Steven H. Baete

Comparison of fitting methods and b‐value sampling strategies for intravoxel incoherent motion in breast cancer

To compare fitting methods and sampling strategies, including the implementation of an optimized b‐value selection for improved estimation of intravoxel incoherent motion (IVIM) parameters in breast cancer.

Time-dependent diffusion in skeletal muscle with the random permeable barrier model (RPBM): Application to normal controls and chronic exertional compartment syndrome patients

The purpose of this work was to carry out diffusion tensor imaging (DTI) at multiple diffusion times Td in skeletal muscle in normal subjects and chronic exertional compartment syndrome (CECS) patients and analyze the data with the random permeable barrier model (RPBM) for biophysical specificity.

Stimulated echo diffusion tensor imaging and SPAIR T2 -weighted imaging in chronic exertional compartment syndrome of the lower leg muscles.

To evaluate the performance of diffusion tensor imaging (DTI) in the evaluation of chronic exertional compartment syndrome (CECS) as compared to T2‐weighted (T2w) imaging.

A model-based reconstruction for undersampled radial spin-echo DTI with variational penalties on the diffusion tensor.

Radial spin‐echo diffusion imaging allows motion‐robust imaging of tissues with very low T2 values like articular cartilage with high spatial resolution and signal‐to‐noise ratio (SNR). However, in vivo measurements are challenging, due to the significantly slower data acquisition speed of spin‐echo sequences and the less efficient k‐space coverage of radial sampling, which raises the demand for accelerated protocols by means of undersampling. This work introduces a new reconstruction approach for undersampled diffusion‐tensor imaging (DTI). A model‐based reconstruction implicitly exploits redundancies in the diffusion‐weighted images by reducing the number of unknowns in the optimization problem and compressed sensing is performed directly in the target quantitative domain by imposing a total variation (TV) constraint on the elements of the diffusion tensor. Experiments were performed for an anisotropic phantom and the knee and brain of healthy volunteers (three and two volunteers, respectively). Evaluation of the new approach was conducted by comparing the results with reconstructions performed with gridding, combined parallel imaging and compressed sensing and a recently proposed model‐based approach. The experiments demonstrated improvements in terms of reduction of noise and streaking artifacts in the quantitative parameter maps, as well as a reduction of angular dispersion of the primary eigenvector when using the proposed method, without introducing systematic errors into the maps. This may enable an essential reduction of the acquisition time in radial spin‐echo diffusion‐tensor imaging without degrading parameter quantification and/or SNR. Copyright

Multiple‐echo diffusion tensor acquisition technique (MEDITATE) on a 3T clinical scanner

This article describes the concepts and implementation of an MRI method, the multiple‐echo diffusion tensor acquisition technique (MEDITATE), which is capable of acquiring apparent diffusion tensor maps in two scans on a 3T clinical scanner. In each MEDITATE scan, a set of RF pulses generates multiple echoes, the amplitudes of which are diffusion weighted in both magnitude and direction by a pattern of diffusion gradients. As a result, two scans acquired with different diffusion weighting strengths suffice for accurate estimation of diffusion tensor imaging (DTI) parameters. The MEDITATE variation presented here expands previous MEDITATE approaches to adapt to the clinical scanner platform, such as exploiting longitudinal magnetization storage to reduce T2 weighting. Fully segmented multi‐shot Cartesian encoding is used for image encoding. MEDITATE was tested on isotropic (agar gel), anisotropic diffusion phantoms (asparagus) and in vivo skeletal muscle in healthy volunteers with cardiac gating. Comparisons of accuracy were performed with standard twice‐refocused spin echo (TRSE) DTI in each case and good quantitative agreement was found between diffusion eigenvalues, mean diffusivity and fractional anisotropy derived from TRSE DTI and from the MEDITATE sequence. Orientation patterns were correctly reproduced in both isotropic and anisotropic phantoms, and approximately for in vivo imaging. This illustrates that the MEDITATE method of compressed diffusion encoding is feasible on the clinical scanner platform. With future development and employment of appropriate view‐sharing image encoding, this technique may be used in clinical applications requiring time‐sensitive acquisition of DTI parameters such as dynamical DTI in muscle. Copyright

Validation of surface-to-volume ratio measurements derived from oscillating gradient spin echo on a clinical scanner using anisotropic fiber phantoms

