CT pulmonary angiogram (protocol)

The computed tomography pulmonary angiogram (CTPA/CTPE) is a commonly performed diagnostic examination to exclude pulmonary emboli (PE). Each radiology department will have a slightly different method for achieving the same outcome, i.e. adequate enhancementdiagnostic density of the main pulmonary trunkartery and and its branches. 

There are two principal approaches for performing a CTPA of high diagnostic quality:

• test bolus
  • a: a small ‘test’ quantity of contrast is injected and sequential axial slices at a set region of interest are acquired to calculate the time of peak contrast enhancement and determine an optimal scan delay
• bolus tracking
  • : ​​sequential axial slices at a set region of interest are conducted during the contrast injection until a threshold enhancement is met, triggering a diagnostic scan ¹

NB: This article is intended to outline some general principles of protocol design. The specifics will vary depending on CT hardware and software, radiologists' and referrers' preference, institutional protocols, patient factors (e.g. allergy) and time constraints. 

Indications

Suspected pulmonary embolism: acute or chronic.

Purpose

This technique is based on the detection of filling defects in the pulmonary arterial vasculature⁶, so acquisition at the right time is of vital importance. The study is considered optimal when the pulmonary arteries are opacified and the aorta is not. Late acquisition will make it difficult to differentiate between pulmonary arterial and pulmonary venous branches.

Contraindications 

• previous severe reactions to iodinated contrast
• non-compliance

Technique

Bolus tracking

• patient position
  • supine with their arms above their head
• scout 
  • apices to diaphragm
• scan extent 
  • apices to diaphragm
• scan direction
  • caudocranial
• contrast injection considerations
  • monitoring slice (region of interest)
    • below the carina at the level of the pulmonary trunk with an ROI on the pulmonary artery
  • threshold
    • 100 HU
  • volume
    • 60 mL of non-ionic contrast with a 100 mL saline chaser at 4.5/5 mL/s
• scan delay
  • minimal scan delay
• ​respiration phase
  • ​inspiration

Test bolus

• patient position
  • supine with their arms above their head
• scout 
  • apices to diaphragm 
• scan extent 
  • apices to diaphragm 
• scan direction
  • caudocranial 
• contrast injection considerations
• test bolus
  • contrast volume
    • 20 mL of non-ionic contrast with a 10 mL saline chaser at 4.5/5 mL/s
  • monitoring slice (region of interest) 
    • below the carina at the level of the pulmonary trunk with an ROI on the pulmonary artery
    • monitor contrast enhancement peak over time via a time-enhancement curve
• calculating scan delay
  • as the time-enhancement curve will only begin recording after the scan delay. A; a widely accepted formula for calculating the scan delay is ¹: 
    • peak contrast enhancement (time-enhancement curve) + scanner's diagnostic scan delay
• contrast volume (diagnostic scan)
  • 60 mL of non-ionic contrast with a 100 mL saline chaser at 4.5/5 mL/s
• scan delay
  • peak contrast enhancement (time-enhancement curve) + scanner's diagnostic scan delay
• ​respiration phase
  • ​inspiration

Practical points

• what constitutes asis considered a diagnostic CTPA based on enhancementmain pulmonary artery density varies from site-to-site210 ^6,9 to 300 HU ⁵ with 250 HU a commonly accepted value ^7,8
  • the density can be theoretically as low as 93 HU for the detection of acute PE ⁶
  • measurement should be performed with a round ROI covering at least 50% of the main pulmonary artery lumen ⁹
• changing the scan direction to caudocranial has been shown to better demonstrate the lower lobes whilst alleviating artefact from the contrast bolus in the SVC ¹
• use of monoenergetic reconstruction of CTPA studies with suboptimal enhancement has been shown to increase enhancement of the pulmonary trunk ³
• use of high-pitch-scanning in non-obese patients can see a reduction in contrast use to as low as 12 mL ⁴