I get this question all the time: why should I pay thousands
of dollars for a medical grade monitor to
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| Looking at medical monitors at a trade show |
diagnose digital X-ray (CR/DR), if I
can buy a very nice looking commercial grade off-the-shelf, (COTS) monitors at
the local computer store. I have boiled this argument down to 6 important reasons
based on critical arguments, which are (hopefully) simple to understand and allow
you to convey this to your radiologists or administrators who have little
technical or physics background.
1. A commercial grade monitor does not show all of the
critical anatomical information. As the name implies, the COTS monitors are
intended for office automation, to display documents to appear like a printed
page. Therefore performance attributes
are weighted heavily to being as bright as possible so that text is easily
resolved with minimal eyestrain.
Commercial displays therefore attain maximum luminance way before the
graphic card input reaching its maximum input value. Remember that a typical graphics card can
display 256 different input values, each representing a distinct piece of
valuable diagnostic information. These monitors have been observed to max out as
low as a value of 200, which means values 201 to 255 are being mapped to the
same luminance value ... maximum. This means that 20 percent of all the data is
cropped or simply eliminated.
By contrast, medical grade monitors are
calibrated to map each individual distinct pixel into something you can detect
rather than following the natural response of a graphics card output. Unfortunately,
it is normal for the natural COTS monitor response (un-corrected to DICOM) to
yield the same luminance value (measured) for multiple sequential values, i.e.,
a flat spot in the response curve. These flat spots are especially obvious in
the low range, i.e. the first 160 of the 256 values.
What is the impact of a flat response?
Let’s take as an example, for a commercial grade monitor the pixel values of 101,
102, 103, 104, and 105, could be mapped into a single luminance value on the
screen. That means that if there is a slight nodule, which is identified by a
difference in value between 102 and 105, it will disappear, as there is no
distinction between these values on the monitor. Note that since the better
part of the clinical information from the imaging modalities is in the lower 50
percent of the luminance range, this means that these are in the most critical
areas wherein the ability to resolve pixels at different luminance values is
being compromised.
In conclusion, the potential to miss
critical diagnostic information both at high luminance and due to flat spots in
the response should be the number one reason to not even consider a commercial
grade monitor. Therefore, the first requirement for medical monitors is to insist
on a monitor that is calibrated according to DICOM standards, which truly maps
each of the different pixel values into a luminance value on the screen that is
detectable by the human visual system as noticeably different. It is best to have this calibration done at
manufacturing to have an optimal mapping of the three RGB channels into the
DICOM compliant curve.
2. Many commercial grade monitors don’t have the required
dynamic range. The maximum light output of a monitor is specified using the
units of cd/m2 (candela/square meter). A
good quality commercial display can achieve 300 cd/m2, sometimes
more if you are lucky. The maximum value of 300 cd/m2 would be at the low end of
any medical grade monitor, which might be able to go up to 500 cd/m2 or more. Why do we need this much? The reason is that
when a display is calibrated to DICOM, a percentage of the response is lost in
the mapping process. At 300cd/m and applying DICOM corrections, the maximum
value can be expected to decrease by about 10 percent.
The human eye has a 250:1 contrast ratio at
the ambient conditions of the viewing environment. Assuming the commercial display was DICOM
compliant with aftermarket software, the luminance ratio of the display and the
eye would be very close. However,
ambient light detracts from the ability to see low contrast information. This particular example would need to be in a
low light room to achieve a 250:1 luminance ratio inclusive of ambient
light.
Medical displays are designed to operate
between 400 and 600 cd/m2 as corrected to DICOM with reserve
luminance potential for extended life at those levels. Even if a monitor is calibrated, if there are
not enough points to map the pixel data into, you clip off part of the
information. For example, if you want to map 256 grayscale pixel values but
have only 200 points available, you’ll lose that information. The required dynamic
range depends on where you are going to use the monitor. As you are probably
aware, the lighter, i.e. the brighter the room light, the more information you
are going to lose in the dark, as you simply won’t be able to distinguish
details in the dark.
