Radiological Evaluation of Uncommon Pulmonary Infections

Introduction

The diagnosis and treatment of lung infections depend greatly on the results of the radiological assessment. While common lung diseases like pneumonia have well-known radiological characteristics, some uncommon pulmonary infections have distinctive radiological manifestations. One must be thoroughly aware of their unique radiological findings for a correct diagnosis of these uncommon infections. So, the radiological assessment of rare lung infections must be discussed, emphasizing its significance in clinical practice.

What Is the Radiological Evaluation of Uncommon Pulmonary Infections?

To diagnose and track lung infections, radiological assessment, including chest X-rays and computed tomography (CT) scans, is crucial. Different infections may exhibit particular radiological patterns that aid in identifying and classifying them. Following are a few prominent uncommon pulmonary infections and their radiological characteristics:

Atypical Mycobacterial Infections and Tuberculosis

• Military Pattern: On a chest X-ray or computed tomography (CT) scan, TB (tuberculosis) and atypical mycobacterial infections might appear as a military pattern. This pattern reveals several tiny nodules, resembling millet seeds, dispersed across the lung fields.

• Cavitation: A characteristic of TB and some atypical mycobacterial diseases, cavitation is characterized by air-filled pockets inside lung lesions. Cavities frequently feature thick, crooked walls and may be accompanied by infiltration or consolidation in the vicinity.

Fungal Illnesses

• Cavitary Lesions with Surrounding Halo Sign: Several fungal infections, including coccidioidomycosis and histoplasmosis, can cause cavitary lung lesions with a surrounding halo sign. The "halo sign" is a rim of ground-glass opacity that surrounds the cavity and indicates active inflammation.

• The Tree-in-Bud Pattern: It is characterized by centrilobular nodules joined by linear opacities, which can be evident in infections such as pulmonary aspergillosis on imaging. The branching of the fungus' hyphae within the bronchioles is depicted by this pattern.

Parasitic Diseases

• Eosinophilic Infiltrates: The lungs may develop eosinophilic infiltrates due to specific parasite illnesses, such as pulmonary strongyloidiasis or paragonimiasis. These infiltrates can be connected to peripheral blood eosinophilia and manifest as patchy opacities or consolidation.

Viral Illnesses

• Ground-Glass Opacity: On imaging, some viral diseases, such as respiratory syncytial virus (RSV) and adenovirus, can exhibit a predominant ground-glass opacity pattern. While maintaining lung architecture, ground-glass opacities increase lung attenuation and give off a hazy appearance.

• Consolidation and Airway Inflammation: Bronchial wall thickening or bronchiectasis can result from patchy or diffuse consolidation and airway inflammation brought on by viral infections like influenza or the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Nontuberculous Mycobacterial (NTM) Infections

• Nodules: Lung nodular opacities are a frequent symptom of NTM infections. These nodules can be solid or cavitary in appearance and range in size and distribution. The edges of the nodules may be clearly defined or uneven.

• Bronchiectasis: The term "bronchiectasis," which describes the bronchi's persistent dilatation, is a frequent observation in NTM infections. It frequently coexists with nodular opacities. Indicating branching opacities linked to the dilated bronchi, bronchiectasis may resemble a tree in the bud.

• Consolidation: In NTM infections, areas of consolidation, lung tissue regions filled with inflammatory exudate, can be seen. The extent and location of consolidation depend on the particular NTM species and the stage of the infection, and it can be patchy or lobar.

• Lymphadenopathy: In NTM infections, enlarged lymph nodes may be seen in the mediastinum or hilar areas, indicating local lymph node involvement.

• Pleural Effusion: Advanced or widespread NTM infections may result in pleural effusion, a fluid collection in the pleural cavity. With other NTM species, including the Mycobacterium avium complex (MAC), it is more frequently observed.

Actinomycosis

• Pulmonary Infiltrates: Actinomycosis may show up on a CT scan as pulmonary infiltrates or areas of consolidation. These infiltrates may be patchy, have ill-defined borders, encompass larger lung segments, or both.

• Cavitation: Actinomycosis occasionally causes the development of cavitary lesions inside lung consolidations. These crevices could have uneven walls and be filled with liquid, dirt, or even air.

• Mass-like Opacities: Actinomycosis can cause lung opacities that resemble masses and seem like lung tumors. These opacities might be one or more and can come in various sizes and forms.

• Surrounding Inflammation: Actinomycosis frequently has an attribute known as the "inflammatory halo." This refers to the primary lesion or cavity visible on a CT scan having an enhanced rim of inflammatory tissue surrounding it.

• Extension to Adjacent Structures: Actinomycosis can extend to affect nearby structures, including the chest wall, pleura, or mediastinum, in more severe cases or if the infection spreads. The CT scan aids in determining the severity of the disease and any potential complications.

Pneumocystis jirovecii Pneumonia (PCP)

• Ground-Glass Opacities (GGOs): The hallmark radiological finding in PCP is the presence of ground-glass opacities. Indicating inflammatory alterations and edema within the lung parenchyma manifest as hazy patches of enhanced lung attenuation. GGOs often affect many lung regions bilaterally and diffusely.

• Septal Thickening: PCP can thicken the lung septa and promote interstitial inflammation. This septal thickening is considered unique to PCP and is frequently observed with ground-glass opacities.

• Crazy Paving Pattern: In more severe PCP instances, a "crazy paving" pattern may be seen on a CT scan. It describes the presence of interlobular septal thickening overlaid on a field of ground-glass opacities that resembles a crazy pavement pattern.

Invasive Fungal Infections

• Nodules: Lung nodular opacities can be a sign of invasive fungal infections. These nodules may differ in size, shape, and location. They may be solitary or numerous, with regular or ill-defined margins.

• Halo Sign: One identifying radiological feature in some invasive fungal infections, such as invasive aspergillosis, is the halo sign. It describes a ground-glass halo surrounding a tumor or nodule and indicates active bleeding and inflammation.

• Air Crescent Sign: Another distinct characteristic connected to invasive fungal infections, notably invasive aspergillosis, is the air crescent sign. A fungus ball or necrotic tissue is represented by this as a crescent-shaped pocket of air within a hollow or a nodule.

• Vascular Invasion: Vascular invasion symptoms, such as vessel blockage, intraluminal thrombus, or vessel wall thickening, may be present in invasive fungal infections.

Conclusion

A crucial role in the diagnosis and treatment of rare lung infections is played by radiological assessment. For a precise diagnosis and the best course of treatment, it is crucial to comprehend the distinctive radiological characteristics linked to these illnesses.

Recognizing distinctive patterns, such as miliary opacities in tuberculosis, the halo sign in fungal infections, ground-glass opacities in viral infections, or nodules in parasite infections, helps to narrow the differential diagnosis and direct additional diagnostic testing.

To promote prompt and efficient care, radiologists and clinicians must maintain a high index of suspicion for rare lung infections and be aware of their unique radiological characteristics. The capacity to identify and treat these uncommon lung infections will be significantly improved by ongoing research and developments in radiological techniques.