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Chest ❯ Pulmonary Embolism

ProblemEvaluation of suspected pulmonary embolism

The key to a high accuracy for the detection of a pulmonary embolism (PE) is the protocol for contrast delivery and data acquisition. Although in the past a preset delay (25 sec) may work in most cases it still was not ideal. The protocol we use is to trigger the start of the study when a cursor over the main pulmonary artery hits 200 HU. Some sites use anywhere from 180-210 depending on factors including speed of the scanner as well as contrast injection rate. The key of course is to optimize opacification of the pulmonary arteries and decrease contrast related artifact like beam hardening. Contrast volumes are usually 80-100 ml with injection rates of 4-5 cc/sec. Experienced techs often do not worry about triggering of preset HU values but trigger the scan when they visually see good pulmonary artery enhancement.

PearlsPulmonary embolism studies are often among the most challenging studies to interpret. When done correctly this usually is less of an issue. In the “old days” with 16 slice MDCT there were many false positive and negative studies. This currently is less of an issue when the studies are not correctly. Some specific pearls are;
1.     The key pitfalls for a PE study are a poor injection rate, poor timing of data acquisition (usually too late) or the patient breathing during the study
2.     The images from a PE must be reconstructed with a slice thickness of no greater than 1mm (we use .75mm). It is easy to miss a PE on 3mm thick sections.
3.     Reconstruction in coronal plane is very helpful and occasionally MIP images may be of value.
4.     Review of the lung parenchyma for potential pulmonary infarct is critical.
5.     Review of the cardiac structures to exclude right heart strain. Features of right heart strain include-increased right ventricle (RV)/left ventricular (LV) ratio (>1 in axial plane, >0.9 in 4-chamber reconstruction), flattening of interventricular septum and reflux of contrast material into the IVC and hepatic veins
6.     The extent of PE is important in management especially in a patient with a saddle embolism
7.     If the CT study is unsuccessful there is often a request to repeat the study. Before you repeat the study make certain you will not fail twice. If the patient is not cooperative we will not repeat the study.
8.    Many sites are now using AI programs to help with PE detection. There are several FDA approved products and initial reports are very good.
9.    PE studies as part of a triple rule out (TRO) can be more challenging but are done in select cases. The triple rule out is to rule out PE, aortic dissection and coronary artery disease in a single study.

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