CCD cameras have gained wide acceptance in electron microscopy due to their ease of use, high resolution and excellent linearity. It has been demonstrated that at 120 KeV the DQE and SNR of the latest fiber-optically coupled CCD cameras equals or exceeds that of film over most spatial frequencies [1,2]. At higher accelerating voltages that are often necessary to penetrate thick tomography specimens, the resolution of fiber-coupled cameras is reduced by electron back-scatter in the fiber optic [3] (Figure 1). We have developed a new lens-coupled camera that utilizes a transmission scintillator and 90 degree fold mirror to reduce the effect of electron back-scatter and thus improve DQE at high spatial frequencies. Figure 2 illustrates the basic geometry of the CCD imaging system. The beam, after passing through a Gatan Imaging Filter (GIF) forms a zero-loss energy-filtered image on the far side of a transmission scintillator 90 mm in diameter. The image is reflected by a Beryllium mirror oriented at 45 degrees and transferred to a 4K x 4K back-illuminated CCD with 15 µm square pixels by a 0.25 NA image transfer lens of unit magnification. The entire assembly is coupled rigidly to the TEM column and supported by an additional air spring tied into the TEM air suspension system. DQE was measured at 100, 200, and 300 KeV, 10 primary electrons per pixel (pe/pixel), with a Gatan US4000U fiber-coupled 4K x 4K camera with 15 µm square pixels ( Figure 3) and at 200 KeV with the lens-coupled camera equipped with a front-illuminated 4K x 4K CCD (Figure 4). The backilluminated CCD for the lens-coupled system is currently under development so the effect of its greater quantum efficiency on DQE was estimated. In the fiber-coupled system, there is a 4-fold reduction in DQE at half Nyquist at both 200 and 300 KeV compared with 100KeV. The DQE of the lens-coupled camera at half Nyquist, when corrected to include the enhanced performance of the back-illuminated CCD, is improved by a factor of 2.75 at 200 KeV compared with the US4000U. At 300 KeV, it is expected that the DQE of the lens-coupled system will increase further due to less electron scatter.We will present new DQE data obtained from the back-illuminated CCD at 300 KeV demonstrating the ultimate performance of the system.

Trabecular bone structure and bone density contribute to the strength of bone and are important in the study of osteoporosis. Wavelets are a powerful tool to characterize and quantify texture in an image. In this study the thickness of trabecular bone was analyzed in 8 cylindrical cores of the vertebral spine. Images were obtained from 3 Tesla (T) magnetic resonance imaging (MRI) and micro-computed tomography (μCT). Results from the wavelet based analysis of trabecular bone were compared with standard two-dimensional (2D) structural parameters (analogous to bone histomorphometry) obtained using mean intercept length (MR images) and direct three-dimensional (3D) distance transformation methods (μCT images). Additionally, the bone volume fraction was determined from MR images. We conclude that the wavelet based analyses delivers comparable results to the established MR histomorphometric measurements. The average deviation in trabecular thickness was less than one pixel size between the wavelet and the standard approach for both MR and μCT analysis. Since the wavelet based method is less sensitive to image noise, we see an advantage of wavelet analysis of trabecular bone for MR imaging when going to higher resolution.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006