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Biomedical subjects

A Macovski

Publications and source records attributed to A Macovski.

At least 91 records · Page 5Linked to original sources
PubMed1980

Ultrasonic diagnostic instruments.

The underlying physical principles and current limitations of diagnostic ultrasonic instruments are reviewed. Recently developed ultrasonic imaging devices using pulsed-reflected ultrasound are discussed in detail. These instruments transmit short trains of 1.5- to 10-megahertz sound. Echoes reflected from tissue are converted to electrical signals, which are presented on a display device to outline the contour of tissues and organs within the body. The physical resolution of the system is dependent on several design factors in addition to the transmitted sound frequencies. A resolution volume of approximately 1.5 by 3 by 4 millimeters is achieved optimally with commercially available systems operating at 2.25 megahertz. The various instrument designs are described in the context of clinical usage. Because the sound is diffracted, refracted, and reflected, tghe imaging considerations are different from those of x-ray imaging. Diagnostic devices based on the Doppler principle are distinguished from pulsed-reflected ultrasonic instruments.

Auscultation↗
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↗
PubMed1978

Polychromatic streak artifacts in computed tomography images.

A series of artifacts in computed tomography (CT) images, consisting of streaks coming off edges and flares surrounding the object being scanned, is shown to be due to polychromaticity of the X-ray beam. These artifacts are seldom seen on conventional CT brain scans. They are helpful in understanding artifacts such as those seen around the petrous bone of the skull. Following this, a comparison is made between "single energy" and "dual energy" correction schemes for poly-artifacts. The theoretical assumptions underlying either method are analyzed, and the importance of these concepts in dual energy reconstructions is stressed. A new test phantom is suggested to evaluate different poly-correction schemes. All the simulated projection data in this study are noise free and are reconstructed using an industrial convolution-back projection technique. In addition, reconstructions are shown of the phantom using real data taken with and without a water bag path length compensator.

Mathematics↗
PubMed1976

Energy-selective reconstructions in X-ray computerized tomography.

All X-ray computerized tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. We show that for any material, complete energy spectral information may be summarized by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerized tomography techniques to calculate these constants at every point within a cross-section of an object. For comparable accuracy, patient dose is shown to be approximately the same as that produced by conventional systems. Possible uses of energy spectral information for diagnosis are presented.

Energy Transfer↗
PubMed1975

Real-time imaging with a new ultrasonic camera: part I, in vitro experimental studies on transmission imaging of biological structures.

Results are described of in vitro biological experimentation using a new through-transmission ultrasonic camera system developed at the Stanford Research Institute. The camera system produces real-time focused, orthographic images of a 15 times 15 cm field. The in vitro studies, which demonstrate the diagnostic potential of the new technique, include transmission images of selected excised organs such as liver, kidney, spleen, and uterus. In addition, preliminary studies evaluating ultrasonic image quality and the effect of out-of-focus structures are discussed.

Animals↗
PubMed1975

Real-time imaging with a new ultrasonic camera: part II, preliminary studies in normal adults.

A new transmission ultrasound camera system has provided focused images in real time through the abdomen of adult volunteers at average incident sound intensity levels of less than 1 mW/cm-2. The camera uses a unique acousto-mechanical lens/deflection system and a linear array of piezoelectric transducers to detect the ultrasound image. BONY STRUCTURES AND COSTAL CARTILAGES WERE CLEARLY SEEN. The colon was regularly seen and the kidney could be outlined in several subjects. The stomach and duodenal bulb were demonstrated and peristalsis was observed using a barium contrast material. The camera has the potential of extending the use of ultrasound in clinical medicine by providing an ultrasound soft tissue imaging system analogous to an x-ray fluoroscope. The need for further clinical evaluation and research to improve the images is emphasized.

Adult↗
PubMedDate not supplied

1991 I.I. Rabi Award. Estimating oxygen saturation of blood in vivo with MR imaging at 1.5 T.

The use of magnetic resonance (MR) imaging is investigated for noninvasively estimating the oxygen saturation of human blood (%HbO2) in vivo by means of relaxation characteristics identified in earlier MR spectrometry studies. To this end, a sequence is presented for determining the T2 of vascular blood in regions in which motions of the body and of the blood itself present a major challenge. With use of this sequence on a commercial 1.5-T whole-body imager, the relationship between the T2 and %HbO2 of blood is calibrated in vitro for the conditions expected in vivo. T2 varies predictably from about 30 to 250 msec as %HbO2 varies from 30% to 96%. T2 values measured in situ for vascular blood in the mediastinum of several healthy subjects qualitatively reflected the behavior observed in vitro. Estimates of %HbO2 for these vessels obtained with the in vitro calibration appear reasonable, particularly for venous blood, although difficulties arise in selecting the appropriate calibration factors. These encouraging initial results support a more systematic study of potential sources of error and an examination of the accuracy of in vivo measurements by comparison with direct measurements of %HbO2 in vessels.

