Chap.12 (3) Medical imaging systems: MRI Known as Nuclear Magnetic Resonance NMR Science or black magic? Source: Courtesy of Warner Bros

Principles of MRI Tesla, one T = 10000 Gauss (Magnetic field of earth is 0.5 Gauss) -> 1 T is 20 000 times stronger than earth

MRI Source: Biomed resources

MRI Source: MT Scott Diagnostic imaging

A brief recipe of MRI Put the subject into a strong magnetic field Pass radiowaves through the subject Turn of the radiowaves Recieve radiowaves coming back from the subject Convert the measured RF-data to an image

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

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

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

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

Angle momentum J = m=mvr m v r J

Magnetic momentum µ A I The magnetic momentum vector µ=IA

Precession og relaxation Resonance – thea absorption of energy – occurs when radio frequency energy is applied At the Larmor frequency and causes particles to change state and become excited Spin-lattice decay (T1) and spin-spin decay (T2) Faradays law of induction: If a mmagnetic field is moved through a conductor, a current Will be produced in the conductor

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

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

Ref:www.simplyphysics.com

Larmour frequency The energy difference between the two alignment states depends on the nucleus  E = 2 z Bo  Eh  /2 known as Larmor frequency /2= 42.57 MHz / Tesla for proton Ref: James Voyvodic

Resonance frequencies of common nuclei Note: Resonance at 1.5T = Larmor frequency X 1.5 Ref: James Voyvodic

Electromagnetic Radiation Energy X-Ray, CT MRI

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

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

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

MRI scanner

Relationship between parallell / antiparallell protones : Important MRI equations Larmorequation: ω=γB Relationship between parallell / antiparallell protones : Nn/Ne = ehν/kT =1+410-6 represents net magnetization at room temperature and 1 Tesla

T1 recording

T2 recording

MR images T1 and T2 contrast

3D picture construction ω = γB

T1, T2 and proton-density

Vertical main field Source: Oulun Yliopisto

Extremity MRI

Interventional MRI

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