Revealing sub-voxel motions of brain tissue using phase-based amplified MRI (aMRI).

PURPOSE: Amplified magnetic resonance imaging (aMRI) was recently introduced as a new brain motion detection and visualization method. The original aMRI approach used a video-processing algorithm, Eulerian video magnification (EVM), to amplify cardio-ballistic motion in retrospectively cardiac-gated MRI data. Here, we strive to improve aMRI by incorporating a phase-based motion amplification algorithm. METHODS: Phase-based aMRI was developed and tested for correct implementation and ability to amplify sub-voxel motions using digital phantom simulations. The image quality of phase-based aMRI was compared with EVM-based aMRI in healthy volunteers at 3T, and its amplified motion characteristics were compared with phase-contrast MRI. Data were also acquired on a patient with Chiari I malformation, and qualitative displacement maps were produced using free form deformation (FFD) of the aMRI output. RESULTS: Phantom simulations showed that phase-based aMRI has a linear dependence of amplified displacement on true displacement. Amplification was independent of temporal frequency, varying phantom intensity, Rician noise, and partial volume effect. Phase-based aMRI supported larger amplification factors than EVM-based aMRI and was less sensitive to noise and artifacts. Abnormal biomechanics were seen on FFD maps of the Chiari I malformation patient. CONCLUSION: Phase-based aMRI might be used in the future for quantitative analysis of minute changes in brain motion and may reveal subtle physiological variations of the brain as a result of pathology using processing of the fundamental harmonic or by selectively varying temporal harmonics. Preliminary data shows the potential of phase-based aMRI to qualitatively assess abnormal biomechanics in Chiari I malformation.

Thrombus-mimicking artifacts in two-point Dixon MRI: Prevalence, appearance, and severity.

PURPOSE: To evaluate the incidence and severity of potentially thrombus mimicking, flow-induced misallocation artifacts in a clinical setting. Two-point "Dixon" fat-water separation methods, with bipolar readout gradients, may suffer from flow-induced fat-water misallocation artifacts. If these artifacts occur within blood vessels, they may mimic thrombus. MATERIALS AND METHODS: Two-point Dixon coronal and axial images acquired in 102 consecutive patients were retrospectively evaluated for the presence of flow-induced artifacts in arteries and veins. Artifacts were graded on a 3-point scale (none, mild, severe) by two independent readers. Interreader agreement was evaluated with kappa statistics. RESULTS: Reader 1 reported 63 artifacts in 46 (45%) of the cases (severe in 19 cases, 18.6%). Reader 2 reported 51 artifacts in 43 (42.2%) of the cases (severe in 18 cases, 17.6%). Misallocation of fat and water was apparent in all datasets with severe artifacts, whereas variable signal intensity changes in water and fat images were observed in mild artifacts. Interreader agreement was good for artifacts appearing in coronal images (κ = 0.7) and fair for artifact appearance in axial images (κ = 0.24). CONCLUSION: Our study shows a high incidence of flow-induced mild and severe artifacts in a two-point Dixon method with bipolar readout gradients. This artifact should not be misinterpreted as intravascular thrombus. LEVEL OF EVIDENCE: 3 J. Magn. Reson. Imaging 2017;45:229-236.

Amplified magnetic resonance imaging (aMRI).

PURPOSE: This work describes a new method called amplified MRI (aMRI), which uses Eulerian video magnification to amplify the subtle spatial variations in cardiac-gated brain MRI scans and enables better visualization of brain motion. METHODS: The aMRI method takes retrospective cardiac-gated cine MRI data as input, applies a spatial decomposition, followed by temporal filtering and frequency-selective amplification of the MRI cardiac-gated frames before synthesizing a motion-amplified cine data set. RESULTS: This approach reveals deformations of the brain parenchyma and displacements of arteries due to cardiac pulsatility, especially in the brainstem, cerebellum, and spinal cord. CONCLUSION: aMRI has the potential for widespread neuro- and non-neuro clinical use because it can amplify and characterize small, often barely perceptible motion and can visualize the biomechanical response of tissues using the heartbeat as an endogenous mechanical driver. Magn Reson Med 75:2245-2254, 2016. © 2016 Wiley Periodicals, Inc.

Combined gadoxetic acid and gadofosveset enhanced liver MRI: A feasibility and parameter optimization study.

PURPOSE: Demonstration of feasibility and protocol optimization for the combined use of gadofosveset trisodium with gadoxetic acid for delayed T1-weighted liver MRI. METHODS: Eleven healthy volunteers underwent hepatobiliary phase imaging at 3 Tesla (T) using gadoxetic acid. Multiple breathheld T1-weighted three-dimensional spoiled gradient echo sequences were performed at varying flip angles before and after injection of gadofosveset. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured to determine optimal T1-weighting. Examples of three patients with focal liver lesions were acquired. RESULTS: The addition of gadofosveset to the hepatobiliary phase of gadoxetic acid renders vessels isointense to liver tissue at low flip angles due to increased vessel SNR (P < 0.001). The lowest CNR of liver relative to portal vein (CNR = 15; 95% confidence interval [CI]: -14-44) was observed at a 10º flip angle. The highest CNR of liver relative to muscle (CNR = 214; 95% CI: 191-237) was observed at a 20º flip angle. The combined enhancement leads to homogenously enhanced liver tissue and liver vasculature. Cysts were detected in three volunteers and metastases were detected in two patients. In these anecdotal cases the cysts and metastases stood out as conspicuous focal hypointensities on combined gadoxetic acid and gadofosveset enhanced images. CONCLUSION: Combined gadoxetic acid and gadofosveset enhanced liver MRI is feasible, with low flip angles minimizing contrast between vessels and liver. Further clinical studies are needed to confirm that low flip angles provide an optimal combination of sensitivity and specificity for lesion detection in patients.

Flow-induced signal misallocation artifacts in two-point fat-water chemical shift MRI.

PURPOSE: Two-point fat-water separation methods are increasingly being used for chest and abdominal MRI and have recently been introduced for use in MR angiography of the lower extremities. With these methods, flowing spins can accumulate unintended phase shifts between the echo times. The purpose of this study is to demonstrate that these phase shifts can lead to inaccurate signals in the water and fat images. THEORY AND METHODS: In vitro experiments were conducted at 1.5T and 3.0T using a stenosis-mimicking phantom and a computer-controlled pump to image a range of physiologically relevant velocities. RESULTS: In the phantom images acquired using bipolar readout gradients, fat-water signal inaccuracies were visible in regions of flow, with increasing severity as the flow rate was increased. Additionally, similar effects were observed in regions of high flow in clinical chest and liver exams. In the phantom images, the effect was eliminated by using a dual-pass method without bipolar readout gradients. CONCLUSION: When using fat-water separation methods with bipolar readout gradients, phase shifts caused by the motion of spins can lead to signal inaccuracies in the fat and water images. These artifacts can be mitigated by using approaches that do not use bipolar readout gradients.