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Impressions of Compressions

Posted Oct 23 2008 1:33pm

There has been some talk on Aunt Minnie recently about compression, prompted by a Canadian study that showed you don't lose anything with lossy compression. Think about that one for a moment.

This prompted me to see for myself just how compression might affect an image. There is a ton of information on compression in this setting. Try the Wikipedia article on wavelet compression for starters. Frankly, the topic is so complex, I would have had to dust off my old engineering skills as well as my HP 41c, which I loaned to a physicist friend years ago. So, I thought it would be useful to (picto)graphically demonstrate what varying degrees of compression might do to a CT slice. I began with a DICOM image, the original size of which is 514 KB.

Then, using IrfanView, I applied JPEG 2000 compression (which uses wavelets) in varying degrees, with the results depicted below:

The number after "Lossy" is the degree of compression applied, which can range from 1 to 100, rather like the 1-12 selection offered for the original JPEG. IrfanView's JPEG 2000 compression plug-in uses the LuraWave JP2 module from LuraTech, if you're interested. You do have to keep in mind that I had to convert the images into regular JPEG for display here on, but you still get the idea. The image is pretty much unreadable at over 1:100 compression (I probably should have stated that as 100:1, but it's too late now!) However, I defy anyone to tell me that they can tell much difference between the higher quality compressions and the original. In particular, the lossless image is for all intent and purpose identical to the original, at least to my eye.

There are strong arguments made to store only the original full DICOM image, and this makes sense given the plummeting prices for storage. However, bandwidth, while ever less expensive, is still right up there. For example, we are having to consider the use of Metro Ethernet (MetroE) lines to boost the transmission speeds for our self-owned PACS. This will add somewhere from $50,000 to $100,000 to our yearly PACS expenses. Were we to use lossless compression, or even very high-level lossy compression, this would not be necessary. The question always comes down to this: Will I miss something on a compressed image that I would have seen on the full-fidelity original? Frankly, I don't think so.

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