Continuous Update with Random Encoding (CURE): A New Strategy for Dynamic Imaging

Although dynamic imaging is presently used for various applications, it is still limited by the temporal resolution. In this paper, we present a new technique that uses a random phase-encoding strategy to facilitate faster and smoother update of images and to improve the temporal resolution in dynamic studies. The technique was implemented on a conventional clinical scanner and demonstrated with various in vivo studies. Technical details, simulations, and experimental results are described. Images from experimental studies indicate that the new technique is robust in generating dynamic images and can be potentially utilized for clinical applications.     

A comparison of RIGR and SVD dynamic imaging methods

Several constrained imaging methods have recently been proposed for dynamic imaging applications. This paper compares two of these methods: the Reduced-encoding Imaging by Generalized-series Reconstruction (RIGR) and Singular Value Decomposition (SVD) methods. RIGR utilizes a priori data for optimal image reconstruction whereas the SVD method seeks to optimize data acquisition. However, this study shows that the existing SVD encoding method tends to bias the data acquisition scheme toward reproducing the known features in the reference image. This characteristic of the SVD encoding method reduces its capability to capture new image features and makes it less suitable than RIGR for dynamic imaging applications.     

Frequency resolved single-shot MR imaging using stochastic k-space trajectories

A new single-shot stochastic imaging technique with a random k-space path that provides very selective filtering with respect to chemical shift or off-resonance signals of the investigated tissue is proposed. It is demonstrated that in stochastic imaging only on-resonance compartments are visible whereas frequency shifted compartments cancel to noise that is distributed over the whole image. This method can be used as a single-shot chemical shift selective imaging technique and allows to calculate frequency resolved spectra for each spatial position of the image based on a single signal aquisition. The single-shot stochastic imaging sequence makes high demands on the gradient system and the theoretical k-space trajectory is distorted by imperfect gradient performance. Therefore an additional k-space guided imaging technique that uses the true, measured k-space trajectory to correct artifacts generated by eddy currents and delay times of the rapid switched gradients is presented. In vitro and in vivo measurements demonstrate the successful implementation of single-shot stochastic imaging on a conventional MR scanner with unshielded gradient systems.     

Temporal resolution improvement in dynamic imaging

In some dynamic imaging applications, only a fraction, 1/n, of the field of view (FOV) may show considerable change during the motion cycle. A method is presented that improves the temporal resolution for a dynamic region by a factor, n, while maintaining spatial resolution at a cost of √n in signal-to-noise ratio (SNR). Temporal resolution is improved, or alternatively, total imaging time is reduced by reducing the number of phase encodes acquired for each temporal frame by 1/n. To eliminate aliasing, a representation of the signal from the static outer portion of the FOV is constructed using all the raw data. The k-space data derived from this representation is subtracted from the original data sets, and the differences correspond to the dynamic portion of the FOV. Improved resolution results are presented in phantom studies, and in vivo phase contrast quantitative flow imaging.     

“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.     

Noquist: reduced field-of-view imaging by direct Fourier inversion.

A novel technique called "Noquist" is introduced for the acceleration of dynamic cardiac magnetic resonance imaging (CMRI). With the use of this technique, a more sparsely sampled dynamic image sequence is reconstructed correctly, without Nyquist foldover artifact. Unlike most other reduced field-of-view (rFOV) methods, Noquist does not rely on data substitution or temporal interpolation to reconstruct the dynamic image sequence. The proposed method reduces acquisition time in dynamic MRI scans by eliminating the data redundancy associated with static regions in the dynamic scene. A reduction of imaging time is achieved by a fraction asymptotically equal to the static fraction of the FOV, by omitting acquisition of an appropriate subset of phase-encoding views from a conventional equidistant Cartesian acquisition grid. The theory behind this method is presented along with sample reconstructions from real and simulated data. Noquist is compared with conventional cine imaging by retrospective selection of a reduced data set from a full-grid conventional image sequence. In addition, a comparison is presented, using real and simulated data, of our technique with an existing rFOV technique that uses temporal interpolation. The experimental results confirm the theory, and demonstrate that Noquist reduces scan time for cine MRI while fully preserving both spatial and temporal resolution, but at the cost of a reduced signal-to-noise ratio (SNR).     

