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Fysisk institutt - Rikshospitalet 1 Source: Courtesy of Warner Bros Science or black magic? Chap.12 (3) Medical imaging systems: MRI.

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Presentation on theme: "Fysisk institutt - Rikshospitalet 1 Source: Courtesy of Warner Bros Science or black magic? Chap.12 (3) Medical imaging systems: MRI."— Presentation transcript:

1 Fysisk institutt - Rikshospitalet 1 Source: Courtesy of Warner Bros Science or black magic? Chap.12 (3) Medical imaging systems: MRI

2 Fysisk institutt - Rikshospitalet 2 Principles of MRI

3 Fysisk institutt - Rikshospitalet 3 MRI Source: Biomed resources

4 Fysisk institutt - Rikshospitalet 4 MRI Source: MT Scott Diagnostic imaging

5 Fysisk institutt - Rikshospitalet 5 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

6 Fysisk institutt - Rikshospitalet 6 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

7 Fysisk institutt - Rikshospitalet 7 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

8 Fysisk institutt - Rikshospitalet 8 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

9 Fysisk institutt - Rikshospitalet 9 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

10 Fysisk institutt - Rikshospitalet 10 Angle momentum J = m  =mvr m v rJ

11 Fysisk institutt - Rikshospitalet 11 Magnetic momentum µ A I The magnetic momentum vector µ=IA

12 Fysisk institutt - Rikshospitalet 12 Precession og relaxation

13 Fysisk institutt - Rikshospitalet 13 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

14 Fysisk institutt - Rikshospitalet 14 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

15 Fysisk institutt - Rikshospitalet 15 Ref:www.simplyphysics.com

16 Fysisk institutt - Rikshospitalet 16 Larmour frequency The energy difference between the two alignment states depends on the nucleus  E = 2  z B o  E  h  /2    known as Larmor frequency = MHz / Tesla for proton  /2  = MHz / Tesla for proton Ref: James Voyvodic

17 Fysisk institutt - Rikshospitalet 17 Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5 Ref: James Voyvodic

18 Fysisk institutt - Rikshospitalet 18 MRI X-Ray, CT Electromagnetic Radiation Energy

19 Fysisk institutt - Rikshospitalet 19 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

20 Fysisk institutt - Rikshospitalet 20 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

21 Fysisk institutt - Rikshospitalet 21 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

22 Fysisk institutt - Rikshospitalet 22

23 Fysisk institutt - Rikshospitalet 23 MRI scanner

24 Fysisk institutt - Rikshospitalet 24 Larmorequation: ω=γB Relationship between parallell / antiparallell protones : N n /N e = e hν/kT =1+4  represents net magnetization at room temperature and 1 Tesla Important MRI equations

25 Fysisk institutt - Rikshospitalet 25 T 1 recording

26 Fysisk institutt - Rikshospitalet 26 T 2 recording

27 Fysisk institutt - Rikshospitalet 27 MR images T1 and T2 contrast

28 Fysisk institutt - Rikshospitalet 28 3D picture construction ω = γB

29 Fysisk institutt - Rikshospitalet 29 T1, T2 and proton-density

30 Fysisk institutt - Rikshospitalet 30 Vertical main field Source: Oulun Yliopisto

31 Fysisk institutt - Rikshospitalet 31 Extremity MRI

32 Fysisk institutt - Rikshospitalet 32 Interventional MRI

33 Fysisk institutt - Rikshospitalet 33

34 Fysisk institutt - Rikshospitalet 34 Adv/disadv MRI Adv: No harmful radiation Soft tissue imaging High resolution images of T1 or T2 preferences Disadv: Expensive, large installation with superconducting magnets++ Very strong magnetic field Claustrophobic Not for frozen tissue


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