Hunting a clot: CTPA vs V/Q
When a pulmonary embolism (a clot lodged in the lung's arteries) is suspected, two scans compete. CT pulmonary angiography (CTPA) is a CT timed to the moment contrast fills the pulmonary arteries, so a clot shows up directly as a dark filling defect inside a bright vessel. It is fast, widely available, and also reveals alternative causes of breathlessness. A ventilation–perfusion (V/Q) scan instead uses small amounts of inhaled and injected radiotracer to map where air goes and where blood flows; a clot creates a region that is ventilated but not perfused — a classic V/Q mismatch.
Choosing between them depends on the patient. CTPA is usually the first choice, but it needs iodinated contrast and delivers more radiation to the chest. A V/Q scan avoids iodine and is gentler on the kidneys, making it attractive in pregnancy, kidney impairment, or contrast allergy — but it works best when the chest X-ray is clear, because other lung disease muddies the result.
Wells score (pulmonary embolism) — worked example
A 58-year-old with sudden breathlessness and a swollen, tender calf:
Clinical signs of DVT ................... +3.0
PE is the most likely diagnosis ........ +3.0
Heart rate > 100 /min .................. +1.5
Immobilization / surgery (last 4 wks) .. 0
Previous PE or DVT ..................... 0
Hemoptysis ............................. 0
Active cancer .......................... 0
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TOTAL ................................. 7.5
Two-tier reading:
> 4 -> PE LIKELY -> go straight to CTPA
<= 4 -> PE UNLIKELY -> check D-dimer first
Score 7.5 -> PE likely -> arrange CTPA (no D-dimer needed).PET-CT: imaging metabolism
PET-CT answers a different question: not what tissue looks like, but how busy it is. The patient receives a radioactive sugar (FDG); cells that consume glucose avidly — including many cancers and active inflammation — take it up and glow on the scan. Fused with a CT, the bright ‘hot spots’ are pinned to exact anatomy, marrying function and structure in one image.
Its biggest pulmonary role is in lung cancer: PET-CT helps grade the activity of a suspicious nodule and, vitally, scans the whole body for spread, guiding mediastinal staging and the search for distant disease before treatment is chosen. But it is not perfect — infection and inflammation can light up brightly (false positives), while slow-growing tumors and pure ground-glass lesions may stay quiet (false negatives). PET refines the picture; it rarely replaces tissue.