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Textbook/Clinical imaging and interpretation/CT pulmonary angiography and pulmonary embolism
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CT pulmonary angiography and pulmonary embolism

Select and interpret CTPA within a probability-based pulmonary embolism pathway, recognise technical and clinical limits and act safely on positive, negative or indeterminate results.

29 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
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Shock with suspected PE needs parallel action

Hypotension, syncope, severe hypoxaemia, right-heart failure or cardiac arrest may represent high-risk pulmonary embolism, and transfer for imaging can be unsafe.

Action: Begin resuscitation, call senior emergency, critical-care and specialist help, assess whether immediate bedside cardiac or venous imaging will change treatment, and obtain CTPA only when the patient can undergo it safely without delaying life-saving therapy.

Open the sections you need. The overview is shown first.
01Purpose and principlesWhat the assessment is for and the core concepts behind it.

CT pulmonary angiography uses timed iodinated contrast to opacify the pulmonary arterial tree. Acute embolus appears as a central or eccentric intraluminal filling defect, complete vessel occlusion or a defect surrounded by contrast. Distribution from main arteries to segmental and subsegmental branches affects confidence and clinical context. The examination also assesses right ventricular size, septal contour, reflux and alternative thoracic diagnoses.

CTPA should follow probability assessment, not replace it. In NICE guidance, a two-level Wells score above 4 is PE likely and prompts immediate CTPA; a score of 4 or less enters D-dimer testing, with an age-adjusted threshold considered for people over 50. Interim anticoagulation is given when indicated testing is delayed, after contraindication and bleeding-risk consideration. Pregnancy requires a distinct shared imaging pathway.

A technically adequate negative CTPA substantially lowers probability in the matched population, but image quality and pre-test probability remain essential. Motion, inadequate contrast bolus, transient interruption of contrast and low cardiac output can obscure arteries or simulate defects. Very small isolated subsegmental findings deserve review of image quality, symptoms and DVT evidence because false positives and competing bleeding harms become proportionally important.

Key points

  • Use the two-level PE Wells score before imaging: a score above 4 is PE likely and leads to immediate CTPA in an eligible stable adult.
  • If CTPA cannot be performed immediately in a likely PE and bleeding risk permits, give interim therapeutic anticoagulation while definitive testing is arranged.
  • After an adequate negative CTPA or V/Q study, consider proximal leg ultrasound if DVT is suspected; otherwise stop interim therapeutic anticoagulation without stopping separately indicated anticoagulation, pursue alternatives and give recurrence safety-net advice. Treat a technically inadequate study as indeterminate.
  • When iodinated contrast allergy or severe renal impairment makes CTPA unsuitable, consider V/Q SPECT or V/Q planar imaging with the chest-radiograph pattern and local capability.
  • Read CTPA beyond a binary clot answer: describe central and peripheral distribution, right-heart effects, pulmonary infarction and relevant alternative diagnoses.
  • Emergency contrast-enhanced CT should not be delayed solely awaiting renal blood results when delayed diagnosis threatens life or limb; a senior requester owns that risk decision.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Acute filling defect

A true embolus is visible on contiguous images within an opacified artery and should be confirmed in more than one plane, avoiding motion and mixing artefact.

Central clot burden

Saddle or main pulmonary artery emboli may be dramatic but haemodynamic risk is determined by physiology and right-heart response, not location alone.

Peripheral embolus and infarction

Segmental or subsegmental defects may accompany pleural-based wedge opacity, small effusion or haemorrhage; alternative causes of peripheral opacity remain possible.

Right-heart strainRed flag

Right ventricular enlargement, septal flattening and contrast reflux support pressure overload but can be chronic or caused by other pulmonary vascular disease.

Chronic thromboembolic pattern

Eccentric wall-adherent defects, webs, bands, vessel narrowing and mosaic perfusion suggest chronic disease and require a dedicated specialist pathway.

Technical failureRed flag

Poor arterial enhancement, respiratory motion or streak artefact can make a scan indeterminate; the report must name the limitation and next action.

Red flags requiring action

  • Haemodynamic instability, obstructive shock, cardiac arrest or rapidly worsening oxygenation with suspected PE requires a high-risk pathway rather than routine outpatient imaging.
  • A technically inadequate CTPA cannot exclude embolism; poor contrast timing, motion, low cardiac output and streak artefact must be stated and the diagnostic pathway continued.
  • An adequate negative CTPA or V/Q study enters the NICE negative-imaging branch; a technically inadequate study must be labelled indeterminate and cannot be used to exclude PE.
  • Contrast allergy, pregnancy and severe renal impairment alter modality choice and preparation; they do not justify abandoning investigation of a consequential PE.
  • Right-heart strain on CT can identify adverse risk but is not specific to acute PE and should be integrated with physiology, biomarkers and echocardiography.
03Method and interpretationA systematic approach to the test and its findings.
Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

