Time-resolved MR Imaging Research Articles

Live-view 4D GRASP MRI: A framework for robust real-time respiratory motion tracking with a sub-second imaging latency.

To propose a framework called live-view golden-angle radial sparse parallel (GRASP) MRI for low-latency and high-fidelity real-time volumetric MRI. Live-view GRASP MRI has two stages. The first one is called an off-view stage and the second one is called a live-view stage. In the off-view stage, 3D k-space data and 2D navigators are acquired alternatively using a new navi-stack-of-stars sampling scheme. A 4D motion database is then generated that contains time-resolved MR images at a sub-second temporal resolution, and each image is linked to a 2D navigator. In the live-view stage, only 2D navigators are acquired. At each time point, a live-view 2D navigator is matched to all the off-view 2D navigators. A 3D image that is linked to the best-matched off-view 2D navigator is then selected for this time point. This framework places the typical acquisition and reconstruction burden of MRI in the off-view stage, enabling low-latency real-time 3D imaging in the live-view stage. The accuracy of live-view GRASP MRI and the robustness of 2D navigators for characterizing respiratory variations and/or body movements were assessed. Live-view GRASP MRI can efficiently generate real-time volumetric images that match well with the ground-truth references, with an imaging latency below 500 ms. Compared to 1D navigators, 2D navigators enable more reliable characterization of respiratory variations and/or body movements that may occur throughout the two imaging stages. Live-view GRASP MRI represents a novel, accurate, and robust framework for real-time volumetric imaging, which can potentially be applied for motion adaptive radiotherapy on MRI-Linac.

Dynamic monitoring of vital functions and tissue re-organization in Saturnia pavonia (Lepidoptera, Saturniidae) during final metamorphosis by non-invasive MRI

Magnetic resonance imaging (MRI) is the key whole-body imaging technology for observing processes within a living object providing excellent resolution and contrast between soft tissues. In the present work, we exploited the non-destructive properties of MRI to track longitudinally the dynamic changes that take place in developing pupae of the Emperor Moth (Saturnia pavonia) during the last days before eclosion. While in diapause pupae, body fluid was almost homogeneously distributed over the internal compartments, as soon as wings, legs, flight muscles and the head region were fully developed, a significant redistribution of water levels occurred between thoracic and abdominal regions. During the last two days before eclosion, the developing moths transferred substantial amounts of liquid into the gut and the labial gland, and in case of females, into developing eggs. Concomitantly, the volume of the air sacs increased drastically and their expansion/compression became clearly visible in time-resolved MR images. Furthermore, besides ventilation of the tracheal system, air sacs are likely to serve as volume reservoir for liquid transfer during development of the moths inside their pupal case. In parallel, we were able to monitor noninvasively lipid consumption, cardiac activity and haemolymph circulation during final metamorphosis.

Emerging clinical applications for PET/MR in the field of cardiovascular imaging

In the same way in which PET/CT has been shown to be a powerful multimodality imaging tool, there are compelling reasons for combining PET and MRI. Combined PET-MRI systems, i.e. the simultaneous acquisition of PET and MR images, combine the most sensitive and robust noninvasive clinical technology for molecular imaging (PET) with high-contrast, high-resolution and time-resolved MR imaging. The fusion of these two modalities into one acquisition system has represented a clear breakthrough in the imaging field, and the implementation of these imaging systems for clinical applications still remains a challenge. However, a certain number of pitfalls still remain and must be avoided. In this review, we will first discuss the technological advancements, which take advantage of simultaneous PET and MR acquisitions, such as improved attenuation and motion correction, but also the remaining issues, which must be considered when performing this combined imaging. Second, we will present the first clinical applications of PET-MRI emerging in the field of cardiac imaging. Lastly, we will review promising future clinical applications for PET-MRI in the vascular field.

MRI strain imaging of the carotid artery: Present limitations and future challenges

Rupture of atherosclerotic plaques in the carotid artery is a main cause of stroke. Current diagnostics are not sufficient to identify all rupture-prone plaques, and studies have shown that biomechanical factors improve current plaque risk assessment. Strain imaging may be a valuable contribution to this risk assessment. MRI is a versatile imaging technique that offers various methods that are capable of measuring tissue strain. In this review, MR imaging techniques with displacement (DENSE), velocity (PC MRI), or strain (SENC) encoding protocols are discussed, together with post-processing techniques based on time-resolved MRI data. Although several MRI techniques are being developed to improve time-resolved MR imaging, current technical limitations related to spatial and temporal resolutions render MRI strain imaging currently unfit for carotid plaque strain evaluation.A novel approach using non-rigid image registration of MR images to determine strain in carotid arteries based on black blood cine MRI is proposed in this review. This and other post-processing techniques based on time-resolved MRI data may provide a good estimate of plaque strain, but are also dependent on the spatial and temporal resolution of the MR images. However, they seem to be the most promising approach for MRI based plaque strain analysis in the near future.

