Problem with VOI LUTs in Agfa and Fuji CR and GE DX images, was Re:VOI LUT issues
buzzword wrote in news:comp.protocols.dicom:
> I believe I finally understand the VOI LUT concept, but I am at a loss
> as to how to apply it correctly given the images I have been given.
> THE LUT that comes with the image claims to be 16-bit, but none of the
> values goes higher than 4095. That being said, though, none of my
> original pixel values goes higher than that, either. I have read
> elsewhere on this group that when that happens you are supposed to
> adjust the LUT. Can someone be more specific? There was a thread from
> 2002 where Marco and David were mentioning doing precisely that.
> -carlos rodriguez[/color]
You have encountered the well known "we know what the standard says but
we are going to ignore it and do what we have been doing for almost
a decade regardless" CR vendor bug. Agfa started this, but they are not
the only vendor doing this now; GE and Fuji may have joined the club.
Sadly, one needs to look at the LUT Data, figure out what the maximum
value actually encoded is, and find the next highest power of 2 (e.g.
2^12 in this case), to figure out what the range of the data is
supposed to be. I have assumed that if the maximum value in the LUT
data is less than a power of 2 minus 1 (e.g. 0xebc) then the intent
of the vendor was not to use the maximum available grayscale range
of the display (e.g. the maximum is 0xfff in this case). An alternative
would be to scale to the actual maximum rather than a power of two.
Very irritating, and in theory not totally reliable if one really
intended the full 16 bits and only used, say 15, but that is extremely
unlikely since everything would be too dark, and this heuristic
seems to work OK.
There has never been anything in the standard that describes having
to go through these convolutions. Since the only value in the
standard that describes the bit depth of the LUT values is LUT
Descriptor value 3 and that is (usually) always required to be
either 8 or 16, it mystifies me how the creators' of these images
imagine that the receiver is going to divine the range that
is intended. Further, the standard is quite explicit that this 3rd
value defines the range of LUT values, but as far as I am aware, all
the vendors are ignoring the standard and indeed sending a third value
of 16 in all cases.
This problem is not confined to CR, and is also seen with DX products.
Typically I have seen:
- Agfa CR, which usually (always ?) sends LUTs, values up to 0x0fff
- Fuji CR, which occasionally send LUTs, values up to 0x03ff
- GE DX, for presentation, which always have LUTs, up to 0x3fff
Swissray, Siemens, Philips, Canon and Kodak never seem to send VOI LUTs
at this point (which is a whole other problem). Note that the presence
or absence of a VOI LUT as opposed to window values may be configurable
on the modality in some cases, and I have just looked at what I happen
to have received from a myriad of sites over whose configuration I have
no control. This may be why the majority of Fuji images have no VOI LUTs,
but a few do (or it may be the Siemens system that these Fuji images went
through that perhaps added it). I do have some test Hologic DX images that
are not from a clinical site that do actually get this right (a value
of 12 for the third value and a max of 0xfff).
Since almost every vendor that I have encountered that encodes LUTs
makes this mistake, perhaps it is time to amend the standard to warn
implementor's of receivers and/or sanction this bad behavior. We have
talked about this in the past in WG 6 but so far everyone has been
reluctant to write into the standard such a comment. Maybe it is time
to try again, since if one is not aware of this problem, one cannot
effectively implement display using VOI LUTs, and there is a vast
installed base to contend with.
I did not check presentation states, in which VOI LUTs could also be
encountered, for the prevalence of this mistake, nor did I look at the
encoding of Modality LUT's, which are unusual. Nor did I check digital
mammography images. I would be interested to hear from anyone who has.
PS. The following older thread in this newsgroup discusses this:
PPS. From a historical perspective, the following may be of interest.
In the original standard in 1993, all that was said about this was a
reference to the corresponding such where Modality LUTs are described
"The third value specifies the number of bits for each entry in the
LUT Data. It shall take the value 8 or 16. The LUT Data shall be stored
in a format equivalent to 8 or 16 bits allocated and high bit equal
Since the high bit hint was not apparently explicit enough, a very
early CP, CP 15 (submitted by Agfa as it happens), replaced this with:
"The third value conveys the range of LUT entry values. It shall take
the value 8 or 16, corresponding with the LUT entry value range of
256 or 65536.
Note: The third value is not required for describing the
LUT data and is only included for informational usage
and for maintaining compatibility with ACRNEMA 2.0.
The LUT Data contains the LUT entry values."
That is how it read in the 1996, 1998 and 1999 editions.
By the 2000 edition, Supplement 33 that introduced presentation states
extensively reworked this entire section and tried to explain this in
"The output range is from 0 to 2^n-1 where n is the third value of LUT
Descriptor. This range is always unsigned."
and also added a note to spell out what the output range meant in the
VOI LUT section:
"9. The output of the Window Center/Width or VOI LUT transformation
is either implicitly scaled to the full range of the display device
if there is no succeeding transformation defined, or implicitly scaled
to the full input range of the succeeding transformation step (such as
the Presentation LUT), if present. See C.11.6.1."
It still reads this way in the 2004 edition.
Note that LUTs in other applications than the general VOI LUT allow for
values other than 8 or 16 in the third value of LUT descriptor to permit
ranges other than 0 to 255 or 65535.
In addition, the DX Image Module specializes the VOI LUT
attributes as follows, in PS 3.3 section C.188.8.131.52.5 (added in Sup 32):
"The third value specifies the number of bits for each entry in the LUT
Data (analogous to “bits stored”). It shall be between 10-16. The LUT
Data shall be stored in a format equivalent to 16 “bits allocated” and
“high bit” equal to “bits stored” - 1. The third value conveys the range
of LUT entry values. These unsigned LUT entry values shall range between
0 and 2^n-1, where n is the third value of the LUT Descriptor."
So in the case of the GE DX for presentation images, the third value of
LUT descriptor is allowed to be and probably should be 14 rather than 16.
Re: Problem with VOI LUTs in Agfa and Fuji CR and GE DX images, was Re: VOI LUT issues
Ah, Mr. Clunie...where would I be without you?
Yes, I was able to handle the LUT issue...but I am finding that
applying the usual presentation LUT to scale to 8-bits makes it look
sort of nasty. How would one handle that sort of thing?