Search

We compared the accuracy of five veterinary radiologists when reading 20 radiographic cases on both analog film and in camera-digitized format. In addition, we compared the ability of five veterinary radiologists vs. 10 private practice veterinarians to interpret the analog images. Interpretation accuracy was compared using receiver operating characteristic curve analysis. Veterinary radiologists' accuracy did not significantly differ between analog vs. camera-digitized images (P = 0.13) although sensitivity was higher for analog images. Radiologists' interpretation of both digital and analog images was significantly better compared with the private veterinarians (P < 0.05).

Rationale and Objectives: Use data collected independently at three institutions to compare time to first fixate the true lesion in searching for cancers on mammograms. Examine the fit of the results to a holistic model of visual perception., Materials and Methods: The time required to first fixate a cancer on a mammogram was extracted from 400 eye-tracking records collected independently from three institutions. The time was used as an indicator of the initial perception of cancer. The distribution of first fixation times was partitioned into two normally distributed components using mixture distribution analysis. The true-positive fraction of each component was calculated., Results: About 57% of the cancers had a 95% chance of being fixated in the first second of viewing. The remainder took longer (range, 1.0 to 15.2 seconds). The true-positive fraction was larger for the lesions hit immediately for most of the readers (TPF = 0.63 vs. 0.52, F = 5.88, P = .02) in 68% (13/19) of the readers., Conclusions: The initial detection occurs before visual scanning and, therefore, must be the result of a parallel "global" analysis of the image resulting in an initial holistic, gestalt-like perception. The development of expertise in medical image analysis may consist of a shift in the recognition mechanism from scan-look-detect to look-detect-scan.

Purpose: To test the hypothesis that rapid and accurate performance of the proficient observer in mammogram interpretation involves a shift in the mechanism of image perception from a relatively slow search-to-find mode to a relatively fast holistic mode., Materials and Methods: This HIPAA-compliant study had institutional review board approval, and participant informed consent was obtained; patient informed consent was not required. The eye positions of three full-time mammographers, one attending radiologist, two mammography fellows, and three radiology residents were recorded during the interpretation of 20 normal and 20 subtly abnormal mammograms. The search time required to first locate a cancer, as well as the initial eye scan path, was determined and compared with diagnostic performance as measured with receiver operating characteristic (ROC) analysis., Results: The median time for all observers to fixate a cancer, regardless of the decision outcome, was 1.13 seconds, with a range of 0.68 second to 3.06 seconds. Even though most of the lesions were fixated, recognition of them as cancerous ranged from 85% (17 of 20) to 10% (two of 20), with corresponding areas under the ROC curve of 0.87-0.40. The ROC index of detectability, d(a), was linearly related to the time to first fixate a cancer with a correlation (r(2)) of 0.81., Conclusion: The rapid initial fixation of a true abnormality is evidence for a global perceptual process capable of analyzing the visual input of the entire retinal image and pinpointing the spatial location of an abnormality. It appears to be more highly developed in the most proficient observers, replacing the less efficient initial search-to-find strategies., ((c) RSNA, 2007.)

Two experiments were performed during which adults untrained in the visual arts were shown digital versions of eight paintings by renowned artists. In Experiment 1 participants' written reactions following a single 100 ms glance at each work were found to overwhelmingly reflect an initial holistic impression (i.e. gist) of the structural arrangement and semantic meaning of the paintings. In the second experiment participants' eye movements and verbal reactions were recorded as they evaluated each reproduction for pleasingness. Analyses reveal the relationships between the content and structural organization of the art stimuli and the way viewers select, process and think about information contained in paintings across the time course of an aesthetic experience. The results are interpreted in terms of an information-processing stage model of visual aesthetics according to which perceptual-cognitive processing of an art stimulus begins with the rapid generation of a gist reaction followed by scrutiny of pictorial features directed in a top-down fashion by cognitively-based evaluative processes.

