Michael J. Dennis

Correction for spectral artifacts in cross-sectional reconstruction from x rays.

A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three-dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed-length water-bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross-sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.

Spectral effects on three‐dimensional reconstruction from x rays

Continuous bremsstrahlung spectra were calculated for 120 kVp for constant and sinusoidal potentials. Fluorescent radiation for the tungsten target was added to the bremsstrahlung, and the spectra were attenuated through various filter materials. A drawing of an object to be scanned was divided into an array of small squares in which the composition was assumed to be constant. Transmission data for 120 rays at each of 120 angles spanning a range of 180 degrees were calculated. Two algorithms for the reconstruction of attenuation coefficients from projection data, an algebraic reconstruction technique (ART) and the convolution method, were utilized to reconstruct effective coefficients. The effect of spectral filtration on the quality of the reconstruction was evaluated. Lightly filtered x-ray beams give rise to severe distortions in image quality, with values of the reconstructed coefficients rising toward the periphery of the object. Highly filtered beams give rise to images with less pronounced distortion.

Determination of effective energies in CT calibration

The effective energy of a polychromatic beam for a Computed Tomography (CT) scanner can be measured directly only with difficulty. However, a linear relationship exists between the measured CT numbers and corresponding attenuation coefficients of known materials at the effective energy of the x-ray beam. The effective energy can then be determined by searching all energies for the best linear correlation between the CT numbers and the attenuation coefficients. This can be performed by two methods: graphically, by means of choosing visually the straightest of the fitted lines or, mathematically, by maximizing the correlation coefficient. The energy corresponding to the optimal fit is therefore selected as the effective energy. The latter method was implemented by computer and demonstrated by scanning the AAPM phantom, which contained known materials, and determining the effective energies and the relationship between the linear attenuation coefficients and CT numbers for three commercial units.

Estimation of chemical composition and density from computed tomography carried out at a number of energies.

A method is presented by whichcomputed tomography scans carried out at a number of energies may be utilized to obtain cross-sectional images of density and atomic number in addition to the conventional array of linear attenuation coefficients. This type of analysis has been carried out for various substances of biological relevance. Computer simulated reconstructions of clinical situations suggest that the method shows promise for providing additional diagnostic information and might dispense to some extent with the necessity of injecting contrast agents into the patient.

Correlating computed tomographic numbers with physical properties and operating kilovoltage

The decrease in the EMI values of a dextrose solution which is seen with increasing kVp may be predicted on the basis of its fractional composition by weight.

Preprocessing X-Ray Transmission Data In CT Scanning

While the reconstruction algorithm utilized in computerized tomography (CT) is important, the overall performance of the system is limited by the quality of the measured transmission data which is used as the basis for the reconstruction process. If the projection values derived from the measured data do not adequately represent the line integrals of the linear attenuation coefficients within the slice being scanned, even a perfect reconstruction algorithm will give rise to a distorted image. Phenomena which tend to deteriorate the quality of the measured data, and hence the final image, include the effective finite dimensions of the scanning aperture, distortions introduced by the detector system such as afterglow, and nonlinearities related to the spectral distribution of x-ray photons used in scanning. Computer methods of preprocessing the x-ray transmission data to minimize these distortions are discussed and illustrated.

Treatment planning in Cobalt-60 radiotherapy using computerized tomography techniques.

Cobalt-60 transmission measurements were made through an Alderson phantom utilizing a transverse axial tomographic device and a NaI (Tl) detector. Measurements were made on different sections of the phantom for as many as 162 angles and 120 linear increments. The attenuation coefficients were reconstructed using both convolution and algebraic reconstruction techniques. Three-dimensional isodose distributions were obtained using the reconstructed attenuation coefficients. Comparison with standard treatment plans and measured isodose distribution using TLD techniques suggest that a more accurate isodose distribution may be obtained using the reconstructed attenuation coefficients, particularly in regions involving tissue heterogeneities.

Preprocessing X-Ray Transmission Data in CT Scanning

While the reconstruction algorithm utilized in computerized tomography (CT) is important, the overall performance of the system is limited by the quality of the measured transmission data which is used as a basis for the reconstruction process. If the projection values derived from the measured data do not adequately represent the line integrals of the linear attenuation coefficients within the slice being scanned, even a perfect reconstructruction algorithm will give rise to a distorted image. Phenomena which tend to deteriorate the quality of the measured data, and hence the final image, include the effective finite dimensions of the scanning aperture, distortions introduced by the detector system such as afterglow, and nonlinearities related to the spectral distribution of x-ray photons used in scanning. Computer methods of preprocessing the x-ray transmission data to minimize these distortions are discussed and illustrated.

EFFECTS OF RADIATION PARAMETERS ON COMPUTERIZED TRANSVERSE AXIAL TOMOGRAPHY

Effects of kVp, voltage wave form, added filter, and tube current on three-dimensional reconstruction from x-rays have been investigated in relationship to sampling time,spectral heterogeneity, contrast, quantum noise, and patient dose. Comparison of statistical limitations with and without a water-bath indicates that scanners which employ a water-bath are comparatively inflexible in this regard. A degree of flexibility in the choice of kilovoltage and filter seems both desirable and feasible in scanners which do not use a water-bath. Minimization of patient dose is also limited by statistical factors. Reconstructions from measured data have been carried out to demonstrate the relationship between these factors.