Data compression: effect on diagnostic accuracy in digital chest radiography

High-resolution digital images make up very large data sets that are relatively slow to transmit and expensive to store. Data compression techniques are being developed to address this problem, but significant image deterioration can occur at high compression ratios. In this study, the authors evaluated a form of adaptive block cosine transform coding, a new compression technique that allows considerable compression of digital radiographs with minimal degradation of image quality. To determine the effect of data compression on diagnostic accuracy, observer tests were performed with 60 digitized chest radiographs (2,048 x 2,048 matrix, 1,024 shades of gray) containing subtle examples of pneumothorax, interstitial infiltrate, nodules, and bone lesions. Radiographs with no compression, with 25:1 compression, and with 50:1 compression ratios were presented in randomized order to 12 radiologists. The results suggest that, with this compression scheme, compression ratios as high as 25:1 may be acceptable for primary diagnosis in chest radiology.     

Subperiosteal resorption: effect of full-frame image compression of hand radiographs on diagnostic accuracy.

Image compression is essential to handle a large volume of digital images, including computed tomographic, magnetic resonance, computed radiographic, and digitized images in a digital radiology operation. Developed during the past few years, full-frame bit allocation performed with the cosine transform technique has been proved to be an excellent irreversible image compression method. This article describes the effect, on the accuracy of diagnosis of subperiosteal resorption, of using the hardware compression module to produce hand radiographs. Receiver operating characteristic analysis of the interpretation of 71 radiographs by five observers demonstrated that there is no statistically significant difference in diagnostic accuracy between the original radiographs and compressed and reconstructed images obtained with a compression ratio as high as 20:1.     

Clinical evaluation of irreversible image compression: analysis of chest imaging with computed radiography

To implement a picture archiving and communication system, clinical evaluation of irreversible image compression with a newly developed modified two-dimensional discrete cosine transform (DCT) and bit-allocation technique was performed for chest images with computed radiography (CR). CR images were observed on a cathode-ray-tube monitor in a 1,024 X 1,536 matrix. One original and five reconstructed versions of the same images with compression ratios of 3:1, 6:1, 13:1, 19:1, and 31:1 were ranked according to quality. Test images with higher spatial frequency were ranked better than those with lower spatial frequency and the acceptable upper limit of the compression ratio was 19:1. In studies of receiver operating characteristics for scoring the presence or absence of nodules and linear shadows, the images with a compression ratio of 25:1 showed a statistical difference as compared with the other images with a compression ratio of 20:1 or less. Both studies show that plain CR chest images with a compression ratio of 10:1 are acceptable and, with use of an improved DCT technique, the upper limit of the compression ratio is 20:1.     

Evaluation of a quadtree-based compression algorithm with digitized urograms

The effect of a quadtree-based data-compression algorithm on the diagnostic yield in digitized radiographs was studied for 100 urograms. Each image was digitized and reviewed at nine decreasing compression ratios ranging from 90:1 to 4.2:1, followed by a review of the uncompressed digital images. Four radiologists independently reviewed the digitized images and the original radiographs and agreed on a reference standard of 201 findings. Sensitivity, measured by the number of findings noted on the compressed digital images, decreased with increasing compression ratios at and above the 11:1 level. No loss of sensitivity was noted with a compression ratio of 4.2:1. Sensitivity decreased more precipitously for calcifications than for soft-tissue masses. Only a minimal loss of sensitivity for bilateral renal function was noted, even with high compression ratios. False-positive rates were unaffected by compression. The authors conclude that quadtree compression ratios of 11:1 and higher may result in loss of sensitivity in clinically relevant findings.     