The primary detector of breast cancer is the human eye. Radiologists read mammograms by mapping exogenous and endogenous factors, which are based on the image and observer, respectively, into observer-based decisions. These decisions rely on an internal schema that contains a representation of possible malignant and benign findings. Thus, to understand the hits and misses made by the radiologists, it is important to model the interactions between the measurable image-based elements contained in the mammogram and the decisions made. The image-based elements can be of two types, i.e., areas that attracted the visual attention of the radiologist, but did not yield a report, and areas where the radiologist indicated the presence of an abnormal finding. In this way, overt and covert decisions are made when reading a mammogram. In order to model this decision-making process, we use a system that is based upon the processing done by the human visual system, which decomposes the areas under scrutiny in elements of different sizes and orientations. In our system, this decomposition is done using wavelet packets (WPs). Nonlinear features are then extracted from the WP coefficients, and an artificial neural network is trained to recognize the patterns of decisions made by each radiologist. Afterwards, the system is used to predict how the radiologist will respond to visually selected areas in new mammogram cases.

Objective: This article describes the time course of lesion detection on digital mammograms using data about both eye position and decision time to compare performance between experienced mammographers and trainees. Research indicates that a longer decision time works against performance in the interpretation of chest radiographs because the likelihood of error is increased, particularly for trainees. Is this relation between decision time and performance also true for interpreting mammograms? Is there an optimal decision time-performance trade-off for detecting breast lesions?, Materials and Methods: Six radiology trainees (experience, 302-976 cases) and three mammographers (experience, 3000-5000 cases per year) reviewed 40 test cases. Each test case was represented by two mammograms that showed different views of the same breast. Twenty breasts contained suspicious lesions, and 20 were lesion-free. An interactive computer display system with an eye-head tracker measured the timing of decisions, where visual attention was directed, and how much time was spent fixating on a region of interest for each decision. Eye position was monitored during an initial-decision phase, and decision times were measured throughout a final-decision phase during which suspicious lesions recognized initially were interpreted and localized. Performance was analyzed using localization receiver operating characteristic curves., Results: The time course of interpreting mammograms is similar to that for interpreting chest radiographs. Mammographers detected 71% of the true lesions within 25 sec, and trainees detected 46% within 40 sec. Both a fixation dwell time of 1000 msec and a high level of confidence in the decision were associated with the detection of true lesions for the mammographers but not for the trainees., Conclusion: Mammographers detected most breast lesions by global recognition within 25 sec, but trainees took more time. Prolonging one's search beyond the global recognition phase yielded few new lesions and increased the risk of error.

Rationale and Objectives: Mammographers map endogenous and exogenous factors into decisions whether to report the presence of a malignant finding in a mammogram case. Thus, to understand how image-based elements are translated into observer-based decisions, the authors used spatial frequency analysis to model the areas on mammograms that attracted visual attention, in addition to the areas localized as abnormal., Materials and Methods: Four mammographers read 40 two-view mammogram cases, of which 30 contained at least one malignant lesion visible on one or two views. Their eye positions were recorded during visual search. Once the mammographer felt confident enough to provide an initial impression of the case ("normal" or "abnormal"), the eye position monitoring was turned off and the mammographer indicated, with a mouse-controlled cursor, the location and nature of any malignant findings. Regions that elicited an overt or a covert response by the mammographers were extracted for processing by means of wavelet packets and artificial neural networks., Results: Different decision outcomes yielded different energy representations, in the spatial frequency domain. These energy representations were used by an artificial neural network to predict decision outcome in areas of interest, derived from eye position analysis, on mammograms from new cases. Individual trends were observed for each mammographer., Conclusion: Spatial frequency representation of regions that attracted a given mammographer's visual attention may be useful for characterizing how that mammographer will respond to the visually selected areas.

