Browsing by Author "Irarrazaval, P"

Now showing 1 - 14 of 14
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    3D MR coronary artery segmentation
    (LIPPINCOTT WILLIAMS & WILKINS, 1998) Cline, HE; Thedens, DR; Irarrazaval, P; Meyer, CH; Hu, BS; Nishimura, DG; Ludke, S
    Coronary arteries are segmented from the blood pool using mathematical morphology operations from a 3D magnetic resonance spiral acquisition on a continuously breathing healthy volunteer. The segmented volume is maximal intensity projected at different views to yield coronary angiograms showing the left anterior descending artery (LAD), right coronary artery (RCA), and left circumflex artery (LCX), Magnetic resonance coronary angiography provides a retrospective rotating view of the coronary artery tree that complements oblique reformatted sections.
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    A correction algorithm for undersampled images using dynamic segmentation and entropy based focus criterion
    (ELSEVIER SCIENCE INC, 2002) Lisboa, JC; Guarini, M; Irarrazaval, P
    A post-processing technique is presented for correcting images undersampled in k-space. The method works by taking advantage of the image's background zeros (dynamically segmented through the application of a threshold) to extrapolate the missing k-space samples. The algorithm can produce good quality images from a small set of k-space frequencies with only a few iterations of simple matrix operations, using the image entropy as the focus criterion. It does not require any special patient preparation, extra pulse sequences, complex gradient programming or specialized hardware. This makes it a good candidate for any application that requires short scan times or where only few frequencies can be sampled. (C) 2002 Elsevier Science Inc. All rights reserved.
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    Computer reconstruction of pine growth rings using MRI
    (ELSEVIER SCIENCE INC, 2004) Morales, S; Guesalaga, A; Fernandez, MP; Guarini, M; Irarrazaval, P
    This work explores the use of magnetic resonance imaging (MRI) for nondestructive determination of wood characteristics and for 3D wood modeling. In this context, one of the applications under development is the automatic recognition and reconstruction of rings from transversal images obtained from MRI scanners. The algorithm analyzes a set of transversal MRI images, detecting and reconstructing growth ring edges. The information generated is then interpolated in order to obtain an accurate 3D picture of the log and its fundamental constituents (individual rings, knots, defects, etc). Results also show that the technique has potential for defect recognition, providing a powerful tool for future developments in wood analysis. The results are encouraging and further research is needed to develop automatic detection not only of rings, but also of different types of defects that are of paramount importance in the sawmill and plywood industries. (C) 2004 Elsevier Inc. All rights reserved.
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    Dynamic three-dimensional undersampled data reconstruction employing temporal registration
    (WILEY, 2005) Irarrazaval, P; Boubertakh, R; Razavi, R; Hill, D
    Dynamic 3D imaging is needed for many applications such as imaging of the heart, joints, and abdomen. For these, the contrast and resolution that magnetic resonance imaging (MRI) offers are desirable. Unfortunately, the long acquisition time of MRI limits its application. Several techniques have been proposed to shorten the scan time by undersampling the k-space. To recover the missing data they make assumptions about the object's motion, restricting it in space, spatial frequency, temporal frequency, or a combination of space and temporal frequency. These assumptions limit the applicability of each technique. In this work we propose a reconstruction technique based on a weaker complementary assumption that restricts the motion in time. The technique exploits the redundancy of information in the object domain by predicting time frames from frames where there is little motion. The proposed method is well suited for several applications, in particular for cardiac imaging, considering that the heart remains relatively still during an important fraction of the cardiac cycle, or joint imaging where the motion can easily be controlled. This paper presents the new technique and the results of applying it to knee and cardiac imaging. The results show that the new technique can effectively reconstruct dynamic images acquired with an undersampling factor of 5. The resulting images suffer from little temporal and spatial blurring, significantly better than a sliding window reconstruction. An important attraction of the technique is that it combines reconstruction and registration, thus providing not only the 3D images but also its motion quantification. The method can be adapted to non-Cartesian k-space trajectories and nonuniform undersampling patterns.
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    Fast magnetic resonance coronary angiography with a three-dimensional stack of spirals trajectory
    (WILEY, 1999) Thedens, DR; Irarrazaval, P; Sachs, TS; Meyer, CH; Nishimura, DG
    In this work, three-dimensional (3D) spiral imaging has been utilized for magnetic resonance coronary angiography, Spiral-based 3D techniques can dramatically reduce imaging time requirements compared with 3D Fourier Transform imaging. The method developed here utilized a "stack of spirals" trajectory, to traverse 3D k-space rapidly. Both thick-slab volumes encompassing the entire coronary tree with isotropic resolution and thin-slab volumes targeted to a particular vessel of interest were acquired. Respiratory compensation was achieved using the diminishing variance algorithm. TS-prepared contrast was also applied in some cases to improve contrast between vessel and myocardium, while off-resonance blurring was minimized by applying a linear correction to the acquired data. Images from healthy volunteers were displayed using a curved reformatting technique to view long segments of vessel in a single projection. The results demonstrate that this 3D spiral technique is capable of producing high-quality coronary magnetic resonance angiograms, (C) 1999 Wiley-Liss, Inc.
