Search PubMed⌕ Search

Biomedical subjects

J M Pauly

Publications and source records attributed to J M Pauly.

At least 37 records · Page 2Linked to original sources
PubMed1998

One-shot spatially resolved velocity imaging.

For quantitative velocity measurement, we have developed a technique that acquires full velocity spectra without cardiac gating. After a cylindrical excitation restricts imaging to one spatial dimension, data are acquired while an oscillating gradient is played out. After each excitation, an image of velocity versus spatial location is obtained. For a given spatial location, a series of these images can be used to form an image of velocity versus time. Acquisition times are much shorter than for phase-contrast imaging or Fourier-encoded velocity imaging, obviating the need for cardiac gating. Although a two-shot version of this technique has been presented previously, we have developed a one-shot version that offers higher temporal resolution for a given velocity resolution and superior off-resonance properties.

Aorta, Thoracic↗
PubMed1998

Design of practical T2-selective RF excitation (TELEX) pulses.

Traditional T2-based imaging techniques are geared toward imaging long-T2 species. Traditional techniques are, therefore, not optimal in clinical situations where the information of interest lies in the short-T2 species. T2-selective RF excitation (TELEX) is a technique for obtaining a T2-based contrast that highlights short-T2 values while suppressing long-T2 values-opposite to traditional T2 contrast. Previously, TELEX has been demonstrated qualitatively to highlight only very short-T2 values (T2 approximately 0.001 s). When applied to longer T2 values (T2 > or = 0.01 s), TELEX becomes sensitive to deltaB0 non-uniformities. This restricts its application to problems in which the T2 of interest is very short. In this study, TELEX is characterized quantitatively. Furthermore, a bandwidth broadening scheme is developed that reduces the deltaB0 sensitivity of TELEX. This permits the technique to be applied to longer T2 values. The capabilities and limitations of a practical implementation of TELEX are discussed.

Humans↗
PubMed1998

New real-time interactive cardiac magnetic resonance imaging system complements echocardiography.

OBJECTIVES: We conducted an initial clinical trial of a newly developed cardiac magnetic resonance imaging (CMRI) system. We evaluated left ventricular (LV) function in 85 patients to compare the clinical utility of the CMRI system with echocardiography, the current noninvasive gold standard. BACKGROUND: Conventional CMRI systems require cardiac-gating and respiratory compensation to synthesize a single image from data acquired over multiple cardiac cycles. In contrast, the new CMRI system allows continuous real-time dynamic acquisition and display of any scan plane at 16 images/s without the need for cardiac gating or breath-holding. METHODS: A conventional 1.5T Signa MRI Scanner (GE, Milwaukee, Wisconsin) was modified by the addition of an interactive workstation and a bus adapter. The new CMRI system underwent clinical trial by testing its ability to evaluate global and regional LV function. The first group (A) consisted of 31 patients with acceptable echocardiography image quality. The second group (B) consisted of 31 patients with suboptimal echocardiography image quality. The third group (C) consisted of 29 patients with severe lung disease or congenital cardiac malformation who frequently have suboptimal echo study. Two independent observers scored wall motion and image quality using the standard 16-segment model and rank-order analysis. RESULTS: CMRI evaluation was complete in less than 15 min. In group A, no significant difference was found between ECHO and CMRI studies (p = NS). In group B, adequate visualization of wall segments was obtained 38% of the time using ECHO and 97% of the time using CMRI (p < 0.0001). When grouped into coronary segments, adequate visualization of at least one segment occurred in 18 of 30 patients (60%) with ECHO and in all 30 patients (100%) with CMRI (p < 0.0001). In group C, adequate visualization of the wall segments was obtained in 58% (CI 0.53-0.62) of the time using echocardiography and 99.7% (CI 0.99-1.0) of the time using CMRI (p < 0.0001). CONCLUSIONS: The new CMRI system provides clinically reliable evaluation of LV function and complements suboptimal echocardiography. In comparison with the conventional CMRI, the new CMRI system significantly reduces scan time, patient discomfort and associated cost.