  1. 01
    CT pulmonary angiographyFirst step
    Why
    Confirm or exclude pulmonary arterial emboli and identify right-heart effects or an alternative thoracic diagnosis.
    Interpretation and limitations
    Require adequate arterial opacification and motion control; interpret a negative or small peripheral result with pre-test probability and scan quality.
  2. 02
    Two-level PE Wells score and D-dimer
    Why
    Select adults for immediate imaging or biochemical rule-out within the NICE pathway.
    Interpretation and limitations
    A score above 4 is PE likely and leads to immediate CTPA; a score of 4 or less uses D-dimer, with age adjustment considered over 50.
  3. 03
    V/Q SPECT or V/Q planar scan
    Why
    Assess for mismatched perfusion when iodinated contrast allergy or severe renal impairment makes CTPA less suitable.
    Interpretation and limitations
    A normal scan can exclude clinically significant PE in the appropriate setting; abnormal chest radiography and underlying lung disease increase non-diagnostic patterns.
  4. 04
    Lower-limb proximal venous ultrasound
    Why
    Identify proximal DVT when chest imaging is unsuitable or when a peripheral CTPA finding needs corroboration.
    Interpretation and limitations
    A positive proximal DVT can establish venous thromboembolism treatment, but a negative limited scan does not exclude PE.
  5. 05
    Echocardiography
    Why
    Assess right-heart pressure effects and alternative causes during haemodynamic instability.
    Interpretation and limitations
    Echo supports risk assessment and emergency decisions but a normal study cannot exclude stable PE and right-heart strain is not PE-specific.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked case: PE likelyStable adult proceeds to CTPAFirst stepA haemodynamically stable adult has suspected PE and a two-level Wells score above 4.
  1. 1Check physiological stability, bleeding risk, renal function when available, prior contrast reaction, pregnancy possibility and whether CTPA can be performed immediately.
  2. 2Offer immediate CTPA; if it is delayed, give interim therapeutic anticoagulation when appropriate rather than leaving a likely PE untreated.
  3. 3AlternativeIf an adequate CTPA or V/Q study is negative, consider proximal leg ultrasound when DVT is suspected; otherwise stop interim therapeutic anticoagulation without stopping separately indicated long- or short-term anticoagulation, consider alternative diagnoses, explain that PE is not likely and give recurrence safety-net advice.
  4. 4DefinitiveIf the study is technically inadequate, label it indeterminate and arrange the next definitive test around the stated limitation; seek senior and radiology review when new evidence is genuinely discordant.
02Contrast-limited routeUse VQ imaging when suitableDefinitiveA stable patient needs definitive PE imaging but has severe iodinated-contrast allergy or severe renal impairment.
  1. 1Review chest radiography, pregnancy status, lung disease and local nuclear-medicine availability with radiology.
  2. 2AlternativeUse V/Q SPECT where available or V/Q planar imaging as the alternative identified by NICE and continue interim treatment when indicated.
  3. 3DefinitiveIf the study is non-diagnostic, return to clinical probability and discuss ultrasound, modified CTPA or another definitive strategy rather than declaring PE absent.
03High-risk PEInstability changes the sequenceSuspected PE presents with shock, severe hypoxaemia or cardiac arrest.
  1. 1Resuscitate, obtain immediate senior multidisciplinary help and consider bedside echocardiography and venous ultrasound if they change time-critical treatment.
  2. 2Do not transfer an unstable patient to CT without assessing whether transport and scan delay are safe.
  3. 3When stable enough, use CTPA to confirm anatomy and guide reperfusion or intervention while maintaining haemodynamic support.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
  • Monitor oxygenation, respiratory effort, heart rate, blood pressure and perfusion continuously in suspected high-risk PE.
  • Record contrast agent, dose, reaction and technical adequacy and follow renal function when the clinical risk profile requires it.
  • After a positive result, communicate right-heart strain or other high-risk features directly and link them to the responsible acute team.
  • After an adequate negative CTPA or V/Q study, consider proximal leg ultrasound if DVT is suspected; otherwise stop interim anticoagulation without stopping separately indicated anticoagulation, pursue alternative diagnoses and document recurrence safety-net advice. Continue the imaging pathway when the study is technically inadequate.
  • For chronic thromboembolic features or persistent breathlessness after PE, ensure specialist follow-up considers pulmonary hypertension and residual vascular obstruction.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

Probability gives a negative meaning

The same technically adequate negative scan is more reassuring after a matched pathway than after very high unaddressed clinical probability.

Bolus quality is diagnostic quality

Low cardiac output, deep inspiration and timing error can interrupt contrast in the pulmonary arteries and create an indeterminate examination.

Strain is prognostic context

CT right-heart signs support adverse-risk assessment but cannot by themselves identify shock cause or prove that pressure overload is acute.

Small clots deserve precision

For an isolated subsegmental defect, confirm contiguous-plane visibility and consider DVT, cardiopulmonary reserve and bleeding risk before assuming certainty.

Alternative findings need ownership

Pneumonia, malignancy, oedema and aortic disease discovered on CTPA can explain symptoms and may require urgent pathways even when no embolus is present.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Ordering CTPA without first using the recommended probability and D-dimer pathway in a stable adult.

  2. 02

    Calling an inadequately opacified motion-degraded study negative and stopping investigation.

  3. 03

    Using a normal chest radiograph or echocardiogram to exclude pulmonary embolism.

  4. 04

    Treating every tiny peripheral filling defect as certain without checking multiplanar persistence and artefact.

  5. 05

    Delaying life-or-limb emergency contrast imaging solely for routine renal results despite senior risk assessment.

Practice

Two practice questions

Question 1 of 20 correct
Clinical imaging and interpretationOriginal SBA

Imaging with likely pulmonary embolism

A haemodynamically stable adult has suspected pulmonary embolism and a two-level PE Wells score above 4. There is no contrast allergy or severe renal impairment. What is the best imaging step?

Sources and review status4 sources · checked 13 Sept 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom