Continuously moving table SENSE imaging

Metamaterial-based transmit and receive system for whole-body magnetic resonance imaging at ultra-high magnetic fields

Magnetic resonance imaging (MRI) at ultra-high fields (UHF), such as 7 T, provides an enhanced signal-to-noise ratio and has led to unprecedented high-resolution anatomic images and brain activation maps. Although a variety of radio frequency (RF) coil architectures have been developed for imaging at UHF conditions, they usually are specialized for small volumes of interests (VoI). So far, whole-body coil resonators are not available for commercial UHF human whole-body MRI systems. The goal of the present study was the development and validation of a transmit and receive system for large VoIs that operates at a 7 T human whole-body MRI system. A Metamaterial Ring Antenna System (MRAS) consisting of several ring antennas was developed, since it allows for the imaging of extended VoIs. Furthermore, the MRAS not only requires lower intensities of the irradiated RF energy, but also provides a more confined and focused injection of excitation energy on selected body parts. The MRAS consisted of several antennas with 50 cm inner diameter, 10 cm width and 0.5 cm depth. The position of the rings was freely adjustable. Conformal resonant right-/left-handed metamaterial was used for each ring antenna with two quadrature feeding ports for RF power. The system was successfully implemented and demonstrated with both a silicone oil and a water-NaCl-isopropanol phantom as well as in vivo by acquiring whole-body images of a crab-eating macaque. The potential for future neuroimaging applications was demonstrated by the acquired high-resolution anatomic images of the macaque's head. Phantom and in vivo measurements of crab-eating macaques provided high-resolution images with large VoIs up to 40 cm in xy-direction and 45 cm in z-direction. The results of this work demonstrate the feasibility of the MRAS system for UHF MRI as proof of principle. The MRAS shows a substantial potential for MR imaging of larger volumes at 7 T UHF. This new technique may provide new diagnostic potential in spatially extended pathologies such as searching for spread-out tumor metastases or monitoring systemic inflammatory processes.

Continuously moving table MRI with golden angle radial sampling

PURPOSE

Continuously moving table (CMT) MRI is a high throughput technique that has multiple applications in whole-body imaging. In this work, CMT MRI based on golden angle (GA, 111.246° azimuthal step) radial sampling is developed at 3 Tesla (T), with the goal of increased flexibility in image reconstruction using arbitrary profile groupings.

THEORY AND METHODS

CMT MRI with GA and linear angle (LA) schemes were developed for whole-body imaging at 3T with a table speed of 20 mm/s. Imaging was performed in phantoms and a human volunteer with extended z fields of view of up to 1.8 meters. Four separate LA and a single GA scan were performed to enable slice reconstructions at four different thicknesses.

RESULTS

GA CMT MRI produced high image quality in phantoms and humans and allowed complete flexibility in reconstruction of slices with arbitrary slice thickness and position from a single data set. LA CMT MRI was constrained by predetermined parameters, required multiple scans and suffered from stair step artifacts that were not present in GA images.

CONCLUSION

GA sampling provides a robust flexible approach to CMT whole-body MRI with the ability to reconstruct slices at arbitrary positions and thicknesses from a single scan.

Temporally constrained respiratory gating improves continuously moving table MRI during free breathing

PURPOSE

To evaluate a novel breathing motion correction algorithm for continuously moving table magnetic resonance imaging (CMT-MRI) that optimizes motion consistency in a fixed time span.

MATERIALS AND METHODS

In 22 patients CMT-MRI was performed during free breathing. During a preparatory phase (constant) or continuously during the scan (adaptive) gating thresholds were computed from breathing states that should allow for motion consistent k-space sampling. After data from a first k-space traversal was acquired irrespective of breathing motion, subsequently k-space lines with discordant breathing states were reacquired below the gating threshold. Time constraints of CMT-MRI were respected, because a fixed time was allocated for reacquisition. Image quality and lesion depiction were evaluated on images reconstructed from the first traversal and motion-corrected images.

