Instrumentation Flashcards
Components of MR
4 majour groups
All subsequent components
- RF
- RF waveform controller
- RF ampl
- T/R switch
- Pre-Amp
- Demodulator
- Rx Digital
- Gradient
- Gradient controller, Gx, Gy, Gz
- Magnet
- Sim controller and coil
- RF coil
- Gradient coil
- Main magnet
- Spectrometer/computer
3 Requirements for good magnet
2 types of magnets you could use, uses and field strength?
- High field strength
- Highly unuform over large volume
- Temporaly stable
2 types of magnet
- Permanent magnet - horseshoe system, easier for patent to get in, 0.5T small field strength.
- Superconducting magnet - 1.5-3T field strength, solenoidal design
Design of superconducting magnet:
Materials?
Difference between old and new-designs
Advantage and disadvantage of modern design
What’s a wuench pipe for?
Titanium in copper matrix, temperature needs to be 10K, use liquid helium
Traditional : solenoid windings with Helium and Nitrogen cans, gas cooled shield, outer vaccuum can. But the boil-off is not captured.
New design: reliquify helium vapour boil-off, don’t use Nitrogen. Liquifying compressor called the cold head seits on top of the cryostat. No Helium loss, but 24/7 electricity usage.
If liquid helium gets heated up, there is potential for a lot of gas which needs to be released. Don’t want this to happen in room with patient, use pipe to vent it outside.
2 advantages and 2 disadvanteages for permanent and superconducting magnet design
Permanent
- +ves: Open design & low running cost
- _ves: Only 0.5T & poor stability
Superconducting
- +ves: Very stable & high fields
- _ves: Expensive & high running costs
RF 7 main components and description
- Controller - generates RF ulse shape at right frequency
- amplifier - amplify RF signal
- TR switch - controls whether RF coil is being used to transmit or recieve signal
- coil
- pre-amplifier - high gain low-noise to max SNR
- reciever - reduces signal from 105MHz (fL) to audio waveband
- digitiser - converts analogue signal to digital information for processing and storing
RF Coils
Basic, how does it work?
Surface coil design, best sensitivity?
Birdcage design
RF attenena used to transmit or recieve RF. Tuned circuit operating at FL.
- Coil is loop with inductance L
- Add other components e.g. fixed variable capacitors
- Resonant frequency adjusting via tuning capacitor
- Overall impedence of 50Ohm obtainef via matching capacitor
Surface coul is simplest, flat coil plaed on area of interest. Sensitivity depends on size, small coil is better but limits FOV.
Birdcage - current carrying conductors arranged in circle produce magnetic field apttern that produces standing wave at fL around the end ring. Sinusoidal current distribution.
Gradient coil
Explain basic design
2 desireable properties
- Resistive coils driven by current amplifiers
- Require linear variation in Bz with distance (in 3-directions)
- Need high current density for high gradient strength
- Need Low inductance for fast swithcing
Gradient amplifiers
Requirements
features to meet the requirements
Requirements
- High grad strength (40mT/m)
- Rapid swithching (<200us)
- Temporarly stable
Features
- Constant & high current
- High voltage
- High duty cycle
- Water cooled
Shim system
Why do we need?
How does it work?
Patient in the scanner distorts the magnetic field (difference in succeptability). Correct for these using room temp shim coils.
Measure the field distortion, decompose into spherical harmonics, apply current to each coill to compensate
Spectrometer
What’s it for?
Control system does everything from
- controlling gradients, RF
- Controlling accquisition and reconstruction
- storing images and sending to PACS
