Loren T. Niklason

Tomographic mammography using a limited number of low-dose cone- beam projection images

A method is described for using a limited number (typically 10-50) of low-dose radiographs to reconstruct the three-dimensional (3D) distribution of x-ray attenuation in the breast. The method uses x-ray cone-beam imaging, an electronic digital detector, and constrained nonlinear iterative computational techniques. Images are reconstructed with high resolution in two dimensions and lower resolution in the third dimension. The 3D distribution of attenuation that is projected into one image in conventional mammography can be separated into many layers (typically 30-80 1-mm-thick layers, depending on breast thickness), increasing the conspicuity of features that are often obscured by overlapping structure in a single-projection view. Schemes that record breast images at nonuniform angular increments, nonuniform image exposure, and nonuniform detector resolution are investigated in order to reduce the total x-ray exposure necessary to obtain diagnostically useful 3D reconstructions, and to improve the quality of the reconstructed images for a given exposure. The total patient radiation dose can be comparable to that used for a standard two-view mammogram. The method is illustrated with images from mastectomy specimens, a phantom, and human volunteers. The results show how image quality is affected by various data-collection protocols.

Radiation exposure during radiofrequency catheter ablation of accessory atrioventricular connections.

BackgroundCatheter ablation of accessory atrioventricular (AV) connections has been demonstrated to be effective in more than 85% of patients. One of the risks of this procedure is radiation exposure during the fluoroscopic imaging necessary to guide catheter manipulation. The objective of the present study was to measure the radiation received by patients and physicians during radiofrequency catheter ablation and to estimate the resultant somatic and genetic risks. Methods and ResultsRadiation exposure to patients and physicians was measured during attempts at radiofrequency catheter ablation of accessory AV connections in 31 consecutive patients. Radiation exposure was measured using thermoluminescent sensors placed on the patient and on the physician. Somatic and genetic risks were estimated based on the radiation levels recorded using these sensors. The durations of fluoroscopy and of the catheter ablation procedure were recorded for each patient. Catheter ablation was successful in 28 of 31 patients (90%). Mean + SD duration of fluoroscopy was 44 ± 40 minutes. The largest patient radiation dose was measured over the ninth vertebral body posteriorly (median, 7.26 rem [roentgen equivalents man]; range, 0.31–135.7 rem). Median radiation dose to the thyroid was 0.46 rem (range, 0.06–7.26 rem), and median radiation dose to the posterior iliac crest was 2.43 rem (range, 0.01–8.3 rem). The greatest radiation dose to the operator was recorded at the left hand (99 mrem). Mean radiation dose to the operators eyes was 28 mrem. ConclusionsRadiofrequency catheter ablation of accessory AV connections may result in significant radiation exposure to the patient and to the physician. Each hour of fluoroscopic imaging is associated with a lifetime risk of developing a fatal malignancy of 0.1% and a risk of a genetic defect of 20 per 1 million births. Although these risks must be recognized, they are relatively small compared with the risks associated with alternate approaches to management, including no therapy, antiarrhythmic drug therapy, and surgery.

Breast Tomosynthesis: Present Considerations and Future Applications

Mammography is an effective imaging tool for detecting breast cancer at an early stage and is the only screening modality proved to reduce mortality from breast cancer. However, the overlap of tissues depicted on mammograms may create significant obstacles to the detection and diagnosis of abnormalities. Diagnostic testing initiated because of a questionable result at screening mammography frequently causes patients unnecessary anxiety and incurs increased medical costs. Breast tomosynthesis, a new tool that is based on the acquisition of three-dimensional digital image data, could help solve the problem of interpreting mammographic features produced by tissue overlap. Although the technology has not yet been approved by the Food and Drug Administration, breast tomosynthesis has the potential to help reduce recall rates, improve the selection of patients for biopsy, and increase cancer detection rates, especially in patients with dense breasts. Supplemental material available at radiographics.rsnajnls.org/cgi/content/full/27/S231/DC1.

