Canon global

What kind of mechanism did you employ for running the sensor and capturing images in order to support dynamic imaging?

Tamura

X-ray images are received in the sensor matrix, and for each horizontal line, switching elements (TFT) are driven in sequence from the top with their charges read and then reset in a process that is continuously repeated. The incoming signal for each horizontal line forms a plane in the image processing circuit, and sensors that have been reset accumulate charge for the next image once they have been reset. This process is repeated at high speed to produce dynamic images.

How many sensor elements are there in a single horizontal line?

Watanabe

The first product supporting dynamic imaging had around 5.9 megapixels. Each frame is formed by 2,700 horizontal lines, each made up of around 2,200 pixels.

The maximum speed is 30 frames per second (fps), but does the device also have modes for shooting at lower frame rates?

Takekoshi

Yes. There are modes for 30 fps, 15 fps, 7.5 fps, 3 fps and 1 fps, in addition to a mode that takes one frame every 4 seconds.

Tamura

In addition, there are modes for using four sensor elements for each pixel and modes that change the imaging range, with almost 100 combinations available in all.

Why do you need so many modes?

Takekoshi

The reason is that there are diverse uses for the device in hospitals. As a unique characteristic of Canon's products, by making the imaging panel portable, it offers a range of potential uses and in addition, enables diverse applications for dynamic imaging.

A single device enables a range of imaging uses

What aspects of the circuit design did you find to be difficult when enabling support for dynamic imaging?

Tamura

It is a difficulty inherent to new development, but we faced the issue of the circuit size being increased by several multiples. The surface area and thickness of the imaging device were basically fixed from a usage perspective, and this in turn determined the size of the circuit boards that it could contain. Despite this, we needed to incorporate several times the number of circuits. We wondered what we should do considering that steps to address heat dissipation are also important when dealing with large circuits. It would not be good if the device was too hot for the patient being tested to touch.

So, what kind of design did you employ?

Tamura

We isolated the circuitry and split up the circuit board. Some of the circuitry normally incorporated into the imaging device was moved to the controller box, and the circuit boards were connected using a high-speed serial cable seven meters long. The controller synchronizes the X-ray emitter, the imaging device, and the display PC. The design became more complex due to the circuit board being split up, but this resolved our problem regarding electronics.

Displaying dynamic images in real time has become standard, regardless of the field. Was software technology also a key point in ensuring there were no delays in the display?

That's quite long. I think such a long cable could result in reduced speed when using certain protocols.

Takekoshi

That's right. But thanks to our efforts, we were able to limit the time lag between the subject moving and the image being displayed to between 0.1 and 0.2 seconds.