Fourth-generation Scanner Research Articles

CT fan-beam parametrizations leading to shift-invariant filtering

The problem of two-dimensional tomographic image reconstruction from fan-beam projections via shift-invariant filtering (convolution) followed by backprojection has a solution for two well known fan-beam parametrization classes. These parametrizations are associated either with equidistant collinear detector cells or with equi-angular fan rays. In this paper, the problem of finding all such fan-beam parametrizations is solved. Two new parametrization classes are found, which define new CT reconstruction algorithms. Two interpretations are given for the new parametrizations. First, the associated loci of equidistant detector cells are found by solving numerically the differential equations that characterize them. Application of the numerical approach also unveils a new detector geometry associated with one of the two previously known parametrization classes. Secondly, by mapping the parametrizations onto the familiar third-generation CT geometry, variable resolution samplings of the scan field of view are found. Also, the problem of CT image reconstruction on a fourth-generation scanner is addressed. Although it is found that the corresponding parametrization does not belong to the convolution solution set, a `natural' shift-invariant filter decomposition is described that provides a close approximation to the reconstruction filter.

Problems in recognizing recurrences of craniocervical malignancies in CT

A retrospective study was carried out involving 39 patients with malignant tumours of the head and neck in order to differentiate by CT changes due to local tumour or lymph node recurrences and various post-operative or post-irradiation changes. In these patients, two or more CT examinations of the neck were available, derived from a fourth generation scanner. The suspected lesions were classified according to their type and the appearances correlated with the treatment and clinical findings. There was frequent overlap between the appearances of benign and malignant changes, so that in individual cases a definite decision often proved impossible.

X-Ray Computerized Tomography

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. Computerized tomography (CT) is a new radiological imagingtechnique that measures density and atomic composition inside opaque objects. Arevolutionary advance in medical radiology since 1972, CT has only recentlybeen applied in petrophysics and reservoir engineering. This paper discussesseveral petrophysical applications, including three-dimensional (3D)measurement of density and porosity; rock mechanics studies; correlation ofcore logs with well logs; characterization of mud invasion, fractures, anddisturbed core; and quantification of complex mineralogies and sand/shaleratios. Reservoir engineering applications presented include fundamentalstudies of CO2 displacement in cores, focusing on viscous fingering, gravitysegregation, miscibility, and mobility control. Introduction X-ray CT is a radiological imaging technique first developed in GreatBritain in 1972 by Hounsfield. CT revolutionized medical radiology by producinganatomical images of extraordinary accuracy and clinical detail. Hounsfield wasawarded the Nobel prize in medicine in 1979 for his contributions. Tounderstand the advantages of CT, first consider conventional X-ray radiography, e.g., chest X-rays. Conventional radiographs view an object from only one angleso that shadows from all irradiated matter along a ray path are superimposed onone another (Fig. 1). The attenuation information along the ray path isaveraged together so that localized regions with small attenuation contrast areobscured. By comparison, CT scanners generate cross-sectional image slicesthrough the object by revolving an X-ray tube around the object and obtainingprojections at many different angles (Fig. 2). From a set of these projections, a cross-sectional image is reconstructed by a back-projection algorithm in thescanner's computer. The cross-sectional image of attenuation coefficients isdisplayed on a cathode-ray-tube (CRT) monitor. The beauty of CT is thatattenuation differences as small as 0.1% can be measured accurately within aninterior region of 2 mm(2) or less. With a fourth-generation medical scanner, the entire imaging process is completed in seconds. CT images (3D) can bereconstructed from sequential cross-sectional slices taken as the sample ismoved through the scanner (Fig. 3). Once this 3D data set has been acquired, any plane through the object can be viewed. For example, Figs. 4A and 4B showhow vertical and horizontal slices can be used to separate gravity and viscouseffects during tertiary gas injection. Instrumentation CT scanners have undergone considerable development since 1972. First-generation scanners used a single pencil-beam source and detectorarrangement. Second-generation scanners improved image quality by use ofmultiple detectors in a translate/rotate configuration. A large improvement inspeed occurred in the third-generation scanners, which used a rotate-onlyfan-beam geometry with source and detectors rotated together around the object. Finally, the fourth-generation scanners use a fan-beam geometry with sourcerotating within a fixed ring of high-efficiency detectors. The second- throughfourth-generation medical CT scanners are satisfactory for petroleumengineering applications because they have adequate X-ray energy and dose forscanning core material. Used medical CT scanners are readily available at asmall fraction of the original cost. JPT P. 885^

