Experimental validation of a T2ρ transverse relaxation model using LASER and CPMG acquisitions

The transverse relaxation rate (R-2 = 1/T-2) of many biological tissues are altered by endogenous magnetized particles (i.e., ferritin, deoxyhemoglobin), and may be sensitive to the pathological progression of neurodegenerative disorders associated with altered brain-iron stores. R-2 measurements using Carr-Purcell-Meiboom-Gill (CPMG) acquisitions are sensitive to the refocusing pulse interval (2 tau(cp)), and have been modeled as a chemical exchange (CE) process, while R-2 measurements using a localization by adiabatic selective refocusing (LASER) sequence have an additional relaxation rate contribution that has been modeled as a R-2p process. However, no direct comparison of the R-2 measured using these two sequences has been described for a controlled phantom model of magnetized particles. The three main objectives of this study were: (1) to compare the accuracy of R-2 relaxation rate predictions from the CE model with experimental data acquired using a conventional CPMG sequence, (2) to compare R-2 estimates obtained using LASER and CPMG acquisitions, and (3) to determine whether the CE model, modified to account for R-2p relaxation, adequately describes the R-2 measured by LASER for a full range of tau(cp) values. In all cases, our analysis was confined to spherical magnetic particles that satisfied the weak field regime. Three phantoms were produced that contained spherical magnetic particles (10 mu m diameter polyamide powders) suspended in Gd-DTPA (1.0, 1.5, and 2.0 mmol/L) doped gel. Mono-exponential R-2 measurements were made at 4 T as a function of refocusing pulse interval. CPMG measurements of R-2 agreed with CE model predictions while significant differences in R-2 estimates were observed between LASER and CPMG measurements for short tau c(p) acquisitions. The discrepancy between R-2 estimates is shown to be attributable to contrast enhancement in LASER due to T-2p relaxation. (c) 2006 Elsevier Inc. All rights reserved.