Tobias Block

Self-gated Radial MRI for Respiratory Motion Compensation on Hybrid PET/MR Systems

Accurate localization and uptake quantification of lesions in the chest and abdomen using PET imaging is challenging due to the respiratory motion during the exam. The advent of hybrid PET/MR systems offers new ways to compensate for respiratory motion without exposing the patient to additional radiation. The use of self-gated reconstructions of a 3D radial stack-of-stars GRE acquisition is proposed to derive a high-resolution MRI motion model. The self-gating signal is used to perform respiratory binning of the simultaneously acquired PET raw data. Matching mu-maps are generated for every bin, and post-reconstruction registration is performed in order to obtain a motion-compensated PET volume from the individual gates. The proposed method is demonstrated in-vivo for three clinical patients. Motion-corrected reconstructions are compared against ungated and gated PET reconstructions. In all cases, motion-induced blurring of lesions in the liver and lung was substantially reduced, without compromising SNR as it is the case for gated reconstructions.

Dynamic contrast-enhanced MRI of the prostate with high spatiotemporal resolution using compressed sensing, parallel imaging, and continuous golden-angle radial sampling: Preliminary experience

To demonstrate dynamic contrast‐enhanced (DCE) magnetic resonance imaging (MRI) of the prostate with both high spatial and temporal resolution via a combination of golden‐angle radial k‐space sampling, compressed sensing, and parallel‐imaging reconstruction (GRASP), and to compare image quality and lesion depiction between GRASP and conventional DCE in prostate cancer patients.

Estimating liver perfusion from free-breathing continuously acquired dynamic gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid-enhanced acquisition with compressed sensing reconstruction

ObjectiveThe purpose of this study was to estimate perfusion metrics in healthy and cirrhotic liver with pharmacokinetic modeling of high–temporal resolution reconstruction of continuously acquired free-breathing gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid–enhanced acquisition in patients undergoing clinically indicated liver magnetic resonance imaging. Subjects and MethodsIn this Health Insurance Portability and Accountability Act–compliant prospective study, 9 cirrhotic and 10 noncirrhotic patients underwent clinical magnetic resonance imaging, which included continuously acquired radial stack-of-stars 3-dimensional gradient recalled echo sequence with golden-angle ordering scheme in free breathing during contrast injection. A total of 1904 radial spokes were acquired continuously in 318 to 340 seconds. High–temporal resolution data sets were formed by grouping 13 spokes per frame for temporal resolution of 2.2 to 2.4 seconds, which were reconstructed using the golden-angle radial sparse parallel technique that combines compressed sensing and parallel imaging. High–temporal resolution reconstructions were evaluated by a board-certified radiologist to generate gadolinium concentration-time curves in the aorta (arterial input function), portal vein (venous input function), and liver, which were fitted to dual-input dual-compartment model to estimate liver perfusion metrics that were compared between cirrhotic and noncirrhotic livers. ResultsThe cirrhotic livers had significantly lower total plasma flow (70.1 ± 10.1 versus 103.1 ± 24.3 mL/min per 100 mL; P < 0.05), lower portal venous flow (33.4 ± 17.7 versus 89.9 ± 20.8 mL/min per 100 mL; P < 0.05), and higher arterial perfusion fraction (52.0% ± 23.4% versus 12.4% ± 7.1%; P < 0.05). The mean transit time was higher in the cirrhotic livers (24.4 ± 4.7 versus 15.7 ± 3.4 seconds; P < 0.05), and the hepatocellular uptake rate was lower (3.03 ± 2.1 versus 6.53 ± 2.4 100/min; P < 0.05). ConclusionsLiver perfusion metrics can be estimated from free-breathing dynamic acquisition performed for every clinical examination without additional contrast injection or time. This is a novel paradigm for dynamic liver 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

Contrast-Enhanced Radial 3D Fat-Suppressed T1-Weighted Gradient-Recalled Echo Sequence Versus Conventional Fat-Suppressed Contrast-Enhanced T1-Weighted Studies of the Head and Neck