Quantitative CT measurements: the effect of scatter acceptance and filter characteristics on the EMI 7070

Non-linearities in projection values on computed tomography (CT) scanners cause corresponding errors in derived Hounsfield unit attenuation measurements. Existing commercial machines have been refined for clinical usefulness but not necessarily for quantitative accuracy. The results of measurements on the EMI 7070 fourth-generation scanner are presented and compared with theory with particular attention being paid to the scatter artefact and fan-beam energy profile. It is concluded that, irrespective of any quality assurance protocol, interpatient and interslice errors can be expected to range from 3 to 10% for water-equivalent materials and the intraslice positional dependence of the CT number can vary up to 5% for dense bone-like materials in a uniform phantom. It seems as though the principal cause of this variation is an increase in the effective X-ray energy in the fan beam with increasing distance from the central ray.

Basic Principles of Computed Tomography

This is a well-written book, and, as the title indicates, it is on the basic principles of computed tomography (CT). These basic principles, however, are not those of interpretation and evaluation of CT examinations but rather are the basic physics and mathematics for the concepts of CT. Background radiation physics, radiological technology, and computer theory are part of these discussions. There is little in the way of any clinical discussion other than one chapter on image display. The book nicely goes into the background of conventional tomography and then the background and development of CT. The different types of CT scanning equipment are evaluated, including the various types of geometry used for scanning. A comparison of first, second, third, and fourth generation scanners is discussed. These discussions are followed by a chapter in which there is a comparison of the theories and techniques of image reconstruction. A chapter on practical

Computed tomography of the adrenal glands in Addison's disease.

In 30 cases of chronic adrenal insufficiency (Addison's disease) CT of the adrenal glands was performed using a fourth generation scanner and a 2 mm slice thickness. Adrenal glands were visualized in all the cases. In 26 patients the adrenals were atrophied; the adrenal shape was abnormal in 21 patients. In 15 patients CT disclosed calcifications in one or both glands, which were particularly frequent in patients over the age of 50. Atrophy of adrenal glands was of high occurrence in cases of autoimmune origin.

Prospectively gated cardiac computed tomography

A fourth-generation scanner has been modified to perform prospectively gated cardiac computed tomography (CT). A computer program monitors the electrocardiogram (ECG) and predicts when to initiate the next scan in a gated series in order to acquire all projection data for a desired phase of the heart cycle. The system has been tested with dogs and has produced cross-sectional images of all phases of the cardiac cycle. Eight to ten scans per series were sufficient to obtain reproducible images of each transverse section in the end-diastolic and end-systolic phases. The radiation dose to the skin was approximately 1.4 cGy per scan. The prospectively gated system is more than twice as efficient as a retrospectively gated system in obtaining complete angular projection data for a 10% heart cycle window. A temporal smoothing technique to suppress reconstruction artifacts due to sorting inconsistent projection data was developed and evaluated. Image noise was reduced by averaging together any overlapping projection data. Prospectively gated cardiac CT has also been used to demonstrate that the error in attenuation measured with a single nongated CT scan through the heart can be as large as 50-60 CT numbers outside the heart in the lung field.

Sensitivity of computed tomography in evaluation of pelvic lymph node metastases from carcinoma of bladder and prostate

Pelvic lymph node evaluation by computed tomography was correlated with histologic diagnosis in patients with prostate and bladder carcinoma. Based on computed tomography alone, the stage of the carcinomas would have been underestimated in 38 per cent of cases and overestimated in 6 per cent. Over-all, the sensitivity of pelvic CT scanning in detecting lymph node involvement was only 14 per cent with specificity of 89 per cent. Although this series is relatively small and both second and fourth generation scanners were used, the limitations of computed tomography in recognizing minimal nodal disease or microscopic invasion of adjacent organs is reemphasized.

