David G. Kruger

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. Magn Reson Med 47:224–231, 2002.

Progenitor Cell Therapy in a Porcine Acute Myocardial Infarction Model Induces Cardiac Hypertrophy, Mediated by Paracrine Secretion of Cardiotrophic Factors Including TGFβ1

Administration of endothelial progenitor cells (EPC) is a promising therapy for post-infarction cardiac repair. However, the mechanisms that underlie apparent beneficial effects on myocardial remodeling are unclear. In a porcine model of acute myocardial infarction, we investigated the therapeutic effects of a mixed population of culture modified peripheral blood mononuclear cells (termed hereafter porcine EPC). Porcine EPC were isolated using methods identical to those previously adopted for harvest of EPC in human cell therapy studies. In addition the therapeutic effects of paracrine factors secreted by these cells was evaluated in vitro and in vivo. Intracoronary injection of autologous porcine EPC was associated with increased infarct territory mass and improved regional ventricular systolic function at 2 months compared to control. Treatment with conditioned media derived from autologous EPC was associated with similar improved effects on infarct territory mass and function. Histologic analysis of the infarct territory revealed significantly increased cardiomyocyte size in EPC and conditioned media treated groups, when compared to controls. A paracrine EPC effect was also verified in a pure myocardial preparation in which cardiomyocytes devoid of fibroblast, neuronal and vascular elements directly responded by increasing cell mass when exposed to the same conditioned media. Analysis of conditioned media revealed elevated levels of TGFbeta1 (human 267.3+/-11.8 pg/ml, porcine 57.1+/-6.1 pg/ml), a recognized mediator of hypertrophic signaling in the heart. Neutralizing antibodies to TGFbeta1 attenuated the pro-hypertrophic effect of conditioned media, and use of recombinant TGFbeta1 added to fresh media replicated the pro-hypertrophic effects of conditioned media in vitro. These data demonstrate the potential of paracrine factors secreted from endothelial progenitor cells to induce cardiomyocyte hypertrophy contributing to increased infarct territory LV mass, with favorable medium term effects on regional function following myocardial infarction.

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. Magn Reson Med 51:568–576, 2004.

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. Magn Reson Med 53:110–117, 2005.

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 (4× SENSE, 1.7× PF) consistently provided high‐diagnostic‐quality images with near 1‐mm isotropic resolution in acquisition times of <20 s. Magn Reson Med, 2006.

Correction for gradient nonlinearity in continuously moving table MR imaging

Recently, a number of methods have been demonstrated for large field of view MR imaging using continuous table motion. As with conventional, fixed‐table MRI, the spatial encoding is performed using magnetic field gradients. However, it is demonstrated in this work that as a consequence of every measurement being made at a slightly different displacement between the object and the gradient field, gradient nonlinearities are manifest as blurring in addition to spatial distortion. Moreover, the blurring is spatially dependent. It is also shown that correcting all phase‐encoding steps individually or in groups can reduce these effects. Phantom and in vivo results are shown which demonstrate the effectiveness of the correction. Magn Reson Med 52:181–187, 2004.

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 kY‐kZ 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). Magn Reson Med 55:000–000, 2006.

Intrinsic Signal Amplification in the Application of 2D SENSE Parallel Imaging to 3D Contrast-Enhanced Elliptical Centric MRA and MRV

The relative signal‐to‐noise ratio (SNR) provided by 2D sensitivity encoding (SENSE) when applied to 3D contrast‐enhanced MR angiography (CE‐MRA) is studied. If an elliptical centric phase‐encoding order is used to map the waning magnetization of the contrast bolus to k‐space, the application of SENSE will reduce the degree of k‐space signal modulation, providing a signal amplification A over corresponding nonaccelerated acquisitions. This offsets the SNR loss in R‐accelerated SENSE due to and the geometry (g) factor. The theoretical bound on A is R and is reduced from this depending on the properties of the bolus profile and the duration over which it is imaged. In this work a signal amplification of 1.14–1.23 times that of nonvascular background tissue is demonstrated in a study of 20 volunteers using R = 4 2D SENSE whole‐brain MR venography (MRV). The effects of a nonuniform g‐factor and inhomogeneity of background tissue are accounted for. The observed amplification compares favorably with the value of 1.31 predicted numerically from a measured bolus curve. Magn Reson Med 58:855–864, 2007.

Improved venous suppression and spatial resolution with SENSE in elliptical centric 3D contrast‐enhanced MR angiography

The elliptical centric (EC) view order samples a 3DFT acquisition from the center of k‐space outward, and when applied to contrast‐enhanced MR angiography (CE‐MRA) provides intrinsic venous suppression. This is because the veins enhance several seconds after the scan is initiated, and are thus encoded solely by noncentral k‐space frequencies. A separate method, sensitivity encoding (SENSE), accelerates the k‐space sampling rate by reducing the phase FOV or, equivalently, by increasing the k‐space sampling interval, and has been used to increase spatiotemporal resolution. We hypothesized that by combining SENSE with EC, sampling of central k‐space would be accelerated and the k‐space radius at which the veins first showed contrast enhancement would be increased over a reference scan, thus providing improved venous suppression and spatial resolution without additional scan time. This hypothesis was studied with the use of phantom and carotid CE‐MRA experiments, and the results demonstrated an approximate 25% reduction in venous signal when SENSE was used. Magn Reson Med 52:761–765, 2004.

Real-time imaging and triggering of 3D contrast-enhanced MR angiograms using MR fluoroscopy.

Although a variety of timing techniques and data acquisition strategies have been, used for three-dimensional contrast-enhanced MR angiography, many are still limited by inadequate overall reliability, limited spatial resolution, or complexity. A technique is presented in this work in which contrast arrival is detected in the targeted vasculature in real time using MR fluoroscopy. Upon detection the operator triggers a 3D MR angiographic acquisition which uses an elliptical centric view order. It is shown that the view order intrinsically provides a high degree of venous suppression which in turn allows acquisition times of 30 s or longer. permitting high spatial resolution. The reliability of fluoroscopic triggering in bolus detection is shown to be over 90%. The technique provides high quality contrast-enhanced MR angiograms for many vascular regions.