ISSN:0269-3127    

DOI:10.1118/1.596408

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

There has recently been an increasing interest in the possibility of producing images of electrical impedance within the human body. When an electric current is applied to the body of a voltage distribution is developed across the body surface. This distribution is in part dependent on the internal impedance distribution within the body and its is possible to estimate this distribution from a suitable set of voltage measurements. Because of the nonlinear relationship between the impedance distribution and the voltage distribution at the surface of the body, the reconstruction problem is much more difficult than for other tomographic imaging techniques, but a significant amount of progress has been made, and it is now possible to produce tomographic images ofinvivodistributions of impedance, albeit with low spatial resolution. Future developments should improve image quality.

ISSN:0269-3127    

DOI:10.1118/1.596368

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A theoretical model has been developed to explain the effects of simple linear windowing on the apparent contrasts of signals in displayed digital images. The model predicts, and experimental results demonstrate, that the effective displayed contrast of a digital radiographic signal depends in a complex way upon interactions among the endpoints of the display scale, the signal contrast and noise level of the original data, the window center and display center selected, and the contrast enhancement factor applied. The results obtained from this work apply quantitatively to the highly idealized situation in which (i) a uniform signal is superimposed on a uniform background containing Gaussian pixel‐value noise, and (ii) a linear (or exponential) relationship exists between the optical density of a film display (or the light intensity of a luminous display) and pixel value in some finite range. However, the qualitative effects demonstrated here may be expected to arise in a broad variety of situations involving strong digital contrast enhancement.

ISSN:0269-3127    

DOI:10.1118/1.596369

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An x‐ray imaging system is described that can be used for obtaining arterial blood flow information. The system consists of a linear photodiode array image detector, simple optical and mechanical components, and a data acquisition microcomputer that connect to a conventional x‐ray image intensifier based fluorography system. Flow information is obtained by detecting the movement of a small, locally injected bolus of radio‐opaque contrast agent. This is done by determining the bolus mass, integrated over the cross‐sectional area, at each of 1024 positions along the length of the artery with a sampling rate of up to 200 samples per s. It is shown in a phantom study that the peak flow velocity can be measured with an accuracy of ±5% by detecting the bolus arrival times at each of the 1024 positions. The mean velocity is obtained with similar accuracy using a cross‐correlation technique and a modified form of the Stewart–Hamilton principle. In addition, it is shown that the separation and reattachment points resulting from flow separation near a stenosis can be determined from the bolus clearance times. The locations of these points are consistent with theoretical values for the cosine shaped symmetric 89% stenosis used in this study.

ISSN:0269-3127    

DOI:10.1118/1.596411

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Current edge detection methods used to determine coronary artery dimensions from digital (DSA) images suffer a strong dependence on the system's modulation transfer function (MTF). The videodensitometric algorithms are less sensitive to MTF blurring, yet still result in an overestimation of the vessel size of 10% to 25% for blurring aperture sizes of 50% to 80% of the vessel diameter. We propose a new algorithm to measure the absolute diameter of a vessel which has a lower sensitivity to the system MTF for blurring aperture sizes up to 80% of the vessel diameter. A consequence of the similarity theorem of Fourier transform pairs is that the “width” of the Fourier transform, as characterized by the first zero crossing in frequency space, is inversely proportional to the width of the vessel profile. For an ideal (unblurred) vessel image, the width of the vessel profile is equal to the diameter of the vessel. For a blurred image this is not true. In frequency space, however, the transform of the blurred profile is simply the product of the transformed ideal profile and the system MTF. Thus, if the blurring aperture of the system is below some critical value, the first zero of the unblurred profile will still dominate the transform of the blurred profile. For vessels of circular cross section and a rectangular blurring aperture, this critical aperture size is approximately 80% of the vessel diameter. A more detailed explanation of the theory and calculations involved in this measurement, along with measurements of computer simulated and phantom vessels is presented.

ISSN:0269-3127    

DOI:10.1118/1.596370

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A hybrid reconstruction algorithm (HRA) is derived for fan beam geometry for use with systems utilizing a linear array of detectors. The algorithm uses fan beam geometry with a modified parallel beam reconstruction formula. The parallel beam algorithm is used with fan beam geometry by a simple geometric conversion and a different back projection formula. Correction for center of rotation shift is done during the back projection. Computer simulations are shown, both with the center of rotation shift uncorrected and corrected.

