Radiographic Imaging: Technology for
the Next Millenium
A - Comparing Receptor Systems
P. Langlais DDS, MS, FACD, FICD,
FRCD(C) and Dale A. Miles DDS,
truly hard for the practicing dentist
to begin to sort out the "hype"
from the truth about digital x-ray
imaging systems. What is digital radiographic
imaging? What makes it different from
film? What is the difference between
a CCD and a CMOS receptor? Do I really
care? Why are some systems "wireless"?
And, how do they do that? How many
line pairs/millimeter is good enough?
What is a line pair anyway? Why do
I need 4096 gray levels? What do you
mean by image processing? I thought
that was just the chemistry I use.
Dentists ask all of these questions
each time we talk to a dental group.
Then there’s the ultimate question:
how will digital imaging improve my
practice and my patient’s care?
will attempt to answer all these questions
in this article. In this first part
we’ll discuss hardware; that is, the
sensors (receptors). In the second
part we will present information on
or "image processing" (the
software tools), the part of the system
which truly impacts patient care,
and the part dentists are least familiar
is a "digital" image?
digital image is made up of a binary
language of 0s and 1s arranged in
rows and columns called an image matrix.
Each cell of the image matrix is called
a pixel (short for picture element).
A typical matrix can be seen in Figure
Figure 1 (rows and columns of pixels)
small pixel size for a dental digital
x-ray image is about 40 micrometers
by 40 micrometers. A 40-50 µm pixel
size will result in an image resolution
of about 10-11 line pairs per millimeter
(lp/mm) with a high definition monitor.
Resolution is a measure of small objects
placed close together. The higher
the resolution of a system, the better
the image may appear. In the best
of all worlds, a dentist viewing an
image on a monitor could potentially
separate 8-10 thin pairs of lines
per millimeter of space with the naked
line pair is a measure of the system’s
ability to capture detail; that is
resolution. Manufacturers claim anywhere
from 6 to as much as 22 lp/mm. Does
it really matter? In truth the
human visual system can only resolve
about 8 lp/mm at the best
of times without magnification. So
does 12 lp/mm or 20 lp/mm resolution
really matter? The answer is NO! It
doesn’t matter…all of the commercially
available digital x-ray systems have
sensors which can produce a diagnostically
makes a digital image different from
information for treatment decisions
is available to the clinician in a
digital image since the image can
be subjected to electronic image processing
such as making it lighter (density)
or changing the gray scale (contrast)
without making another exposure.
The amount of image processing the
dentist can do depends on the types
of software "tools" installed in the
individual system. Most software systems
offer the same basic image processing
package; however, some manufacturers
offer additional image processing
features. Several software packages
may be required for the office digital
imaging set-up. Many of the processing
tools available from the manufacturer
are the same as in any desktop publishing
software such as Adobe Photoshop TM.
This ability to study the image more
precisely by employing image processing
makes the digital image that much
more valuable for patient care.
benefits (and differences) over film
are storage and electronic transmission
of the image (teledentistry). Many
of our films, when retrieved from
the patient record years later, are
brown or yellow or some other hue
because of improper fixation in the
processing chemicals. We’ve all experienced
that phenomenon. Digital images stored
on hard drives, CD ROMs, or eventually
DVDs, will maintain their "archival
quality" for decades. KodakTM
guarantees the image fidelity of their
recordable CD ROM material for 100
years! By the way, Kodak still wants
to sell we dentists dental film, but
its medical division is one of the
leading retailers and manufacturers
of CCD devices for camera and radiographic
imaging. You should ask them why they
are holding dentistry back!
ability to send images over existing
telephone lines (teleradiology) exists
today. The insurance industry will
eventually accept image files with
claim submissions for claims adjudication.
Digital images stored at dental school
and other educational sites can now
be accessed for dentists to review
or educate themselves. Just type http://w3.dh.nagasaki-u.ac.jp/tf/atoz.html.
in the URL (Universal Resource Locator)
in your internet account and witness
the wonderful cases with plain film,
CT and even magnetic resonance images
that they have "on-line"!
Alternately, you can type nagasaki
dental school in your browser (Netscape,
is the difference between a CCD and
a CMOS receptor?
are 2 basic methods of acquiring a
digital x-ray image: indirect and
direct. There are now also 3 silicon-based
digital image receptors now to choose
from including the CCD (charge-coupled
device), the CMOS (complimentary metal
oxide semiconductor) and the CID (Charge
indirect method of image acquisition
converts original analog images (dental
x-rays or radiographs) to digital
images usually by a scanning process
using digitization. This is also done
with a CCD device, but it is a linear
detector just like your fax machine,
not a rectangular dectector like the
intraoral CCD. Once digitized the
resultant image can be further processed
like any other digital image. For
this type of "digital" image
a commercial scanner, capable of scanning
600 dpi, with a transparency adapter
in the lid is required. Color slides
may also be scanned in this fashion.
direct method of image acquisition
requires a solid state detector made
of silicon, the infamous "chip",
such as a CCD or CMOS receptor, to
capture the image directly. These
receptors are currently available
for the acquisition of intraoral,
and cephalometric6 images.
The digital system chosen should be
capable of producing images in the
formats used by the office. For example
the general practitioner needing intraoral
and panoramic images should select
a system offering the capability for
solid state detectors are "wired"
to the computer and range in thickness
from 3.2 (DMD’s MPDx) to 8.8mm (ProVision
– DEXIS) . The detectors actual imaging
area is smaller than their outer dimensions
and usually smaller than conventional
dental film. They vary in price from
$5000-$7000 for a #2 size sensor.
