CT Chest — Triple Rule-Out

Lung apices to below the cardiac apex, ECG-synchronised.

Typical, not policy

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.

Premedication

Given to make the acquisition work, not to treat the patient. Doses are typical — confirm against your local protocol and prescribe within your own governance.

Beta-blocker — metoprolol most commonly

Rate control. Motion is the leading cause of a non-diagnostic coronary CT, and the right coronary artery moves most, so a regular rate at or below roughly 60 bpm is part of the protocol rather than an optimisation.

Dose and route: Typically oral metoprolol 50-100 mg about an hour before, and/or intravenous metoprolol in 5 mg increments to a usual maximum of around 15-20 mg, titrated to rate at the scanner. Regimens vary substantially between centres — confirm locally.
When: Oral roughly 60 minutes before; intravenous immediately before, allowing 3-5 minutes between increments to judge effect. Do not begin acquisition until the rate is both low enough and regular.
Do not give if: Avoid in decompensated heart failure, high-grade AV block without a pacemaker, marked bradycardia, and systolic hypotension.
AVOID in asthma or a history of bronchospasm. The BNF position is that beta-blockers should usually be avoided in these patients, and metoprolol’s own label lists severe asthma as a contraindication — cardioselectivity is dose-dependent and falls away at the doses used here (up to 100 mg orally plus 20 mg intravenously). This is the contraindication a radiographer meets most often on an outpatient list, and it belongs with the absolute ones rather than below them.
Suspected aortic dissection is a different scan and a different rationale: rate control there is treatment, not image optimisation.
If unsuitable: Ivabradine is the alternative where the contraindication to beta-blockade is BRONCHOSPASTIC DISEASE, and only there. It is not a general substitute: it shares the conduction and haemodynamic contraindications listed above — sick sinus syndrome, sinoatrial block, second- or third-degree AV block without a pacemaker, resting rate below about 60, significant hypotension, decompensated heart failure — so reaching for it because of those is swapping one contraindicated drug for another. It also acts on the sinus node funny current and therefore does NOTHING in atrial fibrillation, which is the commonest reason a coronary CT is non-diagnostic. Published CCTA regimens are typically 5 mg twice daily for a few days, or a single oral dose. Modern high-pitch and wide-detector systems tolerate higher rates, so scanner capability changes the threshold — which is exactly why this is confirm-locally.

Sublingual glyceryl trinitrate

Coronary vasodilatation. Without it the distal vessels are under-distended and distal disease is systematically over-called — the artefact looks like the finding.

Dose and route: Typically 400-800 micrograms sublingually, as spray or tablet.
When: Roughly 3-5 minutes before acquisition, so the effect is at its maximum during the scan rather than arriving after it.
Do not give if: Absolutely contraindicated with a phosphodiesterase-5 inhibitor — sildenafil or vardenafil within about 24 hours, tadalafil within about 48 — because the combination causes profound hypotension. Also absolutely contraindicated with RIOCIGUAT (Adempas), which is a soluble guanylate cyclase stimulator rather than a PDE5 inhibitor and so is missed by a question that asks only about the latter. Ask by indication as well as by drug name: sildenafil is dispensed as Revatio for pulmonary hypertension, and that patient will not describe it as an erectile-dysfunction tablet. This is the single question that must be asked before it is given.
Avoid in significant aortic stenosis, hypertrophic obstructive cardiomyopathy, and systolic hypotension.
Warn the patient about headache and transient flushing, which are expected rather than adverse.

SCCT guidelines on the performance and acquisition of coronary CT angiography — sublingual nitrate administration.

When to use it

Acute chest pain in a selected patient where coronary disease, acute aortic syndrome and pulmonary embolism are all genuinely on the differential and no single one predominates.

Technique

The protocol is a compromise: opacification of three vascular beds simultaneously is achievable but each is opacified less optimally than in a dedicated study.
Contrast load and radiation dose both exceed a dedicated coronary acquisition.
Society commentary is explicit that a genuinely equal three-way pre-test probability is uncommon; where one diagnosis dominates, the dedicated protocol is the better study.
Bolus tracking as for a dedicated coronary study: monitoring region of interest in the descending thoracic aorta (or the ascending aorta), trigger at roughly 100 HU above baseline, post-trigger diagnostic delay of about 5-8 s to allow the breath-hold instruction. The timing is set for the coronary arteries, and the injection — not the delay — is what buys the pulmonary arterial and aortic opacification.
Where the pulmonary arteries are the weaker part of the differential, some protocols shift the monitoring region of interest to the pulmonary trunk instead and accept slightly earlier coronary timing. Centres differ on which compromise they make, and it is worth knowing which one the local protocol has chosen before reporting a borderline study.
Coverage from the apices means a longer acquisition than a cardiac-only scan, so the injection must be long enough to sustain enhancement across it — this is the practical reason the volume is larger rather than a preference for more contrast.

Where it goes wrong

Ordering a triple rule-out as a way of avoiding a diagnostic decision produces a study that is second-best for all three questions.
Coronary image quality is more vulnerable here because the injection is optimised for a wider window rather than a tight coronary peak.
Extending coverage to the whole chest raises the dose to the breast in a group that is often young.

Clinical questions that reach this study

Suspected acute aortic syndrome Stable chest pain — suspected coronary artery disease Coronary artery calcium scoring for risk stratification Suspected myocarditis Cardiomyopathy — characterisation and assessment Valvular heart disease — assessment and intervention planning

Contrast

Iodinated, intravenous

Typically 90-120 mL of high-concentration non-ionic iodinated contrast (350-400 mgI/mL) at 5-6 mL/s — a larger volume and a longer injection than a dedicated coronary study — delivered biphasically so that the coronary arteries, thoracic aorta and pulmonary arteries are all adequately opacified in one acquisition. A representative regimen is roughly 70-80 mL of neat contrast followed by 30-50 mL of a diluted contrast-saline mix (30:70 or 50:50), then a 40-50 mL saline chaser.

The second, diluted phase is what makes this protocol work: it keeps the right heart and pulmonary arteries opacified for the pulmonary embolism question while the neat leading bolus is peaking in the coronaries, without the dense right atrial contrast that streaks across the right coronary artery.
The contrast load genuinely exceeds a dedicated coronary study — that is a cost of the protocol, not an incidental detail, and it is one of the reasons a triple rule-out should follow a real three-way differential rather than indecision.
Volumes, concentrations and split ratios differ substantially between published triple rule-out protocols and between scanner platforms. These are typical figures; confirm locally.

Acquisition

Breathing: Single inspiratory breath-hold.
Reconstruction: Thin ECG-synchronised reconstructions with coronary curved reformats plus a lung kernel series.
Preparation: Caffeine avoided for several hours; heart-rate control per local pathway, commonly oral or intravenous beta-blockade targeting a regular rate around 60 bpm or below. Sublingual nitrate immediately before acquisition unless contraindicated (recent phosphodiesterase inhibitor, severe aortic stenosis, hypotension). Right antecubital cannula of at least 18-20G; ECG electrodes applied clear of the scan field.

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
Sedation or anaesthesia for a child· nurse pre scan
Kidney function and intravenous iodinated contrast· radiographer at scan

References

SCCT expert consensus: use of coronary CT angiography for patients presenting with acute chest pain to the emergency department (2022).
Halpern EJ. Triple-rule-out CT angiography for evaluation of acute chest pain and possible acute coronary syndrome. Radiology 2009 — biphasic injection technique and the compromise in opacification across the three vascular beds.
SCCT guidelines for the performance and acquisition of coronary computed tomographic angiography — bolus tracking, trigger threshold and post-trigger delay.