The influence of film density on the radiologic detection of lung lesions.

Experiments were conducted on a series of chest radiographs to evaluate the effects of mean film density on reader performance. It was found that the false negative is not measurably affected, in agreement with earlier published data. It was also found, however, that the false positive rate increases with increasing film density. The causes and implications of these results are discussed.

Densitometric measurements of lung nodules on chest radiographs.

A technique is described whereby embedded nodular densities appearing on chest radiographs can be densitometrically measured without the obscurring effects of anatomic structures. The technique consists of rear-projecting a light beam into a chest radiograph and an observe adjusting this beam until the lucency matches that of the nodule's appearance. The radiography is then removed, and the lucency is scanned and analyzed with a microcomputer. Calibration data using simulated nodules show a good accuracy in measuring size, contrast, and edge gradient.

Searching for lung nodules. The guidance of visual scanning.

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.

The effect of verification on the assessment of imaging techniques.

In order to measure the accuracy of a diagnostic imaging technique, the correctness of each observer decision must be determined. This is often difficult in clinical cases, and various strategies are frequently used to approximate the truth. Experiments using chest radiographs as an example are reported to show the pitfalls in those strategies. To assess relative accuracies of three different radiographic techniques, chest radiographs were taken of 66 patients with each of the three techniques, and the films were evaluated by six radiologists. Their findings were then scored by comparing them with the correct decision defined by each of the following methods: majority vote, consensus opinion, expert judgment, feedback review, and clinical/pathologic proof. The findings showed that any one of the techniques could be shown to be better than the other, depending on how the truth was defined. It is concluded, therefore, that strategies that define the presence or absence of disease only by the diagnostic tests under evaluation are inadequate. The truth must be determined by clinical or pathologic proof and follow-up data.

Contrast gradient and the detection of lung nodules.

The effects of edge gradient and blur on the detection pulmonary nodules were studied, using a test series of 175 artificial nodules on 30 chest films. The nodules were of homogeneous texture, about 1 cm in diameter and had a mean contrast of .12. Six observers viewed the film set, scoring both location and confidence. The sensitivity or the probability of a true-positive response was linearly related to the blur and a modified ROC analysis resulted in a family of ROC curves with the index of detectability monotonically decreasing with increased blur.

Visual scanning, pattern recognition and decision-making in pulmonary nodule detection.

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.

Comparison scans while reading chest images. Taught, but not practiced.

Radiology instructors and residents were surveyed for their methods of instruction concerning viewing techniques. A similar group of radiologists had their eye activity measured as they viewed chest images. Image reading techniques are taught to be systematic and directive with comparisons of bilateral features. Yet, most images are read by a free search method; bilateral comparisons comprise less than 4% of the visual activity. Instructors and residents show this discrepancy between instructional techniques and reading practice.

Global and segmented search for lung nodules of different edge gradients.

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.

Computer-displayed eye position as a visual aid to pulmonary nodule interpretation.

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.

Searching for lung nodules. Visual dwell indicates locations of false-positive and false-negative decisions.

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 effect of pulmonary edema on proton nuclear magnetic resonance relaxation times.

This study was done to determine the effect of permeability pulmonary edema on proton nuclear magnetic resonance (NMR) relaxation times. Permeability edema was induced in rats by the intravenous injection of alloxan in saline. Control animals received only saline. The rats were ventilated through a tracheostomy; and after a time sufficient for the edema to become uniform, they were sacrificed. T1, and T2 and extravascular lung water were measured on lung samples. A linear relationship was found between the relaxation times and the extravascular lung water. Any diffuse alveolar process including pulmonary edema can increase proton density as well. The T1 and T2 relaxation times may be used to distinguish among different causes of increased proton density in the lung.

A perceptually based method for enhancing pulmonary nodule recognition.

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.

Sodium nuclear magnetic resonance imaging of neuroblastoma in the nude mouse.

