Accelerated bilateral dynamic contrast‐enhanced 3D spiral breast MRI using TSENSE

Purpose

To assess the ability of adaptive sensitivity encoding incorporating temporal filtering (TSENSE) to accelerate bilateral dynamic contrast‐enhanced (DCE) 3D breast MRI.

Materials and Methods

Bilateral DCE breast magnetic resonance imaging (MRI) exams were performed using a dual‐band water‐only excitation and a “stack‐of‐spirals” imaging trajectory. TSENSE was applied in the slab direction with an acceleration factor of 2. Four different techniques for sensitivity map calculation were compared by analyzing resultant contrast uptake curves qualitatively and quantitatively for 10 patient datasets. In addition, image quality and temporal resolution were compared between unaccelerated and TSENSE images.

Results

TSENSE can increase temporal resolution by a factor of 2 in DCE imaging, providing better depiction of contrast uptake curves and good image quality. Of the different methods tested, calculation of static sensitivity maps by averaging late postcontrast frames yields the lowest aliasing artifact level based on ROI analysis.

Conclusion

TSENSE acceleration combined with 3D spiral imaging is very time‐efficient, providing 11‐second temporal resolution and 1.1 × 1.1 × 3 mm³ spatial resolution over a 20 × 20 × 10 cm³ field of view for each breast. J. Magn. Reson. Imaging 2008;28:1425–1434. © 2008 Wiley‐Liss, Inc.     

A fast Edge‐preserving Bayesian reconstruction method for Parallel Imaging applications in cardiac MRI

Among recent parallel MR imaging reconstruction advances, a Bayesian method called Edge‐preserving Parallel Imaging reconstructions with GRAph cuts Minimization (EPIGRAM) has been demonstrated to significantly improve signal‐to‐noise ratio when compared with conventional regularized sensitivity encoding method. However, EPIGRAM requires a large number of iterations in proportion to the number of intensity labels in the image, making it computationally expensive for high dynamic range images. The objective of this study is to develop a Fast EPIGRAM reconstruction based on the efficient binary jump move algorithm that provides a logarithmic reduction in reconstruction time while maintaining image quality. Preliminary in vivo validation of the proposed algorithm is presented for two‐dimensional cardiac cine MR imaging and three‐dimensional coronary MR angiography at acceleration factors of 2–4. Fast EPIGRAM was found to provide similar image quality to EPIGRAM and maintain the previously reported signal‐to‐noise ratio improvement over regularized sensitivity encoding method, while reducing EPIGRAM reconstruction time by 25–50 times. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Independent slab‐phase modulation combined with parallel imaging in bilateral breast MRI

Independent slab‐phase modulation allows three‐dimensional imaging of multiple volumes without encoding the space between volumes, thus reducing scan time. Parallel imaging further accelerates data acquisition by exploiting coil sensitivity differences between volumes. This work compared bilateral breast image quality from self‐calibrated parallel imaging reconstruction methods such as modified sensitivity encoding, generalized autocalibrating partially parallel acquisitions and autocalibrated reconstruction for Cartesian sampling (ARC) for data with and without slab‐phase modulation. A study showed an improvement of image quality by incorporating slab‐phase modulation. Geometry factors measured from phantom images were more homogenous and lower on average when slab‐phase modulation was used for both mSENSE and GRAPPA reconstructions. The resulting improved signal‐to‐noise ratio (SNR) was validated for in vivo images as well using ARC instead of GRAPPA, illustrating average SNR efficiency increases in mSENSE by 5% and ARC by 8% based on region of interest analysis. Furthermore, aliasing artifacts from mSENSE reconstruction were reduced when slab‐phase modulation was used. Overall, slab‐phase modulation with parallel imaging improved image quality and efficiency for 3D bilateral breast imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Real‐time MR‐thermometry and dosimetry for interventional guidance on abdominal organs