OBJECTIVE Traditional fat-suppressed T1-weighted spin-echo or turbo spin-echo (TSE) sequences (T1-weighted images) may be degraded by motion and pulsation artifacts in head-and-neck studies. Our purpose is to evaluate the role of a fat-suppressed T1-weighted 3D radial gradient-recalled echo sequence (radial-volumetric interpolated breath-hold examination [VIBE]) in the head and neck as compared with standard contrast-enhanced fat-suppressed T1-weighted images. MATERIALS AND METHODS We retrospectively evaluated 21 patients (age range, 9-67 years) who underwent head-and-neck MRI at 1.5 T. Both contrast-enhanced radial-VIBE and conventional fat-suppressed TSE contrast-enhanced T1-weighted imaging were performed. Two radiologists evaluated multiple parameters of image quality, graded on a 5-point scale. Mixed-model analysis of variance and interobserver variability assessment were performed. RESULTS The following parameters were scored as significantly better for the contrast-enhanced radial-VIBE sequence than for conventional contrast-enhanced T1-weighted imaging: overall image quality (p < 0.0001), degree of fat suppression (p = 0.006), mucosal enhancement (p = 0.004), muscle edge clarity (p = 0.049), vessel clarity (p < 0.0001), respiratory motion artifact (p = 0.002), pulsation artifact (p < 0.0001), and lesion edge sharpness (p = 0.004). Interobserver agreement in qualitative evaluation of the two sequences showed fair-to-good agreement for the following variables: overall image quality (intraclass correlation coefficient [ICC], 0.779), degree of fat suppression (ICC, 0.716), mucosal enhancement (ICC, 0.693), muscle edge clarity (ICC, 0.675), respiratory motion artifact (ICC, 0.516), lesion enhancement (ICC, 0.410), and lesion edge sharpness (ICC, 0.538). Excellent agreement was shown for vessel clarity (ICC, 0.846) and pulsation artifact (ICC, 0.808). CONCLUSION The radial-VIBE sequence is a viable motion-robust improvement on the conventional fat-suppressed T1-weighted sequence.

Combination of compressed sensing and parallel imaging for highly-accelerated dynamic MRI

The introduction of compressed sensing methods to speed up image acquisition has received great attention in the Magnetic Resonance Imaging (MRI) community. Compressed sensing exploits the compressibility of medical images to reconstruct unaliased images from undersampled data. Moreover, compressed sensing can be synergistically combined with previously introduced acceleration methods such as parallel imaging, which employs arrays of receiver coils to further increase imaging speed. Over the past three years, we have been working on the combination of compressed sensing and parallel imaging, exploiting the idea of joint multicoil sparsity. In this work, we present a summary of our image acquisition and reconstruction methods for the combination of compressed sensing and parallel imaging, and describe applications to cardiac and body dynamic MRI.

Evaluation of the orbit using contrast-enhanced radial 3D fat-suppressed T1 weighted gradient echo (Radial-VIBE) sequence

