Scanning technique and protocols
For informative diagnostics, it is necessary to acquire 300–450 thin slices (0.9–1.0 mm). A key condition for success is intense and uniform contrast enhancement. Short scan times often result in poor-quality images, especially when complicated by motion artifacts.
There are advanced combined protocols ("all-in-one" / one-stop-shop) where delayed CT venography of the pelvic and lower extremity veins is performed after lung scanning. However, this method yields only a minimal increase in sensitivity compared with isolated CT pulmonary angiography, but radically increases the radiation dose. Therefore, it is not used routinely.
Patients with acute chest pain may benefit from ECG-synchronized CT (to simultaneously rule out PE, aortic dissection, and acute coronary syndrome).
Direct and indirect signs of PE
Radiological symptoms are divided into direct (involving the thrombus itself) and indirect (reflecting the reaction of surrounding tissues).
Acute PE:
- Direct: The filling defect is centrally located or forms an acute angle with the vessel wall. Thrombus density is low (25–40 HU), with sharp contours. The vessel diameter may be enlarged in the obstruction zone.
- Indirect: Areas of reduced perfusion (oligemia), wedge-shaped consolidations of the lung parenchyma, and plate atelectasis. Cardiac findings include right ventricular (RV) dilatation, a thin RV wall, and retrograde contrast reflux into the inferior vena cava (IVC).
Chronic PE:
- Direct: The thrombus forms an obtuse angle with the wall, with intimal irregularities, intraluminal webs (webs), and calcification (65–105 HU). A marked narrowing of the affected vessel is characteristic.
- Indirect: A mosaic attenuation pattern (mosaic pattern), dilatation of the pulmonary trunk (>29 mm), RV wall thickening greater than 4 mm (as a consequence of pulmonary hypertension), and bronchial artery hypertrophy.
Dual-energy CT (DECT) and MRI
Dual-energy CT (DECT) makes it possible to acquire data at different tube voltages and generate color-coded iodine distribution maps. This is an excellent surrogate marker of perfusion: it helps detect small defects in the lung parenchyma and infarction zones (Hampton's hump sign), increasing the overall sensitivity of the method.
Magnetic resonance imaging (MRI) is considered an alternative without ionizing radiation. It has high specificity, but scanning is time-consuming. MR angiography is effective for large branches, but its sensitivity in subsegmental PE drops drastically (down to 21–33%), and the proportion of non-diagnostic scans reaches 30%.
Interpretation errors
False-positive and false-negative CT results are not uncommon. Most often, they are caused by:
- Artifacts. Motion, cardiac pulsation, or beam hardening from dense contrast in the veins can mimic or obscure a thrombus.
- Lymph nodes. Enlarged pulmonary hila lymph nodes on axial slices can easily be mistaken for an embolus.
- Anatomical variants. Pulmonary veins or mucus-filled bronchi are sometimes misidentified as arteries.
- Hemodynamics. Slow blood flow in the setting of pleural effusion or atelectasis impairs vascular enhancement.
How to easily distinguish a thrombus on CT: Acute PE — Acute angle with the vessel wall (and low density); Chronic (old thrombus) — Obtuse angle (and high density/calcifications).
Class and exam questions
How to distinguish a thrombus in the pulmonary artery from an enlarged lymph node?
On axial slices, they may look identical. For accurate differential diagnosis, coronal (frontal) reconstructions must be used—they clearly show whether the defect is inside the vessel or adjacent to it from the outside.
What is the diagnostic strategy for PE in pregnant women?
The first-line method when symptoms are present is compression ultrasound of the lower extremity veins. If the ultrasound is normal, the choice lies between CTPA and perfusion scintigraphy. It is important to consider that CT delivers a high radiation dose to the mother's breasts (10–70 mGy).
Does iodinated contrast affect the fetus during an examination of a pregnant patient?
Fetal radiation exposure during CTPA is minimal at any gestational age. However, iodine from the contrast agent can cross the placenta, so the newborn's thyroid function must be checked during the first week of life.
What else to study on this topic
- Pathogenesis and surgical treatment methods for chronic thromboembolic pulmonary hypertension (CTEPH)
- Principles of dual-energy CT (DECT) and virtual monoenergetic imaging
- Comparison of perfusion scintigraphy and CTPA in the evaluation of pregnant patients
- European Society of Cardiology (ESC) algorithm for suspected acute PE
- Mechanisms of x-ray beam hardening artifact formation
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