Time-resolved 3D contrast-enhanced MRA of an extended FOV using continuous table motion

Improved receiver arrays and optimized parallel imaging accelerations applied to time-resolved 3D fluoroscopically tracked peripheral runoff CE-MRA

OBJECTIVES

Three-station stepping-table time-resolved 3D contrast-enhanced magnetic resonance angiography has conflicting demands in the need to limit acquisition time in proximal stations to match the speed of the advancing contrast bolus and in the distal-most station to avoid venous contamination while still providing clinically useful spatial resolution. This work describes improved receiver coil arrays which address this issue by allowing increased acceleration factors, providing increased spatial resolution per unit time.

MATERIALS AND METHODS

Receiver coil arrays were constructed for each station (pelvis, thigh, calf) and then integrated into a 48-element array for three-station peripheral CE-MRA. Coil element sizes and array configurations for these three stations were designed to improve SENSE-type parallel imaging taking advantage of an increase in coil count for all stations versus the previous 32 channel capability. At each station either acceleration apportionment or optimal CAIPIRINHA selection was used to choose the optimum acceleration parameters for each subject. Results were evaluated in both single- and multi-station studies.

RESULTS

Single-station studies showed that SENSE acceleration in the thigh station could be readily increased from R=8 to R=10, allowing reduction of the frame time from 2.5 to 2.1 s to better image the typically rapidly advancing bolus at this station. Similarly, the improved coil array for the calf station permitted acceleration increase from R=8 to R=12, providing a 4.0 vs. 5.2 s frame time. Results in three-station studies suggest an improved ability to track the contrast bolus in peripheral CE-MRA.

CONCLUSIONS

Modified receiver coil arrays and individualized parameter optimization have been used to provide improved acceleration at all stations in multi-station peripheral CE-MRA and provide high spatial resolution with frame times as short as 2.1 s.

Whole-body continuously moving table fat-water MRI with dynamic B0 shimming at 3 Tesla

PURPOSE

The purpose of this work was to develop a rapid and robust whole-body fat-water MRI (FWMRI) method using a continuously moving table (CMT) with dynamic field corrections at 3 Tesla.

METHODS

CMT FWMRI was developed at 3 Tesla with a multiecho golden angle (GA) radial trajectory and dynamic B0 field shimming. Whole-body imaging was performed with 4 echoes and superior-inferior coverage of 1.8 meters without shims in 90 s. 716 axial images were reconstructed with GA profile binning followed by B0 field map generation using fast three-point seeded region growing fat-water separation and slice-specific 0(th) and 1(st) order shim calculation. Slice-specific shims were applied dynamically in a repeated CMT FWMRI scan in the same session. The resulting images were evaluated for field homogeneity improvements and quality of fat-water separation with a whole-image energy optimized algorithm.

RESULTS

GA sampling allowed high quality whole-body FWMRI from multiecho CMT data. Dynamic B0 shimming greatly improved field homogeneity in the body and produced high quality water and fat only images as well as fat signal fraction and R2 * relaxivity maps.

CONCLUSION

A rapid and robust technique for whole-body fat-water quantification has been developed with CMT MRI with dynamic B0 field correction. Magn Reson Med 76:183-190, 2016. © 2015 Wiley Periodicals, Inc.

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.

Continuously moving table aorto-iliofemoral run-off contrast-enhanced magnetic resonance angiography: image quality analysis in comparison to the multistep acquisition

BACKGROUND

Optimal vessel contrast is a prerequisite for vascular imaging. Consecutive stationary imaging of multiple fields of view is contrary to the continuous contrast material passage through the vascular tree. A continuous acquisition of a magnetic resonance (MR) sequence might overcome this limitation.

PURPOSE

To investigate the image quality of a continuously moving table (CMT) acquisition compared with the established multistep approach for contrast-enhanced magnetic resonance angiography (CE-MRA) of the aorto-iliofemoral run-off.

