High resolution CT (HRCT) scanning of the lungs is sensitive for detecting bronchiectasis. Helical CT scanning may also be used but this delivers a higher radiation dose to the patient.
Essential features of bronchiectasis:
(1) dilation of the bronchus
(2) variable thickening of the bronchial wall
Patterns of bronchiectasis:
(1) cylindrical (uniform dilation)
(2) varicose (beaded appearance)
(3) cystic or saccular
Parameters for the bronchoarterial ratio:
(1) internal diameter of the lumen for a bronchus lying perpendicular to the scanning plane (the external diameter should not be used since it may or may not be thickened)
(2) diameter of the immediately adjacent bronchial artery
bronchoarterial ratio =
= (internal diameter of the bronchus in mm) / (diameter of adjacent bronchial artery)
Kim at al found the bronchoarterial ratio in normal individuals at sea level to be 0.62 +/- 0.13. Using the upper limit of the normal reference range as mean plus 2.5 times the SD, a ratio > 0=0.95 would be increased. At higher elevations the ratio is 0.76 +/- 0.14, meaning that the cutoff would be >= 1.12.
In severe bronchiectasis, the "signet ring" sign will be seen, with the dilated bronchus forming the ring and the adjacent artery forming the signet.
Remy-Jardin et al defined a bronchial as thickened if it is >= 2 times the diameter of a normal bronchus at a corresponding level.
Additional findings in bronchiectasis:
(1) If the bronchiectatic bronchus lies parallel to the plane of the CT cut, then it will show the sides running parallel to each other rather than tapering as it extends to the periphery.
(2) Bronchi may be found lying within 1 cm of the costal or paravertebral pleura; normally the bronchial diameter in these areas is too small to be seen on the scans.
(3) The presence of mucus-plugging may allow peripheral bronchi to be visualized.
Limitations to the bronchoarterial ratio:
• Matsuoka et al found that Japanese > 65 years of age had increased bronchoarterial ratios.
• Normal individuals may show variation in the bronchoalveolar ratio and occasionally a bronchus and artery may show an elevated ratio.
• The diameter of the bronchial artery may vary based on flow and vascular pressures, resulting in false positive or negative ratios.
• Motion or other artifacts may make measurements unreliable.
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