RESULTS

Compared to constant thresholds, gating with adaptive thresholds led to a higher number of reacquired k-space lines (60.1%/41.7%) and a larger fraction of motion consistent final k-space lines (96.6%/78.8%). Adaptive gating induced a significant increase in image quality for all regions affected by breathing motion. Only one of 22 lesions was not depicted on the adaptively corrected images, whereas 15 were readily appreciable.

CONCLUSION

Temporally constrained respiratory gating with adaptive thresholds allows for fully sampled, motion-corrected CMT-MRI acquisitions during free breathing.

Perspectives for preventive screening with total body MRI

Radiology has started to increasingly recognise its potential for screening with the advent of "whole-body" imaging techniques. This article briefly reviews prerequisites for successful screening, presents a quick summary of single-organ screening with magnetic resonance (MR) imaging, and introduces how this knowledge can be integrated into whole-body MR (wb-MR) screening. MR colonography has demonstrated its potential for screening. Wb-MR in the form of wb-MR angiography has already entered both clinical and screening settings; also, the search for metastases with wb-MR has been evaluated and has performed well when compared with other imaging modalities. But screening a group of healthy subjects requires more than feasibility and high accuracy of the screening test; thus, technical and ethical considerations are also presented. Wb-MR is only at its beginning and will in the near future certainly inspire many new research activities as well as transform the radiological market.

Artifact reduction in moving-table acquisitions using parallel imaging and multiple averages

Two-dimensional (2D) axial continuously-moving-table imaging has to deal with artifacts due to gradient nonlinearity and breathing motion, and has to provide the highest scan efficiency. Parallel imaging techniques (e.g., generalized autocalibrating partially parallel acquisition GRAPPA)) are used to reduce such artifacts and avoid ghosting artifacts. The latter occur in T(2)-weighted multi-spin-echo (SE) acquisitions that omit an additional excitation prior to imaging scans for presaturation purposes. Multiple images are reconstructed from subdivisions of a fully sampled k-space data set, each of which is acquired in a single SE train. These images are then averaged. GRAPPA coil weights are estimated without additional measurements. Compared to conventional image reconstruction, inconsistencies between different subsets of k-space induce less artifacts when each k-space part is reconstructed separately and the multiple images are averaged afterwards. These inconsistencies may lead to inaccurate GRAPPA coil weights using the proposed intrinsic GRAPPA calibration. It is shown that aliasing artifacts in single images are canceled out after averaging. Phantom and in vivo studies demonstrate the benefit of the proposed reconstruction scheme for free-breathing axial continuously-moving-table imaging using fast multi-SE sequences.

High-resolution continuously acquired peripheral MR angiography featuring partial parallel imaging GRAPPA

Continuously-moving-table MRI, in contrast to traditional multistation techniques, potentially can improve the scan time efficiency of whole-body applications and provide seamless images of an extended field of view (FOV). Contrast-enhanced MR angiography (CE-MRA) in particular requires high spatial resolution and at the same time has rigid scan time constraints due to the limited arterial contrast window. In this study a reconstruction method for continuously acquired 3D data sets during table movement was combined with a self-calibrated partial parallel imaging algorithm (generalized autocalibrating partially parallel acquisitions (GRAPPA)). The method was applied to peripheral CE-MRA and compared with a standard continuously-moving-table MRA protocol. The gain in scan time was used to increase the data acquisition matrix and decrease the slice thickness. The method was evaluated in five healthy volunteers and applied to one patient with peripheral arterial occlusive disease (PAOD). The protocols were intraindividually compared with respect to the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in selected vessel segments, as well as overall vessel depiction. The combination of the continuously-moving-table technique with parallel imaging enabled the acquisition of seamless peripheral 3D MRA with increased resolution and an overall crisper appearance.