Adolescent↗
PubMed1998

Magnetic resonance imaging of knee cartilage repair.

Cartilage injury resulting in osteoarthritis is a frequent cause of disability in young people. Osteoarthritis, based on either cartilage injury or degeneration, is a leading cause of disability in the United States. Over the last several decades, much progress has been made in understanding cartilage injury and repair. Magnetic resonance (MR) imaging, with its unique ability to noninvasively image and characterize soft tissue, has shown promise in assessment of cartilage integrity. In addition to standard MR imaging methods, MR imaging contrast mechanisms under development may reveal detailed information regarding the physiology and morphology of cartilage. MR imaging will play a crucial role in assessing the success or failure of therapies for cartilage injury and degeneration.

Cartilage, Articular↗
PubMed1997

Angiographic imaging with 2D RF pulses.

Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field-of-view imaging. A rectangular excitation was produced with an "echo-planar" k-space trajectory and a sinc-modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two-dimensional excitation. Aliasing artifacts in small field-of-view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.

Abdomen↗
PubMed1997

Reducing flow artifacts in echo-planar imaging.

Echo-planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, "partial flyback" is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k-space, only the even echoes of the EPI echo-train are used. Partial flyback is shown to improve the readout-flow properties at the expense of a slight worsening of the phase-encode flow and off-resonance properties. We recommend that the flyback region acquire 95% of the energy in k-space. Second, "inside-out" EPI is used to reduce artifacts arising from flow in the phase-encode direction. Data collection begins at the center of k-space, with separate interleaves to acquire the top and bottom, halves of k-space. Partial flyback is combined with partial-Fourier EPI and inside-out EPI. Partial-flyback inside-out EPI has worse off-resonance properties than partial-flyback partial-Fourier EPI but demonstrates better flow properties and does not require partial k-space reconstruction.

Heart↗
PubMed1997

Reduction of motion artifacts in cine MRI using variable-density spiral trajectories.

Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signal-to-noise ratio (SNR), single-shot imaging is not sufficient. Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of kappa-space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable-density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable-density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.

Artifacts↗
PubMed1997

Multifrequency interpolation for fast off-resonance correction.

Field inhomogeneities or susceptibility variations produce blurring in images acquired using non-2DFT k-space readout trajectories. This problem is more pronounced for sequences with long readout times such as spiral imaging. Theoretical and practical correction methods based on an acquired field map have been reported in the past. This paper introduces a new correction method based on the existing concept of frequency segmented correction but which is faster and theoretically more accurate. It consists of reconstructing the data at several frequencies to form a set of base images that are then added together with spatially varying linear coefficients derived from the field map. The new algorithm is applied to phantom and in vivo images acquired with projection reconstruction and spiral sequences, yielding sharply focused images.

Artifacts↗
PubMed1997

Improved automatic off-resonance correction without a field map in spiral imaging.

Non-2DFT k-space readout strategies are useful in fast imaging but prone to blurring when reconstructed off resonance. Field inhomogeneities or susceptibility variations, coupled with a long readout time, are the major sources of this artifact. Correction methods based on a priori off-resonance information such as an acquired field map have been proposed in the literature. An alternative approach estimates the spatially varying off-resonance frequency from the data itself before applying a correction. In this latter approach there is a trade-off between the extent of correction and the chance of increased artifact due to estimation error. This paper introduces an improved algorithm for field map estimation which is both faster and more robust than the existing method. It uses a multi-stage estimation of the field map, starting from a coarse estimate both in frequency and space and proceeds towards higher resolution. The new algorithm is applied to phantom and in vivo images acquired with radial and spiral sequences to give sharper images.

Abdomen↗
PubMed1997

MRI using piecewise-linear spiral trajectory.

