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Biomedical subjects

A Macovski

Publications and source records attributed to A Macovski.

At least 73 records · Page 4Linked to original sources
PubMed1988

Noise reduction in magnetic resonance imaging.

This paper describes a noise-reduction technique applicable to multiple-measurement systems. This method, known as measurement-dependent filtering (or MDF), can be used to advantage in a number of MRI applications. We present the general theory for one of these applications, material-canceled projection imaging. We discuss and show the results of MDF for material-canceled images as well as for heavily T2-weighted spin-echo images and computed T2 images. Significant improvements in SNR are demonstrated while spatial resolution is preserved.

Electricity↗
PubMed1988

Noise-reduced prostatic MR imaging. Work in progress.

Measurement-dependent filtering, a nonlinear noise-reduction technique, was used to improve the signal-to-noise ratio of in vivo T2-weighted magnetic resonance images of the prostate gland. In both normal and abnormal prostates, the technique considerably reduced noise in T2-weighted images. The technique may provide more accurate depiction of regions of benign prostatic hyperplasia and carcinoma in the prostate.

Filtration↗
PubMed1987

MR angiography by selective inversion recovery.

A modified inversion-recovery sequence is introduced which performs subtraction angiography by varying time-of-flight effects of blood flowing into an imaged slab. The selective 180 degrees excitation inverts different regions between measurements to isolate arterial and/or venous blood. On normal human subjects, high-resolution carotid artery angiograms have been obtained.

Carotid Arteries↗
PubMed1986

In vivo spectroscopic magnetic resonance imaging using estimation theory.

The ability to map spectroscopic components for in vivo application is a highly desirable goal. Thusfar it has been unavailable because of the low SNR inherent in the measurement of each voxel. In this paper we deal with this low SNR in two ways. First, estimation theory, using a priori data, is used to estimate the amplitudes in each voxel. Second, the resultant estimates are presented in an image format so that they are readily correlated with anatomical and physiological patterns. A computer simulation is presented of a case where the SNR is -10 dB. At this level, conventional Fourier transform spectroscopy provides meaningless results. Using estimation theory and an imaging format, a simulated lesion is readily seen.

Computers↗
PubMed1985

Volumetric NMR imaging with time-varying gradients.

A novel generalized processing system is described whereby, using time-varying gradients, simultaneous data acquisition can be provided for all points in a volume. This is in sharp distinction to the sensitive-point method where only the null region is imaged. It is shown that time-varying gradients systems provide unique phase modulation functions which, when appropriately decoded, enable every point in the volume to be simultaneously imaged from a single FID signal. Methods are shown using the technique to provide simultaneously acquired, localized NMR spectra. Alternative gradient signals are considered including both periodic and aperiodic waveforms.

Magnetic Resonance Spectroscopy↗
PubMed1984

An implementation of dual energy CT scanning.

We have described a prereconstruction method for dual energy (PREDECT) analysis of CT scans. In theory, this method can (a) eliminate beam hardening and produce an accuracy comparable with monoenergetic scans and (b) provide the effective atomic number and electron density of any voxel scanned. Our implementation proves these statements and eliminates some of the objectionable noise. We constructed a phantom with a cylindrical sleeve-like compartment containing known amounts of high atomic number material simulating a removable skull. Conventional scans, with and without this beam hardener, were done of a water bath containing tubes of high electron and high atomic number material. Dual energy scans were then done for PREDECT. To increase the effective separation of the low and high energy beams by using more appropriate tube filtration, we fabricated a beam filter changer containing erbium, tungsten, aluminum, and steel. We used erbium, tungsten, and steel at high energy and aluminum, steel, and erbium at low energy for data acquisition. The reconstructions were compared visually and numerically for noise levels with the original steel only filtration. We found a decrease in noise down to approximately one-half the prior level when erbium/aluminum or tungsten/aluminum replaced the steel/steel filter. Erbium and tungsten were equally effective. Steel/erbium and steel/aluminum also significantly reduced image noise. The noise in the photoelectric (P) and Compton (C) images is negatively correlated. At any pixel, if the noise is positive in the P image, it is most probably negative in the C. Using this fact, the noise was reduced by postreconstruction processing.

Humans↗
PubMed1983

In vivo stochastic ultrasonic characterisation of abdominal tissues.

