Biomedical subjects

A Macovski

Publications and source records attributed to A Macovski.

98 records · Page 6Linked to original sources

PubMedDate not supplied

Fast minimum variance estimator for limited angle CT image reconstruction.

Many applications of diagnostic cross sectional imaging require that images be reconstructed from a limited number of projections (limited angle). Convolution back projection has been unsuitable in these applications. Methods for reconstruction based on stochastic estimation theory, such as the minimum variance estimator, use a discrete linear measurement model and are suitable for limited angle reconstruction. Unfortunately, the computational requirements of these methods have precluded their use. In this paper, starting from the general minimum variance estimator x = RxyRyy-1y, a computationally efficient (fast) estimator is derived for limited angle reconstruction by choosing Rxy and Ryy in the simplest way consistent with the geometric considerations of data acquisition. Minimum variance has in the past been precluded from use by the large amount of computation required to compute Ryy-1. With the fast estimator, the computation is avoided because Ryy has a particular form that allows factorization of the matrix into a product of matrices, each of which is easily inverted. A demonstration of the estimator for the reconstruction of sharp peaks is provided. Image quality is similar to that obtained with other methods.

Models, Theoretical↗

PubMedDate not supplied

A method for selective tissue and bone visualization using dual energy scanned projection radiography.

Information contained in the x-ray energy spectrum can be used to produce selective radiographic images of bone or soft tissue. A method has been devised to separate bone and soft tissue based upon differences in photoelectric absorption and Compton scattering using an appropriate combination of images obtained with radiographic exposures at 70 KVP and 140 KVP. Since photoelectric absorption is highly dependent upon atomic number, high atomic number materials such as calcium can be easily separated from water density substances. Using a prototype system for line-scanned radiography, selective subtraction of bone or soft-tissue has been implemented. Because this method uses a conventional broad-spectrum x-ray source, it was necessary to develop a nonlinear polynomial approximation to estimate tissue and bone thickness. The model was verified with phantom studies using water and aluminum. The application of this dual-energy bone and soft-tissue separation to chest radiography is demonstrated. This method allows accurate estimation of tissue and bone thickness and should find application to chest radiography for improved lesion detection and for bone mineral assessment.

Animals↗

PubMedDate not supplied

Generalized image combinations in dual KVP digital radiography.

Dual energy basis decomposition techniques apply to single projection radiographic imaging. The high and low energy images are non-linearly transformed to generate two energy-independent images characterizing the integrated Compton/photoelectric attenuation components. Characteristic linear combinations of these two basis images identify unknown materials, cancel known materials, and generate synthesized monoenergetic images. The problems of intervening materials and material displacement are solved in general for a wide class of clinical imaging tasks. The basis projection angle identifies one from a family of energy selective imaging tasks, and such performance measures as the contrast enhancement factor (CEF) and signal to noise ratio (SNR) are expressed as functions of this angle. Algorithms for the decomposition of high and low energy measurements are compared and experimental images are included.

Computers↗

PubMedDate not supplied

Noise reduction methods for hybrid subtraction.

In digital subtraction angiography, hybrid subtraction provides selective vessel images free of soft-tissue motion artifacts but with a lower signal-to-noise ratio (SNR) than temporal subtraction images. An image processing method called measurement-dependent filtering has been developed to enhance the SNR of hybrid images without losing resolution or selectivity. Linear combinations of four images consisting of a pre- and postcontrast dual-energy measurement pair form both the hybrid image and a lower noise but less selective vessel image. The noise-reduced image is derived by combining the low-frequency components of the hybrid image with the high-frequency components of the lower noise image in a variety of ways. The results of the filtering method, when tested on both phantom and clinical data, display images with about the same degree of conspicuity as the hybrid image and a SNR approaching that of the temporal image.

Angiography↗

PubMedDate not supplied

Applications of time-varying gradients in existing magnetic resonance imaging systems.

