Radiology’s Big Challenge: Imaging the “Morbidly Obese”
Acquiring an accurate CT, MR, or sonographic image of a 200-pound patient is usually a straightforward procedure. But what happens when a patient tips the scales at 400, 600, or even 800 pounds?
A study emerging from Harvard Medical School sketches a troubling picture, suggesting that as morbid obesity increases, so,
too, do the number of studies that radiologists have trouble interpreting due to poor image quality.
In doing their research, the Harvard researchers zeroed in on clinical cases in which radiologists had noted a procedural limitation due to a patient’s “body habitus,” or excessive weight or girth. Even when the smaller of these morbidly obese patients were able to squeeze through the CT gantries or MR bores, the resulting images were often subpar, sometimes strikingly so.
Study principal and ACR member Raul N. Uppot, M.D. — assistant radiologist at Massachusetts General Hospital and instructor of radiology at Harvard Medical School — says the implications should give all radiologists pause, considering the skyrocketing numbers of the “morbidly obese” in America today. When the “growing prevalence of obesity impacts a radiologist’s ability to acquire images and to read and interpret radiographs,” it is a problem for all of radiology, not just for the morbidly obese, Uppot declares.
The Harvard team traces this emerging crisis in health care to two key drivers. The first is demographics. According to a 2007 RAND Corp. study, the morbidly obese are this nation’s fastest-growing group of overweight individuals. A recent government study, meanwhile, estimates that fully 5 percent of all adults in the United States today — totaling some 9 million individuals — fit the category of morbidly obese, defined as 50 to 100 percent (or 100 pounds) above a person’s ideal body weight.
A second driver is the recent surge in bariatric procedures, from 13,000 in 1998 to about 200,000 in 2006. If the good news is that greater numbers of morbidly obese individuals are seeking surgical options to potentially life-threatening conditions, the bad news is that many are being stopped in their tracks by a bottleneck within radiology departments. In the world of radiology, size matters.
Uppot notes that bypass patients need both pre-op ultrasound and post-op barium studies to confirm the success of their surgery. Similarly, in the event of a potential complication, slimmer individuals can be given a CT scan, whereas their morbidly obese counterparts greater than 450 pounds — the typical weight capacity of CT units — require other clinical options, such as close serial clinical evaluation or surgical exploration. Physicians with patients too large for conventional CT or MR scanners effectively “take us back to the years before CT scans existed, when the only way to diagnose someone was to look surgically,” he says.
Some modalities perform better than others for testing the morbidly obese. The list below describes the functioning of each modality in evaluating these types of patients.
CT Units: Manufacturers have taken the cue and introduced a new generation of oversized CT, MR, and ultrasound units. These units open a whole new diagnostic realm to the morbidly obese, but Uppot’s impression is they remain few and far between.
Whereas traditional CT scanners have a listed gantry diameter of 70 cm. (27.6 inches) and a table capacity of 450 pounds, the new generation boasts gantry diameters of up to 90 cm. (35.3 inches) and table capacities of up to 680 pounds. In both old and new models, the Harvard researchers note a key caveat: Up to 18 cm. (7 inches) of a CT unit’s listed gantry diameter is taken up by the table itself, resulting in less-than-advertised capacity.
Among the Harvard team’s key findings is this: As long as the morbidly obese patient can fit into a CT scanner, “the image quality is diagnostically acceptable with details of small structures visible even in the most obese patients,” Uppot reports. But, the group did observe increased noise, image cropping, and limited field of view, the latter “result[ing] in beam-hardening artifact in areas where the patient’s body exceeds the field of view.”
MR Scanners: MR scanners are even more confining than traditional CT units. Traditional MR units have a comparatively tight bore diameter of 60 cm. (23.6 inches) and a table capacity limit of just 350 pounds — the weight of an NFL lineman. By contrast, today’s larger models offer up to a 70-cm. bore and a beefier 550-pound capacity. Vertical field open MR scanners solve some issues of accessibility, Uppot notes, but are typically limited by modest field strengths of .3 to 1.0 Tesla.
Radiographs: Uppot also notes that radiographs of the morbidly obese typically result in X-ray beam attenuation, lower image contrast, longer exposure times, and motion artifacts. A few of the Harvard team’s recommendations are to boost the settings from a standard 90-95 kVp to 100 kVp and from 2-2.5 mAs to 4 mAs, use 800 speed film rather than 400 speed, and make proper adjustments to the window and level settings.
Ultrasound: Sonography is the modality most affected by obesity, especially as the patient nears 250 pounds, Uppot’s team found. Fat distribution is a key variable. Excessive subcutaneous fat typically yields poor image quality due to the attenuation of ultrasound beams as they try to reach intraperitoneal structures. This is in contrast to individuals with predominantly intraperitoneal fat, whose thin subcutaneous layer allows for easy penetration of ultrasound beams, Uppot notes.
Fluoroscopy: Fluoroscopy tends to be the diagnostic tool of choice following laparoscopic gastric bypass, but for patients exceeding a fluoroscopy unit’s tight aperture diameter of 45 cm. (18 inches) and table capacity of 350 pounds, Uppot’s team recommends using serial abdominal radiographs or CT, the latter having a table capacity of 100 additional pounds among the traditional units. Other recommendations include using the lowest available frequency transducer (2 MHz), correctly positioning the transducer, and “examining the patient’s previous imaging (CT or MRI) to determine the thickness of subcutaneous fat.”
Design Changes: Given the sheer number of morbidly obese Americans, Uppot believes that manufacturers should push harder on the design front. Although much of this is driven by industry sales forecasts, he believes the engineering know-how exists. “We have devices in radiation oncology with larger gantry diameters, and those patients are intubated, have all this equipment attached, and still fit into these larger gantries,” he says. “It's just that, that technology hasn’t been adapted yet for rapid diagnostic scanning, but I’m confident it will be.”
Cost Issues: The new generation of upsized scanners are costly but can scan patients of all sizes, Uppot notes. Some institutions obtain these devices and become the go-to referral center for obese individuals in an entire state or region, thus tapping a new market while offering invaluable services.
Emotional Impact: Institutions have more than logistical issues to consider. Uppot notes that morbidly obese patients often experience acute emotional distress when told they are too large to receive standard diagnostic testing. Recalling situations in which morbidly obese individuals visited the ER for abdominal pain, he says, “The patient is worried; the family is worried. The patient expects to get a CAT scan or diagnostic scan but can’t fit inside the machine. It's psychologically harming to patients because they suddenly realize, ‘Oh my, what’s going to happen to me?’”