What are the categories of MRI contrast agents?

People often hear the term magnetic resonance. The full name of the magnetic resonance that people usually talk about is magnetic resonance imaging, which is an imaging device widely used in medicine. In fact, there are many classifications of magnetic resonance, including ferromagnetic resonance, cyclotron resonance, paramagnetic resonance, etc. In comparison, magnetic resonance contrast agents are very unfamiliar to most people. The following introduces the classification of magnetic resonance contrast agents.

What are the types of MRI contrast agents?

Magnetic resonance contrast agents indirectly change the signal intensity of tissues through internal and external relaxation effects and magnetic susceptibility effects.

1. According to the enhancement type, it can be divided into two categories: positive and negative contrast agents;

2. According to the biological distribution of contrast agents, they can be divided into contrast agents that are non-specifically distributed in the extracellular space, contrast agents that enter cells or bind to cell membranes, and contrast agents that are distributed in the blood pool.

The most widely used contrast agent at present is Gd-DTPA, whose Chinese name is gadolinium diethylenetriamine pentaacetate or gadopentetate meglumine salt, and its trade name is Magnevist.

The mechanism of action of Gd-DTPA

Gd-DTPA is a paramagnetic substance. Gd3+ has 7 unpaired electrons. Its unpaired electrons are dipoles like protons and have magnetic moments. The electron is very light, but its magnetic moment is about 657 times that of a proton. In the absence of paramagnetic substances, T1 and T2 relaxation of tissues are caused by dipole-dipole interactions between protons, forming local magnetic field fluctuations. In the presence of paramagnetic materials with unpaired electrons, huge local magnetic field fluctuations are generated because the magnetic susceptibility of electrons is about 657 times that of protons. At this time, the movement frequency of most electrons is close to the Larmor frequency, which shortens the T1 and T2 relaxation times of neighboring protons, that is, the so-called dipole-dipole interaction between the proton dipole and the electron dipole is formed, causing the so-called proton magnetic enhancement, which results in the shortening of T1 and T2 relaxation times.

When the concentration of Gd-DTPA is low, the contrast agent has a greater impact on the T1 relaxation time of body tissues because the T1 relaxation time of body tissues is longer. However, as the concentration of Gd-DTPA increases, the T2 shortening effect becomes increasingly obvious. When the concentration of Gd-DTPA is much higher than the clinical dose, T2 is shortened so much that the T2 enhancement masks the T1 enhancement. At this time, if T2 or T2* weighted imaging is used, the part of the tissue containing the contrast agent will appear as a low signal. This situation is called negative contrast. Therefore, high doses of Gd-DTPA can also be used as a negative contrast agent. It can be seen from this that

The effect of MRI contrast agents on tissue signal intensity is closely related to their concentration in the tissue.

Clinical application of Gd-DTPA

Gd-DTPA is an ionic extracellular fluid contrast agent that is not tissue specific but can be used for whole-body MR enhanced scanning. The conventional clinical dosage of Gd-DTPA is 0.1 mmol per kilogram of body weight, and the maximum dose allowed by the FDA is 0.3 mmol per kilogram of body weight.