CT triple rule out (protocol)

A triple rule out (TRO) protocol is a cardiac CT protocol that aims to assess for different problems at the same time in one single examination: obstructive coronary artery disease, aortic dissection or pulmonary embolism.

The approach itself has been continuously under discussion due to difficulties in the practical implementation of a diagnostic scan with respect to opacification and radiation exposure and non-conclusive evidence regarding its efficiency in patient management ¹.

Note: There are different approaches to achieve this goal also with combined non-gated and ECG-gated acquisitions. This article is intended to outline some general principles and outline one approach. Protocol specifics especially medications, contrast doses and decisions on acquisition technique will vary with respect to scanner technology patient factors and institutional protocols.

The medication and contrast doses apply for cardiac CT examinations in adults.

Indications

The typical indication for a triple rule out scan is acute chest pain in an emergency where the following diagnosis is clinically suspected ^1-5:

• acute coronary syndrome (low to intermediate risk)
  • no ECG changes, negative troponin
• aortic dissection
• pulmonary embolism

In situations, where one of the diagnoses is less probable clinically it is advised to adjust the protocol accordingly.

Purpose

The goal of a triple rule out scan is the assessment of the aorta, the coronary arteries, the pulmonary arteries and the captured lung in one examination ^2-4.

Contraindications

Contraindications include the following ^2-4:

• acute ST-elevation myocardial infarction or elevated troponin
• screening of asymptomatic patients
• factors leading to potentially non-diagnostic scans
  • inability to cooperate (e.g. breath-hold instructions, arm elevation etc.)
  • markedly irregular rhythm (e.g. atrial fibrillation)
  • body mass index >39 kg/m²
  • high calcium burden
• contraindications against iodinated contrast media
• pregnancy or uncertain pregnancy status in premenopausal women

Contraindications to ß-blockers include:

• 2^nd or 3^rd-degree atrioventricular block
• systolic blood pressure ≤90 mmHg
• active bronchoconstriction, asthma or bronchospastic disease with regular inhaler use

Contraindications to nitrates include:

• aortic stenosis
• systolic blood pressure ≤90 mmHg

Technical requirements

Minimal technical requirements for a triple rule out protocol are the following ³:

• 64-slice scanner
• detector element width ≤0.625 mm
• option of cardiac CT and ECG-gated triggering

Patient preparation

A triple rule out scan is usually considered in emergencies and there is not much time for preparation. However some fast preparations can still be conducted including checking indications, contraindications not only for contrast media but also medications as ß-blocker and nitrates and obtaining informed consent and instructions on how to breathe ³.

Check heart rate and blood pressure before administration of medications.

Premedication

Premedication includes the following ³:

• administration of nitrates (400-800 µg of sublingual nitroglycerin e.g. 1-2 sprays)
• administration of ß-blocker (to target pulse of ≤60 bpm)
  • e.g. metoprolol 50-100 mg one hour before the exam
  • e.g. metoprolol 5mg iv followed of monitoring for 5 min repeatedly up to 15-20 mg

Technique

• patient position
  • supine with both arms above their head (as comfortable as possible)
  • ECG placement
• acquisition
  • prospective ECG-gated high-pitch acquisition
  • prospective ECG-gated acquisition
  • modulated retrospective ECG-gated acquisition (high radiation)
• tube potential: 100 kVp if patient’s weight ≤100kg or BMI <30kg/m²
• tube current: use automated current adjustment mode
• scout
  • lung apices to below the heart
• ​scan extent
  • 1-2 cm above the aortic arch to below the heart
• scan direction
  • craniocaudal 
• ECG considerations
  • check scan can occur in the agreed-upon portion of the R-R interval within the patient's current heart rate
  • typically in mid-diastole in lower heart rates <70 bpm
  • mid-diastole to end-systole at higher heart rates 70-80 bpm
• contrast injection considerations
  • contrast media: contrast agents with high iodine concentrations (300-400 mg iodine/mL)
  • contrast timing
    • monitoring: ascending aorta
    • bolus tracking (preferred)
    • test bolus (test volume 20 mL at the same flow as the cardiac scan e.g. 5-6 mL/s)
  • optimal opacification of the coronary arteries and aorta usually happens about 10-12s after the pulmonary artery
  • contrast volume
    • triphasic injection 
      • 50-70 mL contrast media at 5-6 mL/s
      • 30-40 mL contrast media or 50 ml contrast media/saline mix (3:2 or 3:1) at 3-4 mL/s
      • 30-40 mL saline chaser at 3 mL/s
    • biphasic injection 
      • 80-100 mL contrast media 5 mL/s
      • 30-40 mL saline chaser 3 mL/s
• respiration phase
  • inspiration
  • for breath-hold consistency, a medium-sized breath is easier to reproduce throughout the examination compared to sharp deep breaths

Practical points

• a high-pitch acquisition can be considered in patients with stable sinus rhythm and a heart rate of less than 60 bpm and a bodyweight ≤100 kg (very low radiation dose)
• a prospective ECG-gated acquisition should be performed in most situations with a regular rhythm, where high-pitch or high-volume scans are not available or contraindicated
• retrospective ECG-gated acquisition can be considered in patients with an irregular rhythm or high heart rate
• for higher heart rates (>70 bpm) consider widening the acquisition window, such as ¹:
  • heart rate <65 bpm: 65-75%
  • heart rate 65-72 bpm: 60-80%  
  • heart rate >72 bpm: 35-80%
• contrast agent: higher concentrations (350-400 mg iodine/mL) give better results
• on the biphasic injection technique, one can consider reducing the flow to 4 mL/s after the first 30-40 mL to reduce the amount of contrast