| Περίληψη: | In current digital imaging systems, new components such as digital display, image
compression, image processing, as well as film digitization and film printing are introduced, in
addition to analogue or digital imaging modalities. Although the current trend is towards fully
digital systems, analog images (films) represent a high percentage of the volume of medical
image data in the interim transition period from analog to digital. Soft copy display of medical
image using CRT monitors, which plays a critical role in diagnostic information extraction, is
one of the most vulnerable to failure and performance degradation component of the display
workstation. Medical film digitizers and film printers play an important transitory role as
analog-to-digital bridges in current PACS. Lossless compression algorithms are widely used
in digital medical imaging systems for efficient image storage, transmission, and display,
while lossy ones, offering significantly higher compression ratios, are emerging. An important
factor for the acceptance of lossy compression in clinical practice is the assessment of
“visually lossless” compression thresholds. The above components, as well as the services
involving them in telemedicine or PACS systems, introduce additional needs of performance
evaluation.
In this thesis a performance evaluation protocol, applicable to each of the new components
of digital medical imaging systems, as well as to telemedicine or PACS services, is
proposed. The proposed protocol is based on a set of transfer characteristics assessment,
such as characteristic curve, high contrast transfer, low contrast transfer, noise, uniformity,
and geometric distortion, common to all components, and additional component specific
parameters (i.e. veiling glare for CRT monitors). The application of the protocol for each
component is based on the use of digital test objects, designed using a software tool, which
enables (a) user-driven selection of patterns, associated to evaluation parameters, to be
included, and (b) user-driven pattern specifications, according to requirements of individual
components.
The protocol has been applied for the assessment of transfer characteristics of: (a) three film
digitizers, (b) two CRT monitors, (c) two film printers, and (d) two compression algorithms,
used or intended for use in digital medical imaging systems, to demonstrate its completeness
with respect to evaluation parameters, its sensitivity (i.e., the capability to differentiate
between components of the same category and with similar performance), and its
adaptability with respect to the specifications of the individual component being tested. Of the
three film digitizers tested two are CCD based, Lumiscan 20 and Agfa DuoScan, and one is
laser based, Lumiscan 75. Measurements performed show superior performance of the Lumiscan 75 digitizer with respect to its characteristic curve and amount of noise introduced.
The Agfa DuoScan has superior performance with respect to spatial resolution. For low
contrast discrimination the Agfa DuoScan has superior performance for low optical densities,
up to 1.8 o.d. units, while for higher optical densities the Lumiscan 75 digitizer show better
performance. The Lumiscan 20 film digitizer has a linear characteristic curve and acceptable
noise handling up to 2.8 o.d. units, but low spatial resolution.
Of the two CRT monitors tested, Sony E500 and Samsung 950p, the former exhibits slight
performance superiority, but they are both characterized by limited performance with respect
to maximum luminance, resolution (high contrast response), and low contrast discrimination.
Of the two film printers tested, AGFA DrayStar 2000 and Kodak DrayView 8100, the latter
shows better performance with respect to characteristic curve, high contrast response and
low contrast discrimination.
The assessment results for the JPEG compression algorithm tested (OTE-TS
implementation) indicate the compression ratio of 15:1, as a visually lossless threshold in
case of low contrast lesion detection in x-ray chest images, in agreement with previous
observer performance studies. The wavelet based compression algorithm tested (Pegasus
Imaging Corp. implementation) is JPEG 2000 compatible with respect to the wavelet filters
used. Its visually lossless threshold, derived from contrast threshold curves associated with
microcalcification cluster detection in mammograms, is at compression ratio 35:1, and it is
lowered at 30:1 if a perceptibility criterion for image quality degradation is applied. The
validation study, based on ROC analysis of observers’ performance, confirmed these values,
as the threshold for cluster detection is at compression ratio 40:1, and it is lowered to 25:1 by
observers’ image quality grading.
Component performance evaluation results indicate that the proposed protocol can be
successfully used for comparative evaluation of film digitizers, film printers and CRT
monitors. In case of compression algorithms, the proposed protocol can be used: (a) as a
preliminary step for testing the algorithm during development, (b) for comparative evaluation
of algorithms, and (c) in the efficient planning of a ROC study with respect to range of
interest of compression ratios.
The wide applicability of the proposed protocol is based on a generic set of common transfer
characteristics with respect to components and services, and is also empowered by the userdriven
design of the digital test objects used. Test objects design is flexible, due to their soft
character, enabling adaptation of the design to the technical specifications of each type of individual component.
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