There is a simple fix for that, the
calibration takes into account the room light and makes sure the lowest pixel
value is mapped to something you can detect. The whole range is shifted, which
is important when using it in a light area such as ER or ICU. Also, it is good
to have some “slack” at the dynamic range, the light source of the monitor will
decrease (compare the output of an old light bulb), and get lower and lower
over time). Therefore, the maximum brightness to facilitate mapping the whole
data range should be about 350 cd/m2
assuming you use it in a dark environment. If you are using it in a bright area
or if you want to make sure you have some slack to facilitate the decrease of
monitor output over a period of let’s say 5 years, you might want to go with
450-500 cd/m2.
3. A medical-grade monitor typically adjusts the output
to compensate for start-up variations in output. The light output of a monitor
varies as the temperature needs to stabilize for about 30-60 minutes. You can
leave them on day and night, or switch them on automatically one hour before
they are going to be used, however, either method will drastically reduce the
lifetime. Better grade medical monitors typically have a feedback mechanism
built in that measures the light output and adjusts the current to the light
source to create the same output. The third requirement therefore is to have a medical
grade monitor with a light output stabilizer.
4. A medical grade monitor can usually keep a record
and keep track of its calibration. One of my students in our PACS training told
me that he had to deliver the calibration record of a specific monitor dated 2
years back for legal reasons, to prove that when an interpretation was made on
a workstation, there was no technical reason that a specific finding was
missed. In addition, you need access to these records on a regular basis, regardless,
to make sure that the monitor is still operating within the acceptable range.
This brings me to another point - many users seem to replace their monitors
after a period of five years. If they are still within the calibration, there
is no reason to do that. Therefore, the fourth requirement for a medical grade
monitor is to make sure that you can retrieve and store the calibration records.
5. A medical grade monitor is typically certified.
There are recommendations that are defined by the ACR for monitors. They are
somewhat technical and in my opinion not worded strongly enough. Also, most
medical grade monitor manufacturers are FDA approved, which is actually only a
requirement in case you are reading digital mammography. If you meet the
requirements stated above, you should be OK, but FDA approval does not hurt.
You can check the FDA website and look up the manufacturer to see if they have
been approved. The fifth (optional) requirement is therefore to be FDA approved.
6. In addition to being able to see all of the grayscale,
which is characterized by the contrast resolution, you also need to be able to
distinguish between the different pixels, i.e. your monitor needs to have the
right spatial resolution to see the individual details. Let’s take a typical CR
chest, which might have an image matrix size of 2000 by 2500 pixels, that
results in 5 million pixels or 5MP. The standard configuration for a diagnostic
monitor to look at X-rays is 3MP, because a physician has the capability to
zoom or use an electronic loupe to see a one-to-one mapping of each image pixel
element on the screen. One could actually argue that you can use a 2MP monitor
as well, and yes that is correct as long as you realize that it will take more
time to make a diagnosis, as you need to zoom more frequently. But if you are
very cost sensitive, for example considering a system that is placed in a
developing country where money is major issue, a 2MP configuration would do.
So, the sixth and final requirement is to have a 3 MP monitor configuration
(assuming time is more important than cost).
Does this mean that a commercial grade monitor cannot be
used? It depends, if you are willing to manually calibrate the monitor and do
this on a regular basis, by running a calibration check and making sure this
can be applied by the monitor, if you make sure to take care of the warm-up
time, if you have a monitor that meets the maximum brightness requirement, if
you keep your calibration records, and are not worried that in case of a legal
dispute the plaintiff does not have enough expertise to challenge you with the
fact that you use sub-standard components that could impact patient care, well…
it is up to you. But I would think twice about it, especially as the price
difference between a good quality medical grade monitor and commercial grade
monitor is not that great compared with the overall cost of a PACS system.
If you are interested to know more details, there is a
video on how to use a
test pattern to check whether a monitor is calibrated or not. And as always, in
our PACS training classes we spend quite a bit of time going over monitor
characteristics and calibration, and include hands-on experience with these, so
if you want to learn more about this, check out our training schedule.
