Phase-encoding Direction Research Articles

Abstract 9975: Midwall Late Gadolinium Enhancement Predicts Mortality in Non-Ischemic Dilated Cardiomyopathy

Introduction: Small studies with composite endpoints show that midwall late gadolinium enhancement (LGE) predicts short term adverse prognosis in dilated cardiomyopathy (DCM). We hypothesized that midwall LGE is an independent predictor of mortality over a long follow-up period. Methods: Consecutive patients with DCM referred for CMR between 2002-2008 were prospectively enrolled. The diagnosis of DCM was made using clinical, CMR and coronary angiographic findings. Patients with ischemic heart disease, primary valvar disease and infiltrative CM were excluded. LGE-CMR at 1.5T (Siemens Sonata or Avanto, Germany) was performed in 2 phase-encoding directions. The presence of midwall LGE was determined by a specialist blinded to all outcome data. The primary endpoint was all-cause mortality. The secondary endpoint was a composite of cardiovascular (CV) mortality or cardiac transplantation. Results: Two hundred and thirty patients (157 male, mean age 50 yrs, mean LVEF 35%) were followed up for a median duration of 44 months. Midwall LGE was identified in 87 (38%) patients. No patient was lost to follow-up. There were 33 deaths (26 CV deaths, 7 non-cardiac) and 7 patients underwent cardiac transplantation. Kaplan-Meier analysis demonstrated a significant association between the presence of midwall LGE and both the primary (p=0.02, Fig.1) and secondary (p=0.003) endpoints. Midwall LGE remained a significant independent predictor of all-cause mortality (HR 2.2, 95% CI 1.1 to 4.7; p=0.035) and CV mortality/transplantation (HR 2.8, 95% CI 1.3 to 6.0; p=0.007) on stepwise Cox regression multivariable analysis allowing for conventional outcome predictors. Conclusions: Midwall LGE is a significant independent predictor of both all-cause mortality and CV mortality/transplantation in DCM. Detection of midwall LGE by CMR represents an important and novel marker for the risk stratification of DCM patients.

Considerations in high‐resolution skeletal muscle diffusion tensor imaging using single‐shot echo planar imaging with stimulated‐echo preparation and sensitivity encoding

Previous studies have shown that skeletal muscle diffusion tensor imaging (DTI) can noninvasively probe changes in the muscle fiber architecture and microstructure in diseased and damaged muscles. However, DTI fiber reconstruction in small muscles and in muscle regions close to aponeuroses and tendons remains challenging because of partial volume effects. Increasing the spatial resolution of skeletal muscle single-shot diffusion-weighted echo planar imaging (DW-EPI) can be hindered by the inherently low signal-to-noise ratio (SNR) of muscle DW-EPI because of the short muscle T(2) and the high sensitivity of single-shot EPI to off-resonance effects and T(2)* blurring. In this article, eddy current-compensated diffusion-weighted stimulated-echo preparation is combined with sensitivity encoding (SENSE) to maintain good SNR properties and to reduce the sensitivity to distortions and T(2)* blurring in high-resolution skeletal muscle single-shot DW-EPI. An analytical framework is developed to optimize the reduction factor and diffusion weighting time to achieve maximum SNR. Arguments for the selection of the experimental parameters are then presented considering the compromise between SNR, B(0)-induced distortions, T(2)* blurring effects and tissue incoherent motion effects. On the basis of the selected parameters in a high-resolution skeletal muscle single-shot DW-EPI protocol, imaging protocols at lower acquisition matrix sizes are defined with matched bandwidth in the phase-encoding direction and SNR. In vivo results show that high-resolution skeletal muscle DTI with minimized sensitivity to geometric distortions and T(2)* blurring is feasible using the proposed methodology. In particular, a significant benefit is demonstrated from a reduction in partial volume effects for resolving multi-pennate muscles and muscles with small cross-sections in calf muscle DTI.

