MR Physics 2 Flashcards
What are the 3 main steps that need to be completed to generate an image?
- Measure the H20 signal in 3D
- Distinguish signal from 3 orthogonal axis
- Use 3 magnetic field gradients: x, y, z
Why are magnetic field gradients needed and what do they do?
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)
How is variation in magnetic field intensity produced?
Produced by a pair of coils placed in each spatial direction
x-axis gradient
y-axis gradient
z-axis gradient
How do gradient coils work?
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
How do we generate an image?
- Measure the H20 signal in 3D
- Define change in signal with respect to location in 3 axis
- Use 3 magnetic field gradients applied at different times
What are the 3 magnetic field gradients applied at different times?
Slice direction
Read direction
Phase direction
Explain slice selection, what axis does slice selection utilise?
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
Explain frequency encoding and what plane?
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
What is phase encoding?
Localises the signal along the single column in the single slice by applying a gradient in the y-axis
- Apply a phase encoding gradient to dephase spins along the vertical axis.
- 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
- 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.
- 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.
What order is the MRI pulse sequence?
RF pulse - excitation
Slice selection
Phase encoding
Frequency/read encoding
== Acquire signal
What is the method of basic image acquisition?
2D spin warp sequence
Explain 2D spin warp sequence
Most common method for spatial localisation
Uses sequential frequency and phase-encoding steps with Fourier transform reconstruction
- Slice is selected Gz
- Frequency encoding gradient Gx - data is acquired for each repetition of the sequence
- Phase encode gradient Gy is applied at different strengths before data acquisition
- Data is stored in a matrix called k-space
What is k-space?
An abstract concept referring to a data matrix of FIDs - array of numbers representing spatial frequencies in the raw MR image
What is FID?
Free induction decay - a dampened oscillation at the resonance frequency recorded when the net magnetization is tipped into the transverse plane
How do the properties of the k-space relate to the image?
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