Apply a phase encoding gradient to dephase spins along the vertical axis.
2. When we switch off the gradient all the segments return to the same frequency but they are now all out of phase with a phase-shift that depends on the position along the column
3. The protons in the same row, perpendicular to the gradient direction, will all have the same phase. This phase difference lasts until the signal is recorded.
4. On receiving the signal, each row of protons will be slightly out of phase. This translates as their signals being more or less out of phase.

To obtain an image, it is necessary to multiply the different dephased acquisitions, which are regularly incremented. For a spin echo sequence with « n » rows, we make « n » acquisitions each with a different phase encoding gradient.

Question 10

What order is the MRI pulse sequence?

Answer 10

RF pulse - excitation
Slice selection
Phase encoding
Frequency/read encoding
== Acquire signal

Question 11

What is the method of basic image acquisition?

Answer 11

2D spin warp sequence

Question 12

Explain 2D spin warp sequence

Answer 12

Most common method for spatial localisation
Uses sequential frequency and phase-encoding steps with Fourier transform reconstruction

1. Slice is selected Gz
2. Frequency encoding gradient Gx - data is acquired for each repetition of the sequence
3. Phase encode gradient Gy is applied at different strengths before data acquisition
4. Data is stored in a matrix called k-space

Question 13

What is k-space?

Answer 13

An abstract concept referring to a data matrix of FIDs - array of numbers representing spatial frequencies in the raw MR image

Question 14

What is FID?

Answer 14

Free induction decay - a dampened oscillation at the resonance frequency recorded when the net magnetization is tipped into the transverse plane

Question 15

How do the properties of the k-space relate to the image?

Answer 15

The centre of k-space contains low spatial frequency information, determining overall image contrast, brightness and general shapes
Lower spatial frequency data have the highest amplitude, giving the greatest changes in grey levels (contrast)

High spatial frequency data (at the periphery) have a lower amplitude - they don’t have effect on contrast or general shape but sharpens the image as they encode the edges
The further from the centre of k-space the data are collected, the higher the spatial frequency information and the better the spatial resolution will be

Question 16

What areas of the k-space have what type of signal?

Answer 16

Centre = maximum signal (contribute mostly to intensity)

Periphery = smallest signal (contributes to detail)

Question 17

What are gradient echo sequences?

Answer 17

Alternative to spin-echo sequences
1. Utilisation of gradient fields to generate transverse magnetisation
2. Flip angles of less than 90 degrees

Generated by the frequency encoding gradient, except that it is used twice in succession and in opposite directions
Used in reverse at first to enforce transverse dephasing of spinning protons and then right after, it is used as a frequency encoding gradient to re-align the dephased protons and hence acquire the signal

Question 18

What is Fourier transform?

Answer 18

Fundamental tool in the decomposition of a complicated signal, allowing us to see clearly the frequency and amplitude components hidden within

In the process of generating an MR image, the Fourier transform resolves the frequency and phase encoded MR signals that compose the k-space

Question 19

How does Fourier transform work?

Answer 19

Single oscillating wave has one frequency
Multiple oscillating waves have multiple frequencies
FT converts data measured in time to frequency
The raw data in k-space has two dimensions in time along the x and y axis
FT applied to both dimensions to extract frequency information

Question 20

How do you increase the resolution of an image? What is a consequence of this?

Answer 20

Increased by acquiring more data points

Easy to increase the read encoding data points without time penalty (fast sampling rate)
More phase encoding data points substantial increases imaging time (one point per TR)

Question 21

What is the dominant method of filling the k-space?

Answer 21

Line-by-line Cartesian method
In this method, each digitised echo completely fills a line of k-space
The echo signal is recorded in the quadrature, so each k-space point contains real and imaginary components
The k-space values on the left side of each row are obtained early in the evolution of the echo while those on the right side are obtained later
The centre of the echo occur near the middle of each row of k-space

Question 22

What are some other methods of sampling k-space?

Answer 22

Radial (start at centre and project outwards)
Spiral
Zig zag

Question 23

What is sample compressed sensing?

Answer 23

A method for accelerating MRI acquisition by acquiring less data through undersampling the k-space

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 reconstructs the missing data