From the Guidelines
To order a CT scan for suspected pulmonary embolism (PE), first assess the patient's clinical probability using validated tools like the Wells score or Geneva score, and for patients with high pretest probability, order a CT pulmonary angiogram (CTPA) with contrast, as recommended by the 2022 update of the ACR Appropriateness Criteria 1. When evaluating a patient with suspected acute PE, it is essential to establish their pretest probability of PE using validated clinical prediction rules, such as the Wells and Geneva rules, or clinician gestalt, as suggested by the American College of Physicians 1. The following steps should be taken:
- Assess the patient's clinical probability using validated tools
- For patients with low pretest probability, consider using the Pulmonary Embolism Rule-Out Criteria (PERC) to determine if imaging is necessary, as recommended by the American College of Physicians 1
- For patients with moderate to high clinical probability, order a CT pulmonary angiogram (CTPA) with contrast, as recommended by the 2022 update of the ACR Appropriateness Criteria 1
- Before ordering, check renal function (eGFR >30 ml/min is preferred) and ensure no contrast allergies exist, as suggested by the European Heart Journal 1
- If the patient has contrast allergies, premedication with prednisone and diphenhydramine may be necessary
- For pregnant patients or those with renal insufficiency, consider ventilation-perfusion (V/Q) scanning as an alternative, as recommended by the American Thoracic Society/Society of Thoracic Radiology Committee on Pulmonary Embolism in Pregnancy 1 Some key points to consider:
- The CTPA uses timed contrast injection to visualize pulmonary arteries when they are optimally opacified, allowing visualization of filling defects that indicate PE
- The patient should be NPO for 4 hours before the scan except for medications and small sips of water
- Ensure IV access with at least an 18-gauge catheter for contrast administration
- The protocol balances the need for accurate diagnosis while minimizing radiation exposure and contrast risks, as suggested by the Journal of the American College of Radiology 1
From the Research
Protocol for Ordering a CT Scan to Rule Out Pulmonary Embolism (PE)
To order a CT scan for ruling out PE, the following steps can be considered:
- Evaluate the patient's clinical probability of having PE using a clinical prediction rule such as the Wells score or the revised Geneva score 2
- If the patient has a low or intermediate clinical probability, a D-dimer test can be performed to further assess the likelihood of PE 3
- If the D-dimer test is negative and the clinical probability is low or intermediate, PE can be safely ruled out without the need for a CT scan 3
- If the D-dimer test is positive or the clinical probability is high, a CT pulmonary angiography (CTPA) can be ordered to confirm or rule out PE 4, 5
- CTPA is the imaging test of first choice for diagnosing PE, and it can be used as a single test to rule out PE in patients with a high clinical probability 5
Considerations for Ordering a CT Scan
When ordering a CT scan to rule out PE, the following considerations should be taken into account:
- The patient's renal function and ability to tolerate contrast media 6
- The patient's history of allergy to contrast media 6
- The availability of alternative imaging modalities, such as ventilation-perfusion scintigraphy or compression ultrasonography, in patients with contraindications to CTPA 5
- The need for a test injection to assess the flow rate and pressure measurements before performing the CT scan 6
Imaging Modalities for Diagnosing PE
The following imaging modalities can be used to diagnose PE: