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 not be less than 1 mm. Further slice thinning will not provide diagnostic benefit, as it will reduce the signal-to-noise ratio, making the image "grainy".
- Role of contrast: Intravenous contrast enhancement is not indicated in HRCT, except for cases of targeted search for concomitant vascular pathologies (e.g., PE). The contrast agent artificially increases the density of lung tissue. This can falsely simulate interstitial changes and extremely complicates the comparison of patient scans over time.
Patient positioning and respiratory phases
The standard examination is performed with the patient in the supine position (supine). The scanner examines the area from the apices to the basal regions strictly at deep inspiration. However, the protocol can be modified for a specific clinical task:
- Prone position (prone): Recommended when early stages of interstitial fibrosis are suspected. It allows true tissue lesions to be separated from gravity-dependent opacity (dependent opacification), which physiologically forms in the posterior basal segments when a person lies on their back. If diffuse disease is already evident on the radiograph, scanning in the prone position loses its meaning.
- Expiratory scanning (expiratory CT): Ordered to detect mosaic attenuation (mosaic attenuation) and "air trapping" (air trapping) in small airway pathologies. Paired examinations (on 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 on expiration can decrease by more than 50%.
Contrast enhancement in modern MSCT
If the patient requires intravenous contrast (tumor staging, aortic assessment, CT angiography), the quality of the examination depends on the patient's parameters (body mass index, cardiac output) and iodine delivery rate.
Historical contrast administration protocols had to be revised due to the transition to modern multi-slice scanners (MSCT) and the introduction of nephrotoxicity reduction protocols. Scanning on modern tomographs is much faster. This means that to achieve peak contrast concentration in the arteries, a high iodine delivery rate is required. If the physician is forced to reduce the total volume of the administered drug, they must compensate for this by increasing the injection rate.
Evolution of scanning protocols
In the era of single-slice tomographs, performing volumetric (three-dimensional) lung scanning with thin slices in a single breath-hold was technically impossible. Physicians had to use a sampling technique (sampling) — the scanner took non-contiguous thin slices, skipping 10–20 mm of tissue between them.
Modern MSCT (MDCT) scanners lack this drawback and easily image the entire lung volume in a single breath-hold, even if the patient suffers from tachypnea. The historical step-by-step method of acquiring non-contiguous slices is no longer mandatory for HRCT today.
Singkatnya: jawaban atas pertanyaan topik
What CT patterns are characteristic of interstitial lung diseases?
Several main HRCT patterns are distinguished for the differential diagnosis of interstitial lung diseases.
- Reticular pattern — includes fibrotic and non-fibrotic changes; irreversible reticulation may appear as the "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 tissue aeration.
- UIP pattern (UIP) — associated with idiopathic and non-idiopathic diseases, and is a negative prognostic factor.
- NSIP pattern (NSIP) — frequently found in systemic connective tissue diseases.
- Cysts — characteristic of pulmonary Langerhans cell histiocytosis (PLCH).
What 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) — especially in phenotypes with predominant airway involvement.
- Bronchiolitis obliterans (obliterative bronchiolitis) — including the constrictive type, often found in severe bronchiectasis, cystic fibrosis, or as a post-infectious complication (M. pneumoniae and adenovirus pathogens).
- Chronic hypersensitivity pneumonitis (HP) — identifying air trapping is important for differential diagnosis with usual interstitial pneumonia.
Which reconstruction algorithms are used to assess the mediastinum on CT?
Low spatial resolution algorithms are used to assess mediastinal structures on CT.
They reduce visual noise and improve image contrast at the cost of some loss of spatial resolution, making them optimal for visualizing soft tissues and vessels.
Additionally, for optimal visualization of the mediastinum, a gray-scale adjustment setting is used — windowing (windowing). Unlike the lung parenchyma, the mediastinum requires narrower windows (narrower windows), and the window center level is usually set midway between the density of the structure being examined and the density of the surrounding tissue.
Soal-soal dari kelas dan ujian
Mengapa CT paru dilakukan dalam posisi tengkurap (prone)?
This is necessary to differentiate early interstitial fibrosis from the gravity-dependent increase in parenchymal density that normally occurs in the posterior basal segments in supine patients.
Mengapa penggunaan kontras dihindari pada 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.
Bagaimana cara mengonfirmasi trakeomalasia pada CT?
Paired scans are performed at peak inspiration and at the end of expiration. The diagnosis is confirmed if excessive airway collapse is visualized (e.g., the cross-sectional area of the trachea decreases by more than half).
Untuk apa proyeksi minimum intensity projection (minIP) digunakan?
minIP reconstruction, especially when viewed in a narrow electronic window, highlights and makes the mosaic attenuation pattern significantly more noticeable in small airway disease.
Topik lain untuk dipelajari
- Minimum intensity projection (minIP): construction algorithms and application
- "Ground-glass opacity" sign and thickening of interlobular septa: morphological basis
- "Air trapping" pattern and differential diagnosis of mosaic attenuation
- Perubahan densitas parenkim paru yang bergantung pada gravitasi
- Perhitungan kecepatan pengiriman iodium pada CT-angiografi modern
Topik lain di bagian «Radiologi diagnostik organ toraks»
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