An enormous percentage of Americans are becoming overweight, obese, or morbidly obese . Obese population directly impacts the health care system with an increased incidence of diabetes and heart disease. The impact of obesity is also indirectly felt by radiology departments . Routinely used imaging modalities, for example, have limited ability to accommodate patients with very large body habitus. Loss of quality of images acquired from overweight patients is another problem faced by radiologists during clinical interpretations .
In this issue of Academic Radiology , Larson et al. reported the impact of body mass index (BMI, the weight in kilograms divided by the square of the height in meters) on the detection of pleural thickening in subjects who had prior asbestos exposure.
Asbestos is a major cause of both benign and malignant pleural disorders . Although the role of asbestos as a health hazard has long been recognized , its use is not restricted in many countries . The identification of pleural thickening, either localized or diffuse, is important because it is a biomarker of prior significant asbestos exposure, which itself is a major risk factor for mesothelioma, an aggressive malignancy that has 5-year survival rates as low as 1% .
The detection of pleural thickening is currently made on chest radiographs. However, Larson et al. have shown that this approach has limitations in subjects with high BMI. Studying patients who were imaged by radiographs and high-resolution computed tomography (median interval 30 days between imaging tests), the authors showed that elevated BMI (and the associated subpleural fat deposition) can actually result in a false-positive diagnosis of pleural thickening and therefore prior asbestos exposure. This is important because obesity is a rising epidemic worldwide and because a false-positive diagnosis of prior asbestos exposure could have critical implications for patients in terms of their future clinical management, medico-legally, and psychologically.
Overall, Larson et al.’s study points toward the frequent issue in medicine of balancing the use of two-dimensional imaging techniques, which are inexpensive but may lack sensitivity/specificity versus the use of three-dimensional imaging modalities, which are expensive, but may be more sensitive/specific for disease detection. For example, in the musculoskeletal imaging realm, the diagnosis of osteoporosis is currently made form two-dimensional bone densitometry, which is inexpensive, but has low sensitivity for the detection of patients with low bone strength at risk for fracture . As a consequence, resources are being invested into the development of advanced bone strength assessment techniques to improve the detection of bone fragility and assessment of fracture risk .
The role of BMI in radiology could very well be a more general issue not necessarily limited to radiographs. Large body habitus can degrade image quality in all medical imaging modalities, sometimes making it difficult to obtain accurate clinical interpretations. In ultrasound imaging, a thick layer of fat can significantly attenuate the beam intensity, especially at high frequencies used for abdominal imaging. In computed tomography imaging, X-ray attenuation can affect the image quality, albeit to a lesser degree compared to plain film radiography. The attenuation can be somewhat compensated by increasing the tube current and decreasing the gantry speed, but with the expense of increased radiation dose. In positron emission tomography and single photo emission computed tomography, image quality is affected because of increased scatter and photon attenuation in subjects with large body mass, especially when using lower energy isotopes. In magnetic resonance imaging, the signal-to-noise ratio and spatial resolution get adversely affected (albeit to a lesser degree compared to other modalities) by the large field of view required to avoid aliasing artifacts when imaging patients with large body size.
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