MRI: Image Formation & Instrumentation Flashcards Preview

Medical Physics: Imaging w/ Non-Ionising Radiation > MRI: Image Formation & Instrumentation > Flashcards

Flashcards in MRI: Image Formation & Instrumentation Deck (31)
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How can 1-dimentional spatial information be encoded into the MR signal?

Altering the magnetic field using a magnetic field gradient alters the resonant frequency of the sample in that direction.
The magnitude of the frequency change depends on the sample's position in the gradient.
This is the "read-out gradient".


On what does the amplitude of each frequency from a simple MR scan depend?

The total concentration of nuclei at that point in space.


An ideal Fourier Transform samples from -inf to +inf, how is this dealt with in an MR sequence?

Negative gradient applied before sampling to simulate "negative time".
Length of gradient should be 1/2 of sampling time.


How is the second spatial dimension encoded into an MR signal?

"Phase Encoding gradient".
Orthogonal to Read-Out.
Repeat sequence but reduce amplitude of PE gradient,
The phase of the signal is modified dependent on position orthogonal to the Read-Out gradient.
2D-Fourier Transform to convert modulation into an image.


How is the third spatial dimension encoded into an MR image?

Slice Selection gradient applied during RF pulse only excites protons at a certain point along the z-axis. All signals come from that slice.


What determines the slice thickness from a slice selection gradient?

Gradient steepness.
RF pulse bandwidth.


What are the four main components of an MR scanner?

Gradient System.
Magnet/Shim System.
Radio-Frequency System.
Spectrometer-Computer System.


What are the requirements of the magnet system?

High field strength
Highly uniform over a large volume
Temporally stable to 1 part in 10^8
Sensitive to frequency shifts of <1Hz


What are some of the cryogen issues associated with a superconducting magnet?

As the magnet contains large volume of liquid helium so must be thermally isolated to minimise boil off.
The gas has a low thermal heat capacity, so there is a potential for a lot of gas to escape if heat is input - quench.
Quench pipe to vent gas to the atmosphere.


How is passive shimming used to increase homogeneity in the magnet?

Structures in the local environment can distort the homogeneity in the magnet field. Small ferromagnetic plates are arranged inside the scanner bore to increase homogeneity
This process performed once during the installation.


Why must the fringe field be managed using magnetic shielding?

Can cause interactions with the environment, which affect the image quality, and the people and equipment in the local environment.


What are the three options for magnetic shielding?

Site magnet is a large open space with controlled access with the fringe field.
Place magnet in a steel shield to constrain the field.
Actively shielded scanner design.


What are the advantages of a permanent magnet?

Open design - less claustrophobia.
Low running costs.


What are the disadvantages of a permanent magnet?

Low field strength (<0.5T)
Very heavy


What are the advantages of a superconducting magnet?

Very stable
High field strength


What are the disadvantages of a superconducting magnet?

Expensive to purchase
High running costs


What are the components of the RF system?

RF Waveform Controller
RF Amp
Tx/Rx Switch
RF Coil
Demodulator (High field systems)
RF Reciever system


What is the purpose of the RF Waveform Controller?

Generates the RF pulse shape at the correct frequency.


What is the purpose of the RF amplifier?

Linear amplifier to amplify the basic low voltage RF pulse shape.
Tuned to work at the resonant frequency.


What is the purpose of the Transmit Recieve (Tx/Rx) switch?

Controls whether the RF coil is being used to transmit or receive signal.
During transmission ensures no power goes into the receiver.


What is the purpose of the RF coil?

Transmits the RF pulse into the patient and picks up the emitted RF signal.


What is the purpose of the preamplifier?

High gain low noise amplifier to maximise SNR.


What is the purpose of the RF receiver system?

Reduces the signal from around the Larmor frequency (10s MHz) to audio waveband (100s kHz bandwidth).


What is the purpose of the Digitiser?

Converts the analogue signal to digital information for processing and storage.


How does the sensitivity of an RF surface depend on size?

Small size - Higher sensitivity
Limited Field of View


What are the requirements of gradient coils?

Variation in Bz with distance.
High gradient strength - high current density
Low inductance - fast switching


How are eddy currents induced in the gradient coils?

Gradient coils created rapidly varying magnetic fields
Field extend inside and outside the coil structure
Coils are mounted in the scanner bore (a metal tube)
Interaction between external field and the bore induce Eddy currents.


How are eddy currents in the coils controlled?

Self-Shielded gradient coils
Adding a secondary winding on a second larger coaxial cylinder, wound in opposite direction to cancel out field.


What are the requirements for a gradient amplifier?

Must create high gradient strength.
Must establish gradient rapidly.
Must be temporally stable.


What are the characteristics of a gradient amplifier?

Constant, High current (400-600A)
High voltage (800V)
High duty cycle (ideally 100%)
Water cooled.