The use of proton resonance frequency shift–based magnetic resonance (MR) thermometry for interventional guidance on abdominal organs is hampered by the constant displacement of the target due to the respiratory cycle and the associated thermometry artifacts. Ideally, a suitable MR thermometry method should for this role achieve a subsecond temporal resolution while maintaining a precision comparable to those achieved on static organs while not introducing significant processing latencies. Here, a computationally effective processing pipeline for two‐dimensional image registration coupled with a multibaseline phase correction is proposed in conjunction with high‐frame‐rate MRI as a possible solution. The proposed MR thermometry method was evaluated for 5 min at a frame rate of 10 images/sec in the liver and the kidney of 11 healthy volunteers and achieved a precision of less than 2°C in 70% of the pixels while delivering temperature and thermal dose maps on the fly. The ability to perform MR thermometry and dosimetry in vivo during a real intervention was demonstrated on a porcine kidney during a high‐intensity focused ultrasound heating experiment. Magn Reson Med 63:1080–1087, 2010. © 2010 Wiley‐Liss, Inc.     

A new approach to autocalibrated dynamic parallel imaging based on the Karhunen‐Loeve transform: KL‐TSENSE and KL‐TGRAPPA

TSENSE and TGRAPPA are autocalibrated parallel imaging techniques that can improve the temporal resolution and/or spatial resolution in dynamic magnetic resonance imaging applications. In its original form, TSENSE uses temporal low‐pass filtering of the undersampled frames to create the sensitivity map. TGRAPPA uses a sliding‐window moving average when finding the autocalibrating signals. Both filtering methods are suboptimal in the least‐squares sense and may give rise to mismatches between the undersampled k‐space raw data and the corresponding coil sensitivities. Such mismatches may result in aliasing artifacts when imaging patients with heavy breathing, as in real‐time imaging of wall motion by MRI following a treadmill exercise stress test. In this study, we demonstrate the use of an optimal linear filter, i.e., the Karhunen‐Loeve transform filter, to estimate the channel sensitivity for TSENSE and acquire the autocalibration signals for TGRAPPA. Phantom experiments show that the new reconstruction method has comparable signal‐to‐noise ratio performance to traditional TSENSE/TGRAPPA reconstruction. In vivo real‐time cardiac cine experiments performed in five healthy volunteers post‐exercise during rapid respiration show that the new method significantly reduces the chest wall aliasing artifacts caused by respiratory motion (P < 0.001). Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Parallel spectroscopic imaging reconstruction with arbitrary trajectories using k‐space sparse matrices

Parallel imaging reconstruction has been successfully applied to magnetic resonance spectroscopic imaging (MRSI) to reduce scan times. For undersampled k‐space data on a Cartesian grid, the reconstruction can be achieved in image domain using a sensitivity encoding (SENSE) algorithm for each spectral data point. Alternative methods for reconstruction with undersampled Cartesian k‐space data are the SMASH and GRAPPA algorithms that do the reconstruction in the k‐space domain. To reconstruct undersampled MRSI data with arbitrary k‐space trajectories, image‐domain‐based iterative SENSE algorithm has been applied at the cost of long computing times. In this paper, a new k‐space domain‐based parallel spectroscopic imaging reconstruction with arbitrary k‐space trajectories using k‐space sparse matrices is applied to MRSI with spiral k‐space trajectories. The algorithm achieves MRSI reconstruction with reduced memory requirements and computing times. The results are demonstrated in both phantom and in vivo studies. Spectroscopic images very similar to that reconstructed with fully sampled spiral k‐space data are obtained at different reduction factors. Magn Reson Med 61:267–272, 2009. © 2009 Wiley‐Liss, Inc.     

Contrast‐enhanced MR angiography using time resolved interleaved projection sampling with three‐dimensional cartesian phase and slice encoding (TRIPPS)

Undersampled projection reconstruction (PR) techniques provide contrast enhanced MR angiography (CE‐MRA) with high temporal resolution, but sensitivity to eddy current, gradient error and off‐resonance effects. It is desirable to combine the time efficiency of undersampled PR acquisition with the robustness of Cartesian imaging. In this work we present a technique designed to do this termed time resolved projection sampling with three‐dimensional (3D) Cartesian phase and slice encoding (TRIPPS), where 3D Cartesian k‐space is partitioned into multiple half projections in the ky‐kz plane. The phase and slice encoding are performed along predefined center‐out radial trajectories. The whole set of half projections is interleaved into multiple groups of half projections, with each group sparsely but uniformly covering the ky‐kz space. A view sharing sliding window reconstruction algorithm is adapted to reconstruct the dynamic images. The feasibility of the TRIPPS technique for CE‐MRA was demonstrated on the renal, pulmonary, and intracranial vasculatures of healthy volunteers with a high temporal resolution of 2 s/frame. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Accelerated 3D catheter visualization from triplanar MR projection images