MATERIAL AND METHODS

Institutional review board approved this retrospective interindividual study of 60 consecutive patients referred to CE-MRA for peripheral arterial disease. Thirty patients underwent CE-MRA using the routine multistep acquisition and 30 patients were scanned using the CMT technique at 1.5 Tesla. All patients received a fixed contrast dose of 25 mL gadoterate meglumine. A quantitative analysis was performed to assess the relative contrast of 10 vascular segments from the proximal abdominal aorta to the distal calf arteries. A qualitative evaluation of three separate vascular regions (abdomen and pelvis, thighs, and calves) was performed. Two radiologists graded independently arterial vessel conspicuity, venous contamination, presence of artifacts, and diagnostic confidence on a 4-point scale. Overall scan time, including all localizer scans, was recorded. Statistical differences were tested using the Wilcoxon signed-rank test with Bonferroni correction.

RESULTS

No significant differences were found between the continuously moving table acquisition and the multistep acquisition with regard to the relative vascular contrast and the qualitative image criteria. The agreement between both readers was significant (Kendall tau rank correlation coefficient, 0.373). The absolute reader agreement was 71.4%. The mean overall scan time was 12 min 44 s for the CMT protocol and 21 min 41 s for the multistep protocol.

CONCLUSION

Aorto-iliofemoral run-off CE-MRA acquired with CMT technique provides a high image quality equivalent to a multistep technique at an overall scan time reduction of 41.3%.

Time-resolved bolus-chase MR angiography with real-time triggering of table motion

Time-resolved bolus-chase contrast-enhanced MR angiography with real-time station switching is demonstrated. The Cartesian acquisition with projection reconstruction-like sampling (CAPR) technique and high 2D sensitivity encoding (SENSE) (6x or 8x) and 2D homodyne (1.8x) accelerations were used to acquire 3D volumes with 1.0-mm isotropic spatial resolution and frame times as low as 2.5 sec in two imaging stations covering the thighs and calves. A custom real-time system was developed to reconstruct and display CAPR frames for visually guided triggering of table motion upon passage of contrast through the proximal station. The method was evaluated in seven volunteers. High-spatial-resolution arteriograms with minimal venous contamination were consistently acquired in both stations. Real-time stepping table contrast-enhanced MR angiography is a method for providing time-resolved images with high spatial resolution over an extended field-of-view.

Peripheral vasculature: high-temporal- and high-spatial-resolution three-dimensional contrast-enhanced MR angiography

PURPOSE

To prospectively evaluate the feasibility of performing high-spatial-resolution (1-mm isotropic) time-resolved three-dimensional (3D) contrast material-enhanced magnetic resonance (MR) angiography of the peripheral vasculature with Cartesian acquisition with projection-reconstruction-like sampling (CAPR) and eightfold accelerated two-dimensional (2D) sensitivity encoding (SENSE).

MATERIALS AND METHODS

All studies were approved by the institutional review board and were HIPAA compliant; written informed consent was obtained from all participants. There were 13 volunteers (mean age, 41.9; range, 27-53 years). The CAPR sequence was adapted to provide 1-mm isotropic spatial resolution and a 5-second frame time. Use of different receiver coil element sizes for those placed on the anterior-to-posterior versus left-to-right sides of the field of view reduced signal-to-noise ratio loss due to acceleration. Results from eight volunteers were rated independently by two radiologists according to prominence of artifact, arterial to venous separation, vessel sharpness, continuity of arterial signal intensity in major arteries (anterior and posterior tibial, peroneal), demarcation of origin of major arteries, and overall diagnostic image quality. MR angiographic results in two patients with peripheral vascular disease were compared with their results at computed tomographic angiography.

RESULTS

The sequence exhibited no image artifact adversely affecting diagnostic image quality. Temporal resolution was evaluated to be sufficient in all cases, even with known rapid arterial to venous transit. The vessels were graded to have excellent sharpness, continuity, and demarcation of the origins of the major arteries. Distal muscular branches and the communicating and perforating arteries were routinely seen. Excellent diagnostic quality rating was given for 15 (94%) of 16 evaluations.

