P3D Re: 3-D Ultra-sound (was 3-D X-Rays)

[Tom Hubin <thubin@xxxxxxxxx>]

<snip part about grainy results>

> Registration is also difficult because the
> ultrasound transceiver is hand-held, so there is no fixed point of
> reference (unlike, say, CT and MRI which are very precisely and
> repeatably aligned so long as the patient lies still).  Unfortunately
> you can get rotational errors (roll, pitch, yaw) in addition to simple
> translation in one or more axes.

Once upon a time I earned my keep by designing noncontact distance
measuring equipment. I used lasers and CCD cameras but other methods are
possible. If you have 3 points on the handheld ultrasound device that
you can locate in space then you know where the device is and how it is
oriented. 

It could be as simple as 3 strings tied to 3 points on the device. The
other end of each sting would pass through a known point. The 3 strings
then form the edges for the triangular sides of a pyramid. The
triangular base is formed by the 3 known string feed points. When you
crunch the numbers there are always 2 possible places where the
ultrasound device could be. But one of those is on the wrong side of the
base. This is called trilateration since distances are involved rather
than angles. It is not triangulation. 

Tom Hubin
thubin@xxxxxxxxx


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