ISSN:0269-3127    

DOI:10.1118/1.596371

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Conventional chest radiography is the most effective tool for lung cancer detection and diagnosis; nevertheless, a high percentage of lung cancer tumors are missed because of the overlap of lung nodule image contrast with bone image contrast in a chest radiograph. Two different energy subtraction strategies, dual exposure and single exposure techniques, were studied for decomposing a radiograph into bone‐free and soft tissue‐free images to address this problem. For comparing the efficiency of these two techniques in lung nodule detection, the performances of the techniques were evaluated on the basis of residual tissue contrast, energy separation, and signal‐to‐noise ratio. The evaluation was based on both computer simulation and experimental verification. The dual exposure technique was found to be better than the single exposure technique because of its higher signal‐to‐noise ratio and greater residual tissue contrast. However, x‐ray tube loading and patient motion are problems.

ISSN:0269-3127    

DOI:10.1118/1.596372

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have previously reported a motion immune dual‐energy subtraction technique in which x‐ray tube voltage and x‐ray beam filtration are switched at 30 Hz between 60 kVp (2.0‐mm Al filter) and 120 kVp (2.0‐mm Al+2.5‐mm Cu filtration). In this paper we consider the suitability of these dual‐energy images for quantitative measurements of iodine thickness and volume. Optimized iodine signal‐to‐noise ratio(S/N) was measured as a function of phantom thickness. Using a fixed mAs, the S/N of the dual‐energy images was found to decrease by sevenfold as lucite thickness increased from 10 to 25 cm. For the same increase in lucite thickness S/N for time subtraction images decreased by fivefold. Image quality in two human volunteers was subjectively judged to be good. In order to quantitate physiological parameters such as ejection fraction and left ventricular volume, energy dependent corrections for scatter and veiling glare, beam hardening, detector nonuniformity, heel effect, and uncanceled bone signals were developed. Since the dual‐energy technique does not conpletely cancel bone, a preinjection dual‐energy subtraction image was used to estimate integrated bone contributions to iodine volume measurements. In a phantom measurement simulating exercise ventriculography, the known (Vk) and videodensitometrically measured (Vm) volumes of 19 mg/cm3solution of iodine were related byVm=0.95Vk+1.50 cm3(r>0.99).

ISSN:0269-3127    

DOI:10.1118/1.596418

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Statistical considerations related to the measuring and analyzing procedures in computed tomography (CT) scans indicate that determination of a 2.5% change in spinal mineral content (SMC) which is assumed necessary in clinical practice, requires a 0.9% combined precision in setting the location and direction of the scan plane through the vertebral body, and defining the region of interest. In single energy examinations the short‐term precision of CT scanners was found to be sufficiently high to achieve this goal, but the variability due to subjective finding of scan direction requires the automation of slice selection and additional averaging of data from more than one scan slice. The precision of calcium images was much lower, and one conclusion of this study is that dual energy technology is not suitable for longitudinal examinations, where small changes in SMC have to be determined. The SMC of the vertebrae T12, Ll, L2, and L3 as measured in 41 subjects was compared and a significant decrease in trabecular bone in single energy examinations and increase in cortical bone was found from T12 to L3. This suggests that the comparison of SMC data should be done over time for each vertebra separately, in order to preserve diagnostic information.

ISSN:0269-3127    

DOI:10.1118/1.596417

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A methodology for evaluating the accuracy and reproducibility of calculatedT1's andT2's has been developed and applied to a General Electric 1.5 T Signa System. Specifically studied were absolute accuracy and temporal stability of image derivedT1's andT2's as well as the dependence of calculated relaxation times on position in the scan plane, position along the axis of the magnet bore, the scan plane orientation (axial, sagittal, coronal), and the center frequency. The results of this study of the accuracy and reproducibility of image relaxation times have direct bearing on the design of clinical investigations assessing the diagnostic usefulness of these values.

ISSN:0269-3127    

DOI:10.1118/1.596373

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY