Magnetism: The effect of contrast agents on MRI signals

Protons have an intrinsic magnetic moment coupled to the direction of their spins. During MRI, a magnetic field is used to align the spins of protons, and, as a result of this, the sample acquires a net, measurable magnetization parallel to the static applied field. Once the protons are aligned with the static field, radio-frequency (RF) photons are then applied to the protons, which cause them to enter a higher energy state, with their spins aligned antiparallel to the field. The steps are outlined in Figure 1.

Figure 1. Alignment of proton spins before the measurement begins, after application of a static magnetic field, and after application of a radio-frequency (RF) field.

The applied RF field is then removed, and the protons gradually return to the lower-energy, aligned state, emitting photons with the same energy as the exciting pulse. This relaxation occurs gradually, with the number of spins aligned opposite the static field decreasing exponentially in time. The time constant, known as T1, depends on composition and structure of the tissue.

In addition, neighboring spins in the sample tend to become aligned with one another due to pairwise magnetic interactions, which further improves the MRI signal. After the removal of the RF, this spin-spin alignment exponentially decays with a different time constant, T2, which also depends on material properties. The dependence of T1 and T2 on structural properties gives rise to contrast between different tissues in MRI.

In many applications, “contrast agents” consisting of fluids with particularly short T1 times are used in order to differentiate otherwise similar tissues. Experimental results indicate a relationship between the observed relaxation timescale and the concentration of contrast agent used, as shown in Figure 2.

Figure 2. The strength of the MRI signal along two, perpendicular measurement axes allows T1 and T2 to be separately observed for different concentrations of contrast agent. A: Measurement of the signal along one direction allows T1 to be observed for two different concentrations of contrast agent X. B: Measurement along another, orthogonal direction allows T2 to be observed for different concentrations of contrast agent Y.

Data adapted from: Yoo, B., & Pagel, M. D. (2007). An overview of responsive MRI contrast agents for molecular imaging. Frontiers in Bioscience: a Journal and Virtual Library, 13, 1733-1752.