Introduction: Low-cost optical microscopes are under development by several groups to bring point-of-care malaria diagnosis services to under-served communities in low-resource settings. We describe a Fourier ptychography microscope (FPM) system based on an open-source design and a method of measuring spatial resolution in terms of the optical modulation transfer function (MTF) using images of the 1951 USAF resolution test device.
Methods: The FPM system uses a Raspberry Pi computer and 196-LED matrix light source. FPM images were reconstructed using in-house Python code. The MTF was determined from bar-patterns using a least-squares analysis to fit a square wave, including odd-harmonic terms, to image profiles through the bars. These were normalized to larger uniform regions of the pattern and combined to generate the MTF.
Results: Total component cost of the FPM was less than $200. The theoretical diffraction limit imposed by the pupil function of this system was 280 cycles/mm, slightly less than the measured MTF 10% frequency of 300 cycles/mm. The 10% frequency in FPM images was 550 cycles/mm.
Conclusions: The USAF test pattern provides a practical method for assessing FPM performance in terms of the achieved MTF of the $200 Raspberry Pi based Fourier ptychography microscope. The limiting frequency in FPM images, 550 cycles/mm, was slightly less than the Nyquist sampling cut-off frequency of 670 cycles/mm imposed by pixel spacing.
Digital radiographic imaging systems are becoming more widely deployed in low-resource settings, potentially reducing the inequitable access to medical imaging that persists today. Even when resources are made available to install digital x-ray equipment, challenges remain with respect to ongoing maintenance and recommended quality assurance programs. Recent studies have indicated that a significant fraction of radiographic installations in Africa are not assessed at recommended intervals due to the lack of high-cost x-ray exposure meters. As a result, errors in x-ray exposure parameters (e.g. current, exposure time, or kilovoltage) can lead to suboptimal image quality, repeated exams, and unnecessary radiation exposure to patients and staff. We have developed a low-cost solution for routine x-ray quality assurance measurements, which takes advantage of commercial-off-the-shelf (COTS) electronic components integrated with a low-power microprocessor controller. The device employs four sensitive phototransistors connected to a multiplexed 16-bit analog-to-digital converter. Light input to the optical sensors is provided by a rare-earth phosphor screen (Lanex regular), which emits green light under exposure to x-rays. Estimation of spectral properties is enabled by the use of aluminum filters on two of the four photosensors. Acquisition is controlled by an open-source microprocessor (Arduino Nano 33 BLE) and the total cost of all components is less than $100 USD. Comparisons against a commercial sensor indicate that the optical-based measurements of x-ray exposure are linear and accurate to within ±3%, over the range from 60 to 140 kVp. This project demonstrates the feasibility of providing accurate, robust solid-state x-ray exposure measurements in low-resource settings.
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