Improved quantitative myocardial T₂ mapping: Impact of the fitting model

Abstract

Purpose To develop an improved T2 prepared (T2prep) balanced steady-state free-precession (bSSFP) sequence and signal relaxation curve fitting method for myocardial T2 mapping. Methods Myocardial T2 mapping is commonly performed by acquisition of multiple T2prep bSSFP images and estimating the voxel-wise T2 values using a 2-parameter fit for relaxation. However, a 2-parameter fit model does not take into account the effect of imaging pulses in a bSSFP sequence or other imperfections in T2prep RF pulses, which may decrease the robustness of T2 mapping. Therefore, we propose a novel T2 mapping sequence that incorporates an additional image acquired with saturation preparation, simulating a very long T2prep echo time. This enables the robust estimation of T2 maps using a 3-parameter fit model, which captures the effect of imaging pulses and other imperfections. Phantom imaging is performed to compare the T2 maps generated using the proposed 3-parameter model to the conventional 2-parameter model, as well as a spin echo reference. In-vivo imaging is performed on eight healthy subjects to compare the different fitting models. Results Phantom and in-vivo data show that the T2 values generated by the proposed 3-parameter model fitting do not change with different choices of the T2prep echo times, and are not statistically different than the reference values for the phantom (P = 0.10 with three T2prep echoes). The 2-parameter model exhibits dependence on the choice of T2prep echo times and are significantly different than the reference values (P = 0.01 with three T2prep echoes). Conclusion The proposed imaging sequence in combination with a 3-parameter model allows accurate measurement of myocardial T2 values, which is independent of number and duration of T2prep echo times.

T2 mapping with magnetization‐prepared 3D TSE based on a modified BIR‐4 T2 preparation

“…Akcakaya et al have proposed a saturation preparation module to eliminate T 1 relaxation effects when combining a T 2 preparation with a gradient echo readout. Using an additional saturation preparation scan they were able to remove T 1 weighting effects during the gradient echo readout in T 2 quantification of the myocardium with good noise performance . Therefore, the proposed approach for the use of an additional saturation preparation scan uses a similar principle.…”

Joint myocardial T1 and T2 mapping using a combination of saturation recovery and T2‐preparation

“…A composite saturation pulse, based on Sung and Nayak , with 1 kHz bandwidth and 10 ms duration is used. T 2 prep uses nonselective opening and closing 90° hard pulses with 2.3 kHz bandwidth and 0.44 ms duration, to minimize T 2 * effects that might occur during the pulse , while the refocus pulses are weighted in a MLEV opposing phase pairs scheme to compensate for RF pulse shape imperfection . Additionally, for TE prep = 0, a 90° followed immediately by a −90°, followed by a crusher gradient was used, similar to the one proposed in Huang et al , shown to reduce the effect of B 0 and B 1 variations of the excitation and refocusing pulses of the T 2 prep sequence, and improving the T 2 quantification accuracy .…”

“…This model is given by: where the parameters, A and B do not depend on the saturation time, T sat or the T 2 prep time, TE T2P . As detailed in Akcakaya et al , the B parameter captures the effect of the imaging pulses, and is a function of the steady‐state magnetization, as well as sequence parameters. The A parameter is a function of the signal at full‐recovery, and sequence parameters (flip angle, number of pulses, repetition time, etc).…”

“…Finally, T 1 mapping was also performed using SASHA T 1 mapping sequence of the same duration, which is a saturation recovery based technique. For comparison T 2 maps, a conventional T 2 mapping sequence was used with 3 T 2 prep echo times (0, 25, 50 ms), whose acquisitions are separated by a 4‐s rest period to allow for sufficient recovery, as well as an image acquired immediately after saturation pulse to simulate a very long T 2 prep echo time (i.e., T 2 prep = ∞) . The duration of this acquisition was 4 heartbeats plus 8 s of rest period.…”

“…When both native and postcontrast myocardial T 1 maps are acquired, they can be used to measure the extracellular volume fraction , which has shown utility for detection of diffuse myocardial fibrosis . On the other hand, quantitative myocardial T 2 mapping enables detection of inflammation and edema .…”