MRI Instrumentation Flashcards
Main sections of MRI instrument
Radio frequency
Gradient system
Shim controller
Magnet
Spectrometer/computer
Radio-frequency section in order
RF Waveform controller
RF amp
T/R switch
Pre amp
Demodulator (in high field systems only)
Rx Digital
Requirement for magnet
High magnetic field strength
Highly uniform
Temporally stable
Permanent vs superconducting magnet
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
Superconductivity
Niobium/titanium in a copper matrix
Tc~10K
Convenient to use liquid helium
Conventional vs refrigerated magnet design
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
Quench
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.
RF coil
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.
Coil designs
Golay Gx, Gy - go partway around
Maxwell Gz - go all the way around in a loop
Gradient system requirements
High gradient strength ~40mT/m
Established rapidly
Temporally stable
Shim system
Object in scanner distorts B field, correct this using shim coils - adjusts field to be as uniform as possible. Performed on every subject.
Control system consists of
PACS
Console (host)
Master control and reconstruction
Controllers (RF,Grad,Rx)
Role of RF controller
Generate pulse at correct f
Role of RF amplifier
Amplify basic low voltage RF pulse shape
Role of T/R switch
Control if coil is being used to transmit or receive signal, ensure no power goes to receive during transmission
Role of preamplifier
Maximise signal strength
Role of RF receiver system
Reduce signal from around larmor frequency to audio wavband
Role of digitiser
Converts analogue signal to digital information for processing and storage
Required qualities for test objects and test conditions
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
3 layers of test object
Grid - check geometry
Flood field - uniformity
Wedges - slice position/profile
Noise in MR images
Random fluctuations in image background (and over object) ideally only from electrical noise - generated inherently in receiver electronics
Instrumental factors of SNR
RF chain in particular:
Poor calibration in RF chain or amplifier problem
RF coil performance
Preamp problem
Method 1 testing SNR
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
Method 2 testing SNR
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
Issue with method 2 of SNR
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
Test intensity uniformity
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)
Instrumental factors affecting uniformity
RF coil problem (channels not behaving equally)
Shim problem (poor B0 homogeneity)
Magnet uniformity
How to test stability and how it originates
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.
Instrumental factors stability
RF amplifier problem (fluctuating phase angle) - RF excitation phase stability
Gradient or shim amplifier problem
Instrumental factors - image geometry
Gradient amplifier problem (axes not equally calibrated)
Shim problem
Slice profile/thickness/offset
Use overlapping wedge area
Look at slice thickness and offset.
Measured from image through overlapping wedges - location of band moves with slice
Image geometry
Use grid to check for distortions - holes are a known distance apart, can use measuring tool to determine if they are the same length.
Areas to assess for QA
SNR
Stability
Uniformity
Geometry
How does shim system work
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
Name 3 hazards when imaging, not specific to a patient
Static magnetic field
Time varying magnetic fields from either MRI gradients or RF irradiation
Cryogens
Contrast agents
