J. Andrew Derbyshire

Centering the projection reconstruction trajectory: Reducing gradient delay errors†

The projection reconstruction (PR) trajectory was investigated for the effect of gradient timing delays between the actual and requested start time of each physical gradient. Radial trajectories constructed with delayed gradients miss the center of k‐space in an angularly dependent manner, causing effective echo times to vary with projection angle. The gradient timing delays were measured in phantoms, revealing delays on the x, y, and z gradients which differed by as much as 5 μsec. Using this one‐time calibration measurement, the trajectories were corrected for gradient delays by addition of compensatory gradient areas to the prephasers of the logical x and y readout gradients. Effective projection‐to‐projection echo time variability was reduced to less than 1 μsec for all imaging orientations. Using corrected trajectories, artifacts were reduced in phantom images and in volunteer studies. This correction should potentiate greater clinical use of the PR trajectory. Magn Reson Med 50:1–6, 2003. Published 2003 Wiley‐Liss, Inc.

Real‐time imaging of two‐dimensional cardiac strain using a harmonic phase magnetic resonance imaging (HARP‐MRI) pulse sequence

The harmonic phase (HARP) method provides automatic and rapid analysis of tagged magnetic resonance (MR) images for quantification and visualization of myocardial strain. In this article, the development and implementation of a pulse sequence that acquires HARP images in real time are described. In this pulse sequence, a CINE sequence of images with 1‐1 spatial modulation of magnetization (SPAMM) tags are acquired during each cardiac cycle, alternating between vertical and horizontal tags in successive heartbeats. An incrementing train of imaging RF flip angles is used to compensate for the decay of the harmonic peaks due to both T1 relaxation and the applied imaging pulses. The magnitude images displaying coarse anatomy are automatically reconstructed and displayed in real time after each heartbeat. HARP strain images are generated offline at a rate of four images per second; real‐time processing should be possible with faster algorithms or computers. A comparison of myocardial contractility in non‐breath‐hold and breath‐hold experiments in normal humans is presented. Magn Reson Med 50:154–163, 2003.

Undersampled projection reconstruction for active catheter imaging with adaptable temporal resolution and catheter‐only views

In this study undersampled projection reconstruction (PR) was used for rapid catheter imaging in the heart, employing steady‐state free precession (SSFP) contrast. Active catheters and phased‐array coils were used for combined imaging of anatomy and catheter position in swine. Real‐time imaging of catheter position was performed with relatively high spatial and temporal resolution, providing 2 × 2 × 8 mm spatial resolution and four to eight frames per second. Two interactive features were introduced. The number of projections (Np) was adjusted interactively to trade off imaging speed and artifact reduction, allowing acquisition of high‐quality or high‐frame‐rate images. Thin‐slice imaging was performed, with interactive requests for thick‐slab projection images of the signal received solely from the active catheter. Briefly toggling on catheter‐only projection images was valuable for verifying that the catheter tip was contained within the selected slice, or for locating the catheter when part of it was outside the selected slice. Magn Reson Med 49:216–222, 2003. Published 2003 Wiley‐Liss, Inc.

Unsupervised estimation of myocardial displacement from tagged MR sequences using nonrigid registration

We propose a fully automatic cardiac motion estimation technique that uses nonrigid registration between temporally adjacent images to compute the myocardial displacement field from tagged MR sequences using as inputs (sources) both horizontally and vertically tagged images. We present a new multisource nonrigid registration algorithm employing a semilocal deformation model that provides controlled smoothness. The method requires no segmentation. We apply a multiresolution optimization strategy for better speed and robustness. The accuracy of the algorithm is assessed on experimental data (animal model) and healthy volunteer data by calculating the root mean square (RMS) difference in position between the estimated tag trajectories and manual tracings outlined by an expert. For the ∼20000 tag lines analyzed (45 slices over 20–40 time frames), the RMS difference between the automatic tag trajectories and the manually segmented tag trajectories was 0.51 pixels (0.25 mm) for the animal data and 0.49 pixels (0.49 mm) for the human volunteer data. The RMS difference in the separation between adjacent tag lines (RMS_TS) was also assessed, resulting in an RMS_TS of 0.40 pixels (0.19 mm) in the experimental data and 0.52 pixels (0.56 mm) in the volunteer data. These results confirm the subpixel accuracy achieved using the proposed methodology. Magn Reson Med 2007.