Purpose: To determine whether unreported retrospectively identified cancers on mammograms receive prolonged visual attention and can be reliably detected in a blinded review., Materials and Methods: Four experienced mammographers performed a blinded review of a test set of 20 retrospective cases where the cancer was not detected until the next mammographic evaluation, 10 prospective cases where the cancer was initially detected, and 10 cancer-free cases. Two views were digitized and displayed on a workstation. The experiment consisted of an initial impression, during which eye position was monitored, and a final impression, during which viewers zoomed on regions of interest and localized suspicious lesions. Eye-position data were analyzed to determine whether retrospectively visible cancers attracted attention to the same degree as prospectively visible cancers. The initial impression used 1,000 msec as the eye-fixation dwell criterion for detecting a lesion., Results: Initially, 70% of retrospective cancers and 50% of prospective cancers did not attract prolonged visual attention. In prospective cases, detailed examination significantly improved the mean receiver operating characteristic area, from.73 to.88 (P <.01), but in retrospective cases, the mean receiver operating characteristic area barely increased, from.60 to.68, due to a high true-positive-to-false-positive ratio., Conclusion: At blinded review, detection of retrospectively visible cancers was significantly inferior to that of prospective cancers. It cannot be assumed that retrospectively identified cancers are intrinsically detectable, because they do not draw prolonged visual attention during visual search for breast cancers.

In this pilot study the authors examined areas on a mammogram that attracted the visual attention of experienced mammographers and mammography fellows, as well as areas that were reported to contain a malignant lesion, and, based on their spatial frequency spectrum, they characterized these areas by the type of decision outcome that they yielded: true-positives (TP), false-positives (FP), true-negatives (TN), and false-negatives (FN). Five 2-view (craniocaudal and medial-lateral oblique) mammogram cases were examined by 8 experienced observers, and the eye position of the observers was tracked. The observers were asked to report the location and nature of any malignant lesions present in the case. The authors analyzed each area in which either the observer made a decision or in which the observer had prolonged (>1,000 ms) visual dwell using wavelet packets, and characterized these areas in terms of the energy contents of each spatial frequency band. It was shown that each decision outcome is characterized by a specific profile in the spatial frequency domain, and that these profiles are significantly different from one another. As a consequence of these differences, the profiles can be used to determine which type of decision a given observer will make when examining the area. Computer-assisted perception correctly predicted up to 64% of the TPs made by the observers, 77% of the FPs, and 70% of the TNs.

Rationale and Objectives: The authors evaluated the influence of perceptual and cognitive skills in mammography detection and interpretation by testing three groups representing different levels of mammography expertise in terms of experience, training, and talent with a mammography screening-diagnostic task., Materials and Methods: One hundred fifty mammograms, composed of unilateral cranial-caudal and mediolateral oblique views, were displayed in pairs on a digital workstation to 19 radiology residents, three experienced mammographers, and nine mammography technologists. One-third of the mammograms showed malignant lesions; two-thirds were malignancy-free. Observers interacted with the display to indicate whether each image contained no malignant lesions or suspicious lesions indicating malignancy. Decision time was measured as the lesions were localized, classified, and rated for decision confidence., Results: Compared with performance of experts, alternative free response operating characteristic performance for residents was significantly lower and equivalent to that of technologists. Analysis of overall performance showed that, as level of expertise decreased, false-positive results exerted a greater effect on overall decision accuracy over the time course of image perception. This defines the decision speed-accuracy relationship that characterizes mammography expertise., Conclusion: Differences in resident performance resulted primarily from lack of perceptual-learning experience during mammography training, which limited object recognition skills and made it difficult to determine differences between malignant lesions, benign lesions, and normal image perturbations. A proposed solution is systematic mentor-guided training that links image perception to feedback about the reasons underlying decision making.