Awards and Prizes↗
PubMedDate not supplied

Lipid-suppressed single- and multisection proton spectroscopic imaging of the human brain.

Spectroscopic images of the brain have great potential in disease diagnosis and treatment monitoring. Unfortunately, interfering lipid signals from subcutaneous fat and poor water suppression due to magnetic field inhomogeneities can make such images difficult to obtain. A pulse sequence that uses inversion recovery for lipid suppression and a spectral-spatial refocusing pulse for water suppression is introduced. In contrast to methods that eliminate fat signal by restricting the excited volume to lie completely within the brain, inversion-recovery techniques allow imaging of an entire section without such restrictions. In addition, the spectral-spatial pulse was designed to provide water suppression insensitive to a reasonable range of B0 and B1 inhomogeneities. Several data processing algorithms have also been developed and used in conjunction with the new pulse sequence to produce metabolite maps covering large volumes of the human brain. Images from single- and multisection studies demonstrate the performance of these techniques.

Brain↗
PubMedDate not supplied

Characterization of atherosclerosis with a 1.5-T imaging system.

It is shown that a conventional 1.5-T magnetic resonance (MR) imaging system can help characterize some of the key components of atherosclerotic plaque ex vivo. Fresh human aorta with atheromata was suspended in solutions of agarose and manganese chloride and heated to body temperature. The specimens were imaged with modified Dixon and projection-reconstruction imaging sequences. The specimens were then examined histologically to obtain direct correlation between images, spectra, and histologic characteristics. The results show that vessel wall and plaque components can be identified by means of their MR characteristics and correlated with their histologic appearance. The authors were able to identify normal vessel wall components, such as adventitial lipids and smooth muscle. They were also able to identify and localize plaque components such as fibrous tissue, calcification, lipids, and possible areas of hemorrhage and hemosiderin deposition.

Aged↗
PubMedDate not supplied

Clinical application of Compton and photo-electric reconstruction in computed tomography: preliminary results.

Although computed tomography has demonstrated some promise in the direction of quantative radiology, valuable information related to the varying response of tissues to x-rays of different energy is still not utilized on a routine basis. Advancements in a method proposed by Alvarez and Macovski for decomposing dual-spectra CT projection data into its material-dependent Compton and photo-electric components are described. Results are presented to demonstrate that such a separation can be performed. Reconstructed images of separated Compton and photo-electric data obtained from clinical scans are shown. With the improvements described, the Compton images begin to approach the quality of conventional reconstructions with evidence of improved polychromatic correction. The photo-electric data, while separable, suffers from unacceptable noise level. Analysis of this difficulty is presented, with recommendations for future improvement by careful selection of the effective energy of the low energy spectrum. The encouraging results suggest that this technique warrants continued development and evaluation.

Brain↗
PubMedDate not supplied

Intravenous angiography using scanned projection radiography: preliminary investigation of a new method.

The use of digital subtraction techniques combined with fluoroscopy has rekindled interest in arteriography using intranvenous injections of contrast media. A new method is proposed for intravenous angiography in which an x-ray source and xenon detector array from a computed tomographic (CT) scanner are used to scan a region of interest to produce projection image. In order to provide adequate visualization of small concentrations of iodine in blood vessels, a subtraction scheme is used to remove the contribution from overlapping soft tissue and bone. Initial experiments with a temporal subtraction algorithm on phantoms have demonstrated the ability to image simulated blood vessels of 1.7-mm diameter containing dilute diatrizoate with an iodine concentration of 3.7 mg/cc, at an exposure of less than 100 mR. Vascular structures 5-8 mm in diameter have been imaged in dogs with iodine concentrations of less than 37 mg/cc using temporal subtraction. Principal advantages of the method over other film or fluoroscopic subtraction techniques are: 1) wide dynamic range an low noise of the (CT) detectors, providing excellent iodine sensitivity; 2) high scatter rejection; and 3) efficient utilization of x-ray dose.

Angiography↗
PubMedDate not supplied

Noise analysis in isolation of iodine using three energies.

The Poisson noise inherent in multispectral x-ray imaging systems is formulated to give the SNR under a limited exposure constraint. The SNR value is maximized with respect to the exposure partition among different x-ray energies. The study shows how to choose suitable spectra and the dose weights. The robustness of these dose weights is also demonstrated for systems of varying bone and tissue contents.

Bone and Bones↗
PubMedDate not supplied

Information and artifact in computed tomography image statistics.

In conventional computed tomography images only the average CT number, which is a first-order statistical parameter, is used to characterize the tissues by giving an estimate of tissue density. Second order statistical parameters such as the signal variance and cross-correlation function have also been used to obtain additional information to discriminate between certain tissues and lesions. However, the contribution of quantum noise to the signal variance and cross-correlation function creates, for the conventional CT patient dose, a background signal often larger than the signal containing the information about tissue structure. The misleading information, called "artifacts", in second-order image statistics caused by quantum noise, is studied.

Information Theory↗