‘Limited By Body Habitus’


Vol. 19 •Issue 21 • Page 20
‘Limited By Body Habitus’

Thirty percent of U.S. adults are considered obese. America’s obesity epidemic is causing compromised image quality and stressing imaging systems

Obesity is more than a cosmetic problem. That’s a lesson many hospitals are learning as they invest in larger operating tables, wheelchairs, beds and other equipment to accommodate the 30 percent of American adults that the Centers for Disease Control and Prevention (CDC) counts as obese.

Imaging is also feeling the strain. In radiology, obese patients that do not fit in equipment have set off a ripple effect of negative consequences ranging from a growing backlog of patients to inconclusive diagnostic imaging exams.

Excess fat, compromising image quality, often requires further testing and increased hospitalization time. It also stresses imaging equipment due to increased power output and more rapid burnout of parts, such as X-ray tubes.

The obesity epidemic, furthermore, increasingly is hampering radiologists’ ability to diagnose and treat these medical conditions in patients using ultrasound, X-rays, CT, MRI and even mammography.

People are considered obese with a body mass index (BMI) of 30 or greater, weighing about 30 pounds more than their ideal weight. Those with a BMI of 40 or greater are considered extremely obese, defined as at least 100 pounds overweight.

Many serious medical conditions have been linked to obesity, including type 2 diabetes, heart disease, high blood pressure, stroke and various cancers.

Quantifying the problem

According to the CDC, obesity has grown dramatically in the last 20 years, a fact Raul N. Uppot, MD, a staff radiologist at Massachusetts General Hospital (MGH), has quantified for the imaging world.

To quantify how obesity affects diagnostic imaging quality, Dr. Uppot and fellow researchers assessed all radiology exams performed at MGH between 1989 and 2003.

The team looked through about 5 million radiology reports, searching for the words “limited by body habitus,” a term radiologists use for inconclusive tests due to obesity.

Obesity doubled the number of inconclusive diagnostic imaging exams over a 15-year period, the researchers concluded in a study featured in the August issue of Radiology.

While 0.10 percent of inconclusive exams were due to patient size in 1989, by 2003 the number had jumped to 0.19 percent, despite advances in imaging technology, Dr. Uppot said, which strongly correlates with the increase in obesity in Massachusetts from 9 percent in 1991 to 16 percent in 2001.

The researchers then looked at each modality in each year. By 2003, the modality that yielded the most difficulties in rendering a diagnosis was abdominal ultrasound (1.90 percent), followed by chest X-ray (0.18 percent), abdominal CT, abdominal X-ray, chest CT and MRI (all anatomic regions included).

“In daily practice you start to see a problem in imaging with weights as low as 250 to 300 pounds.” Dr. Uppot said. “That’s not an overwhelming weight and not everyone that weighs 250 pounds has problems. But as the weight increases, the problems get progressively worse.”

Too much fat makes it difficult, for example, to distinguish a benign fibroid tumor from ovarian cancer or to properly examine a fetus.

For exams that require radiation exposure, such as X-ray and CT, the power can be increased on standard machines in an attempt to acquire a higher-quality image. However, this leads to an undesirable increase in radiation dose.

Incomplete exams related to obesity also can lead to serious consequences, such as misdiagnosis or failure to assign a diagnosis at all.

Ultrasound and X-ray

Ultrasound is the modality most impacted by obesity because thick layers of fat are barriers to beam penetration, resulting in degraded image quality, said Levon Nazarian, MD, professor of radiology at Thomas Jefferson University in Philadelphia.

In ultrasound, the resolution of the image depends on the frequency, but higher frequencies do not penetrate the body as well, Dr. Nazarian said. The larger the body, the more the beam gets scattered.

“It’s harder to find what you’re looking for and harder to get a crisp picture,” he said.

It’s also harder on the sonographer.

John Dlugosz, RDMS, said sonographers want the probe to be as close as possible to the structure they’re trying to image. Excessive fat means they have to push deeper into the skin.

“You feel it in your shoulder, wrist and elbow,” Dlugosz said. “There is a cumulative effect if you’re constantly scanning obese patients. You tire more quickly.”

Dlugosz estimated that close to 50 percent of patients he scans each day are obese.

Harmonic imaging, a tool that lets ultrasound send in beams at higher frequencies, has been bandied about as a possible solution to imaging bariatric patients. To that end, Toshiba America Medical Systems added expanded differential tissue harmonic imaging to its Aplio XV premium package. The upgrade offers improved image resolution and allows users to scan heavier patients due to its better tissue penetration.

Dr. Nazarian, however, has doubts about how much harmonic imaging will help.

“Ultimately, it’s hard for me to envision how it can be optimized in larger patients,” he said. “The best solution I can see is to lose weight.”

