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

Abstract The transverse relaxation rate 1 / T 2 of several batches of water suspensions of magnetite nanoclusters of average effective size 2 R eff between 37 and 475 nm has been studied experimentally as a function of volume fraction fv, inter-echo time 2 τ CP , and waiting time tw. It is found that T2 for suspensions with 2 R eff around or larger than 80 nm increases significantly with tw, and the tw dependence is enhanced by decreasing τ CP . The initial relative T2 increase rate is roughly proportional to fv for each batch of suspensions at a fixed value of τ CP and roughly proportional to R eff − 1 at given values of τ CP and fv. After extrapolating tw to 0, 1 / T 2 ∝ f v λ ( λ ≈ 1 ) occurs for all batches of suspensions at given value of τ CP , where λ increases slightly with increasing τ CP for each batch of suspension. 1 / T 2 ∝ τ CP κ ( κ > 0 ) occurs roughly for all suspensions, where κ increases slightly with increasing fv and greatly with increasing 2 R eff . Such fv and τ CP dependences are qualitatively predicted by a well accepted formula for the echo-refocusing limit. However, the experimental results for the batch with largest 2 R eff , which is well located in the echo-limiting regime, agree better with a simplified formula. The theoretical static dephasing upper limit of 1/T2 may be reached for suspensions of 2 R eff around 100 nm at τ CP = 3.2 ms , and the experimental echo-limiting regime starts at a smaller 2 R eff 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.