One major obstacle for MR‐guided catheterizations is long acquisition times associated with visualizing interventional devices. Therefore, most techniques presented hitherto rely on single‐plane imaging to visualize the catheter. Recently, accelerated three‐dimensional (3D) imaging based on compressed sensing has been proposed to reduce acquisition times. However, frame rates with this technique remain low, and the 3D reconstruction problem yields a considerable computational load. In X‐ray angiography, it is well understood that the shape of interventional devices can be derived in 3D space from a limited number of projection images. In this work, this fact is exploited to develop a method for 3D visualization of active catheters from multiplanar two‐dimensional (2D) projection MR images. This is favorable to 3D MRI as the overall number of acquired profiles, and consequently the acquisition time, is reduced. To further reduce measurement times, compressed sensing is employed. Furthermore, a novel single‐channel catheter design is presented that combines a solenoidal tip coil in series with a single‐loop antenna, enabling simultaneous tip tracking and shape visualization. The tracked tip and catheter properties provide constraints for compressed sensing reconstruction and subsequent 2D/3D curve fitting. The feasibility of the method is demonstrated in phantoms and in an in vivo pig experiment. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Multiple field of view MR fluoroscopy.

This work describes a real-time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real-time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very-narrow-FOV imaging to decrease imaging time. Very-narrow-FOV images are acquired on MR receiver channels that collect guidewire or catheter data. These very-narrow-FOV images provide very high frame rate continuous, real-time imaging of the interventional devices (25 fps). Large-FOV images are formed from receiver channels that collect anatomical data from standard imaging surface coils, and simultaneously provide a dynamic, frequently updated roadmap. These multiple-FOV images are displayed together, improving visualization of interventional device placement.     

Comparison of three accelerated pulse sequences for semiquantitative myocardial perfusion imaging using sensitivity encoding incorporating temporal filtering (TSENSE)

PURPOSE: To investigate the parallel acquisition technique sensitivity encoding incorporating temporal filtering (TSENSE) with three saturation-recovery (SR) prepared pulse sequences (SR turbo fast low-angle shot [SR-TurboFLASH], SR true fast imaging with steady precession [SR-TrueFISP], and SR-prepared segmented echo-planar-imaging [SR-segEPI]) for semiquantitative first-pass myocardial perfusion imaging. MATERIALS AND METHODS: In blood- and tissue-equivalent phantoms the relationship between signal intensity (SI) and contrast-medium concentration was evaluated for the three pulse sequences. In volunteers, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and normalized upslopes (NUS) were calculated from signal-time curves (STC). Moreover, artifacts, image noise, and overall image quality were qualitatively evaluated. RESULTS: Phantom data showed a 40% increased linear range of the relation between SI and contrast-medium concentration with TSENSE. In volunteers, TSENSE introduced significantly residual artifacts and loss in SNR and CNR. No differences were found for NUS values with TSENSE. SR-TrueFISP yielded highest SNR, CNR, and quality scores. However, in SR-True-FISP images, dark-banding artifacts were most pronounced. NUS values obtained with SR-TrueFISP were significantly higher and with SR-segEPI significantly lower than with SR-TurboFLASH. CONCLUSION: Semiquantitative myocardial perfusion imaging can significantly benefit from TSENSE due to shorter acquisition times and increased linearity of the pulse sequences. Among the three pulse sequences tested, SR-TrueFISP yielded best image quality. SR-segEPI proved to be an interesting alternative due to shorter acquisition times, higher linearity and fewer dark-banding artifacts.     

Adaptive sensitivity encoding incorporating temporal filtering (TSENSE).

A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k-space lines in sequential images and exploits the property that the outer portion of the field-of-view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k-space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency-shifted to the band edge and thus may be further suppressed by temporal low-pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low-pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real-time nonbreath-held cardiac MR imaging during exercise induced stress are presented.     

