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Oct 07, 2020

Time Delay Summation in Photon Counting

- By Jonathan Schock

Frame based imaging and line scanning

When using a pixelated, radiation sensitive detector, usually the information of all pixels is read out at the same moment in time, forming an image. If a series of sequential images is taken, one image of such a series is referred to as a ‘frame’. With fast moving objects, the speed at which the frames can be taken, referred to as frame rate, imposes an upper limit to the speed at which the object can still be visualised  sharply and clearly.

When using a frame-based read out, the data transfer speed between the detector and any analysis system dictates an upper limit to the frame rate.

Detectors which have only one active line circumvent that problem. As data transfer per frame is limited to a single line, the frame rates can be higher. Result -> a higher overall scan speed.

By combining numerous single-line frames together the resulting image can be made arbitrarily large, allowing the scanning of very long objects. However, the speed gain comes with a tradeoff in image statistics, as each point in the sample is seen by exactly one pixel during the entire scan.

Increasing statistics and maintaining speed

To increase image statistics of a line scanning system whilst maintaining the speed, a second scanning line can be added. The signal accumulated in the second line can be added to the signal from the first line, doubling imaging statistics. To avoid smearing due to the motion of the sampled object, the acquisition of the second line must be timed such that it acquires the exact same point of the sample as the first line while passing over it. When adding several more lines, this concept is known as time delayed integration (TDI), or when applied to photon counting detectors as time delayed summation (TDS). A visualisation of the concept is shown in the following video:

 

Direct Conversion detectors deliver up to 512 detector lines in TDS mode.

Achieving imaging speeds of up to 20,000 lines per second.

The combination of photon counting quantitative imaging and the principle of time delayed summation brings new levels of imaging speeds to the market.

With an added benefit – not only does TDS enable imaging of fast-moving objects, it also enables images to be taken of objects with low X-ray dose.