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Biomedical subjects

A Macovski

Publications and source records attributed to A Macovski.

At least 55 records · Page 3Linked to original sources
PubMed1992

Pulsed saturation transfer contrast.

In vivo 1H conventional NMR image contrast generation usually relies on the macroscopic T1 and T2 relaxation parameters of the tissues of interest. Recently cross-relaxation related image contrast has been reported by Wolff and Balaban in animal models. Due primarily to the broad lineshape of the intended saturation spin pool and the use of off-resonance irradiation, high specific absorption rate and an auxiliary RF amplifier have been necessary to produce these images. The relatively long spin-lattice relaxation property of this spin pool, however, suggests the use of pulse methods to achieve saturation. In this paper, we show that short-T2 spin pools can be selectively saturated with short intense RF pulses. Cross-relaxation time constants can be measured using the technique of saturation recovery. In vivo magnetization-transfer-weighted images can be produced using pulses on commercial whole-body imagers without additional hardware.

Adipose Tissue↗
PubMed1992

Fast spiral coronary artery imaging.

A flow-independent method for imaging the coronary arteries within a breath-hold on a standard whole-body MR imager was developed. The technique is based on interleaved spiral k-space scanning and forms a cardiac-gated image in 20 heartbeats. The spiral readouts have good flow properties and generate minimal flow artifacts. The oblique slices are positioned so that the arteries are in the plane and so that the chamber blood does not obscure the arteries. Fat suppression by a spectral-spatial pulse improves the visualization of the arteries.

Adipose Tissue↗
PubMed1992

MR imaging of lung parenchyma: a solution to susceptibility.

The authors have developed a pulse sequence for imaging lung parenchyma with projection reconstruction magnetic resonance (MR) imaging that reduces the effects of motion and susceptibility. In this study, the projection reconstruction technique was further modified by optimizing MR signal frequencies for reconstructing the images. This was done by means of one of two methods. With the first method, a susceptibility map was derived from the raw image data and this map was used to indicate the optimal frequencies for reconstructing the images. The second method of susceptibility correction was a postprocessing technique in which the optimal reconstruction frequencies were selected with use of specific focusing criteria to generate the least blurred image. The effect of using susceptibility map correction on a phantom was demonstrated, and both of these methods were used to improve the visibility of pulmonary structures on images of subjects with normal and abnormal lungs.

Humans↗
PubMed1991

Flow-independent magnetic resonance projection angiography.

The performance of current, flow-based sequences for imaging vasculature using MR is severely restricted in regions with inherently slow flow. We address this problem with a flow-independent imaging method. Specifically, we generate projection images of blood in the limbs while suppressing signal from all other tissues (primarily skeletal muscle, bone marrow, and subcutaneous fat) using a flow-compensated, water-selective, short TI inversion recovery sequence with a long echo time. We experimentally evaluate the effectiveness of this sequence and present in vivo results clearly demonstrating the method's potential.

Adipose Tissue↗
PubMed1991

Multiple-readout selective inversion recovery angiography.

We have developed a variation of selective inversion recovery (SIR) angiography that allows us to obtain a collection of several angiograms within the same acquisition time previously required to obtain a single image. In basic SIR, a single readout is performed after the tagging inversion pulse. In multiple-readout SIR, a succession of readout pulses is applied following the inversion pulse. By varying the gradients appropriately during the successive readouts, we can obtain a set of multiple projection-angle angiograms, or, by appropriately spacing the readouts throughout the cardiac cycle, we can obtain a set of time-resolved angiograms. This technique allows us to obtain additional spatial or temporal information without increasing total scan time. A sequence of increasing flip-angle read pulses is used to maintain a constant signal level across the images. A trade-off exists between SNR and the number of images acquired.

Blood Vessels↗
PubMed1991

A reduced power selective adiabatic spin-echo pulse sequence.

