Download The first fully-digital C-arm. 21st century mobile X-ray imaging.

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White Paper No. 03/2010
The first fully-digital C-arm.
21st century mobile X-ray imaging.
Ziehm Imaging’s product strategy is heavily focused on the continuous enhancement of mobile C-arm functionality, mobility and network capabilities. Innovation
milestones include the replacement of
conventional electron-based image intensifiers with digital flat-panels. Débuting at RSNA 2001 (annual congress held
by the Radiological Society of North
America) in Chicago, Ziehm Vision FD’s
fully-digital C-arm was developed
specifically for applications demanding
the highest levels of image quality and
precision. These include 3D CAS (Computer-Aided Surgery) and angiographic
applications. The highly dynamic and
distortion-free images are the foundation
for a 3D reconstruction procedure, specially developed by Ziehm Imaging. And
thanks to the elliptical path of the detector
and the X-ray tube, optimum image geometry is ensured – regardless of the angle.
In order to be able to fully understand the significant benefits of the digital flat-panel detector, it is
necessary to describe the characteristics and
functional principles of conventional imaging systems first. Several consecutive conversion steps
are required for generating usable image information from X-ray photons. This conversion system (Fig. 1) comprises the analog X-ray image
intensifier, a tube camera or alternatively a CCD
(charge-coupled device) camera, an analog/
digital converter and a digital/analog converter,
image processors and a TV monitor.
Monitor
D/A
A/D
Image processing/
storage
Camera
Image intensifier
Patient
X-ray generator
Fig. 1: Conventional video chain
Conventional video chain – image
intensifier with CCD camera
The electron-optical X-ray image intensifier is an
evacuated metal/glass tube which consists of
four basic elements. These are the input screen
(fluorescent screen and photocathode), the electrostatic lenses (electrode voltage 30 kV), the
accelerating anode and the output screen (fluorescent screen). The individual components of the
image intensifier are illustrated in Fig. 2. The
fluorescent input screen (with a layer consisting
of caesium iodide doped with natrium – CsI:Na)
absorbs the X-ray photons and converts their
energy to light photons. These light photons hit
the photocathode and are emitted from there as
photo electrons, proportionally to the brightness
of the input screen. A high potential difference (up
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Fig. 2: Conventional image intensifier
be emitted. Due to their strong acceleration, the
electrons produce approximately 50 times as
many light photons on the output screen. [Curry].
A tube camera consists of a vacuum tube having a
diameter of 2.5 cm and a length of approx. 15 cm
which is surrounded by control coils. Immediately
behind the glass input window, the radiolucent
conductive layer and the light-sensitive semiconductor layer are located. The semiconductor layer
is scanned row by row by the electron beam which
can be fine-tuned by the control coils (Fig. 3).
Depending on the brightness at each scanned
position, a higher or lower signal electricity is
generated and transmitted as an analog video
signal via the analog/digital converter to the image processing system and then further on to the
monitor. [Curry] The tube camera was used in
mobile C-arms until about 1996 (Fig. 4).
Conventional video chain – image
intensifier with CCD camera
Fig. 3: Schematic representation of a
tube camera
Fig. 4: C-arm with tube camera
to 30 kV) which is applied between the photocathode and the accelerating anode pulls the electrons away from the photocathode very quickly.
The electrostatic lenses drive the electrons flying
from the cathode to the anode precisely to the
fluorescent output screen. The electrons which
hit the output screen again cause light photons to
Since the tube camera featured higher dynamics
and a better resolution than the CCD camera (at
least according to its specifications), the significant benefits offered by the latter (in addition to
its lower price) could only be explored empirically.
Yet it was possible to compensate the theoretic
limitations reflected in the CCD camera‘s technical data by means of electronic signal amplification to such an extent that the resulting image
quality finally surpassed that of the tube camera.
On modern C-arms equipped with 1 k systems,
approx. 1 million photodiodes form a field of only
7.68 mm x 7.68 mm in the silicon-based video chip
(Fig. 5). A single photodiode is as small as
7.5 μm x 7.5 μm. A CCD camera has the ability to
store electric charges in cells (photodiodes) and
to transport them after a row-by-row readout
process – at a rate of max. 25 frames per second
– as an analog signal via the analog/digital converter to the image processing system. The distribution of those charges to the individual cells
and the electrons stored in them represent the
stored image. Compared to tube cameras, CCD
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above is mature, significant improvements can no
longer be expected. This technology level exhibits
some typical characteristics. One example is the
distortion caused by the terrestrial magnetic field
or other, artificially-created magnetic fields, on
the image intensifier. Only a technology leap to
fullydigital flat-panel detectors equipped with
highly-integrated solidstate electronics will bring
about substantial further development and improvements in image accuracy and dynamics.
