MRIcomponents: primary magnet, gradient magnet, radiofrequency coils, and computersystem. Primary magnetic field: superconducting magnet; hydrogen atoms alignparallel or antiparallel to the primary field (longitudinal magnetisation). Agreater proportion of the hydrogen atoms align parallel to the primary magneticstate (low energy) than antiparallel (high energy).
The net magnetic vector istherefore in the direction of the primary magnetic field. Precession: protonsspin around the long axis, and the rate is termed the Larmor frequency.Gradient coils create secondary magnetic fields.
They are arranged inopposition to each other to generate positive and negative gradients for x, y,and z coils. This allows spatial encoding for MRIs, i.e. localization (Plewesand Kucharczyk 2012). The RF coils transmit the RFs and receive signals,designed for specific body regions. This RF coil transmits a secondary MFpulse, which results in the disturbance of the proton alignment. Some lowenergy parallel flips to a high-energy state, decreasing longitudinalmagnetization. The protons then become synchronized and precess in phase.
As aresult, the net magnetization vector turns towards the transverse plane, i.e.right angles to the primary magnetic field (transverse magnetization). The RFcoil receives signals to create images. As protonsresume their normal state in the PMF prior to the transmission of the RF pulse,i.e. relaxation.
Relaxation in the longitudinal axis is T1 relaxation (z axis),and relaxation in the transverse axis is T2 relaxation (x-y axis). After the RFpulse several protons flip back to their original state, parallel to the z-axismagnetic field (T1 or spin lattice relaxation). T1 relaxation will differ indifferent tissues (fat vs H2O). T2 relaxation (spin spin relaxation): after RFpulse protons that were in phase begin to dephase out of the Larmor frequencyin the transverse (x-y axis). T1 increases and T2 decreases over time. Thechanging magnetic moment of the net magnetic vector results in free inductiondecay. This induces an electrical signal. The signal received from the RF coilis in the transverse plane and reduces as the net magnetic vector moves to thelong or z axis.
Thecomputer receives the RF signal and performs an analog-to-digital conversion.The digital signal representing the imaged body part is stored in a temporaryimage space (k-space). This stores digitized MR signals during dataacquisition. The digital signal is then sent to an image processor, where amathematical formula (Fourier transformation) is applied, and the image of theMRI scan is displayed on a monitor.
