Basic protocol and scanning phases
In the standard setup, the patient lies supine on the scanner table with arms raised overhead (supine position). The study begins with a scout projection (scout projection) in anteroposterior and lateral views, which is necessary to plan the region of interest and optimize the radiation dose.
The timing of intravenous enhancement is chosen based on the clinical task:
- 20–30 seconds: arterial phase for optimal visualization of the central pulmonary vessels.
- 65 seconds: portal venous phase (portal venous phase), which is mandatory if chest CT is performed concurrently with abdominopelvic scanning.
To eliminate motion artifacts from breathing, the classical protocol involves two short breath-holds. However, modern fast multislice CT (MSCT) scanners offer a single volumetric acquisition option (single volumetric acquisition). In this mode, the scanner generates isotropic voxels, enabling accurate multiplanar reconstructions.
Factors of high-quality enhancement
Enhancement is not a one-size-fits-all approach. Older protocols (e.g., administration of 100 ml of contrast at 2.5 ml/s) had to be revised due to the introduction of ultrafast scanners and the need to reduce nephrotoxicity. Enhancement quality is affected by:
- Patient parameters: body weight, body surface area, and cardiac output.
- Iodine delivery rate: a key factor depending on the contrast volume and injector flow rate. The faster the scanner, the higher the iodine injection rate must be.
- Timing: to accurately hit the target phase, automated bolus tracking (automated bolus triggering) or the test-bolus method is used.
CT pulmonary angiography (CTPA)
This is a specialized protocol aimed at diagnosing pulmonary embolism (PE). It requires the acquisition of 300–450 slices with a thickness of approximately 0.9–1.0 mm. The goal is to achieve an attenuation of 300–350 HU within the arterial lumen.
The physician evaluates direct signs (partial filling defects or thrombi completely occluding the lumen) and indirect signs (pulmonary infarctions, enlargement of the pulmonary trunk and right heart chambers due to acute pulmonary hypertension). The main technical challenge of CTPA is the short data acquisition window, meaning that even minor timing errors or patient movements result in diagnostic failure.
Considerations in children and pregnant women
In newborns and infants, short blood transit times and small contrast volumes often lead to suboptimal vascular opacification. To overcome this, the contrast agent is diluted with normal saline to 70–80 concentration and the injection rate is reduced. The right arm is the optimal venous access site, as this eliminates dense artifacts from the left brachiocephalic vein obscuring the branches of the aortic arch.
When pulmonary embolism is suspected in pregnant women, the risk of undiagnosed pathology outweighs the risks of fetal irradiation (fetal dose during CTPA is negligible). However, maternal breast tissue radiation is a concern (10–70 mGy). It is important to remember that iodine from the contrast agent can impair fetal thyroid function; therefore, neonatal thyroid screening is required during the first week of life.
When contrast is contraindicated
Intravenous contrast enhancement is strongly discouraged during high-resolution CT (HRCT / HRCT) unless there are additional clinical indications (e.g., suspected acute pulmonary embolism).
Iodinated contrast increases the X-ray attenuation of the lung parenchyma. This artificial enhancement can easily mimic a ground-glass opacity (ground-glass opacification). As a result, the physician may falsely diagnose interstitial disease or misinterpret disease progression in a patient with pre-existing fibrosis.
To remember the three pillars of quality contrast enhancement, use the rule of three P's: Patient (weight and cardiac output), Product (iodine delivery rate), and Protocol (bolus timing).
Class and exam questions
Why is a triple-rule-out CT protocol necessary?
This ECG-gated protocol is used in patients presenting with acute chest pain. In a single scan, it rules out three life-threatening conditions: acute coronary syndrome, aortic dissection, and pulmonary embolism.
Why should contrast administration be avoided in high-resolution chest CT?
Contrast artificially increases lung density, mimicking the pathological ground-glass pattern. This significantly complicates the evaluation of interstitial lung diseases.
Does it make sense to perform CT venography along with CTPA?
The one-stop-shop technique with a delayed phase for deep leg veins yields only a marginal gain in sensitivity while substantially increasing the radiation dose. It is not routinely used.
What else to study on this topic
- Features of dual-energy CT (DECT) for pulmonary perfusion evaluation
- ECG-gated CT angiography protocols
- CT imaging of carcinoid tumors and atelectasis
- Role of MRI in staging Pancoast tumors
- Radiological signs of congenital heart disease (CHD)
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