Hierarchical Manifold Learning for Regional Image Analysis

We present a novel method of hierarchical manifold learning which aims to automatically discover regional properties of image datasets. While traditional manifold learning methods have become widely used for dimensionality reduction in medical imaging, they suffer from only being able to consider whole images as single data points. We extend conventional techniques by additionally examining local variations, in order to produce spatially-varying manifold embeddings that characterize a given dataset. This involves constructing manifolds in a hierarchy of image patches of increasing granularity, while ensuring consistency between hierarchy levels. We demonstrate the utility of our method in two very different settings: 1) to learn the regional correlations in motion within a sequence of time-resolved MR images of the thoracic cavity; 2) to find discriminative regions of 3-D brain MR images associated with neurodegenerative disease.

Time-resolved gadolinium-enhanced MR imaging of the breast: Impact on accuracy of diagnosis of breast carcinoma.

33 Background: The purpose of the study is to differentiate benign and malignant breast lesions based on very early enhancement of the lesion on time-resolved gadolinium-enhanced MR imaging. Methods: 100 women with breast lesions were examined at 1.5 T (Siemens, Erlangen, Germany), by fast T1-weighted GRE images every 5 s for 1 min after injection of Gd-DOTA (Dotarem, Guerbet, France), followed by 5 spacially-resolved MR image series every 30 s. Images were subtracted; maximum-intensity-projection images were obtained; and images were randomized and reviewed by two blinded readers, who reviewed only the spacially-resolved MR images and gave BIRADS diagnosis. After one month, they evaluated only the time-resolved MR images and noted the time of first appearance of enhancement. Results: The patients had 249 enhancing lesions: 66 malignant on histopathology and 183 benign on basis of histopathology or 1-year imaging follow-up. On time-resolved MR imaging, 15 lesions enhanced at 0-5 s (all malignant), 23 at 5-10 s (21 malignant, 2 benign), 34 at 10-15 s (28 malignant, 6 benign), and 117 at 15-20 s (2 malignant, 115 benign). All malignant lesions enhanced before 20 s. Taking 15s as the cut-off point, early enhancement had sensitivity of 96.9%, specificity of 95.6%, positive predictive value of 88.8%, negative predictive value of 98.8%, and accuracy of 95.9% for diagnosis of carcinoma. Enhancement before 15 s corrected BIRADS diagnosis of the spacially-resolved MR imaging in 14 lesions (5.6%). Conclusions: Lesion enhancement in the first 15s on time-resolved MR imaging is a useful sign for the diagnosis of breast carcinoma.

Breast MRI at Highfield

Puropose: Breast MR imaging has an established role for staging primary cancer and for diagnosing recurrent tumor. Recently, it has also been suggested as screening tool in specific high risk cohorts. In particular in a screening setting, a high PPV is mandatory. One way to improve the technique's PPV is by improving morphologic analysis of enhancing lesions by higher matrix imgaing. MR systems operating at stronger magnetic fields (e.g. 3.0T) may offer the higher SNR that is necessary for high-matrix time-resolved MR imaging. On the other hand, high field systems may go along with specific difficulties that are due to stronger susceptibility effects, longer T1 relaxation times, and higher RF deposition. Aim of this study was first, to establish a clinically useful and stable pulse sequence for bilateral dynamic contrast enhanced breast MRI at 3.0T that would provide about the same diagnostic information as breast MRI at 1.5T. Second, we strived to establish pulse sequences that would provide a higher spatial resolution compared to what is currently attainable at 1.5T.

Time‐resolved 3D contrast‐enhanced MRA of an extended FOV using continuous table motion

A method is presented for acquiring 3D time-resolved MR images of an extended (>100 cm) longitudinal field of view (FOV), as used for peripheral MR angiographic runoff studies. Previous techniques for long-FOV peripheral MRA have generally provided a single image (i.e., with no time resolution). The technique presented here generates a time series of 3D images of the FOV that lies within the homogeneous volume of the magnet. This is achieved by differential sampling of 3D k-space during continuous motion of the patient table. Each point in the object is interrogated in five consecutive 3D image sets generated at 2.5-s intervals. The method was tested experimentally in eight human subjects, and the leading edge of the bolus was observed in real time and maintained within the imaging FOV. The data revealed differential bolus velocities along the vasculature of the legs.

5474067 Method for time-resolved MR-imaging

5474067 Method for time-resolved MR-imaging

Time-resolved MR imaging by automatic data segmentation.

A method for time-resolved imaging that provides a flexible trade-off between imaging time and temporal resolution is presented. It is based on a view order selection technique that automatically segments the acquired raw data into appropriate temporal frames. When used with cardiac monitoring and phase-contrast imaging, data similar to that obtained with a conventional gated phase-contrast sequence are acquired rapidly. For many applications, the temporal resolution can be reduced enough to permit imaging within a breath-hold interval, while still allowing accurate time-averaged flow quantitation. This is a general technique that can be implemented within a variety of pulse sequences and can resolve other motion cycles, including the respiratory cycle.