Chest radiography: comparison of high-resolution digital displays with conventional and digital film

This study was performed to compare the performances of observers using three display formats for chest radiography. The display formats were conventional radiographs, digitized radiographs (2,048 X 2,048 X 12 bits) printed on laser film, and digitized radiographs (2,048 X 2,048 X 12 bits) displayed on a high-resolution (2,560 X 2,048 X 12-bit) gray-scale display. The test set for the study consisted of 163 cases. Sixty-four of the cases were normal, whereas the 99 remaining cases demonstrated one or more common radiographic abnormalities. Nine abnormalities were selected for analysis: costophrenic angle blunting, interstitial disease, atelectasis, pneumothorax, parenchymal mass, consolidation, obstructive disease, hilar/mediastinal mass, and apical scarring. Six experienced general radiologists participated in the evaluation. Receiver operating characteristic curves were generated for each abnormality and display format. The results indicate that, while the three display formats are equivalent for the detection of some abnormalities, detectable differences in observer performance may be seen even at 2,048 X 2,048 X 12 bits for the detection of obstructive disease, pneumothorax, interstitial disease, and parenchymal masses.     

Evaluation of diagnostic accuracy of CRT monitor display for personal computer in the detection of small lung nodules: with particular emphasis on comparison between JPEG and wavelet compression

PURPOSE: To compare observer performance on cathode-ray-tube(CRT) monitors for personal computers with that on conventional radiographs in the detection of small lung nodules. MATERIALS AND METHODS: Fifty-eight normal chest radiographs and 58 chest radiographs with a small lung nodule were selected. Ten radiologists examined the original conventional films on a viewbox and digitized (8 bit) uncompressed and compressed images of the same patient on a color CRT monitor with a matrix of 1,600 x 1,200, and rated the presence of lung nodules with a five-level scale of confidence. The methods of compression used in this study were the JPEG and wavelet methods, with compression ratios of 6:1 and 15:1. Results were analyzed by receiver operating characteristic methods. RESULTS: There was no significant difference between film and digitized uncompressed and compressed images obtained by the JPEG and wavelet methods with a compression ratio of 6:1. No statistically significant difference was detected between film and digitized image with wavelet compression at 15:1. However, detection was less accurate on digitized images with JPEG compression at 15:1. CONCLUSION: Digitized (8 bit) uncompressed and compressed images with a compression ratio of 6:1 are acceptable for the detection of small lung nodules. Digitized compressed images at a compression ratio of 15:1 are also acceptable when the wavelet method is used.     

High-performance wavelet compression for mammography: localization response operating characteristic evaluation

PURPOSE: To evaluate the accuracy of a visually lossless, image-adaptive, wavelet-based compression method for achievement of high compression rates at mammography. MATERIALS AND METHODS: The study was approved by the institutional review board of the University of South Florida as a research study with existing medical records and was exempt from individual patient consent requirements. Patient identifiers were obliterated from all images. The study was HIPAA compliant. An algorithm based on scale-specific quantization of biorthogonal wavelet coefficients was developed for the compression of digitized mammograms with high spatial and dynamic resolution. The method was applied to 500 normal and abnormal mammograms from 278 patients who were 32-85 years old, 85 of whom had biopsy-proved cancer. Film images were digitized with a charge-coupled device-based digitizer. The original and compressed reconstructed images were evaluated in a localization response operating characteristic experiment involving three radiologists with 2-10 years of experience in reading mammograms. RESULTS: Compression rates in the range of 14:1 to 2051:1 were achieved, and the rates were dependent on the degree of parenchymal density and the type of breast structure. Ranges of the area under the receiver operating characteristic curve were 0.70-0.83 and 0.72-0.86 for original and compressed reconstructed mammograms, respectively. Ranges of the area under the localization response operating characteristic curve were 0.39-0.65 and 0.43-0.71 for original and compressed reconstructed mammograms, respectively. The localization accuracy increased an average of 6% (0.04 of 0.67) with the compressed mammograms. Localization performance differences were statistically significant with P = .05 and favored interpretation with the wavelet-compressed reconstructed images. CONCLUSION: The tested wavelet-based compression method proved to be an accurate approach for digitized mammography and yielded visually lossless high-rate compression and improved tumor localization.     

