2D axial moving table acquisitions with dynamic slice adaptation

A method for axial multi‐slice imaging during continuous table motion has been developed and implemented on a clinical scanner. Multiple axial slice packages are acquired consecutively and combined to cover an extended longitudinal FOV. To account for the table motion during the acquisition, the RF pulse frequencies are continuously updated according to the actual table velocity and slice position. Different strategies for the spatial‐temporal acquisition sequence with extended FOV are proposed. They cover different regimes of scan requirements regarding table velocity, used scan range, and slice resolution. The method is easy to implement and compatible with most kinds of sequences. The robustness of the proposed approach has been tested in phantom studies and healthy volunteers using T1‐, T2‐, and STIR‐weighted multi‐slice techniques that are based on gradient and turbo spin echo sequences and compared to a stationary approach usually used in clinical routine. The method provides artifact free gradient echo based images during continuous table motion, while for turbo spin echo sequences limitations in choosing table translations occur due to gradient non‐linearity effects. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.     

Continuously moving table data acquisition method for long FOV contrast-enhanced MRA and whole-body MRI.

A method is presented in which an extended longitudinal field of view (FOV), as required for whole-body MRI or MRA peripheral runoff studies, is acquired in one seamless image. Previous methods typically either acquired 3D data at multiple static "stations" which covered the extended FOV or as a series of 2D axial sections. The method presented here maintains the benefits of 3D acquisition while removing the discrete nature of the multistation method by continuous acquisition of MR data as the patient table moves through the desired FOV. Although the technique acquires data only from a homogeneous central volume of the magnet at any point in time, by spatially registering all data it is possible to extend the FOV well beyond this volume. The method is demonstrated experimentally with phantoms, in vivo angiographic animal studies, and in vivo human studies.     

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.     

Floating table isotropic projection (FLIPR) acquisition: a time-resolved 3D method for extended field-of-view MRI during continuous table motion.

In this work, 3D vastly undersampled isotropic projection (VIPR) acquisition is used simultaneously with continuous table motion to extend the superior/inferior (S/I) FOV for MR angiograms. The new technique is termed floating table isotropic PR (FLIPR). The use of 3D PR in conjunction with table motion obviates the need to locate and prescribe imaging volumes containing the major blood vessels over the large superior-inferior (S/I) ranges encountered in whole-body imaging. In addition, the FLIPR technique provides extended anterior-posterior (A/P) abdominal coverage, isotropic spatial resolution, and temporal resolution. In volunteer studies, FLIPR MR angiograms with 1.6-mm isotropic spatial resolution that approached whole body in extent were acquired in less than 2 min.     

Whole‐body diffusion‐weighted imaging with a continuously moving table acquisition method: Preliminary results

In this study, a method for whole‐body diffusion‐weighted imaging (wbDWI) during continuous table motion has been developed and implemented on a clinical scanner based on a short‐Tau inversion recovery echo‐planar DWI sequence. Unlike currently available multistation wbDWI, which has disadvantages such as long scanning times, poor image quality, and troublesome data realignment, continuously moving table wbDWI can overcome these technical problems while extending the longitudinal field of view in MRI systems. In continuously moving table wbDWI, images are acquired consecutively at the isocenter of the magnet, having less geometric distortions and various possibilities of spatial and temporal coverage of an extended field of view. The acquired images, together with an apparent diffusion coefficient analysis, show that continuously moving table wbDWI can be used by appropriately adapting the table velocity, scan range, radiofrequency coils, slice resolutions, and spatio‐temporal acquisition schemes according to various clinical demands. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Extended field-of-view imaging with table translation and frequency sweeping.

A new method for MRI of an extended field of view (FOV) has been developed and validated. The method employs concurrent MR data acquisition and patient table motion. Table motion-induced image artifacts are minimized by sweeping the frequency of the receiver at a rate matching the table's speed. Multiple regional images are collected and combined to reconstruct the full FOV. The imaging parameters and table speed are chosen to ensure that each regional image of the subject is collected while the corresponding anatomy is in the useable imaging volume of the scanner. Additional strategies are applied to further reduce field inhomogeneity-induced artifacts, especially distortions due to gradient field nonlinearity. The method is robust and can be easily incorporated into most multislice 2D and volumetric 3D imaging pulse sequences. It is anticipated that this technique will be useful for a variety of applications, including angiographic runoffs, whole-body screening, and short-magnet imaging.     

Whole-body magnetic resonance imaging featuring moving table continuous data acquisition with high-precision position feedback.

