CR uses an imaging plate coated with
storage phosphors to capture x-rays as they pass through the
patient. Trace amounts of impurities are added to the phosphor
materials in a process called "doping," to alter their crystalline
form and physical properties. When irradiated, the enhanced
phosphors absorb and store x-ray energy in gaps in their altered
crystal structure. This trapped energy comprises a latent image;
when stimulated by additional light energy of the proper wavelength,
the trapped energy is released.
In modern CR systems, storage phosphors commonly are stimulated with
a low-energy laser to release visible light wherever x-rays have
been absorbed. This light is captured and converted into an
electrical signal, which is converted to data that can be
transmitted to remote systems or locations, displayed on
laser-printed films or softcopy workstations and stored digitally.
An X-ray with Vision
Other than the absence of film and chemical processing, from the
technician's and the patient's perspectives, computed radiography -
including equipment for capture - works very much like conventional
film-screen radiology. The difference is in the benefits.
Phosphor plates, like film, are stored in cassette format. In
fact, existing analog equipment, from generators to x-ray tubes,
examination tables and upright chest exam systems, can be used with
a CR system. Technicians simply insert a CR cassette instead of a
film cassette, take the x-ray, and then transfer the exposed
cassette with x-ray images into the CR unit that scans and
translates the contents into data, to be sent to soft copy display,
archives, or hard copy print-out.
Compared to conventional film-screen capture, CR technology
speeds image availability and can reduce image retakes and
duplication costs, to boost workflow and productivity. CR also
offers more options for displaying, sharing and storing images.
From the core precepts of storage phosphor-based image
generation, improvements in phosphor screen coating, optics and
scanning systems, and image data processing have increased the
sophistication of computed radiography. Kodak scientists'
contributions in many areas have helped make it possible for
self-contained hardware and software systems to handle every step
from image acquisition to display. In general, CR systems are
getting smaller and faster, to handle heavy patient loads in tight
spaces, such as emergency rooms or intensive care units.
The Next Step
Kodak experts are continuing to refine aspects of CR to address
industry challenges including the types of exams possible, such as
"long length" imaging. Not long ago, physicians requiring
hip-to-foot, full spine or other "long length" images needed to use
film-based image capture - along with attendant chemical processing
and film handling. Kodak recently introduced fully automatic
stitching software and cassette positioning system that delivers
images up to 17 inches wide by 51 inches long (43 x 129 cm), with
few, if any, visible seams. The results can be viewed in soft copy
or printed on film using a Kodak DryView laser imager for viewbox
display.
In an ongoing commitment to digital imaging, Kodak continues to
refresh CR technology quickly, applying its innovation and expertise
to meet the needs of hospital radiology centers and diagnostic
facilities - whether they are high-traffic, cutting-edge teaching
schools, or low-volume operations with specific cost requirements.
Since 2000, Kodak has introduced three DirectView CR systems for
single- or multi-plate image capture and high-volume image
generation and processing, with additional new product offerings
expected by the end of 2003.
Efficiency - better workflow and greater productivity - remains a
top priority for healthcare providers. For those facilities that
have adopted CR, further advances in system capacity and image
processing capability contribute to quantity and quality of images,
so that the time to produce images is reduced, and the number of
radiography sessions completed is increased. Kodak pioneered remote
operations panel display equipment that allows radiology technicians
to perform most system functions while away from the main CR unit.
Images may be reviewed or accessed from an examination room as if
the technician were using the primary CR system itself. Other
software makes it possible for demographic data to be entered by
department administrative personnel, to make the best use of
technician's time.
Advances in image science also make it possible for radiology
centers to improve operations without huge capital investment; for
example, Kodak recently introduced third-generation software for
image enhancement and processing. Along with these improvements, the
cost has come down, and CR systems with better functionality are
available at half the price typical just five or six years ago.
These changes further encourage adoption of CR by healthcare
providers.
As CR was becoming more standard in hospitals and diagnostic
centers, Kodak scientists began work several years ago on the next
wave of radiographic imaging technology: Digital, or direct,
radiography (DR). DR systems feature a detector that has the
capability to produce images very quickly, either through a similar
phosphor stimulation process or by a special material that converts
x-ray energy directly to a charge that is read by a semiconductor.
Indeed, adoption of Kodak CR technology could help customers more
quickly embrace DR technology. Kodak is commercializing this next
generation of radiography technology to leverage the familiar
functional aspects of its CR systems, so that DR will be easy to
incorporate into existing radiology practices and workflow.