Interactive continuously moving table (iCMT) large field-of-view real-time MRI

Continuously moving table (CMT) MRI is a new method that is capable of generating 3D, seamless, large field-of-view (FOV) images by acquiring readouts along the patient superior-inferior axis as the subject is translated through the scanner. For applications that require artifact-free images, such as arterial-phase contrast-enhanced (CE) angiography of the legs, a major challenge is to match the MR data acquisition and patient table motion with the dynamics of blood flow in the region of interest (ROI). Instead of restricting the CMT to predetermined constant table speeds, we adopted a more general approach in which the table motion is decoupled from the phase-encoding order. In our approach the table moves adaptively and in response to operator-provided feedback obtained from viewing real-time preview (or fluoroscopic) images. This interactivity is accomplished by integrating high temporal-spatial resolution encoding of the table position with real-time hybrid-space filling and image reconstruction. Experimental results obtained using our prototype interactive CMT (iCMT) system on a peripheral vascular phantom and five healthy volunteers demonstrate the feasibility of this robust and rapid imaging method for acquiring 3D large-FOV continuous images with patient-specific adaptive table motion profiles.

Numerical equilibration of signal intensity and spatial resolution in time-resolved continuously moving table imaging

Time-resolved continuously moving table imaging techniques have been previously developed to observe a dynamically changing phenomenon over an extended field-of-view. The acquisition involves differential k-space sampling and view sharing. Since the table is continuously moving during data acquisition, the k-space for any longitudinal position is sampled only sparsely for the first reconstruction timeframe and is progressively more fully sampled for subsequent frames. Consequently, the signal intensity increases and the lateral spatial resolution improves from frame to frame even for static materials, which can mask true dynamically changing phenomena. This work provides a description of this effect and a means for signal correction in the early reconstruction frames, thus permitting any residual variation in signal intensity to be primarily attributed to true dynamic processes. The method is tested experimentally on a static phantom and in a peripheral vascular study designed to observe the leading edge of the contrast bolus.

Autocalibrated coil sensitivity estimation for parallel imaging

Parallel imaging has proven to be a robust solution to the problem of acquisition speed in MRI. These methods are based on extracting spatial information from an array of multiple surface coils in order to speed up image acquisition. One of the most essential elements of any parallel imaging method is the information describing the coil sensitivity distribution throughout the sample. This paper covers some of the advanced methods to obtain coil sensitivity-related information, focusing particularly on the class of methods referred to as autocalibrating. These methods all acquire the data for coil sensitivity estimation directly before, during or directly after the reduced data acquisition. After a review of standard methods for coil sensitivity estimation, some of the basic and advanced autocalibrating methods are reviewed, and some example applications shown.

Whole-body MRI and PET-CT in the management of cancer patients

Mortality rate, prognosis, and treatment outcome of cancer patients depend strongly on the detection of malignancy at an early stage and efficient monitoring of the disease. Multimodality diagnostic approaches are now widely applied for tumor detection, staging, and follow-up. However, the introduction of whole-body imaging modalities into clinical practice has substantially expanded diagnostic options. PET-CT has increased diagnostic accuracy by providing "anatometabolic" information by fusing tumor glucose-uptake measures from the PET examination and accurate delineation of anatomical structures given by spiral CT. Since PET-CT is associated with high doses of ionizing radiation, it is used in mainly tumor staging and screening within the scope of tertiary prevention. Here promising results have been reported for various tumor entities. MRI provides excellent tissue contrast, detailed morphological information and lack of ionizing radiation. MRI has been employed for the assessment of focal pathologies in specific anatomical regions. Whole-body MRI scanners using multiple receiver channels with parallel acquisition techniques now allow tumor screening from head to toe within substantially shorter examination times and without compromises in image resolution. We report our experience with these two novel techniques and discuss their benefits and drawbacks in terms of systemic tumor screening.

