Compensation of slice profile mismatch in combined spin‐ and gradient‐echo echo‐planar imaging pulse sequences

Abstract

Combined acquisition of gradient-echo and spin-echo signals in MRI time series reveals additional information for perfusion-weighted imaging and functional magnetic resonance imaging due to differences in the sensitivity of gradient-echo and spin-echo measurements to the properties of the underlying vascular architecture. The acquisition of multiple echo trains within one time frame facilitates the simultaneous estimation of the transversal relaxation parameters R2 and R2*. However, the simultaneous estimation of these parameters tends to be incorrect in presence of slice profile mismatches between signal excitation and subsequent refocusing pulses. It is shown here that improvements in pulse design reduced R2 and R2* estimation errors. Further improvements were achieved by augmented parameter estimation through the introduction of an additional parameter δ to correct for discordances in slice profiles to facilitate more quantitative measurements. Moreover, the analysis of time-resolved acquisitions revealed that the temporal stability of R2 estimates could be increased with improved pulse design, counteracting low contrast-to-noise ratios in spin-echo-based perfusion and functional MRI.

Assessment of a combined spin- and gradient-echo (SAGE) DSC-MRI method for preclinical neuroimaging

“…The SAGE-derived R 2 and ${\mathrm{R}}_2^{\ast }$ time-courses were obtained using nonlinear least squares fits to a piecewise function (Eq. (1)) as previously described [15]. Due to slice profile imperfections between the excitation and refocusing pulses [15], the signal intensities S I 0 and S II 0 were permitted to differ in the fitting.…”

“…(1)) as previously described [15]. Due to slice profile imperfections between the excitation and refocusing pulses [15], the signal intensities S I 0 and S II 0 were permitted to differ in the fitting. The root mean square percentage error (RMSPE) was calculated as a measure of model goodness-of-fit.…”

“…Combined sequences have been proposed to obtain both SE and GE contrast, although early implementations either were not well suited for dynamic studies or were sensitive to T 1 effects that can induce errors in the DSC measurements [1,2,11–14]. Recently, a T 1 -insensitive combined spin- and gradient-echo (SAGE) EPI method was proposed to simultaneously obtain absolute R 2 , ${\mathrm{R}}_2^{\ast }$ and T 1 -weighted dynamic time-courses [15,16]. …”

“…In previous implementations, parallel imaging was used to reduce the EPI readout time. The derived measures of R 2 and ${\mathrm{R}}_2^{\ast }$ from SAGE acquisitions have been previously validated in phantoms and healthy human subjects [15,16]. In contrast to traditional single-echo DSC-MRI, SAGE-based measures in stroke and tumor patients were found to be insensitive to contrast agent (CA) leakage-induced T 1 effects in regions exhibiting a disrupted blood–brain barrier [16].…”

Evaluation of a multiple spin- and gradient-echo (SAGE) EPI acquisition with SENSE acceleration: Applications for perfusion imaging in and outside the brain

“…More recently, Schmiedeskamp et al . developed a multiple echo spin- and gradient-echo (SAGE) EPI sequence for perfusion measurements in the brain (27,28). The SAGE acquisition permits the simultaneous measurement of R 2 and R 2 * during a dynamic event, such as CA distribution.…”