Suspected acute pulmonary embolism

ACR AC Suspected Pulmonary Embolism (2022 update); ATS/STR 2011; ESC 2019

Two studies answer this question — CT pulmonary angiography and perfusion scintigraphy — and both are rated appropriate. Which one is right depends on the chest radiograph, on renal function and on whether the patient is pregnant. This is the card where withholding imaging does the most harm.

Breathlessness, pleuritic pain, haemoptysis, tachycardia or unexplained hypoxia in a patient whose clinical probability score and D-dimer have already made imaging appropriate.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Haemodynamic state

The fact that lets a pathway waive its own requirements. A crashing patient does not wait for a score.

Pre-test probability

Which validated rule the referrer applied, and what it gave. Each of these is also a calculator here.

D-dimer

Whether it was done at all is as informative as the value.

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CTPA — Standard Bolus-Tracked

CT Pulmonary Angiogram (CTPA)

What we'd amend, and why

The bolus is tracked on the pulmonary trunk rather than the aorta because the pulmonary arteries are the first vascular bed the contrast reaches after the right heart — trigger on the aorta and the pulmonary arteries are already washing out by the time the table moves. High flow rate through a large cannula keeps the bolus tight, and the saline chaser both pushes the tail of the bolus through and clears dense contrast out of the superior vena cava, which is what otherwise streaks across the right pulmonary artery. A lung kernel series is not optional: an alternative diagnosis is found in a large minority of studies that are negative for embolus, and that is much of the value of the examination.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
Pregnancy here selects a pathway, not a refusal. Conceptus dose from either CTPA or perfusion scintigraphy is far below any deterministic threshold; the live radiation question is maternal breast dose, which is what tips a normal-radiograph patient towards scintigraphy. Untreated pulmonary embolism remains a leading cause of maternal death.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Worth asking the referrer (2)

None of these hold the request up. They sharpen the protocol or the plan that follows.

Is the patient pregnant, recently postpartum, or could she be?
It changes the sequence of studies and the CT protocol, not merely the consent conversation.
Are there renal risk factors, and is there a recent eGFR?
Unlike most contrast questions, an answer of "yes" here has a real alternative that fully answers the question rather than a degraded version of the same scan.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Haemodynamically unstable, suspected high-risk PE

Role	Study & protocol	Why this answers the question
First line	Transthoracic echocardiography Transthoracic echocardiography — standard adult study usually appropriate	This is the one presentation where the scanner is not the first move. A patient too unstable to travel gets bedside echocardiography, and right ventricular pressure overload in that context is enough to justify reperfusion without waiting for CT. Sending a peri-arrest patient to the scanner to satisfy a pathway is how they arrest in the scanner. Insisting on a pre-test probability score first is a category error: those scores were derived in stable patients and the answer would not change management here.
Second line	CT Pulmonary Angiogram (CTPA) CTPA — Standard Bolus-Tracked usually appropriate	Confirmation follows stabilisation rather than preceding it, and it still answers the anatomical question — clot burden, and whatever else the chest holds.

Low clinical probability with a negative D-dimer

Role	Study & protocol	Why this answers the question
First line	CT Pulmonary Angiogram (CTPA) CTPA — Standard Bolus-Tracked usually not appropriate	The rule-out pathway has already answered the question. A low clinical probability with a D-dimer below the threshold carries a negative predictive value high enough to stop, and scanning anyway buys a radiation dose, a contrast load and a meaningful chance of finding something incidental that generates its own follow-up. The right answer to this request is that it has already been answered. Four cautions, and each of them is a way this verdict can be wrong. A negative D-dimer is NOT reassuring at high clinical probability — the combination that excludes is a low probability score and a negative D-dimer together. An age-adjusted threshold has to be declared as such rather than read against the conventional cut-off. The score has to be a validated pulmonary embolism rule, because a low band from a rule built for a different disease is not a pre-test probability for this one. And none of it transfers to pregnancy or the puerperium: D-dimer rises physiologically, Wells and Geneva were not derived in pregnant women, and the pregnancy-adapted algorithms are separate instruments — so a pregnant patient never reaches this arm and is answered on the pregnancy pathway instead.

