A pilot look on the events occurring in MRI
1) Patient is put into the bore
of a strong magnet.
2) Radio frequency waves are transmitted into the patient for a extremely short time [2~10 ms], then turned off.
3) Radio waves re-transmitted by the patient are then received in the Receive coil.
4) Measured RF data are then transformed to image.
MRI Uses Three Principal Magnetic Fields
nStatic High Field (B0)
–Creates or
polarizes signal
–1000 Gauss to
100,000 Gauss
§Earth’s field is 0.5 G
nRadiofrequency Field (B1)
–Excites or
perturbs signal into a measurable form
–On the order of
O.1 G but in resonance with MR signal
–RF coils also
measure MR signal
–Excited or
perturbed signal returns to equilibrium
§Important for contrast mechanisms to take place.
nGradient Fields
–1-4 G/cm
–Used to image
& determine spatial position of MR signal
Back to MRI, Let's dissect this title,
Magnetic
žAny moving charged
particle generates a magnetic dipole moment (field) around.
žApplying this concept
to the atom excludes neutrons from magnetic field production (only protons
& electrons are charged). Being of an extremely small mass, the electron is
negligible, so only protons will be considered as the source of magnetization
in MRI.
žLet's, then, consider
the proton as a simple bar magnet
Which nuclei are NMR active?
žTo be NMR active, a nucleus needs to have 2 properties:
—Spin
—charge
žPairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons have spin
—Good MR nuclei are 1H, 13C, 19F, 23Na, 31P
What protons are we talking about?
So as to image the body, we should consider
the following conditions:
ž1-Abundance:
The most abundant protons in our bodies are the hydrogen protons which are present in almost all our body chemistry e.g., water, fat, carbohydrates, proteins, vitamins,…etc. If imaged, hydrogen is a good reflection of our body anatomy & pathology.
ž2-Ease of study:
Being the simplest atom (the first in the periodic table), hydrogen is easy to study. We have three isotopes for hydrogen, Protium , Deuterium, & Tritium. The simplest of which (simplest of the simplest) is Protium which is the isotope present in our bodies. Its nucleus contains one proton with no neutrons. We can consider this nucleus as a Proton.
3-High energy content:
nThe energy profile of the proton falls in two types: Potential energy & Kinetic energy.
nThe potential energy can be understood according to Blanc's Quantum Theory which states that: Matter & Energy are inter-convertible.
nMatter according to Blanc, contains Energy which can be expressed as 4 quantum numbers one of which is called Spin quantum number (I). For hydrogen, the spin quantum number=1/2
nThe number of energy states, a proton can assume when put in a magnetic field is given by the equation:
No of Energy (spin) states= 2 x spin quantum No +1
nFor hydrogen proton, the No of spin states thus = (2 x 1/2) +1 =2.
nHence, we have 2 spin states one called high energy state & the other is called low energy state according to their energy threshold.
nThe number of protons in the low energy state should exceed that in the high energy state & the energy diagram would be as follows:
žThis looks like the water fall, that is, the energy in the top of the water fall is greater than at the bottom & the difference ∆E creates electricity (in our condition ∆E creates the MR image).
nOn the other hand, kinetic energy can be understood by considering the proton spinning which looks like the spinning top. If you watch the spinning top, you see it spinning vertically.
nBut by the time, it begins to wobble (precess) changing the spinning axis (widening the rotation diameter gradually) till the top touches the earth.
n This energy difference is directly proportional to the magnet field strength because the stronger the magnet, the bigger the number of protons it can align in the low energy state (along the magnet axis) & hence the energy difference between the 2 states increases (i.e. image data increases). This is the source of the Potential Energy
0 التعليقات:
الرموز التعبيرية