Technical parameters of HRCT
To obtain high-quality images in HRCT, two conditions are critical: thin collimation and the use of a "sharp" reconstruction algorithm (sharp algorithm). This approach eliminates image smoothing and emphasizes the contours of anatomical structures, which is absolutely essential for detecting the "ground-glass opacity" sign (ground-glass opacification), assessing subsegmental bronchi, and identifying thickened interlobular septa.
- Scan thickness: The optimal value should be no less than 1 mm. Further slice thinning offers no diagnostic benefit as it decreases the signal-to-noise ratio, making the image "grainy".
- The role of contrast: Intravenous contrast enhancement is not indicated in HRCT, except when specifically searching for concurrent vascular pathologies (e.g., PE). The contrast agent artificially increases the density of the lung tissue. This can falsely simulate interstitial changes and severely complicates the comparison of serial patient scans over time.
Patient positioning and breathing phases
The standard examination is performed with the patient in the supine position (supine). The scanner acquires images from the apexes to the lung bases strictly at deep inspiration. However, the protocol may be modified for a specific clinical task:
- Prone position (prone): Recommended when early-stage interstitial fibrosis is suspected. It helps differentiate true tissue involvement from gravity-dependent opacification (dependent opacification), which physiologically develops in the posterior-basal segments when the patient lies on their back. If diffuse disease is already evident on the radiograph, scanning in the prone position loses its utility.
- Expiratory scanning (expiratory CT): Ordered to detect mosaic attenuation (mosaic attenuation) and air trapping (air trapping) in small airway pathologies. Paired scans (inspiration and expiration) also serve as a non-invasive method to confirm excessive airway collapse. For example, in relapsing polychondritis and the development of tracheomalacia, the cross-sectional area of the trachea during expiration can decrease by more than 50%.
Contrast administration in modern MSCT
If a patient requires intravenous contrast (tumor staging, aortic assessment, CT angiography), the quality of the study depends on patient parameters (body mass index, cardiac output) and the iodine delivery rate.
Historical contrast administration protocols had to be revised due to the transition to modern multislice scanners (MSCT) and the implementation of nephrotoxicity reduction protocols. Scanning on modern tomographs is much faster. This means that achieving peak arterial contrast concentration requires a high iodine delivery rate. If the physician is forced to reduce the total volume of administered contrast, they must compensate by increasing the injection rate.
Evolution of scanning protocols
In the era of single-slice scanners, performing volumetric CT scanning of the lungs with thin slices in a single breath-hold was technically impossible. Physicians had to use a sampling technique (sampling) where the scanner acquired non-contiguous thin slices, skipping 10–20 mm of tissue between them.
Modern MSCT (MDCT) scanners lack this limitation and easily capture the entire lung volume in a single breath-hold, even if the patient suffers from tachypnea. The historical step-and-shoot method of acquiring non-contiguous slices is no longer mandatory for HRCT today.
Briefly: answers to topical questions
What CT patterns are characteristic of interstitial lung diseases?
Several key HRCT patterns are distinguished for the differential diagnosis of interstitial lung diseases.
- Reticular pattern — includes fibrotic and non-fibrotic changes; irreversible reticulation may manifest as a "ground-glass opacity" sign (ground-glass opacity).
- Nodular pattern — reflects pathological involvement of various structures (centrilobular, perilymphatic, or random nodules).
- Mosaic attenuation pattern (mosaic attenuation) — reflects regional differences in lung parenchymal aeration.
- UIP pattern (UIP) — associated with idiopathic and non-idiopathic diseases and serves as a negative prognostic factor.
- NSIP pattern (NSIP) — frequently seen in systemic connective tissue diseases.
- Cysts — characteristic of pulmonary Langerhans cell histiocytosis (PLCH).
Which diseases cause the "air trapping" sign on HRCT?
The "air trapping" sign on HRCT occurs in small airway pathologies accompanied by hypoventilation. Diseases and conditions causing this sign include:
- Obstructive small airway diseases — confirmed by pulmonary function test results.
- Chronic obstructive pulmonary disease (COPD) — particularly in the airway-dominant phenotype.
- Bronchiolitis obliterans (obliterative bronchiolitis) — including the constrictive type, frequently encountered in severe bronchiectasis, cystic fibrosis, or as a post-infectious complication (pathogens M. pneumoniae and adenovirus).
- Chronic hypersensitivity pneumonitis (HP) — identifying air trapping is crucial for differential diagnosis with usual interstitial pneumonia.
What reconstruction algorithms are used to evaluate the mediastinum on CT?
Low spatial resolution algorithms are used to evaluate mediastinal structures on CT.
They reduce visual noise and improve image contrast at the cost of some loss in spatial resolution, making them optimal for visualizing soft tissues and vessels.
Additionally, gray-scale adjustment—windowing (windowing)—is used to optimally display the mediastinum. Unlike the lung parenchyma, the mediastinum requires narrower windows (narrower windows), and the window level is typically set midway between the density of the structure of interest and the surrounding tissue density.
Class and exam questions
Why is prone chest CT performed?
This is necessary to differentiate early interstitial fibrosis from the gravitationally dependent increase in parenchymal density that normally occurs in the posterobasal segments in supine patients.
Why is contrast avoided in HRCT?
The contrast agent falsely increases the X-ray density of the lung tissue. This can mimic the "ground-glass opacity" sign and greatly hinders the evaluation of disease progression over time.
How to confirm tracheomalacia on CT?
Paired images are acquired at maximum inspiration and at the end of expiration. The diagnosis is confirmed if excessive airway collapse is visualized (e.g., the tracheal lumen area decreases by more than half).
What is the minimum intensity projection (minIP) used for?
minIP reconstruction, especially when viewed in a narrow electronic window, highlights and significantly accentuates the mosaic attenuation pattern in small airway disease.
What else to study on this topic
- Minimum intensity projection (minIP): reconstruction algorithms and application
- "Ground-glass opacity" sign and interlobular septal thickening: morphological basis
- "Air trapping" pattern and differential diagnosis of mosaic attenuation
- Gravity-dependent density changes of the pulmonary parenchyma
- Calculating the iodine delivery rate in modern CT angiography
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