A Mobile Computed Tomographic Scanner with Intraoperative and Intensive Care Unit Applications

INTRODUCTION A mobile computed tomographic scanner has been developed in which the scan plane is selected by means of gantry translation, rather than by translation of the patient table. This permits computed tomographic scanning in situ of any patient who is positioned on a radiolucent surface that fits within the inner diameter of the gantry. We report the design of and initial experience with this scanner as used with adapters for intraoperative and bedside computed tomography (CT). METHODS The scanner is equipped with wheels, draws power from wall outlets (120 V, 20 A) in combination with batteries, and has a translating gantry. Preclinical studies of image quality were performed with phantoms. An operating table adapter was built for use with a radiolucent cranial fixation device. A bedside adapter was built that holds the head and shoulders of a patient in the intensive care unit. RESULTS The preclinical phantom studies showed satisfactory image spatial resolution (0.8 mm) and low-contrast resolution signal-to-noise relative standard deviation (0.37%). Experience to date with 12 patients has confirmed the feasibility of intraoperative CT on demand. Experience to date with 26 patients has confirmed the feasibility of routine bedside CT in the intensive care unit. CONCLUSION With these adaptations, mobile CT may increase the efficiency of intraoperative scanning by making it available to multiple operating rooms without committing it to any room for an entire operation and may increase the efficiency and safety of CT of critically ill patients who currently need to leave the intensive care unit to travel to a fixed CT installation and back.

Digitization requirements in mammography: Effects on computer‐aided detection of microcalcifications

We have developed a computerized method for detection of microcalcifications on digitized mammograms. The program has achieved an accuracy that can detect subtle microcalcifications which may potentially be missed by radiologists. In this study, we evaluated the dependence of the detection accuracy on the pixel size and pixel depth of the digitized mammograms. The mammograms were digitized with a laser film scanner at a pixel size of 0.035 mm x0.035 mm and 12-bit gray levels. Digitization with larger pixel sizes or fewer number of bits was simulated by averaging adjacent pixels or by eliminating the least significant bits, respectively. The SNR enhancement filter and the signal-extraction criteria in the computer program were adjusted to maximize the accuracy of signal detection for each pixel size. The overall detection accuracy was compared using the free response receiver operating characteristic curves. The results indicate that the detection accuracy decreases significantly as the pixel size increases from 0.035 mm x 0.035 mm to 0.07 mm x 0.07 mm (P < 0.007) and from 0.07 mm x 0.07 mm to 0.105 mm x 0.105 mm (P < 0.002). The detection accuracy is essentially independent of pixel depth from 12 to 9 bits and decreases significantly (P < 0.003) from 9 to 8 bits; a rapid decrease is observed as the pixel depth decreases further from 8 to 7 bits (P < 0.03) or from 7 to 6 bits (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Image compression in digital mammography: Effects on computerized detection of subtle microcalcifications

Our previous receiver operating characteristic (ROC) study indicated that the detection accuracy of microcalcifications by radiologists is significantly reduced if mammograms are digitized at 0.1 mm x 0.1 mm. Our recent study also showed that detection accuracy by computer decreases as the pixel size increases from 0.035 mm x 0.035 mm. It is evident that very large matrix sizes have to be used for digitizing mammograms in order to preserve the information in the image. Efficient compression techniques will be needed to facilitate communication and archiving of digital mammograms. In this study, we evaluated two compression techniques: full frame discrete cosine transform (DCT) with entropy coding and Laplacian pyramid hierarchical coding (LPHC). The dependence of their efficiency on the compression parameters was investigated. The techniques were compared in terms of the trade-off between the bit rate and the detection accuracy of subtle microcalcifications by an automated detection algorithm. The mean-square errors in the reconstructed images were determined and the visual quality of the error images was examined. It was found that with the LPHC method, the highest compression ratio achieved without a significant degradation in the detectability was 3.6:1. The full frame DCT method with entropy coding provided a higher compression efficiency of 9.6:1 at comparable detection accuracy. The mean-square errors did not correlate with the detection accuracy of the microcalcifications. This study demonstrated the importance of determining the quality of the decompressed images by the specific requirements of the task for which the decompressed images are to be used. Further investigation is needed for selection of optimal compression technique for digital mammograms.