Applying computer-assisted detection schemes to digitized mammograms after JPEG data compression: an assessment

RATIONALE AND OBJECTIVES: The authors' purpose was to assess the effects of Joint Photographic Experts Group (JPEG) image data compression on the performance of computer-assisted detection (CAD) schemes for the detection of masses and microcalcification clusters on digitized mammograms. MATERIALS AND METHODS: This study included 952 mammograms that were digitized and compressed with a JPEG-compatible image-compression scheme. A CAD scheme, previously developed in the authors' laboratory and optimized for noncompressed images, was applied to reconstructed images after compression at five levels. The performance was compared with that obtained with the original noncompressed digitized images. RESULTS: For mass detection, there were no significant differences in performance between noncompressed and compressed images for true-positive regions (P = .25) or false-positive regions (P = .40). In all six modes the scheme identified 80% of masses with less than one false-positive region per image. For the detection of microcalcification clusters, there was significant performance degradation (P < .001) at all compression levels. Detection sensitivity was reduced by 4%-10% as compression ratios increased from 17:1 to 62:1. At the same time, the false-positive detection rate was increased by 91%-140%. CONCLUSION: The JPEG algorithm did not adversely affect the performance of the CAD scheme for detecting masses, but it did significantly affect the detection of microcalcification clusters.     

Evaluation of JPEG and wavelet compression of body CT images for direct digital teleradiologic transmission

PURPOSE: To determine acceptable levels of JPEG (Joint Photographic Experts Group) and wavelet compression for teleradiologic transmission of body computed tomographic (CT) images. MATERIALS AND METHODS: A digital test pattern (Society of Motion Picture and Television Engineers, 512 x 512 matrix) was transmitted after JPEG or wavelet compression by using point-to-point and Web-based teleradiology, respectively. Lossless, 10:1 lossy, and 20:1 lossy ratios were tested. Images were evaluated for high- and low-contrast resolution, sensitivity to small signal differences, and misregistration artifacts. Three independent observers who were blinded to the compression scheme evaluated these image quality measures in 20 clinical cases with similar levels of compression. RESULTS: High-contrast resolution was not diminished with any tested level of JPEG or wavelet compression. With JPEG compression, low-contrast resolution was not lost with 10:1 lossy compression but was lost at 3% modulation with 20:1 lossy compression. With wavelet compression, there was loss of 1% modulation with 10:1 lossy compression and loss of 5% modulation with 20:1 lossy compression. Sensitivity to small signal differences (5% and 95% of the maximal signal) diminished only with 20:1 lossy wavelet compression. With 10:1 lossy compression, misregistration artifacts were mild and were equivalent with JPEG and wavelet compression. Qualitative clinical findings supported these findings. CONCLUSION: Lossy 10:1 compression is suitable for on-call electronic transmission of body CT images as long as original images are subsequently reviewed.     

Comparison of digital and conventional musculoskeletal radiography: an observer performance study

Interpretations of 122 musculoskeletal radiographs were compared with interpretations of their digital counterparts at a resolution of 1,024 X 840 X 12 bits. Images were evaluated by four readers and included subtle and nonsubtle abnormalities and normal findings. Joint receiver operating characteristic (ROC) analysis results were averaged over all readers and demonstrated no statistically significant difference between the two imaging methods. High interobserver variability limited identifiable differences. Review of cases with subtle findings on the digital system at a resolution of 2,048 X 1,680 X 12 bits (2.5 line pairs per millimeter) revealed adequate visualization of the abnormality in every case. It is concluded that a resolution of 1,024 X 840 X 12 bits is adequate for the interpretation of many musculoskeletal abnormalities but that a resolution of 2,048 X 1,680 X 12 bits is needed for visualization of some subtle abnormalities.     

Gastrointestinal examinations with digital radiography.

