MRI Physics

THE BASIC PROCESS

The way MR images are generated is complicated and is much harder to understand than plain radiography, CT and ultrasound. It has strong underpinnings in physics which must be understood before any real sense of 'how it works' is gained.

What follows is a very abbreviated, 'broad strokes' description of the process. Essentially, the process can be broken down into four parts:

1. Preparation

2. Excitation

3. Spatial encoding

4. Signal acquisition

Preparation

The patient is placed in a static magnetic field produced by the magnet of the MR scanner. In living tissues there are a lot of hydrogen atoms included in water molecules or in many different other molecules. The proton, the nucleus of hydrogen, possesses an intrinsic magnetisation called spin. The spin magnetization vector precesses (rotates) around the magnetic field at a frequency called the Larmor frequency, which is proportional to the magnetic field intensity. The resulting magnetisation of all protons inside the tissues aligns parallel to the magnetic field. The parallel magnetisation scales with the magnetic field intensity, basically at 3T it will be twice the value obtained at 1.5T. Additional preparation sequences can also be performed to manipulate the magnetisation and so the image contrast, e.g. inversion preparation.

The Larmor or precessional frequency in MRI refers to the rate of precession of the magnetic moment of the proton around the external magnetic field. The frequency of precession is related to the strength of the magnetic field, B0.

The precessional frequency of nuclei of a substance placed in a static magnetic field B0 is calculated from the Larmor Equation:

ω = γB

where ω is the Larmor frequency in MHz, γ is the gyromagnetic ratio in MHz/tesla and B is the strength of the static magnetic field in tesla.

Excitation

During the image acquisition process, a radio frequency (RF) pulse) is emitted from the scanner. When tuned to the Larmor frequency, the RF pulse is at resonance: it creates a phase coherence in the precession of all the proton spins. The duration of the RF pulse is chosen such that it tilts the spin magnetisation perpendicularly to the magnetic field.  When a receiving coil (an electrical conductor) is put in the vicinity of the tissue, the transverse magnetisation, that still rotates as the Larmor precession, will generate an electric current in the coil by Faraday induction: this is the nuclear magnetic resonance (NMR) signal. The NMR signal is attenuated due to two simultaneous relaxation processes. The loss of coherence of the spin system attenuates the NMR signal with a time constant called the transverse relaxation time (T2). Concurrently, the magnetisation vector slowly relaxes towards its equilibrium orientation that is parallel to the magnetic field: this occurs with a time constant called the spin-lattice relaxation time (T1). The contrast in MR images originates from the fact that different tissues have, in general, different T1 and T2 relaxation times; as this is especially true for soft tissues, it explains the excellent soft tissue contrast of MR images.

Spatial encoding

Spatial encoding of the MRI signal is accomplished through the use of magnetic field gradients (smaller additional  magnetic fields with an intensity that linearly depends on the spatial location): spins from protons in different locations do precess at slightly different rates. The portion of the gradient coils and the associated current that is perpendicular to the main magnetic field cause a force (Lorentz force) on the coils. The gradients are turned on and off very quickly in this process causing them to vibrate and producing the majority of the acoustic noise during an MR image acquisition.

Signal acquisition

When using magnetic field gradients, the obtained NMR signal contains different frequencies corresponding to the different tissue spin positions and is called the MRI signal. After sampling, the analog MRI signal is digitised and stored for processing, which consists of a separation of the signal contributions from different spatial locations represented by pixels in the final image. This is achieved by a mathematical operation called a Fourier transform.

Standard exam

Multiple image sets are obtained in a standard exam protocol (which varies from facility to facility). Exam times vary according to the part of the anatomy being studied, pathology expected, radiologist preferences, and the scanner hardware and software used. Occasionally, a contrast medium may be used to enhance images, this will also usually prolong the scan time. Typically, exams are ordered without and with contrast for comparison purposes. Very rarely, and only in certain circumstances are exams ordered with contrast only. After the MRI exam the patient is removed from the scanner and given post-procedure instructions.