An X-ray Light Valve image of the skull
The X-Ray Light Valve used in conjunction with a simple optical scanner is the next step in digital radiographic imaging as a cost-effective alternative to the Active Matrix Flat-Panel flat panel imager. It can be used to produce digital images of revolutionary quality in radiography and medical technology that parallel its counterpart, the Active Matrix Flat Panel system.
A schematic diagram of the Active Matrix Flat Panel Imager (AMFPI) system
The Active Matrix Flat-Panel flat panel imager was the latest in X-ray imaging technology until researchers developed the X-ray light valve. The Active Matrix Flat-Panel Imager works in two main ways, by direct conversion and then by indirect conversion. In the process of direct conversion, incoming X-rays are converted to charge carriers with an amorphous Selenium (a-Se) layer. Then an electric field is used to move them into an embedded array of thin-film of transistors, amplifiers, and analogue-to-digital converters. The digital signal can be displayed as an image, processed and stored electronically.
Amorphous selenium in the indirect conversion process
The second process of indirect conversion involves the X-rays hit a phosphor layer that emits light in response. An array of photo-diodes converts the light to electrical signals that can be converted to digital signals to be displayed, processed and stored as image.The Active Matrix Flat Panel System has great advantages that make it a revolutionary asset to the field of medical imaging, technology and radiography. It produces very high resolution images with very little disturbance from electrical noise along the transistor circuitry that load the electrical signal of the image and transmit to the digital analyzer to be converted into digital format. The high speed acquisition of this data in the conversion of the image in its analogue state in form of electrical signals through the array of multiple transistors to the digital analyzer, allows real-time fluoroscopic imaging, to account the dynamics and concurrent changes in the body being imaged. However, these advantages apply specifically to the direct conversion approach. The indirect conversion method on the other hand produces images with lower resolution due to scattering of light in the phosphor layer. Overall, the main inconvenience the Active Matrix Flat Panel Imager poses is the enormous cost of producing, implementing and maintaining the equipment and system in the clinical environment. At $200,000.00, it is a very expensive method of imaging in radiography and diagnostic medicine, a cost that patients bare in paying for treatment. Very few hospitals and people in the developed countries can actually afford it and it is not even an option for hospitals and people in developing or underdeveloped countries.
The X-ray Light Valve, if perfected would be an order of magnitude less expensive at $20,000 making it much cheaper to produce and install in hospitals, and a more cost-feasible technique in medical imaging and radiography. This translates into reduced expenses in the maintenance and upkeep of such a complex system, and an affordability that makes this method of medical imaging more easily and readily available and accessible to medical institutions less capable of financing the Active Matrix Flat Panel Imager such as those in developing nations, without compromising or compensating the cutting-edge technology of high resolution fluoroscopic digital images the Active Matrix Flat Panel Image provides. The X-ray Light Valve uses a-Se to convert X-rays to charge without actually measuring the electric charge signal directly like the Active Matrix Flat Panel Imager does. It uses of a birefringent liquid crystal to receive and read the electro-optical effects caused by the interaction of the X-rays with the amorphous selenium to form electron-hole pairs.
A magnified surface view of a birefringent liquid crystal

An electric potential is generated across the amorphous selenium using a positively charged electrode at the surface of the posterior surface of birefringent liquid crystal and a negatively charged electrode on the photon receiving end of the amorphous selenium. The electrodes effectively sandwich the amorphous selenium and the birefringent crystal. The bias voltage applied across the amorphous selenium layer separates the electrons-hole pairs created from the interaction between the X-ray photons and the amorphous selenium into electrons and holes.

The electric field then causes the electrons and holes to drift apart to either electrode, with the electrons going to the interior surface of the birefringent liquid crystal in direct contact with the amorphous selenium. Since the X-rays interact with the body first, they hit the amorphous selenium at different energies that depending on how the photons interact with the part of the body being imaged. This causes different intensities or concentrations of electrons-hole pairs to be formed in the amorphous selenium depending on the varying energies at which the X-ray photons leave the body part undergoing radiography, after interacting with it. This causes variations in the charge distribution at the interior surface of the birefringent crystal that translates into the analogue form of the image. The scanner receives the double refracted light through the birefringent liquid crystal from the analogue image in the form of variations in distribution of charge, simultaneously converting it to a digital image as all optical scanners do.
A simple optical scanner






~ by nigus21 on December 10, 2009.


  1. Now, I’m certainly not as versed as you in the language of digital radiographic imaging, but I seem to remember there being a problem with X-rays and the nuclei of living cells. That’s why we have to wear those lead bibs at the dentist, to protect parts of our anatomy that are sensitive to the X-rays. Does the same apply to this new X-ray valve, I wonder? You made no mention of it here and it could be that I’m behind, and the current methods have already dealt with that problem. Furthermore, you mentioned that the direct conversion method of acquiring an X-ray image is superior to the indirect method, but that the Active Matrix Flat Panel uses both. It will be interesting to see where the X-ray Valve technology takes us.

    • Yes, X-rays have an ionizing effect that is dangerous to body cells and therefore tissue. They alter the structure of DNA molecules that eventually lead to cell/ tissue death, hence killing of cancerous cells. Unfortunately, healthy tissues are also sensitive to ionizing radiation. The struggle in radiography and oncology is producing an effective means and technology that can use ionizing radiation to kill of cancerous cells, reduce morbidity (affectation of healthy surrounding cells and tissue), without compromising the effectiveness of the treatment. In that sense the XLV is probably no different from the AFPMI, because the exposure levels are the same. However, the XLV makes a world of difference in terms of the cost, due to intrinsic mechanical differences in the technology itself, as explained in the article, because it operates optically as opposed to electronically like in the AFPMI.

  2. I’m curious as to how this might work for other x-ray based technologies. For example, x-ray crystallography works much like a medical x-ray except that it studies the diffraction pattern of x-rays off a molecule (many really, but that’s not too important for this discussion). Would this technology provide the same level of resolution as current systems? Could we make it better?

    Also, does it require any different power of x-rays? Much like in the body, biological molecules in crystals tend to degrade when hit with high energy photons. Its generally better to hit them with more photons over a short amount of time than the other way around, so I’m curious as to the “durability” of these valves, or their ability to read x-rays quickly.

  3. I did not understand the harmful effects of using x-rays? Why go risk using X-rays for medical imaging knowing that they have harmful effects on human tissues. Why cant we use CT scans and MRI?

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