The cathode ray tube of a workstation for use with digital mammograms was calibrated with a photometer to produce an input-output characteristic curve similar to the perceptually linear curve defined by a current display standard. Then, a test pattern consisting of bars of increasing intensity containing disks of decreasing contrast was used by an observer to estimate the minimal detectable contrast (MDC) at different levels of display luminance. The MDC was modeled by a parabola. The shape of the parabola was determined by the observer's perceptual responses, and the range was determined by the maximum and minimum pixel values of the breast parenchyma. As each mammogram was displayed, the contour of the breast was automatically found and pixels within the breast image were sampled to determine the pixel values that were used to compute the maximum and minimum pixel values. The parabola was integrated to determine the look-up table for the initial MDC-tempered display of the mammogram. Preliminary observer performance tests showed no significant differences in the accuracy and speed of three radiologists who read a set of mammograms when the MDC-tempered display was compared with the perceptually linear display.

Rationale and Objectives: The goal of this study was to determine if radiologists possess superior visual search and analysis skills compared with those of laypeople., Materials and Methods: In two experiments, radiologists and laypeople searched one of two complex pictorial scenes for hidden targets. Eye position was recorded during the search. Two measures of performance were obtained: accuracy of detecting targets as measured by using alternative free response receiver operating characteristic analysis and visual search efficiency as measured by using eye position analysis., Results: There were no statistically significant differences in detection performance between radiologists and laypeople for either of the search tasks. Radiologists took longer on average to search the images and to first fixate on the targets than did the laypeople. For both groups, true-positive and false-positive decisions were associated with longer dwell times than true-negative decisions. As with radiology search tasks, false-negative decisions were also associated with longer dwell times than true-negative decisions., Conclusion: Performance on two visual search and detection tasks indicate that radiologists do not possess superior visual skills compared with laypeople. Radiology expertise is more likely to be a combination of specific visual and cognitive skills derived from medical training and experience in detecting and determining the diagnostic importance of radiographic findings.

Rationale and Objectives: The authors investigated how training and experience affect the performance of observers searching mammograms for breast masses., Methods: Eye positions of mammographers, mammography technologists, mammography residents, and laypersons were compared to scan paths generated by a simulated scanner as each searched nine two-view digital mammogram pairs for breast masses., Results: Analysis of time-to-hit data revealed that mammographers and mammography technologists with the most extensive training and experience had the fastest search times in the detection and confirmation of a breast mass on two views. Scanning patterns of less-experienced mammography residents were less efficient due to wider dispersion of visual attention between potential breast masses and perturbations in breast parenchyma. Because laypersons lacked both training and experience in mammography, bright blobs in the breast image were considered to be intuitively valid target candidates and these features distracted the search by capturing visual attention., Conclusion: Experience reading normal and abnormal mammograms plays a critical role in training radiologists. Experience combined with training provides the basis for generating efficient visual search strategies and developing distinctive conceptual criteria for perceptual differentiation and interpretation of true breast masses from image artifacts and structured noise that mimics breast abnormalities.

Rationale and Objectives: To determine whether observer performance in the localization of tubes and catheters on computed radiography (CR) chest images can be improved by using an automatic image-processing algorithm., Methods: Comparisons were made of CR hard-copy, CR soft-copy, and CR enhanced soft-copy images obtained with an image-processing algorithm. The enhanced images used gray-level optimization and nonlinear unsharp masking to emphasize the edges of the devices. Chest radiologists (n = 4), general radiologists (n = 4), and interns (n = 6) read 45 images that contained endotracheal tubes, pulmonary artery catheters, and central venous catheters., Results: Chest radiologists had the smallest mean interobserver localization variability (4 mm), followed by general radiologists (6 mm) and interns (8 mm). Localization variability was greatest for hard-copy images and least for enhanced soft-copy images., Conclusion: Use of an automatic imaging-processing algorithm reduced localization variability and enabled the medical interns to perform at approximately the same level as the chest radiologists.

Patients over 50 years old with intrauterine low density on enhanced computed tomography were analyzed. Uterine volume and volume of intrauterine low density were calculated. Intrauterine low density was expressed as a percent of uterine volume. At 1-year follow-up, 23 (63.9%) had uterine malignancy and 13 (36.2%) had benign findings. All patients whose intrauterine low density exceeded 35% of the total uterine volume had a malignancy (p < 0.001). If intrauterine low density exceeds 35% of uterine volume, evaluation of uterine malignancy should be performed regardless of symptoms.