Structured noise in computed tomography: effects of periodic error sources.

The artifact in computed tomography (CT) images due to cyclic projection errors, such as errors due to periodic fluctuations in x-ray intensity, is derived and verified by computer simulation. Depending upon the relative phase of the error between projections, the artifact is shown to be described by a Bessel function (or functions) of the radial argument which is sinusoidally modulated as a function of angle. Because of the nature of Bessel functions, the artifact is essentially zero up to some minimum radius. When the effects of sampling are neglected, a single fundamental artifact is shown to occur. In fourth-generation scanners, the fundamental artifact (neglecting sampling) will occur at a detector fan angle of about about 39 degrees (depending on the ratio of the axis-to-detector, source-to-axis distances). The radius of appearance of this fundamental artifact is independent of the frequency of the periodic error signal and will only be visible in fourth-generation CT scanners with detector fan angles greater than about 39 degrees. The effects of sampling are derived and illustrated by simulation for first-, third-, and fourth-generation CT-scanner geometries. It is shown that the effect of sampling is to cause an infinite number of such artifacts to be superimposed in the final image. The radius of appearance of all but the fundamental artifact are shown to be dependent on the frequency of the periodic signal. It is shown that by judicious choice of the sampling parameters relative to the frequency of the periodic error, the artifact can be effectively eliminated.

An Anatomical Aid for the Evaluation of Computed Tomography Scans: A Photographic Cross-section Atlas of the Head and Neck with CT Images and Graphic Reconstructions

Computed tomography (CT) has already changed practice of neuroradiology and neurosurgery. Improved technology and high resolution of third- and fourth-generation scanners have increased potential of CT of head and neck. The authors of this well-prepared and detailed atlas have attempted to cover the missing anatomic details of conventional CT scanning of brain. Anatomically, they have succeeded. Photographs of thin-sectioned specimens are superb. Classic anatomic labeling is precise. Graphic reconstructions of clinically relevant structures give three-dimensional detail applicable to reconstruction in longitudinal and axial planes. Unfortunate, and readily acknowledged by authors, is use of an early generation scanner, whose images pale next to excellent photographic sections. Additionally, CT images through neck are not included. The authors believe that their atlas will appeal to neurologists and neurosurgeons rather than radiologists. However, with capabilities of newer scanners and increasing number

Theoretical Prediction of X-Ray CT Image Quality Using Contrast-Detail Diagrams

A theoretical model is developed to calculate contrast-detail diagrams(CDD) that can be used to express the image quality of an X-ray computerized tomography (CT) system. Using the model, CDDs are calculated for third and fourth generation scanners. The quantitative relations between the CT system characteristics and the consequent image quality are presented by means of calculated results.

Computed tomography of the normal thymus.

Recognition of variations in size, shape, and density of the normal thymus on computed tomographic (CT) scans is of paramount importance, lest it be misinterpreted as an abnormal mediastinal mass. Studying patients subsequently proved free of active chest disease, we analyzed 154 CT scans of the mediastinum, performed on a fourth-generation scanner, to determine the incidence of visualization and appearance of the normal thymus. The thymus was seen in 100% of patients under age 30, 73% of patients between 30 and 49 years, and in 17% of patients over 49 years of age. The thickness of the thymus showed a definite decrease in size with increasing age; although the width showed a similar general tendency, a wide variation was noted within each age group. In younger patients, the density of the thymus was similar to that of muscle; the attenuation values progressively decreased in older patients, finally approaching that of fat.

Direct coronal section CT of the retroperitoneum: normal anatomy without contrast

The advent of the general purpose CT scanner saw the introduction of direct coronal and off-lateral sagittal sections (1). This became possible because the aperture inside the detectors was considerably larger than that in the original dedicated head scanner, which proved particularly valuable in localizing tumors, diagnosing and showing the extent of chronic subdural hematomas, and demonstrating the suprasellar extension of pituitary adenomas (2) and basal aneurysms. The apertures of third and fourth generation scanners are even larger and, with suitable positioning, direct coronal sections of the retroperitoneum become possible. This paper describes the appearances of the normal anatomy without contrast. Subsequent papers will discuss appearances with contrast and in some pathological states.