Vascular injury after whole thoracic x-ray irradiation in the rat.

PURPOSE: To study vascular injury after whole thoracic irradiation with single sublethal doses of X-rays in the rat and to develop markers that might predict the severity of injury. METHODS AND MATERIALS: Rats that received 5- or 10-Gy thorax-only irradiation and age-matched controls were studied at 3 days, 2 weeks, and 1, 2, 5, and 12 months. Several pulmonary vascular parameters were evaluated, including hemodynamics, vessel density, total lung angiotensin-converting enzyme activity, and right ventricular hypertrophy. RESULTS: By 1 month, the rats in the 10-Gy group had pulmonary vascular dropout, right ventricular hypertrophy, increased pulmonary vascular resistance, increased dry lung weights, and decreases in total lung angiotensin-converting enzyme activity, as well as pulmonary artery distensibility. In contrast, irradiation with 5 Gy resulted in only a modest increase in right ventricular weight and a reduction in lung angiotensin-converting enzyme activity. CONCLUSION: In a previous investigation using the same model, we observed that recovery from radiation-induced attenuation of pulmonary vascular reactivity occurred. In the present study, we report that deterioration results in several vascular parameters for

Pulmonary arterial morphometry from microfocal X-ray computed tomography.

The objective of this study was to develop an X-ray computed tomographic method for pulmonary arterial morphometry. The lungs were removed from a rat, and the pulmonary arterial tree was filled with perfluorooctyl bromide to enhance X-ray absorbance. At each of four pulmonary arterial pressures (30, 21, 12, and 5.4 mmHg), the lungs were rotated within the cone of the X-ray beam that was projected from a microfocal X-ray source onto an image intensifier, and 360 images were obtained at 1 degrees increments. The three-dimensional image volumes were reconstructed with isotropic resolution with the use of a cone beam reconstruction algorithm. The luminal diameter and distance from the inlet artery were measured for the main trunk, its immediate branches, and several minor trunks. These data revealed a self-consistent tree structure wherein the portion of the tree downstream from any vessel of a given diameter has a similar structure. Self-consistency allows the entire tree structure to be characterized by measuring the dimensions of only the vessels comprising the main trunk of the tree and its immediate branches. An approach for taking advantage of this property to parameterize the morphometry and distensibility of the pulmonary arterial tree is developed.

Microfocal X-ray CT imaging and pulmonary arterial distensibility in excised rat lungs.

The objective of this study was to develop an X-ray computed tomographic method for measuring pulmonary arterial dimensions and locations within the intact rat lung. Lungs were removed from rats and their pulmonary arterial trees were filled with perfluorooctyl bromide to enhance X-ray absorbance. The lungs were rotated within the cone of the X-ray beam projected from a microfocal X-ray source onto an image intensifier, and 360 images were obtained at 1 degrees increments. The three-dimensional image volumes were reconstructed with isotropic resolution using a cone beam reconstruction algorithm. The vessel diameters were obtained by fitting a functional form to the image of the vessel circular cross section. The functional form was chosen to take into account the point spread function of the image acquisition and reconstruction system. The diameter measurements obtained over a range of vascular pressures were used to characterize the distensibility of the rat pulmonary arteries. The distensibility coefficient alpha [defined by D(P) = D(0)(1 + alphaP), where D(P) is the diameter at intravascular pressure (P)] was approximately 2.8% mmHg and independent of vessel diameter in the diameter range (about 100 to 2,000 mm) studied.

An iterative approach to the beam hardening correction in cone beam CT.

In computed tomography (CT), the beam hardening effect has been known to be one of the major sources of deterministic error that leads to inaccuracy and artifact in the reconstructed images. Because of the polychromatic nature of the x-ray source used in CT and the energy-dependent attenuation of most materials, Beer's law no longer holds. As a result, errors are present in the acquired line integrals or measurements of the attenuation coefficients of the scanned object. In the past, many studies have been conducted to combat image artifacts induced by beam hardening. In this paper, we present an iterative beam hardening correction approach for cone beam CT. An algorithm that utilizes a tilted parallel beam geometry is developed and subsequently employed to estimate the projection error and obtain an error estimation image, which is then subtracted from the initial reconstruction. A theoretical analysis is performed to investigate the accuracy of our methods. Phantom and animal experiments are conducted to demonstrate the effectiveness of our approach.