Fig. 5: CCD camera video chip
The use of digital flat-panel detectors
on mobile C-arms
CCD camera
housing
Fig. 6: Ziehm Vision with CCD camera
cameras require much less space, since the coils
which control the electron beam are no longer
needed [Curry] (compare sizes of the image intensifier housings in Fig. 4 and Fig. 6). In addition,
they respond more quickly and more directly and
are less sensitive to overexposures. We have explained how X-ray photons are converted first
to visible light and subsequently to accelerated
electrons before they hit the output screen of the
image intensifier. The emitted light photons are
output by the camera as an analog signal, which
is then converted again by an analog/digital converter prior to storage or image processing. If an
analog CRT monitor is used for image display,
another digital/analog conversion is required.
Each of these conversion processes implies a
loss of the original image information, thus decreasing the obtainable image quality. In a conventional video chain, image quality depends on
several components, whereby the weakest element determines the overall performance of the
entire chain. Since the technology presented
The aim was to reduce the number of conversion
steps required for imaging to a minimum, while at
the same time keeping the amount of losses and
errors involved as small as possible. An imaging
chain realized with a digital flat-panel detector is
much more compact and simple than a conventional one, as can be seen already from its graphical representation (Fig. 7, as opposed to Fig. 1).
Before explaining the detector technology used
on fully-digital C-arms in detail, we should briefly
differentiate the possible concepts with regard to
their usability. Basically, there are two different
types of digital detectors: ones that work with direct detection of the X-rays, and ones that work
with indirect detection. With a direct detector, the
X-ray photons which hit an amorphous selenium
(a-Se) plate are directly converted to electronhole pairs (Fig. 8). In the selenium layer, a bias
voltage drives the charge carriers above a pixel
electrode into the pixel to be collected. With direct
conversion, there is no light scatter. A-Se detectors therefore distinguish themselves by an
excellent Detective Quantum Efficiency (DQE) of
up to 65 % and an extremely good contrast resolution. However, this technology has one feature
which impedes its use in mobile systems. The
different expansion coefficients of glass and selenium produce tension cracks at temperatures
below 10 °C and above 70 °C. In addition to that,
the selenium layer loses its amorphous structure
at temperatures above 70 °C and adopts an irre-
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TFT flat-screen
Image processing/
storage
Digital flat-panel
detector
Patient
X-ray generator
Fig. 7: Video chain with digital flat-panel
detector
versible crystalline structure. Only stationary
radiological equipment with its permanently
available mains voltage allows for continuous
control of the permissible temperature range,
thus guaranteeing disturbance-free operation at
reasonable costs. With indirect detection (Fig. 9),
the X-ray photons hit a scintillator consisting of
caesium iodide doped with natrium (CsI:Na),
which converts them to light photons and sends
them to the sensor matrix (photodiodes) made of
amorphous silicon (a-Si) which lies behind it.
Since silicon is not sufficiently sensitive to X-ray
photons, the CsI scintillator is required for energy
conversion (similar to its use in an image intensifier). The individual sensors of a cell are activated
via an address line, and the resulting X-ray profile
is scanned row by row and transmitted to the image processing system. Fig. 10 shows the functional principle in a simplified way. Since this
detector concept has a higher resistance to environmental influences, it is also suitable for use in
mobile C-arms.
Integration of the a-Si detector into
the C-arm
Fig. 8: Functional principle of an a-Se detector
Fig. 9: Structure of an a-Si detector with
CsI scintillator
For this reason, Ziehm Imaging decided to integrate an a-Si detector into the Ziehm Vision – the
C-arm model which was especially designed for
high-end applications. After just 6 months of development, the first C-arm equipped with a digital
flat-panel detector – the Ziehm Vision FD (Fig. 11) –
was ready to be presented to the public at the
RSNA 2001 (Radiological Society of North America) in Chicago. The detector is available with pixel
area options of 19.8 cm x 19.8 cm or 30 cm x 30 cm,
containing 1024 px x 1024 px or 1536 px x 1536 px
with a size of 194 μm each. The storage depth is
14 bits, thus allowing the display of up to
16,384 levels of grey. With a maximum frame rate
of 25 frames per second, more than 30 million
pixels per second are read out and processed in
the highly-integrated electronics using a bus width
of 14 bits, and are then displayed on the TFT flatscreens at an image quality which could not be
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obtained before (Fig. 12). The a-Si detector is able
to capture the object information with practically
no losses. An outstanding image quality, yet
achieved with less dose and hence reduced radiation exposure for patients and doctors, is the
result of this revolutionary imaging chain.