CONCLUSION

The feasibility of performing high-diagnostic-quality time-resolved 3D contrast-enhanced MR angiography of the peripheral vasculature by using CAPR and eightfold accelerated 2D SENSE has been demonstrated.

Controlled experimental study depicting moving objects in view-shared time-resolved 3D MRA

Various methods have been used for time-resolved contrast-enhanced magnetic resonance angiography (CE-MRA), many involving view sharing. However, the extent to which the resultant image time series represents the actual dynamic behavior of the contrast bolus is not always clear. Although numerical simulations can be used to estimate performance, an experimental study can allow more realistic characterization. The purpose of this work was to use a computer-controlled motion phantom for study of the temporal fidelity of three-dimensional (3D) time-resolved sequences in depicting a contrast bolus. It is hypothesized that the view order of the acquisition and the selection of views in the reconstruction can affect the positional accuracy and sharpness of the leading edge of the bolus and artifactual signal preceding the edge. Phantom studies were performed using dilute gadolinium-filled vials that were moved along tabletop tracks by a computer-controlled motor. Several view orders were tested using view-sharing and Cartesian sampling. Compactness of measuring the k-space center, consistency of view ordering within each reconstruction frame, and sampling the k-space center near the end of the temporal footprint were shown to be important in accurate portrayal of the leading edge of the bolus. A number of findings were confirmed in an in vivo CE-MRA study.

Peripheral Vascular Disease: Comparison of Continuous MR Angiography and Conventional MR Angiography—Pilot Study

The aim of this study was to prospectively assess the accuracy of three-dimensional magnetic resonance (MR) angiography for evaluation of stenosis in the peripheral arterial system with a continuous moving table technique, with conventional MR angiography as reference. This study was approved by the local institutional review board; informed consent was obtained. Five healthy male volunteers (mean age, 27 years; range, 24-35 years) and four men and one woman (mean age, 63 years; range, 46-78 years) with peripheral arterial occlusive disease were examined. Images obtained with both techniques showed excellent concordance (Cohen kappa = 0.75). Images obtained with a conventional protocol had higher quality compared with those obtained with the continuous technique (mean, 1.07 +/- 0.25 [standard deviation] vs 1.58 +/- 0.6; P < .05); small vessels appeared sharper on them. For detection of significant stenosis and occlusion, accuracy, sensitivity, and specificity of the continuous technique were 92.8%, 100%, and 89.2%, respectively.

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.

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.

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.

Contrast-enhanced MR Angiography of the Peripheral Vasculature with a Continuously Moving Table and Modified Elliptical Centric Acquisition

This study was approved by the institutional review board and was HIPAA compliant. All subjects provided written informed consent, and subject confidentiality was protected. The purpose of this study was to prospectively evaluate the feasibility of integrating a modified elliptical centric (EC) acquisition with a continuously moving table technique to acquire high-spatial-resolution contrast material-enhanced magnetic resonance (MR) angiograms of the peripheral vasculature. Incorporation of two-dimensional homodyne reconstruction modified the EC view order, allowing improved spatial resolution per unit time while retaining the advantage of venous suppression intrinsic to the EC technique. Spatial resolution was dynamically improved when the table reached the distal-most station. The modified view order provided improved spatial resolution in phantom examinations compared with that in standard examinations. Peripheral MR angiograms were generated in a group of 13 volunteers (eight women; five men; age range, 51-72 years; mean age, 58.5 years +/- 7.9 [standard deviation]) at 1.5 T. Four arterial regions were evaluated on a five-point scale (scores ranged from 0 to 4; a score of 4 was considered excellent); venous suppression was also evaluated. The mean arterial scores exceeded 3.0 for all regions. There was no venous signal or only superficial venous signal in 10 of the 13 cases.

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.

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.