Lateralization of amygdala activation in fMRI may depend on phase-encoding polarity

Susceptibility artifacts along the phase-encoding (PE) direction impact the activation pattern in the amygdala and may lead to systematic asymmetries. We implemented a triple-echo echo-planar imaging (EPI) sequence, acquiring opposite PE polarities along left-right PE direction in a single shot, to investigate its effects on amygdala lateralization. Twelve subjects viewed emotional faces to evoke amygdala activation. A region of interest analysis revealed that the lateralization of amygdala responses depended on the PE polarity thus representing a pure method artifact. Alternating PE with multi-echo EPI reduced the artifact. Lateralized fMRI activation in areas with magnetic field inhomogeneities need to be interpreted with caution.

Parallel imaging with asymmetric acceleration to reduce Gibbs artifacts and to increase signal‐to‐noise ratio of the gradient echo echo‐planar imaging sequence for functional MRI

Parallel imaging with accelerated acquisition was noted to pronounce Gibbs artifacts which appear as ripples propagated in the phase-encoding (PE) direction near the susceptibility-affected region in echo-planar imaging (EPI). Using the extended EPI sequence, which collected extended readouts outside the regular data sampling time, the pronounced Gibbs artifact was analyzed and found to be caused by an increased echo shift in the pre-echo time (T(E)) of accelerated parallel imaging. This was also confirmed by theoretical derivation of the echo shift caused by the inplane susceptibility gradient in the PE direction (ISG(PE)). A new EPI sequence was developed to reduce the Gibbs artifact and to restore the signal level toward that of nonaccelerated parallel imaging by asymmetrically accelerating only the post-T(E) sampling time and by using the extended EPI in the pre-T(E). The nonaccelerated portion in the pre-T(E) used the delay for the optimum blood oxygen level dependent (BOLD) sensitivity at 3 T, maintaining the same slice coverage as the symmetrical acceleration in both pre-T(E) and post-T(E). The increased data sampling points resulted in an increase of the signal-to-noise ratio (SNR). The restored signal and enhanced SNR of the proposed method were confirmed to deliver a better BOLD functional MRI (fMRI) result in the breath holding experiment.

Loss of fine structure and edge sharpness in fast‐spin‐echo carotid wall imaging: Measurements and comparison with multiple‐spin‐echo in normal and atherosclerotic subjects

To test whether the k-space acquisition strategy used by fast-spin-echo (FSE) is a major source of blurring in carotid wall and plaque imaging, and investigate an alternative acquisition approach. The effect of echo train length (ETL) and T(2) on the amount of blurring was studied in FSE simulations of vessel images. Edge sharpness was measured in black-blood T(1) W and proton density-weighted (PDW) carotid images acquired from 5 normal volunteers and 19 asymptomatic patients using both FSE and multiple-spin-echo (Multi-SE) sequences at 3 Tesla (T). Plaque images were classified and divided in group α (tissues' average T(2) ∼ 40-70 ms) and group β (plaque components with shorter T(2) ). Simulations predicted 26.9% reduction of vessel edge sharpness from Multi-SE to FSE images (ETL = 9, T(2) = 60 ms). This agreed with in vivo measurements in normal volunteers (27.4%) and in patient group α (26.2%), while in group β the loss was higher (31.6%). FSE significantly reduced vessel edge sharpness along the phase-encoding direction in T(1) W and PDW images. Blurring was stronger in the presence of plaque components with short T(2) times. This study shows a limitation of FSE and the potential of Multi-SE to improve the quality of carotid imaging.

