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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