OBJECTIVE Contrast-enhanced fat-suppressed T1 weighted (T1W) two-dimensional (2D) turbo spin echo (TSE) and magnetization-prepared gradient echo (MPRAGE) sequences with water excitation are routinely obtained to evaluate orbit pathology. However, these sequences can be marred by artefacts. The radial-volume-interpolated breath-hold examination (VIBE) sequence is a motion-robust fat-suppressed T1W sequence which has demonstrated value in paediatric and body imaging. The purpose of our study was to evaluate its role in assessing the orbit and to compare it with routinely acquired sequences. METHODS A Health Insurance Portability and Accountability Act-compliant and institutional review board-approved retrospective study was performed in 46 patients (age range: 1-81 years) who underwent orbit studies on a 1.5-T MRI system using contrast-enhanced Radial-VIBE, MPRAGE and 2D TSE sequences. Two radiologists blinded to the sequence analysed evaluated multiple parameters of image quality including motion artefact, degree of fat suppression, clarity of choroidal enhancement, intraorbital vessels, extraocular muscles, optic nerves, brain parenchyma and evaluation of pathology. Each parameter was assessed on a 5-point scale, with a higher score indicating the more optimal examination. Mix model analysis of variance and interobserver variability were assessed. RESULTS Radial-VIBE demonstrated superior quality (p < 0.001) for all orbit parameters when compared with MPRAGE and 2D TSE. Interobserver agreement demonstrated average fair-to-good agreement for degree of motion artefact (0.745), fat suppression (0.678), clarity of choroidal enhancement (0.688), vessels (0.655), extraocular muscles (0.675), optic nerves (0.518), brain parenchyma (0.710) and evaluation of pathology (0.590). CONCLUSION Radial-VIBE sequence demonstrates superior image quality when evaluating the orbits as compared with conventional MPRAGE and 2D TSE sequences. ADVANCES IN KNOWLEDGE Radial-VIBE employs unique non-Cartesian k-space sampling in a radial or spoke-wheel fashion which provides superior image quality improving diagnostic capability in the evaluation of the orbits.

Joint reconstruction of simultaneously acquired MR-PET data with multi sensor compressed sensing based on a joint sparsity constraint

State-of-the-art MR-PET scanners allow simultaneous data acquisition. However, image reconstruction is performed separately and results are only combined at the visualization stage. PET images are reconstructed using a variant of EM and MR data are reconstructed using an inverse Fourier transform or iterative algorithms for parallel imaging or compressed sensing. We propose a new iterative joint reconstruction framework based on multi-sensor compressed sensing that exploits anatomical correlations between MR and PET. Joint reconstruction is motivated by the fact that MR and PET are based on the same anatomy. High resolution MR information can be used to enhance the PET reconstruction and MR artifacts are not present in the PET image. Therefore a dedicated reconstruction can exploit the incoherence of artifacts in the joint space. Our approach uses a nonlinear constrained optimization problem. In each iteration OSEM enforces data consistency of the current solution with measured PET rawdata. An l1-norm based joint sparsity term exploits anatomical correlations. MR data consistency is enforced with the MR forward operator, consisting of coil sensitivity modulation and a (non-uniform) Fourier transform. Data were acquired on a clinical 3T MR-PET unit (Siemens Biograph mMR). 10 mCi 18F-FDG were injected followed by a 60min list mode scan. 3D MP-RAGE was used for MR data acquisition: TR=2300ms, TE=2.98ms, TI=900ms, FA=9°, acceleration factor 2, 24 ACS lines, 256 matrix, voxel size=1×1×1mm3, 192 slices. Joint MR-PET reconstruction improves resolution in PET images when structures are aligned with MR. PET signal information cannot be improved in regions showing no distinctive MR contrast, but it is also not influenced falsely. The availability of simultaneously-acquired MR and PET data will also enable incorporation of dynamic correlations and motion correction into the joint reconstruction framework. We expect that this provides additional enhancements to the information content of multimodality studies.

Validation of highly accelerated real-time cardiac cine MRI with radial k-space sampling and compressed sensing in patients at 1.5T and 3T

To validate an optimal 12‐fold accelerated real‐time cine MRI pulse sequence with radial k‐space sampling and compressed sensing (CS) in patients at 1.5T and 3T.

Comparison of conventional DCE-MRI and a novel golden-angle radial multicoil compressed sensing method for the evaluation of breast lesion conspicuity.

To compare a novel multicoil compressed sensing technique with flexible temporal resolution, golden‐angle radial sparse parallel (GRASP), to conventional fat‐suppressed spoiled three‐dimensional (3D) gradient‐echo (volumetric interpolated breath‐hold examination, VIBE) MRI in evaluating the conspicuity of benign and malignant breast lesions.