The third in the series of the
Revolution™ Digital Flat Panel Education will cover the
specifics of the GE Revolution™ Digital Flat Panel Detector.
Discover the principles, technology and design structure of the
GEs Digital Flat Panel in this issue.
Result of extensive corporate R&D
since 1985, the GE Revolution™ Digital Flat Panel (DFP) detector
replaces the film in Mammography and Rad applications as well as the
analog image intensifier, with its camera optics, pickup tube or CCD
camera, and analog-to-digital converter, in Cardio-Vascular
Using a common technology platform that requires only limited
customization for each application, GE pioneered the deployment of
DFP detectors in Mammography (1999), in Rad (1999) and in Cardiac
Characterized by a very high Detective Quantum Efficiency, the GE
Revolution™ detector captures nearly all the information available
at its entrance and transfers it with almost no degradation to the
observer. For all applications, the result is outstanding image
quality at reduced dose.
The principle of the flat-panel
detector is illustrated in the drawing below.
the GE Revolution™ Digital Flat Panel Detector.
The cesium iodide (CsI) scintillator absorbs x-ray photons,
converting their energy into light photons emission. This light is
then channeled toward the amorphous silicon photodiode array where
it causes the charge of each photodiode to be depleted in proportion
to the light it receives. Each of these photodiodes is a picture
element (pixel); the spatial sampling of the image, which is the
first step in image digitization, is thus performed exactly where
the image is formed, whereas it is realized almost at the end of the
chain in an Image Intensifier
(see more in part 2 of the education series). The electronic
charge required to recharge each photodiode is then read by
ultra-low-noise proprietary electronics and converted into digital
data that are then sent to a real-time image processor. In the GE
cardiac system, over 30 million pixels per second are read out,
processed, and displayed in real time.
Amorphous Silicon panel coated with CsI scintillator.
The heart of the flat panel digital detector consists of a
two-dimensional array of amorphous silicon photodiodes and thin-film
transistors (TFTs), all deposited on a single substrate.
Utilizing thin film technology similar to that used in the
fabrication of integrated circuits, layers of amorphous silicon and
various metals and insulators are deposited on a glass substrate to
form the photodiodes and TFTs matrix, as well as the
interconnections, and the contacts on the edges of the panel.
The CsI scintillator, which converts x-ray photons into visible
light photons, is deposited directly on top of the amorphous silicon
Using a proprietary process, it is grown in very thin needles (5µm
width) that channel the light photons towards the photo-diode, like
a fiber optics would do. This allows one to increase the thickness
of the CsI, and thus to stop and detect more X-rays, without
degrading spatial resolution because of wide-spreading light scatter
as observed in typical radiographic phosphor screens.
Electron microscope views of
CsI needles that constitute the scintillator layer.
The photo-diode comprising each pixel is
used as a bucket for electrons and each TFT behaves as a switch to
access the associated photo-diode. The TFT conductive state is
controlled through the voltage applied by scan electronics modules
to matrix rows.
When a TFT is conductive, the charge of the corresponding
photo-diode can be measured through a matrix column by the readout
electronics modules and converted to a digital value by the analog
to digital converter attached to each colomn.
The second step of image digitization after spatial sampling: pixel
quantification, is thus also performed next to image formation, and
not at the end of a long transformation chain like in an Image
Intensifier-based system. (for more details on Image Intensifier
see part 2 in the Digital Flat Panel education series).
Flat Panel and Imaging
Scan modules and readout modules are GE proprietary designs and use
state-of-the-art high density packaging technology to minimize
sources of noise. Associated with the optimized design of the
amorphous silicon flat panel, the electronic noise generated in the
entire detection chain, from the photo-diode to the output of the
analog-to-digital converter, is equivalent to the signal generated
by a single X-ray photon. Thus, the read-out noise added by the
panel is significantly less important then the quantum noise in
X-ray imaging. The image quality is therefore limited only by the
X-ray quantum noise, i.e. by the dose, and not by the detector
performance. This low noise performance, which is particularly
important in fluoro where very low dose is required. Combined with
other advantages of the flat panel detector, such as large dynamic,
response stability over dose variations and time, response
uniformity over the entire image area, and absence of distortion, it
provides a breakthrough in image quality. All this adds not only to
intrinsic image quality but also and opens new opportunities for
further image processing.
In cardio-vascular imaging,
information is typically associated with small objects such as
arteries, stents, guide wires, and catheters - objects that overlap
each other and large organs with different contrasts such as lungs
or diaphragm. Because the display has a finite number of gray
levels, representing the organs at their acquired brightness levels
may compromise the representation of smaller objects of clinical
interest. At GE, we have developed state-of-the art computational
methods to represent the information in an intelligent manner, so
that features of interest are allocated optimal display values. This
requires that the original image be captured with high fidelity over
its entire dynamic range.
As a result, the detector gives images a unique look and feel. This
allows diagnostic information to be presented with optimal
utilization of display properties and human visual perception. This
technology also provides the ability to selectively enhance the
contrast of objects such as stents regardless of the anatomical
background against which they are acquired, providing better
visibility of object details across the entire image, regardless of
the background anatomy.
The family of digital detectors
manufactured by GE is based on a common technology platform whose
heart is a two-dimensional amorphous silicon array of
photo-diodes and thin-film transistors deposited on a single
piece substrate and directly coated with needle grown Cesium
Iodide. The technology platform strategy forced the design to be
able to answer the most challenging needs of each application, such
as large field of view for chest Rad, high resolution for
Mammography, real time and low noise image acquisition for Cardiac.
This strategy has several advantages:
- fast introduction of the successive detectors customized for
each application; today, more than 1,100 systems are installed
worldwide and give GE a unique know-how in Digital detectors,
- easy cross-fertilization between customized oanel formats and
- enables each customer to benefit from developments made to the
panel for other applications; this offers the ability to enjoy
performances that exceed the demands of today’s practice and open
the way to new breaking-through applications.
The high performances of the amorphous silicon flat panel are
complemented by the proprietary electronics for detector control and
readout. Associating one Analog-to-Digital converter with each of
the 1024 or 2048 pixels forming a single image row is a good example
of a design without compromise to minimize noise sources in all
All that results in a final design which offers Image Quality
performances as well as simplicity, with a single large sensitive
area requiring neither tile stitching with the associated lost
pixels, nor detector motion prohibiting fast acquisition, and thus a
reliability demonstrated by the most large and diverse installed
base of Digital Detectors.