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Medical Physics > MRI Instrumentation > Flashcards

MRI Instrumentation Flashcards

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1
Q

Main sections of MRI instrument

A

Radio frequency
Gradient system
Shim controller
Magnet
Spectrometer/computer

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2
Q

Radio-frequency section in order

A

RF Waveform controller
RF amp
T/R switch
Pre amp
Demodulator (in high field systems only)
Rx Digital

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3
Q

Requirement for magnet

A

High magnetic field strength
Highly uniform
Temporally stable

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4
Q

Permanent vs superconducting magnet

A

Permanent: open design and low running costs but low field only (<0.5T), heavy and lack of stability
Superconducting: very stable and high fields but expensive and high running costs

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5
Q

Superconductivity

A

Niobium/titanium in a copper matrix
Tc~10K
Convenient to use liquid helium

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6
Q

Conventional vs refrigerated magnet design

A

Conventional design needed topping up with liquid N and He, boil off not captured, lost to space
Modern systems reliquify He vapour, topped up yearly. No helium loss but 24/7 electricity usage

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7
Q

Quench

A

System needs a quench pipe to vent gas to atmosphere, contains a lot of liquid He that has low thermal heat capacity, a lot of gas if hated up.

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8
Q

RF coil

A

RF antenna used to transmit RF pulse or receive MR signal.
Simplest is surface coil, flat coil. Sensitivity depends on size, smallest have highest but small field of view.

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9
Q

Coil designs

A

Golay Gx, Gy - go partway around
Maxwell Gz - go all the way around in a loop

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10
Q

Gradient system requirements

A

High gradient strength ~40mT/m
Established rapidly
Temporally stable

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11
Q

Shim system

A

Object in scanner distorts B field, correct this using shim coils - adjusts field to be as uniform as possible. Performed on every subject.

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12
Q

Control system consists of

A

PACS
Console (host)
Master control and reconstruction
Controllers (RF,Grad,Rx)

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13
Q

Role of RF controller

A

Generate pulse at correct f

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14
Q

Role of RF amplifier

A

Amplify basic low voltage RF pulse shape

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15
Q

Role of T/R switch

A

Control if coil is being used to transmit or receive signal, ensure no power goes to receive during transmission

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16
Q

Role of preamplifier

A

Maximise signal strength

17
Q

Role of RF receiver system

A

Reduce signal from around larmor frequency to audio wavband

18
Q

Role of digitiser

A

Converts analogue signal to digital information for processing and storage

19
Q

Required qualities for test objects and test conditions

A

Test objects need high temperature stability (T1 and T2 can change with T) and minimal susceptibility effects.
Scanner tested under routine conditions, TR, TE and relaxation times mimicking tissues

20
Q

3 layers of test object

A

Grid - check geometry
Flood field - uniformity
Wedges - slice position/profile

21
Q

Noise in MR images

A

Random fluctuations in image background (and over object) ideally only from electrical noise - generated inherently in receiver electronics

22
Q

Instrumental factors of SNR

A

RF chain in particular:
Poor calibration in RF chain or amplifier problem
RF coil performance
Preamp problem

23
Q

Method 1 testing SNR

A

Use uniform flood field region
Collect 2 images with identical parameters
Subtract them
Measure signal (S) as mean across 5 ROIs in basic image
Measure SD of noise in same ROIs in difference image

SNR = sq.rt 2 x S/average of SD_noise

24
Q

Method 2 testing SNR

A

Less robust

Use flood field region
Collect one image
Measure signal (S) as mean across 5 ROI in basic image
Measure SD of noise in area outside object

Image is signal magnitude, affects noise characteristics, gaussian becomes Rayleigh

SNR = (0.66 x mean signal)/SD of noise

25
Q

Issue with method 2 of SNR

A

Requires background noise to be unfiltered and assumes that noise away from object is same as noise in object, may not be true if filters are applied, parallel imaging applied, multichannel array coil used

26
Q

Test intensity uniformity

A

Require uniform signal over field of view
Use uniform flood field
Collect image
Measure intensity profile across image (should be flat)
Measure SD of signal over largest area of object (uniform intensity produces narrow distribution)

27
Q

Instrumental factors affecting uniformity

A

RF coil problem (channels not behaving equally)
Shim problem (poor B0 homogeneity)
Magnet uniformity

28
Q

How to test stability and how it originates

A

Multiple phase encoding steps used
Assumes object remains unchanged and system is stationary
Instabilities create variations in signal amplitude and/or phase - phase encoding artefacts
Test using multiple spin echo times using long TE - allows enhanced effects.

29
Q

Instrumental factors stability

A

RF amplifier problem (fluctuating phase angle) - RF excitation phase stability
Gradient or shim amplifier problem

30
Q

Instrumental factors - image geometry

A

Gradient amplifier problem (axes not equally calibrated)
Shim problem

31
Q

Slice profile/thickness/offset

A

Use overlapping wedge area
Look at slice thickness and offset.
Measured from image through overlapping wedges - location of band moves with slice

32
Q

Image geometry

A

Use grid to check for distortions - holes are a known distance apart, can use measuring tool to determine if they are the same length.

33
Q

Areas to assess for QA

A

SNR
Stability
Uniformity
Geometry

34
Q

How does shim system work

A

The B0 distribution will be measured within the prescribed volume of interest, decomposed into spherical harmonics and currents applied to the various shim coils to compensate the distortion of the field

35
Q

Name 3 hazards when imaging, not specific to a patient

A

Static magnetic field
Time varying magnetic fields from either MRI gradients or RF irradiation
Cryogens
Contrast agents

Medical Physics (16 decks)

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