How is an MR image produced

MRI uses a magnetic field rather than X-rays/ionising radiation to produce an image. To perform an MRI scan, the patient is placed in very strong magnetic field (superconducting magnet) of the MRI scanner. The hydrogen nuclei/protons within the body are subjected to bursts of radiofrequency (RF). The hydrogen nuclei in the body take up the RF energy, which is subsequently released again as they relax. This emitted energy is measured using an external RF coil. This signal can be localised to an exact location in the body and varies depending on the physical composition of the emitting body part. These signals are built up into MR images. MRI depends on the interaction between several factors:

1. Hydrogen nuclei (single protons) within tissues, mostly within water molecules.

2. A strong and uniform external magnetic field (0.15-2 T).

3. Pulses of RF/radiowaves.

The hydrogen nuclei act like tiny spinning bar magnets (magnetic moments) and within the MRI scanner they align themselves parallel or anti-parallel to the external magnetic field. If radiowaves/RF of a critical frequency (the resonant or Lamor frequency) are generated, some of the nuclei absorb energy causing them to change their orientation relative to the external magnetic field. This causes rotation of the net magnetisation vector to rotate through a certain angle - flip angle, e.g. 90 degrees. The greater the strength and duration of the RF pulse, the greater the flip angle. At the same time, all of the nuclei begin to spin in phase with one another. When the RF is turned off, the nuclei start to relax towards their resting state. The magnetisation vector returns to its original orientation (T1 relaxation). Immediately following the RF pulse, the individual magnetic moments are rotating in phase. Simultaneously with T1 relaxation, there is dephasing of the spins (T2 relaxation). As relaxation occurs, the signal decays. Every tissue has its own T1 and T2 relaxation rates, which depend on the chemical and physical properties of that tissue. T2 relaxation or spin-spin relaxation is a much more rapid process. The small alternating magnetic field, perpendicular to the external field, induces an electrical current in receiver coils placed close to the patient. This current is amplified into an MRI signal.

In MRI, the field gradients are employed to make the MRI signal contain spatial information. The gradient field is superimposed on the main magnetic field for spatial encoding. There are three orthogonal gradient coils - in the transverse (X and Y) and longitudinal (Z) planes. This allows localisation of the signal which can then be translated onto the final image. It is the gradient coil which produces the loud banging during an MRI study. The strength of the signal from a given point in the patient

determines the shade of grey of the corresponding pixel on the image. High signal tissue appears white and low signal black, with a spectrum in between.

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