Mass effect – medical imaging and obesity

In 2003, a young radiology fellow at Massachusetts General Hospital noticed a surprising anomaly. While looking through films with a colleague, he realised that a certain number of readings were qualified with the statement, "limited by body size".

This suggested that the image quality was not what it might have been; the patients in question faced a barrier to diagnosis in the form of their own body mass. "I was shocked by the frequency with which it happened," says Dr Raul Uppot, now an assistant professor at Harvard Medical School. "I wondered if anyone had ever studied the impact of obesity on acquiring diagnostic-quality images, so I decided to attempt to quantify that."

The hospital, as it turned out, had a strong informatics department, with past radiology reports electronically searchable as far back as 1989. By entering the search term, "limited by body size", Uppot was able to plot trends in its usage over the previous ten years. It became apparent that there had been a steady increase, correlating strongly with rising rates of obesity in the state of Massachusetts.

As patients became heavier, radiologists were having ever more trouble interpreting their scans. The second question concerned modalities. Not all diagnostic imaging is made alike, and each variant faces specific issues.

Uppot wanted to find out what the ramifications were for ultrasound, X-rays, CAT scans and MRI scans. When a patient was obese, did some modalities fare better than others in terms of obtaining a reliable image?

Sound reasoning

"It turned out that the one that was most affected was ultrasound, because there's a direct relationship: the ultrasound has to penetrate through the patient and bounce back from the organs to give us an image," he explains. "The more subcutaneous fat someone has, the deeper the penetration, and the lower quality the image becomes. We then started asking questions about whether there are issues beyond interpretation. Are there patients who won't even fit on the scanners because they are so large?"

Through talking to technologists, radiologists, patients and referring physicians, the team began to collate the various diagnostic hurdles that existed for obese patients, and then started looking into solutions. Over the subsequent decade, Uppot has continued to engage with this subject by holding talks, passing on his findings and even persuading manufacturers to increase the weight limit on their CAT scanners.

The topic continues to beleaguer radiology departments. While US rates of obesity are beginning to flatten, the patient group whose scans are compromised by their bulk has not gone away.

Evidently, we are not just talking here about the super-obese, that rare class of patients for whom no scan can be obtained at all. They do exist, and for people in excess of 600lb (272kg) diagnostic imaging is essentially impossible, creating issues on a whole different scale.

Individual concerns

Far more prevalent are those patients who, at perhaps 250 or 300lb (113 or 136kg), would be placed in the moderately obese category and can easily be accommodated by a CAT scanner. For these people, the difficulties are more nuanced. Depending on the distribution of their fat, they may be no harder to image than an average-sized patient, or they might return blurry, noisy scans.

"The first question you ask as a radiologist is whether or not they can get on the scanner, and then you go through that process," says Uppot. "Once they are on, the next concern is the likelihood of getting a good diagnostic image.

"These are the questions we must address with an obese patient, compared with a patient who is of what we'd consider a healthy weight."

Patients whose fat is mainly intraperitoneal (between the organs) are generally straightforward to image using ultrasound. If the fat has accumulated subcutaneously, however, it will be difficult to obtain the desired quality as the ultrasound first has to penetrate a thick layer of adipose tissue.

In this case, says Uppot, the solution comes down simply to the laws of physics. As he explains, "There's a direct relationship between the thickness of the tissue and the frequency of the ultrasound probe.

"If you want to get a better image quality in an obese person, you have to do one of two things: you either have to decrease the thickness it has to penetrate through, such as by pushing the probe a little deeper into the subcutaneous tissue, or you can try to decrease the frequency of the ultrasound, going from a 3MHz transducer to a 2MHz, or even lower."

While the guidance for X-rays is broadly similar - radiographers should adjust their settings to increase the photon energy - the other modalities face different sorts of pitfalls.

With MRI it is more a question of trying to squeeze the patient into the scanner without exceeding the field of view, all the while ensuring their skin does not touch the inner board and keeping claustrophobia to a minimum. In this case, best practice simply involves taking extra care, investing in a larger scanner wherever possible.

PET, meanwhile, is dogged by the question of radiation dose. Although the recommended dosage is scaled up by weight, there is an upper limit beyond which safety becomes an issue.

This upper limit may not be sufficient for extremely obese patients, as most of the radioactive energy is deposited in their adipose tissues, degrading the image quality.

Biopsies may also be a problem, in that the needle may not be long enough to reach the necessary spot (the same applies to the catheter used to drain the abscess). Finally, sedation can be difficult on occasion, as the amount of drug required might in certain cases be enough to compromise their airways.

It's an extensive and troubling list of challenges, which throw up no easy answers. In many cases, the only option is to switch modality.

"In moderately sized patients, if you can't get an ultrasound or X-ray, you can always get a CAT scan," says Uppot. "This is a very versatile machine which for the most part you can use as a last resort. If you can get some kind of scan, you can at least work with that and make a treatment plan."

East meets West

For extremely obese patients, the choices are more perturbing. If a hospital is still using a regular-sized CAT scanner (with a weight limit of 450lb), it may be that the patient needs to be observed in hospital and clinically followed using laboratory data.

In a worst case scenario, diagnosis may involve a return to the bad old days of exploratory surgery - physically cutting the patient open to see what is wrong, and potentially exposing them to unnecessary risks in the process.

While this is not a common scenario, it can have grave consequences and even lead to lawsuits. "Since I've been talking about this issue, several lawyers have contacted me wanting me to testify regarding suits in patients where things were missed, either because an image could not be acquired, or because the quality was not good and therefore the diagnosis was not correct," says Uppot. "Both the doctors defending and the prosecution representing wanted to get an expert opinion. So these issues do exist, and particularly at the extremes, patients over 550lb, big errors can occur - things can be missed."

It is difficult to get a handle on the scale of the problem, not least because radiologists don't document when they've turned down a patient for a CAT scan. Uppot, however, is optimistic that such incidents will become less common in the future, at least in the US and Western Europe.

"I think the prevalence will be tied to the rise or fall of obesity in this country," he says. "My hope is that as we learn more about the biology of obesity and come up with ways to prevent or treat it, it will not be so big an issue. But what I am seeing is countries that have recently adopted the Western lifestyle are now facing the same issues we experienced ten years ago. I get calls from people in India, China and the Middle East asking what we have come up with: they are approaching what we have here."

As the global obesity crisis deepens, bariatrics is becoming an increasingly critical field of medicine. Radiologists therefore need to remain cognisant of the challenges faced by this patient group and the steps that can be taken. After all, an accurate diagnosis is the first and most important step towards obtaining optimal medical care.