Clinical meaning
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 estrogen use, no prior DVT/PE, no unilateral leg swelling, no recent surgery) can safely exclude PE in low-probability patients without D-dimer testing. If PERC criteria are not fully met, the Wells Score quantifies pre-test probability and guides D-dimer testing or CT pulmonary angiography. Thunderclap headache (maximal intensity within seconds to 1 minute) mandates emergent non-contrast CT head to evaluate for subarachnoid hemorrhage; CT sensitivity approaches 98% within 6 hours of onset but decreases thereafter, necessitating lumbar puncture for xanthochromia if CT is negative. New focal neurological deficits require emergent brain imaging to distinguish ischemic stroke (CT within 20 minutes of arrival for thrombolytic eligibility window) from hemorrhagic stroke, mass lesion, or abscess. Acute abdomen with peritoneal signs warrants CT abdomen/pelvis with IV contrast to identify perforation, obstruction, mesenteric ischemia, or appendicitis. The ACR Appropriateness Criteria and Choosing Wisely campaign provide evidence-based frameworks that match clinical scenarios to the most appropriate imaging modality, balancing diagnostic yield against radiation dose (ALARA principle: As Low As Reasonably Achievable), contrast nephrotoxicity risk, cost, and the potential for incidental findings that may trigger unnecessary downstream testing.