Tipping the Scales: Molecular Imaging & The Obese Patient

The obesity epidemic continues to climb and has topped 300 million people globally and growing. The ripple effects through the healthcare system reach far and wide, and include rising costs and increasing prevalence of cancer, heart disease and diabetes. Obesity also presents a hefty pain point for molecular imaging departments, impacting safety, efficiency and diagnostic quality.

Many nuclear medicine departments are aware of, but not necessarily prepared for, the challenges of imaging obese patients, according to Abdelhamid H. Elgazzar, MD, from the department of nuclear medicine at Kuwait University in Safat, Kuwait. The issues extend from radioisotope dosing, image acquisition and interpretation, which may be hindered by soft-tissue attenuation and inadequate coverage.

The challenges begin when the obese patient, defined as body mass index (BMI) of 30 or more, presents for an exam. Safety of both the patient and technologist may be compromised as the patient is transferred from the stretcher or wheelchair to the table. The patient’s weight may exceed table capacity, which may damage equipment. The University of Rochester Medical Center (URMC) in N.Y., found tables on its older SPECT cameras were damaged by the extra burden of obese patients, confirms Ronald G. Schwartz, MD, director of nuclear cardiology.

The second primary challenge relates to diagnostic quality, which is reduced for many types of nuclear medicine studies in obese patients. Consider:

Cadmium-zinc-telluride (CZT) SPECT myocardial perfusion imaging was nondiagnostic in 81 percent of patients with a BMI of 40 or more; CT attenuation correction reduced the rate of nondiagnostic studies to 55 percent (J Nucl Med 2012;53(9):1401-1406).

Technologist-reported problems in FDG-PET imaging of overweight and obese patients included difficult IV access (15 percent), difficult patient positioning (10 percent), patient motion (7 percent), incomplete study (1 percent) and potential higher radiation exposure to the technologist because of additional time spent near the patient (J Nucl Med Technol 2007;35(2):80-83).

Sentinel lymph node mapping was unsuccessful in 12.4 percent of patients with a BMI greater than 30 compared with 1.9 percent of those with a BMI less than 30 (Eur J Surg Oncol 2004;30(5):475-479).

“The objective is to minimize the impact of obesity on the outcome of the study and patient care, but in certain patients, it may be impossible to resolve the problems,” explains Elgazzar. Multiple strategies at various points in the imaging chain can chink away at the challenge.

However, many unanswered questions remain. Molecular imaging of obese patients is an area that needs additional research, continues Elgazzar, who started a year-long sabbatical in February to focus on pressing research issues, including the impact of obesity on bone scanning and iodine-131 whole-body scanning and appropriate PET tracers for obese patients. Upcoming research may help to determine if substituting 18F-Fluoride PET/CT for 99mTc-MDP bone scanning can improve image quality in obese patients.

As researchers aim to improve diagnostic quality of nuclear imaging studies among obese patients, vendors also are tackling the challenge. Many have increased table capacity, introduced open gantry cameras and widened the field of view on some systems.

Changing practice

Shopping for a new system with this population in mind can reap both patient care and practice dividends, according to Schwartz.

In October 2012, URMC replaced two sodium iodide SPECT cameras with one CZT system designed to better accommodate obese patients. “A CZT camera is much more expensive than  a sodium iodide system. The argument for CZT is it can serve as a cost-effective standard of care for rapid assessment in the emergency department and it can better meet the needs of underserved obese patients,” says Schwartz.

Since the camera deployment, URMC has imaged 161 obese patients with the camera, with only one to two nondiagnostic exams, a rate that outperforms the previous research on CZT SPECT imaging of obese patients. In addition, early data demonstrate the images correlate with angiographic data, says Schwartz.

He credits URMC’s success to multiple factors. The configuration resembles a dentist’s chair with a C-arm, which allows technologists to position the device around the patient. It features parallel-hole collimation, rather than pinhole collimation, providing a larger field of view. The department uses effective attenuation correction with the system, an older technique that changes the patient’s position during imaging to determine if an anterior wall defect improves when the patient moves from supine to upright.

In addition to improving image quality, the camera has increased efficiency in nuclear cardiology imaging. Sodium iodide camera studies lasted approximately 17 minutes while the CZT camera exams last nine minutes. Many obese patients can be imaged at a normal radioisotope dose, starting at 4-6 mCi for stress imaging. The dose may be lowered and exam time extended if the physician wants to minimize further radiation exposure. For example, URMC obtained “gorgeous” images of a 41-year-old morbidly obese women during a 36-minute, low-dose study, says Schwartz.

Other molecular imaging exams of obese patients may routinely require extended exam times. Researchers suggest a 265-pound adult needs a PET acquisition time 2.3 times longer than a 132-pound person to obtain the same signal-to-noise ratio (J Nucl Med 2005;46:1825–1834). Similarly, a four-to-five hour delay of bone scanning after injection can clear soft-tissue activity and improve target-to-non-target ratio, according to Elgazzar and colleagues. However, modifications to attempt to optimize imaging of obese patients can hamper workflow, particularly if a department is unaware or unprepared.

Nevertheless, the right system paired with informed workflow can tip the scales in the department’s favor. “The CZT camera is breathing new life into nuclear cardiology,” says Schwartz. The department has seen its SPECT referrals increase from multiple providers, including interventional cardiologists, following a local television news spot.

In addition to a modest community marketing effort, URMC has tweaked its patient intake process; administrative assistants direct all patients weighing more than 250 pounds to the CZT system.

The link

There are no hard-and-fast answers for the multiple challenges of imaging obese patients. However, one process is essential. “It’s crucial to take safety precautions and devise a strategy for preparation and interpretation of studies for this population,” says Elgazzar.

He and his colleagues at Kuwait University developed a patient work-up form that includes height and weight. With this information, nuclear medicine physicians can devise an appropriate strategy to improve the likelihood of a diagnostic quality exam. A patient may be directed to a specific camera, or a mobile camera may be used if the patient should be imaged on a stretcher or hospital bed. In other cases, the physician may order increased dosing or instruct the technologist to increase the length of time between injecting the tracer and starting the scan. Finally, additional staff may be called to assist with moving the patient.

Although obesity presents a burgeoning challenge to nuclear medicine departments, it can be contained through careful communication, preparation and smart infrastructure investments. Doing so not only better serves patients but also may improve the bottom line.