摘要:

A clinical ultrasound scanner has been integrated with a digital data acquisition system to record echo signals for off‐line processing of quantitative acoustic backscatter images. The method used to determine backscatter coefficients accounts for experimental factors related to the beam directivity function, the transmitting and receiving electronics, and the attenuation path of the beam. After characterization and calibration of the ultrasound scanner according to the data processing requirements, the quantitative backscatter coefficient for tissue‐mimicking phantoms are within 14% of a value predicted by scattering theory. On five normal volunteers, preliminaryinvivoliver images of the acoustic backscatter coefficient are obtained. Results from this study are compared to previously publishedinvitroresults.

ISSN:0269-3127    

DOI:10.1118/1.596789

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to aid radiologists in the diagnosis of pneumothorax from chest radiographs, an automated method for detection of subtle pneumothorax is being developed. The computerized method is based on the detection of a fine curved‐line pattern, which is a unique feature of radiographic findings of pneumothorax. Initially, regions of interest (ROIs) are determined in each upper lung area, where subtle pneumothoraces commonly appear. The pneumothorax pattern is enhanced by the selection of edge gradients within a limited range of orientations. Rib edges included in this edge‐enhanced image are removed, based on the locations of posterior ribs that are determined separately. A subtle curved line due to pneumothorax is then detected by means of the Hough transform. The detected pneumothorax pattern is marked on the chest image displayed on a CRT monitor. With the present computer method applied to 50 chest images (28 normals and 22 abnormals with pneumothorax), we were able to detect 77% of pneumothoraces, with 0.44 false‐positives per image.

ISSN:0269-3127    

DOI:10.1118/1.596790

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A detailed analysis of the effects of temperature changes, over time and between array elements, on the generation of circular artifacts in images produced by x‐ray computed tomography was reported. We give formulas for calculating—according to the x‐ray energy, detector sensitivity, and observed image quality (contrast and spatial resolution)—the maximum offset temperature coefficient and maximum gain temperature coefficient that will allow circular‐artifact‐free imaging. A temperature‐controlled and insulated solid‐state x‐ray detector array, consisting of Gd2O2S:Pr,Ce,F ceramic scintillators coupled to crystal Si pin‐photodiodes and designed to meet the requirements for these coefficients, produced high‐resolution artifacts‐free CT images of a phantom head.

ISSN:0269-3127    

DOI:10.1118/1.596919

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Contrast‐detail curves were constructed for liver computed tomographic (CT) images using an objective method. Stimuli were created by superimposing disks at specified locations on sets of 92 normal liver CT images. Bright and dark disks of 9 sizes and 36 possible image contrasts were used. Sets of 92 stimuli were rendered on film at five window widths (64, 128, 256, 512, and 1024 HU). The contrast‐detail (CD) curve flattened substantially for disks larger than 7‐mm diameter, and its slope (on a log‐log plot) was less than predicted from signal‐detection theory. Manipulation of display window manipulation had little impact on this disks' visibility. The results indicate that human observers have difficulty visualizing large, low‐contrast details on liver CT scans, and suggest that narrowing the display window will have little effect on this limitation.

ISSN:0269-3127    

DOI:10.1118/1.596791

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The expectation maximization (EM) algorithm has received considerable attention in the area of positron emitted tomography (PET) as a restoration and reconstruction technique. In this paper, the restoration capabilities of the EM algorithm when applied to radiographic images is investigated. This application does not involve reconstruction. The performance of the EM algorithm is quantitatively evaluated using a “perceived” signal‐to‐noise ratio (SNR) as the image quality metric. This perceived SNR is based on statistical decision theory and includes both the observer's visual response function and a noise component internal to the eye–brain system. For a variety of processing parameters, the relative SNR (ratio of the processed SNR to the original SNR) is calculated and used as a metric to compare quantitatively the effects of the EM algorithm with two other image enhancement techniques: global contrast enhancement (windowing) and unsharp mask filtering. The results suggest that the EM algorithm's performance is superior when compared to unsharp mask filtering and global contrast enhancement for radiographic images which contain objects smaller than 4 mm.

ISSN:0269-3127    

DOI:10.1118/1.596895

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In some medical imaging applications, it is necessary to visualize only the center of the field of view with optimal quality. For example, often in interventional radiographic procedures only the region directly adjacent to the catheter tip must be well seen. A new imaging approach which reduces the number of photons exposing the patient outside a region of interest (ROI), while allowing the photon fluence to be maintained or increased in the ROI, may make more optimal use of the total integral radiation dose to the patient as well as enable increased contrast and reduced artifacts in the ROI. A demonstration is given with an angiographic phantom, for an ROI which is less than 10% of the total field of view and where the periphery receives 6% of the ROI exposure. Contrast is improved about 30% in the ROI, and yet the images are adequate in the periphery for visualizing high contrast reference features while there is a reduction in total integral patient dose. Details of the technique are discussed along with requirements for clinical implementation.

