Purpose To investigate performance in detectability of small (≤1 cm) low-contrast hypoattenuating focal lesions by using filtered back projection (FBP) and iterative reconstruction (IR) algorithms from two major CT vendors across a range of 11 radiation exposures. Materials and Methods A low-contrast detectability phantom consisting of 21 low-contrast hypoattenuating focal objects (seven sizes between 2.4 and 10.0 mm, three contrast levels) embedded into a liver-equivalent background was scanned at 11 radiation exposures (volume CT dose index range, 0.5-18.0 mGy; size-specific dose estimate [SSDE] range, 0.8-30.6 mGy) with four high-end CT platforms. Data sets were reconstructed by using FBP and varied strengths of image-based, model-based, and hybrid IRs. Sixteen observers evaluated all data sets for lesion detectability by using a two-alternative-forced-choice (2AFC) paradigm. Diagnostic performances were evaluated by calculating area under the receiver operating characteristic curve (AUC) and by performing noninferiority analyses. Results At benchmark exposure, FBP yielded a mean AUC of 0.79 ± 0.09 (standard deviation) across all platforms which, on average, was approximately 2% lower than that observed with the different IR algorithms, which showed an average AUC of 0.81 ± 0.09 (P = .12). Radiation decreases of 30%, 50%, and 80% resulted in similar declines of observer detectability with FBP (mean AUC decrease, -0.02 ± 0.05, -0.03 ± 0.05, and -0.05 ± 0.05, respectively) and all IR methods investigated (mean AUC decrease, -0.00 ± 0.05, -0.04 ± 0.05, and -0.04 ± 0.05, respectively). For each radiation level and CT platform, variance in performance across observers was greater than that across reconstruction algorithms (P = .03). Conclusion Iterative reconstruction algorithms have limited radiation optimization potential in detectability of small low-contrast hypoattenuating focal lesions. This task may be further complicated by a high degree of variation in radiologists' performances, seemingly exceeding real performance differences among reconstruction algorithms. © RSNA, 2018 Online supplemental material is available for this article.

The usefulness of a CT for determining perforation in acute appendicitis is limited, and methods to improve precision in identifying patients with complicated appendicitis should be explored as this may help for improving risk prediction for failure of treatment with antibiotic therapy and help guide patients and providers in shared decision-making for treatment options.

We report a near-field scanning optical microscopy ͑NSOM͒ probe that enables high-resolution imaging of living cells under physiological buffered conditions. The hybrid design combines a conventional fiber optic near-field probe with a standard atomic force microscopy cantilever. Imaging of fluorescent latex spheres suspended in an acetate matrix demonstrates the subdiffraction limited fluorescence and topography capabilities of the tips. The reduced spring constant of the hybrid tip is also shown to be amenable to measurements on living cells. Near-field scanning optical microscopy ͑NSOM͒ is a scanning probe technique that utilizes specially fabricated probes to deliver light down to the nanometric dimension.1,2 NSOM provides opportunities to simultaneously measure both optical and topographic features of samples with subdiffraction limited spatial resolution. These capabilities have been utilized to probe thin films, solid-state devices, and due to the unique electromagnetic fields emerging from the nearfield aperture, single molecule orientations.2 The potential impact of NSOM is arguably the greatest in the biological sciences, where there is a well-developed history of using fluorescence probes to tag specific proteins or structures. This field seems particularly well suited to take advantage of the single molecule fluorescence sensitivity, high spatial resolution, and simultaneous force information that NSOM offers. However, current applications of NSOM to biological samples are largely limited to isolated protein samples, model membranes, or chemically fixed biological cells. 2-5The extension to viable, unfixed biological tissues has previously proven problematic.The difficulty in conducting NSOM measurements on viable biological tissues arises from the forces generated in maintaining the NSOM tip close to the specimen. Highresolution NSOM measurements require that the tip be held within nanometers of the sample while scanning. This necessitates the implementation of a feedback system for sensing the sample surface and maintaining the tip-sample gap. [6][7][8] Traditionally, a force feedback approach is implemented in which the tip is either dithered laterally or vertically to the sample surface, depending on probe geometry. The dampening in the amplitude of the oscillating NSOM tip as the tip interacts with the sample surface is monitored and used to hold the tip-sample gap constant during scanning. While straightforward and highly successful for most applications, the large forces generated using conventional fiber optic NSOM probes often damage fragile biological samples such as living cells.A number of approaches have been reported in attempts to circumvent the large forces generated under normal NSOM feedback operation and thus open applications in the biological sciences. These have largely involved either changing the feedback mechanism utilized to hold the tip close to the sample or modifying the probes themselves to lower the spring constant and thus the forces generated in force feedback. 2,5,9 ...

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