How to Compress Picture

The change from the cine film to digital methods of image exchange and archival is primarily motivated by the ease and flexibility of handling digital image information instead of the film media. While preparing this step and developing standards for digital image communication, one has to make absolutely sure that also the image quality of coronary angiograms and ventriculograms is maintained or improved. Similar requirements exist also in echocardiography.

Regarding image quality, the most critical step in going from the analog world (cine film or high definition live video in the catheterization laboratory) to the digital world is the digitization of the signals. For this step, the basic requirement of maintaining image quality is easily translated into two basic quantitative parameters:

* the rate of digital image data transfer or data rate (Megabit per second or Mb/s)
* and the total amount of digital storage required or data capacity (Megabyte or MByte).

As a specific example, the spatial resolution of the cine film is generally assumed to be equivalent to a digital matrix of at least 1000 by 1000 pixels, each with up to 256 gray levels (8 bit or one byte) of contrast information (see Syllabus Unit 1). The following table derives from this principal parameter some examples for requirements on digital image communication and archival in a catheterization laboratory with low to medium volume.
Film Digital
Spatial resolution 4 linepairs/mm 1024*1024 pixels
Data capacity per image 1 Megabyte (MByte)
Data rate 30 images per seconds 30 MByte per second

Data capacity per patient exam 2,400 images 2,400 MByte
Media one film four CD-R
Data for 10 years 30,000 films 120,000 CD-R

Table 1 Scenario for replacement of cine film by digital imaging with high resolution






From Table 1 we see, that in this scenario the enormous data rate of 30 Megabyte per second has to be supported. This is much faster than even advanced ATM networks (offering less than 20 Mbyte/s or 160 Mbit/s). Looking for existing off-line media, real-time display from CD-R would require a CD-R player with a data rate of 200X, while the fastest players available presently deliver 50X (1X stands for a data rate of 150 KByte per second). The total amount of data or the ‘data capacity’ required in this scenario is even more frightening (see Table 1).

Computer technology, however, provides flexible principles for processing large amounts of information. Among the algorithms available is image data reduction or ‘image compression’. The principal approach in data compression is the reduction of the amount of image data (bits) while preserving information (image details). This technology is a key enabling factor in many imaging and multimedia concepts outside of medicine. So one has to ask if cardiology really will have to cope with these enormous and totally uncommon requirements concerning digital data rates and digital data capacity (Table 1), or if image compression can also be applied without problems in cardiac imaging.

At a closer look one observes that ad hoc approaches to image data compression have been applied in most digital imaging systems for the catheterization laboratory all the time. An example is recording the x-ray images with a smaller matrix of just 512 by 512 pixels (instead of the 1024 by 1024 pixel matrix often applied for real-time displays). In order to objectively assess these and other techniques of image data compression, some systematic knowledge of the tradeoffs implied in different modes of image data reduction is mandatory.
How to Compress Picture