William R. Brody

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.

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.

Iterative convolution backprojection algorithms for image reconstruction from limited data

Image-reconstruction algorithms implemented on existing computerized tomography (CT) scanners require the collection of line integrals that are evenly spaced over 360 deg. In many practical situations, some of the line integrals are inaccurately measured or are not measured at all. In these limited-data situations, conventional algorithms produce images with severe streak artifacts. Recently, several other image-reconstruction algorithms were suggested, each tailored to a specific type of limited-data problem. These algorithms make minimal use of a priori knowledge about the image; only one has been demonstrated with real x-ray data. We present a new operator framework that treats all types of limited-data image-reconstruction problems in a unified way. From this framework we derive iterative convolution backprojection algorithms that make no restrictions on the location of missing line integrals. All available a priori information is incorporated by constraint operators. The algorithm has been implemented on a commercial CT scanner. We present examples of images reconstructed from real x-ray data in two limited-data situations and demonstrate the use of additional a priori information to reduce streak artifacts further.

Digital Subtraction Angiography

Digital subtraction angiography (DSA), utilizing computerized x-ray imaging equipment for image acquisition and subtraction, will provide substantial improvements in blood vessel imaging. With the high contrast sensitivity of these systems, visualization of major blood vessels using intravenous injections of radiographic contrast media is possible on a routine basis. Current technology, using digitized video fluoroscopy and temporal subtraction, may in the future be upgraded using energy subtraction, or a combination of time and energy (hybrid subtraction). Applications of line-scanned radiography to DSA have been demonstrated but improvements in frame rate and resolution of the line-scanned technology is needed for future systems.

Iterative Reconstruction-Reprojection: An Algorithm for Limited Data Cardiac-Computed Tomography

Cardiac X-ray computed tomography (CT) has been limited due to scanning times which are considerably longer (1 s) than required to resolve the beating heart (0.1 s). The otherwise attractive convolution-backprojection algorithm is not suited for CT image reconstruction from measurements comprising an incomplete set of projection data. In this paper, an iterative reconstruction-reprojection (IRR) algorithm is proposed for limited projection data CT image reconstruction. At each iteration, the missing views are estimated based on reprojection, which is a software substitute for the scanning process. The standard fan-beam convolution-backprojection algorithm is then used for image reconstruction. The proposed IRR algorithm enables the use of convolution-backprojection in limited angle of view and in limited field of view CT cases. The potential of this method for cardiac CT reconstruction is demonstrated using computer simulated data.

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.

The use of computed tomography in the diagnosis of coronary artery bypass graft patency.

In a preliminary evaluation of the use of dynamic computed tomography (CT) for the detection of patent aortocoronary bypass grafts, 30 patients were scanned either during the early postoperative peroid or after graft patency was determined by angiography. To visualize the proximal grafts, CT scans were taken through the aortic root following an intravenous bolus injection of contrast medium. Patent bypass grafts to the left anterior descending and right coronary arteries were demonstrated in 77.5%, while posterior grafts to the circumflex and obtuse marginal coronary arteries were detected in 40%. With its pontential for extracting dynamic events, CT scanning provides a new, noninvasive modality for the diagnosis of patent bypass grafts, which heretofore have only been visualized by selective angiography.

An Experimental Evaluation of Central vs. Peripheral Injection for Intravenous Digital Subtraction Angiography (IV-DSA)

At a given radiation dosage and field of view, five variables are under meaningful control for intravenous digital subtraction angiography (IV-DSA): concentration and quantity of contrast media injected, volume of injectate, rate of injection, and site of injection. Some controversy exists regarding the selection of a central vs. a peripheral injection site for IV-DSA. This study determined the influence of the site of injection on the peak and width of the arterial time-concentration curve produced by contrast media. Using a noninvasive, in vivo, quantitative x-ray measurement method, 36 separate injections (10 ml of ioxaglate at 8 ml/sec) were administered into the cephalic vein, subclavian vein, and main pulmonary artery in dogs. Injection sites were varied using a Latin-square experimental design. Cardiac output, central blood volume and the peak and width of the contrast media time-concentration curves were measured. The average peak enhancement was greatest for the pulmonary artery injection site. Normalizing peak and width values to make the pulmonary artery values 100%, the average peak values for injections into the subclavian vein and cephalic vein were 93% and 56%, and the average widths were 141% and 163%, respectively. These data support the use of a more central injection site for optimizing IV-DSA examinations.

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 =R xy R yy −1 y , a computationally efficient (fast) estimator is derived for limited angle reconstruction by choosing R xy and R yy 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 R yy −1 . With the fast estimator, the computation is avoided because R yy 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.

Effect of volume and rate of contrast medium injection on intravenous digital subtraction angiographic contrast medium curves

The image quality of temporal (mask mode) intravenous digital subtraction angiography is directly dependent on the shape of arterial time-concentration curves produced by the intravenous injection of contrast medium. Curves that are narrow and tall minimize motion artifact (misregistration) and maximize contrast enhancement (pre- and postcontrast differences). To determine the effects of rate and volume of injection of contrast medium on intravenous digital subtraction angiographic curves, ioxaglate (Hexabrix), a monoacidic ionic dimer, was injected into large mongrel dogs. Quantitative measurements of opacification were made over time in the femoral arteries using a modified General Electric CT/T scanner. Peak opacification was directly proportional to the volume of contrast medium injected. Curve width was not affected by increasing volume of injection. At rates below a critical point, slower injection rates produced progressively shorter and wider arterial time-concentration curves. Above that critical point, increasing the rate of injection did not affect either curve width or curve peak.