Bronchiectasis

• Bronchiectasis is a clinico-radiological disease characterized by abnormal and permanent dilatation of the bronchial tree associated with respiratory symptoms (productive cough and/or recurrent bronchial infection) 1, 2.

• However radiological bronchiectasis may also be present in asymptomatic individuals –particularly the elderly 1.

• Bronchiectasis may be described as a final common pathway for multiple diseases that are associated with chronic excessive bronchial inflammation 3, closely linked to bacterial infection 1. The pathogenesis of bronchiectasis may be considered according to a vicious cycle concept of chronic bronchial infection, structural lung disease , impaired mucociliary clearance and inflammation 1.

• Radiological signs are optimally seen on CT and are abnormal widening of the bronchi, with lack of normal tapering of the airways as they pass distally . There may be with thickening of the walls +/- irregular walls.

• The most used criterion for bronchiectasis on CT is the airway-artery ratio – that is increase in the diameter of the airway compared to an adjacent artery 4. However, standard reference values are lacking, and while inter-observer variability for diagnosis is good for highly abnormal findings, there is more variability for early or mild disease 5.

• There is considerable variation in the measurement of airway diameter on CT in relation to the degree of lung volume below total lung capacity (TLC) at which scanning is performed, as well as other factors such as mucus plugging and whether the outer or inner diameter of bronchi are measured. There is thus a need for standardization of protocols and measurements Computed Tomography5. Airway caliber is also dependent on age 6.

Classification of Bronchiectasis

A broad classification of bronchiectasis defines (a) Cystic Fibrosis (CF) bronchiectasis and (b) Non-Cystic Fibrosis (NF) bronchiectasis. 4

1. CF Bronchiectasis. Cystic Fibrosis is a congenital disease that is associated with sticky mucus, repeated pulmonary infections and resulting lung damage.

2. Non-CF Bronchiectasis 3, 7, 8. This has multiple causes associated with a combination of inflammation and obstruction/impaired clearance, including (N.B this is not a comprehensive list):

• Idiopathic

• Primary Ciliary Dyskinesia (Kartagener Syndrome)

• Immunodeficiency

• Post-infective

1. particularly repeated bacterial infections

2.  tuberculosis

• Allergic and Autoimmune

1. Allergic Bronchopulmonary Aspergillosis

2. Connective Tissue Diseases

• Obstruction

1. Chronic Obstructive Pulmonary Disease

2. Obstructing bronchial neoplasms

3. Inhaled foreign bodies

• Congenital , for example

1. Pulmonary sequestration

2. Alpha-1-antitrypsin deficiency

• Traction bronchiectasis due to, for example, pulmonary fibrosis

A classification based on macro morphology identifies the following types of bronchiectasis 4:

1. Cylindrical

2. Varicose

3. Cystic (saccular)

4. Traction (sometimes considered as a separate morphology)

Radiological Diagnosis of Cause of Bronchiectasis

Although radiological findings alone are of limited value in diagnosing the cause of bronchiectasis, the morphological type (see above), the distribution within the lungs, and concurrent radiological abnormalities may help narrow the differential diagnosis 4

• Relatively insensitive for bronchiectasis, but usually the initial investigation to exclude other causes for the patient's symptoms 9, 10

• Compared to CT, CXR has a reported sensitivity of 88% and specificity of 74% 11, 12

• May be normal in mild disease and underestimates the severity and extent of the disease 9, 10, 13, 14 Bronchiectasis patients commonly have abnormal CXR appearances but changes are often non-specific. 3. Findings suggestive of bronchiectasis include “tram track” appearance of dilated bronchi radiating from the hila, bronchial wall thickening and nodular or tubular opacities representing mucous impaction 15, 16 however, these signs may also represent COPD, asthma or lower respiratory tract infection 13. There may also be evidence of chronic lower airway infection such as calcifications or infiltrates 17. A baseline CXR is recommended in all bronchiectasis patients with repeat CXR based on clinical need, but CT is recommended to establish the diagnosis 13.