Evaluation of acquired cholesteatoma with PROPELLER diffusion imaging

PurposeWas to evaluate the role of periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) diffusion imaging in evaluation of middle ear acquired cholesteatomas (primary and recurrent) and differentiating them from middle ear granulation tissue. Patients and methodsForty two patients with suggested acquired cholesteatomas were included in this prospective study, they underwent spiral CT and MRI (PROPELLER diffusion) for all. The two radiologist analyzed the imaging data of plain spiral CT and PROPELLER diffusion MR images. Sensitivity and specificity were assessed. Results were compared with endoscopic and surgical results, which were regarded as the standard reference. ResultsTen patients had middle ear granulation tissue, 21 patients had acquired primary cholesteatomas, and 11 patients had acquired recurrent (post operative) cholesteatomas. PROPELLER diffusion MR successfully diagnosed and differentiated between granulation tissue and cholesteatomas (primary and recurrent. Sensitivity (lesions detection) and specificity (lesions characterization) of PROPELLER diffusion MR was 100%. In recurrent cholesteatomas, plain CT detected abnormal densities without any differentiation resulting in 0% specificity while in primary cholesteatomas, CT successfully diagnosed them based on associated bone erosions/destruction. ConclusionPROPPLER diffusion Imaging is both sensitive and specific for detection and characterization of primary and recurrent cholesteatomas as it lack susceptibility and chemical-shift artifacts, and ghosts in the phase-encoding direction which occur in Echo-planar diffusion, it can detect small cholestatoma as 3mm. It provides the highest sensitivity, specificity for detection and characterization of acquired cholesteatomas.

Data-driven optimization and evaluation of 2D EPI and 3D PRESTO for BOLD fMRI at 7 Tesla: I. Focal coverage

Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) is commonly performed using 2D single-shot echo-planar imaging (EPI). However, single-shot EPI at 7 Tesla (T) often suffers from significant geometric distortions (due to low bandwidth (BW) in the phase-encode (PE) direction) and amplified physiological noise. Recent studies have suggested that 3D multi-shot sequences such as PRESTO may offer comparable BOLD contrast-to-noise ratio with increased volume coverage and decreased geometric distortions. Thus, a four-way group-level comparison was performed between 2D and 3D acquisition sequences at two in-plane resolutions. The quality of fMRI data was evaluated via metrics of prediction and reproducibility using NPAIRS (Non-parametric Prediction, Activation, Influence and Reproducibility re-Sampling). Group activation maps were optimized for each acquisition strategy by selecting the number of principal components that jointly maximized prediction and reproducibility, and showed good agreement in sensitivity and specificity for positive BOLD changes. High-resolution EPI exhibited the highest z-scores of the four acquisition sequences; however, it suffered from the lowest BW in the PE direction (resulting in the worst geometric distortions) and limited spatial coverage, and also caused some subject discomfort through peripheral nerve stimulation (PNS). In comparison, PRESTO also had high z-scores (higher than EPI for a matched in-plane resolution), the highest BW in the PE direction (producing images with superior geometric fidelity), the potential for whole-brain coverage, and no reported PNS. This study provides evidence to support the use of 3D multi-shot acquisition sequences in lieu of single-shot EPI for ultra high field BOLD fMRI at 7T.

FMRI retinotopic mapping at 3 T: Benefits gained from correcting the spatial distortions due to static field inhomogeneity

fMRI retinotopic mapping usually relies upon Fourier analysis of functional responses to periodic visual stimuli that encode eccentricity or polar angle in the visual field. Generally, phase estimations are assigned to a surface model of the cerebral cortex and borders between retinotopic areas are eventually determined following ad hoc phase analysis on the surface model. Assigning functional responses to a surface model of the cortex is particularly sensitive to geometric distortions of the 3D functional data due to static field inhomogeneity. Here, we assess and document the benefits gained from correcting the fMRI data for these effects, under standard experimental conditions (echo-planar imaging, 3.0-T field strength) and with well-chosen acquisition parameters (regarding slice orientation and phase-encoding direction). While it appears that, in the absence of correction, errors in the estimates of the borders between low-order visual areas do not then significantly exceed the variance of statistical origin, about half of the functional responses in a retinotopic experiment are misassigned to neighboring functional areas. Therefore, correction of the effects due to geometric distortions is important in any retinotopic mapping experiment and by extension in any fMRI experiment on the visual system.

