Excerpt: Health insurers are increasingly relying on outside firms to help rein in the skyrocketing costs of imaging scans like MRIs. But when these middlemen clash with doctors about what tests are needed, consumers can get caught in the crossfire. Big insurers including Aetna Inc., WellPoint Inc. and Cigna Corp. have hired so-called radiology benefits managers, or RBMs. Health plans say they want to ensure that doctors use high-tech scans only when it is clear that patients will benefit.

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