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Archive through January 17, 200717-01-07  10:32 amNick Gibson15
 
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 Link to this message Tom Naunton Morgan  posted on Wednesday, January 17, 2007 - 10:43 am Edit Post Delete Post Print Post
This is fascinating and seems to explain the confusion. Thanks for that. What is informed wisdom's advise for compressing CTs for remote reporting? I assume that one shouldn't compress plain films but am slightly in the dark!! Tom Naunton Morgan
 Link to this message Dave Harvey  posted on Wednesday, January 17, 2007 - 12:57 pm Edit Post Delete Post Print Post
Tom,

In general, clinical studies so far show that you CAN compress plain films and CT using moderate LOSSY compression without losing any significant clinical information. There is an on-going Canadian study to clarify where the cut-off exists, and to test different algorithms, but no studies so far have shown any loss of diagnostic accuracy at about 10:1 for low resolution images (CT, MR etc.) and at 20:1 for high matrix images such as plain films, using standard JPEG (12 bit where necessary). So, contrary to what you might think, plain films can sensibly be compressed MORE than CT/MR etc.

Nick,

Not "treading on my toes" at all - I don't "own" this subject! .....at least we gave the same answer!

Dave
 Link to this message Nicola Strickland  posted on Wednesday, January 17, 2007 - 01:40 pm Edit Post Delete Post Print Post
Dear Dave Which are the DICOM recognised forms of lossy compression? Where do the various wavelet compression algorithms stand in all this please? Thank you Nicola
 Link to this message Dave Harvey  posted on Wednesday, January 17, 2007 - 02:43 pm Edit Post Delete Post Print Post
OK - here's the full list - including lossless. I include the DICOM transfewr syntax UIDs for completeness, but most people probably don't need to know them!

JPEG Lossy
==========
Moderately well supported in practice. 2 forms:

"Baseline": Only allows 8 bit data : 1.2.840.10008.1.2.4.50
"Extended": Allows 12 bit data : 1.2.840.10008.1.2.4.51
This second version allows use for rewindowable images - CT is only 12 bits anyway, and MR can be "rescaled" to fit if necessary


JPEG Lossless
============
Very good industry support, and used internally by many systems. There are 2 forms, one having slightly restricted options to make life easier for simple implementations, but therefore with slightly less compression. That is the one widely used in cardiology systems.

"Full" : best compression: 1.2.840.10008.1.2.4.57
"Restricted" (PSV=1) : cardiology : 1.2.840.10008.1.2.4.70


Run-Length Encoding (RLE)
=========================
Simplistic lossless method only suitable where there are large areas of black (e.g. suitably encoded U/S). Rarely used
1.2.840.10008.1.2.5


JPEG 2000
=========
Only slowly coming into use. Unlike older JPEG, this uses the SAME basic mechanism for both lossy & lossless, the only difference being that lossy is a "truncated" version of the lossless data stream. This IS a wavelet algorithm, and is the only standardised interoperable wavelet method. Despite the hype, there is little difference in practice between usable compression ratios for original JPEG and JPEG 2000, and some concerns have been raised that artefacts in JPEG 2000 look "natural" (and are therefore dangerous) as opposed to JPEG, where artefacts are "blocky" and obvious. The major potiential benefit of JPEG 2000 is for streaming applications, but few vendors have yet to implement non-proprietary streaming using it.
Two different transfer syantxes to allow negotiation:
Lossless : 1.2.840.1.2.4.90
Lossless OR lossy : 1.2.840.10008.1.2.4.91


JPEG-LS
=======
Added a few years back, with a few influential backers, but never really used:
1.2.840.10008.1.2.4.80
1.2.840.10008.1.2.4.81


MPEG
====
This is the same as used for DVDs, digital TV etc., and is for video, including endoscopy etc. It gives MUCH higher compression ratios for the same quality, as it uses the frame-to-frame similarities in the data, unlike the other methods, which work "one frame at a time". Slowly coming into use:
1.2.840.10008.1.2.4.100


There have also been multiple "Private" transfer syntaxes/compression methods over the years, offering wavelet before JPEG 2000 became available. Though salemen remain stubbornly keen on claiming that THEIR wavelet mechanism is however SPECIAL, there is little if any evidence to suggest that any proprietary versions are any better than JPEG 2000, and they certainly wouldn't have enough benefit to offset the loss of standardised interoperability. They are already falling out of use, being superceded by JPEG 2000, and this will continue.

Is this perhaps too much information?

Dave
 Link to this message Keith Foord  posted on Wednesday, January 17, 2007 - 03:48 pm Edit Post Delete Post Print Post
The less 'techy' amongst us may like to see the SCAR review of the excellent Canadian work on compression for radiology images. They promise more information (also see a previous posting in this topic) but for CT it looks as if (from this work and others) that 5:1 JPEG2000 and for CR 20:1 JPEG200 are OK. There is an interesting legal caveat in the SCAR report and as Canadian law is similar to UK law this interpretation should be heeded!