A novel setup for whole-body MR imaging with moving table continuous data acquisition has been developed and evaluated. The setup features a manually positioned moving table platform with integrated phased-array surface radiofrequency coils. A high-precision laser position sensor was integrated into the system to provide real-time positional data that were used to compensate for nonlinear manual table translation. This setup enables continuous 2D and 3D whole-body data acquisition during table movement with surface coil image quality. The concept has been successfully evaluated with whole-body steady-state free precession (TrueFISP) anatomic imaging in five healthy volunteers. Seamless coronal and sagittal slices of continually acquired whole-body data during table movement were accurately reconstructed. The proposed strategy is potentially useful for a variety of applications, including whole-body metastasis screening, whole-body MR angiography, large field-of-view imaging in short bore systems, and for moving table applications during MR-guided interventions.     

Helical MR: continuously moving table axial imaging with radial acquisitions.

A technique for extended field of view MRI is presented. Similar to helical computed tomography, the method utilizes a continuously moving patient table, a 2D axial slice that remains fixed relative to the MRI magnet, and a radial k-space trajectory. A fully refocused SSFP acquisition enables spatial resolution comparable to current clinical protocols in scan times that are sufficiently short to allow a reasonable breathhold duration. RF transmission and signal reception are performed using the RF body coil and the images are reconstructed in real time. Experimental results are presented that illustrate the technique's ability to resolve small structures in the table-motion direction. Simulation experiments to study the steady-state response of the fully refocused SSFP acquisition during continuous table motion are also presented. Finally, whole body images of healthy volunteers demonstrate the high image quality achieved using the helical MRI approach.     

Novel reconstruction method for three‐dimensional axial continuously moving table whole‐body magnetic resonance imaging featuring autocalibrated parallel imaging GRAPPA

Continuously moving table MR imaging has been successfully evaluated for whole‐body tumor staging and metastasis screening. In previous studies it was demonstrated that three‐dimensional (3D) slab‐selective excitation with lateral readout can provide very efficient k‐space coverage when the longitudinal field of view (FOV) is limited. To reduce respiratory artifacts, data acquisition in the thoracoabdominal region of the patient typically must be performed during one single breath hold. This consequently restricts acquisition time and thus spatial resolution. In this work, a novel reconstruction method is introduced for axial 3D moving table data acquisition with lateral readout. The method features table position correction completely in k‐space and is compatible with autocalibrated parallel imaging (GRAPPA). Parallel imaging can be applied to increase spatial resolution while maintaining the breath‐holding time. A sophisticated protocol for whole‐body moving table MRI was developed. The impact of gradient nonlinearity on the featured imaging method was evaluated in phantom and volunteer experiments. Finally, the protocol was optimized toward minimizing residual artifacts. Moving table whole‐body MRI with lateral readout was performed in 5 healthy volunteers and was compared with lateral readout data acquired with a GRAPPA accelerated protocol providing increased spatial resolution. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

Sliding multislice (SMS): a new technique for minimum FOV usage in axial continuously moving-table acquisitions.

A novel technique for axial continuously moving-table scans is described that minimizes the required extension of the scanner's field of view (FOV) along the direction of table motion (z) by applying a segmented multislice acquisition technique. Any anatomical slice is acquired by applying the same phase-encoding steps at the same spatial positions along the scanner FOV. The full k-space data set of any anatomical slice is collected while the slice moves through the scanner from one scan position to the next. Simultaneous acquisition of multiple slices is realized by shifting the acquisition trajectories of different slices in time. It is demonstrated how the image artifact behavior that relates to varying imaging properties along the distance the table traverses during the acquisition of any given anatomical slice can be optimized simultaneously for all images. Discontinuities between the images along the slice axis are avoided because all z-dependent scan properties are encoded identically for all slices. Flexible spatial acquisition patterns are proposed to enable data oversampling and overlapping slice acquisitions at reduced table speeds. A framework of equations is presented by which matched parameter combinations for sliding multislice acquisitions can be applied to both single- and multiecho sequences. The new technique is validated on phantom and in vivo measurements using a T1-weighted fast low-angle shot (FLASH) sequence as well as a T2-weighted multi-spin-echo sequence of variable echo train lengths.     

Compensation of breathing motion artifacts for MRI with continuously moving table

Breathing motion is one of the main sources of artifacts in MRI acquisitions that can severely impair diagnosis. In MRI with continuously moving table, the application of common motion compensation approaches such as breath holding or the synchronization of the measurement with the breathing motion can be problematic. In this study, a technique for the reduction of breathing‐motion artifacts for MRI with continuously moving table is presented, which reconstructs motion‐consistent volumes from data acquired during free breathing. Axial images are acquired rapidly compared to the period of the breathing motion and consistently combined using a combination of rigid and nonrigid slice‐to‐volume registration. This new technique is compared to a previously reported artifact reduction method for MRI with continuously moving table that is based on the same acquisition scheme. While the latter method only suppresses ghosting artifacts, the new technique is shown to additionally reduce blurring, misregistrations, and signal cancellations in the reconstructed images. Magn Reson Med 63:701–712, 2010. © 2010 Wiley‐Liss, Inc.     