Continuously moving table MRI with SENSE: application in peripheral contrast enhanced MR angiography

An integration of SENSitivity Encoding (SENSE) with continuously moving table (CMT) MRI for extended field-of-view (FOV) acquisitions is described. In this work, the approach in which receiver coils are attached to the object and move in synchrony with the scanner table is considered. Technical issues dealing with the implementation of SENSE-CMT are addressed, including coil calibration, correction for non-uniform magnetic gradients, and specific reconstruction steps. An explanation of combining SENSE with gradient non-linearity correction is given, as the latter becomes necessary in CMT acquisitions where a large sampling FOV is used. It is hypothesized that SENSE can provide at least a 2-fold improvement in lateral spatial resolution compared to non-accelerated CMT acquisitions. The hypothesis is tested in phantoms, where the effectiveness of both SENSE and gradient non-linearity correction to improve spatial resolution is shown. The SENSE-CMT technique is further demonstrated in vivo with contrast-enhanced MR angiography of the peripheral vasculature.

Continuously moving table 3D MRI with lateral frequency-encoding direction

A method is presented for 3D MRI in an extended field of view (FOV) based on continuous motion of the patient table and an efficient acquisition scheme. A gradient-echo MR pulse sequence is applied with lateral (left-right (L/R)) frequency-encoding direction and slab selection along the direction of motion. Compensation for the table motion is achieved by a combination of slab tracking and data alignment in hybrid space. The method allows fast k-space coverage to be achieved, especially when a short sampling FOV is chosen along the direction of table motion, as is desirable for good image quality. The method can be incorporated into different acquisitions schemes, including segmented k-space scanning, which allows for contrast variation with the use of magnetization preparation. Head-to-toe images of volunteers were obtained with good quality using 3D spoiled gradient-echo sequences. As an example of magnetization-prepared imaging, fat/water separated images were acquired using chemical shift selective (CHESS) presaturation pulses.

Variable field of view for spatial resolution improvement in continuously moving table magnetic resonance imaging

An approach is described in which the field of view (FOV) along the Y (right/left) phase encoding direction can be dynamically altered during a continuously moving table (CMT) coronal acquisition for extended FOV MRI. We hypothesize that with this method, regions of the anatomy exhibiting significantly different lateral widths can be imaged with a matching local FOV(Y), thereby improving local lateral spatial resolution. k-space raw data from the variable-FOV CMT acquisition do not allow simple Fourier reconstruction due to the presence of a mixture of phase encodes sampled at different Deltak(Y) intervals. In this work, we employ spline interpolation to reregister the mixed data set onto a uniformly sampled k-space grid. Using this interpolation scheme, we present phantom and peripheral contrast-enhanced MR angiography results demonstrating an approximate 45% improvement in local lateral spatial resolution for continuously moving table acquisitions.

Recovery of phase inconsistencies in continuously moving table extended field of view magnetic resonance imaging acquisitions

MR images formed using extended FOV continuously moving table data acquisition can have signal falloff and loss of lateral spatial resolution at localized, periodic positions along the direction of table motion. In this work we identify the origin of these artifacts and provide a means for correction. The artifacts are due to a mismatch of the phase of signals acquired from contiguous sampling fields of view and are most pronounced when the central k-space views are being sampled. Correction can be performed using the phase information from a periodically sampled central view to adjust the phase of all other views of that view cycle, making the net phase uniform across each axial plane. Results from experimental phantom and contrast-enhanced peripheral MRA studies show that the correction technique substantially eliminates the artifact for a variety of phase encode orders.

Continuously moving table SENSE imaging

A combination of continuously moving table imaging and parallel imaging based on sensitivity encoding (SENSE) is presented. One specific geometry is considered, where the receiver array is fixed to the MR magnet and does not move with the table, which allows for head-to-toe imaging with a small total number of coils. Sensitivity maps are defined for the enlarged virtual field of view and are composed according to the k-space sampling scheme such that established parallel reconstruction techniques are applicable to good approximation. In vivo experiments show the feasibility of this approach, and simulations determine the application range. Three-dimensional head-to-toe imaging of volunteers is performed in 77 s with a SENSE reduction factor of 2 in a virtual field of view of 1800 x 460 x 100 mm(3).