The average concentration of sodium is known to be elevated in some tumors relative to normal tissues, and necrosis is suspected of being a possible cause. We have performed in vivo sodium-23 magnetic resonance imaging (MRI) of IMR-5 neuroblastoma in the athymic nude mouse on a 1.9-Tesla, small-bore animal imaging system. We compared the sodium images with histologic analysis for necrosis and with proton images, chemical measurements of water and blood content, and sodium and potassium concentrations. We found that the sodium concentrations determined by MRI were proportional to the fraction of the tumor tissue that was necrotic. Correlation coefficients varied from 0.65 to 0.78, depending upon how the data were selected. With further refinement it is possible that the sodium concentration measurements determined noninvasively by MRI may have applications as part of clinical diagnosis and staging of soft tissue tumors.

Searching for lung nodules. A comparison of human performance with random and systematic scanning models.

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.

Magnetic resonance imaging of fluorine in rats infused with artificial blood.

An MRI pulse sequence has been developed that enables the visualization of a perfluorocarbon (PFC) emulsion in the vascular system of rats. Images were made at 0.12T on a clinical imaging system using a small receiver coil, at intervals of approximately 2 hours, two days, two weeks, and two months after replacement of 50% of total blood volume. The most successful technique produced PA projections of the entire torso for both the fluorine and proton components. Direct comparison allowed identification of PFC in heart, lung, liver, spleen, and large vessels both in vivo and postmortem. Potential clinical applications to vascular imaging are discussed.

Nodule detection with and without a chest 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.

Variability of myocardial signal on magnetic resonance images.

The diagnosis of myocardial disease by magnetic resonance (MR) imaging depends on accurate measurement of myocardial signal intensity. The authors performed 15 experiments in four rabbits at 1.9 T with spin-echo MR imaging to study the variability of myocardial signal intensity throughout the cardiac cycle and to measure myocardial T2 values. Variability in signal from the myocardium throughout the cardiac cycle was observed in all experiments. During systole, a significant increase in myocardial signal was noted, when data acquisition was performed with electrocardiogram (ECG)-gating and controlled ventilation (P = .02). An inverse relationship between myocardial signal and phase noise was found, indicating the motion-related nature of the variation of myocardial signal. A similar inverse relationship was observed in images obtained from a normal human volunteer. Ex vivo myocardial T2 values of rabbit myocardial tissue were significantly higher than the in vivo values (P = .003), reflecting residual motion despite cardiac gating and controlled ventilation.

A silicone gel phantom suitable for multimodality imaging.

A silicone gel phantom material is described. It can be molded into any shape and it can contain internal cavities. The material is selfsealing allowing the cavities to be filled with liquids. A simple geometric phantom consisting of a rectangular solid with a single internal spherical cavity was fabricated and imaged using MRI, CT, and PET. Properties important for imaging are described.

Water content and NMR relaxation time gradients in the rabbit kidney.

Magnetic resonance images were made of rabbits at 1.4 tesla for the purpose of measuring the T1 relaxation times of the kidney in vivo with the animal in different states of hydration. In the normally hydrated animal, the outer cortex has a T1 of about 600 msec and there was a steep T1 gradient with an increase to about 1200 msec at the tip of the inner medulla. The relaxation time gradient was not affected by overhydration with intravenous fluid, but dehydration by withholding water for 24 hours decreased the values in the inner medulla. The T1 and T2 relaxation times and total water content were measured in vitro on tissue samples. The values for both T1 and T2 correlated with the regional renal water content.

Computed tomography, magnetic resonance imaging, and pathologic observations of the effects of intrathecal metrizamide and iohexol on the canine central nervous system.

A two-group crossover design experiment was used in eight dogs to compare the effects of intrathecally injected (0.25 mL/kg, 180 mg I/mL) metrizamide and iohexol. Additional acute observations were made in four of the dogs following a high dose (0.45 mL/kg of 300 mg I/mL) of each agent. In computed tomographic observations, there was progressive passage of each contrast medium across the interface between the cerebrospinal fluid and the brain, and statistically identical levels of each contrast medium were present in the cerebral gyri. There was a similar passage of contrast medium across the interface between the cerebrospinal fluid and the spinal cord. The rate of passage of each agent into the brain parenchyma suggested their active transport rather than a simple diffusion. In both T2-weighted magnetic resonance imaging and histopathologic studies, there was no evidence of cerebral edema following either high or low doses of each agent. In T1-weighted images made following the intravenous injection of gadolinium-DTPA (0.1 mmol/kg), the blood-brain barrier was intact.