In this department, a new feature of Applied Radiology, we report on the latest developments and concerns in the PACS & teleradiology arena. This month’s installment describes the benefits and detriments of grayscale monitor workstations used in PACS systems for the reading of CT, MRI, digital fluoroscopy, computed radiography, ultrasound, and nuclear medicine images.
Grayscale workstations are an essential element of any PACS or
teleradiologysystem. Gray-scale monitors are the electronic display
screens of any PACStermed soft-copy display used in these
workstations. Image fidelity forgrayscale monitors is measured by
the physical characteristics of luminance,dynamic range,
distortion, resolution, and noise. Psychophysical
techniques,including receiver-operator characteristics analysis
using clinical images andtesting with phantom contrast-detail
patterns to determine threshold contrast,are used to measure the
user's response to the display of images.
Display protocols are the sequence in which images are presented
to the useron the grayscale monitors. The two technologies used for
displaying digitalclinical images are grayscale workstations and
laser film printers.
Grayscale workstations with cathode ray tubes (CRTs) are used in
the displayof digital imaging data. Images are initially displayed
on a set of CRTs forthe proper selection of desired information;
laser film printers are used togenerate the films from which the
primary diagnosis is made and to archivethese films into the
patient's film jacket.
Research continues on the issue of high-resolution grayscale
displays forprimary diagnosis, particularly for the format used in
chest radiography (14¥ 17 inch) or full breast digital mammography
(8 ¥ 10 inch, 10 ¥12 inch, 4k ¥ 5k ¥ 16 bits). Studies have
compared reader responsesfor conventional analog screen-film
radiographs, laser printed digital images,and soft-copy readings.
Results of such studies indicate that readers find thesystems
nearly equivalent, but some users continue to favor films displayed
ona view box. Others favor reading from grayscale workstations.
Currently, thetrend is towards soft-copy radiology reading due to
the advantages ofinteractive grayscale workstation retrieval,
display, manipulation, archiving,and rule-driven workflow
The grayscale monitor has a number of deficiencies compared to
filmdisplayed on a view box, however. The luminance of the CRT
display screen isless than that of the view box by a factor of 10,
and the phosphor granularityof a grayscale display limits the
delectability of variations in contrast inthe displayed image.
Additionally, a grayscale monitor is limited toapproximately 2,400
scan lines because of its electron gun and deflectioncircuits, and
the bandwidth of the video amplifiers. Finally, the dynamic rangeof
a grayscale display is narrower than the optical density range of
the filmdisplayed on a viewer box. The light scattered in the glass
faceplate of thegrayscale monitor, a phenomenon known as veiling
glare, limits the dynamicrange and modulation transfer function of
Independent of these limitations, grayscale monitors are used in
PACS; theseare called soft-copy readings. These monitors can be
used for reading of CT,MRI, digital fluoroscopy, computed
radiography, ultrasound, and nuclearmedicine images.
The single largest concern of radiologists in the use of
grayscaleworkstations is the lack of user-friendly display
protocols and how they fitinto the workflow of the department. To
combat this, vendors and theirengineering staffs have focused on
providing workstations with features theybelieve are essential for
basic reviews of imaging for a variety of imagingmodalities. Such
capabilities include: 1) placement of patients' examinationsin all
modalities into a single folder; 2) patient selection
mechanisms(allowing the ability to select data, the entire patient
folder, specificpatient exams, a specific exam series, or multiple
folders for examinations andseries of examinations); (3) study
information displays (to display studyinformation and provide
access to detailed information); and (4) results reportdisplays (to
display transcribed result reports thatare associated with
aparticular patient exam. While the radiologist has the choice of
the spectrumof these display functions, all are not needed for each
examination reading.Rather, the need for throughput and a few of
these functions, carefully chosen,is the key to the radiologist's
desire for a friendly and rapid displayprotocol.
Modeling of throughput
To avoid the radiologist being disappointed in the grayscale
workstation'sthroughput and integration into the department's
workflow, it is necessary toconstruct models that will predict how
these parameters will behave. One suchmodel that has been developed
is called the resource utilization analysis andit is used to
determine how the grayscale workstations will fit into theworkflow.
table 1 (a typical CT laser film reading protocol) and table 2
(atypical PACS CT grayscale workstation reading) illustrate a
resourceutilization model. This model is simple to construct. It
will identify anypossible bottleneck in a series of steps that
could compromise a complete job.To construct such a table, first
select the resources to be modeled (identifiedby the columns
labeled tech, modality, printer, resident, radiologist and filmroom
personnel). The first column identifies the steps (need not be
sequential)to accomplish one complete reading, and the last column
identifies the averagetime required to complete each identified
step (one could also record thevariance about the average value).
Then for each step in the table, theresources used in that step are
indicated by a "l" (being used) and a"0" (not being used). For
example, the first step "patientexam" uses the resources of the
"tech" and the"modality" for a total of 25.653 minutes. Then, from
this table thethroughput per minute for each resource is
calculated. This is accomplished byassuming that a resource is 100%
utilized and hence the maximum throughput perminute for the "tech"
is 1/(25.653 + 6.520) or 0.031 jobs/minute. Thesmallest throughput
of the resources is the bottleneck (0.015 job/minute forthe "film
room personnel." Table 2 is calculated in the same mannerso that
the bottleneck is the "tech" (0.031 jobs/minute).
This modeling method is easy to achieve and determines the
bottleneck of theresources for integrating the workflow into the
PACS and comparing thesoft-copy to the film reading (table 1). The
"disruption" step is theamount of time that results from phone
calls and other interruptions to thereader.
This process of modeling the workflow allows a method for
seeking answers to"what if" questions. For example, in table 1, the
question of addinga second technologist would increase throughput
and shift the bottleneck to themodality. One could also compute the
"cost-per-throughput" to allowfor the cost of resources and the
impact on throughput.
An alternative means of modeling is with a state diagram (figure
1). Inthis, the states of the model (workstation, window level,
sign in, etc.) andthe transactions between each state are
identified. The steps used in thismodel are as follows: l) power
up; 2) sign in; 3) call up work list; 4) selectcase #l; 5) select
display protocol; 6) send case elsewhere 7) select windowlevels; 8)
select start mode; 9) read out case; 10) review consultation
report;11) sign case; and 12) log out. However, such modeling is
difficult for theradiologist, though it is easy for the vendors
The radiologist can become an integral part of the PACS
acquisition processby modeling the desired operation of the
grayscale workstation and how it isintegrated into the workflow.
Though the models shown in tables 1 and 2 andfigure 1 are simple to
construct, they are important in assuring that theworkstation is an
integral part of the workflow and that the workflow resultsin an
adequate throughput. Such models, when presented to the vendor,
willassist in determining how best to achieve a "user
friendly"grayscaleworkstation for your particular departmental
needs and preferences. AR