Compared to E-speed film, these receptors
generally require about 50% less radiation.
CCD and CMOS receptors have a wider
exposure latitude than film, but much
less than the PSP (photostimualble
phosphor) plates (which we’ll discuss
next). Inadequate exposure time results
in increased noise in the digital
image. Noise is analogous to the graininess
in film-based images, but the cause
in digital imaging is the collection
of too few x-ray photons. However,
unlike an "underexposed"
x-ray film, a lot of the image information
can be recovered in the digital image.
The information is there, it just
needs "enhancing". With
a film image, the halide is washed
off and unrecoverable.
two types of solid state detectors
available are the CCD and the CMOS
sensors. Both were developed along
with the transistor in the 1960’s,
but neither was as commercially viable
as the transistor back then. It took
the advent of the computer to make
CCD and CMOS technology more technically
and commercially viable. CMOS chips
are used in every computer, and their
manufacturing process is very mature.
Therefore they can be made cheaper
than CCD chips but, as yet, have not
been adequately tested for x-ray image
capture. CMOS chips contain some RAM
operation circuitry and a microprocessor
on the same silicon chip (Figure 2).
2 – CMOS chip with components
the small area called "imager"
is actually dedicated to image reception.
there are concerns by some that the
noise level will be greater with CMOS
sensors than with CCD, and that the
use of some of the "chip real
estate" or area leaves less sensor
area available for image capture.
This might lead to a less image information
in an x-ray system. They appear to
be more suited for commercial products
such as digital cameras and video
cameras which operate in bright light
conditions. In the industry, there
is as yet no consensus as to which
sensor is better. Table 2 outlines
the advantages/disadvantages of these
sensors in terms of their technical
specifications. Bold terms in the
table indicate which detector has
an advantage in that area.
2 - Modified from Technical Issues
in Digital Imaging: Dr.D.A. Miles
Bold words indicate an advantage of
to x rays
Fixed pattern noise
While the time from
image capture to acquisition and display
on the monitor is only a few seconds
with CCDs, many systems require that
the operator go to the monitor for
one or several steps before exposing
the next area. These steps include
trying to adjust the density or contrast
before deciding to re-expose. This
occurs with some frequency especially
with existing AC type machines with
older timers. Sometimes the dentist
must reorient the image; that is,
flip or rotate it, as the detector
cannot recognize or separate left
from right or upper from lower images.
Also the keyboard and mouse need to
be covered with a barrier material
like clear plastic wrap. This series
of steps may take from one to several
minutes per image. After using the
detector, it must be disinfected with
a high level or tuberculocidal agent.
Special barrier envelopes are available
for detectors which must also cover
about 6-8 inches of the cord. CCD
detectors are somewhat delicate and
must be handled with care, as replacement
is expensive. Detectors should preferably
be used with DC type intraoral machines
with 1/100sec exposure intervals for
the shorter exposure times.
Wired Vs. Wireless " How do they
the solid state detectors, PSP (photostimulable
phosphor) systems are "wireless".
Three manufacturers offer imaging
systems which use (PSP) plates; Soredex
(Helsinki, Finland, DigoraTM),
Digident (Israel, CD-DentTM)
and Gendex (Milwaukee, WI, DenOptixTM).
These plates have no wire to the computer
and resemble in every way intraoral
film including size, thickness, rigidity
and receptor placement. The cost is
about $30-$50 per plate.
the exposure, the phosphors in the
plate enter into an excited state
proportional to the amount of radiation
exposure. However, unlike conventional
screen phosphors used in panoramic
systems, the PSPs do not immediately
fluoresce but only store the image
information. The plates must be placed
into a scanner ($12-25,000) and excited
by a laser which then causes them
to fluoresce. The fluorescence is
captured as an electronic wave form
and converted to a digital image (analog
to digital conversion) by the computer.
The resultant digital image can then
be viewed on a monitor in about 30
seconds to 5.5 minutes depending upon
the systems. The time from capture
to viewing is one to several minutes
and varies with film size or number
of films being scanned. The image
resolution is from 6 lp/mm to 9 lp/mm
depending on the product. PSP plates
are not as sensitive to exposure time
variations as film because of their
wide exposure latitude. This latitude
is even greater than the solid state
systems. So, though they lack the
resolution of film or CCD/CMOS systems,
they compensate by having an extremely
wide exposure latitude (wider gray
scale). PSP plates also come in 5
in. X 12. in panoramic and 8 in. X
10 in.cephalometric sizes. The resolution
of the image is 6 or 9 lp/mm for the
two available systems.
plates are exactly the same size as
the corresponding intraoral film including
the thickness (about 1.6 mm.). They
are soft and pliant much like film.
There is no need to go to the computer
between exposures even to check for
proper density as the plates will
be properly exposed over a wide range
of exposure times. However each plate
must be shaken out of it's barrier
pouch and scanned individually or
as a group by the laser scanner and
imported into the computer. Thus the
total time to process each image can
take several minutes. The infection
control step is carried out after
exposing all of the plates. Each is
carefully shaken out of the barrier
envelope onto a clean surface, wiped
with a disinfectant and then fed into
the laser scanner. After use, the
plates are placed into new barrier
envelopes for the next patient. PSP
plates are not delicate and they can
be replaced for a fraction of the
cost of detectors. Plates may be the
system of choice for use with older
AC x-ray machines.