Improved diffusion‐weighted single‐shot echo‐planar imaging (EPI) in stroke using sensitivity encoding (SENSE)

Abstract

Diffusion-weighted single-shot EPI (sshEPI) is one of the most important tools for the diagnostic assessment of stroke patients, but it suffers from well known artifacts. Therefore, sshEPI was combined with SENSitivity Encoding (SENSE) to further increase EPI's potential for stroke imaging. Eight healthy volunteers and a consecutive series of patients (N ‫؍‬ 8) with suspected stroke were examined with diffusion-weighted SENSE-sshEPI using different reduction factors (1.0 ≤ R ≤ 3.0). Additionally, a high-resolution diffusion-weighted SENSEsshEPI scan was included. All examinations were diagnostic and of better quality than conventional sshEPI. No ghostings or aliasing artifacts were discernible, and EPI-related image distortions were markedly diminished. Chemical shift artifacts and eddy current-induced image warping were still present, although to a markedly smaller extent. Measured direction-dependent diffusion-coefficients and isotropic diffusion values were comparable to previous findings but showed less fluctuation. We have demonstrated the technical feasibility and clinical applicability of diffusion-weighted SENSE-sshEPI in patients with subacute stroke. Because of the faster k-space traversal, this novel technique is able to reduce typical EPI artifacts and increase spatial resolution while simultaneously remaining insensitive to bulk motion. Key words: magnetic resonance imaging; diffusion imaging; brain; stroke; parallel imaging Diffusion-weighted imaging (DWI) has become a major tool for the diagnostic assessment of stroke patients (1). In the acute stage, DWI is extremely valuable for treatment decisions because of its high sensitivity to early ischemic damage (2). Thereafter, contributions to the diagnostic work-up come from observing the characteristic course of diffusional abnormalities from infarction (3,4). Most often, single-shot echo-planar imaging (sshEPI) has been the preferred technique for performing DWI of stroke patients because it allows rapid image acquisition and is much less susceptible to ghosting artifacts than conventional multishot techniques. These ghosting artifacts are primarily the consequence of random phase errors, which are caused by bulk motion during the diffusion-preparation period. However, image quality of sshEPI can also suffer from problems such as image blurring due to the long EPI readout interval, and artifacts due to EPI's high susceptibility to resonance offsets (e.g., lipid chemical shift, magnetic susceptibility gradients, or B 0 inhomogeneities). In this context, image degradation is likely to occur, especially at the base of the skull and in infratentorial parts of the brain, and may therefore critically impair the detection and delineation of ischemic lesions in these regions.In this study, diffusion-weighted sshEPI was used together with SENSitivity Encoding (SENSE) (5) to overcome the aforementioned limitations of sshEPI. SENSE-sshEPI reduces the train of gradient echoes in combination with a faster k-space traversal per unit time. The resultant increased...

Diffusion‐weighted imaging of the liver: Comparison of navigator triggered and breathhold acquisitions

“…SS EPI sequences are very fast, however, suffer from limited image quality, mostly related to limited signal to noise ratio (SNR) especially at higher b-values, and limited spatial resolution, which constitute an obstacle for its widespread use in clinical practice. Techniques such as parallel imaging (25,26) and pulse triggering (27) have been proposed to improve SS EPI image quality and precision of ADC measurement.…”

“…Breathhold SS EPI sequences are fast, however, suffer from low SNR especially at higher b-values. Techniques such as parallel imaging (25,26) and pulse triggering (27) have been proposed to improve diffusion image quality and precision of ADC measurement. Respiratory triggering is another method that can be proposed to improve image quality of DWI.…”

“…SS EPI sequences are very fast, however, suffer from limited image quality, mostly related to limited signal to noise ratio (SNR) especially at higher b-values, and limited spatial resolution, which constitute an obstacle for its widespread use in clinical practice. Techniques such as parallel imaging (25,26) and pulse triggering (27) have been proposed to improve SS EPI image quality and precision of ADC measurement.There are several recent reports on the use of respiratory triggered acquisitions for liver DWI (15,16,[28][29][30][31]. However, only two studies have specifically compared different diffusion acquisition schemes for liver imaging (28,30).…”

Diffusion‐tensor MRI: theory, experimental design and data analysis – a technical review

“…72,73 An interesting approach is to use a mutual information criterion to determine a warp that maximizes the overlap within a series of diffusion-weighted images. 57,74 Other post-processing methods that have been proposed include correcting the phase map using a model of the effects of the eddy current on it, 75,76 and mapping the eddy current induced fields directly. 77,78 Magnetic susceptibility effects Large discontinuities in bulk magnetic susceptibility, such as those occurring at tissue-air interfaces, produce local magnetic field gradients that notoriously degrade and distort DWIs, particularly during echo-planar imaging.…”

“…It is clear that, at higher field strengths, Figure 2 are typical of distortions using single-shot EPI at 3T, diffusion weighted SENSE acquisitions appear to mitigate these artifacts. 76 …”