Richard M. Slone

JPEG 2000 compression of medical imagery

A multi-institution effort was conducted to assess the visual quality performance of various JPEG 2000 (Joint Photographic Experts Group) lossy compression options for medical imagery. The purpose of this effort was to provide clinical data to DICOM (Digital Imaging and Communications in Medicine) WG IV to support recommendations to the JPEG 2000 committee regarding the definition of the base standard. A variety of projection radiographic, cross sectional, and visible light images were compressed-reconstructed using various JPEG 2000 options and with the current JPEG standard. The options that were assessed included integer and floating point transforms, scalar and vector quantization, and the use of visual weighting. Experts from various institutions used a sensitive rank order methodology to evaluate the images. The proposed JPEG 2000 scheme appears to offer similar or improved image quality performance relative to the current JPEG standard for compression of medical images, yet has additional features useful for medical applications, indicating that it should be included as an additional standard transfer syntax in DICOM.

PREOPERATIVE AND POSTOPERATIVE IMAGING IN THE SURGICAL MANAGEMENT OF PULMONARY EMPHYSEMA

For patients with emphysema, imaging studies have been useful for diagnostic purposes and for preoperative patient selection for surgical intervention, such as bullectomy, lung transplantation, and LVRS. Chest radiography is useful in evaluating hyperinflation. Inspiratory and expiratory films are used to estimate diaphragmatic excursion and air-trapping. CT scan is used to evaluate the anatomy and distribution of emphysema throughout the lungs, providing information clinically unobtainable by other means. Both imaging techniques are useful for detecting other disease processes. Radionuclide lung scanning also provides an estimate of target areas, volume occupying but nonfunctioning lung. Cohort studies utilizing these imaging techniques have demonstrated associations between preoperative characteristics and postoperative outcome. The imaging studies, especially the chest radiograph, have also played an important role in postoperative management. Many other imaging options are available, such as HRCT scan, quantitative CT scan, and single photon emission CT scan. Other techniques, such as MR imaging, may play a future role as well.

Perceived Fidelity of Compressed and Reconstructed Radiological Images: A Preliminary Exploration of Compression, Luminance, and Viewing Distance

The authors’ goal was to explore the impact of image compression algorithm and ratio, image luminance, and viewing distance on radiologists’ perception of reconstructed image fidelity. Five radiologists viewed 16 sets of four hard-copy chest radiographs prepared for secondary interpretation. Each set included one uncompressed, and three compressed and reconstruted images prepared using three different algorithms but the same compression ratio. The sets were prepared using two subjects, four compression ratios (10∶1, 20∶1, 30∶1, 40∶1), and two luminance levels (2,400 cd/m2, standard lightbox illumination, and 200 cd/m2, simulating a typical CRT display). Readers ranked image quality and evaluated obviousness and clinical importance of differences. Viewing distances for image screening, inspection, and comparison were recorded. At 10∶1 compression, the compressed and uncompressed images were nearly indistinguishable; the three algorithms were very similar, and differences were rated “not obvious” and “not important.” At higher compression, readers consistently preferred uncompressed images, with notable differences between algorithms. The obviousness and clinical importance of differences were rated higher at lightbox luminance. Viewing distances appeared to be idiosyncratic

Filmless digital chest radiography within the radiology department

The technical purposes of this work were to develop improvements in the methodology for assessing the physical performance of CRT monitors and display controller systems and to explore image processing techniques to make soft- and hard-copy image quality visually similar. The clinical purpose was to determine whether, with proper image processing, soft-copy presentations of digital chest radiographs could become equivalent to hard-copy for visualizing normal and pathological features. The luminance characteristic curve, luminance uniformity, modulation transfer function, and noise power spectra of the CRT monitors as well as video waveforms of a display controller were measured. Posteroanterior and lateral chest radiographs were acquired by a dedicated thorax imaging system with a selenium detector and processed using a previously optimized algorithm for printing on film. A Laplacian pyramid filter was employed to compensate for the mid- to high-frequency contrast losses in the soft-copy presentation. Five chest radiologists directly compared the soft- and hard-copy presentations in eighteen patients with CT-proven pathologies. Based on 99 percent confidence intervals, the soft-copy images were preferred for seven of the fourteen anatomic categories and image contrast, and the hard-copy images were preferred for brightness and image granularity. There were no preferences for the depiction of pathologies, spatial resolution, and the remaining anatomic categories. After determining the physical properties of the CRT monitors, image processing operations can be defined to produce soft-copy renditions of soft-copy displays for primary diagnosis to make digital radiography more cost- effective and to encourage additional development of filmless image interpretation and management in a PACS.