Basic imaging properties and clinical usefulness of an upgraded digital radiography system were evaluated. The system, which has 1,024 x 1,024 and 2,048 x 2,048 matrices, was upgraded with smaller focal spots (0.3 and 0.8 mm) and reduced thickness of the photoconductive layer of the video camera. Screen-film and digital images (with and without postprocessing) of the upper and lower gastrointestinal (GI) tract were used in the clinical evaluation. Overall modulation transfer functions of the upgraded digital system were comparable to those of the screen-film system, especially at the lower spatial frequency. Threshold contrasts of the two systems were similar despite a 50% reduction in incident exposure for the digital system. For the upper GI tract, digital images processed with unsharp masking techniques were comparable in quality to screen-film images before and after upgrade of the system. For the lower GI tract, screen-film images were better than digital images, except for those produced with a 2,048 x 2,048 matrix with unsharp masking. Further evaluation of the system for examination of other parts of the body seems warranted.     

The influence of liquid crystal display (LCD) monitors on observer performance for the detection of nodular lesions on chest radiographs

Purpose To access the influence of liquid crystal display (LCD) monitors on the detectability of nodular lesions depicted on chest radiographs by comparing them with a high-resolution cathode ray tube (CRT) monitor. Material and methods Ten radiologists interpreted 247 soft-copy images on LCD monitors with pixel arrays of 1,024×1,280, 1,200×1,600, 1,536×2,048 and 2,048×2,560, and a CRT monitor with a pixel array of 2,048×2,560, and were asked to indicate their individual confidence levels regarding the presence of a nodule. These images were chest radiographs with and without a lung nodule from the “Standard Digital Image Database” created by the Japanese Society of Radiological Technology. The luminance distributions of all monitors were adjusted to the same, and the ambient illumination was 200 lux. Observer performance was analyzed in terms of the receiver-operating characteristics. Results No significant statistical differences in nodule detection performance were found among the four LCD monitors and the CRT monitor. Conclusion The nodule detection performance on the LCD monitors with a spatial resolution higher than a matrix size of 1,024×1,280 was found to be equivalent to that on the high-resolution CRT monitor.     

ROC assessment of compressed wrist radiographs

The aim of this paper is to validate a compression scheme applied on a medical image database of digitized wrist radiographs. The compression scheme adapts itself to local statistical properties of the images. The diagnostic quality of the reconstructed images is evaluated using a ROC protocol involving five medical experts. The results of this evaluation enable us to validate the compression scheme on this database with a compression ratio of 40 (0.2 bits per pixel).     

Effect of manual compared with reference point superimposition on image quality in digital subtraction radiography

The aim of this study was to compare a new subtraction program based on positioning of reference points (RP) in the two images with the classic manual (M) superimposition of the images during recording. The experiments were performed on 22 dry mandibles from domestic pigs. A bone chip was prepared from the cortex with seven edges: 0.26, 0.30, 0.36, 0.42, 0.49, 0.55 and 0.72 mm thick respectively. Each mandible was radiographed at 0 degrees vertical angulation without the bone chip. The bone chip was then fixed to the lingual plate for a series of radiographs during which the vertical angulation was varied from 0 degrees to 10 degrees at 1 degree intervals. After completion of this series of exposures, amalgam fillings were placed in the two molars on the right side and a second series obtained. The radiographs were digitized (512 x 512 x 8 bit resolution), stored in a personal computer and then subtracted in a random order by the two methods. The RP method was found to be superior to the M method for all the angulations evaluated; as the images were more homogeneous as shown by the smaller SD in the grey-scale histogram. The best results were obtained on images with amalgam-filled teeth. Significantly more bone edges were seen with the RP method, especially when amalgam fillings were present. The RP method may therefore be superior to the M in clinical trials using subtraction radiography.     