Preliminary study of 3.0 T contrast-enhanced whole heart coronary MR angiography using 32-channel coils with high acceleration factor

Objective To evaluate the diagnostic accuracy of 3.0 T contrast enhanced (CE) whole heart coronary MRA ( CE MRA ) using 32-channel coils with high acceleration factor. Methods Sixty patients with suspected coronary artery disease who were scheduled for coronary angiography (CAG)underwent CE CMRA at 3.0 T MRI scanner. A 32-channel receiver coil was used for data acquisition. For image acquisition, an ECG-triggered, navigator-gated, inversion-recovery prepared, segmented gradient-echo sequence was used with an acceleration factor of three in the phase-encoding direction using GRAPPA reconstruction. Gd-BOPTA (0.15 mmol/kg body weight) was intravenously administered at a rate of 0. 3 ml/s. The diagnostic accuracy in detecting significant stenoses ( ≥50% of vessel lumen) was evaluated using χ2 test with X-ray angiography as the reference. Results Whole-heart CE CMRA was successfully completed in 56 patients who were scheduled for CAG. The averaged imaging time was ( 6. 0 ± 1.3 ) min.3.0 T CE CMRA using 32 channel coils correctly identified significant CAD in 28 patients and correctly ruled out CAD in 23 patients. The sensitivity and specificity were 93. 3% and 88.5% respectively.Conclusion Combined with dedicated 32-channel coils, 3.0 T CE CMRA allows significant reduction in imaging speed and reduced dose of the contrast agent. These improvements resulted in substantially improved overall accuracy of CE CMRA in detecting coronary artery disease. Key words: Magnetic resonance angiography; Coronary disease; Contrast media

PROPELLER for motion-robust imaging of in vivo mouse abdomen at 9.4 T

In vivo high-field MRI in the abdomen of small animals is technically challenging because of the small voxel sizes, short T(2) and physiological motion. In standard Cartesian sampling, respiratory and gastrointestinal motion can lead to ghosting artefacts. Although respiratory triggering and navigator echoes can either avoid or compensate for motion, they can lead to variable TRs, require invasive intubation and ventilation, or extend TEs. A self-navigated fast spin echo (FSE)-based periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) acquisition was implemented at 9.4 T to enable high-resolution in vivo MRI of mouse abdomen without the use of additional navigators or triggering. T(2)-weighted FSE-PROPELLER data were compared with single-shot FSE and multi-shot FSE data with and without triggering. Single-shot methods, although rapid and robust to motion, demonstrated strong blurring. Multi-shot FSE data showed better resolution, but suffered from marked blurring in the phase-encoding direction and motion in between shots, leading to ghosting artefacts. When respiratory triggering was used, motion artefacts were largely avoided. However, TRs and acquisition times were lengthened by up to approximately 20%. The PROPELLER data showed a 25% and 61% improvement in signal-to-noise ratio and contrast-to-noise ratio, respectively, compared with multi-shot FSE data, together with a 35% reduction in artefact power. A qualitative comparison between acquisition methods using diffusion-weighted imaging was performed. The results were similar, with the exception that respiratory triggering was unable to exclude major motion artefacts as a result of the sensitisation to motion by the diffusion gradients. The PROPELLER data were of consistently higher quality. Considerations specific to the use of PROPELLER at high field are discussed, including the selection of practical blade widths and the effects on contrast, resolution and artefacts.