Whole‐heart imaging using undersampled radial phase encoding (RPE) and iterative sensitivity encoding (SENSE) reconstruction

Whole‐heart isotropic nonangulated cardiac magnetic resonance (CMR) is becoming an important protocol in simplifying MRI, since it reduces the need of cumbersome planning of angulations. However the acquisition times of whole‐heart MRI are prohibitive due to the large fields of view (FOVs) and the high spatial resolution required for depicting small structures and vessels. To address this problem, we propose a three‐dimensional (3D) acquisition scheme that combines Cartesian sampling in the readout direction with an undersampled radial scheme in the phase‐encoding plane. Different undersampling patterns were investigated in combination with an iterative sensitivity encoding (SENSE) reconstruction and a 32‐channel cardiac coil. Noise amplification maps were calculated to compare the performance of the different patterns using iterative SENSE reconstruction. The radial phase‐encoding (RPE) scheme was implemented on a clinical MR scanner and tested on phantoms and healthy volunteers. The proposed method exhibits better image quality even for high acceleration factors (up to 12) in comparison to Cartesian acquisitions. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Radial sliding‐window magnetic resonance angiography (MRA) with highly‐constrained projection reconstruction (HYPR)

Sufficient temporal resolution is required to image the dynamics of blood flow, which may be critical for accurate diagnosis and treatment of various intracranial vascular diseases, such as arteriovenous malformations (AVMs) and aneurysms. Highly‐constrained projection reconstruction (HYPR) has recently become a technique of interest for high‐speed contrast‐enhanced magnetic resonance angiography (CE‐MRA). HYPR provides high frame rates by preferential weighting of radial projections while maintaining signal‐to‐noise ratio (SNR) by using a high SNR composite. An analysis was done to quantify the effects of HYPR on SNR, contrast‐to‐noise ratio (CNR), and temporal blur compared to the previously developed radial sliding‐window technique using standard filtered backprojection or regridding methods. Computer simulations were performed to study the effects of HYPR processing on image error and the temporal information. Additionally, in vivo imaging was done on patients with angiographically confirmed AVMs to measure the effects of alteration of various HYPR parameters on SNR as well as the fidelity of the temporal information. The images were scored by an interventional radiologist in a blinded read and were compared with X‐ray digital subtraction angiography (DSA). It was found that with the right choice of parameters, modest improvements in both SNR and dynamic information can be achieved as compared to radial sliding‐window MRA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Accelerated cardiac perfusion imaging using k‐t SENSE with SENSE training

In k‐t sensitivity encoding (SENSE), MR data acquisition performed in parallel by multiple coils is accelerated by sparsely sampling the k‐space over time. The resulting aliasing is resolved by exploiting spatiotemporal correlations inherent in dynamic images of natural objects. In this article, a modified k‐t SENSE reconstruction approach is presented, which aims at improving the temporal fidelity of first‐pass, contrast‐enhanced myocardial perfusion images at high accelerations. The proposed technique is based on applying parallel imaging on the training data in order to increase their spatial resolution. At a net acceleration of 5.8 (k‐t factor = 8, training profiles = 11) accurate representations of dynamic signal‐intensities were achieved. The efficacy of this approach as well as limitations due to noise amplification were investigated in computer simulations and in vivo experiments. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

“PINOT”: Time‐resolved parallel magnetic resonance imaging with a reduced dynamic field of view

This article introduces a novel method named “Parallel Imaging and Noquist in Tandem” (PINOT) for accelerated image acquisition of cine cardiac magnetic resonance imaging. This method combines two prior information formalisms, the SPACE‐RIP implementation of parallel imaging and the Noquist method for reduced‐data image reconstruction with prior knowledge of static and dynamic regions in the field of view. The general theory is presented, and supported by results from experiments using time‐resolved two‐dimensional simulation data and retrospectively sub‐sampled magnetic resonance imaging data with acceleration factors around 4. A signal‐to‐noise ratio analysis and a comparison study with TSENSE and k‐t SENSE show that PINOT performs favorably in preserving edge detail, at a cost in signal‐to‐noise ratio and computational complexity. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Real‐time geometric distortion correction for interventional imaging with echo‐planar imaging (EPI)