We introduce a selective adiabatic pulse sequence suitable for generating selective spin-echoes for both MR imaging and spectroscopy. The technique is simple; one uses the echo generated by any pair of identical selective adiabatic inversion pulses. The nonlinear phase across the slice is compensated perfectly by the second pi pulse. This compensation is immune to RF inhomogeneity and nonlinearity. For imaging applications, we concentrate on a reduced-power version of the pulse sequence in which time is traded off variably for RF amplitude in the presence of a time-varying gradient. This technique, known as variable-rate excitation, mildly degrades the off-resonant slice profile when applied to amplitude-modulated pulses. We present theoretical explanations and experimental results that show that the variable-rate adiabatic pulses are immune to off-resonant degradation of the magnitude normally encountered in MR imaging.

Brain↗
PubMed1991

1H spectroscopic imaging using a spectral-spatial excitation pulse.

Excellent water suppression is required to perform in vivo 1H spectroscopic experiments. However water suppression is difficult due to both B0 and RF inhomogeneities. These inhomogeneities are particularly troublesome in spectroscopic imaging experiments where water suppression is required throughout some large region of interest. In this paper, we propose the use of spectral-spatial excitation pulses for such experiments. These two-dimensional pulses are shown to provide water suppression that is insensitive to a range of B0 and RF variations while simultaneously providing spatial localization. Experimental results including images (with voxel volumes ranging from 3.4 to 1.5 cc) of various brain metabolites from both a normal volunteer and a patient with a metastatic lung carcinoma are presented.

Brain↗
PubMed1991

Coronary angiography using fast selective inversion recovery.

Using a fast version of selective inversion recovery, we have obtained coronary angiograms of normal volunteers showing the proximal portions of the left coronary artery. Blood is tagged in the aortic root at end systole using a 2D inversion pulse. After a wash-in time of 300-600 ms, the coronary vessels are imaged with a 2- to 3-cm-thick slab in either axial or oblique projection. The scan is completed within a breathhold in 24 heartbeats.

Coronary Vessels↗
PubMed1991

Spectroscopic imaging with multidimensional pulses for excitation: SIMPLE.

Proton spectroscopy and spectroscopic imaging in the human brain require the elimination of both water and lipid signals. Strong lipid signals from subcutaneous fat are usually eliminated by confining the excited volume to lie wholly within the skull. Water suppression, however, can be difficult due to both B0 and RF inhomogeneities, which are particularly troublesome in imaging experiments where a relatively large region-of-interest (ROI) is typical. In this paper, we discuss the use of multidimensional selective-excitation pulses (e.g., pulses that are simultaneously selective along two axes) to both define the ROI and provide the necessary water suppression. Pulse sequences providing three-dimensional localization along with water suppression that is insensitive to a range of B0 and RF inhomogeneities are described. Spectra and spectroscopic images (voxel volume = 3.4 cc. acquisition time = 38 min) of various 1H metabolites from a patient with an astrocytoma show clear differences between normal and cancerous tissues and demonstrate the ability of these techniques to be used in vivo.

Astrocytoma↗
PubMed1991

Rapid, fully automatic, arbitrary-volume in vivo shimming.

MR spectroscopy and many MR imaging methods benefit from a well-shimmed magnet. We have developed a pulse sequence which enables fast and accurate measurement of three-dimensional field maps in vivo, and a data analysis package that allows calculation of shim currents to optimally shim arbitrary selected volumes. A data link to the shim power supply allows automatic update of currents. No intervention by the operator is required. Typical in vivo shimming time is less than 5 min. Performance analysis, phantom, and in vivo results are presented.

Brain↗
PubMed1991

Lung parenchyma: projection reconstruction MR imaging.

Magnetic resonance (MR) imaging of lung parenchyma is limited by the low proton density and short T2 in the lung as well as the effects of susceptibility and motion. The MR imaging appearance of lung parenchyma was investigated with a pulse sequence that offers some solutions to these problems. This sequence employs projection reconstruction (PR) acquisition gradients and a section-selective excitation pulse designed to eliminate the need to refocus and to allow low-frequency k-space data to be collected with minimal delay. Echo times as short as 50 microseconds can be achieved, producing a proton-density-weighted image. An excised inflated lung specimen and specimens from human subjects with normal lungs (n = 3), pulmonary arteriovenous malformations (n = 1), bronchogenic carcinoma (n = 1), and bullous lung disease with lung metastases (n = 1) were examined. Signal intensity from lung parenchyma and visibility of pulmonary structures were superior on images obtained with the PR MR imaging technique compared with spin-echo images.