Combination of 2D sensitivity encoding and 2D partial fourier techniques for improved acceleration in 3D contrast-enhanced MR angiography

Sensitivity encoding (SENSE) and partial Fourier (PF) techniques both reduce MRI acquisition time. Two-dimensional SENSE uses coil sensitivities to unfold aliasing in the phase/slice-encoding plane. One-dimensional PF and homodyne reconstruction are routinely applied in the frequency/phase-encoding plane to compensate for nonsampled k-space of the presumed real magnetization. Recently, a modified 3D elliptical centric acquisition was proposed to facilitate 2D-PF and homodyne reconstruction on an undersampled phase/slice-encoding plane. In this work we hypothesized that this 2D-PF technique can be combined with 2D-SENSE to achieve a greater acceleration factor than what each method can provide separately. Reconstruction of data whereby SENSE and PF are applied along the same axes is described. Contrast-enhanced MR angiography (CE-MRA) results from experiments using four receiver coils in phantom and volunteer studies are shown. In 11 volunteer studies, the SENSE-PF-homodyne technique using sevenfold acceleration (4x SENSE, 1.7x PF) consistently provided high-diagnostic-quality images with near 1-mm isotropic resolution in acquisition times of <20 s.

Real-time tracking of contrast bolus propagation in continuously moving table MR angiography

The recent introduction of variable-velocity control for Continuously Moving Table Magnetic Resonance Imaging has introduced the ability for interactive examination across extended fields of view. A common application of this method is contrast-enhanced angiography, where propagation of an intravenously injected contrast agent is tracked throughout the peripheral vascular tree. Whereas current methods for performing contrast bolus tracking are entirely manual, we discuss the complete automation of this process through the use of real-time image processing. Specifically, we provide a coupled intensity-correction procedure and modified Fast Marching method for rapid segmentation of contrast enhanced vasculature in CE-MRA and discuss the incorporation of this process into a framework for fully automated and adaptive control of table motion for real-time tracking of contrast bolus propagation through the lower peripheral vasculature.

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.

Undersampled elliptical centric view-order for improved spatial resolution in contrast-enhanced MR angiography

Although contrast-enhanced MR angiography (CE-MRA) has been successfully developed into a routine clinical imaging technique, there is still need for improved spatial resolution in a given acquisition time. Undersampled projection reconstruction (PR) techniques maintain spatial resolution with reduced scan times, and the elliptical centric (EC) view order provides high quality arterial phase images without venous contamination. In this work, we present a hybrid elliptical centric-projection reconstruction (EC-PR) technique to provide spatial resolution improvement over standard EC in a given time. The k-space sampling was performed by undersampling the periphery of the k(Y)-k(Z) phase encoding plane of an EC view order in a PR like manner. The sampled views were maintained on a rectilinear grid, and thus reconstructed by standard 3DFT. The non-sampled views were compensated either by zero-filling or performing a 2D homodyne reconstruction. Compared to a fully sampled k-space, the EC-PR sequence acquired in the same scan time provides a resolution improvement of about two, as shown by point spread function analysis and phantom experiments. The hypothesis that EC-PR provides improved resolution while retaining diagnostically adequate SNR was tested in 11 CE-MRA studies of the popliteal and carotid arteries and shown to be true (P < 0.03).

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).

Dual-velocity continuously moving table acquisition for contrast-enhanced peripheral magnetic resonance angiography

Acquisition of MR angiographic data of the peripheral vasculature during continuous table motion offers certain advantages over fixed station approaches, such as the elimination of wasted time moving between stations and the ability to form a seamless image of the extended field of view. However, it has recently been demonstrated that there is an approximate twofold reduction in contrast bolus velocity as it moves from the thighs to the calves. This can potentially cause a mismatch of the moving table with the contrast peak, resulting in the table outpacing the contrast bolus distally. In this work we describe a modification to the continuous table motion technique allowing two table velocities: a high (ca. 3.6 cm/sec) velocity from the abdomen to the thighs and a low (ca. 1.6 cm/sec) velocity distally. Implications of the nonconstant velocity on k-space sampling are described, and it is shown that lateral resolution is improved for the low-velocity region. Correction for table deceleration during the transition time between high and low velocities is demonstrated. Contrast-enhanced studies in 15 volunteers are free of table-motion-related artifact and suggest improved depiction of the contrast bolus distally.