Purpose: To prospectively compare efficiency and outcome of a standard film-only system with those of a digital picture archiving and communication system (PACS)., Materials and Methods: The film-only system, which used either analog film or computed radiography (CR) hard copy, was compared with a PACS, which used CR images displayed on a multiviewer in the radiology department and a workstation in the medical intensive care unit. A random sample of nonroutine, bedside chest radiographs was studied., Results: Within 20 minutes of completion of radiography, 246 of 328 (75%) of the images were available at the workstations; it took 1.8 hours for 238 of 317 (75%) of the images to be displayed on the multiviewer. When the workstation was used, the staff did not access the image information earlier, but clinical actions were initiated more promptly in response to imaging findings. Consultation with radiologists decreased from 507 of 561 (90%) images with hard copies to 70 of 249 (28%) with the workstation., Conclusion: Use of a PACS improves the delivery of chest images, facilitates the initiation of clinical actions, and decreases input by radiologists.

Purpose: To use the eye position recordings of observers as they read chest images to advance understanding of the mechanism of "satisfaction of search," a phenomenon in which the detection of one abnormality interferes with the detection of other abnormalities., Materials and Methods: Eight radiologists examined chest images that contained simulated pulmonary nodules and native abnormalities. Accuracy in detecting nodules and native abnormalities and eye position, gaze duration, and total search time were recorded., Results: Nodule detectability was lower on native abnormality-containing images than it was on normal images (P < .005). Native-abnormality detectability was not negatively affected by the nodules. Most missed nodules were fixated; only those on images without native abnormalities, however, received prolonged visual attention., Conclusion: The satisfaction of search phenomenon is an important source of error in the detection of subtle abnormalities but not of obvious abnormalities. Obvious abnormalities capture visual attention and decrease vigilance for more subtle abnormalities.

Rationale and Objectives: We aimed to determine if the characteristics and principles of visual search described for the detection of pulmonary nodules apply to extremity fractures., Methods: The eye positions of staff orthopedic radiologists, radiology residents, and medical students were monitored as they searched hand and wrist X-ray images for fractures and a chest image for nodules., Results: More systematic scanning patterns were observed for experienced observers than inexperienced observers. Positive decisions for bone images were associated with prolonged gaze durations; prolonged gaze durations were significantly longer for false-negative versus true-negative decisions. Intercluster jump distances were found to be greater for chest images than bone images., Conclusions: A search for bone fractures can be qualitatively characterized by classifying observer scan paths, dwell times, and jump distances. Gaze duration can be a useful predictor of bone image locations containing potential missed fractures. Perceptual feedback could aid observers in the detection of inconspicuous fractures.

Purpose: To evaluate the role of biopsy-proved benign peripheral zone hypoechoic lesions of the prostate gland, ultrasonographic (US) characteristics at follow-up, prostate-specific antigen (PSA) levels, and digital rectal examination (DRE) in prediction of cancer risk., Materials and Methods: Retrospective analysis was performed for 105 consecutive patients with 148 benign hypoechoic lesions discovered at transrectal US (TRUS) and diagnosed with US-guided needle biopsy. At least one repeat TRUS study was performed in each patient., Results: Among the benign lesions, 72% changed at follow-up TRUS, either disappearing or becoming smaller, less hypoechoic, and more vague. Cancer developed in 13% of patients. In 93% of patients in whom cancer developed, the appearance changed in the peripheral zone at follow-up TRUS. In this patient population, the positive predictive value for development of cancer was 16% with a changing TRUS appearance, 19% with an abnormal DRE result, and 27% with an elevated level of PSA; only the latter was statistically significant., Conclusion: The PSA value, alone or in combination with a changing TRUS appearance, is the best indicator for development of cancer.