• There is poor correlation with infective exacerbations of bronchiectasis and radiographic changes 18 and exacerbations are usually defined by clinical signs and symptoms 13

• CT Chest is the investigation of choice for diagnosing bronchiectasis 2, 13

• CT can identify classify bronchiectasis into different morphologies including cylindrical, cystic and varicoid 8, 16

• Typical features of bronchiectasis on CT include: 8, 13, 19-23

• Lack of tapering of the bronchial lumina 

• Dilated bronchi with internal diameter greater than that of the adjacent pulmonary artery

• The potential pitfall for this sign is that some lung diseases are associated with decreased caliber of pulmonary arteries (eg due to hypoxia) which may lead to a false-positive impression of bronchiectasis. Conversely, a suboptimal inspiratory effort during CT scan may not fully dilate the airways causing an underestimation of bronchiectasis. 23

• Visualized bronchi within 1 cm of the pleura

• An inherent problem with this sign is in that some diseases associated with bronchiectasis the disease is predominantly central rather than peripheral 23.

• Mucus-filled dilated bronchi

• An attempt to standardize the criteria for the radiological definition and diagnosis of non-CF bronchiectasis by a consensus of international expert clinicians found an inner or outer airway-artery diameter ratio of 1.5 or more, a lack of airway tapering and visibility of airways in the lung periphery were the criteria that led to highest diagnostic confidence on CT 24.

• Radiographic findings can also point towards the underlying cause 8, 22, 23, 25, 26 albeit having limited value in doing so 27. The morphological type (cylindrical, varicose, cystic, traction), the distribution within the lungs, and concurrent radiological abnormalities may help narrow the differential diagnosis 4 although no aetiology is identified in a large proportion 27.

• CT protocols:

• Conventional HRCT with 1mm thick slices at 10mm intervals was considered the gold standard for diagnosis of bronchiectasis 13, but it has been mostly replaced by multislice CT (or multidetector CT, MDCT) which can be reconstructed into thin slices to assess parenchymal and bronchial detail, and continuous slices to assess for small nodules. 

• Multislice CT has a faster acquisition time and higher resolution when reconstructed in different planes. 

• MDCT has been shown to be more accurate for diagnosing bronchiectasis compared to conventional HRCT 28 and is preferred 2.

• Volumetric CT with multiplanar reconstructions is recommended 5 and allows for assessment of continuity of bronchial structures and differentiation of bronchiectasis form cystic lung disorders 4, 28

• The caliber of the airways and accompanying pulmonary artery branches depend on lung volumes at the time of acquiring the CT scan. If CT is performed at lung volumes less than total lung capacity (TLC), bronchiectasis may be under-diagnosed. Lung volume standardization for CT has been suggested 4, 5. This may include the possible use of spirometer control or pre-scanning spirometer training of the patient 5 but is limited in clinical practice.

Assessment of Bronchiectasis Severity 23

Semi-quantitative Assessment:

• Several visual scoring methods have been used (for example the Bhalla score and the Reiff score 4. However, these are suboptimal in assessing both bronchiectasis extent and severity. These scoring systems are of limited value in clinical practice due to variable observer reliability.

• Some systems , for example the Bronchiectasis Radiologically Indexed CT Score (BRICS), use multidimensional scores which account for clinical, imaging and functional measurements.

Computer analysis of Bronchiectasis:

Automated computer tools will likely replace visual analysis 5; these allow automated measurement of airway-artery ratios of multiple pairs of these structures.

MRI 

MRI in bronchiectasis is currently largely a research tool.

Use of MRI was reviewed in 2022 by Pakzad et al. 23. Studies utilizing quantitative MRI focus primarily on lung ventilatory defects. Differences in non- contrast-enhanced magnetic resonance (MR) signal intensity between inspiratory and expiratory MR scans has been found to correlate with lung function measures in patients with cystic fibrosis.  The inhalation of inert hyperpolarized noble gases such as 3He during MRI acquisition has been shown to accentuate the MR spin magnetization of the noble gas, which increases the MR signal-to-noise ratio within the lungs.