A new generation of high power gradient systems which allow much faster MR imaging as well as shorter echo times has recently become available. Some of these high-speed gradient systems impose limits on the percentage of time during which the gradient can change in amplitude (slewing duty cycle). While this limitation may be immaterial to many 2DFT and echo planar imaging methods, a traditional circular spiral trajectory is difficult to use on these systems because its gradient waveforms change during the entire course of the trajectory so that the slewing duty cycle during the readout period is 100%. We describe a piecewise-linear spiral trajectory which is composed of linear segments and rounded corners. This trajectory reduces the slewing duty cycle while maintaining the desirable imaging properties of circular spirals including interleaving by simple gradient rotation. For one representative example, the slewing duty cycle is reduced to 46%. A conventional gridding method was used for image reconstruction, but a new numerical algorithm to calculate the density compensation factor was required. Use of piecewise-linear spiral trajectories reduces the impact imposed by limited gradient slewing duty cycle.

Image Processing, Computer-Assisted↗
PubMed1997

Real-time interactive MRI on a conventional scanner.

A real-time interactive MRI system capable of localizing coronary arteries and imaging arrhythmic hearts in real-time is described. Non-2DFT acquisition strategies such as spiral-interleaf, spiral-ring, and circular echo-planar imaging provide short scan times on a conventional scanner. Real-time gridding reconstruction at 8-20 images/s is achieved by distributing the reconstruction on general-purpose UNIX workstations. An X-windows application provides interactive control. A six-interleaf spiral sequence is used for cardiac imaging and can acquire six images/s. A sliding window reconstruction achieves display rates of 16-20 images/s. This allows cardiac images to be acquired in real-time, with minimal motion and flow artifacts, and without breath holding or cardiac gating. Abdominal images are acquired at over 2.5 images/s with spiral-ring or circular echo-planar sequences. Reconstruction rates are 8-10 images/s. Rapid localization in the abdomen is demonstrated with the spiral-ring acquisition, whereas peristaltic motion in the small bowel is well visualized using the circular echo-planar sequence.

Abdomen↗
PubMed1997

Short echo time projection reconstruction MR imaging of cartilage: comparison with fat-suppressed spoiled GRASS and magnetization transfer contrast MR imaging.

PURPOSE: To evaluate short echo time (TE) projection reconstruction magnetic resonance (MR) imaging in the detection of cartilage lesions. MATERIALS AND METHODS: Twenty-seven cartilage regions of 10 human patellar specimens were examined with the following MR sequences: short TE projection reconstruction (repetition time msec/TE msec, 400/0.15), fat-suppressed three-dimensional spoiled gradient-recalled acquisition in the steady state (Spoiled GRASS) (50/10, 60 degrees flip angle), and magnetization transfer contrast (MTC) subtraction (400/6). MR findings were correlated with histopathologic grading of the cartilage. RESULTS: For detection of cartilage lesions, sensitivity of projection reconstruction imaging (100%) was significantly greater (P = .03) than that of MTC (62%) but not significantly greater (P > .05) than that of Spoiled GRASS (81%) imaging. Accuracy of projection reconstruction was significantly greater than that of MTC (P = .004) and Spoiled GRASS (P = .03) imaging. Unmasking of collagen fibers was most predictive of abnormal signal intensity of the cartilage with all sequences. CONCLUSION: In vitro, short TE projection reconstruction MR imaging provides superior delineation of cartilage lesions when compared with two other sequences. On Spoiled GRASS and MTC images, signal intensity of the superficial layer of cartilage is not a reliable sign for surface integrity.

Aged↗
PubMed1996

Decomposition of inflow and blood oxygen level-dependent (BOLD) effects with dual-echo spiral gradient-recalled echo (GRE) fMRI.

Image contrast with gradient-recalled echo sequences (GRE) used for fMRI can have both blood oxygen level-dependent (BOLD) and inflow components, and the latter is often undesirable. A dual-echo technique can be used to differentiate these mechanisms, because modulation of signal from inflow is common to both echoes, whereas susceptibility and diffusion-related signal losses are larger in the second echo. An efficient dual-echo interleaved spiral sequence was developed for use with a conventional scanner. It uses a k-space trajectory that spirals out from the origin while the first echo is collected, then spirals back in while collecting the second echo. Decomposition of the data provides separate images of the inflow and T2-weighted components. Results demonstrate the decomposition with phantom experiments and with photic stimulation in normal volunteers.

