Metallic implants, such as pedicle screws, are commonly used in orthopedic surgery to fixate fractures, replace arthritic joints, and to align and immobilize vertebra. In the United States alone, there were 325,000 spinal fusions performed in 2003 and 450,000 primary or revision total knee arthroplasties performed in 2002 (1). The current standard imaging test for complications associated with metallic implants is plain radiography. For accurate diagnosis, radiography requires the X-ray beam to be oriented exactly parallel to the bone-implant interface: any obliquity of the X-ray beam can obscure the radiolucent area (2). A better substitute for two-dimensional radiograph is crosssectional imaging that images the entire bone-implant interface in three dimensions. However, the physical characteristics of metallic implants cause difficulties with crosssectional imaging techniques. Computed tomography (CT) and invasive CT myelography suffer from metal-induced streak/beam-hardening artifacts and data loss throughout the field-of-view (FOV) (3). In addition, radiography and CT are relatively insensitive to soft tissue abnormalities and bone marrow edema caused by infection, which are important complications to be evaluated in patients with implants.Magnetic resonance imaging (MRI) is potentially the best imaging modality for diagnosing patients with metallic implants because of its superior soft tissue contrast (4). However, MRI near metallic implants is hampered by severe artifacts, which stem from large metal-induced field inhomogeneities (5), local gradient-induced eddy currents on metal surfaces (6), and radiofrequency (RF) shielding effects (7). Among these, metal-induced field inhomogeneities are responsible for the most severe artifacts. It is well understood that steep field gradients near metal objects result in increased intra-voxel dephasing and a severely shortened T * 2 . As a result, MRI near metallic implants inevitably involves spin-echo (SE) sequences, which refocus the dephased spins. However, SE techniques still suffer from spatially dependent artifacts (e.g., signal voids and pile-ups), owing to a nonlinear frequencyposition mapping caused by metal-induced field inhomogeneities. In the absence of field inhomogeneities, the mapping between a spin's precession frequency and its spatial location is a linear function (dotted line in Fig. 1), and the slope of the linear mapping is determined by the gradient amplitude. When metal-induced field inhomogeneities (dashed line of a bell shape in Fig. 1) superimpose upon the frequency induced by the gradient, the resulting frequency-position mapping becomes highly nonlinear (solid line in Fig. 1), which causes problems in sliceselective excitation and frequency encoding during the readout.As illustrated in Fig. 1, an RF pulse with 1 kHz bandwidth centered at 11.5 kHz is designed to excite a 3-mmthick slice centered at location 3.3 cm. However, because of the nonlinear frequency-position mapping, the RF pulse excites a much thinner slice centered at location ...
Purpose: To enable accurate MR parameter mapping with accelerated data acquisition, utilizing recent advances in constrained imaging with sparse sampling. Theory and Methods: A new constrained reconstruction method based on low-rank and sparsity constraints is proposed to accelerate MR parameter mapping. More specifically, the proposed method simultaneously imposes low-rank and joint sparse structures on contrast-weighted image sequences within a unified mathematical formulation. With a pre-estimated subspace, this formulation results in a convex optimization problem, which is solved using an efficient numerical algorithm based on the alternating direction method of multipliers. Results: To evaluate the performance of the proposed method, two application examples were considered: i) T2 mapping of the human brain, and ii) T1 mapping of the rat brain. For each application, the proposed method was evaluated at both moderate and high acceleration levels. Additionally, the proposed method was compared with two state-of-the-art methods that only use a single low-rank or joint sparsity constraint. The results demonstrate that the proposed method can achieve accurate parameter estimation with both moderately and highly undersampled data. Although all methods performed fairly well with moderately undersampled data, the proposed method achieved much better performance (e.g., more accurate parameter values) than the other two methods with highly undersampled data. Conclusions: Simultaneously imposing low-rank and sparsity constraints can effectively improve the accuracy of fast MR parameter mapping with sparse sampling.
Purpose To address phase and amplitude errors for multi-point water–fat separation with “bipolar” acquisitions, which efficiently collect all echoes with alternating read-out gradient polarities in one repetition. Materials and Methods With the bipolar acquisitions, eddy currents and other system nonidealities can induce inconsistent phase errors between echoes, disrupting water–fat separation. Previous studies have addressed phase correction in the read-out direction. However, the bipolar acquisitions may be subject to spatially high order phase errors as well as an amplitude modulation in the read-out direction. A method to correct for the 2D phase and amplitude errors is introduced. Low resolution reference data with reversed gradient polarities are collected. From the pair of low-resolution data collected with opposite gradient polarities, the two-dimensional phase errors are estimated and corrected. The pair of data are then combined for water–fat separation. Results We demonstrate that the proposed method can effectively remove the high order errors with phantom and in vivo experiments, including obliquely oriented scans. Conclusion For bipolar multi-echo acquisitions, uniform water–fat separation can be achieved by removing high order phase errors with the proposed method.
Multiecho sequences provide an efficient means to acquire multiple echoes in a single repetition, which has found applications in spectroscopy, relaxometry, and water-fat separation. By replacing the fly-back gradients in unipolar multiecho sequences with alternating readout gradients, bipolar multiecho sequences greatly reduce both echo-spacing and repetition interval. This offers many attractive advantages, such as shorter scan times, higher SNR efficiency, more robust field map estimation, reduced motion-induced artifacts, and less sensitivity to short T * 2 . However, the alternating readout gradients cause several technical problems, including delay effects and image misregistrations, which prevent direct application of existing water-fat separation methods. This work presents solutions to address these problems, including a post-processing method that shifts k -space data to correct k -space echo misalignment, an image warping method that utilizes a low-resolution field map to remove field-inhomogeneity-induced misregistration, and a k -space water-fat separation method that eliminates chemicalshift-induced artifacts in separated water and fat images. In addition, a noise amplification factor, which characterizes the noise present in separated images, is proposed to serve as a useful guideline for choosing imaging parameters or regularization parameters in the case of ill-conditioned separation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.