When Is Imaging Mandatory?

Mandatory imaging refers to clinical scenarios in which diagnostic imaging is considered the standard of care because failure to image carries an unacceptable risk of missing a time-sensitive or life-threatening diagnosis. The decision to image is guided by validated clinical decision rules (CDRs) that stratify patient risk and identify those who require imaging versus those who can be safely observed. These CDRs were developed through large prospective studies to standardize care, reduce missed diagnoses, and avoid unnecessary radiation exposure. The Canadian C-Spine Rule and NEXUS criteria guide cervical spine imaging after trauma: the Canadian rule evaluates three sequential questions (any high-risk factor such as age >65 or dangerous mechanism, any low-risk factor permitting range-of-motion assessment, and ability to actively rotate the neck 45 degrees bilaterally). Patients who fail any step require imaging. The Ottawa Ankle and Knee Rules apply similar logic to extremity trauma, identifying specific bony tenderness patterns and functional criteria (ability to bear weight for four steps) that mandate radiography. For pulmonary embolism, the PERC Rule (8 criteria including age <50, HR <100, SpO2 >94%, no hemoptysis, no...