The noninvasive assessment of lung function using imaging is increasingly of interest for the study of lung diseases, including chronic obstructive pulmonary disease (COPD) and asthma. Hyperpolarized gas MRI (HP MRI) has demonstrated the ability to detect changes in ventilation, perfusion, and lung microstructure that appear to be associated with both normal lung development and disease progression. The physical characteristics of HP gases and their application to MRI are presented with an emphasis on current applications. Clinical investigations using HP MRI to study asthma, COPD, cystic fibrosis, pediatric chronic lung disease, and lung transplant are reviewed. Recent advances in polarization, pulse sequence development for imaging with Xe-129, and prototype low magnetic field systems dedicated to lung imaging are highlighted as areas of future development for this rapidly evolving technology.
Highly-constrained back-projection (HYPR) is a technique for the reconstruction of sparse, highly-undersampled time-resolved image data. A novel iterative HYPR (I-HYPR) algorithm is presented and validated in computer simulations. The reconstruction method is then applied to cerebral perfusion MRI simulated as a radial acquisition and contrast-enhanced angiography of the head to assess feasibility in accelerating acquisitions requiring high temporal resolution and accurate representation of contrast kinetics. The I-HYPR algorithm is shown to be more robust than standard HYPR in these applications in which the sparsity condition is not met or in which quantitative information is required. Specifically, iterative reconstruction of undersampled perfusion and contrast-enhanced angiography data improved accuracy of the representation of contrast kinetics and increased the temporal separation of arterial and venous contrast kinetics. In time-resolved imaging, the classic tradeoff between temporal and spatial resolution motivates the investigation of faster acquisition methods. One such method that has recently gained popularity is undersampled projection imaging. Angular undersampling decreases acquisition time in projection imaging while maintaining image resolution by varying the sample density in k-space (1). However, in standard projection reconstruction methods, large undersampling factors lead to streak artifacts that may render the reconstructed images unsuitable for quantitative analysis.One recently-developed reconstruction method for undersampled time-resolved images is the highly-constrained back-projection (HYPR) reconstruction (2). In the HYPR method, a priori information from an image, known as the composite, is used to constrain the reconstruction to locations with signal. The composite is typically reconstructed from sufficiently-sampled time-averaged data using conventional filtered back-projection (FBP) or regridding. In conventional noniterative HYPR, unfiltered backprojection of highly-undersampled projection data is used to apply the temporal signal changes to the composite image. In the absence of motion and for sufficiently sparse data sets, this approach has been shown to mitigate the problem of streak artifacts while depicting temporal dynamics with good fidelity. For example, HYPR has been shown to perform well in time-resolved angiography in which the data generally meet these conditions (2). However, it has been shown that HYPR reconstruction results in inaccurate representation of signal changes in more spatially-and temporally-complex data (3). These errors would be particularly problematic in perfusion and diffusion data in which highly-accurate reconstructions of each timepoint are essential for accurate measurement of quantitative functional maps. In addition, angiographic applications that demand high temporal resolution in circumstances in which blood vessels nearly overlap, such as arteriovenous malformations (AVM), may also be challenging.To address the sparsity limitation of the s...
Diffusion‐weighted imaging, a contrast unique to MRI, is used for assessment of tissue microstructure in vivo. However, this exquisite sensitivity to finer scales far above imaging resolution comes at the cost of vulnerability to errors caused by sources of motion other than diffusion motion. Addressing the issue of motion has traditionally limited diffusion‐weighted imaging to a few acquisition techniques and, as a consequence, to poorer spatial resolution than other MRI applications. Advances in MRI imaging methodology have allowed diffusion‐weighted MRI to push to ever higher spatial resolution. In this review we focus on the pulse sequences and associated techniques under development that have pushed the limits of image quality and spatial resolution in diffusion‐weighted MRI.
In diffusion-weighted imaging, multi-shot acquisitions are problematic due to inter-shot inconsistencies of the phase caused by motion during the diffusion-encoding gradients. A model for the motion-induced phase errors in DW-MRI of the brain is presented in which rigid-body and non-rigid-body motion are separated. In the model it is assumed that non-rigid-body motion is due to cardiac pulsation, and that the motion patterns are repeatable from beat to beat. To test the validity of this assumption, the repeatability of non-rigid-body motion-induced phase errors is quantified in three healthy volunteers. Non-rigid-body motion-induced phase was found to significantly correlate (p < 0.05) with pulse-oximeter waveforms in approximately 83 percent of the pixels tested across all slices and subjects.
A method is presented for high-resolution 3D imaging of the whole lung using inhaled hyperpolarized (HP) He-3 MR with multiple half-echo radial trajectories that can accelerate imaging through undersampling. A multiple half-echo radial trajectory can be used to reduce the level of artifact for undersampled 3D projection reconstruction (PR) imaging by increasing the amount of data acquired per unit time for HP He-3 lung imaging. The point spread functions (PSFs) for breath-held He-3 MRI using multiple half-echo trajectories were evaluated using simulations to predict the effects of T 2 * and gas diffusion on image quality. Results from PSF simulations were consistent with imaging results in volunteer studies showing improved image quality with increasing number of echoes using up to 8 half-echoes. The 8-half-echo acquisition is shown to accommodate lost breath-holds as short as 6 sec using a retrospective reconstruction at reduced resolution and also to allow reduced breath-hold time compared with an equivalent Cartesian trajectory. Obstructive lung disease is highly heterogeneous (1), requiring 3D coverage at sufficient resolution to detect loci of disease. Hyperpolarized gas (HP) He-3 MRI with 3D isotropic resolution will likely improve depiction and analysis of lung structures and ventilation defects by reducing partial volume effects and providing true cubic voxel size for segmentation and measurement. However, challenges remain, as the polarization is non-recoverable, and sampling time is limited by the breath-hold duration.HP He-3 imaging has been performed using non-Cartesian 2D trajectories such as spiral (2) and undersampled projection acquisition to allow depiction of HP He-3 in the lungs using short breath-holds (3-6). These techniques provide a method of detecting regional ventilation differences without the use of the ionizing radiation associated with CT (7). Breath-hold spin-density imaging of the lungs in 3D has been demonstrated using Fourier techniques (8), a stack of spirals (9), and cyclindrical techniques (10). The use of functional HP He-3 data has shown exciting prospects in the setting of radiation treatment planning for lung cancer, for which 3D isotropic data will aid in registering anatomic with functional images of the lungs (11). A 3D stack of stars acquisition comprised of undersampled 2D projection acquisition in-plane and Fourier encoding in the third dimension has also been demonstrated to provide improved 3D visualization for short breath-holds as well as detection of gas trapping (6). However, each of these methods has tradeoffs with respect to spatial and temporal resolution as well as sensitivity to motion for depiction of dynamic processes, complex lung structure, and accommodating loss of breath-hold during the acquisition.A fully 3D projection acquisition trajectory and projection reconstruction (PR) has advantages for HP He-3 lung imaging as it allows the flexibility not only to perform spin-density imaging at isotropic resolution, but also to acquire functional informatio...
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