Imaging Obese Patients a Weighty Task

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 - Rhino

More than one-third of Americans are obese and the number continues to climb. In June, the American Medical Association classified obesity as a disease, but the condition has challenged healthcare providers for decades, with radiology departments attempting to shoulder the weight of the obesity crisis and the logistical and technical hurdles it creates.

Even as imaging equipment has evolved to accommodate more weight and girth and software has been refined, there are still issues with image quality and radiation dose that can hinder the ability to make accurate diagnoses. 

“Obese patients can be difficult to image. There’s an extra layer of tissue between the machines and the organs of interest, so assessment may be impaired,” says Martin Gunn, MBChB, associate professor of radiology at the University of Washington School of Medicine in Seattle.

Poor image quality is a potential problem in all imaging modalities. “The overhanging rule is the more tissue you have, the harder it is for ultrasound and X-ray beams to penetrate,” says Raul Uppot, MD, of Harvard Medical School in Boston.

In response to the obesity epidemic and the demand for imaging services, manufacturers have adapted their equipment to allow for excess load. Tables can handle more weight—some machines can withstand more than 650 pounds—and gantry openings have gotten larger to allow for wider girth.

The changes, while making it possible to scan patients who once would have been impossible to image, are not cure-alls. Not every facility, for example, is able to acquire the necessary equipment.

“Hospitals that haven’t been able to get new equipment and are still using older equipment are still stuck with older weight limits,” says Leigh Shuman, MD, staff radiologist at Lancaster (Pa.) General Hospital. “We assume that because vendors have built new equipment that everyone’s got it, but that’s not true.”

Facilities with newer equipment may be able to provide imaging services to more patients, but radiology departments still face numerous obstacles once the patient gets to the imaging suite.

“There’s wear and tear on staff, and the number of medical professionals who have back injuries from moving obese patients is increasing,” Shuman says. “And many radiology departments try to have equipment for lifting like slings, but getting people trained to use them is expensive and can strain budgets.”

Gunn says his institution relies on cranes to help position patient to take the strain off staff. “We don’t put in new scanners anymore without putting in a crane at the same time.”

Once the patient is on the table, another challenge presents itself as the gantry aperture may be too small for the patient to fit through. Over the past several years, manufacturers have increased the gantry diameter to accommodate patients with a larger girth.

If a patient is still too large, however, radiologists may have to try other modalities, such as low field open MRI or ultrasound—which has no weight limitations via the scanner, however larger size makes penetrating extra soft tissue more challenging and may yield poor quality images, Uppot notes. They also may have to send patients to other facilities that have bariatric CT scanners.

Uppot and colleagues have had to turn to imaging equipment designed for other purposes. “We have a CT scanner that has a gantry opening 80 centimeters in diameter that we use for interventional purposes to fit patient and interventional equipment. It is also used to accommodate the occasional large patient who does not fit on our standard CT scans,” he says.

Larger gantry size, while allowing the patient to get into the machine, introduces another imaging obstacle. “We can’t increase the size of the gantry forever, because once we increase it, we increase the distance between the organs we need to see and the x-ray tube,” says Gunn. “Sometimes, we can only see part of the patient.”

Poor image quality & diagnostic dilemmas

Uppot says different tactics may be required to help with diagnosis, including in-hospital observation and even exploratory surgery.

Each modality presents different challenges for radiology staff, whether related to image quality or some other issue. X-ray images of larger patients often have foggy areas that hide critical details. One problem, Shuman explains, is created because photons get deflected by the extra tissue.

“We’ve gone about as far as we can go to eliminate scatter radiation,” says Shuman. “There are grids that try to minimize scatter radiation, but we can’t eliminate it.”

He adds that while many radiologists will still utilize x-rays with obese patients, their value is limited. “We get precious little information from them. We may have to keep moving from test to test.”

CT scanning equipment is more forgiving, however.

“Of all the modalities we have, the best bet in an obese person is a CT scanner,” Uppot argues. “If the person can fit, we can usually modify the settings to get diagnostic-quality images.”

Manufacturers have built in ways to adjust the amount of radiation based on patient size, which has enabled the acquisition of better-quality images, but at a potential cost to the patient. Radiation doses often have to be higher than for non-obese patients.

In a 2012 study published in Physics in Medicine and Biology, Ding et al used phantom models of different weights to demonstrate that increasing tube potential of a CT scanner from 120 to 140 kVp for an obese individual increased radiation doses by 56 percent for organs in the scan field and 62 percent for organs outside the scan field. 

Technology improvements helpful, but not cure-alls

To counter radiation risks, manufacturers have developed software to optimize dose. Image quality can now be improved without increasing radiation. Similar gains have been made related to the use of contrast material.

“Technology now allows us to better tailor volume and rate of injection of contrast to patient size,” Gunn adds. “In larger patients, we need greater volumes of contrast with CT scans.”

Some manufacturers are now designing their CT scanners to be automated and optimize the image quality in patients of any size.

But Shuman says there still may be no skirting the issue of image quality. “For studies that require rapid imaging, we really have to up the radiation dose, and seeing fine details is difficult. It’s easy to see if there’s a kidney stone, but when you need to see small structures, it’s very difficult.”

Scatter radiation also is an issue with CT, most often because of imaging artifacts and unclear images, Shuman says. 

Patients who are very heavy, he also explains, may place too much stress on the equipment motors and, if their weight exceeds manufacturer-indicated weight limits, void the machine’s warranty.

MRI also has improved in terms of increased table weight allowances and gantry size as well as offering technological options that help yield better images.

“There’s also the advantage of open MRI magnets that we don’t have with CT scanning,” Gunn says.

As with CT scanners, the increased gantry size on MRI systems often compromises image quality. “The problem is when you make the MRI gantry bigger, you decrease the strength of the signal because the distance between the coil and the organ in question is increased. As a result, the image quality goes down,” Shuman says.

Since it is critical to get the desired organ or body part in the middle of the magnetic field with MRIs, the large patient can make it difficult to position the organ appropriately, according to Shuman.

Last resort

According to recent reports in the British press, U.K. hospitals have had to resort to using imaging equipment designed for zoo animals in some cases. Both Shuman and Uppot say they haven’t heard of that exact scenario happening in the U.S., but Shuman noted that he is familiar with some cases of patients who were sent to local veterinary schools where larger scanners were available.

Despite the imaging enigma often posed by obese patients, best practices and the accompanying technology are improving. Gunn says many facilities are moving toward having protocols for imaging obese patients. On the technology front, each facility needs to determine what investments to make in newer equipment.

“Each institution has to look at what their patient population is like and how many patients they are turning away and make a decision on how much to invest in larger scanners,” Uppot says. “In Massachusetts, obesity is not as big a problem as in other parts of the country, but in other states where obesity rates are much higher, they may have to address how to render accurate diagnoses with imaging obese patients.”

Imaging has still not caught up with the rate of obesity in the U.S., sometimes leaving radiologists frustrated in the end. “People die because they are morbidly obese and we can’t find a diagnosis,” Shuman says. “It’s not common, but it happens.”