Pregnant or recently postpartum

Role	Study & protocol	Why this answers the question
First line	Chest radiograph Chest radiograph — PA (± lateral) usually appropriate	The radiograph is not the diagnostic test — it is the switch that selects one. Perfusion scintigraphy is interpreted as a comparison between perfusion defects and the parenchyma; if the parenchyma is abnormal, a perfusion defect cannot be attributed to embolus and the study is non-diagnostic. So a normal radiograph makes scintigraphy usable and its low breast dose worth having, while an abnormal radiograph makes scintigraphy uninterpretable and selects CTPA, which will also characterise whatever the radiograph showed. It takes minutes, the fetal dose is negligible, and it occasionally supplies the alternative diagnosis outright.
Reasonable alternative	Ventilation/perfusion scintigraphy V/Q or perfusion-only scintigraphy usually appropriate	With a normal radiograph, perfusion imaging alone answers the question at a fraction of the maternal breast dose of a CT acquisition, and the ventilation component can be withheld unless the perfusion study is abnormal. The physiological argument for it is strongest in exactly this group: young, glandular breast tissue that is proliferating under pregnancy hormones is the tissue most sensitive to the radiation CT delivers, and the conceptus dose is trivially small from either test.
Reasonable alternative	CT Pulmonary Angiogram (CTPA) CTPA — Reduced Dose, Pregnancy-Adapted usually appropriate	The definitive study when the radiograph is abnormal, when scintigraphy is unavailable or non-diagnostic, or when an alternative thoracic diagnosis is genuinely in play. Cardiac output rises substantially in pregnancy, so the bolus arrives early and clears fast — the pregnancy-adapted protocol exists because standard timing produces a mistimed study in this group, not because the dose alone needed cutting. A pregnant patient who needs a CTPA should have one; the documented harm in this population is under-imaging, not over-imaging.

Nothing in this pathway withholds imaging. The radiograph is taken alongside the definitive study on the same attendance, not instead of it, and it exists only to decide which definitive study is interpretable.
Sources genuinely disagree on the default. ATS/STR recommends scintigraphy first where the radiograph is normal; ACR rates CTPA and V/Q equally appropriate and uses the radiograph to choose; the ESC notes that planar V/Q is inconclusive in a substantial proportion and that CTPA is more available and offers alternative diagnoses. Local availability of a same-day perfusion service usually decides it.
Where a leg is symptomatic, proximal venous ultrasound first can establish venous thromboembolism and end the question with no radiation at all.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Pulmonary Angiogram (CTPA) CTPA — Standard Bolus-Tracked usually appropriate	The bolus is tracked on the pulmonary trunk rather than the aorta because the pulmonary arteries are the first vascular bed the contrast reaches after the right heart — trigger on the aorta and the pulmonary arteries are already washing out by the time the table moves. High flow rate through a large cannula keeps the bolus tight, and the saline chaser both pushes the tail of the bolus through and clears dense contrast out of the superior vena cava, which is what otherwise streaks across the right pulmonary artery. A lung kernel series is not optional: an alternative diagnosis is found in a large minority of studies that are negative for embolus, and that is much of the value of the examination.
Reasonable alternative	Ventilation/perfusion scintigraphy V/Q or perfusion-only scintigraphy	A genuine substitute rather than a consolation prize: scintigraphy answers the same clinical question by a different physiological route — regional perfusion rather than intraluminal filling defect — and needs no iodinated contrast at all. That makes it the correct study, not a compromise, where contrast must be avoided for renal or allergic reasons. Its weakness is parenchymal disease, which is why the chest radiograph governs whether it can be interpreted.
Problem solving	Ultrasound venous Doppler (limb) Venous duplex ultrasound for deep vein thrombosis	Where a leg is clinically involved, finding proximal deep vein thrombosis establishes venous thromboembolism and commits the patient to the same anticoagulation, so the chest study adds nothing to the immediate decision. Useful when the patient is too unstable to move to CT, or when contrast is contraindicated and scintigraphy is unavailable.