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    Fast three-dimensional k-space trajectory design using missile guidance ideas
    (WILEY, 2004) Mir, R; Guesalaga, A; Spiniak, J; Guarini, M; Irarrazaval, P
    Three-dimensional (3D) k-space trajectories are needed to acquire volumetric images in MRI. While scan time is determined by the trajectory efficiency, image quality and distortions depend on the shape of the trajectories. There are several 3D trajectory strategies for sampling the k-space using rectilinear or curve schemes. Since there is no evidence about their optimality in terms of image quality and acquisition time, a new design method based on missile guidance ideas is explored. Since air-to-air missile guidance shares similar goals and constraints with the problem of k-space trajectory design, a control approach for missiles is used to design a 3D trajectory. The k-space is divided into small cubes, and each one is treated as a target to be sampled. The main goal is to cover the entire space as quickly and efficiently as possible, with good performance under different conditions. This novel design method is compared to other trajectories using simulated and real data. As an example, a trajectory that requires 0.11 times the number of shots needed by the cylindrical 3DFT acquisition was designed. This trajectory requires more shots (1.66 times) than the stack of spirals, but behaves better under nonideal conditions, such as off-resonance and motion. (C) 2004 Wiley-Liss, Inc.
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    Flow properties of fast three-dimensional sequences for MR angiography
    (ELSEVIER SCIENCE INC, 1999) Irarrazaval, P; Santos, JM; Guarini, M; Nishimura, D
    To reduce the scan time of time of flight or phase contrast angiography sequences, fast three-dimensional k-space trajectories can be employed. The best 3D trajectory depends on tolerable scan time, readout time, geometric flexibility, flow/motion properties and others, A formalism for flow/motion sensitivity comparison based on the velocity k-space behavior is presented. It consists in finding the velocity k-space position as a function of the spatial k-space position. The trajectories are compared graphically by their velocity k-space maps, with simulations and with an objective computed index. The flow/motion properties of various 3D trajectories (cones, spiral-pr hybrid, spherical stack of spirals, 3DFT, 3D echo-planar, and shells) were determined. In terms of flow/motion sensitivity the cones trajectory is the best, however, it is difficult to use it for anisotropic resolutions or fields of view. Tolerating more flow sensitivity, the stack of spirals trajectory offers more geometric flexibility. (C) 1999 Elsevier Science Inc.
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    Magnetic resonance imaging for nondestructive analysis of wine grapes
    (AMER CHEMICAL SOC, 2004) Andaur, JE; Guesalaga, AR; Agosin, EE; Guarini, MW; Irarrazaval, P
    Magnetic resonance imaging (MRI) was used to study the growth and ripening of grape berries for three varieties. The results show that this technique allows the visualization of internal characteristics of berries using noninvasive procedures in order to obtain the volume and degreesBrix distribution within a cluster. Samples of Cabernet Sauvignon, Carmenere, and Chardonnay varieties collected over the 2002 season were analyzed. Calibration models were developed to correlate soluble solids (degreesBrix) against spin-lattice relaxation time t(1) and spin-spin relaxation time t(2). The correlation of degreesBrix and t(1) was R-2 = 0.75 for Cabernet Sauvignon, R-2 = 0.8 for Carmenere, and R-2 = 0.65 for Chardonnay. In the case of t(2) the correlation was significantly lower. Reconstruction techniques for the three-dimensional representation of clusters were developed, allowing an interactive visualization of the bunches. The method also provides volume measurements of single berries and their distribution within the cluster with an accuracy of 3% and R-2 = 0.98. These results show the potential of MRI in the wine industry for both monitoring and research. Not only does it provide quantitative information about the berries such as Volume and degreesBrix distributions, but it can also be used to support the sampling procedures by providing a better cluster characterization.
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    MRI fast tree log scanning with helical undersampled projection acquisitions
    (ELSEVIER SCIENCE INC, 2002) Contreras, I; Guesalga, A; Fernandez, MP; Guarini, M; Irarrazaval, P
    Magnetic Resonance Imaging opens an alternative way to analyze wood structures using a non-destructive technology. It provides high resolution, compound-based contrast manipulation and increased data acquisition flexibility. The technique is particularly useful for tree logs, since they present several characteristics that can be used to reduce the long scan time. This study proposes a method that takes advantage of the log cylindrical symmetry, acquiring transverse 1-D projections with a helical and undersampled pattern. Linear interpolation is used to estimate the skipped data and slice images are reconstructed by filtered backprojection. The sequence is improved using selective multi-pass scanning, without major variations of the scan time. Computer simulations and experimental results show that the proposed technique can increase the scan speed by a factor of 6, while maintaining the ability to identify typical tree log characteristics. (C) 2002 Elsevier Science Inc. All rights reserved.
