L. A. Lehmann

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.

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.

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.

Iodine Imaging Using Three Energy Spectra

The visualization of blood vessels using noninvasive administration of contrast agents is clearly a desirable clinical goal. It opens up the opportunity for mass screening of vessel disease and the study of asymptomatic people who are at high risk. A number of approaches have been taken to view vessels noninvasively. In this paper we present some initial considerations of a system using measurements at three broad energy spectra.

Data Compression Of X-Ray Images By Adaptive Differential Pulse Code Modulation (DPCM) Coding

The intolerance of medical images to error places special demands on data reduction techniques. Quantization in the spatial or transform domains, widely studied in other data compression applications, is irreversible and therefore not applicable to medical image coding. Rather, we must rely on effective prediction and prediction error coding. This paper compares seven differential pulse code modulation (DPCM) predictors on the basis of average prediction error and channel induced error propagation. A Laplacian model of prediction error distribution suggests a simple family of variable length codes and an algorithm for adaptively selecting that code which best suits the anticipated predictor performance.

Noise And Material Residues In Dual-Energy Basis Decomposition Radiography

Intervening tissues in projection radiography often seriously interfere with the observability of other low contrast structures. Dual energy techniques resolve this difficulty by removing a selected substance from the image, enhancing the relative contrast of other materials. Undesired materials will remain; these residues, as well as quantum noise, limit the accuracy of material isolation. Parameterizations of the attenuation coefficient lead to simple expressions for the signal-to-noise ratio (SNR) in the dual energy radiograph. The SNR depends on the absorption path, the imaging energies, and the cancelled material. The magnitude of material residue depends only on the atomic numbers and may be an important consideration in the selection and development of new contrast agents.

Limitations to accuracy in dual energy digital fluoroscopy

A digital fluoroscopy system has many desirable features for energy selective imaging. Unfortunately it has significant problems which may prevent it from being used for this application. Two techniques for overcoming the limitations of digital fluoroscopy are presented. The first attacks the lack of absolute measurements in data from digital fluoroscopy. The nonlinear processing used in energy selective imaging requires absolute data. Techniques are presented which use relative measurements and still allow selective material imaging. Digital fluoroscopy is normally used for subtraction imaging so that errors in the data tend to cancel. By using a generalization of subtraction imaging called hybrid subtraction, the errors introduced by using fluoroscopic data in energy selective imaging can be significantly reduced.

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.

The Energy Factor In Radiographic Imaging

The photon energy spectrum has played a relatively minor role in classical radiographic imaging. Radiologists have used different regions of the spectrum for different studies. For example; lower energies are used for mammography to emphasize calcifications, and higher energies are used for chest studies to minimize rib shadows. These effects, however, are relatively subtle compared to those of selective energy imaging to be described in this paper. We will describe the application of energy selectivity to computerized tomography and projection radiography. In the case of computerized tomography the technique eliminates the non-linear spectral-shift artifact, and provides the average atomic number and density of each pixel. In the case of projection radiography, any intervening material can be removed to facilitate visualization of some desired structure. In addition, as with CT, an unknown lesion can be identified based on its average atomic number.