Pitfalls

Accepting "CT chest with contrast" for suspected PE. That protocol is timed for the systemic venous return at around 60 seconds; by then the pulmonary arteries have washed out and subsegmental clot is invisible. The study looks normal and reads as reassurance.
Vetting a request where the rule-out pathway was never applied — imaging a low-probability patient with a negative D-dimer generates incidental findings and anticoagulation decisions without answering anything.
Treating a technically poor CTPA as negative. Opacification below roughly 200 HU in the pulmonary trunk, or transient interruption of contrast from a deep inspiration, produces a study that cannot exclude segmental disease and should be reported as non-diagnostic.
Choosing scintigraphy without looking at the chest radiograph — parenchymal disease makes perfusion defects uninterpretable and wastes both the appointment and the dose.
Cancelling rather than switching when renal function is flagged. For this question there is a radiation-based alternative that is fully diagnostic, which is not true of most contrast indications.
Applying the ordinary score-plus-D-dimer rule-out to a pregnant or recently postpartum woman. D-dimer rises through normal gestation and does not settle for weeks after delivery, and neither Wells nor Geneva was derived or validated in pregnancy; the algorithms that were — the pregnancy-adapted YEARS and Geneva pathways — are different rules with different criteria. Pulmonary embolism remains a leading direct cause of maternal death and the documented harm in this group is under-imaging, so a scored-out pregnant patient still needs the pregnancy pathway rather than a discharge.
Reading a negative CT pulmonary angiogram as closing the question in a patient with a high clinical probability. Discordance between a high score and a negative scan should prompt a look at the technical quality of the study and at the legs, and a conversation — not a discharge letter.
Reporting the clot and not the right ventricle. Right ventricular dilatation, judged as a right-to-left ventricular diameter ratio above about one on the axial images, together with septal bowing and reflux of contrast into the hepatic veins, is what separates a low-risk embolus from one that needs monitoring or reperfusion. It is available on every CT pulmonary angiogram at no extra dose, and omitting it hands the clinician half the study.

Priors — what to pull first

Look for a CTPA in the preceding days before repeating one — a request generated by a rising D-dimer on a patient already anticoagulated for a known embolus rarely changes management.
A previous CTPA is the comparator that distinguishes residual chronic thrombus from new acute embolus, and their appearances differ: acute clot is central and expands the vessel, chronic clot is eccentric, adherent and often calcified or webbed.

What makes a good request

A good request states the pre-test probability score used, the D-dimer result (and whether an age-adjusted or pregnancy-adapted threshold was applied), the renal risk factors, and whether the patient is pregnant or within six weeks postpartum.
Imaging is the wrong answer when the rule-out pathway has not been completed: a low probability score with a negative D-dimer, or a negative PERC assessment, ends the question without a scan.
The reverse also holds. In a haemodynamically unstable patient with suspected high-risk PE, the pathway compresses — bedside echocardiography showing right ventricular strain may be enough to treat, and confirmatory CT follows stabilisation.

Scoring this once it is done

The classification and risk tools this question ends in.

Wells Score for Pulmonary Embolism Revised Geneva Score for Pulmonary Embolism PERC Rule (Pulmonary Embolism Rule-out Criteria)Age-Adjusted D-dimer (ADJUST-PE)PESI / sPESI (Pulmonary Embolism Severity Index)Qanadli CT Pulmonary Obstruction Index RV/LV Diameter Ratio on CTPA eGFR (CKD-EPI 2021 Creatinine)

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Transthoracic echocardiography — standard adult study CTPA — Standard Bolus-Tracked Chest radiograph — PA (± lateral)V/Q or perfusion-only scintigraphy CTPA — Reduced Dose, Pregnancy-Adapted Venous duplex ultrasound for deep vein thrombosis

Confirm locally

CTPA — Standard Bolus-Tracked: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.