Creative solutions cope with imaging of obese patients

Conventional wisdom holds that CT is the modality of choice for imaging obese patients. Findings from a pair of studies conducted in Chicago and Boston, however, led researchers to stress the need for creative solutions in this group in two education exhibits at the RSNA meeting.

"CT is an attractive modality for imaging obese patients due to its versatility, availability, and speed. However, the protocols available for CT scanners do not address optimal imaging methods for massively obese patients, especially for routine (e.g., nonbariatric surgery) indications," said radiologists from the University of Chicago led by Dr. Michael Vannier.

Wide-bore CT scanners, some up to 85 cm, are now available. Some provide high-capacity tables for patients weighing up to 615 pounds (270 kg). ImPACT published a report on these scanners earlier in 2005, the authors said.

Maximum dose per section is required for very obese patients. This can be achieved by decreasing the rotation time and increasing the section collimation and mAs.

"Image noise can be reduced by reconstructing thicker sections (up to 1 cm) and by using soft convolution kernels. With multislice scanning, low pitch factors are required to enable high mAs settings," Vannier said. "Some authors have suggested that additional contrast is needed, but we found higher doses are not required, provided the image noise is as low as possible."

The group included its recommended protocol for body imaging (e.g., chest, abdomen, and pelvis):

• 140 kVp, manual technique (>400 mAs)

• 0.75 to 1 sec/rotation

• 32 x 1.25 mm (using 40- and 64-slice scanners)

• 16 x 1.5 mm (16-slice systems)

• axial, coronal, sagittal series with 5-mm slice thickess at 4-mm intervals

• 50-cm FOV, 512 x 512 matrix

In another education exhibit, Dr. Raul N. Uppot and colleagues from the department of radiology at Massachusetts General Hospital took a multimodality approach.

For fluoroscopy, the weight limit is 350 pounds and girth limit is 45 cm. The respective figures are 350 pounds and 60 cm for closed-bore MRI, 450 pounds and 70 cm for abdominal CT, and 500 pounds and 44.7 cm for vertical field/open MRI.

Maximizing equipment settings is also important. Positioning, attenuation, large surface area, field-of-view, and artifacts must all be addressed to optimize image quality.

A common problem in x-ray and angiography examinations is inadequate penetration of the x-ray beam due to attenuation and depth. This results in increased noise, decreased contrast, and motion artifacts. The area of examination may also exceed the size of the largest plate (14 x 17 inch). Greater penetration may be achieved by increasing the kVp setting of the chest x-ray from 95 to 100, the mAs setting from two to four, and film development speed from 400 to 800. Using a grid and multiple plates may help.

For ultrasound scans of obese patients, using the lowest frequency transducer (2 mHz) and pushing on the transducer can help, the authors said. For nuclear examinations, using the highest field gamma cameras, increasing the imaging time (and therefore counts), and putting patients on stretchers can yield better results.

Possible solutions in CT include changing from fixed to automatic mAs, decreasing gantry rotation speed from 0.5 to 1 sec, and checking images before cropping. For MR examinations, the authors suggest maximizing the FOV and cushioning the patient to avoid minor burns.