Efficiency and significant benefits
As the efficiency of such a detector is determined
by a large number of inter-dependent parameters, its characteristics can be described using a
variety of methods. But surely the best method
consists in choosing variables which correlate
with the interests of the user and are related to
visible image quality. For such a user-focused assessment, the Detective Quantum Efficiency
(DQE) and the dynamic range as well as a qualitative examination of the distortion prove to be the
most suitable parameters. The first one of these
parameters – the DQE – describes the efficiency
with which the image information is transmitted
between entering and leaving the detector. A detector with a DQE of 1 or 100 % does not have any
losses during information transfer, hence representing the theoretical optimum. The DQE is defined as the quotient of SNROut (signal-to-noise
ratio at the detector output) and SNRIn (signal-tonoise ratio at the detector input). The detector
used in our new model reaches a DQE of 0.65
or 65 %, which is a very good value. The higher
the detective quantum efficiency, the better the
image quality at a given dose. In other words: by
increasing the detective quantum efficiency, it is
possible to achieve a dose reduction while the image quality remains constant. With rising spatial
frequency (measured in line pairs per millimetre
– lp/mm) and a high MTF (Modular Transfer Function), any noise in the image increases, thus
resulting rather in a degeneration of the image
quality that is visible by the user. By using the DQE
as a variable, these conflicting goals are avoided
and the behaviour is quantified by a single
function. The ability to visualise small and lowcontrast objects is thus qualified by the DQE.
Fig. 10: Functional principle of an (a-Si)
flat-panel detector
Fig. 11: Ziehm Vision FD – the first mobile
C-arm with a digital a-Si detector
Fig. 12: Ziehm Vision FD with monitor cart and
TFT flatscreen monitors
Visualising high-density anatomic elements with
low intensity (high absorption of the X-ray photons) and low-density elements with high intensity (low absorption of the X-ray photons) together
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in one X-ray image in a clearly distinguishable
way has always been a prime concern in X-ray
diagnostics. This has now become possible thanks
to the high dynamics inherent in digital detector
technology. Human anatomy provides many examples where it is necessary to display very
bright and very dark areas simultaneously. Filigree arteries or vessels can be visualised with
high contrast even if they are superimposed by
another anatomical structure or organ. Calcifications, coagulations or lesions become clearly
visible. Spine, lung and tumour imaging can be
greatly enhanced. Fig. 13 shows an example of
high dynamics. The same image, acquired with a
conventional image intensifier, shows a major
oversaturation or overexposure of the circle
located in the centre. Contrast decreases, the
contours become blurred, and so-called “blooming“ appears (Fig. 14). Greatly improved and artefact-free visualisation possibilities also open up
in vascular surgery and cardiology when using
Digital Subtraction Angiography (DSA) – a method
developed and introduced by Ziehm Imaging in
which contrast-filled vessels are displayed without any interfering background.
operation of mobile C-arms – the operating
theatre – is usually not shielded against the electromagnetic radiation emitted by MRI equipment.
With the Ziehm Vision FD it is possible to generate
completely distortion-free images of the human
body irrespective of any electromagnetic influences, be they of natural or artificial origin. Proof
of this significant benefit is furnished by the image
of the fine-hole lead phantom, which is now displayed absolutely distortion-free on the TFT
screen (Fig. 16). Combining the significant benefits
of the first fully-digital C-arm described in detail
above, i.e. its high detective quantum efficiency,
extra-ordinarily high dynamics and absolutely
distortion-free image display, with the compre-
Distortion-free imaging – ideal
prerequisites for 3D reconstructions
Another special feature of digital detectors derives from the fact that image acquisition is accomplished without any electron optics. Neither
the earth‘s natural magnetic field nor artificially
generated magnetic fields affect the imaging
chain, giving surgens maximum flexibility when
positioning the C-arm. When a lead phantom with
fine holes is imaged with a conventional image intensifier (Fig. 15), the hole pattern appears with
the typical S-distortion caused by the terrestrial
magnetic field. Not even the algorithm that has
been implemented especially for its compensation is able to completely eliminate the distortion
of these fine contours. Very strong artificial magnetic fields are created e.g. by Magnetic Resonance Imaging systems (MRI). The typical site of
Fig. 13: Ziehm Vision FD: Illustration of the
high dynamic range
Fig. 14: The same image acquired with
a conventional image intensifier
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hensive applications of 3D reconstruction or of
Computer-Aided Surgery (CAS) results in further
considerable benefits for both surgeon and
patient. Ziehm Images uses an FBP (filtered back
projection) algorithm from Feldkamp, Davis &
Kress for 3D reconstruction. The same algorithm
is also used for some larger, stationary CT
systems.
