Functionality of an MRT

Flussdiagrammknoten

• 1. Patient

• 2. Irradiation of an RF pulse

• 3. Switching off the RF pulse

• 4. Patient transmits signal

• 5. Image construction

• Functional principle  of an MRT

• 1952 Nobelprice for Bloch and Purcell

• First MR images in the 70´s

• Strong magnetic Field of 1.5 T - 3 T

• Based on the interaction of a magnetic field and radiofrequency impulses

• Magnet field of the MRT is always "ON"

• Consists of 70% water H2O

• Hydrogen (H) is a proton = stable, electrically positively charged

• Turns around the horizontal axis - has a spin

• The nuclear spin is the total angular momentum of an atomic nucleus around its center of gravity

• Electrical energie is created and induced an magnetic field

• Proton behaves like a rod magnet

• Introduction of protons into an external magnetic field (MR tomographs)

• A19734.000 2 Big (binary/octet-stream)

• Abb10 (binary/octet-stream)

• Protons align themselves

• External magnetic field runs in longitudinal direction of the magnet/patient direction

• Protons align parallel or antiparallel to the external magnetic field. There are more and more protons aligned in parallel

• Have different energy levels

• More energy is needed for antiparallel alignment

• Difference is really small (with the background of the strong external magnetic field) = NET MAGNETISATION

• This is what MRI works with !!!

• Points in longitudinal direction of the magnet

• LONGITUDINAL MAGNETISATION

• No use of ionized radiation

• Magnetic field of the earth 50 µT

• Move like a gyroscope

• Precession / Lamour precession is the frequency the proton rotates at = precession frequency (depending on the strength of the external magnetic field)

• Describe as a vector

• Moves with the precession frequency on its own axis

• Disturbance/deflection to make the longitudinal magnetization measurable !

• Patient

• Short strong electromagnetic pulse

• In order to bring protons out of balance, energy must be exchanged with them

• HOW?

• RF - pulse must have the same frequency as the protons

• Frequency describe by the Lamourfrequency

• Only then is an energy exchange possible

• RF- pulse in resonance with the protons (resonate)

• 1. Irradiation of an RF - pulse

• 2. Energy exchange with the protons

• 3. A part of the protons absorbs the energy

• 4. Alignment of the protons in antiparallel direction increases

• Longitudinal magnetisation and ↓ = > Net magnetisation vector ↓

• Minimal differently precise protons => sychronize

• "in phase"

• Addition of magnetic vectors

• Development of transverse magnetization

• Precise also on its own axis

• Induced an electric current

• Measureabe with a cpi = MR -SIGNAL

• Proton system strives for a state with low energy input

• System returns to initial state

• Transversal magnatisation ↓

• Longitudinal magnetisation ↑

• Photons go out of phase again

• Transversal Relaxation T2

• Construction

• Longitudinal Relaxation T1