Purpose: To determine the effectiveness of pelvic sonography as a screening test for ectopic pregnancy., Materials and Methods: Pelvic sonograms were prospectively analyzed in 1,427 consecutive patients with a serum level of the beta subunit of human chorionic gonadotropin of over 1,500 IU/L., Results: Sonograms were diagnostic in 1,158 patients and indeterminate in 269. When indeterminate studies were considered falsely negative, the diagnostic accuracy was 81%. Twenty-four percent of patients with indeterminate studies were subsequently proved to have ectopic pregnancy. In ectopic pregnancy (n = 103), the most common finding was a complex adnexal mass (specificity = 92% [P < .001]). The sensitivity and specificity of screening sonography for ectopic pregnancy were 99% and 84%, respectively., Conclusion: Pelvic sonography is an effective screening test for ectopic pregnancy. Having a one in four chance of harboring an ectopic pregnancy, patients with indeterminate studies require close follow-up. The presence of a complex adnexal mass is a strong predictor of ectopic pregnancy.

Objective: Vesicourethral anastomotic strictures are an important complication of radical prostatectomy for prostatic cancer. Their formation has been attributed to extravasation of urine at the anastomosis and to the surgical technique used to construct the anastomosis. Our study examines whether the formation of a vesicourethral anastomotic stricture correlates with (a) contrast extravasation seen on postoperative voiding cystourethrograms and (b) the surgical technique used to construct the vesicourethral anastomosis. We also describe the postoperative appearances of the anastomosis., Materials and Methods: One hundred and forty-two patients who underwent radical retropubic prostatectomy at our institution between June 1, 1987, and December 31, 1991, were included in the study. Of these, 101 had a direct end-to-end vesicourethral anastomosis, and 41 had their anastomosis constructed with traction sutures (Vest procedure). Voiding cystourethrograms were obtained 3 weeks after the prostatectomy. The appearance of the anastomosis, the presence of extravasation of contrast material at the anastomotic site, and the relationship of the subsequent formation of an anastomotic stricture to extravasation were evaluated. The influence of the surgical technique used to construct the vesicourethral anastomosis on the development of anastomotic strictures was analyzed., Results: Contrast extravasation at the anastomotic site was seen in 14 (14%) of 101 patients who had a direct procedure and in three (7%) of 41 patients who had a Vest procedure. No relationship was found between contrast extravasation and subsequent formation of a stricture. Anastomotic strictures occurred in 16 (16%) of 101 patients who had a direct anastomosis and in 12 (29%) of 41 patients who had a Vest procedure. The surgical technique used to construct the vesicourethral anastomosis influenced the appearance of the vesicourethral anastomosis on cystourethrograms., Conclusion: Contrast extravasation at the anastomotic site is not infrequently seen on voiding cystourethrograms obtained after radical retropubic prostatectomy and resolves with continued drainage via a Foley catheter. As long as catheters are left in place until anastomotic healing is complete, extravasation of contrast material (implying urine extravasation at the anastomotic site) does not influence the subsequent formation of anastomotic strictures. Anastomoses that heal more slowly are no more likely to develop strictures than normally healing ones. Construction of the vesicourethral anastomosis by using the Vest procedure is a significant risk factor for stricture formation.

Under tachistoscopic viewing conditions, precuing the location of potential lung tumor targets in chest X-ray images was less effective than precuing followed by bounding the region of interest (ROI) with a circle directly on the image. Detection performance increased as the image was systematically masked so that its size approximated that of the circled ROI. When viewing time was extended to allow shifts in eye position, circling the ROI was found to restrict the dispersion of fixations and increase the accuracy of fixating the target tumor. When targets were placed outside the ROI, the circle inhibited their detection relative to detection of targets inside the circled region. These findings suggest that cuing by circling restricts target detection to the ROI, and by doing so reduces the interfering effects of outside distractors that complete with the target for attention.

