A 2011 literature review looked at the problem as it related to one particular condition: Difficulties in diagnosing pulmonary embolism in the obese patient:
A literature review.
They conclude that there is a problem - not only are many CT scanners unable to handle morbidly obese patients (although this is improving), there are no guidelines for when this poses a problem:
With the increasing prevalence of morbidly obese patients,
clinicians will more frequently be faced with obese patients
with signs and symptoms suggestive of PE who either cannot
receive thoracic imaging at their hospital because of
their size or the imaging is inconclusive [...]. The
clinician in this situation must first decide whether they
should transfer the patient to another facility with a ‘heavyweight’
CT scanner. There are no guidelines to aid the physician
in this situation. Locating these facilities is difficult
since there is no website or national registry cataloging
these CT scanners. In addition, risks are involved with such
transfers, especially the risk of re-embolization during
travel. Often there are questions about whether third party
payors will provide reimbursement for these transfers.
Finally, even though the patient may be transferred for the
study, there is no guarantee that the study will be diagnostic
since image quality may be poor, as described previously.
So, there is no protocol to contact local zoos.
Further, they reference another paper that considered the zoo angle for CTPA. (
CT pulmonary angiography is a particular subset of CT (computer tomography) diagnostic test.
In 2008, Ginde et al. published a study in which they
surveyed 136 US academic hospitals and a random sampling
of all non-federal USA hospitals with emergency
departments to determine the availability of CT scanners
capable of imaging obese patients. They located two 500
lb (227 kg) weight capacity scanners in their sample of
hospitals and eight in the 136 academic centers they contacted.
In addition, they contacted 145 zoos and 28 veterinary
schools to see if any of these facilities had scanners
used for large animal studies which could be used for
obese humans. Only two zoos had CT scanners; both
would not image humans. Sixteen of the veterinary
schools had large weight capacity scanners but only four
would consider scanning humans. Most of the veterinary
schools had policies specifically prohibiting imaging
human patients. Therefore, the rumor that zoos and veterinary
schools will perform CTPA on very obese patients
with suspected PE is false.
I checked the abstract of the cited 2008 paper and it concurred, on an even broader scale - all CT and MRI imaging:
Animal facilities are not a viable alternative for diagnostic imaging of human patients.
This is not evidence that it never happens - anecdotes would counter that - but merely that, as of 2008, it wasn't a common occurrence and wasn't a recommended practice.