Continuous radial data acquisition for dynamic MRI

Since image acquisition times in MRI have been reduced considerably over recent years, several new important application areas of MRI have appeared. In addition to pure static anatomic information, the evolution of a dynamic process may be visualized by a sequence of temporal snapshots of the process acquired within a short time period. This makes applications like interactive or interventional MRI as well as the acquisition of additional functional information feasible. For high temporal resolution, all these applications require a quasi real-time image acquisition during the time the interaction or dynamic process evolves. We present an approach to realtime imaging using a continuous radial acquisition scheme. The intrinsic advantages of radial or projection reconstruction (PR) techniques are used to minimize motion-related image distortions. Modifications of the acquisition scheme as well as dedicated reconstruction techniques are used to further reduce the temporal blurring due to the finite acquisition time of one entire data set in our approach. So far we have used this technique for the visualization of active joint motion.     

Implementation of a fast gradient-echo SVD encoding technique for dynamic imaging

In the paper, the results of a fast gradient-echo implementation of the singular value decomposition (SVD) encoding technique for dynamic imaging are presented. The method used is an adaptation with several critical modifications of a keyhole-type approach previously proposed but not implemented. The method was tested by imaging the events following injection of a contrast agent into a phantom, producing a series of dynamic image updates. It is demonstrated that, for this type of application, the SVD encoding technique adequately follows dynamic changes with even a small number of encodes. The result is comared qualitatively to that obtained by standard Fourier-based keyhole imaging and is shown to provide improved spatial resolution of dynamic events when updating with the same number of encodes.     

Echo-Time-Encoded Burst Imaging (EBI): A Novel Technique for Spectroscopic Imaging

A new technique for rapid spectroscopic imaging is presented. The proposed experiment enables a complete mapping of the two-dimensional reciprocal space k_x, k_o, and thus the acquisition of a 1D spectroscopic image in a single scan. The properties of the pulse sequence, based on the use of a burst of low flip angle pulses, are analyzed in the framework of linear response theory, and it is shown that chemical shift information may be introduced into the spatially encoded echoes. First experimental results are presented demonstrating that 32 x 32 proton spectroscopic images may be acquired within less than 1 min with a conventional imaging system.     

Simulation of spatial and contrast distortions in keyhole imaging

Keyhole imaging is a scheme introduced to improve temporal resolution in dynamic contrast-enhanced MRI by a factor of four or more. A “full” acquisition before contrast administration is followed by truncated acquisitions sensitive primarily to changes in image contrast. Simulations of the point-spread functions that obtain, and their effect on contrast and spatial resolution, reveal significant degradation only for the smallest objects. Our simulations also address the feasibility of three-dimensional keyhole imaging, and demonstrate a potential 16-fold increase in temporal resolution. This suggests roles for keyhole imaging in conventional (nondynamic) precon-trast and postcontrast studies and other applications.     

Ultrafast imaging: Principles,pitfalls, solutions,and applications

Ultrafast MRI refers to efficient scan techniques that use a high percentage of the scan time for data acquisition. Often, they are used to achieve short scan duration ranging from sub‐second to several seconds. Alternatively, they may form basic components of longer scans that may be more robust or have higher image quality. Several important applications use ultrafast imaging, including real‐time dynamic imaging, myocardial perfusion imaging, high‐resolution coronary imaging, functional neuroimaging, diffusion imaging, and whole‐body scanning. Over the years, echo‐planar imaging (EPI) and spiral imaging have been the main ultrafast techniques, and they will be the focus of the review. In practice, there are important challenges with these techniques, as it is easy to push imaging speed too far, resulting in images of a nondiagnostic quality. Thus, it is important to understand and balance the trade‐off between speed and image quality. The purpose of this review is to describe how ultrafast imaging works, the potential pitfalls, current solutions to overcome the challenges, and the key applications. J. Magn. Reson. Imaging 2010;32:252–266. © 2010 Wiley‐Liss, Inc.     

K-space substitution: A novel dynamic imaging technique

A rapid dynamic imaging sequence has been developed in which only the 32 phase encoding steps that encode low spatial frequencies are collected for each dynamic image. These are substituted into a previously acquired, 128 × 128 raw data set prior to image reconstruction. In this way the dynamic information is retained while the overall appearance is improved in comparison with images obtained by zero filling to 128 × 128, leading to better qualitative evaluation. The limited k-space sampling means that the technique is most effective for large homogeneous areas of signal change since fine changes in contrast are imperfectly recorded.     

Parallel imaging reconstruction for arbitrary trajectories using k-space sparse matrices (kSPA).