Humans↗
PubMed1990

Simultaneous spatial and spectral selective excitation.

Using a k-space interpretation of small-tip excitation, a single excitation pulse has been designed that is simultaneously selective in space and resonant frequency. An analytic expression for the response of this pulse has been derived. The pulse has been implemented on a 1.5-T imaging system. The pulse has been applied to a rapid gradient-echo imaging sequence that forms both water and fat images within a breath-holding interval. These rapid images are free of the chemical shift artifacts at organ boundaries that typically afflict conventional rapid images. The pulse can be applied to a variety of other sequences, such as multislice water/fat sequences and rapid k-space scanning sequences.

Adipose Tissue↗
PubMed1989

Computing material-selective projection images in MR.

We detail a robust, general method for computing projection images of individual materials in a volume as linear combinations of MR projection images with different material-dependent weightings. Signal per unit volume for each material in each raw image is acquired directly for accurate cancellation of undesired, overlapping materials. The weighted sum of the input images is determined to maximize the signal-to-noise ratio (SNR) and minimize inhomogeneity effects in the material-selective images. We tested the implementation experimentally in both phantom and human studies, producing selective images with reasonable SNRs and material isolation. With further development of sequences to rapidly acquire input images having greater material differentiability, we envision the application of the selective projection imaging format to screening studies searching over large volumes for diseased tissues.

Humans↗
PubMed1989

Low-frequency restoration.

In magnetic resonance imaging, if the object of interest is known to be spatially bounded within the image field-of-view, then the high-intensity, low spatial frequencies can be determined by postprocessing. This allows the system receiver gain to be increased, thereby decreasing the quantitation noise from the analog-to-digital converters. No imaging sequence modifications are required.

Animals↗
PubMed1989

Water referencing for spectroscopic imaging.

A water referencing algorithm for addressing the spectroscopic imaging problems of low SNR and main field inhomogeneities is proposed. Using the location of the water peak from each voxel and additional a priori information results in a parametric estimation problem. Optimum estimates of the desired metabolite concentration can then be computed and displayed in an image format. The algorithm is shown to be very stable in the presence of noise and is insensitive to Bo inhomogeneity. A detailed error analysis as well as extensions to the basic data model are also discussed. Results from both 1H and 31P experiments are presented to verify the predicted good performance even with extremely low signal-to-noise ratio data.

Algorithms↗
PubMed1989

A fast spectroscopic imaging method using a blipped phase encode gradient.

Many methods of chemical shift imaging have been described recently. In most cases, these methods couple resolution and imaging time. The most flexible methods use time-varying gradients to cover a large region of k space on each excitation. We present here a new time-varying gradient method that offers a decrease in scan time (when SNR is sufficient), simplifies the reconstruction problem by retaining an essentially rectilinear sampling grid, and makes efficient use of scan time by minimizing gradient reversals. Implementation on a standard high-field imaging system (GE Signa) is discussed, and experimental results are shown. An application of the method to the generation of water reference data sets is described.

Fourier Analysis↗
PubMed1988

Considerations of magnetic resonance angiography by selective inversion recovery.

In the selective inversion recovery method for projection angiography, upstream blood is tagged by an inversion excitation and then allowed to flow into the imaged region. The subtraction of this first image from a second image acquired without the tagging leaves a signal from only the selectively tagged blood. Pulse sequence design involves consideration of the duration of the blood transit interval, excitation timing and cardiac gating, static material suppression, inversion excitation pulses, and flow compensation. Each of these considerations must be viewed with respect to the particular application. The method has demonstrated potential application to areas such as the carotid arteries, aortic arch, and peripheral vessels.

Arteries↗