Imaging

Pulmonary Embolism

Preliminary Diagnosis: Pulmonary Embolism (PE)

I. What imaging technique is first-line for this diagnosis?

Computed tomographic angiography (CTA) utilizing PE protocol.

II. Describe the advantages and disadvantages of this technique for diagnosing pulmonary embolism.

Advantages

  • Diagnostic for pulmonary embolisms of clinically significant size.

  • Relatively quick imaging procedures.

  • Requires minimal patient cooperation.

  • Offers excellent anatomic detail of the thorax, including lung parenchyma, airways, and chest wall.

  • Can help differentiate between chronic and acute pulmonary embolic events.

Disadvantages

  • Exposes the patient to a large amount of ionizing radiation.

  • Exposes the patient to the risk of contrast induced nephropathy.

  • Exposes the patient to the risk of contrast induced allergic reaction.

III. What are the contraindications for the first-line imaging technique?

  • Known contrast allergy.

  • Pregnancy, especially during the first two trimesters.

  • Creatinine > 1.6. Exceptions can be made on a case-by-case basis.

IV. What alternative imaging techniques are available?

  • Ventilation perfusion (V/Q) nuclear medicine scan.

  • Conventional angiography.

  • Ultrasound of the lower extremities, with and without Doppler.

V. Describe the advantages and disadvantages of the alternative techniques for diagnosing pulmonary embolism.

Ventilation perfusion (V/Q) nuclear medicine scan

Advantages

  • May be used in circumstances when intravenous iodinated contrast is contraindicated.

  • Preferred in a pregnant patient (utilizing only the perfusion portion of the exam with half the dose of Technetium 99m-MAA. The ventilation portion of the exam is omitted to reduce radiation dose to the pregnant patient).

  • Can be diagnostic for pulmonary embolus.

  • Uses a very small dose of ionizing radiation, much less than CTA.

Disadvantages

  • The exam takes about an hour to complete. The patient must be able to tolerate lying supine for the length of the exam.

  • May be more difficult to interpret and less accurate than CTA.

  • Requires the appropriate handling of radioactive nuclear tracers.

  • The hospital must be equipped with proper waste disposal for nuclear tracers.

Conventional angiography

Advantages

  • Diagnostic gold standard for pulmonary embolism.

  • Can be therapeutic if direct intraluminal thrombolysis is indicated.

  • Can determine flow dynamics and pressures within the pulmonary arteries.

Disadvantages

  • Invasive and associated with a higher morbidity than CTA or V/Q scans.

  • Does not demonstrate the anatomical detail of a CT or MRI.

  • Requires the use of iodinated contrast, which can be associated with an allergy or contrast induced nephropathy.

  • Uses a high dose of radiation.

  • Expensive.

  • Requires a fluoroscopy suite and interventional radiology.

  • Sedation is recommended.

Ultrasound of the lower extremities with and without doppler

Advantages

  • This adjunctive imaging exam can help rule in or out deep venous thrombosis as an underlying cause for pulmonary embolism.

  • Lack of ionizing radiation, particularly advantageous when imaging a pregnant patient.

  • Lack of contrast administration, particularly advantageous when imaging a patient with renal failure or creatinine > 1.6.

  • Relatively cheap and fast real time imaging evaluation.

  • Noninvasive.

  • Ultrasound of the lower extremities, with and without Doppler, is a useful imaging study in pregnant patients, patients with renal abnormality, patients in which CTA or VQ scans are relatively contraindicated, and patients with negative/equivocal imaging findings.

Disadvantages

  • The quality of the exam is operator dependent.

  • Large body habitus and/or the presence of central lines or overlying bandages secondary to trauma or recent procedure can limit visibility of the lower extremity veins and can thereby reduce the sensitivity and/or specificity of the exam.

VI. What are the contraindications for the alternative imaging techniques?

Ventilation perfusion (V/Q) nuclear medicine scan

  • Pregnancy is a contraindication for the ventilation portion of the exam.

  • While not an absolute contraindication, breastfeeding patients must be counseled not to breastfeed for approximately 24 hours.

Conventional angiography

  • Coagulopathy, i.e., bleeding diathysis.

  • Pregnancy.

  • Creatinine > 1.6.

  • Allergy.

References

Bettmann, MA. "ACR Appropriateness Criteria® acute chest pain--suspected pulmonary embolism.". J Thorac Imaging. vol. 27. 2012. pp. W28-31.

http://www.acr.org/~/media/186992e39e754270bb3abbf44a5a5801.pdf
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