Irreversible JPEG 2000 compression of abdominal CT for primary interpretation: assessment of visually lossless threshold

To estimate the visually lossless threshold for Joint Photographic Experts Group (JPEG) 2000 compression of contrast-enhanced abdominal computed tomography (CT) images, 100 images were compressed to four different levels: a reversible (as negative control) and irreversible 5:1, 10:1, and 15:1. By alternately displaying the original and the compressed image on the same monitor, six radiologists independently determined if the compressed image was distinguishable from the original image. For each reader, we compared the proportion of the compressed images being rated distinguishable from the original images between the reversible compression and each of the three irreversible compressions using the exact test for paired proportions. For each reader, the proportion was not significantly different between the reversible (0–1%, 0/100 to 1/100) and irreversible 5:1 compression (0–3%). However, the proportion significantly increased with the irreversible 10:1 (95–99%) and 15:1 compressions (100%) versus reversible compression in all readers (P < 0.001); 100 and 95% of the 5:1 compressed images were rated indistinguishable from the original images by at least five of the six readers and all readers, respectively. Irreversibly 5:1 compressed abdominal CT images are visually lossless and, therefore, potentially acceptable for primary interpretation. This study was supported by Seoul R&BD Program, Republic of Korea (project number, 10675).     

JPEG2000 compression of thin-section CT images of the lung: effect of compression ratio on image quality

PURPOSE: To assess retrospectively the effect of the Joint Photographic Experts Group 2000 (JPEG2000) compression ratio on the quality of thin-section computed tomographic (CT) images. MATERIALS AND METHODS: In this institutional review board-approved investigation (protocol 238/2004), thin-section CT images were subjected to irreversible JPEG2000 compression by using five compression ratios (3:1, 5:1, 7:1, 9:1, and 11:1). Three radiologists independently evaluated 60 thin-section CT images, of various diseases, that were obtained with single-detector (weighted dose index, 14.4 mGy) and multidetector (weighted dose index, 9.8 mGy) CT. Toggling between the original and compressed images, readers had to identify the original image by using a forced-choice two-alternative model and to subjectively rank the quality of what they believed to be the compressed image. To assess the reader's ability to distinguish the compressed from the original image, a binomial test was used. Bonferroni correction was applied for all multiple tests. RESULTS: Images compressed with a ratio of 3:1 were not distinguishable from original images (P > .2 for all readers). With use of the 5:1 ratio, minor differences in appearance between the compressed and original images were seen by one of the three readers. With use of higher compression ratios (>/=7:1), all readers (P < .001) recognized the original image. The quality of more than 90% of the images compressed with a 7:1 or higher ratio was substantially degraded. Single-detector and multidetector CT results were not significantly different. CONCLUSION: The highest ratio that yielded visually lossless compression of thin-section CT images was 3:1. With the 5:1 ratio, there was minor image quality loss, while use of higher compression ratios (>/=7:1) caused substantial degradation of image quality and potential loss of diagnostic information.     

Subtle lung cancers: impact of edge enhancement and gray scale reversal on detection with digitized chest radiographs

The authors studied the impact of edge enhancement and gray scale polarity reversal on the detection of subtle lung cancers. Three experienced readers reviewed 46 biopsy-proved subtle lung cancers and 46 normal controls on chest radiographs that had been digitized into a 1,024 X 1,536-pixel matrix 8 bits deep. Receiver-operating characteristics (ROC) analysis of 1,656 pooled observations indicated that performance was best with the unmodified images (ROC area = 0.83), degraded by moderate enhancement of medium frequencies (ROC area = 0.80), and markedly impaired by severe enhancement of low frequencies (ROC area = 0.69). Gray scale polarity reversal further degraded performance (unenhanced ROC area = 0.74; moderately enhanced ROC area = 0.76; severely enhanced ROC area = 0.76). The authors conclude that edge enhancement and gray scale polarity reversal can impair the detectability of subtle lung cancers on digitized radiographs of medium resolution.     