A diffusion measurement in the short‐time surface‐to‐volume ratio (S/V) limit (Mitra et al., Phys Rev Lett. 1992;68:3555) can disentangle the free diffusion coefficient from geometric restrictions to diffusion. Biophysical parameters, such as the S/V of tissue membranes, can be used to estimate microscopic length scales non‐invasively. However, due to gradient strength limitations on clinical MRI scanners, pulsed gradient spin echo (PGSE) measurements are impractical for probing the S/V limit. To achieve this limit on clinical systems, an oscillating gradient spin echo (OGSE) sequence was developed. Two phantoms containing 10 fiber bundles, each consisting of impermeable aligned fibers with different packing densities, were constructed to achieve a range of S/V values. The frequency‐dependent diffusion coefficient, D(ω), was measured in each fiber bundle using OGSE with different gradient waveforms (cosine, stretched cosine, and trapezoidal), while D(t) was measured from PGSE and stimulated‐echo measurements. The S/V values derived from the universal high‐frequency behavior of D(ω) were compared against those derived from quantitative proton density measurements using single spin echo (SE) with varying echo times, and from magnetic resonance fingerprinting (MRF). S/V estimates derived from different OGSE waveforms were similar and demonstrated excellent correlation with both SE‐ and MRF‐derived S/V measures (ρ  ≥  0.99). Furthermore, there was a smoother transition between OGSE frequency f and PGSE diffusion time when using teffS/V=9/64f , rather than the commonly used teff = 1/(4f), validating the specific frequency/diffusion time conversion for this regime. Our well‐characterized fiber phantom can be used for the calibration of OGSE and diffusion modeling techniques, as the S/V ratio can be measured independently using other MR modalities. Moreover, our calibration experiment offers an exciting perspective of mapping tissue S/V on clinical systems.

Radial q-space sampling for DSI

Diffusion spectrum imaging (DSI) has been shown to be an effective tool for noninvasively depicting the anatomical details of brain microstructure. Existing implementations of DSI sample the diffusion encoding space using a rectangular grid. Here we present a different implementation of DSI whereby a radially symmetric q‐space sampling scheme for DSI is used to improve the angular resolution and accuracy of the reconstructed orientation distribution functions.

Dynamic diffusion-tensor measurements in muscle tissue using the single-line multiple-echo diffusion-tensor acquisition technique at 3T

When diffusion biomarkers display transient changes, i.e. in muscle following exercise, traditional diffusion‐tensor imaging (DTI) methods lack the temporal resolution to resolve the dynamics. This article presents an MRI method for dynamic diffusion‐tensor acquisitions on a clinical 3T scanner. This method, the Single‐Line Multiple‐Echo Diffusion‐Tensor Acquisition Technique (SL‐MEDITATE), achieves a high temporal resolution (4 s) by rapid diffusion encoding through the acquisition of multiple echoes with unique diffusion sensitization and limiting the readout to a single line volume. The method is demonstrated in a rotating anisotropic phantom, a flow phantom with adjustable flow speed and in vivo skeletal calf muscle of healthy volunteers following a plantar flexion exercise. The rotating and flow‐varying phantom experiments show that SL‐MEDITATE correctly identifies the rotation of the first diffusion eigenvector and the changes in diffusion‐tensor parameter magnitudes, respectively. Immediately following exercise, the in vivo mean diffusivity (MD) time courses show, before the well‐known increase, an initial decrease that is not typically observed in traditional DTI. In conclusion, SL‐MEDITATE can be used to capture transient changes in tissue anisotropy in a single line. Future progress might allow for dynamic DTI when combined with appropriate k‐space trajectories and compressed sensing reconstruction. Copyright

Accelerated radial diffusion spectrum imaging using a multi-echo stimulated echo diffusion sequence

Diffusion spectrum imaging (DSI) provides us non‐invasively and robustly with anatomical details of brain microstructure. To achieve sufficient angular resolution, DSI requires a large number of q‐space samples, leading to long acquisition times. This need is mitigated here by combining the beneficial properties of Radial q‐space sampling for DSI with a Multi‐Echo Stimulated Echo Sequence (MESTIM).

MRI assessment of the thigh musculature in dermatomyositis and healthy subjects using diffusion tensor imaging, intravoxel incoherent motion and dynamic DTI.

IntroductionDermatomyositis (DM) is an idiopathic inflammatory myopathy involving severe debilitation in need of diagnostics. We evaluated the proximal lower extremity musculature with diffusion tensor imaging (DTI), intravoxel incoherent motion (IVIM) and dynamic DTI in DM patients and controls and compared with standard clinical workup. MethodsIn this IRB-approved, HIPAA-compliant study with written informed consent, anatomical, Dixon fat/water and diffusion imaging were collected in bilateral thigh MRI of 22 controls and 27 DM patients in a 3T scanner. Compartments were scored on T1/T2 scales. Single voxel dynamic DTI metrics in quadriceps before and after 3-min leg exercise were measured. Spearman rank correlation and mixed model analysis of variance/covariance (ANOVA/ANCOVA) were used to correlate with T1 and T2 scores and to compare patients with controls.ResultsDM patients showed significantly lower pseudo-diffusion and volume in quadriceps than controls. All subjects showed significant correlation between T1 score and signal-weighted fat fraction; tissue diffusion and pseudo-diffusion varied significantly with T1 and T2 score in patients. Radial and mean diffusion exercise response in patients was significantly higher than controls.ConclusionStatic and dynamic diffusion imaging metrics show correlation with conventional imaging scores, reveal spatial heterogeneity, and provide means to differentiate dermatomyositis patients from controls.Key Points• Diffusion imaging shows regional differences between thigh muscles of dermatomyositis patients and controls.• Signal-weighted fat fraction and diffusion metrics correlate with T1/T2 scores of disease severity.• Dermatomyositis patients show significantly higher radial diffusion exercise response than controls.