Measurement and correction of stimulated echo contamination in T2-based iron quantification

Abstract

The purpose of this study was to characterize the effects of stimulated echo contamination on MR-based iron measurement derived from quantitative T2 images and develop a method for retrospective correction. Two multiple spin echo (MSE) pulse sequences were implemented with different amounts of stimulated echo contamination. Agarose-based phantoms were constructed that simulate the relaxation and susceptibility properties of tissue with different concentrations of dispersed (ferritin-like) and aggregated (hemosiderin-like) iron. Additionally, myocardial iron was assessed in nine human subjects with transfusion iron overload. These data were used to determine the influence of stimulated echoes on iron measurements made by an MR-based iron quantification model that can separately measure dispersed and aggregated iron. The study found that stimulated echo contamination caused an underestimation of dispersed (ferritin-like) iron and an overestimation of aggregated (hemosiderin-like) iron when applying this model. The relationship between the measurements made with and without stimulated echo appears to be linear. The findings suggest that while it is important to use MSE sequences with minimal stimulated echo in T2-based iron quantification, it appears that data acquired with sub-optimal sequences can be retrospectively corrected using the methodology described here.

MR characterization of hepatic storage iron in transfusional iron overload

“…We noticed that in the present study, the estimated relaxivity of the MnCl 2 phantom was approximately 59.1 s À1 /mM at 1.5T, which is lower than the published values of 74.2 s À1 /mM measured with a single spin echo sequence (22) and 71.4 s À1 /mM measured with the MSE sequence (38). We believe this underestimation to be mainly due to stimulated echoes introduced by the imperfect slice selection profile and reduced refocusing RF pulse flip angle (19,(30)(31)(32)(33). When the refocusing slice thickness is increased by an empirically determined factor of three, the relaxivity then becomes consistent with the literature values.…”

“…The in vivo image data in the present study were acquired using the vendor-supplied, nonoptimized MSE sequence, i.e., without increasing the slice selection thickness for the refocusing RF pulses. This difference prompted investigation of the relationship between RR 2 and A values derived from the present and optimized MSE sequences (32,33). A linear regression analysis (r ¼ 0.99) verified that the stimulated echo effects contributed to an underestimation of RR 2 (Fig.…”

“…This difference prompted investigation of the relationship between RR 2 and A values derived from the present and optimized MSE sequences (32, 33). A linear regression analysis ( r = 0.99) verified that the stimulated echo effects contributed to an underestimation of RR 2 (Figure 2a) and an overestimation of A (Figure 2b).…”

“…We demonstrate that the contributions of dispersed and aggregated iron to the transverse relaxation rate (R 2 ) can be separated using the parameters RR 2 and A. Finally, the influnce of the stimulated echo (31) caused by the present MSE sequence on RR 2 and A (32, 33), and on iron measurement are analyzed based on phantom simulations. We hypothesize that tissue iron quantification based on the two parameters, RR 2 and A, rather than the single parameter, R 2 or R 2 *, will improve estimates of total storage iron.…”

“…Finally, stimulated echoes may occur for MSE sequences with inaccurate RF pulse flip angles and imperfect slice selection profile (31). To minimize these, an optimized MSE sequence was implemented on the same MR system with an increased slice selection thickness for the refocusing pulses (19,30,32,33). While the in vivo iron measurement was based on the vendor's default (nonoptimized) MSE sequence, we evaluated our method on phantoms with both nonoptimized and optimized MSE sequence to assess potential systematic effects.…”