ISSN:0269-3127    

DOI:10.1118/1.596792

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This report describes noise statistics in 2DFT MR images, expanding the earlier work of Henkelman and others to include variably asymmetric sampling and conjugate synthesis reconstruction. The effects of low‐order polynomial and Fourier phase correction used with conjugate synthesis are also explicitly considered. This analysis shows that complex images obtained by conjugate synthesis have an elliptical noise distribution, with the smaller axis corresponding to the imaginary image channel. Derivations and simulations predict a ratio of mean to standard deviation in the background of magnitude images varying from the known value ofπ/(4−π)(approximately 1.91) for full symmetry to2/(π−2)(approximately 1.32) at fully asymmetric or half‐echo sampling; these predictions are validated over a range of asymmetry by experimental measurements. These results are important for predicting and interpreting image noise when using asymmetric sampling.

ISSN:0269-3127    

DOI:10.1118/1.596920

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Variations in tube output, film processing, and radiologist's preferences affect the screen–film combination that is appropriate for any particular mammographic facility. A technique to test a variety of screen–film combinations for screening mammography is described. Films are selected for testing because of their sensitometric characteristics. Dose and clinical reliability are established with phantoms before the screen–film combinations are used to image consecutive patients having bilateral examinations. The mammograms selected for evaluation are those with similar optical density ranges, and which also may be compared to available previous mammograms or which have unusual mammographic findings. All radiologists reading mammograms at a facility independently score the selected cases. Scores of “unacceptable,” “acceptable,” or “outstanding” are assigned to four basic imaging characteristics: sharpness, contrast, visibility of skin line, and noise. Interobserver variations by this method require normalization, unlike ROC analysis which is not applicable for this data because of the absence of proved pathologic diagnoses. The testing of 5 films and two screens using 42 patient examinations required 2 h of time from each radiologist. It took 7 h of the physicist's time to pretest the 5 films, select the 42 acceptable examinations for testing by the radiologists, and summarize the data.

ISSN:0269-3127    

DOI:10.1118/1.596793

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Off‐focus radiation (OFR) has previously been described by many investigators [R. Thoraeus, Acta Radiologica18, 753 (1937); L. Mallet and R. Maurin, Radiology48, 628–632 (1947); J. F. Timmer, Medicamundi19(2), 52–54 (1974); G. U. Rao, Appl. Radiol.3(3), 45–49 (1974); R. Birch, Br. J. Radiol.49, 951–955 (1976); W. W. Roeck, AAPM Symposium, 217–247 (1991)]. Off‐focus radiation is frequently manifested on radiographs by the appearance of faint images of anatomical structuresoutsidethe collimated field of interest, i.e., soft tissues, ear lobes and hair, which we will refer to as “penumbral images.” The observable small detail resolution within these penumbral images led to the assumption that minute sources of increased radiation intensity (secondary microfocal spots) must be present outside the area of the primary focal spot. The existence of these multiple secondary microfocal spots has been established and their locations within the area of the anode surface from which the general OFR originates was identified. The number, size, and distribution of the secondary focal spots vary over time and their magnitudes vary widely. The source of the well‐focused electron beams creating the spots is attributed to the cold cathode emission principle.

ISSN:0269-3127    

DOI:10.1118/1.596794

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Transmission imaging is receiving increasing attention in SPECT due to the need to compensate for nonuniform attenuation in cardiac‐chest SPECT. The quality of a transmission image has an important effect on the measured attenuation distribution. To improve image quality, knowledge of the performance characteristics of a transmission imaging system is essential. The characteristics, spatial resolution, detection efficiency, photon flux, and exposure to the object, of a transmission imaging system consisting of a gamma camera and a uniform sheet source have been studied. The results demonstrate that spatial resolution of a transmission imaging system can be improved by use of a high‐resolution source collimator at the price of a moderate decrease in detection efficiency, in comparison to the uncollimated case. Also, the source collimator significantly reduces the photon flux and exposure to the object. This investigation suggests that a high‐resolution collimator be used with an intense sheet source to improve spatial resolution and reduce statistical noise with low exposure to the patient. This research further suggests that the amount of source activity is determined by the requirement of image quality, detection geometry, and allowed absorbed dose to the patient.

ISSN:0269-3127    

DOI:10.1118/1.596752

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY