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X-ray light valve (XLV)

Robert MacDougall

Why develop the XLV?

Many radiology departments are making an effort to become completely filmless, and are thus replacing traditional film and screen systems with digital technologies. Most current digital X-ray detectors, however, make a compromise between cost and image quality. The X-ray light valve (XLV) has the potential to be relatively inexpensive to manufacture and use while providing image quality that equals or surpasses that of detectors now in use.

The XLV has three main components:

  1. an X-ray detector made of amorphous selenium (a-Se)
  2. a liquid crystal (LC) cell
  3. an optical scanner as a readout system

Using these inexpensive technologies, we construct the XLVs in our clean room facility (pictured above).

The feasibility of the XLV as a digital X-ray imaging system has been established. On the left is an X-ray image of a pirahnna acquired with the XLV system. Current work is focused on imaging an anthropomorphic chest phantom to evaluate image quality.

Applications

We anticipate adapting the XLV to all types of medical X-ray imaging including general radiography, mammography, fluoroscopy and portal imaging.

How the XLV works

  • What an XLV is

The XLV consists of an a-Se layer sandwiched with a liquid crystal (LC) cell (see figure below). This cell is similar to those found in typical LC displays (LCDs). We are using the LC cell in a reflective mode, where the readout light is reflected back toward the observer after passing through the cell; so, in the figure below, readout occurs from the bottom.

  • X-ray exposure

X-rays (shown as purple arrows) interact in the a-Se layer, creating electrons and holes (black and white circles). The applied electric field (red and black lines) causes the electrons and holes to move to opposite surfaces of the a-Se layer.

  • What an LC cell does

An LC cell is an electrically controlled 'optical valve.' The cell allows a controllable amount of the input light to pass through the cell. The amount of transmission (or reflectance) depends on the electric field across the cell in that region. In a typical LCD, the amount of light that is allowed to pass through the cell is controlled by the electronics on the back of the cell. In the XLV, the optical properties of the LC are controlled instead by the amount of charge that has been collected on the surface of the a-Se layer at the interface with the LC cell.

  • Collected charges open and close the 'valve'

The holes collected at the interface with the LC produce a spatial variation in the electric field across the cell, with more charge and therefore a higher field occurring where X-rays have interacted in the a-Se layer and less charge and hence a lower field where they did not. This variation in the electric field across the cell results in more light transmission through the cell in regions where X-rays have interacted in the a-Se.

  • Reading out the 'collected charge' image

Upon X-ray exposure, a visible image is produced in the LC cell, which acts as an analogue display. We then use an optical scanner to digitize the image for archiving and viewing on a monitor.