Overview

Mycobacterium avium complex (MAC) lung disease is an infection caused by a group of bacteria (MAC) that typically reside in soil, water, and aerosols. The bacteria can enter the system through ingestion or inhalation, but inhalation is more common. When inhaled, MAC may cause an infection in the lungs. Immunocompromised individuals are particularly at risk, with the potential for the disease to spread to other organs, and even enter the bloodstream.¹

Signs of MAC lung disease (MAC-LD) include a chronic cough, fever, shortness of breath, and hemoptysis.² The infection may sometimes remain asymptomatic,³ or it may progress over several years.⁴ Symptoms of MAC-LD overlap with those of several other common respiratory infections, such as tuberculosis. For accurate identification of the disease, your healthcare specialist will employ a range of diagnostic tools, including phlegm analysis, chest imaging, and clinical evaluation.¹ 

Chest imaging using High-Resolution Computed Tomography (HRCT) is a crucial step in the investigation and monitoring of cases of MAC-LD and other lung diseases. It provides detailed information about structural and functional issues in the lungs.⁵ CT features can give indications on the disease type,¹ progression,^6,7, and the patient’s response to treatment. HRCT’s sensitivity also allows for the early detection of new abnormalities, making this imaging technique a powerful tool in the diagnostic process.⁸

This article highlights the utility of HRCT in all the clinical phases of MAC-LD, from diagnosis to treatment to prognosis. 

Understanding high-resolution CT (HRCT)

What is HRCT?

An HRCT is a highly sensitive version of a Computed Tomography (CT) scan. A CT scan applies the same principles as an X-ray radiograph, where a high-energy light beam (X-ray) is passed through the body and detected on the other side. Due to the beam’s high intensity, it will pass easily through lower-density materials like soft tissue; however, it will lose intensity when encountering denser materials, such as bones, bronchi, and specific abnormal lung structures. This process casts a negative image of the internal structure of the body on the light detector. 

HRCT provides a more detailed view of the lung architecture than an X-ray or a standard CT scan. It does so by taking and processing a series of X-ray images at different angles. The image processing will produce a picture of a thin slice of your lungs, which can be either a frontal view or a top view.⁹

Benefits of using HRCT in MAC lung disease

If you are dealing with a suspected case of MAC-LD, your healthcare provider will likely ask for a radiograph first. Chest X-rays are fast and affordable and can reveal features often present in MAC-LD cases, such as abnormal growths or opacities. However, their utility is limited by their sensitivity, as CTs tend to be better at detecting the location and nature of abnormalities, especially in the early stages of the infection.¹⁰ Chest X-rays are also unable to narrow the diagnosis to the type of MAC-LD, an important step in identifying the appropriate treatment and prognosis.¹¹

Common HRCT findings in MAC lung disease

MAC-LD manifests in the lungs with similar features to other lung infections. Regardless, HRCT can be used to reveal specific characteristics of the infection that are potential indicators for disease progression, prognosis, and even the possible impact of the disease on the patient’s quality of life.¹² 

When looking at lung HRCT scans of a MAC-LD patient, a radiologist will likely look for: 

• Nodular patterns
• Signs of bronchiectasis
• Cavities 
• Opacities

Nodular patterns

In general, lung nodules refer to any abnormal growth in your lungs. In cases of MAC-PD, nodules may specifically refer to obstructions in the bronchi caused by inflammation and cell death. This condition is known as cellular bronchiolitis.⁶ These nodules will limit airflow and may deform and scar the bronchioles depending on the size¹³ at which they form. The nodules in MAC-PD may appear in specific patterns, often near each other.⁶

Small nodules (Tree-in-bud appearance)

Nodules in MAC-PD may appear close together in specific patterns.⁶ Particularly common is the tree-in-bud appearance. This pattern results from the spread of the infection from the bronchi into the bronchioles. As the disease progresses into the respiratory system, more infected areas present growths. These growths might deform the bronchioles.¹ HRCT can identify the presence, size, and location of nodules.¹²

HRCT can also aid in identifying the nature of the growths, as not all may originate from a lung infection. CT scans can be used as a non-invasive technique to determine if a nodule or growth is malignant or benign.¹⁴

Bronchiectasis

Bronchiectasis is a chronic lung disease that can appear as a result of a MAC lung infection or other lung conditions. It manifests as a loss in lung function caused by mucus pooling in the bronchi.¹⁵ Symptoms of bronchiectasis include recurring lung infections and cough, as well as limited airflow.¹¹

Signs of bronchiectasis in HRCT include: 

• Dilated airways 
• Clusters of thin-walled cysts¹⁵
• Mucus plugging in airways¹¹

HRCT is considered the most effective method for identifying bronchiectasis, making it an essential part of the diagnostic process in MAC-LD, as well as other lung diseases. 

