Experimental study on transverse relaxation rate of protons in water suspensions of magnetite nanoclusters: Dependence of cluster sizes, volume fraction, inter-echo time, and waiting time

The transverse relaxation rate 1/ T2 of several batches of water suspensions of magnetite nanoclusters of average effective size 2 Reff between 37 and 475 nm has been studied experimentally as a function of volume fraction f v, inter-echo time 2 τCP, and waiting time t w. It is found that T 2 for suspensions with 2 Reff around or larger than 80 nm increases significantly with t w, and the t w dependence is enhanced by decreasing τCP. The initial relative T 2 increase rate is roughly proportional to f v for each batch of suspensions at a fixed value of τCP and roughly proportional to Reff-1 at given values of τCP and f v. After extrapolating t w to 0, 1/ T2∝fvλ(λ≈1) occurs for all batches of suspensions at given value of τCP, where λ increases slightly with increasing τCP for each batch of suspension. 1/ T2∝τCPκ(κ>0) occurs roughly for all suspensions, where κ increases slightly with increasing f v and greatly with increasing 2 Reff. Such f v and τCP dependences are qualitatively predicted by a well accepted formula for the echo-refocusing limit. However, the experimental results for the batch with largest 2 Reff, which is well located in the echo-limiting regime, agree better with a simplified formula. The theoretical static dephasing upper limit of 1/T 2 may be reached for suspensions of 2 Reff around 100 nm at τCP=3.2ms, and the experimental echo-limiting regime starts at a smaller 2 Reff than the simulated one. All these phenomena have been explained based on existing knowledge, but further experimental and theoretical studies are necessary to fully understand them. © 2012 Elsevier B.V. All rights reserved.