MRI Flashcards
What is MRI?
a spectroscopic imaging technique used in medical settings to produce images of the inside of the human body
How does MRI work?
absorption + emission of energy of the RF EM Spectrum
Adv of MRI (4)
- Utilises non ionizing radition
- ability to image in any plane
- very low incidents of side effects
- ability to diagnose, visualize + evaluate various illnesses
Components of MRI (4)
- Magnet - powerful uniform magnetic field
- Gradient magnet (lower in strength)
- RF equipment
- Powerful Computer
Field strength of MRI magnet
.5 - 2.0 T
Types of magnets used in MRI (4)
- Resistive Magnets
- Permanent Magnets
- Super conducting magnets
- Gradient Magnets (used to create variable field)
Proton and neutron spins are known as …
… nuclear spins
What is spin?
A small magneti field, either + or - and mathematical value of 1/2
An unpaired component has a spin of 1/2 and two particles …
… with opposite spis cancel out
In NMR, the magnetic field is produced by …
… the unpaired nuclear spins
Why are Protons important to MIR? (4)
- +’ve charge
- spin about central axis
- a moving (spinning) charge creates a magnetic field
- straight arrow (vector) indicates the direction of the magnetic field
When placed in a large magnetic field, hydrogen atoms have …
… a strong tendancy to align in the direction of the magnetic field
The magnetic field runs down the …
… centre of the tube, so the hydrogen protons will align at either the head or feet
Majority of protons will …
… cancel out, but the net number of protons is sufficient to produce an image
MRI machine applies a RF pulse that is …
… specific to hydrogen
RF pulse are applied through a coil that is …
… specific to the part of the body being scanned
The gradient magnets are …
… rapidly turned on and off which alters the magnetic field
Larmor Equation
What does the Larmor Frequency?
Frequency need to excite transverse magnetization (precession)
When the RF pulse is turned off, the hydrogen protons …
… slowly return to their natural alignment within the magnetic field and release their excess stored energy
What happens to the released energy? (3)
- Released as heat
- Exchanged and absorbed by other protons
- Released as Radio Waves
Measuring the MR Signal (3)
- Moving proton vector induces a signal in the RF antenna
- Signal is picked up by coil and sent to computer
- Computer uses Fourier transform to convert to an image
Magnetisation State (2)
A
- Protons spinning in nature, w/o external mag field.
- Directions of spins are random and cancel out each other
- Net magnetisation is 0
B
- Large magnetic field, B_0, spins align themselves either
- against (high energy state)
- along (low energy state)
- Slight abundance of spins aligned in low energy state
Proton Alignment (4)
A - spin of a proton aligned to B0 in the z-axis
B - An external perturbing magnetic field, B1, knocks the vector out its axis, which is now aligned at a new angle w.r.t B0
C - As the pertubing field B1 is removed, the vector gradually starts returning back to its original state
D - begins to wobble
Resonance Eqn
v - resonant frequency
gamma - gyromagnetic ratio
B_0 - magnetic field
T1 Parameter
the spin-lattic relaxation time-scale for the longitudinal magnetisation to come back to its original value
T2 Parameter
the spin-spin relxation time for the transverse magnetisation to come back to its initial value
T1 weighted imaging has …
… higher spatial resolution
T2 weighted imaging has …
…. higher tissue contrast
What does the image represent?
Every tissue layer has a different volume of water thus a different number of hydrogen atoms which produce different images based on the # hydrogen atoms