Cavities

A lung cavity refers to an isolated gas-filled structure inside the lung tissues. They are not unique to MAC-LD, as they often develop in cases of tuberculosis.¹⁶  However, the location of the lesion might indicate the type of infection, as cavities in MAC-PD tend to form in the upper lobe of the lung.¹ They appear as darker or opaque regions in HRCT images of the lung. 

Consolidation

Consolidation is a common finding in patients with MAC-LD.¹² Consolidations refer to structures in the lungs that appear as hazy and opaque areas, or ground-glass opacities, as seen on HRCT imaging.¹⁷ The opacity is a result of the presence of liquid in the lungs.¹⁸ 

How HRCT helps doctors manage MAC lung disease

Differential diagnosis 

Cases of MAC-LD can be divided into two categories:

• Nodular Bronchiectatic Disease: This type of infection is characterised by small nodules in combination with bronchiectasis.⁶ It has a slow progression and is considered the less severe case⁷
• Fibro-Cavitary disease: This type of infection is more common in caucasian males and is characterised by clusters of cavities in the upper area (lobes) of the lung.¹ It manifests primarily in patients with pre-existing lung conditions and progresses quickly⁷

While Nodular Bronchiectatic disease may be monitored without the need for antibiotic treatment, MAC-LD cases of the fibro-cavitary type warrant immediate intervention due to their rapid progression.⁷ Additionally, although fibro-cavitary disease can be diagnosed with an X-ray, bronchiectasis identification requires an HRCT scan.¹¹ These two factors make HRCT the ideal tool for differentiating between the two diseases, thereby providing the appropriate care and limiting symptom worsening.¹

CT is also often used to distinguish benign nodules, such as those that accompany MAC-LD, from other malignant masses.¹⁴ Identifying cancerous growths can further improve the personalisation of the patient's treatment plan and optimise outcomes.  

Monitoring progression

Healthcare providers may use CT checkups to monitor changes through the course of the disease. HRCT can not only show the progression of disease, but also measure how the lung function changes as the disease evolves. Performing functionality tests, such as the ones performed with HRCT, is crucial for monitoring the disease burden.¹²

Guiding treatment

Different types of MAC infections require various treatments. For example, mild cases of nodular bronchiectasis disease may be managed by monitoring alone, while fibro-cavitary disease requires prompt intervention.⁷  Disease severity also influences the management plan - acute infections will usually be treated with three antibiotics, rather than the two recommended for mild to moderate cases.¹

As both severity and infection type can be analysed by CT imaging, HRCT is a powerful tool in tailoring the treatment course to the patient’s needs. 

HRCT has demonstrated some utility in predicting treatment outcomes.⁸ Studies have also shown that HRCT has the potential to serve as a tool to predict the natural, untreated course of MAC-LD in patients.^6,7 This potential application, if incorporated into the clinical setting, has the potential to ease and further personalise the treatment plan for MAC-LD patients.

Summary

High-Resolution CT (HRCT) is a key tool in diagnosing and managing Mycobacterium avium complex lung disease (MAC-LD). It's a detailed imaging technique that can identify specific features of the disease, such as nodules, bronchiectasis and cavities, which can be missed on standard chest X-rays. This level of detail allows doctors to differentiate between forms of MAC-LD, like the slower-progressing nodular bronchiectatic type and the more aggressive fibro-cavitary form, in order to tailor the treatment plan accordingly.

HRCT not only supports diagnosis but also plays a crucial role in monitoring how the disease progresses. It helps healthcare providers evaluate whether the infection is stable, worsening, or improving with treatment. This can guide decisions about planning treatment, including whether aggressive antibiotic therapy is necessary.

While HRCT involves some exposure to radiation, its benefits in managing MAC-LD often outweigh the risks, especially when it comes to accurate diagnosis and timely treatment adjustments.

If you are dealing with MAC-LD, maintaining open communication with your healthcare provider is essential. Ask about what your scan results mean, how frequently you should be monitored, and what treatment options are appropriate for your condition. Staying informed will help you take an active role in managing your lung health and working with your care team toward the best possible outcome.