Ferromagnetic particles as magnetic resonance imaging temperature sensors

Results

Characterization of Gd particles

In our experiments, we employed gadolinium particles prepared by a mechanical method. Using scanning electron microscope (SEM) images, we estimated the particle’s major axis average length as 4.8 μm with a standard deviation of 2.7 μm. An SEM image of the particles and the corresponding histogram with the major axis length distribution is shown in Fig. 1.

Figure 1: Properties of Gd particles.

(a) Scanning electron microscope image of particles. (b) Histogram of particles’ major axis length distribution as calculated from a scanning electron microscope image.

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To determine the stability of the Gd particles in an aqueous solution, the NMR linewidth (measurements discussed below) of Ringer’s solution-agar gel, with a 2.75 mM l−1 concentration of Gd, was monitored over 20 months. A gradual decrease of the linewidth, by 8 Hz per month, was determined.

Magnetization measurements

In zero applied field, Gd is characterized by a transition from the ferromagnetic to a paramagnetic state around 20 °C (ref. 33). As we will see, in the large fields produced in typical MRI systems (1.5 and 3.0 T), the transition is shifted to higher temperatures and broadened out. Gd also possesses a large magnetic moment, creating large changes in the NMR linewidth. The use of Gd for this application was suggested earlier in a study using a solid Gd wire34. Our theoretical calculations for magnetization as function of temperature and field, M(T,H), are in good agreement with the experiments and show that the dipolar field produced by the ferromagnetic particles increases the NMR linewidth as temperature is reduced.

Figure 2 shows the results of magnetization measurements of metallic gadolinium particles using a superconducting quantum interference device (SQUID) at selected magnetic fields and the corresponding theoretical calculations of magnetization. The temperature-dependent measurements of the magnetization were carried out at fields of 364 mT, used later in the NMR measurements, and at fields of 1.5 and 3.0 T, typical in clinical MRI settings. The Curie temperature of Gd particles was determined to be 19 °C using the magnetization measurements at 1 mT. It is clear from Fig. 2 that an increase of magnetic field shifts the transition to the paramagnetic state towards higher temperatures and potentially makes gadolinium particles a useful temperature-sensitive contrast within our projected target range.

Figure 2: Magnetization of Gd particles versus temperature.

Data from superconducting quantum interference device measurements (squares) and theoretical calculations (solid lines). The magnetization measurements in large d.c. fields of 1.5 and 3.0 T (typical fields for clinical magnetic resonance imaging systems) show a substantial change near body temperature (37 °C).

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