Rationale and Objectives: Feedback of chest areas receiving prolonged gaze durations significantly increases nodule detection performance. Why feedback circling enhances performance when other cueing methods produce equivocal results was assessed., Methods: Chest and noise images with nodule targets were used to determine: what type of cue is most effective; whether circling influences the way the eye samples the target; whether circling limits processing of distracting information outside its boundary., Results: Circling improves performance more than cues with less complete boundaries and increases the accuracy and frequency with which nodules are fixated. Outside distractors were detected less often with than without the circle present., Conclusions: Circling isolates the abnormal region from the rest of the image, making disembedding and integration of nodule features more likely and insulates this region from distractors. The facilitative effects of circling are generalizable to other images in which low contrast targets are embedded in noisy backgrounds.

Four subjects searched a chest image for lung nodules using an eye position-dependent display that presented the image of a nodule selectively to either the central or peripheral visual field, while preserving the appearance of the chest. The time required to scan the image and fixate the nodule was shortest for nodules that were both reported and accessible to the peripheral vision. A stepwise concentric reduction in the size of the peripheral field that could access the nodules only affected search performance when the field was less than 5 degrees. (A chest image subtends about 25 degrees.) These data support the hypothesis that the optimal scanning strategy for lung nodules consists of spacing fixation clusters 5 degrees apart, and that peripheral vision only acts as an adjunct in guiding the gaze to inconspicuous nodules.

Approximately 30% of nodules are missed during the initial reading of chest radiographs. Eye-position recordings have shown that most nodules that are missed receive prolonged visual attention. A computer algorithm was developed that uses eye-position and gaze-duration times to identify locations on the chest image likely to contain missed nodules. These locations are highlighted on the displayed image to give visual feedback. The current study tested whether visual feedback was an effective aid to nodule detection. Six radiology residents searched 40 chest images for nodules while their eye-position and gaze-duration times were recorded. Half received displayed visual feedback and half were given a second view without feedback. Two months later the two groups returned and viewed the images in the opposite condition to counterbalance for possible practice effects. Performance of readers who were given feedback showed an average of 16% improvement as measured by the alternative free response operating characteristic (AFROC) curve area, A1. Performance of the same readers given a second look without feedback did not improve.

Six subjects--three radiologists and three laypersons--were shown a hidden figure picture, a sonogram, a tomogram, and two standard radiographic images while their eye movements were recorded. They were asked to summarize each of the images in one sentence. The distribution of visual attention was found to be related to the assignment of meaning to the image. When the interpretation of the image was correct, visual attention focused on the main subject or radiological abnormality. When the viewers found the images to be ambiguous, visual attention focused on dominant pictorial features such as borders or textured areas. These features were then used to generate a meaningful perception in response to the request for a summary statement. The data are interpreted in terms of a theory that holds that picture perception begins with a parallel global characterization of the scene that uses data from both the retina and memory to form a visual concept. According to this theory, it may be necessary to have seen radiological abnormalities in the past to be able to form an appropriate visual concept for a current abnormality.

Eye movements were recorded while four subjects searched a set of 60 films, 24 normal and 36 abnormal for pulmonary nodules. Error rates, scanning patterns and the dwell time of fixation clusters on normal and nodule-containing areas of the film were studied. Using the assumption that prolonged dwell time indicates intensive processing of visual data, a model was developed for nodule detection that includes four steps: orientation, scanning, pattern recognition and decision-making. False-negative errors were divided into three classes: scanning errors, recognition errors and decision-making errors. Of 20 false-negative errors, 30% were considered scanning, 25% recognition and 45% decision-making.

Two experimental approaches examined observers' ability to characterize shape symmetry based upon the information obtained within a single fixation. Exp. I tested observers' ability to discriminate among three levels of symmetry on the basis of a 25-msec. presentation. Results indicated that identification accuracy was higher for symmetrical and asymmetrical shapes than for mixed shapes, which contained both symmetrical and asymmetrical components. Exp. II tested observers' ability to detect a given level of symmetry by means of a signal-detection approach. Symmetrical shapes were detected better than asymmetrical shapes. Detectability of mixed shapes was lower than that for both symmetrical and asymmetrical shapes. The results of Exps. I and II demonstrate that observers can characterize shape symmetry on the basis of information available within a single fixation. The data are supportive of the hypothesis that a global response to the over-all configuration of the shape serves to facilitate form perception.