Brain Mapping↗
PubMed1996

Diffusion-weighted interleaved echo-planar imaging with a pair of orthogonal navigator echoes.

This work describes a diffusion-weighted (DW) interleaved echo-planar imaging (IEPI) method for use on either conventional whole-body scanners or scanners equipped with high-speed gradient and receiver hardware. In combination with cardiac gating and motion correction with a pair of orthogonal navigator echoes, the presented method is time-efficient, compensates for patient motions, and is less sensitive to image distortions than single-shot methods. The motion-correction scheme consists of correction for constant and linear phase terms found from the orthogonal navigator echoes. The correction for the linear phase term in the phase-encode direction includes gridding the data to the Cartesian grid. The DW IEPI was used to image a phantom rotating about the slice-select direction, and motion correction was performed to eliminate ghost artifacts arising from motion in either the readout- or phase-encoding directions. DW IEPI with motion correction was performed on a normal volunteer and on a patient with a 26-day-old region of ischemia over much of the right hemisphere.

Algorithms↗
PubMed1995

Magnetic resonance fluoroscopy using spirals with variable sampling densities.

The imaging of dynamic processes in the body is of considerable interest in interventional examinations as well as kinematic studies, and spiral imaging is a fast magnetic resonance imaging technique ideally suited for such fluoroscopic applications. In this manuscript, magnetic resonance fluoroscopy pulse sequences in which interleaved spirals are used to continuously acquire data and reconstruct one movie frame for each repetition time interval are implemented. For many applications, not all of k-space needs to be updated each frame, and nonuniform k-space sampling can be used to exploit this rapid imaging strategy by allowing variable update rates for different spatial frequencies. Using the appropriate reconstruction algorithm, the temporal updating rate for each spatial frequency is effectively proportional to the corresponding k-space sampling density. Results from a motion phantom as well as in in vivo gadolinium diethylenetriaminopentaacetic acid (Gd-DTPA) bolus tracking studies in a rat model demonstrate the high temporal resolution achievable using these techniques as well as the tradeoffs available with nonuniform sampling densities. This paper focuses on the acquisition of real-time dynamic information, and all images presented are reconstructed retrospectively. The issues of real-time data reconstruction and display are not addressed.

Animals↗
PubMed1995

Spiral imaging on a small-bore system at 4.7T.

Spiral imaging has a number of advantages for ultrafast data acquisition. However, implementation on high-field small-bore systems requires carefully addressing the issues of inhomogeneity-induced blurring and gradient hardware constraints. In this paper, spiral imaging on a 40-cm-bore 4.7T CSI Omega System (Bruker Instruments) is discussed. A constant-voltage gradient waveform design algorithm is developed to reduce readout times as well as minimize waveform distortions due to gradient amplifier nonlinearities. Residual errors are then measured and taken into account in the image reconstruction procedure. Multiple spiral interleaves as well as a multifrequency reconstruction algorithm are used to decrease blurring of off-resonance spins. Both phantom and in vivo images demonstrate the performance of the resulting pulse sequences.

Adipose Tissue↗
PubMed1995

MR spectroscopic imaging of collagen: tendons and knee menisci.

Water molecules associated with collagen have short transverse (T2) relaxation times. Projection-reconstruction techniques are able to achieve an echo time (TE) much shorter than conventional techniques, allowing imaging of tissues with T2 < 5 ms. Using these techniques, a conventional 1.5-T MRI human imaging system can directly image collagen-associated water from knee menisci and tendons in normal volunteers and patients. Long-T2 suppression improves the contrast between these structures and the surrounding tissue with long-T2 relaxation times. Spectroscopic imaging provides improved lipid/water registration and information about chemical composition and relaxation times. Direct imaging of tendons and menisci may provide more information about these structures and provide a new way to assess both injury and repair.

Collagen↗