Enhanced imaging in low dose electron microscopy using electron counting - PubMed (original) (raw)

Enhanced imaging in low dose electron microscopy using electron counting

G McMullan et al. Ultramicroscopy. 2009 Nov.

Abstract

We compare the direct electron imaging performance at 120keV of a monolithic active pixel sensor (MAPS) operated in a conventional integrating mode with the performance obtained when operated in a single event counting mode. For the combination of sensor and incident electron energy used here, we propose a heuristic approach with which to process the single event images in which each event is renormalised to have an integrated weight of unity. Using this approach we find enhancements in the Nyquist frequency modulation transfer function (MTF) and detective quantum efficiency (DQE) over the corresponding integrating mode values by factors of 8 and 3, respectively.

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Figures

Fig. 1

Fig. 1

Examples of single electron events due to 120 keV incident electrons as recorded on the Vanilla sensor. The pixel values are both dark subtracted and gain corrected. Their values lie between -15 and 241 ADC units and they are illustrated by linearly mapping their values to a grey scale in which the incident electrons are shown as darker. A typical area with just readout noise is shown in (s) and, for reference, the zero value is illustrated in (t). The integrated value of each event is given in the lower right-hand corner of each panel. The average integrated value for 120 keV electrons was found to be 375 ADC units. In the images shown the peak value in an individual pixel is 241 ADC units and is found in event (c). The peak value in the weakest event shown in (r) is 49 ADC units.

Fig. 2

Fig. 2

Images of a 300×300 pixel area of the detector illuminated by the edge shadow pattern used to calculate the MTF. The pattern is formed by partially blocking a uniform beam with a 2 mm diameter Pt rod placed in the pointer position of the microscope. (a) Shows a typical frame in a series used to calculate the edge image via single electron events. As in Fig. 1, darker areas indicate the single electron events. (b) Shows the events selected from the frame shown in (a). The circles have been drawn around the seed pixel associated with each event. The use of a threshold meant that some events are not counted and where two incident electrons are too close they are counted as a single event. (c) Shows the edge image obtained by summing the contributions from single electron events contained in 68 550 frames such as shown in (a). The events have been renormalised using the procedure described in the text. (d) Shows the corresponding edge image obtained using the MAPS detector in the integrating mode in which the individual dark subtracted and gain normalised frames are simply added. The insets in (c) and (d) show an 8× magnified view of an area of the edge with a scale bar representing 8 pixels, i.e. 200μm.

Fig. 3

Fig. 3

Comparison of (a) MTF (ω) and (b) DQE (ω) obtained using integrating and single electron event modes. Results obtained using the integrating mode are shown as the solid (—) lines. Single electron mode results in which renormalised events are used are shown in dashed (-) lines and single electron mode results obtained using the events’ peak positions are shown as dotted (⋯) lines.

Fig. 4

Fig. 4

Comparison of integrating and single electron mode shadow images of an EM grid obtained using the same total number of electrons. The integrating mode and single electron mode images are shown in (a) and (c), respectively, with the darker areas representing areas exposed to electrons. A quadrant of the Fourier transforms of the images shown in (a) and (c) are given in (b) and (d), respectively. The circles in (d) indicate the spots from one of the [10] directions whose amplitudes and signal to noise ratios were measured and used in generating Fig. 5.

Fig. 5

Fig. 5

Enhancement in (a) MTF (ω) and (b) DQE (ω) performance obtained by using renormalised single event imaging over the conventional integrating mode imaging. The dashed lines in (a) and (b) show the corresponding enhancements taken from the results given in Fig. 3. The circles (∘) show results obtained from the grid shadow images. The MTF enhancement was obtained from the ratio of amplitudes of the spots as indicated in Fig. 4 measured in single event and integrating modes. The DQE enhancement was obtained from the corresponding square of the ratio of signal-to-noise for the spots.

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