Although the concept of receiving MR signal using multiple coils simultaneously has been known for over two decades, the technique has only recently become clinically available as a result of the development of several effective parallel imaging reconstruction algorithms. Despite the success of these algorithms, it remains a challenge in many applications to rapidly and reliably reconstruct an image from partially-acquired general non-Cartesian k-space data. Such applications include, for example, three-dimensional (3D) imaging, functional MRI (fMRI), perfusion-weighted imaging, and diffusion tensor imaging (DTI), in which a large number of images have to be reconstructed. In this work, a systematic k-space-based reconstruction algorithm based on k-space sparse matrices (kSPA) is introduced. This algorithm formulates the image reconstruction problem as a system of sparse linear equations in k-space. The inversion of this system of equations is achieved by computing a sparse approximate inverse matrix. The algorithm is demonstrated using both simulated and in vivo data, and the resulting image quality is comparable to that of the iterative sensitivity encoding (SENSE) algorithm. The kSPA algorithm is noniterative and the computed sparse approximate inverse can be applied repetitively to reconstruct all subsequent images. This algorithm, therefore, is particularly suitable for the aforementioned applications.     

A Modified EPI sequence for high‐resolution imaging at ultra‐short echo time

A robust modification of echo‐planar imaging dubbed double‐shot echo‐planar imaging with center‐out trajectories and intrinsic navigation (DEPICTING) is proposed, which permits imaging at ultra‐short echo time. The k‐space data is sampled by two center‐out trajectories with a minimal delay achieving a temporal efficiency similar to conventional single‐shot echo‐planar imaging. Intersegment phase and intensity imperfections are corrected by exploiting the intrinsic navigator information from both central lines, which are subsequently averaged for image reconstruction. Phase errors induced by inhomogeneities of the main magnetic field are corrected in k‐space, recovering the superior point‐spread function achieved with center‐out trajectories. The minimal echo time (<2 msec) is nearly independent of the acquisition matrix permitting applications, which simultaneously require high spatial and temporal resolution. Examples of demonstrated applications include anatomical imaging, BOLD‐based functional brain mapping, and quantitative perfusion imaging. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Detection of hepatocellular carcinoma: comparison of ferumoxides-enhanced and gadolinium-enhanced dynamic three-dimensional volume interpolated breath-hold MR imaging

The purpose was to compare the diagnostic accuracy of ferumoxides-enhanced MR imaging and gadolinium-enhanced dynamic MR imaging using three-dimensional (3D) volume interpolated breath-hold examination (VIBE) for the detection of hepatocellular carcinoma (HCC). Forty-nine patients with 61 HCCs, who underwent ferumoxides-enhanced and gadolinium-enhanced dynamic MR imaging, were included prospectively in this study. Ferumoxides-enhanced MR imaging was performed 24 h after completion of the dynamic study using 3D-VIBE. Three radiologists independently interpreted the images. The diagnostic accuracy was evaluated using the receiver-operating characteristic method, and the sensitivity of each imaging technique was compared using McNemars test. The mean diagnostic accuracy of dynamic MR imaging (Az=0.95) was higher than that of ferumoxides-enhanced MR imaging (Az=0.90), but failed to reach a statistical significance (P=0.057). The mean sensitivity of dynamic MR imaging (90.7%) was significantly superior to that of ferumoxides-enhanced MR imaging (80.9%, P=0.03). Furthermore, for lesions smaller than 15 mm, the mean sensitivity of dynamic MR imaging was significantly higher than that of ferumoxides-enhanced MR imaging (85.2% vs. 69.2%, P<0.05). Dynamic MR imaging showed a trend toward better diagnostic accuracy for than ferumoxides-enhanced MR imaging for the detection of HCCs.     

Dynamic MRI with projection reconstruction and KWIC processing for simultaneous high spatial and temporal resolution.

A method for dynamic imaging in MRI is presented that enables the acquisition of a series of images with both high temporal and high spatial resolution. The technique, which is based on the projection reconstruction (PR) imaging scheme, utilizes distinct data acquisition and reconstruction strategies to achieve this simultaneous capability. First, during acquisition, data are collected in multiple undersampled passes, with the view angles interleaved in such a way that those of subsequent passes bisect the views of earlier ones. During reconstruction, these views are weighted according to a previously described k-space weighted image contrast (KWIC) technique that enables the manipulation of image contrast by selective filtering. Unlike conventional undersampled PR methods, the proposed dynamic KWIC technique does not suffer from low image SNR or image degradation due to streaking artifacts. The effectiveness of dynamic KWIC is demonstrated in both simulations and in vivo, high-resolution, contrast-enhanced imaging of breast lesions.     

