Basic principles of MRI

Nuclei with an odd number ofprotons (such as hydrogen) posses a property called spin angular momentum—that is, the nucleus spins around its axis. Since the odd nuclei are positively charged, the spinning motion causes a magnetic momentum around it and acts as a small magnet. The strength of this magnetic moment is a property of the type of nucleus. Hydrogen nuclei possess a large magnetic moment, and are very abundant in the human body thereby making hydrogen the nucleus of choice for MRI. In the absence of an externally applied magnetic field (Bo) these individual magnetic moments have no preferred orientation, but when placed in strong external magnetic field these magnetic moments align with the orientation of the external field. However, the spins do not exactly align but are at an angle to the external magnetic field (fig 32.1). This causes the spin to precess around the axis of the external magnetic field with a unique (resonant) frequency.

The unique frequency of this precession is governed by the Larmor frequency equation f = g x M, where f = frequency, g = gyro-magnetic ratio (unique for each nucleus), and M = strength of magnetic field. The spins precess at random and give rise to a rather small secondary magnetic field (net tissue magnetisation, M) which at equilibrium is aligned longitudinally, along the axis of the main magnetic field (Bo) which is much larger, so that tissue magnetisation is "overruled" by the main magnetic field Bo, making tissue magnetisation undetectable in the longitudinal axis. To "detect" this tissue magnetisation to produce an MR image it is necessary to disturb this equilibrium. An RF pulse emitted from the RF transmitter coil with a resonant frequency equal to the unique frequency of the precessing spins rotates the net longitudinal tissue magnetisation into the transverse plane (X-Y plane) and synchronises the precession (fig 32.2). This allows the transverse magnetisation to be detected and measured. Termination of the RF

pulse causes the perturbed nuclei to return (relax) to the original longitudinal alignment in the magnetic field and incoherent precession. As they relax a signal is emitted which is detected by the RF receiver. This is the MR signal from which the image is reconstructed.

Different tissues relax at different rates, thus providing contrast between tissues. This signal needs to be processed to allow three dimensional location of the source (tissue protons) of the signal. A supplemental magnetic field gradient is applied (by the gradient coils) which causes a predictable variation of the magnetic field and thus a predictable resonant frequency of protons along an axis. This allows exact location of protons enabling precise image reconstruction.

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