With a simple C Program it is easy to generate arbitrary intensity modulated DMLC files. The images below show my first experiments with a home-made Leaf Motion Calculator.
The process is the following:
The number of shapes in a DMLC file depends on the complexity of the image, the number of shades and the intensities of the different shades of grey, and can be up to 320. Such a text file has around 400k. The treatment for the following image was set up for 6MV, 600 MU/min, 250 MU total with no buildup. Treatment time was half a minute.
This was my first try. The next thing was to improve imaging contrast and to extend the size to 40 by 32 cm, which is the largest area the MLC can cover (leaves can move 16 cm over the central axis). This can be accomplished by splitting up the image into three parts and treating them sequentially. With a little buildup the total number of Monitor Units can be reduced to around 500.
The problem now is to find out the right number of Monitor Units for each of the three slices. The picture above has three parts, 12 cm, 12 cm and 8 cm wide, from left to right. It was exposed with 200, 200 and 133 MU. As one can see, the middle part is a little bit too dark. My program also counts the total number of 'photons' for each file (left: 2934, middle: 3072, right: 1974), so if MU and photons are proportional, it should come out correctly ... But anyway, this one looks already quite nice !
This one is technically the same as the last one, but the Lumisys scan has a higher resolution. It is quite fascinating to see that a total number of 600 to 800 MLC shapes can be treated in less than 5 minutes.
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Last modified: March 18, 1999 HK