Limitations of stimulated echo acquisition mode (steam) techniques in cardiac applications

Abstract

Stimulated echoes are widely used for imaging functional tissue parameters such as diffusion coefficient, perfusion, and flow rates. They are potentially interesting for the assessment of various cardiac functions. However, severe limitations of the stimulated echo acquisition mode occur, which are related to the special dynamic properties of the beating heart and flowing blood. To the well-known signal decay due to longitudinal relaxation and through-plane motion between the preparation and the read-out period of the stimulated echoes, additional signal loss is often observed. As the prepared magnetization is fixed with respect to the tissue, this signal loss is caused by the tissue deformation during the cardiac cycle, which leads to a modification of the modulation frequency of the magnetization. These effects are theoretically derived and corroborated by phantom and in vivo experiments.

In vivo SNR in DENSE MRI; temporal and regional effects of field strength, receiver coil sensitivity and flip angle strategies

“…In DENSE, local strain variation may cause spatial variations of SNR due to strain induced intravoxel dephasing [23]. However, as no large strain variations were found in the healthy volunteers in this short axis slice, strain induced dephasing would not explain the SNR differences found between the distal and proximal regions in this study.…”

Correction of through‐plane deformation artifacts in stimulated echo acquisition mode cardiac imaging

“…The deformation strain is defined here as the change of the tissue length per unit length in the through-plane direction. The exact relation between local modulation frequency and the through-plane strain can be described by the equation (10): ͑x,y;t͒ ϭ m 1 ϩ ε͑x,y;t͒ [1] Another way to view this change in the modulation frequency is to consider it intravoxel dephasing that reduces the signal intensity from this voxel as reported in (2,7,12). Alternatively, the frequency change can be considered a shift of the stimulated echo in k-space, as in the work of Osman et al (10).…”

“…Moreover, a shift that exceeds 1/L (outside the shaded area) causes the signal intensity to slightly increase due to the side lobe of the sinc function (dealing with magnitude images implies that all signal values to be nonnegative). Equation [4] then provides a good explanation for the reported artifact in STEAM images of the heart (2,12). Note that for the rest of this paper, we will assume the case of rectangular imaging slice, but the method is applicable to more varieties of slice profiles.…”

“…As reported in a number of articles (2,(7)(8)(9)11,12) when applying STEAM for cardiac imaging, the contraction (or stretching) of the cardiac tissue results in intravoxel dephasing of the magnetization, which leads to a substantial loss of the myocardial signal. Fischer et al (12) proposed a method to correct for through-plane motion at each cardiac phase by adapting the demodulation frequency to the anticipated amount of tissue deformation.…”

“…Fischer et al (12) proposed a method to correct for through-plane motion at each cardiac phase by adapting the demodulation frequency to the anticipated amount of tissue deformation. However, this solution has limitations because of the heterogeneity of contraction of the myocardium, which makes it difficult for a single demodulation frequency to compensate for a range of contraction.…”