Many MR‐guided interventional procedures rely on fast imaging sequences for providing images in real‐time with a precise relation between the target position in the image and its true position. Echo‐planar imaging (EPI) methods are very fast but prone to geometric distortions. Here, we propose a correction method designed for real‐time conditions, adapting existing approaches based on dual EPI acquisition with varying echo times. The method is demonstrated with MR‐thermometry for guiding thermal therapies. The proposed approach imposes a small penalty in acquisition speed but adds negligible latency to data processing, an important element for interventions of mobile organs. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Improved temporal resolution in cardiac imaging using through‐time spiral GRAPPA

Previous work has shown that the use of radial GRAPPA for the reconstruction of undersampled real‐time free‐breathing cardiac data allows for frame rates of up to 30 images/s. It is well known that the spiral trajectory offers a higher scan efficiency compared to radial trajectories. For this reason, we have developed a novel through‐time spiral GRAPPA method and demonstrate its application to real‐time cardiac imaging. By moving from the radial trajectory to the spiral trajectory, the temporal resolution can be further improved at lower acceleration factors compared to radial GRAPPA. In addition, the image quality is improved compared to those generated using the radial trajectory due to the lower acceleration factor. Here, we show that 2D frame rates of up to 56 images/s can be achieved using this parallel imaging method with the spiral trajectory. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Super‐resolved spatially encoded single‐scan 2D MRI

Single‐scan MRI underlies a wide variety of clinical and research activities, including functional and diffusion studies. Most common among these “ultrafast” MRI approaches is echo‐planar imaging. Notwithstanding its proven success, echo‐planar imaging still faces a number of limitations, particularly as a result of susceptibility heterogeneities and of chemical shift effects that can become acute at high fields. The present study explores a new approach for acquiring multidimensional MR images in a single scan, which possesses a higher built‐in immunity to this kind of heterogeneity while retaining echo‐planar imaging's temporal and spatial performances. This new protocol combines a novel approach to multidimensional spectroscopy, based on the spatial encoding of the spin interactions, with image reconstruction algorithms based on super‐resolution principles. Single‐scan two‐dimensional MRI examples of the performance improvements provided by the resulting imaging protocol are illustrated using phantom‐based and in vivo experiments. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Accelerating SENSE using compressed sensing

Both parallel MRI and compressed sensing (CS) are emerging techniques to accelerate conventional MRI by reducing the number of acquired data. The combination of parallel MRI and CS for further acceleration is of great interest. In this paper, we propose a novel method to combine sensitivity encoding (SENSE), one of the standard methods for parallel MRI, and compressed sensing for rapid MR imaging (SparseMRI), a recently proposed method for applying CS in MR imaging with Cartesian trajectories. The proposed method, named CS‐SENSE, sequentially reconstructs a set of aliased reduced‐field‐of‐view images in each channel using SparseMRI and then reconstructs the final image from the aliased images using Cartesian SENSE. The results from simulations and phantom and in vivo experiments demonstrate that CS‐SENSE can achieve a reduction factor higher than those achieved by SparseMRI and SENSE individually and outperform the existing method that combines parallel MRI and CS. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Fast functional brain imaging using constrained reconstruction based on regularization using arbitrary projections

This work describes a novel method for highly undersampled projection imaging using constrained reconstruction by Tikhonov‐Phillips regularization and its application for high temporal resolution functional MRI (fMRI) at a repetition time of 80 ms. The high‐resolution reference image used as in vivo coil sensitivity is acquired in a separate acquisition using otherwise identical parameters. Activation studies using a standard checkerboard activation paradigm demonstrate the inherent high sensitivity afforded by the possibility to separate activation‐related effects from “physiological noise.”. In this first proof‐of‐principle of the constrained reconstruction based on regularization using arbitrary projections (COBRA) technique, experiments are performed in a single‐slice mode, which allows for a comparison with fast single‐slice echo‐planar imaging (EPI) at equal temporal resolution. The COBRA method can be extended to three‐dimensional (3D) encoding without severe penalty in temporal performance. Analysis of the global signal change demonstrates the excellent reproducibility of COBRA compared to standard EPI. Activation analysis is considerably improved by the possibility to remove electrocardiogram (ECG)‐related and breathing‐related signal fluctuations by physiological correction of each individual breathing and ECG cycle, respectively. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.