Biomedical subjects

A Macovski

Publications and source records attributed to A Macovski.

At least 19 recordsLinked to original sources

PubMed1992

Inhomogeneity correction for in vivo spectroscopy by high-resolution water referencing.

One of the most common sources of distortion in in vivo spectroscopy is the inhomogeneity of the main magnetic field. This effect is particularly problematic when performing spectroscopic imaging, as the shim cannot be simultaneously optimized for all voxels. In this paper we present a technique to measure inhomogeneity rapidly, then show how to use the measurement to improve the analysis of the spectrum. This technique can be applied in conjunction with any spectroscopic localization method and any spectral quantitation algorithm. We present results from spectroscopic imaging of phantoms, then show application to a single-voxel water-suppressed proton brain study. We find that the quantitation of the in vivo spectrum is made immune to inhomogeneous line broadening.

Brain↗

PubMed1992

Fast angiography using selective inversion recovery.

We have developed an enhancement of selective inversion recovery that allows us to obtain high-resolution angiograms in reduced scan time. By applying several read pulses following each tagging inversion pulse, we can obtain several phase encodes in each cardiac cycle, thereby reducing the total scan time required for a complete image. Using this technique, high-resolution angiograms can be obtained in as little as 15 s. Because the phase encodes are collected in short bursts separated by long pauses, care must be taken to maintain uniform signal weighting across phase-encoding views and avoid ghosting. We use an increasing flip-angle sequence to equalize signal level weighting across the readouts. The phase encodes are collected in a special order to minimize ghosting. A postprocessing technique is used to further reduce signal nonuniformity between phase encodes. This fast angiography technique can significantly reduce artifacts due to patient motion during scanning and is especially useful for imaging vasculature in regions of the body where respiratory motion is a problem.

Algorithms↗

PubMed1992

Effects of RF amplifier distortion on selective excitation and their correction by prewarping.

In a magnetic resonance imaging system, an RF power amplifier is employed to boost an RF pulse to sufficient strength to excite the nuclear spins in a subject. The nonideal behavior of this amplifier distorts a selective-excitation pulse, and this distortion in turn degrades the slice profile. We have found two types of nonideal behavior particularly troublesome: nonlinearity and incidental phase modulation. One of their effects is the introduction of an unwanted "skirt" in the out-of-slice region of a slice profile. We present an effective method of correction in which a selective-excitation pulse is prewarped to compensate for the distortion.

Algorithms↗

PubMed1992

Two-dimensional selective adiabatic pulses.

Using the technique of separable k-space excitation, we have designed a two-dimensional selective adiabatic pulse that inverts magnetization from a square region in the xy plane with insensitivity to RF variations. We also have designed a two-dimensional adiabatic pulse that inverts selectively in frequency and in one spatial dimension. The pulses should be useful for both MR imaging and spectroscopy. We present experimental results to demonstrate that the two-dimensional adiabatic pulses are feasible on commercial MR imaging systems.

Humans↗

PubMed1992

Twisting radial lines with application to robust magnetic resonance imaging of irregular flow.

A problem with magnetic resonance angiograms of vessels containing irregular flow is that flow-induced dephasing can result in voids in the image. These void regions are susceptible to misinterpretation as regions of stenosis or other vessel pathology. Flow-induced dephasing can be minimized by using radial lines to cover k space. However, radial lines provide a very nonuniform, and hence, inefficient, coverage of k space. By twisting the outer portions of the radial trajectories, undistorted images of very rapid and turbulent flow can be obtained with a reasonable number of RF excitations.

Blood Flow Velocity↗

PubMed1992

Deblurring for non-2D Fourier transform magnetic resonance imaging.

For several non-2D Fourier transform imaging methods, off-resonant reconstruction does not just cause geometric distortion, but changes the shape of the point spread function and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. We introduce here a method that automatically removes blur introduced by magnetic field inhomogeneity and susceptibility without using a resonant frequency map, making these imaging methods more useful. In this method, the raw data are modulated to several different frequencies and reconstructed to create a series of base images. Determination of degree of blur is done by calculating a focusing measure for each point in each base image and a composite image is then constructed using only the unblurred regions from each base image. This method has been successfully applied to phantom and in vivo images using projection-reconstruction and spiral-scan sequences.

Algorithms↗

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↗

PubMed1979

Nonlinear polychromatic and noise artifacts in x-ray computed tomography images.

The variance of the image noise in computed X-ray transmission tomography (CT) due to quantum noise is in a first approximation a nonlinear function of X-ray attenuation. Beam hardening in CT is also a nonlinear function of attenuation. We present a theoretical study of both phenomena. Computer simulations and numerical results show that both nonlinear dependencies have quite similar effects on image quality. We also show how the two-dimensional distribution of the noise variance in a CT image is a weighted superposition of images obtained by backprojecting integer powers of the noiseless projection data corresponding to the scanned object. The streak-like pattern in the image noise due to the anisotropic nature of the noise cross-correlation function is discussed. We also discuss how these nonlinear phenomena affect noise filtering and tissue characterization using statistical parameters.

Electronics, Medical↗