© Eastman Kodak Company, 2003. Kodak, DirectView and DryView are
trademarks.
http://www.kodak.com
CR v Film screen imaging - process
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| The Diagnostic Imaging Chain in CR
Compare this with the standard film screen Imaging
Chain
Compare FS and CR Storage Phosphors
Advantages of CR over FS systems
Ref: Kodak Training CD
Exposure Techniques in CR
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Having reviewed the exposure factor considerations we now need to
look at the differences with CR.
mA - It is no longer true that mAs controls image density,
in FS where the same medium is used for detection and display the 2
are intimately linked. However in CR where detection and
display are separate processes they are not linked. For a
given CR image receptor mAS still determines the number of quanta
that are absorbed and , therefore noise, the image processing
controls the output signal and hence image density in CR.
kV - Kv still controls contrast BUT a linear response of
CR (Exposure v density) means that matching the subject contrast to
the Kv and film screen combination is much less important. The prime
influence on contrast in CR comes from the
"lookup tables" Kv along with tube filtration still controls the
incident x-ray spectrum and therefore, subject contrast and scatter,
image processing can partially compensate for loss of subject
contrast.
This difference between Film Screen imaging and Computed
radiography and the transition from FS to CR imaging is something
radiographers find difficult and when they produce a light or
dark image are tempted to repeat the image using more or less
exposure respectively in order to correct the image - this is the
INCORRECT way to approach the problem, the method should be to
check the Exposure index to see if enough exposure has been received
by the imaging plate to permit image manipulation then check and
adjust the image using the image processing functions of the system.
Scatter
As with FS systems scatter plays an important role - in fact
scatter and its control is more important in CR than in FS
radiography
CR Systems and Scatter
Here we compare the absorption spectra of FS screens (green
line) and CR image screens, (pink line) note the absorption
peak (K edge) of CR screens to the considerably lower due to
the primary absorber Barium of CR compared with Gadolinium of FS
screens, hence the greater sensitivity to scattered radiation
energies.
Now look at the primary and secondary or scatter spectra for a
typicaly 80 Kv exposure, the scattered x-rays have lower energies
than the primary beam, the storage phosphor CR absorbtion peak falls
in the middle of the scattered engies range indicating that it will
absorb a fair amount of these energies - compare this with the Lanex
regular screen phosphor.
CR storage phosphor screens absorb more x-ray scatter than most
conventional FS combinations under the same exposure conditions.
This may explain observations that image quality of some images
where there is a lot of scatter eg (Mediatinum in non grid chest
images and the abdomen of large patients) is lower with CR
than conventional FS radiography.
Summary of Comparison of CR and FS phosphorus
Lookup table,
table allowing a display system to map pixel values into colours or
grey scale values with a convenient range of brightness and
contrast. Thus, a narrow range of input pixel intensities may be
mapped onto the available range of output intensities. Rather than
using the pixel values directly, the value is instead used as an
address into a lookup table where the content of the table at that
address defines the output grey-scale value .
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Consider now the imaging cycle events
 On x-ray
exposure the latent image is formed by trapping electrons in the
screen, half of the energy disappears immediately in the form of
light, the remaining is trapped in metastable storage states in the
screen.
When exposed to X-rays, the europium
atoms in the phosphor crystalline lattice are ionized (converted
from 2+ to 3+), liberating a valence electron. These electrons are
raised to a higher energy state in the conduction band (see solid
and photoconduction for an explanation of conduction band). Once in
the conduction band, the electrons travel freely until they are
trapped in a so-called F-centre in a metastable state with an energy
level slightly below that of the conduction band, but higher than
that of the valence band. The number of trapped electrons is
proportional to the amount of X-rays absorbed locally. The trapped
electrons constitute the latent image. Due to thermal motion, the
electrons will slowly be liberated from the traps, and the latent
image should therefore be read without too much delay. At room
temperature, the image should, however, be readable up to 8 hours
after exposure.
The wide-latitude response of the storage phosphors means that a
high-quality image can be produced regardless of the relative amount
of exposure to the plate. The laser beam in the CR reader scans
across the imaging plate, the phosphors are excited and release the
energy they have stored. This energy is emitted from the plate as a
violet blue glow. The strength of this glow is directly proportional
to the amount of radiation absorbed. The phosphor glow is captured
in the scanner and converted into a digital image. Because visible
(“photo”) light excites (“stimulates”) the phosphors to glow (“luminesce”),
this process is known as photo stimulated luminescence and the
phosphors are often called photostimulable phosphors or storage
phosphors.
The laser scan does not extract all the energy stored in the
crystals so the storage phosphor plate must be erased. After the
plate is scanned it enters the eraser where it is flooded with
bright fluorescent light. This intense light removes any residual
energy remaining on the plate so that it can be used again. At the
end of the erase cycle, the operator removes the storage phosphor
plate from the eraser drawer and reloads it into a cassette. There
is no evidence that the storage phosphors’ exposure or erasure
response changes over time. |
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