In the last decade, magnetic resonance imaging (MRI) has become pivotal in the staging and investigation of urological malignancy and has had a transformative effect on prostate cancer care pathways.

Basic Principles

Nuclei, made up of protons and neutrons, are charged particles with a specific motion or ‘precession.’ When a human body is placed in a strong magnetic field, many of the free, randomly aligned hydrogen nuclei align themselves with the direction of the magnetic field. This behaviour is termed Larmor precession. To generate a magnetic resonance (MR) image, a radio‐frequency pulse with a frequency equal to the Larmor frequency is applied perpendicular to the magnetic field, causing the net magnetic moment to tilt away from the direction of the magnetic field. Once the radio‐frequency signal is halted, the nuclei realign themselves with their net magnetic moment parallel to the strong magnetic field. During this ‘relaxation’, the nuclei lose energy and emit their own radiofrequency signal, referred to as the ‘free‐induction decay (FID) response signal.’ The FID response signal can then be measured by a field coil placed around the body being imaged. This measurement can be reconstructed to generate three‐dimensional MR images.

There are two types of relaxation: longitudinal (T1) and transverse (T2). T1 measures the time taken for the magnetic moment of the displaced nuclei to return 63% to thermal equilibrium. Water and cerebrospinal fluid (CSF) have long T1 values, appearing dark on T1 weighted images, whereas fat has a short T1 value and appears bright. T1‐weighted imaging (T1WI) is particularly useful in identifying post‐biopsy haemorrhage and detecting the status of lymph nodes and skeletal metastases, especially in combination with IV gadolinium‐based contrast. T2 on the other hand measures the time required for the FID response signal to decay.

Clinical Applications