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    Off-resonance correction using an estimated linear time map
    (ELSEVIER SCIENCE INC, 2002) Akel, JA; Rosenblitt, M; Irarrazaval, P
    Images acquired in the presence of magnetic field deviations and reconstructed without taking into account the off-resonance, are distorted and corrupted with artifacts. Several post-processing algorithms have been developed for correcting the distortion when it is not possible to fix the field inhomogeneities. These off-resonance correction methods are, in general, slow and computing intensive. To make them faster they are usually adapted to a particular situation or approximated. One of these approximations is to assume that the field map is linear. Although this assumption makes the algorithm fast and robust it is not well suited for arbitrary field maps. On the other hand, there are k-space trajectories with an almost linear time map (time at which each k-space value is acquired), such as 2DFT and EPI. This paper presents an algorithm for off-resonance correction based on a linear time map approximation. This approximation allows a fast algorithm that takes advantage of the almost linearity of the time map and uses the whole field map to correct the images. The proposed correction algorithm reduces the off-resonance induced artifacts while being fast. The linear approximation of the time map needs to be done only once for each trajectory because it does not depend on the acquired image or field map data. The method can also be extended to a multi-plane approximation for sequences with more complex time maps. (C) 2002 Elsevier Science Inc. All rights reserved.
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    Scan time reduction with an adaptive field of view
    (ELSEVIER SCIENCE INC, 2005) McLeish, K; Irarrazaval, P
    In magnetic resonance imaging (MRI), there is always a drive toward reducing the acquisition time. In volume imaging, time is often spent in acquiring data where there exists no signal because the imaging volume is larger than the object. In this paper, a method is presented for scan time reduction using an adaptive field of view (FOV). Multislice images are acquired with the FOV in the phase encoding direction of each slice determined by measurements made on the initial localization survey scan. Depending on the region of interest, an optimized FOV is also determined so that scan time is reduced in comparison to a normal scan while improving image resolution. The method is simple to implement and requires no additional hardware. Typical reductions in scan time are on the order 9-14%. (C) 2005 Elsevier Inc. All rights reserved.
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    Three dimensional k-space trajectory design using genetic algorithms
    (ELSEVIER SCIENCE INC, 2003) Sabat, S; Mir, R; Guarini, M; Guesalaga, A; Irarrazaval, P
    Image quality and total scan time in MRI are determined in large part by the trajectory employed to sample the Fourier space. Each trajectory has different properties like coverage of k-space, scan time, sensitivity to off-resonance conditions, etc. These properties are often contradictory, therefore a universal optimal trajectory does not exist and ultimately, it will depend on the image characteristics sought. Most trajectories used today are designed based on intuition and k-space analysis more than with optimization methods. This work presents a 3D k-space trajectory design method based on Genetic Algorithm optimization. Genetic Algorithms have been chosen because they are particularly good for searching large solution spaces. They emulate the natural evolutionary process allowing better offsprings to survive. The objective function searches the maximum of the trajectory's k-space coverage subject to hardware constraints for a fixed scanning time using the trajectory's torsion as its optimization variable.
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    Three-dimensional flow-independent peripheral angiography
    (WILEY-BLACKWELL, 1997) Brittain, JH; Olcott, EW; Szuba, A; Gold, GE; Wright, GA; Irarrazaval, P; Nishimura, DG
    A magnetization-prepared sequence, T-2-Prep-IR, exploits T-1, T-2, and chemical shift differences to suppress background tissues relative to arterial blood, The resulting flow-independent angiograms depict vessels with any orientation and flow velocity, No extrinsic contrast agent is required, Muscle is the dominant source of background signal in normal volunteers. However, long-T-2 deep venous blood and nonvascular fluids such as edema also contribute background signal in some patients. Three sets of imaging parameters are described to address patient-specific contrast requirements, A rapid, spiral-based, three-dimensional readout is utilized to generate high-resolution angiograms of the lower extremities. Comparisons with x-ray angiography and two-dimensional time-of-flight angiography indicate that this flow-independent technique has unique capabilities to accurately depict stenoses and to visualize slow flow and in-plane vessels.
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    Undersampling k-space using fast progressive 3D trajectories
    (WILEY, 2005) Spiniak, J; Guesalaga, A; Mir, R; Guarini, M; Irarrazaval, P
    In 3D MRI, sampling k-space with traditional trajectories can be excessively time-consuming. Fast imaging trajectories are used in an attempt to efficiently cover the k-space and reduce the scan time without significantly affecting the image quality. In many applications, further reductions in scan time can be achieved via undersampling of the k-space; however, no clearly optimal method exists. In most 3D trajectories the k-space is divided into regions that are sampled with shots that share a common geometry (e.g., spirals). A different approach is to design trajectories that gradually but uniformly cover the k-space. In the current work, successive shots progressively add sampled regions to the 3D frequency space. By cutting the sequence short, a natural undersampled method is obtained. This can be particularly efficient because in these types of trajectories the contribution of new information by later shots is less significant. In this work the performance of progressive trajectories for different degrees of undersampling is assessed with trajectories based on missile guidance (MG) ideas. The results show that the approach can be efficient in terms of reducing the scan time, and performs better than the stack of spirals (SOS) technique, particularly under nonideal conditions.