Why is MRI so popular? Flashcards
Why MRI?
- Based on magnetic resonance of water molecules
- Non-ionising
- Full 3D imaging technique
- Has excellent contrast in soft tissue
- Very versatile
What are the MRI Field strength?
1T = 1 Tesla 0.00005T = Average Earth's field 1.5T = Typical clinical MR scanner 3T = becoming standard MR in UK 7T = many in US, Europe, Asia, Australia 10.5T = Highest human MR scanner
What does MRI look at?
Molecular level =10-9 m
Atomic level =10-10 m
Nucleus of hydrogen atom=10-13 m
Where does spin rotate (or precess)?
About applied magnetic field B0 i.e. along z axis
What is the frequency of this precession proportional to?
Applied field
What does each water molecule within body have?
Small magnet associated with it
What is Larmor equation?
v= γB0
What does MRI rely on?
Magnetic properties of hydrogen atom to produce images
What is H nucleus composed of?
- Single proton
- Spinning charged particle
- Produces a magnetic field = magnetic moment
How are the protons orientated?
Randomly orientated with no applied field
What are the MRI components?
- Primary magnet
- Gradient magnet
- Radiofrequency (RF) coils
- Computer system
What does the primary magnetic field refer to?
The strength of the static permanent field
e.g. 1.5 or 3T
How does hydrogen atoms align?
parallel or antiparallel to primary field (B0)
known as longitudinal magnetisation
How does a greater proportion of the hydrogen protons align?
Direction parallel to the primary magnetic field or low energy state than align anti-parallel to the primary magnetic field (high energy state)
net result = net magnetic vector (M)
What is precession?
The protons spin on their axis
protons spin around long axis of primary magnetic field
What is the precession rate termed?
Larmor frequency
What is in phase?
When protons precess together?
What is out of phase?
When protons precess separately
What does frequency change with?
In proportion to the magnetic field strength
At 1.5T = 63.9 MHz
What do gradient coil generate?
Secondary magnetic field over the primary field
located within the bore of the primary magnet
They are arranged in opposition to each other to produce positive and negative poles
Why do gradient coil give MRI the capacity to do?
Image directionally along the z, x and y axis
What do gradient magnet alter?
the strength of primary magnetic field changing the precession frequency between slices
this can be used for slice selection and localised in the x, y and z axis called spatial encoding of MR images
Where does the z gradient run along?
Long axis to produce axial images
Where does the y gradient run along?
Along the vertical axis to produce coronal images
Where does the x gradient run along?
Along the horizontal axis to produce sagittal images
What is RF coils used for ?
Transmitting radio frequency or RF pulse and receiving signals in MR
What are RF coils designed for?
Specific body regions e.g. Head Body Knee Shoulder Wrist Ankle produce best diagnostic images
What is RF coil used to transmit/
Second magnetic field/RF pulse which results in a disturbance of the proton alignment
How is longitudinal magnetisation decreased?
Some low energy parallel protons flip to a high energy state
When does net magnetization vector turn toward transverse plane?
When protons become synchronised and precess in phase
What is the transverse plane?
Right angles to the primary magnetic field = transverse magnetisation
What is radio frequency or RF coil for?
Receive signal to create images as protons resume their normal state in primary magnetic field prior to the transmission of the RF pulse = this is called relaxation
What is relaxation in the longitudinal axis?
T1 relaxation
What is relaxation in the transverse axis?
T2 relaxation
What happens after the RF pulse?
Several protons flip back to their low energy state parallel to primary magnetic field z axis giving energy to surroundings the lattice
This results in changes to the longitudinal relaxation known as T1/ or spin lattice relaxation
What increases with time?
Magnetisation
T1 curve
What is spin-spin relaxatin?
After RF pulse, protons that were in phase begin to dephase out of larmor frequency in the tranverse (x-y) plane
Reduction in tranverse magnetisation
Why does spins dephase much quicker than T2?
Inhomogeneities in the magnetic field (B0)
What is T2*?
T2 relaxation + field inhomogeneities
What is net magnetic vector?
Sum of longitudinal and transverse magnetisation
Where does the net magnetic vector spiral around?
Z axis with net precession
What results in free induction decay (FID)?
Changing magnetic moment of net magnetic vector
Induces an electrical signal
Signal received by the RF coil in transverse plane
What is contrast ?
Difference in appearance of different tissues in an image
What is X-ray contrast based on?
Transmission
What is one of the most important characteristic of MRI?
Range of contrasts available
Can visualise aspect of tissue structure, composition and function
What is magnetic susceptibility?
Degree of magnetization of a material in response to applied magnetic field
What is magnetisation transfer?
physical process by which macromolecules and their closely associated water molecules (the “bound” pool) cross-relax with protons in the free water pool.
What are the advantages of MRI?
- MRI does not use ionizing radiation;
- MRI has a much greater range of available soft tissue contrast;
- MRI scanning can be performed in any imaging plane;
- MRI uses safer contrast agents;
- MRI can produce quantitative images that reflect underlying biophysical properties;
