Obese emergency patients may benefit from ultrasound

Despite challenges, ultrasound remains useful in obese trauma patients. Using the lowest frequency ultrasound probe along with tissue harmonic imaging improves image quality, according to an education exhibit presented at RSNA 2009.

"Ultrasound is often worthwhile attempting, despite individual variations in the distribution and sound attenuation of fat," said lead author Dr. Michael Modica, a radiologist at the University of Washington in Seattle. "The ultrasound machine should be set to ‘penetrate' mode. Position the patient to move the fat away from the sound beam. The aorta and kidney can be scanned with a coronal or posterior oblique approach in some obese patients."

In ultrasound, both the thickness of subcutaneous fat and the sound-attenuating properties of fat present difficulties. Sound attenuation increases exponentially with fat thickness, Modica said. Furthermore, ultrasound attenuation is frequency-dependent; the greater the frequency, the higher the sound attenuation, he said. For instance, a sound wave emitted from a 7-MHz transducer is attenuated by 50% after travelling through 1 cm of fat. When imaging an obese patient with 8 cm of subcutaneous fat, 94% of the original sound wave is attenuated before it reaches the peritoneal cavity.

Lowering the frequency of the transducer decreases attenuation, allowing more of the primary beam to penetrate the subcutaneous fat and reach peritoneal organs and deep vascular structures. When tissue harmonic imaging is used, harmonic beams are produced beyond the body wall, which reduces the defocusing effect of the body wall, produces better penetration in obese patients, and improves lesion detection, Modica said.

For radiography, he recommends using smart collimation and antiscatter grids and tightly coning the region of interest.

In CT, position the patient so the region of interest is in the separation of variables (SOV). Bundle the patient to increase his or her ‘roundness,' and if possible, use a wider, extrapolated reconstruction field-of-view and algorithms for truncation artifact reduction-and consider iterative reconstruction, Modica said. To avoid severe photon starvation, consider the generator and tube capacity in all future CT purchases, and optimize CT protocols for obese patients by increasing kVp , mAs, and slice thickness, and reduce pitch to improve the signal-to-noise ratio, Modica advised. As well as using specific protocols for children and small patients, design specific scanning protocols for obese patients.

Improvements in image quality often come by compromising on radiation dose, particularly in the obese patient, and preliminary studies imply significant increase in CT dose when comparing thin, overweight, and obese patients, he said.