A similar phenomenon is at work with X-ray where technologists can help assuage the problem by increasing the development speed following film acquisition and taking images in quadrants, as single images are often insufficient for large patients.

Siemens Medical Solutions has made it easier for bariatric patients to fit into the machine with the release of its AXIOM Luminos TF, a fluoroscopy system featuring a wide table and high weight limit.

The system can support up to 600 pounds in the locked horizontal position and 500 pounds with table movement.

CT and MRI

CT and MRI do well in terms of imaging obese people, as long as they fit into the equipment, Dr. Uppot said.

But many CT and MRI machines have weight limitations of the imaging table and the size of the gantry or bore opening. Standard CT tables can accommodate up to 450 pounds, and MRI machines can fit patients weighing up to 350 pounds.

“There is a danger of artifacts because of noise, beam hardening artifact, when the patient’s body exceeds the field of view, or near field artifact when the patient’s body touches the bore,” said Dr. Uppot, who wrote an article that will be published in the February issue of the American Journal of Roentgenology about what radiologists can do to improve image quality on current equipment.

“Each modality is affected differently and each has a different solution,” he said. “We want to show what radiologists can do to improve image quality.”

CT protocols must be tailored for obese patients by adjusting the imaging parameters. For CT, the mAs, kVp, total scan time and contrast volume can be adjusted to help image large patients.

Repositioning can also help avoid hardening artifacts that result when an obese patient touches the gantry or exceeds the field of view.

“The amount of radiation used on obese people is not tremendously higher, but because you have incremental increases, there is increased exposure,” he said.

Newer equipment has helped improve scan and reconstruction field of views. GE Healthcare, Philips Medical Systems, Siemens and Toshiba all offer large-bore CTs with gantry aperture diameters of 80 centimeters or more. Higher table weight limits of up to 650 pounds also have entered the market. Siemens has also introduced a dual-source CT with more energy to penetrate through the patient.

“Everyone is seeing that this is a problem that has to be addressed,” Dr. Uppot said. “As hospitals consider their routine imaging equipment upgrades, obesity and imaging equipment designed to accommodate larger patients must now be a factor they should consider when making the purchase.”

As a senior CT clinical education specialist for Siemens, James Swartz, BS, RT(R)(CT), said imaging obese patients is a topic that often arises.

With an 82-centimeter large gantry bore and 82-centimeter extended field of view, Siemens’ Somatom Sensation Open can accomodate patients weighing up to 615 pounds, but technologists still need to determine the correct mass values and techniques to get the best results.

“There are a lot of situations where the study is underpenetrated and the radiologist can’t read the image because there was too much scatter,” Swartz said. “You can’t differentiate between organs or tell if there is new disease process.”

In MR, where bariatric patients typically can’t fit in a high-field closed magnet, Siemens has responded with the industry’s first open-bore 1.5 tesla MRI system.

The Magnetom Espree offers multiple advantages over vertical field magnets, according to Siemens marketing manager Anne Sheehan. The Espree offers a 550-pound table weight limit, which is equal to or more than any vertical field open. The open bore diameter of 70 centimeters offers 10 centimeters more vertical room than vertical field opens, Sheehan said.

The 1.5 tesla magnet offers up to four times more signal-to-noise for obese pa-tients, which means better image quality and shorter exam times.

“Now obese patients can get high-end applications that are not done well or not at all on low-field, vertical magnets, like spectroscopy, cardiac, breast imaging and peripheral runoffs,” Sheehan said.

Dr. Uppot agreed that the 1.5 tesla magnet allows results in a higher quality image.

“A lot of institutions that have installed this machine have become de facto places to image obese patients,” he said.

There are benefits for technologists, too.

Lightweight coils allow easy positioning on large patients and the user interface allows virtual centering, if the patient is not quite positioned correctly.

Mammography

Mammography isn’t usually affected by obesity’s characteristic imaging complication of attenuation, but compressing and repositioning large, fatty breasts can be challenging.

A report by researchers at the University of Washington School of Medicine was published in the Archives of Internal Medicine in 2004. It found that obese women had more than a 20 percent increased risk of false-positive mammography results compared with underweight or normal weight women.

Overweight and obese women were also more likely to be recalled for additional tests after adjusting for variables like age and breast density, the study found.

Achieving a normal weight may improve screening mammography performance, the researchers concluded.

Conclusion

Health care providers generally agree that health care costs are reaching new levels, driven in part by hospitals and imaging departments’ need to invest in new technology and equipment to accommodate larger patients.

“This is something every radiologist has been facing for the last 10 years, but now it has become an issue that can be talked about because there is objective information,” Dr. Uppot said. “This is an important topic for radiologists who want to know what to do to help their patients.”

Sarah Long is a New England-based freelance writer. She can be reached at sarahglong@gmail.com.

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