Which medical imaging procedure contributes most to the US population's cumulative radiation exposure?

Myocardial perfusion imaging.

Below is a table from the New England Journal of Medicine which lists the 20 medical imaging procedures that contributed most to a study population's (5 large US health care markets) cumulative radiation dose.

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Source

Fazel, R. et al. "Exposure to Low-Dose Ionizing Radiation from Medical Imaging Procedures" NEJM. 27 Aug 2009.