Transcript MRI
Chap.12 (3) Medical imaging systems: MRI Science or black magic? 1 Fysisk institutt - Rikshospitalet Source: Courtesy of Warner Bros Principles of MRI 2 Fysisk institutt - Rikshospitalet MRI Source: Biomed resources 3 Fysisk institutt - Rikshospitalet MRI 4 Source: MT Scott Diagnostic imaging Fysisk institutt - Rikshospitalet 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 5 Fysisk institutt - Rikshospitalet 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 6 Fysisk institutt - Rikshospitalet 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 7 Fysisk institutt - Rikshospitalet 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 8 Fysisk institutt - Rikshospitalet 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 9 Fysisk institutt - Rikshospitalet Angle momentum J = m=mvr J m r v 10 Fysisk institutt - Rikshospitalet Magnetic momentum µ A I The magnetic momentum vector µ=IA 11 Fysisk institutt - Rikshospitalet Precession og relaxation 12 Fysisk institutt - Rikshospitalet Vector direction • The magnetic momentum and the angle momentum vector is aligned to the spinaxis. µ=γJ Where γ is the gyromagnetic ratio, constant for a given nucleus 13 Fysisk institutt - Rikshospitalet 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 14 Fysisk institutt - Rikshospitalet • Ref:www.simplyphysics.com 15 Fysisk institutt - Rikshospitalet Larmour frequency The energy difference between the two alignment states depends on the nucleus E = 2 z Bo Eh /2 known as Larmor frequency /2= 42.57 MHz / Tesla for proton 16 Fysisk institutt - Rikshospitalet Ref: James Voyvodic Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5 17 Fysisk institutt - Rikshospitalet Ref: James Voyvodic Electromagnetic Radiation Energy X-Ray, CT MRI 18 Fysisk institutt - Rikshospitalet 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 19 Fysisk institutt - Rikshospitalet 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 RFfrequecy • Eksternal RF-waves can be sent into the subject in order to disturb the hydrogen nucleus • Disturbed hydrogen nuclei will generate RFsignals with the same frequency – which can later be detected 20 Fysisk institutt - Rikshospitalet 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 21 Fysisk institutt - Rikshospitalet 22 Fysisk institutt - Rikshospitalet MRI scanner 23 Fysisk institutt - Rikshospitalet Important MRI equations • Larmorequation: ω=γB • Relationship between parallell / antiparallell protones : Nn/Ne = ehν/kT =1+410-6 represents net magnetization at room temperature and 1 Tesla 24 Fysisk institutt - Rikshospitalet T1 recording 25 Fysisk institutt - Rikshospitalet T2 recording 26 Fysisk institutt - Rikshospitalet MR images T1 and T2 contrast 27 Fysisk institutt - Rikshospitalet 3D picture construction ω = γB 28 Fysisk institutt - Rikshospitalet T1, T2 and proton-density 29 Fysisk institutt - Rikshospitalet Vertical main field Source: Oulun Yliopisto 30 Fysisk institutt - Rikshospitalet Extremity MRI 31 Fysisk institutt - Rikshospitalet Interventional MRI 32 Fysisk institutt - Rikshospitalet 33 Fysisk institutt - Rikshospitalet 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 34 Fysisk institutt - Rikshospitalet