Using phase information in ultrasonic backscatter for in vivo liver analysis.

In recent studies, it has been shown that information about scatterer spacing can be obtained from analyzing the phase of the ultrasound echo from various media. Such information proves to be useful when examining the ultrasonic backscatter from well-organized tissue, such as the liver. By quantifying the deviations in scatterer spacing and/or varying degrees of regularity, conclusions may be drawn about the underlying pathology of the tissue. This paper examines the physical basis of how the scatterer locations affect the phase of the data. Computer simulations were performed that mimic various scattering conditions and that display the effects of differing degrees of regularity, as well as increases in a diffuse random background scattering component. Results of studies on a phantom are also included to investigate and display the phase response under well-controlled scattering conditions. Finally, in vivo data taken from liver scans were analyzed. In this work, it was shown that the phase of the backscattered signal holds valuable information regarding the pathological state of liver tissue. It is suggested that this simple examination of the phase can be refined into a technique to be used as a method to consistently detect the onset of pathological change.

Comparisons of the Rayleigh and K-distribution models using in vivo breast and liver tissue.

There is a strong interest in finding out which statistical model is the most appropriate for describing the envelope of the backscattered ultrasonic echoes from different types of tissues. The Rayleigh model is commonly employed, but this requires conditions, such as the presence of large number of randomly located scatterers with fairly uniform cross-sections, that are not always met. However, our research indicates that a model based on the K-distribution may provide a better fit to empirical data over a range of scattering conditions than the standard Rayleigh model. In this study, we looked at the K-distribution as a descriptor of the backscattered envelope of the breast and liver tissues (in vivo). By examining data from various tissue regions, a goodness-of-fit test (a least squares error method) was used to determine whether a Rayleigh or K-distribution model is more appropriate. From a large group of patients and volunteer scans (a total of 72 subjects), the fit between the K-distribution and the data is shown to have a much smaller error than the Rayleigh model.

Studies on ultrasonic scattering from quasi-periodic structures.

This study extends the work done on nonuniform phase statistics by including additional results based on quasi-periodic scattering. Three parameters are used to predict the presence of regular structure within the region of interest. The signal-to-noise ratio of phase and the chi (2) statistic resulting from conducting a goodness of fit test are two parameters used to verify whether the phase signal followed a uniform distribution. A third parameter, the power spectral density (PSD), was studied and its ability to provide information on the level of periodicity present was analyzed. Computer simulations and experiments on tissue mimicking phantoms were carried out, the results of which indicate that the parameters introduced in this paper have good potential in providing a better understanding of scattering from a collection of quasi-periodic scatterers.

Characterization of ultrasonic B-scans using non-Rayleigh statistics.

The envelope of the ultrasonic echo backscattered from tissues is modeled using non-Rayleigh statistics, namely the K-distribution. We show that the K-distribution can be used to describe the envelope of the echo, and its parameters may be used to distinguish between different regions in ultrasonic B-scan images. The validity of the model is tested using phantoms. Preliminary results indicate that the parameters of the K-distribution may be used to separate targets, in which the number of scatterers or the scattering cross-sections differ from that of the background. The method of employing the parameters of the K-distribution appears to be better than the methods based solely on the signal-to-noise ratio.

Thyroid and thyroxine effects on adrenoreceptors in relation to circadian activity.

Experiments were conducted to ascertain if changes in central adrenergic receptors could be associated with altered circadian activity patterns induced by thyroparathyroidectomy (TPX) and thyroxine. An initial experiment used TPX and sham-operated rats that had been exposed to dim red light for 7 months. The alpha and beta receptor densities were compared in the suprachiasmatic nuclei (SCN), preoptic (PO), septum, and caudate-putamen. TPX animals showed significant reductions in beta 1 and beta 2 receptor densities in SCN and PO, and alpha 1 densities in SCN, but no other changes. A second experiment, lasting 4 months, examined the effects of thyroxine, which has been shown to reverse the period-shortening effects of TPX surgery. Thyroxine significantly increased beta 1 receptors in both the SCN and ventromedial hypothalamus (VMH), the only regions that displayed significant reductions in TPXs during the second experiment. Increases of sevenfold and threefold were observed in the SCNs of TPXs and shams, respectively, but thyroxine's action in the VMH was limited to TPX animals, an effect that mimics thyroxine's action on circadian activity rhythms.