This paper describes several applications of magnetic resonance imaging (MRI) with time-varying gradients within the framework of existing imaging systems. An alternative form of slice selection is shown where a time-varying gradient is used, during the reception interval, to isolate the slice of interest (the reconstruction of the slice itself is treated by the conventional techniques). This system allows for precise control of the slice, thinner slices (less than 1 mm) and the ability to select multiple, closely packed slices using a single imaging sequence. All this is done by postprocessing so the slice(s) of interest can be selected after the measurements have been completed. It is also shown how time-varying gradients can be used to generate the signals required for conventional projection-reconstruction and two-dimensional Fourier transform techniques (yet achieving higher resolution and using a resonant gradient system). In addition to cross-sectional imaging, this same approach provides a simple system for projection imaging of the entire volume. All of these techniques constitute a good starting point for the exploitation of time-varying gradients for fast, high-resolution MRI with existing imaging systems.

Filtration↗

PubMedDate not supplied

Vessel imaging using dual-energy tomosynthesis.

A method has been developed that combines dual-energy subtraction and tomosynthesis for vessel imaging in intravenous angiography. This paper describes the procedure for doing tomosynthesis on a fan-beam rotational-motion system and gives the point responses of the imaging system. Phantom studies show that dual-energy tomosynthesis improves the visualization of desired vessels lying on a selected plane. The results may be feasible for some applications such as clinical diagnosis of coronary artery diseases.

Coronary Angiography↗

PubMedDate not supplied

Dual-energy x-ray projection imaging: two sampling schemes for the correction of scattered radiation.

In addition to the familiar problems of reduced contrast and signal-to-noise ratio (SNR) in the single energy case, dual-energy subtractions in the presence of scattered radiation suffer further degradations from: (1) artifacts due to nonuniform subtraction of scatter, and (2) a serious deterioration of the signal of interest. To determine the expected performance of scatter correcting schemes, we simulated energy subtractions performed in the presence of scatter. We discuss scatter's detrimental effects on contrast and SNR in these simulations and the expected improvements from scatter corrections to within 5% to 10%. We introduce two sampling schemes for the correction of scatter. Each scheme requires two measurements, and each involves placing an x-ray opaque sampling grid between the source and the object. In the first method, the grid is an array of lead disks present only during one measurement. Using these samples we generate an estimate of the scatter field and then subtract it from the second measurement yielding a scatter corrected image. In the second method, the grid is an array of lead strips present during both measurements but displaced between measurements by one-half of a strip spacing to completely sample the image. From the two measurements we generate an image to be corrected, an estimate of the scatter field, and a scatter corrected image. In phantom studies implemented on a digital fluoroscopy system, we observed for single energy images of blood vessel phantoms improved contrast and field uniformity. For scatter corrected selective material cancellations in human phantoms we observed improved contrast and significant reduction in artifacts. In both cases we observed no significant loss in SNR. These results facilitate the implementation of efficient large area detectors for dual-energy imaging.

Algorithms↗

PubMedDate not supplied

Two interpolating filters for scatter estimation.

We have previously reported on a dual-measurement sample-and-estimate technique for scatter correction. In this paper, we present a scatter-correction technique that uses the previous sampling scheme but a different method of estimation. To provide samples of the scatter directly, an array of small, uniformly spaced lead disks is placed immediately before the object during only the first measurement. Interpolating from these samples we form an estimate of the scatter. We subtract this estimate from the second measurement to form a scatter-corrected image. Previously, we used least-squares interpolation to estimate the scatter. Because the samples are uniformly spaced, classical sampling theory motivated the investigation of interpolating filters for scatter estimation. To form the scatter image, we convolved the sample set with two different interpolating filters--a sinc function from classical sampling theory and a jinc function because the scatter function is radially symmetric. Using phantoms as objects, we applied both filters for scatter correction in vessel imaging and energy-subtraction imaging. Initial corrected images contained an artifact attributed to aliasing. We modified the filter widths to reduce the aliasing. Although improvements in image quality were measured and the artifact was less pronounced, the artifact was still present. We present the phantom results obtained with this class of filters and discuss methods for its improved performance.

Algorithms↗