Reduction of fast spin echo cusp artifact using a slice‐tilting gradient

A "featherlike" artifact, termed a cusp artifact, is sometimes seen along the phase-encoding direction in sagittal or coronal fast spin echo images. This artifact arises from the spins, at a location distant from the magnet isocenter, that are excited and aliased to the field of view because their precession frequency is similar to those at the isocenter. Such a situation is created due to a combination of excessive gradient nonlinearity and rapid change of the main magnetic field near the edge of the magnet where the artifact-producing spins are located. A novel technique is proposed to reduce this artifact, in which a fast spin echo pulse sequence is modified to slightly tilt the slice selected by the radiofrequency excitation pulse away from the slice selected by the radiofrequency refocusing pulses. At the edge of the field of view, the incomplete overlap between the slices selected by the excitation and refocusing pulses effectively reduces the signals from the artifact-prone region. In contrast, the slices overlap substantially within the field of view so that the signals are largely retained. This slice-tilting technique has been implemented on two commercial MRI scanners operating at 3.0 T and 1.5 T, respectively, and evaluated on phantoms and human spine and extremities using clinical protocols. Both phantom and human results showed that the technique decreased the strength of the cusp artifact by at least 65% and substantially limited the spatial extent of the artifact. This technique, which can be further enhanced by a simple postprocessing step, offers significant advantages over a number of other techniques for reducing the fast spin echo cusp artifact. It can be implemented on virtually any scanner without hardware modification, complicated calibration, sophisticated image reconstruction, or patient-handling alteration.

Reducing distortions in diffusion‐weighted echo planar imaging with a dual‐echo blip‐reversed sequence

The inherent distortions in echo-planar imaging that arise due to inhomogeneities in the static magnetic field can lead to difficulties when attempting to obtain structurally accurate diffusion-tensor imaging data. Parallel acceleration techniques can reduce the magnitude of these distortions but do not remove them entirely. Images can be corrected using a measured field map, but this is prone to error. One approach to correcting for these distortions, referred to here as "blip-reversed" echo-planar imaging, involves collecting a second set of images with the phase encoding reversed. Here, a novel approach to collecting blip-reversed echo-planar imaging data for diffusion-tensor imaging is presented: a dual-echo sequence is used in which the phase-encoding direction of the second echo is swapped compared to the first echo. This allows benefits of the blip-reversed approach to be exploited, with only a modest increase in scan time and, due to the extra data acquired, no significant loss of signal-to-noise efficiency. A novel approach to recombining blip-reversed data is also presented, which involves refining the measured field map, using an algorithm to minimize the difference between the corrected images. The field map refinement is also applicable to conventionally acquired blip-reversed sequences.

O‐space imaging: Highly efficient parallel imaging using second‐order nonlinear fields as encoding gradients with no phase encoding

Recent improvements in parallel imaging have been driven by the use of greater numbers of independent surface coils placed so as to minimize aliasing along the phase-encode direction(s). However, gains from increasing the number of coils diminish as coil coupling problems begin to dominate and the ratio of acceleration gain to expense for multiple receiver chains becomes prohibitive. In this work, we redesign the spatial-encoding strategy in order to gain efficiency, achieving a gradient encoding scheme that is complementary to the spatial encoding provided by the receiver coils. This approach leads to "O-space" imaging, wherein the gradient shapes are tailored to an existing surface coil array, making more efficient use of the spatial information contained in the coil profiles. In its simplest form, for each acquired echo the Z2 spherical harmonic is used to project the object onto sets of concentric rings, while the X and Y gradients are used to offset this projection within the imaging plane. The theory is presented, an algorithm is introduced for image reconstruction, and simulations reveal that O-space encoding achieves high encoding efficiency compared to sensitivity encoding (SENSE) radial k-space trajectories, and parallel imaging technique with localized gradients (PatLoc), suggesting that O-space imaging holds great potential for accelerated scanning.

