Quality Resource Guide
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Digital Radiography 6th Edition
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www.metdental.com
overexposure. While this is convenient for the
radiographer, it poses a potential risk for the patient
in terms of increased exposure to ionizing radiation
without a diagnostic benefit. Underexposure of
PSP plates does not result in images that are too
light, but rather in images that are noisy.
This
is the result of post-processing techniques that
can ensure proper brightness and contrast, but
that cannot compensate for insufficient signal.
Although manufacturers have put safeguards in
place, exposure tables or calibrated x-ray unit
preset buttons remain important for consistent
selection of the minimum exposure time required to
obtain a high quality image.
Manufacturers of solid-state detectors report
dose reductions of up to 90% compared to film.
These dose savings usually relate to the dose
reduction per image in comparison with D-speed
film. Although there are few reports on the actual
reduction in absorbed dose per patient as a result
of digital imaging, the reported dose reduction is
somewhat less when the results are compared to
F-speed film.
Solid-state and PSP systems for extraoral imaging
require exposures similar to those needed for 400
speed screen-film systems.
Systems Integration and
Compatibility
The development of digital imaging systems
for dental radiography has largely been driven
by industry. Manufacturers have adopted and
developed technologies according to individual
needs and philosophies. As a result, image formats
among systems from different vendors are not
standardized and image archival, retrieval and
display systems are not compatible. One can always
export and import images in generic image formats,
such as JPEG (Joint Photographic Experts Group)
and TIFF (Tagged Image File Format), however,
such processes are cumbersome, and information
associated with the image is lost. It has long been
recognized that the adoption of a standard for
transferring images and associated information
between different digital imaging devices is
necessary. Through the efforts of a large number of
professional organizations the Digital Imaging and
Communications in Medicine (DICOM) standard
was developed. The DICOM standard addresses
the interoperability of medical and dental imaging
and information systems. Images saved in DICOM
format store pertinent image information, patient
information and system information. This assures
a higher level of data integrity and allows seamless
transfer of images and related information between
DICOM conformant devices. Most dental imaging
systems now conform to at least part of the DICOM
standard.
Quality Assurance
The use of digital imaging systems implies
significant changes in how radiographs are
acquired, stored, retrieved and displayed.
Quality
assurance (QA) issues specifically related to film
and chemistry become obsolete. Other QA issues
stay the same, such as the need for correct
detector positioning and proper beam alignment.
The use of exposure tables or preset exposure
times for specific sites also remains important,
despite the ability to adjust image brightness and
contrast.
Whereas digital imaging eliminates a
number of significant QA issues, it also introduces
a number of new ones. The most important of
these are addressed below.
Electronic Image Display
Viewing dental radiographs as an electronic image
on a monitor instead of as a film on a view box is
sometimes perceived as very different despite the
fact that the image content is virtually identical.
When setting up a digital imaging system, the
selection of display hardware and software requires
some thought and evaluation. A host of technical
properties define the ultimate image quality of a
monitor. Most mid-range and high-end desktop
computers are configured with flat panel monitors
that are suitable for digital radiography. Besides
properties as size, resolution and refresh rate, the
brightness and contrast of a monitor are important
factors in a dental office that is typically well-lit.
Current laptop displays are of sufficient quality to
be used for typical dental diagnostic tasks, but their
brightness is often inferior to stand-alone monitors.
Also, the viewing angle of some laptop displays is
limited, and the observer needs to be positioned
squarely in front of the display for optimal viewing.
The software interface used for presenting
radiographic images as well as other diagnostic and
demographic information is an important element of
a digital imaging system. The requirements vary
according to the diagnostic task and practice
pattern. Important functions such as zooming,
scrolling and multiple image comparison must
meet the needs and preferences of the practitioner.
These functions are generally not as fast or flexible
as shifting a film mount around on a view box.
Comparing the different approaches of the various
vendors is an important aspect in selecting a digital
imaging system.
The visibility of electronic displays is degraded by
many of the same elements which degrade viewing
of film images.
Bright background illumination from
windows or other sources of ambient light reduce
visual contrast sensitivity. It should be noted that
it is more difficult to block ambient light around
a monitor than it is around a film on a view box.
Images are best viewed in an environment where
lighting is subdued and indirect.
Performance Assessment
The diagnostic imaging chain, from exposure to
interpretation, contains a number of links, each
of which has the ability to be the limiting factor.
Whereas each of the components, like the x-ray
source, the detector and the image display, requires
individual quality control protocols, it is also good
practice to assess the imaging system as one.
Using objective methods to measure sensitivity,
spatial resolution, contrast resolution, dynamic
range, noise and artifacts can help clinicians
monitor their imaging system over time and detect
potential degradation. Although not mainstream in
most practices, test phantoms have been developed
specifically for this purpose.
Hard Copies
As digital technology has become mainstream, the
need to print digital images for sharing with other
clinicians or with third parties is decreasing rapidly.
When images must be printed, it is imperative
to use a printing system that is designed for its
intended use and to follow the manufacturer’s
recommendations. It is always preferable to transfer
images digitally when possible. The main types of