Reduction of field of view for dynamic imaging

This paper describes a simple technique that improves the temporal resolution for certain dynamic imaging applications. The technique is based on the assumption that the image to image intensity changes sought in dynamic imaging studies ire sometimes localized, and a smaller field of view can be used to reduce imaging time. Technical details and experimental results are presented. Experimental results show that this technique works reasonably well for in vivo applications.     

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.     

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.     

High-resolution steady-state free precession coronary magnetic resonance angiography within a breath-hold: parallel imaging with extended cardiac data acquisition.

Coronary artery imaging data are conventionally acquired in a single imaging frame during mid-diastole. The data acquisition window must be sufficiently short to avoid cardiac motion artifacts. A short data acquisition window results in decreased imaging efficiency and limited spatial resolution. Parallel imaging may lessen these limitations, but requires highly accurate coil sensitivity. The purpose of this work was to increase the imaging efficiency and spatial resolution in coronary artery imaging using parallel imaging with an extended acquisition window. External coil calibration data were acquired before and after a short mid-diastolic period of accelerated imaging data acquisition. It was assumed that residual cardiac motion in the extended acquisition window would not impede accurate estimation of coil sensitivity since only low spatial frequency signals were acquired for coil calibration. Experimental studies were performed in five healthy volunteers at 3 T using steady-state free precession sequence. Statistical comparison was made between the proposed method and conventional data acquisition for visual quality of image and vessel sharpness. The proposed technique demonstrated higher visual grading and improved vessel sharpness. The proposed method is a new approach to enhance the imaging efficiency and spatial resolution in coronary artery imaging.     

Restricted field of view magnetic resonance imaging of a dynamic time series.

A restricted field of view (rFOV) approach for imaging a dynamic time series of volumes of limited spatial extent within a larger subject is described. The shorter readout with rFOV-MRI can be exploited to either limit image artifacts or increase spatial resolution. To accomplish rFOV imaging of a multislice volume for a dynamic series, an outer volume suppression (OVS) preparation that saturates signal external to a cylinder through the subject is followed by slice-selective excitation and a spiral readout. The pass- and stopband efficiencies of the OVS in an agar gel phantom were 97% (+/-1.5%) and 3% (+/-1%), respectively. Profiles of the temporal signal-to-noise ratio (SNR) were measured in a phantom and an adult brain. The rFOV sequence reduced distortions from off-resonance signal and T2*-induced blurring compared to a conventional sequence. Sequence utility is demonstrated for high-resolution rFOV functional MRI (fMRI) in the visual cortex. The rFOV sequence may prove to be useful for other multislice dynamic and high-resolution imaging applications.     

Reduced field of view and undersampled PR combined for interventional imaging of a fully dynamic field of view.

Active catheter imaging was investigated using real-time undersampled projection reconstruction (PR) combined with the temporal filtering technique of reduced field of view (rFOV). Real-time rFOV processing was interactively enabled during highly undersampled catheter imaging, resulting in improved artifact suppression with better temporal resolution than that obtained by view-sharing. Imaging with 64 to 32 projections provided a resolution of 2 x 2 x 8 mm, and four to eight true frames per second. Image artifacts were reduced when rFOV processing was applied to the undersampled images. A comparison with Cartesian rFOV showed that PR image quality is less susceptible to aliasing that results from rFOV imaging with a wholly dynamic outer FOV. Simulations and MRI experiments demonstrated that PR rFOV provides significant artifact suppression, even for a fully dynamic FOV. The near doubling of temporal resolution that is possible with PR rFOV permits accurate monitoring of highly dynamic events, such as catheter movements, and arrhythmias, such as ventricular ectopy.     

3D magnetization-prepared true-FISP: a new technique for imaging coronary arteries.

The purpose of this work was to develop an ECG-triggered, segmented 3D true-FISP (fast imaging with steady-state precession) technique to improve the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of breath-hold coronary artery imaging. The major task was to optimize an appropriate magnetization preparation scheme to permit saturation of the epicardial fat signal. An alpha/2 preparation pulse was used to speed up the approach to steady-state following a frequency-selective fat-saturation pulse in each heartbeat. The application of dummy cycles was found to reduce the oscillation of the magnetization during data acquisition. The fat saturation and magnetization preparation scheme was validated with simulations and phantom studies. Volunteer studies demonstrated substantially increased SNR (55%) and CNR (178%) for coronary arteries compared to FLASH (fast low-angle shot) with the same imaging time. In conclusion, true-FISP is a promising technique for coronary artery imaging.