Temporal subtraction for detection of solitary pulmonary nodules on chest radiographs: evaluation of a commercially available computer-aided diagnosis system

PURPOSE: To evaluate the usefulness of a commercially available computer-aided diagnosis (CAD) system that incorporates temporal subtraction for the detection of solitary pulmonary nodules on chest radiographs by readers with different levels of experience. MATERIALS AND METHODS: Sixty pairs of chest radiographs in 30 patients with newly detected solitary pulmonary nodules and 30 normal cases, all confirmed with serial chest computed tomography (CT), were obtained from screen-film or digital radiographic systems and were digitized (spatial resolution, 0.171 mm/pixel). Temporal subtraction images were produced with an iterative image-warping technique. Five chest radiologists and five residents evaluated both image sets for solitary nodules: set A, current and prior radiographs with temporal subtraction images, and set B, current and prior radiographs only. Assessment was performed with receiver operating characteristic (ROC) analysis of the images on a monitor (pixel size, 1,280 x 1,024) equipped with the system. The reading time needed by each reader was recorded in each case. RESULTS: For the chest radiologists, no statistically significant difference was found between set A (area under the ROC curve [A(z)] = 0.934) and set B (A(z) = 0.964). For the residents, however, observer performance in set A (A(z) = 0.907) was superior to that in set B (A(z) = 0.855) (P <.05). For both groups, the mean reading time per case for set A (chest radiologists, 16.7 seconds; residents, 15.7 seconds) was significantly (P <.05) shorter than that for set B (chest radiologists, 20.4 seconds; residents, 26.2 seconds). CONCLUSION: For the detection of solitary pulmonary nodules, the CAD system with temporal subtraction can promote efficiency for established chest radiologists and improvement in accuracy for less experienced readers.     

Acute cerebral infarction: effect of JPEG compression on detection at CT

PURPOSE: To evaluate the effect of Joint Photographic Experts Group (JPEG) compression ratios of 10:1 and 20:1 on detection of acute cerebral infarction at computed tomography (CT). MATERIALS AND METHODS: CT images obtained in 25 patients with acute cerebral infarction and 25 patients with no lesions were compressed by means of a JPEG algorithm at ratios of 10:1 and 20:1. Normal and abnormal sections (on original and compressed images) were reviewed by using a color soft-copy computed monochrome cathode ray tube monitor. Five observers rated the presence or absence of a lesion with a 50-point scale (0, definitely absent; 25, equivocal; and 50, definitely present). Diagnostic accuracy was evaluated with receiver operating characteristic (ROC) curve analysis. Significant difference was defined as a P value less than.05 for the area tested with a two-tailed paired Student t test. RESULTS: At ROC analysis, no statistically significant difference was detected for all cases considered together (Az [area under the ROC curve] = 0.887 +/- 0.038 [mean +/- SD] on noncompressed images, Az = 0.897 +/- 0.038 on 10:1 compressed images, and Az = 0.842 +/- 0.073 on 20:1 compressed images; P >.05). CONCLUSION: JPEG compression at ratios of 10:1 and 20:1 was tolerated in the detection of acute cerebral infarction at CT.     

Clinical trial of digital teleradiology in the practice of emergency room radiology

The application of digital teleradiology (DTR) to radiologic examinations performed in the emergency room was evaluated. A total of 919 examinations (ten computed tomographic; the rest, radiographic) were transmitted to a radiology resident at another hospital emergency room. The 512 X 512 images were reviewed by an attending radiologist and compared with another attending radiologist's interpretation of the original films. Cases with discrepant interpretations were analyzed. Inadequate DTR image quality was responsible for clinically significant discrepancies in 14 of 897 cases (1.6%) available for follow up. Problem areas such as the detection of pneumothorax and abdominal calcifications were identified. Retransmission of optically zoomed images of areas of concern and repeat radiographs of overpenetrated films are suggested to improve DTR performance.