Multicontrast sequences with continuous table motion: a novel acquisition technique for extended field of view imaging.

A novel acquisition technique called multicontrast imaging is presented that provides multiple datasets of different image contrasts covering an extended field of view within one measurement procedure. The technique uses a continuously moving table and is based on the repetitive acquisition of axial volume sections while the patient moves through the scanner once. To compensate for the table motion during the measurement, adaptive slice shifting is applied. Multicontrast imaging is designed to combine the comfort of a moving table examination with the high time efficiency of a multitask protocol and can be used for generating differences in both contrast and spatial parameters of the acquired data. The technique and its properties are demonstrated on healthy human volunteers.     

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.     

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.     

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.     

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.     

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.     

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.     

Whole-body 3D water/fat resolved continuously moving table imaging

PURPOSE: To study the feasibility of three-dimensional (3D) whole-body, head-to-toe, water/fat resolved MRI, using continuously moving table imaging technology. MATERIALS AND METHODS: Experiments were performed on nine healthy volunteers, acquiring 3D whole-body head-to-toe data under continuous motion of the patient table. Two different approaches for water/fat separation have been studied. Results of a three-point chemical shift encoding and a spectral presaturation technique were compared with respect to image quality and performance. Furthermore, fast, low-resolution, whole-body water/fat imaging was performed in two minutes total scan time to derive patient-specific parameters such as the total water/fat ratio, the intraperitoneal/extraperitoneal fat ratio, and the body mass index (BMI). RESULTS: Good water/fat separation with decent image quality was obtained in all cases. The three-point chemical shift encoding approach was found to be more efficient with respect to signal-to-noise ratio (SNR) and acquisition time. CONCLUSION: Whole-body water/fat sensitive MRI using continuous table motion is feasible and could be of interest for clinical practice. Some improvements of the method are desirable.     

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.     

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).     

Continuously moving table time‐of‐flight angiography of the peripheral veins

Time‐of‐flight (TOF) MR angiography allows for noninvasive vessel imaging. To overcome the limited volumetric coverage of standard TOF techniques, the aim of this study was to investigate the combination of TOF and continuously moving table (CMT) acquisitions for peripheral vein imaging based on image subtraction. Two acquisition strategies are presented: a simple two‐step method based on 2‐fold CMT acquisition and an advanced one‐step method requiring only one continuous scan. Image quality of both CMT TOF techniques was evaluated by semiquantitative image grading and by signal‐to‐noise ratio and contrast‐to‐noise ratio analysis for veins of the upper and lower leg in 10 healthy volunteers. Results were compared to a standard stationary two‐dimensional (2D) TOF multistation acquisition. Image grading revealed good image quality for both CMT TOF methods, thereby confirming the feasibility of axial 2D CMT TOF to assess the veins of the lower extremities during a single scan. Quantitative evaluation showed no significant difference in signal‐to‐noise ratio and contrast‐to‐noise ratio compared to the stationary experiment. Additional measurements in three patients with postthrombotic changes and varicosities demonstrated the clinical applicability of the presented methods. CMT TOF provides promising results and permits the detection of various pathologic changes of the venous system. Magn Reson Med 63:1219–1229, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

Towards automatic patient positioning and scan planning using continuously moving table MR imaging

A concept is proposed to simplify patient positioning and scan planning to improve ease of use and workflow in MR. After patient preparation in front of the scanner the operator selects the anatomy of interest by a single push‐button action. Subsequently, the patient table is moved automatically into the scanner, while real‐time 3D isotropic low‐resolution continuously moving table scout scanning is performed using patient‐independent MR system settings. With a real‐time organ identification process running in parallel and steering the scanner, the target anatomy can be positioned fully automatically in the scanner's sensitive volume. The desired diagnostic examination of the anatomy of interest can be planned and continued immediately using the geometric information derived from the acquired 3D data. The concept was implemented and successfully tested in vivo in 12 healthy volunteers, focusing on the liver as the target anatomy. The positioning accuracy achieved was on the order of several millimeters, which turned out to be sufficient for initial planning purposes. Furthermore, the impact of nonoptimal system settings on the positioning performance, the signal‐to‐noise ratio (SNR), and contrast‐to‐noise ratio (CNR) was investigated. The present work proved the basic concept of the proposed approach as an element of future scan automation. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

Principles of whole-body continuously-moving-table MRI

Continuously-moving-table (CMT) imaging is a new and promising approach to virtually extend the field-of-view (FOV) in currently available MRI systems. It shows high potential to improve a number of applications that require a large FOV, such as whole-body contrast-enhanced angiography and contrast-optimized whole-body head-to-toe imaging. In this work, an overview of the different approaches to CMT imaging is given. Basic principles of two- and three-dimensional (2D and 3D) techniques are discussed, with emphasis on performance and image-artifact issues. Potential clinical applications and further desirable improvements are outlined.     

Increasing efficiency of parallel imaging for 2D multislice acquisitions

Parallel imaging algorithms require precise knowledge about the spatial sensitivity variation of the receiver coils to reconstruct images with full field of view (FOV) from undersampled Fourier encoded data. Sensitivity information must either be given a priori, or estimated from calibration data acquired along with the actual image data. In this study, two approaches are presented, which require very little or no additional data at all for calibration in two‐dimensional multislice acquisitions. Instead of additional data, information from spatially adjacent slices is used to estimate coil sensitivity information, thereby increasing the efficiency of parallel imaging. The proposed approaches rely on the assumption that over a small range of slices, coil sensitivities vary smoothly in slice direction. Both methods are implemented as variants of the GRAPPA algorithm. For a given effective acceleration, superior image quality is achieved compared to the conventional GRAPPA method. For the latter calibration lines for coil weight computation must be acquired in addition to the undersampled k‐spaces for coil weight computation, thus requiring higher k‐space undersampling, that is, a higher reduction factor to achieve the same effective acceleration. The proposed methods are particularly suitable to speed up parallel imaging for clinical applications where the reduction factor is limited to two or three. Magn Reson Med 2009 © 2009 Wiley‐Liss, Inc.