Two experimental systems have been used for digitization of waveforms backscattered from normal and pathologic abdominal tissues in vivo. Initial studies were performed with a digitization system which allowed only A-mode display of areas of interest before and during waveform recording. A new interactive system allows for digitization of waveforms from areas of interest in abdominal organs, selected with the aid of real-time B-mode images of a mechanical sector scanner. Statistical parameters of these waveforms, derived in the amplitude and frequency domains, show potential utility for diagnosis of diffuse diseases of the liver and spleen.

Abdomen↗
PubMed1982

Stochastic frequency-domain tissue characterization: application to human spleens "in vivo'.

Ultrasonic waveforms backscattered from normal and pathologic human spleens were recorded in vivo using a high-speed digital data acquisition system. A frequency-domain analysis was performed for 50 digitized waveforms from each spleen, in which autocorrelations of frequency spectra were employed to measure the mean spacing among adjacent tissue scatterers. Mean scatterer spacing for normal spleens averaged 1.10 +/- 0.14 mm, whereas abnormal, diffusely of these measurements are in accord with those expected from splenic histology. The techniques described may be valuable in ultrasonic diagnosis of the spleen and other organs.

Adult↗
PubMed1982

Renal imaging with dual energy projection radiography.

Applications of dual energy radiography to renal imaging were evaluated using an experimental system for line-scanned projection radiography. This system combines digital radiographs simultaneously obtained at two differing x-ray energies (85 and 135 kVp) to create images in which materials of a desired mean atomic number are selectively cancelled. A preliminary evaluation of the technique was performed for three renal imaging problems: detection and characterization of renal masses of low radiographic contrast, evaluation of renal artery stenosis and unilateral renal ischemia, and detection of renal calculi. In all three applications, the dual energy technique produced results indicating potential advantages over conventional radiographic studies.

Adenocarcinoma↗
PubMed1981

Initial results with prereconstruction dual-energy computed tomography (PREDECT).

Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstructions that accurately separate the total attenuation coefficient into two values, representing the Compton and photoelectric contributions. The images are obtained without polychromatic distortion and with an acceptable dose. PREDECT was used to scan objects and solutions of varying atomic number and electron density, specimens of normal and pathologic brain and other body tissues, and nine patients. The values for Compton and photoelectric attenuation of the different specimens were distinctive enough to provide "tissue signatures" of potential clinical usefulness. Eight of the nine patients studied provided acceptable images, which produced some tissue characterization. PREDECT appears to represent an advance over the previously used postreconstruction methods; areas of greatest potential are differential diagnosis, improved detection of abnormalities, and elimination of the polychromatic artifact.

Adult↗
PubMed1981

Intravenous arteriography using scanned projection radiography.

Visualization of arteries using intravenous injections of contrast material requires an imaging system capable of (a) excellent sensitivity to dilute concentrations of contrast media and (b) adequate temporal resolution to minimize the effects of motion during the exposure. A line-scanned radiography system based on a CT fan-beam detector (high-pressure xenon ionization chamber) was used for phantom and animal studies of intravenous arteriography to demonstrate the potential advantages of this method. Even though line-scanned systems require long scan times compared to existing radiographic methods, they are capable of showing rapidly moving arteries without blurring. Concentrations of 3 mg/ml of iodine could be seen in vessels 2 mm in diameter. Phantom studies using kVp switching showed that bone or soft tissue could be selectively cancelled.

Angiography↗
PubMed1981

Ultrasonic characterization of abdominal tissues via digital analysis of backscattered waveforms.

Ultrasonic waveforms backscattered from normal and abnormal livers and spleens were recorded in vivo with the aid of a high-speed data acquisition system. Frequency-domain analysis of the digitized waveforms was employed to derive values for mean spacing of ultrasonic scatterers in tissue. Amplitude-domain analysis yielded measures of mean amplitude and variance, an indication of the distribution of amplitude values. Studies indicated that cirrhotic liver is characterized by a higher mean amplitude, a broader amplitude distribution, and a larger mean scatterer spacing than normal liver. Amplitude- and frequency-domain statistics for livers with other diffuse disease processes or tumor, as well for abnormal spleens. also differed from those of normal tissues. These data give information which may not be evident from B-mode sonograms, and which appears to be related to histological features.

Computers↗