Asymmetric spin‐echo imaging of magnetically inhomogeneous systems: Theory, experiment, and numerical studies

Abstract

The ability of the asymmetric spin-echo (ASE) pulse sequence to provide different degrees of spin-echo (SE)-type and gradient-echo (GE)-type contrast when imaging media containing magnetic inhomogeneities is investigated. The dependence of the ASE signal on the size of magnetic field perturbers is examined using theory, computer simulations, and experiment. A theoretical prediction of the ASE signal is obtained using the Anderson-Weiss mean field theory, the results of which are qualitatively supported by computer simulations and experimental studies. It is shown that the ASE sequence can be used to tune the range of perturber sizes that provide the largest contributions to susceptibility contrast effects.

Cerebral Hemodynamics in CADASIL Before and After Acetazolamide Challenge Assessed With MRI Bolus Tracking

“…31 Second, in contrast to that observed with spin-echo sequences, the asymmetrical spin-echo signal changes due to the tracer passage is sensitive to local field inhomogeneities, which can also change between subjects in the region of MCA. 21 Third, our asymmetrical spin-echo sequence chosen to obtain the maximal sensitivity to microcirculatory blood volumes with a minimal dose of contrast agent is intermediate between the spin-echo and gradient-echo sequences. 21,22 This sequence remains strongly sensitive to signal variations within large vessels as shown by our CBF or CBV maps.…”

“…21 Third, our asymmetrical spin-echo sequence chosen to obtain the maximal sensitivity to microcirculatory blood volumes with a minimal dose of contrast agent is intermediate between the spin-echo and gradient-echo sequences. 21,22 This sequence remains strongly sensitive to signal variations within large vessels as shown by our CBF or CBV maps. Therefore, it is likely that there was different contamination of the tissue signal variations (presumed to be at the capillary level) by signal changes coming from large arteries or veins, particularly at the cortical level, despite our precautions to exclude the regions containing the largest vessels along the cortical rim.…”

“…This sequence was chosen to detect sufficient signal variations with a single dose of 0.1 mmol/kg gadolinium within the affected white matter of the patients. 21 It was also preferred to the gradient echo EPI sequence, which is generally assumed to be more sensitive to signal changes within large vessels. 21,22 A 128ϫ128 acquisition matrix was used with a 24ϫ24-cm field of view, leading to an in-plane resolution of 1.875ϫ1.875 mm 2 .…”

“…21 It was also preferred to the gradient echo EPI sequence, which is generally assumed to be more sensitive to signal changes within large vessels. 21,22 A 128ϫ128 acquisition matrix was used with a 24ϫ24-cm field of view, leading to an in-plane resolution of 1.875ϫ1.875 mm 2 . Seven slices, 5 mm thick, were acquired every 1.6 seconds from the cerebellar level up to the MR images obtained in 1 control subject (top row) and 1 patient with CADASIL.…”

Spin-Echo Resting-State Functional Connectivity in High-Susceptibility Regions: Accuracy, Reliability, and the Impact of Physiological Noise

“…Purely T 2 -weighted techniques such as HASTE (Poser and Norris, 2007) and RASER (Goerke et al, 2011) aim to eliminate T 2 * weighting, but introduce high-power deposition, necessitating longer repetition times (TR) that are nonideal for fcMRI applications. Alternatively, asymmetric spin echo (Stables et al, 1998), dual-echo gradient-spin echo (Halai et al, 2014;Schwarzbauer et al, 2010), or 3D GRASE (Feinberg et al, 2008) can be used to compensate for signal loss while boosting BOLD signal contrast. However, these approaches also reduce acquisition speed and are not available on all commercial MR scanners.…”