Charge-Coupled Device (CCD) Camera in Time-Delay Integration
Principles of Operation
Amnis® cytometers employ a time-delayed integration (TDI), charge-coupled device (CCD) camera. This is the same technology used in medical radiography and military applications to capture images of moving objects.
Fast imaging is required to obtain enough events for statistical significance. Traditional flash on the fly photography sacrifices sensitivity in order to image rapidly moving objects. Amnis® employs a custom camera which is operated using a technique called time-delay-integration (TDI), a specialized detector readout mode that preserves sensitivity and image quality even with fast relative movement between the detector and the objects being imaged. The TDI detection technology of the CCD camera allows up to 1000 times more signal to be acquired from cells in flow than from conventional frame imaging approaches. Velocity detection and autofocus systems maintain proper camera synchronization and focus during the process of image acquisition.
As with any CCD, image photons are converted to photocharges in an array of pixels. During TDI operation, the photocharges are continuously shifted from pixel to pixel down the detector, parallel to the axis of flow, integrating the signal over multiple pixels. By synchronizing the photocharge shift rate with the velocity of the flowing cell, the effect is similar to physically panning a camera. With TDI, image streaking is avoided despite signal integration times that are orders of magnitude longer than those of conventional flow cytometry. Each pixel is digitized with 12 bits of intensity resolution, providing a minimum dynamic range of three decades per pixel.
- Charge-coupled device (CCD) camera provides 5- to 10-fold increase in detection sensitivity over photomultiplier tubes
- Time-delayed integration (TDI) preserves image quality and sensitivity in fast-moving objects