November 26, 2011

Recently I found myself lying on an oversized tongue depressor, being transported feet first through a giant, humming doughnut called a PET scan machine. Which fortunately found no malignancy, to get that out of the way right at the start.

The process requires 45 minutes of lying perfectly still on the back, arms at the sides and hands in pockets. This allows plenty of time to reflect not just on the transitory nature of man, but also on the design of PET scan machines.

For one thing, the tiny label that warns you not to blind yourself by looking at the laser is printed upside down. For another, the hole through which you are being drawn with such excruciating slowness is pretty damn small. I am six feet long but not, at 160 pounds, very wide. The smooth sides of the doughnut grabbed my shirtsleeves and pulled them up.

One out of five adult Walmart customers — I’m guessing here, but I doubt if I’m far off — could no more have gotten through that opening than an orange could squeeze through the neck of a milk bottle. Life-threatening compression or major surgery would be required.

I asked mysonthedoctor about this afterwards and he said it was indeed a problem. “Sometimes they have to send patients to the zoo where they have larger machines,” he said. “It can be psychologically devastating.”

The zoo? Off to the internets, where that part turned out to be something of an urban legend:

“We found that only 1 percent of zoos had CT scanners at all,” [Dr. Ginde] said. “And neither of them said they would image human patients.” About 15 percent of the veterinarian schools had scanners, “but almost all of them said they had policies prohibiting human patients because there were medical and legal issues about them not being licensed.” So, except for one or two isolated cases, no human has ever been scanned by a machine meant for animals.

But the underlying problem is no legend. Many existing PET, CT or MRI tables can’t handle patients weighing more than 450 pounds, and one manufacturer reports:

For instance, it is difficult for some obese patients to fit in the openings of standard MRI scanners so they can be examined. These patients are often referred to an open MRI scanner that presents physicians with a Catch-22 situation: While the open MRI scanner can physically accommodate larger patients, these scanners typically have much less power and images of obese patients tend to be of diminished quality.

The technical description for such images is LBBH (Limited By Body Habitus). Just so you know.



Posted by Jerome Doolittle at November 26, 2011 06:39 PM
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My understanding is that as the opening gets larger, the machine must be disproportionately more powerful.

Perhaps the issue can be addressed by legislation. Since the US government can define pizza as a vegetable ("Congress Steps Up to the Plate…"), maybe they solve the problem by defining a 27-inch MRI as a 38inch one? :)

Posted by: John Anthony Curran on November 27, 2011 8:18 PM

I would note that in the USA the price for laying on the giant tongue depressor is about $1200. In Europe a visit to one usually runs less than $400. I think GE is the company that makes a good many of these devidevices. Coincidentally, a corporation that Ronald Reagan worked for to get their workers to believe a bunch of crazy ideas.

Posted by: Buck on November 28, 2011 7:19 PM
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