Gadobenate dimeglumine-enhanced liver MR imaging: value of dynamic and delayed imaging for the characterization and detection of focal liver lesions

The aim of this study was to determine the value of delayed-phase imaging (DPI) of gadobenate dimeglumine (Gd-BOPTA)-enhanced MR imaging for the evaluation of focal hepatic tumors compared with precontrast imaging and early dynamic phase imaging. The MR images were obtained in 48 patients with 98 focal hepatic tumors. Three-dimensional gradient-echo (GRE) imaging obtained before and 30, 60, and 1 h after administration of 0.1 mmol/kg of gadobenate dimeglumine. Each image set was analyzed qualitatively (lesion detection, conspicuity, delineation, and enhancement pattern on DPI) and quantitatively [signal-to-noise ratio (SNR), tumor–liver contrast-to-noise ratio (CNR)]. Improved lesion-to-liver contrast during the dynamic phase imaging was observed compared with precontrast images. The DPI showed a homogeneous enhancement of liver parenchyma and distinctive enhancement features of focal liver lesions: metastases (85%) showed a target shaped enhancement, and hepatocellular carcinomas (HCCs) showed an inhomogeneous (58%) or homogeneous enhancement (21%). The DPI showed better performance for the detection of metastases than other images by increasing lesion delineation (p<0.05). The absolute CNR of metastasis measured from periphery of the tumors on DPI was greater than precontrast and arterial phase imaging (p<0.05). The Gd-BOPTA during both dynamic and delayed phases provides valuable information for the characterization of focal liver lesions, and furthermore, Gd-BOPTA-enhanced DPI contributed to the improved detection of liver metastasis compared to precontrast and early dynamic imaging.     

Single‐shot T1 mapping using simultaneous acquisitions of spin‐ and stimulated‐echo‐planar imaging (2D ss‐SESTEPI)

The conventional stimulated‐echo NMR sequence only measures the longitudinal component while discarding the transverse component, after tipping up the prepared magnetization. This transverse magnetization can be used to measure a spin echo, in addition to the stimulated echo. Two‐dimensional single‐shot spin‐ and stimulated‐echo‐planar imaging (ss‐SESTEPI) is an echo‐planar‐imaging‐based single‐shot imaging technique that simultaneously acquires a spin‐echo‐planar image and a stimulated‐echo‐planar image after a single radiofrequency excitation. The magnitudes of the spin‐echo‐planar image and stimulated‐echo‐planar image differ by T₁ decay and diffusion weighting for perfect 90° radiofrequency and thus can be used to rapidly measure T₁. However, the spatial variation of amplitude of radiofrequency field induces uneven splitting of the transverse magnetization for the spin‐echo‐planar image and stimulated‐echo‐planar image within the imaging field of view. Correction for amplitude of radiofrequency field inhomogeneity is therefore critical for two‐dimensional ss‐SESTEPI to be used for T₁ measurement. We developed a method for amplitude of radiofrequency field inhomogeneity correction by acquiring an additional stimulated‐echo‐planar image with minimal mixing time, calculating the difference between the spin echo and the stimulated echo and multiplying the stimulated‐echo‐planar image by the inverse functional map. Diffusion‐induced decay is corrected by measuring the average diffusivity during the prescanning. Rapid single‐shot T₁ mapping may be useful for various applications, such as dynamic T₁ mapping for real‐time estimation of the concentration of contrast agent in dynamic contrast enhancement MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

k‐t PCA: Temporally constrained k‐t BLAST reconstruction using principal component analysis

The k‐t broad‐use linear acquisition speed‐up technique (BLAST) has become widespread for reducing image acquisition time in dynamic MRI. In its basic form k‐t BLAST speeds up the data acquisition by undersampling k‐space over time (referred to as k‐t space). The resulting aliasing is resolved in the Fourier reciprocal x‐f space (x = spatial position, f = temporal frequency) using an adaptive filter derived from a low‐resolution estimate of the signal covariance. However, this filtering process tends to increase the reconstruction error or lower the achievable acceleration factor. This is problematic in applications exhibiting a broad range of temporal frequencies such as free‐breathing myocardial perfusion imaging. We show that temporal basis functions calculated by subjecting the training data to principal component analysis (PCA) can be used to constrain the reconstruction such that the temporal resolution is improved. The presented method is called k‐t PCA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.