Ten radiologists were shown a series of 10 normal and 10 abnormal chest films under two viewing conditions: a 0.2-second flash and unlimited viewing time. The results were compared in terms of verbal content, diagnostic accuracy, and level of confidence. The overall accuracy was surprisingly high (70% true positives) considering that no search was possible. Performance improved as expected with free search (97% true positives). These data support the hypothesis that visual search begins with a global response that establishes content, detects gross deviations from normal, and organizes subsequent foveal checking fixations to conduct a detailed examination of ambiguities. The total search strategy then consists of an ordered sequence of interspersed global and checking fixations.

Detection of small tumors in chest x-ray films was studied under tachistoscopic viewing conditions designed to stimulate single fixations varying in dwell time, found in free search. Two questions were asked: How much dwell time is required to detect a nodule (experiment 1)? How effective are peripheral inputs in the detection of a nodule (experiment 2)? Our findings indicate that a dwell time of 300 ms was sufficient to detect 85% of the nodules when they were viewed directly. Detection accuracy was reduced by one-half when the tumor was located 5 degrees from the axis of gaze. Taken together with data from eye movement experiments, these results provide useful estimates of perceptual and cognitive parameters of visual search.

Chest films were inspected by staff and resident radiologists for lung nodules under two viewing conditions: segmented search, in which films were divided into six sections and viewed piecemeal, and global search, in which the complete film was presented and viewed in its entirety. Nodules varied in edge gradient from sharp to fuzzy. Nodules with sharper edges were identified faster, more frequently, and with higher confidence than nodules with less sharp edges. Segmented search did not increase the probability of nodule detection; rather, it led to an increase in the number of false-positives, resulting in lower overall performance when compared with global search. The results are interpreted according to a visual search model which calls for the use of comparative scanning strategies to differentiate nodule-containing areas from nodule-free areas on chest films.

The contrast sensitivity of the retina is greatest in the center and decreases rapidly toward the periphery. Therefore, the detection of low-contrast lung nodules depends upon the manner in which the image is sampled by retinal receptors as eye fixations jump across the image during scanning. The scanning performance of two radiologists was compared with two computed models, a systematic and a random scanner. Although radiologists do not seem to have random scanning patterns, their coverage of the image was matched more closely by the random model. This suggests that radiologists employ a scanning strategy that is designed to cover the image of the lungs in a minimum time using the smallest possible visual field. The visual field size that is most effective in detecting nodules during search has a radius of 3.5 degrees visual angle. Nodule detection may be limited by basic neurologic constraints on human scanning performance.

Eye position recordings made while radiologists searched chest images for lung nodules showed that regions falsely reported positive or suspicious received prolonged visual attention. Correlation of regional fixation dwell time with independent ratings of image features indicated that more than 90% of false-positive decisions were caused by some perturbation in the image that aroused the suspicion of the viewer. The remainder apparently arose from within the viewer. Most missed nodules (false-negative reports) also received prolonged visual attention, implying an active decision not to perceive a nodule. The data are interpreted to show that roughly one task-related decision is made during each second of scanning a radiograph. This departs from the central assumption of the traditional signal-detection model based upon one decision per image.

The detectability of nodules displayed on uniform backgrounds was compared with their detectability on chest images by using receiver operating characteristic (ROC) curves. The images were displayed using a digital television system. Two conditions, chest image and uniform background, were compared at three different levels of added random noise. The viewing conditions were made as similar as possible by setting the luminance of the uniform background equal to the luminance of the lung in the chest image. Nodule detectability was significantly lower for the chest image than for the uniform background at each level of added random noise. The anatomic structure of the chest image interferes with the structural integrity of the nodule image resulting in lower detection performance.