Monday, November 12, 2012

Ultrasound Phantoms: War were declared

The ultrasound phantoms work.  They have a few limitations, but for certain applications, these cheap DIY phantoms are going to be far superior to expensive commercial phantoms.  In fact, part of their DIY nature makes them inherently superior.  Creating a phantom for a special purpose, such as vessel cannulation, foreign body identification, or needle procedures, is simple.

One of the big issues that has come up is bacterial/fungal growth on the phantom.  This should be expected, as the phantom is constructed of gelatin and sugar-free psyllium.  The system is essentially a giant petri dish.  I needed to find a way to inhibit microbial growth.  Not just inhibit, but stop.  Completely and totally.

I lucked out in a couple of ways.  I tried mixing some bleach into the material when it was on the stovetop.  The mixture foamed up a fair amount.  This wasn't a big issue with the first two layers, as pouring the next layer seemed to dissolve the previously settled foam.  The final layer settled with a very large amount of foam.  This foam is a random mixture of air and gelatin, and almost completely sono-opaque.

This is the external surface of the simulant, and represented a major problem for the phantom.  It was impossible to image past this interfering layer.  I tried scraping the surface foam off, but to no avail.
I spent three shifts at the hospital trying to find a way to make the system workable.  At this point I had intentionally inoculated the surface with dust and dirt to attempt encourage microbial growth. At the three day point, no growth, but no apparent solution.

Ending my shift one morning, I had my "Eureka" moment.  One of the workmen was removing floor tiles by heating them with a propane torch.  Between my broken Spanish and his broken English, I was able to convey that I wanted to borrow his propane torch.  I quickly heated the surface of the phantom, melting the top foam layer.  It liquified and appeared to completely remove the foam layer.

I waited ten anxious minutes, visibly bouncing.  I placed the ultrasound transducer and was rewarded with two easily visible vessel simulants.  Cannulation was almost exactly the same (the new material seems somewhat more friable).

Six weeks later, the phantom was still visibly free of microbial growth (I don't have the patience or material to actually test for microbial growth beyond naked eye visualization).

I'm stuck at the next step.  I'm either casting a human arm and making a mould, or I'm developing some method to snake the vessels through the simulant to increase the difficulty in cannulation.

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