Patient-Adaptive Reconstruction and Acquisition in Dynamic Imaging with Sensitivity Encoding (PARADISE)

MRI of the human heart without explicit cardiac synchronization promises to extend the applicability of cardiac MR to a larger patient population and potentially expand its diagnostic capabilities. However, conventional nongated imaging techniques typically suffer from low image quality or inadequate spatio‐temporal resolution and fidelity. Patient‐Adaptive Reconstruction and Acquisition in Dynamic Imaging with Sensitivity Encoding (PARADISE) is a highly accelerated nongated dynamic imaging method that enables artifact‐free imaging with high spatio‐temporal resolutions by utilizing novel computational techniques to optimize the imaging process. In addition to using parallel imaging, the method gains acceleration from a physiologically driven spatio‐temporal support model; hence, it is doubly accelerated. The support model is patient adaptive, i.e., its geometry depends on dynamics of the imaged slice, e.g., subjects heart rate and heart location within the slice. The proposed method is also doubly adaptive as it adapts both the acquisition and reconstruction schemes. Based on the theory of time‐sequential sampling, the proposed framework explicitly accounts for speed limitations of gradient encoding and provides performance guarantees on achievable image quality. The presented in‐vivo results demonstrate the effectiveness and feasibility of the PARADISE method for high‐resolution nongated cardiac MRI during short breath‐hold. Magn Reson Med, 2010.

Real-time blood flow imaging using autocalibrated spiral sensitivity encoding†

A novel spiral phase contrast (PC) technique was developed for high temporal resolution imaging of blood flow without cardiac gating. An autocalibrated spiral sensitivity encoding (SENSE) method is introduced and used to reconstruct PC images. Numerical simulations and a flow phantom study were performed to validate the technique. To study the accuracy of the flow measurement in vivo, a high‐resolution cardiac experiment was performed and a subset of undersampled SENSE reconstructed data were reconstructed. Good agreement between the velocity measurement from the fully‐sampled and undersampled data was achieved. Real‐time experiments were performed to measure blood velocity in the ascending aorta and aortic valve, and during a Valsalva maneuver. The results demonstrate the potential of this technique for real‐time flow imaging. Magn Reson Med, 2005. Published 2005 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. Magn Reson Med 47:53–60, 2002.

Virtual dye angiography: flow visualization for MRI-guided interventions.

In magnetic resonance imaging‐guided cardiovascular interventional procedures, it is valuable to be able to visualize blood flow immediately and interactively in selected regions. In particular, it is useful to assess normal or pathological communications between specific heart chambers and vessels. Phase‐contrast velocity mapping is not suitable for this purpose as it requires too much data and is not capable of determining directly if blood originating in one location travels to a nearby location. This article presents a novel flow visualization method called virtual dye angiography that enables visualization of blood flow analogous to selective catheter angiography. The method uses two‐dimensional radio frequency pulses to achieve interactive, intermittent, targeted saturation of a localized region of the blood pool. The flow of the saturated spins is observed directly on real‐time images or, in an enhanced manner, using ECG synchronized background subtraction. The modular nature of the technique allows for easy and seamless integration into a real‐time, interactive imaging system with minimal overhead. We present initial results in animals and in a healthy human volunteer. Magn Reson Med, 2011.

Visualization of active devices and automatic slice repositioning ("SnapTo") for MRI-guided interventions.

The accurate visualization of interventional devices is crucial for the safety and effectiveness of MRI‐guided interventional procedures. In this paper, we introduce an improvement to the visualization of active devices. The key component is a fast, robust method (“CurveFind”) that reconstructs the three‐dimensional trajectory of the device from projection images in a fraction of a second. CurveFind is an iterative prediction‐correction algorithm that acts on a product of orthogonal projection images. By varying step size and search direction, it is robust to signal inhomogeneities. At the touch of a key, the imaged slice is repositioned to contain the relevant section of the device (“SnapTo”), the curve of the device is plotted in a three‐dimensional display, and the point on a target slice, which the device will intersect, is displayed. These features have been incorporated into a real‐time MRI system. Experiments in vitro and in vivo (in a pig) have produced successful results using a variety of single‐ and multichannel devices designed to produce both spatially continuous and discrete signals. CurveFind is typically able to reconstruct the device curve, with an average error of approximately 2 mm, even in the case of complex geometries. Magn Reson Med 63:1070–1079, 2010.

HTGRAPPA: Real-time B1-weighted image domain TGRAPPA reconstruction†

The temporal generalized autocalibrating partially parallel acquisitions (TGRAPPA) algorithm for parallel MRI was modified for real‐time low latency imaging in interventional procedures using image domain, B1‐weighted reconstruction. GRAPPA coefficients were calculated in k‐space, but applied in the image domain after appropriate transformation. Convolution‐like operations in k‐space were thus avoided, resulting in improved reconstruction speed. Image domain GRAPPA weights were combined into composite unmixing coefficients using adaptive B1‐map estimates and optimal noise weighting. Images were reconstructed by pixel‐by‐pixel multiplication in the image domain, rather than time‐consuming convolution operations in k‐space. Reconstruction and weight‐set calculation computations were parallelized and implemented on a general‐purpose multicore architecture. The weight calculation was performed asynchronously to the real‐time image reconstruction using a dedicated parallel processing thread. The weight‐set coefficients were computed in an adaptive manner with updates linked to changes in the imaging scan plane. In this implementation, reconstruction speed is not dependent on acceleration rate or GRAPPA kernel size. Magn Reson Med, 2009.