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Choosing the appropriate imaging test

Test selection rests on a defined clinical question, clinical decision rules and the ACR criteria — the right test is the one that answers the question at the lowest dose, and sometimes the answer is no imaging at all.

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Why it matters in the exam

The ACR criteria rate each test in a clinical scenario on a 1–9 scale: 7–9 — usually appropriate; 4–6 — may be appropriate; 1–3 — usually not appropriate; alongside the rating is a relative radiation level (RRL). Decision rules prevent unnecessary tests: Canadian CT Head and PECARN in head trauma, Canadian C-spine and NEXUS for the neck, Ottawa for the ankle and knee, Wells and D-dimer for pulmonary embolism and venous thrombosis. Headache without red flags, low back pain for less than 6 weeks without red flags, and simple syncope — no routine imaging.

Classic choices: right upper quadrant pain — US; right lower quadrant pain in an adult — CT, and in a child or in pregnancy — US; pelvic pain in a woman — transvaginal US; renal colic — low-dose non-contrast CT; macroscopic haematuria — CT urography; suspected deep vein thrombosis — compression US; suspected pulmonary embolism with non-low probability or a positive D-dimer — CTPA; sudden severe headache — non-contrast CT; suspected stroke — CT and CTA; a finding that changes only the report and not management — does not justify a test.

Incidental findings and screening: thyroid nodule by TI-RADS — aspiration (FNA) for TR3 from 2.5 cm, TR4 from 1.5 cm and TR5 from 1 cm; a nodule that is solid, hypoechoic, taller than wide, with irregular margins or microcalcifications — suspicious. A typically benign nodule — cystic or spongiform. Low-dose CT for lung cancer screening (USPSTF 2021): age 50–80, 20 pack-years or more, and a current smoker or one who quit within the last 15 years. Incidental adrenal findings from 1 cm — work-up by HU and washout. Lung nodule — Fleischner. In children — Image Gently; in adults — Image Wisely: every test needs justification, and a protocol adapted to the patient.

Key points

• ACR: 7–9 appropriate, 4–6 may be, 1–3 not.
• Decision rules before ordering a test.
• Right upper quadrant — US; right lower in an adult — CT.
• TI-RADS: TR5 from 1 cm, TR4 from 1.5, TR3 from 2.5.
• Lung screening: 50–80, 20 pack-years, low-dose CT.

Summary table

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Sources

• ACR Appropriateness Criteria: Methodology and Rating Scale, 2023 — Appropriateness categories and relative radiation level
• USPSTF Recommendation: Screening for Lung Cancer, 2021 — Low-dose CT eligibility
• ACR Thyroid Imaging Reporting and Data System (TI-RADS) White Paper, 2017 — FNA and follow-up thresholds

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.