Anne Lindsay Hall

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.

The technical characteristics of matched filtering in digital subtraction angiography

The technical characteristics of a new digital fluorographic image processing method called matched filtering are presented. This technique, a type of extensive temporal integration, takes a weighted sum of images acquired during passage of a contrast bolus through some area of interest. The weight of each image is governed by the magnitude of the contrast bolus in that image. An essential requirement of the matched filter is that its integral be zero. It is shown for equal exposure rates and typical bolus characteristics that matched filtering provides a factor of two higher signal-to-noise ratio (SNR) than conventional methods for bolus transit times of 10 s or higher. Equilvalently, matched filtering can yield images with quality comparable to conventional digital subtraction angiography (DSA) at a factor of four less patient exposure. The SNR obtained with matched filtering is shown to be within 30% of an ideal bound. Comparisons of matched filtering to standard recursive methods and simple integration are made. Experimental canine studies are presented which compare matched filtering with conventional DSA.

Experimental System For Dual Energy Scanned Projection Radiography

Developing digital radiography techniques provides greater diagnostic information while utilizing less invasive procedures and/or decreased patient dose. An experimental scanned projection radiography system has been built using a CT detector and data acquisition system to provide increased contrast resolution and flexibility in data manipulation. Modifications to the basic system allow dual energy scanning, and subtraction algorithms relying on the energy dependence of the mass attenuation coefficient have been implemented.

Beam Hardening, Noise, and Contrast Considerations in Selective Iodine Digital Radiography

Temporal and dual beam energy subtraction are discussed in conjunction with selective imaging of iodine. Several different kinds of energy subtraction are described and their relative sensitivities to noise are predicted. Effects due to beam hardening are demonstrated for temporal and energy subtraction. Hardening errors are more severe for the latter case by about a factor of three. A possible hardening correction is presented for energy subtraction and an experimental example is provided. Techniques for choosing optimized spectra and data handling for dual beam energy subtraction are presented. These include qualitative principles for spectral selection, identification of the individual noise contributions from the two beams, use of weighted least squares fits, and beam hardening and exposure reduction through the use of filtration.

Contrast Sensitivity Of Digital Fluorographic Systems

Several aspects of iodine sensitivity of digital fluorographic systems are discussed as applied to digital subtraction angiography. Expected iodine signal magnitudes are reviewed and shown to be in the range of 1-10 mg/cm2. An experimental example of spurious contrast in iodine sensitivity measurements is2 provided. Imaging of a 1 mm wide 0.25 mg/cm simulated iodinated vessel is demonstrated. Such iodine amounts are thought to be near the limit of the sensitivity of digital fluorographic systems. The concept of matched filtering is proposed for generating the equivalent of temporal subtraction images but using low intensity continuous x-ray exposures. The first experimental matched filter canine image is presented using a total exposure for the procedure about one-sixth of that used for pulsed exposure images of comparable quality.

DUAL-kVp RADIOGRAPHY.

The use of information contained in the transmitted x-ray spectrum provides the capability for selective removal of substances of a particular mean atomic number in projection radiographs. Using a prototype system for line-scanned digital radiography, x-ray images were produced with two different x-ray spectra by modulating the kVp applied to the x-ray source and filtering the x-ray beam. Using a previously described Compton/photoelectric decomposition algorithm, subtraction images are made with bone or soft-tissue (water) shadows removed. Applications to chest, abdominal and skeletal imaging in-vivo are demonstrated.