Posted by Yu-Shen Lin on May 5, 2008 07:24 PM|Permalink
Comments
1. Explain in more detail how the relaxivity changes as a function of contrast agent?
2. In the expression provided in 1, what would be different for contrast agents as nanoparticles (c.f. Lee's paper) versus agents (e.g. Europium) that directly bind to cell surface of a target cell?
2. Typically, the nanoparticle based contrast agents (e.g. iron oxide nanopartiles) are used for T2-weight image. T2 contrast agent will increase the T2 relaxation rate and make the image darker. The metal complex typed contrast agents (e.g. Gd(DTPA)) are for T1-weighted image. Then T1 contrast agent will enhance the T1 relaxation rate and make image brighter.
For the question one, it is a long story. Sorry about that. I try my best to explain it. Metal ions with one or more unpaired electrons are paramagnetic and have a permanent magnetic moment and the T1 and T2 relaxation times are reduced by the magnetic dipolar interactions between paramagnetic metal and water proton or long-range interactions through the magnetic susceptibility of a metal ion or superparamagnetic particles. Thus, paramagnetic metal ions and superparamagnetic particles are the most common contrast agents used presently. However, due to the relatively high toxicity, metal ions can not be used directly as contrast agents for the body. For safety, chelating the metals to the ligands is the most popular and desirable methods for designing MRI contrast agents. The efficiency of MRI contrast agent is defined by relaxivity (r). The definition of relaxivity is the enhancement of proton relaxation rate in aqueous solution per unit of concentration in mM. Hence, T1 relaxivity (r1) and T2 relaxivity (r2) are very important information to evaluate the enhancement ability of MRI contrast agents.
For r1, R1 = r1 [CA] + R1_tissue
For r2, R2 = r2 [CA] + R2_tissue
Where R1 (1/T1) and R2 (1/T2) are relaxation rates of proton, [CA] is the concentration of the contrast agent in mmol/L (mM). Consequently, r1 and r2 have the units of mM-1 s-1. The quantities of r1, r2, R1_tissue, and R2_tissue are dependent of both temperature and magnetic field strength.
In more a general response to question 3, MRI resolution is typically limited by several different factors. One the factors is the strength the strength of the magnetic field. Also the background noise of the sample, which would be effected by how specific your probe is, would also lower the resolution.
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Comments
1. Explain in more detail how the relaxivity changes as a function of contrast agent?
2. In the expression provided in 1, what would be different for contrast agents as nanoparticles (c.f. Lee's paper) versus agents (e.g. Europium) that directly bind to cell surface of a target cell?
3. What limits the resolution of MRI?
Posted by: Edgar Arriaga | May 6, 2008 11:50 PM
2. Typically, the nanoparticle based contrast agents (e.g. iron oxide nanopartiles) are used for T2-weight image. T2 contrast agent will increase the T2 relaxation rate and make the image darker. The metal complex typed contrast agents (e.g. Gd(DTPA)) are for T1-weighted image. Then T1 contrast agent will enhance the T1 relaxation rate and make image brighter.
3. The typical resolution of MRI is 25–100 μm.
Posted by: Yu-Shen Lin | May 7, 2008 10:50 AM
For the question one, it is a long story. Sorry about that. I try my best to explain it. Metal ions with one or more unpaired electrons are paramagnetic and have a permanent magnetic moment and the T1 and T2 relaxation times are reduced by the magnetic dipolar interactions between paramagnetic metal and water proton or long-range interactions through the magnetic susceptibility of a metal ion or superparamagnetic particles. Thus, paramagnetic metal ions and superparamagnetic particles are the most common contrast agents used presently. However, due to the relatively high toxicity, metal ions can not be used directly as contrast agents for the body. For safety, chelating the metals to the ligands is the most popular and desirable methods for designing MRI contrast agents. The efficiency of MRI contrast agent is defined by relaxivity (r). The definition of relaxivity is the enhancement of proton relaxation rate in aqueous solution per unit of concentration in mM. Hence, T1 relaxivity (r1) and T2 relaxivity (r2) are very important information to evaluate the enhancement ability of MRI contrast agents.
For r1, R1 = r1 [CA] + R1_tissue
For r2, R2 = r2 [CA] + R2_tissue
Where R1 (1/T1) and R2 (1/T2) are relaxation rates of proton, [CA] is the concentration of the contrast agent in mmol/L (mM). Consequently, r1 and r2 have the units of mM-1 s-1. The quantities of r1, r2, R1_tissue, and R2_tissue are dependent of both temperature and magnetic field strength.
Posted by: Yu-Shen Lin | May 7, 2008 11:00 AM
In more a general response to question 3, MRI resolution is typically limited by several different factors. One the factors is the strength the strength of the magnetic field. Also the background noise of the sample, which would be effected by how specific your probe is, would also lower the resolution.
Posted by: Jonathan Dozier | May 7, 2008 08:39 PM