Dynamic viewing protocols for diagnostic image comparison

There is an ongoing need to evaluate the impact of various digital image processing and display variables on diagnostic image quality. In most cases, evaluation includes comparison of images, often multiple versions of the same image. In order to improve speed and sensitivity, new protocols were developed to enhance a radiologists ability to detect subtle changes in images and provide a means to quantify differences in a standard fashion. The protocols make use of the rapid sequential display of registered images on a single high- resolution CRT (a.k.a., flicker) and 2X magnification in order to increase observer sensitivity. The flicker technique was implemented in the form of an image comparison workstation (ICW) that was designed to facilitate the evaluation of different image processing options. The ICW was developed with capabilities to interactively control the rate of flicker between image pairs (up to 5 Hz), the degree of image magnification (1X to 4X), and the selection of the region of interest (ROI). Three specific protocols were developed based on the flicker technique, two forms of forced-choice and a rank-ordering protocol employing a reference set comprised of images with varying degrees of spatial-resolution degradation. All three protocols were exercised as part of an observe study whose goal was to establish visually lossless compression levels for JPEG and a wavelet-transform based algorithm. The results indicate that, for high resolution digitally acquired posteroanterior (PA) chest radiographs presented to observers at 2X magnification on a 2K X 2.5K addressable pixel monochrome display, the visually lossless thresholds for both JPEG and wavelet occur in the range of 2.0 to 1.5 bits-per- pixel (approximately equals 10:1). These results are a conservative estimate of the visually lossless threshold because of the sensitive nature of the experimental methodology.

Significance of exposure data recognizer modes in computed radiography

To take advantage of the wide latitude of computed radiography (CR), the AC series manufactured by Fuji uses an exposure data recognizer (EDR) system which may be operated in one of three modes: automatic, semi-automatic (semi auto), and fixed. This study evaluated the performance of these EDR modes in comparison with a conventional screen-film combination. Multiple images of foot and skull phantoms were obtained and measurements made of the resultant film density, density variability and contrast. The auto mode could correct for inaccurate exposures but introduced additional variability to CR film density. Semi auto mode had improved consistency but could result in suboptimal data processing parameters. For constant exposures, fixed mode had a consistency comparable to screen-film combinations but could not compensate for radiation dose variations. Selection of an optimal mode of operation requires an understanding of how the EDR affects CR performance and depends on the clinical problem at hand.

Comparison of hard and soft copy viewing of computed radiography portable chest radiographs

Soft-copy display is emerging as a practical means of efficiently interpreting portable computed radiography exams. This study comparing the soft and hard-copy presentation of portable chest radiographs was undertaken to evaluate radiologist preference and confidence prior to implementing filmless soft-copy reading of portable chest radiographs. Seven radiologists with substantial previous experience interpreting portable chest radiographs directly compared 126 hard- and soft-copy presentations of computed radiography chest radiographs obtained over a two week period from a cardiac care unit. The radiologists first viewed the soft-copy on 1k X 1k monitors, using magnification and windowing tools when desired. Immediately following interpretation, a hard-copy produced from the same 2k data set and processed with identical parameters was reviewed and subjective comparisons recorded regarding the visibility and definition of lung pathology, soft tissues, bone detail and catheters. The use of magnification and windowing tools was recorded. The resulting data showed little perceived difference between the hard- and soft-copy images. The unmodified soft-copy images were considered equivalent for diagnostic purposes in regards to lung pathology in 78 percent, soft tissue 84 percent, bone detail 68 percent and catheters 76 percent. The hard-copy was more frequently considered better for bone detail and catheter visualization. The soft-copy was more frequently considered better for visualizing lung pathology and soft tissue structures. Changing the window settings and magnification comparison of images eased their concerns about the adequacy of soft-copy presentation of computed radiography chest images at least the equivalent of hard-copy for depiction of normal anatomy and pathologic features in most categories. The next step is to optimize the soft-copy display parameters for routine viewing and then for specific clinical questions.

Optimizing computed radiography processing parameters for different examinations, display types, and local preferences

Both computed radiography systems at our institution offer control over the tonal and spatial frequency characteristics of the images. Our radiologists considered the default processing parameters to be suboptimal for many examinations, but the range of possible combination is tremendous. Our purpose was to develop an efficient methodology for determining the optimal processing parameters to achieve the desired image appearance for specific examinations and display types.

Optimizing the tonescale display of computed radiography images

Computed radiography (CR) systems transform exposures incident on the imaging plates to image densities using examination specific tonescale display algorithms. A procedure is proposed which permits CR users to optimize these algorithms based on the premise that image contrast should be optimized. This procedure was applied to portable chest x-rays on a medical intensive care unit. Changes made to the display parameters supplied by the manufacturer resulted in an improved quality of the displayed image.