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Source: Courtesy of Warner Bros
Science or black magic?
Chap.12 (3) Medical imagingChap.12 (3) Medical imagingsystems: MRIsystems: MRI
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Principles of MRI
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MRI
Source: Biomed resources
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MRI
Source: MT Scott Diagnostic imaging
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A brief recipe of MRI
1. Put the subject into a strong magnetic field
2. Pass radiowaves through the subject
3. Turn of the radiowaves
4. Recieve radiowaves coming back from the subject
5. Convert the measured RF-data to an image
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Elements contributing to a MRI
• The quantitative properties of the nuclear spin
• The radiofrequency (RF) exitation properties
• Relaxationproperties of the tissue
• Magnetic field strength and gradients
• Thte timing of the gradients, RF-pulses and signal detection
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Prerequisites for depicted nucleus
• A nucleus that is to be pictured must have both: – Spin– Charge
Nucleus with even protonnumbers cannot be used because the spin will cancel each other
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Single-proton
• A single proton has a charge on the surface which is sufficient to form a small current-loop and generates a magnetic momentum µ
• The proton has also a mass that creates an angle-moment J due to the spin
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Hydrogenatoms
• The hydrogenatom is the only large element in the body able to be depicted with MRI. (C, O and N have all even numbers in the proton number).
• Hydrogen is everywhere in the body, primarily combined to water
= All MRI are in fact a picture of hydrogen
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Angle momentum
JJ = m = m=m=mvvrr
mm
vv
rr
JJ
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Magnetic momentum
µ
A
I
The magnetic momentum vector µ=IA
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Precession og relaxation
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Vector direction
• The magnetic momentum and the angle momentum vector is aligned to the spin-axis.
µ=γJ
Where γ is the gyromagnetic ratio, constant for a given nucleus
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Proton interaction with magnetism
• Loaded particles spinning is constructing their own little magnetic field.
- Will line up in the same direction as an external magnetic field
Spinning particles with a mass have an angle momentum – The angle momentum works as a gyroscope and counteracts
changes of the spin direction
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• Ref:www.simplyphysics.com
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Larmour frequencyThe energy difference between
the two alignment states depends on the nucleus
E = 2 z Bo
Eh
/2known as Larmor frequency
/2= 42.57 MHz / Tesla for proton= 42.57 MHz / Tesla for proton
Ref: James Voyvodic
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Resonance frequencies of common nuclei
Note: Resonance at 1.5T = Larmor frequency X 1.5
Ref: James Voyvodic
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MRI
X-Ray, CT
Electromagnetic Radiation Energy
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Magnetization• Sum of all contributions from each nucleus• Large magnetic fields create a big magnetization
M • Temperature dependency • To be able to measure the magnetization, we will
have to disturb it • The quantity of energy supplied (durability for the
RF-pulse at the resonance frequency) will decide how far the nuclei will be pushed away from B
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Radiofrequency field• RF fields are used to manipulate the
magnetization for a specific atom in a specific position
• The hydrogen nucleus is tuned to a certain RF-frequecy
• Eksternal RF-waves can be sent into the subject in order to disturb the hydrogen nucleus
• Disturbed hydrogen nuclei will generate RF-signals with the same frequency – which can later be detected
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To record an MRI signal• Needs a receive coil tuned in to the same RF-requency as
the excitasjonscoil
• Measure net magnetization
• The signal oscillates at the resonansfrequency when the net magnetization vector rotates in the room
• Signalamplitude will be weakened when the netto magnetization returns to the B-direction
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MRI scanner
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• Larmorequation: ω=γB
• Relationship between parallell / antiparallell protones :
Nn/Ne = ehν/kT =1+410-6
represents net magnetization at room temperature and 1 Tesla
Important MRI equations
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T1 recording
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T2 recording
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MR imagesT1 and T2contrast
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3D picture construction
ω = γB
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T1, T2 and proton-density
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Vertical main field
Source: Oulun Yliopisto
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ExtremityMRI
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Interventional MRI
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Adv/disadv MRIAdv:No harmful radiationSoft tissue imagingHigh resolution images of T1 or T2 preferences
Disadv:Expensive, large installation with superconducting magnets+
+Very strong magnetic fieldClaustrophobicNot for frozen tissue