Recovery of signal loss due to an in-plane susceptibility gradient in the gradient echo EPI through acquisition of extended phase-encoding lines

In gradient echo imaging the in-plane susceptibility gradient causes an echo shift which results in signal loss. The loss of signal becomes more severe in gradient echo EPI, due to the low amplitude of the gradient which is applied in the phase-encoding direction during a long echo train. As the readout gradient amplitude is set to be very high in gradient echo EPI, the echo shift in the readout direction is negligible compared to that in the phase-encoding direction. Traditionally, a z-shimming technique has been applied to the phase-encoding direction of gradient echo EPI to restore the lost signal. This technique, however, requires a significant increase of scan time, as is also the case with the through-plane z-shimming technique. A new approach that allows one to restore the lost signal is to acquire additional phase-encoding lines beyond the regular phase-encoding range. The extension of the phase-encoding lines prior to the regular phase-encoding range exploits the delay time for optimum echo time of the BOLD sensitivity. Therefore, scan time is increased only for the extended phase-encoding lines posterior to the regular phase-encoding range. This technique has been confirmed experimentally by imaging human subject's heads at 3T.

Simulating the effects of time-varying magnetic fields with a realistic simulated scanner

Transient magnetic fields induce changes in magnetic resonance (MR) images ranging from small, visually undetectable effects (caused, for instance, by neuronal currents) to more significant ones, such as those created by the gradient fields and eddy currents. Accurately simulating these effects may assist in correcting or optimising MR imaging for many applications (e.g., diffusion imaging, current density imaging, use of magnetic contrast agents, neuronal current imaging, etc.). Here we have extended an existing MR simulator (POSSUM) with a model for changing magnetic fields at a very high-resolution time-scale. This simulator captures a realistic range of scanner and physiological artifacts by modeling the scanner environment, pulse sequence details and subject properties (e.g., brain geometry and air-tissue boundaries). The simulations were validated by using previously published experimental data sets. In the first dataset a transient magnetic field was produced by a single conducting wire with varying current amplitude (between 17 μA and 765 μA). The second was identical except that current amplitude was fixed (at 7.8 mA) and current timing varied. A very close match between simulated images and experimental data was observed. In addition, these validation results led to the observation that the current-induced effects included ringing in the image, which extended away from the conductor, primarily in the phase-encode direction. This effect had previously not been noticed in the noisy, experimentally-acquired images, demonstrating one way in which simulated images can provide potential insight into imaging experiments.

Quantitative evaluation of k‐space reordering schemes for compatible dual‐echo arteriovenography (CODEA)

Compatible dual-echo arteriovenography (CODEA) is a recently developed technique for simultaneous acquisition of time-of-flight MR angiogram (MRA) and blood oxygenation level-dependent MR venogram (MRV) using an echo-specific k-space reordering scheme. In this study, we evaluated and compared the image quality of CODEA MRA/MRV implemented with two different schemes of echo-specific k-space reordering: one along the 1st phase-encode direction (one-dimensional) only and the other along both phase-encode directions (two-dimensional). Our results showed that use of the two-dimensional reordering scheme improved contrast-to-noise ratio of small arteries by approximately 8%, although not statistically significant (P > 0.1). Contrast-to-noise ratio of the CODEA MRAs was better than that for the non-CODEA dual-echo MRA without k-space reordering (contrast-to-noise ratio increased in large arteries by approximately 10% and small arteries by approximately 45%; P < 0.1). Contrast-to-noise ratio of the CODEA MRAs was comparable with that of the conventional single-echo MRA for large arteries but reduced by approximately 20% for small arteries. Contrast-to-noise ratio of veins on the CODEA MRVs was equivalent to that of the conventional single-echo and the non-CODEA dual-echo MRVs. However, some veins in the CODEA MRVs showed stronger contrast than those in the single-echo MRV in relation to the contrast of neighboring arterial signals.