The link is : http://www.siimweb.org/assets/6D50192B-239D-413E-96CE-04933B9C17F0.pdf

Or here it is downloaded and renamed for this group's convenience.
application/pdf
SCARWinter2006.pdf (688.4 k)
 Link to this message Ed McDonagh  posted on Wednesday, January 17, 2007 - 03:52 pm Edit Post Delete Post Print Post
Hi Dave.

Comprehensive information, nicely presented.

One small typo to correct - CT is only 8 bits, rather than 12.

Ed
 Link to this message Ed McDonagh  posted on Wednesday, January 17, 2007 - 04:13 pm Edit Post Delete Post Print Post
Doh! Apology needed I think.
Spoke too soon - 8 bits made Dave's statement make more sense, but 12 bits (as he said) is correct.

In fact, with the new CT scanners using extended CT number range, it can be even more than 12 bit.

Ed
 Link to this message Dave Harvey  posted on Wednesday, January 17, 2007 - 04:19 pm Edit Post Delete Post Print Post
Ed,

NO!!! - CT is at least 12 bits !!! (and if more can be rescaled like MR)

[Reference: DICOM PS3.3 C.8.2.1.1.5 : "For CT Images, Bits Stored (0028,0101) shall have the Enumerated Values of 12 to 16."]

Keith,

Where does your 5:1 figure come from? (which is barely more than the 3:1 available lossless!) The Canadian SCAR article (and all others I have previously seen) have found 10:1 OK for CT.

Dave
 Link to this message Dave Harvey  posted on Wednesday, January 17, 2007 - 04:27 pm Edit Post Delete Post Print Post
Ed,

Apology accepted (I wrote the last message before your 2nd comment appeared). To clarify my original comment (which was perhaps not well explained):

The 12 bit (extended) JPEG mode allows enough bit depth to be preserved for meaningful re-windowing, which is not true for the 8 bit mode. Most CT data is 12 bits, and in fact most MR (though nominally 16 bits) also fits into a 12 bit range. For data that does not, they can and should be rescaled prior to compression - whilst this does potentially lose a few "least significant bits", in the context of lossy compression anyway, this is unlikely to be significant.

Most of the other compression types listed support any number of bits (including JPEG 2000 lossy), the only exception being MPEG which is limited to 8 bits per colour.

Dave
 Link to this message Keith Foord  posted on Wednesday, January 17, 2007 - 04:34 pm Edit Post Delete Post Print Post
Dave,

Can't remember at present, but I think it was from a recent ER paper on chest CT. I'll do a bit of browsing and see if I can find the ref.

K
 Link to this message Keith Foord  posted on Wednesday, January 17, 2007 - 04:43 pm Edit Post Delete Post Print Post
Dave,

Try http://www.springerlink.com/content/8m1309v33807u6r2/?p=642867989f1a4381ab319b81 f125f282&pi=3
for the abstract

K
 Link to this message Dave Harvey  posted on Wednesday, January 17, 2007 - 05:12 pm Edit Post Delete Post Print Post
Keith,

That article is looking at a slightly different question from that normally asked.....it is looking at whether the images are "distinguishable" as opposed to the more clinically relevant measure of "equal diagnostic use", which is what more sophisticated studies such as the Canadians are using. As an example of the difference, it is often commented (as in the SIIM article) that moderately compressed images have a little less noise, and are sometimes "Preferred". Such images would be rated as distinguishable (and fail the test on this paper) but pass diagnostically based tests.

Dave
 Link to this message Keith Foord  posted on Wednesday, January 17, 2007 - 05:55 pm Edit Post Delete Post Print Post
Aye, well there you go. I guess we (RCR IT Committee?)are going to have to determine which way to fall off the fence! Whichever way it is going to have to be legally defensible and defensible to radiologists who might just perceive a difference and be unsettled?
The final outcome of the excellent Canadian work may determine this.
 Link to this message Cynthia E. Keen  posted on Thursday, September 13, 2007 - 02:18 am Edit Post Delete Post Print Post
Dr. David Ross reported "almost finished" interim findings at SIIM 2007. This was reported by Erik Ridley of AuntMinnie.com.

http://www.auntminnie.com/index.asp?Sec=sup&Sub=pac&Pag=dis&ItemId=77123

Dr. Ross said he will be reporting the final findings at the forthcoming Canadian Association of Radiologists annual meeting.

Cynthia
 Link to this message Arronlee  posted on Monday, April 21, 2014 - 04:29 am Edit Post Delete Post Print Post
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 Link to this message Susanna Moore  posted on Thursday, May 15, 2014 - 03:33 am Edit Post Delete Post Print Post
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 Link to this message David Clunie  posted on Thursday, May 15, 2014 - 01:42 pm Edit Post Delete Post Print Post
Yet another old thread getting resurrected and spammed by self serving posts from commercial vendors touting their products.

It does remind me though, that this thread never did get updated to include a pointer to an ESR position paper on the subject of lossy compression (which proposed the term "Diagnostically Acceptable Irreversible Compression" (DAIC)), which can be found at "http://www.i3-journal.org/cms/website.php?id=/en/index/read/image_compression.ht m" or "http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3259360/". Re-reading it to today I find a few funky statements in it, but it is a pretty thorough summary of our discussions at the workshop in Spain, and it is probably the most up to date summary of the state of the art.
 
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