In conventional MRI contrast is often derived from differences in the spin-lattice relaxation time constant, T1, in different tissues. Diseased tissue often exhibits markedly different T1 values compared to healthy tissue. By choosing appropriate imaging parameters these differences can be exploited to make diseased tissue stand out in an MR image.
In addition to differences in the values of T1, it has been shown that the variation of T1 with magnetic field - known as T1 dispersion, varies between healthy and abnormal tissue. By looking at T1 dispersion it may be possible to detect disease much sooner than would be otherwise possible. Unfortunately it is not possible to study this T1 dispersion using conventional MRI scanners as they operate at a fixed magnetic field strength.
At Aberdeen University we can address this problem by using a special scanner in which the B0 field can be rapidly varied during a pulse sequence. This technique, known as Fast Field-Cycling MRI (FFC-MRI), allows us to collect information relating to a wide range of magnetic field strengths using a single instrument. This scanner consists of a permanent magnet which provides a 'detection' field of 59 mT, at which the NMR signals are measured, and a coaxial resistive offset magnet which allows field-cycling through field compensation.
A typical field-cycling pulse sequence. The sample is first polarised in a strong magnetic field B0P. Immediately after this the field is ramped to the field of interest B0E. The sample then relaxes at this field for a time referred to as the evolution period. Finally the field is returned to detection field where the NMR signals are detected.
The technique of fast field-cycling NMR is not a new one; FFC-NMR relaxometry is a well established method of studying field dependent relaxation effects. What makes our system unique is that is combines FFC-NMR, which is usually only performed on small samples, with MRI, allowing the field-cycling technique to be used on human sized samples while retaining all the advantages of MRI.
In FFC-MRI the basic pulse sequence consisting of a polarisation, evolution and detection period is the same as with FFC-NMR. However during the detection period we now apply field gradients in order to generate images. These images can then be analysed to provide information on T1 dispersion.
An image of a volunteer's thighs. During a FFC-MRI relaxometric imaging experiment an image is collected for every evolution field of interest. Analysis of the image intensity variation in these images yields a graph of R1 vs proton Larmor frequency (note R1 = 1/T1). In this example the graph on the right is the R1 dispersion corresponding to the region of interest shown in the left hand image. The arrows point to the so called 'quadrupole peaks', where enhanced relaxation takes place due to 14N-1H cross relaxation.
For more information on FFC-MRI visit the group website.