Both technological innovations – the fully-digital
detector and the 3D procedure – are integrated
using inhouse expertise. The evolution of the
scan procedure – from what was originally a strict
circular orbit to the full benefits of an elliptical
path – pays testimony to the innovative drive of
Ziehm Imaging. The distortion- free imaging characteristics, including insensitivity to magnetic
fields, the resulting fully-digital 3D system is capable of producing a 3D reconstruction which is
absolutely free from artefacts (Fig. 17). Furthermore, the integrated CAS interface, Ziehm NaviPort, enables its adaptation to the navigation systems of various wellknown software providers. In
navigated, Computer-Aided Surgery, distortionfree imaging plays a key role. It is the only way to
ensure that surgical instruments can be precisely
positioned. And exact positioning of the surgical
instruments has a direct impact on the outcome of
the interventional surgical procedure. When using
a conventional image intensifier, a very time-consuming pre-operative image calibration – which
serves as a basis for the algorithm used for distortion compensation – with the help of a so-called
‚de-warping‘ grid plate mounted on the image intensifier is therefore essential (Fig. 18). On some
systems, the de-warping grid plate is still visible
on the resulting X-ray image; on others, it
is compensated by image interpolation, thus
making it invisible to the viewer. Neither of the two
methods is very satisfactory for the user, since
both undoubtedly involve a certain loss of image
information. If the surgeon navigates with the help
of the distortion-free, fully-digital detector, this
complicated procedure as well as the de-warping
grid plate itself are no longer necessary. By
Fig. 15: Image acquired with a conventional
image intensifier
Fig. 16: The distortion-free image acquired
with the Ziehm Vision FD
enhancing the precision in image generation, it is
possible to shorten the duration of the interventional procedure, thus cutting down the costs as
well. Furthermore, the surgeon gains more security, which again contributes to facilitating his/her
work and to minimising situation-related stress.
This increase in interventional quality directly
benefits the patient.
Summary and outlook
It was a long way from the first conventional video
chain to the nowadays feasible fully-digital imaging chain. Many steps of development and sophistication were necessary to get there. Access to
global competence resources which were avail-
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able within the group enabled the company to
choose critically from a variety of innovative detector concepts. The arrival of digital flat-panel
detectors in mobile radiology marks a quantum
leap in the technological development of modern
medical imaging equipment. Targeted research
in the field of photo semiconductor technology
results in enormous benefits both for the patient
and the surgeon. In particular, the combination
of digital detection with the complex applications
of 3D reconstruction or Computer-Aided Surgery
(CAS) opens up new horizons in areas such as
cardiac angiography or neurosurgery. Elevated
detective quantum efficiency, high dynamics and
truly distortionfree imaging clear the way for new
levels of quality and accuracy in image display. An
increase in interventional efficiency and quality
combined with simultaneous cost savings will be
the positive results. Instrumentarium Imaging
Ziehm aspires to assist the users by providing
them with technological support mainly for advancing their surgical techniques. The scope
ranges from imaging which is individually matched
to certain applications via multi-modality imaging
to userguided, intelligent robotics. Today, the
C-arm forms part of the IT management used in
modern hospitals and will even represent the local PACS (Picture Archiving and Communication
System) by adaptation of a workstation specially
designed for that purpose. The C-arm communicates with the existing IT infrastructure via a
standard DICOM interface.
Fig. 17: 3D-Reconstruction using
flat-panel technology
Fig. 18: Conventional image intensifier with
de-warping grid plate mounted on it
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Bibliography
[Curry] Christensen’s physics of diagnostic radiology. – 4th ed. / Thomas S. Curry III, James E. Dowdey,
Robert C. Murry, Jr., 1990; 166-169, 175-179
Contact
Peter Berauer, Director Research
[email protected]
Günter Stelzer
Director Special Projects and Education
[email protected]
Ziehm Imaging GmbH
Donaustrasse 31, 90451 Nuremberg
Phone +49.(0) 9 11.21 72-0
Fax +49.(0) 9 11.21 72-390