CTPA — Standard Bolus-Tracked
Lung apices to below the costophrenic angles, covering the whole pulmonary arterial tree to segmental and subsegmental level.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Suspected acute pulmonary embolism with a positive D-dimer or a clinical probability that makes imaging appropriate.
- Suspected PE in a patient in whom D-dimer is uninformative.
- Assessment of clot burden and right heart strain when it will change management.
- Suspected chronic thromboembolic disease, as a first-line anatomical study alongside functional imaging.
Technique
- Bolus tracking with the monitoring region of interest in the main pulmonary trunk is the usual approach. Reported triggers vary: commonly around 100 HU above baseline, with some protocols using a rise of about 60 HU above baseline blood pool, followed by a post-trigger diagnostic delay of roughly 3-5 s. That delay is deliberately short — the monitoring vessel IS the target vessel, so anything longer starts to sample the systemic arterial phase.
- A test bolus is an accepted alternative; published comparisons of bolus tracking against test bolus reach different conclusions about which yields fewer suboptimal studies, so local practice governs. A test bolus of about 15-20 mL at the diagnostic rate, monitored at the pulmonary trunk every 1-2 s, gives the individual peak, and the diagnostic scan starts at that peak.
- Where neither is available, a blind fixed delay of around 15-20 s from the start of injection is the published fallback. It is materially less reliable, and least reliable in exactly the patients most likely to have a pulmonary embolus — the breathless, the tachycardic and those with right heart strain, in whom transit time is abnormal.
- Scanner generation changes these numbers: a wide-detector or high-pitch system acquires the whole chest in 1-2 s and needs a tighter, shorter-delay bolus than a platform taking 8-10 s over the same coverage.
- Adequate opacification is conventionally taken as attenuation above roughly 200 HU in the pulmonary trunk.
Where it goes wrong
- Transient interruption of contrast: a deep inspiration or Valsalva draws unopacified blood from the inferior vena cava into the right heart and dilutes the pulmonary arterial bolus. This is a breathing-instruction failure, not a scanner failure.
- A small or distal cannula that cannot take 4 mL/s is the single commonest cause of a repeat study.
- Triggering late produces an aortic-phase study in which the pulmonary arteries are already washing out.
- Respiratory motion blurs exactly the segmental and subsegmental vessels where equivocal filling defects are called.
- Requesting a routine contrast-enhanced chest CT when the question is PE gives a study timed for the mediastinum and not for the pulmonary arteries.
Clinical questions that reach this study
Contrast
Typically 50-100 mL of non-ionic iodinated contrast at 4 mL/s or more through a cannula of at least 20G, followed by a saline chaser.
- The saline chaser both preserves the bolus and reduces streak from dense contrast in the superior vena cava.
Acquisition
- Breathing
- A shallow inspiratory breath-hold, or quiet respiration in the breathless patient. A deep breath with Valsalva is actively discouraged.
- Reconstruction
- Thin axial reconstructions with coronal and sagittal reformats; a lung kernel series is standard because an alternative diagnosis is found in a large minority of negative studies.
- Preparation
- Cannula of at least 20G, antecubital or larger vein, capable of 4 mL/s or more. Breath-hold at end-inspiration but WITHOUT a deep Valsalva, which drives unopacified inferior vena caval blood into the right heart and interrupts the bolus.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- CT angiography, bolus-trackedNo fixed delay. A monitoring ROI is placed in the target vessel (commonly the aorta) and acquisition triggers at a set attenuation rise — a ~100 HU threshold is widely used — followed by a short diagnostic delay of a few seconds for table movement and breath-hold instruction. Confirm locally.
This is not a separate physiological phase so much as a technique for hitting one reliably. The transferable principle is that a fixed delay assumes an average circulation, and the patients who most need vascular imaging — the shocked, the failing, the arrhythmic, the aneurysmal — are precisely those whose circulation time is furthest from average. Tracking the bolus in the target vessel replaces that assumption with a measurement, so peak arterial opacification is achieved in the individual patient. The trade-off is that the technique commits the scan to whatever the monitoring ROI sees: a badly placed ROI, a mistimed breath-hold or a threshold reached by a contralateral vein produces a systematically mistimed study rather than a slightly degraded one.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Prior contrast reaction and elective premedication· nurse pre scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- Child-sized technique and contrast dose· radiographer at scan
- Pregnancy status before an ionising exposure· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan