Image Weighting and Instrumentation Flashcards
Imaging requires what pulses to be transmitted
RF pulses
All the pulses have to flip
the NMW at some angle with respect to static magnetic field B0
T2 time for fat is
shorter than CSF
Fat T1 & T2 times are
250 and 80
White matter T1 & T2 times are
800 and 90
CSF T1 & T2 times are
2400, and 280
T2 transverse is always
shorter than longitudinal T1 time
Repetition time is
the time from the application of one RF pulse to the release of the next pulse
Echo time is
the time from the application of RF pulse to the peal of the signal induced in the coil
Echo time determines how
much decay of transverse magnetisation is allowed to occur before signal is read
How are images obtained
Obtained thru contrast with T1 recovery; T2 decay and proton/spin density
T1 has
short TE & TR times
T2 has
long TE & TR times
Proton Density image is
one where the contrast depends on the number of protons per unit volume (minimise both T1 & t2 effects)
What are the processes required to make a MRI
- Nuclear alignment
- RF excitation of protons
- Spatial encoding
- Image formation
What is the hardware to make these processes
Main maget (B0); gradient coils; RF source, and image processer/comp system
What is the purpose of the main magnet?
To generate static magnetic field and promote homogenity throughout the device - must also be stable
Larmar Equations is
precessional freq = gyromatic ratio x magnetic field strength (B0)
Advantages for high field strength
signal from patient increases; faster scan times; probe physiological processes
Disadvantages of high field strengths
bioeffects; heating; nerve stimulation; cost; sitting issues; artifacts
What are the main categories of magnets
Permament; resistive and superconducting
Advantages of main magnet
no power supply; low maintenance; large bore
Disadvantages of main magnet
low SNR; long scan times; siting - heavy
Resistive magnet
less field strength; current passes thru solenoid,
Main advantages of resistive
good uniformity, i.e. generates similar field strengths from one end to the other
* light weight compared to permanent magnet
* lower capital costs
* can be turned off in an emergency
Disadvantages for resistive magnets
- requires continuous electric power to produce magnetic field - > cost
- produces a significant amount of heat
- creates significant stray magnetic fields -> safety
- low inherent SNR
Supreconducting magnets
Superconducting coils allow electric current to flow without any resistance at temperatures below -
2500C (23 K)
Quenching is
where higher temperatures cause loss of superconductivity and resistance - uncontrolled can lead to explosive boiling & damage
Computer performs image construction where it
performs Fourier transformations (FT) on the FID before displaying images
An image artefact is an
unwanted pattern or structural appearance that does not represent the actual anatomy
What are the sources of MRI artefacts
Instrumentation, patient related artefacts, metallic susceptibility, system related artefacts
Zipper artefact is caused by the
leakage of the RF signals into scanner room - faults in faraday cage door seal, unshielded equipment in MRI
Motion artefact is caused by
random motion during the image sequence which results in the blurring of the image
Source of motion can include
anterior abdominal or thoracic wall during respiration, flow related from arteries/veins, eye movements, patient moving
Magnetic susceptibility artefact occurs
as a result of the microscopic gradients or variation in the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility - metal objects insidebuman body
Magic angle artefact is
when the tissues with tightly bound collagen lie at an angle of 55 to the main static magnetic field
How does magic affect the area
restricts the molecular motion of water molecules and elongation of T2 times
A patient undergoing MRI will see that
both long and trans relaxation processes occur at the same time