Digital Breast Imaging: Tomosynthesis and Digital Subtraction Mammography

Advances in the computer technology and the introduction of new digital imaging detectors offer the potential for digital image acquisition and several new mammography techniques, such as tomosynthesis and digital subtraction mammography. Tomosynthesis is a method of obtaining tomographic images of a breast. In tomosynthesis, any number of tomographic planes may be reconstructed from a set of images obtained as the X-ray source is moved in an arc above the breast. By shifting and adding the information obtained at different source positions, any plane of the breast can be brought into a sharp focus, while structures outside this selected plane are blurred. This may lead to improved lesion detection, especially in dense breast tissue. Thus, tomosynthesis may play a role in improving breast cancer screening and lesion characterization. Digital subtraction mammography is a method of breast angiography. It is performed by obtaining a digital radiographic image before, and one or more digital radiographic images after the injection of a contrast agent such as iodine. The pre- and post-contrast images are subtracted, resulting in an image of the vascular structures in the breast. Because breast cancer lesions have increased vascularity, digital subtraction mammography may play an important role in improving lesion detection, characterizing lesions, monitoring response to therapy, and determining lesion extent.Thus, both of these new digital techniques have the potential to address the major limitation of conventional mammography, namely the difficulty in detecting cancer in radiographically dense breasts.

Photographic unsharp masking in chest radiography.

A photographic unsharp masking technique for improving the latitude of chest radiographs without sacrificing image contrast or detail is described. An unsharp mask film, prepared from a scout film of the patients chest, is placed between the film and the front (entrance) screen in the cassette. A second radiograph then is recorded using technique factors that provide a well-penetrated view of the central mediastinum, etc. The unsharp mask absorbs light from the screen in those areas of the chest that normally are well penetrated, preventing overexposure of these areas and resulting in an improved balance of densities across the chest image. Improvement of contrast by a factor of 2 is demonstrated for mediastinal and retrocardiac structures with no loss of contrast in the central lung fields. Nodule detection studies with a chest phantom and simulated nodules suggest that a single unsharp masked film provides higher nodule detection rates than a pair of films consisting of a normally penetrated and an overpenetrated view, possibly because of facilitation of visual search patterns and contrast/brightness adaptation mechanisms of the visual system. Initial clinical studies indicate that unsharp masking may provide additional useful clinical information.

Automated segmentation of regions of interest on hand radiographs

Most radiologists do not use texture information contained in the trabecular patterns of hand radiographs to diagnose erosive changes and demineralization due to systemic inflammatory diseases that affect the skeletal system. However, high-resolution digitization achievable by a laser digitizer now makes it possible to access texture information that may not be perceived visually. We are studying the feasibility of computer-assisted early detection of these processes with particular attention to patients with hyperparathyroidism. In this paper the methods used to extract a region of interest (ROI) for texture analysis are discussed. The techniques include multiresolution sensing, automatic adaptive thresholding, detection of orientation angle, and projection taken perpendicular to the line of least second moment. The methods were tested on a database of 50 pairs of hand radiographs. We segmented the middle and the index fingers with an average success rate of 83% per hand. For the segmented finger strips, we located ROIs on both the middle and the proximal phalanges correctly over 84% of the times. Texture information was collected in the form of a concurrence matrix within the ROI. This study is a prelude to evaluating the correlation between classification based on texture analysis and diagnosis made by experienced radiologists.

Computer‐aided diagnosis: Detection and characterization of hyperparathyroidism in digital hand radiographs

An automated method is developed for the detection and staging of skeletal changes due to hyperparathyroidism on digitized hand radiographs. Subperiosteal bony resorption, particularly along the radial margins of the middle and proximal phalanges, is among the earliest manifestations of secondary hyperparathyroidism. In order to quantify the severity of bone resorption in these regions, the computer method analyzes the roughness of the phalangeal margins, as projected on the radiograph. The regions of interest, which contain the phalanges, are obtained from the digitized hand radiographs by an image preprocessor. The radial margin of each phalanx is detected by a model-guided boundary-tracking scheme. The roughness of these boundaries is then quantified by the mean-square variation and the first moment of the power spectrum. A receiver operating characteristic (ROC) study comparing the computer detection of hyperparathyroidism with the diagnosis by three experienced skeletal radiologists was performed by evaluating 84 hand images from 22 patients. Our present computer method can achieve a true-positive rate of 94% and a true-negative rate of 92%. Such a computer-aided diagnosis system may assist radiologists in their assessment of primary and secondary hyperparathyroidism, since it is both accurate and not subject to either intra- or interobserver variations.