Multiband multislice GE‐EPI at 7 tesla, with 16‐fold acceleration using partial parallel imaging with application to high spatial and temporal whole‐brain fMRI

Parallel imaging in the form of multiband radiofrequency excitation, together with reduced k-space coverage in the phase-encode direction, was applied to human gradient echo functional MRI at 7 T for increased volumetric coverage and concurrent high spatial and temporal resolution. Echo planar imaging with simultaneous acquisition of four coronal slices separated by 44mm and simultaneous 4-fold phase-encoding undersampling, resulting in 16-fold acceleration and up to 16-fold maximal aliasing, was investigated. Task/stimulus-induced signal changes and temporal signal behavior under basal conditions were comparable for multiband and standard single-band excitation and longer pulse repetition times. Robust, whole-brain functional mapping at 7 T, with 2 x 2 x 2mm(3) (pulse repetition time 1.25 sec) and 1 x 1 x 2mm(3) (pulse repetition time 1.5 sec) resolutions, covering fields of view of 256 x 256 x 176 mm(3) and 192 x 172 x 176 mm(3), respectively, was demonstrated with current gradient performance.

Imagerie de diffusion : moelle et rachis

Due to its excellent sensitivity, MR imaging is invaluable for the evaluation of lesions of the cord and spine. Several studies dedicated to diffusion-weighted MR evaluation of the cord and spine have been published. While diffusion-weighted MR imaging of the brain is routinely performed, it is seldom performed when imaging the spine due to serious limitations. While anatomical limitations may not be changed, the voxel size, phase-encoding direction, mode of k-space filling, and acceleration factor are all parameters that can be optimized in order to routinely obtain diffusion-weighted imaging of the spine on 1.5T and 3T scanners.

Motion correction using an enhanced floating navigator and GRAPPA operations

A method for motion correction in multicoil imaging applications, involving both data collection and reconstruction, is presented. The floating navigator method, which acquires a readout line off center in the phase-encoding direction, is expanded to detect translation/rotation and inconsistent motion. This is done by comparing floating navigator data with a reference k-space region surrounding the floating navigator line, using a correlation measure. The technique of generalized autocalibrating partially parallel acquisition is further developed to correct for a fully sampled, motion-corrupted dataset. The flexibility of generalized autocalibrating partially parallel acquisition kernels is exploited by extrapolating readout lines to fill in missing "pie slices" of k-space caused by rotational motion and regenerating full k-space data from multiple interleaved datasets, facilitating subsequent rigid-body motion correction or proper weighting of inconsistent data (e.g., with through-plane and nonrigid motion). Phantom and in vivo imaging experiments with turbo spin-echo sequence demonstrate the correction of severe motion artifacts.

Influence of cardiac motion on diffusion-weighted magnetic resonance imaging of the liver

To assess cardiac motion-induced signal loss in diffusion-weighted magnetic resonance imaging (DWI) of the liver using dynamic DWI. Three volunteers underwent dynamic coronal DWI of the liver under breathholding, in the diastolic (DWI(diast)) or systolic (DWI(syst)) cardiac phase, and with motion probing gradients (MPGs) in phase encoding (P, left-right), frequency encoding (M, superior-inferior), or slice select (S, anterior-posterior) direction. Liver-to-background contrasts (LBCs) of DWI(syst) were compared to those of DWI(diast), for both the left and right liver lobes, using nonparametric tests. Signal decrease ratios (SDRs) were calculated as (1-(LBCDWI(syst)/LBCDWI(diast))) x 100%. DWI(syst) was further analyzed to determine which direction of MPGs was most affected by cardiac motion. In the left liver lobe, LBCs of DWI(syst) (median 3.35) were significantly lower (P < 0.0001) than those of DWI(diast) (median 4.84). In the right liver lobe, LBCs of DWI(syst) (median 4.17) were also significantly lower (P < 0.0001) than those of DWI(diast) (median 5.35 ). SDRs of the left and right liver lobes were 25.5% and 17.3%, respectively. In DWI(syst), the significantly lowest (P < 0.05) LBCs were observed in the M direction (left liver lobe) and P direction (right liver lobe) of MPGs. Signal intensity of both liver lobes are affected by cardiac motion in DWI. In the left liver lobe, signal loss especially occurs in the superior-inferior direction of MPGs, whereas in the right lobe, signal loss especially occurs in the left-right direction of MPGs.