We have investigated methods to generate T1 dispersion plots from precisely-defined local volumes within larger samples. This could be used, for example, to generate a T1 dispersion graph of tumour tissue, even if the tumour was deep inside an organ of the body. In this way, the T1 dispersion data would pertain only to the tumour, and would not be "contaminated" by the T1 dispersion of surrounding normal tissue.
In order for this to work, we had to develop (a) a method of selecting the desired volume for T1 dispersion measurement on pilot MR images of the sample (or patient) under study; (b) a reliable method of limiting the NMR signals to the image-selected volume of the sample (or patient); and (c) methods of analysing and displaying the T1 dispersion plots.
Localisation of the NMR signals (and hence the T1 dispersion data) in three dimensions was accomplished using a technique borrowed from NMR spectroscopy, called "PRESS". It involves applying selective 90° and 180° radiofrequency pulses in the presence of orthogonal magnetic field gradients. For T1 measurement the PRESS module is incorporated into an inversion-recovery pulse sequence, using an adiabatic fast passage (AFP) pulse for improved inversion efficiency. The pulse sequence is applied in an interleaved manner (AFP-on, AFP-off) to allow T1 calculation by a 2-point method.
|Field-cycling inversion-recovery pulse sequence using PRESS localisation. Spins are inverted using the AFP pulse, then relaxation occurs at field B0E, followed by selective excitation and echo formation. The sequence is repeated at a range of B0E values to build up a T1 dispersion curve.|
In order to measure a localised T1 dispersion curve, one or more pilot images are collected, on which regions-of-interest are drawn using the mouse. These define the cuboid volume from which the NMR signals are produced. The scanner's software then calculates the necessary magnetic field gradient values (G1, G2 and G3 in the above diagram), then runs the pulse sequence to generate the data. An example of a pilot image and a resulting R1 dispersion plot (R1 = 1/T1) are shown below.
|Pilot image (59 mT) through thighs of a volunteer, showing in red the region-of-interest where it intersects the image plane. R1 dispersion curve on the right was obtained from the selected volume. The quadrupole peaks due to immobile protein within muscle can clearly be seen.|