Large-Pixel Architectures Aid Radiographic Imaging

Dave Litwiller

Much of the effort in CMOS image sensor development to date has focused on circuit architectures and on fabrication processes for small devices. Typical applications in this category include cell-phone camera modules, Web cameras and video camera sensors. Compared with CCD technology, the challenges of small CMOS image sensors, especially in high-volume applications, are well-documented and involve issues related to fill factor, noise performance, dark current, uniformity, image lag, microlenses and rolling shutter.

There is, however, a class of applications for which CMOS image sensor technology is more readily suited: large-area, large-pixel image sensors, which in recent years have received less attention than their small-sensor counterparts. In large-pixel devices, a number of traditional CMOS image sensor challenges and trade-offs are reduced or eliminated, making it simpler to achieve imaging goals such as:

* High fill factor. The optically insensitive circuit overhead in each pixel does not scale up with the size of the pixel.

* Uniform response across the image sensor. The pixel capacitance is large compared with the capacitance variations inherent in the readout structure of the sensor, which helps optimize uniformity of response across the imager.

* High sensitivity. This is possible without microlenses because of the high fill factor.

When one further considers the application characteristics of radiographic imaging in particular, a number of other traditional CMOS sensor limitations also decline in importance. For instance, noise performance is dominated by the statistics of x-ray photon arrival, rather than by the noise floor (dark current shot noise plus random noise) of the image sensor itself.

Dark current also is less of a potential problem because imaging frame and data rates are slow enough, relative to the processing power of cost-effective companion PCs, to allow the effective use of dark-field subtraction.

End users do not have to compromise on image lag to suppress noise performance, because the signal-to-noise ratio is governed by the variations inherent in typical x-ray exposures rather than by the noise floor of the image sensor. In addition, because x-ray sources in many radiographic applications are pulsed or shuttered, the image sensor does not have to deliver the same function.
System producers and end users benefit from large-area, large-pixel image sensors in such applications because reduction optics are often expensive or optically inefficient. In many cases, it is impractical to use small image sensors in conjunction with demagnification optics in the form of lenses or fused fiber optic tapers. For these reasons, designers seeking to deliver value in performance-driven digital radiography systems tend to migrate to large-area image sensors.

Although CCDs rely on a higher- performance technology than CMOS image sensors, the latter have several inherent strengths that can capably meet the needs of imaging applications such as digital radiography.