In the past few months, I've been heavily involved with upgrading
some CAT scanners that date from the late 70's. I've mostly been involved
with the PDP-11 side, but bolted onto the PDP-11 (or, actually, the other
way around) there's a custom piece of hardware (two dozen hex-height
cards) that does all the heavy number crunching. Backprojection is
what the process is called, and it's basically a deconvolution of the
data measured by the individual detectors to make the pretty picture
of your insides that a CAT scanner displays.
The power and speed of this classic hardware is quite impressive,
considering it's nearly 20 years old now. We've been timing other
approaches to backprojection, and if we implement the same algorithm
that the custom hardware is doing but on a modern 533 MHz Pentium III,
we discover that the custom hardware is twice as fast as the Pentium.
There are some other competing technologies, making use of DSP chips,
which promise to be faster, but at a substantial development cost.
Is this a fair comparison? Maybe not! The Pentium system costs roughly
$2000, while the custom backprojection hardware was several hundred
thousand dollars back in the late 70's.
The PC boards that make up the backprojector make use of a unique
technology: the PC boards themselves are just plain old two-layer boards,
with the actual PC traces carrying only power and ground. All the actual
wiring is done by fine machine-laid wiring between pads, with all
the wire layers "potted" in a plastic compound. Obviously, reworking
such a PC board is extremely difficult, but it gives impressive component
densities for the time. The modern equivalent of this is multi-layer
PC boards, naturally.
Of course, there's another sort of power involved too: the custom
backprojection hardware requires +5V at 350 Amps, and some installations
have multiple backprojectors!
--
Tim Shoppa Email: shoppa_at_trailing-edge.com
Trailing Edge Technology WWW: http://www.trailing-edge.com/
7328 Bradley Blvd Voice: 301-767-5917
Bethesda, MD, USA 20817 Fax: 301-767-5927
Received on Mon Oct 04 1999 - 16:06:40 BST