Introduction

What is fibrosing mediastinitis?

Fibrosing mediastinitis (FM) is a rare and progressive disorder involving the growth of fibrous tissue within the mediastinum (the portion of the chest between both lungs) and hilar regions (the area between the upper and lower lobes of each lung). Although benign, the growth of fibrous tissue can cause narrowing of the bronchovascular structures, such as pulmonary arteries and veins, superior vena cava and the airways.

Some of the known causes of FM include:¹

• Idiopathic - unknown cause
• Histoplasma capsulatum- a species of dimorphic fungus commonly found in the United States
• Mycobacterium tuberculosis – the bacteria involved in tuberculosis
• Sarcoidosis
• Radiation therapy
• Drugs such as methysergide therapy

There are two main types:

• Focal: the more common subtype associated with abnormal immune response to Histoplasma capsulatum infection. These individuals are typically young; however, it can affect any age
• Diffuse: thought to be a more idiopathic form associated with autoimmunity, usually seen in middle/elderly patients and is more commonly reported in men

Early diagnosis of FM is important for several reasons. Firstly, FM can progressively worsen over time causing complications from the compression of vital structures in the chest cavity. Early detection can prevent complications from becoming severe. This can also help develop a long-term treatment plan.

Imaging allows clinicians to visualise the extent of FM non-invasively. It gives the precise location and how it’s affecting the surrounding structures. This indicates the full impact of the disease and allows the planning of interventions and monitoring of the disease progression over time. Without imaging, diagnosing fibrosing mediastinitis would be far more challenging, and treatment would likely be less targeted and effective.

Understanding commonly used terms

These are medical terms associated with FM:

• Fibrosis – thickening or scarring of tissue
• Enlarged lymph nodes – a part of the lymph system which allows for the movement of lymph fluid around the body. The nodes work to filter foreign substances from the body and also contain immune cells that help our body fight infections. During times of infection or malignancy, these nodes can swell and appear enlarged
• Mediastinal – the region between the two lungs
• Hilar – where the nerves and blood vessels from surrounding regions attach to an organ
• Calcification – the build of calcium in a region which causes tissue hardening
• Histoplasmosis- an infection caused by inhaling fungus spores
• Tracheobronchial narrowing – narrowing of the trachea, bronchi and bronchioles (regions that transport air from the environment to the lungs)
• Endobronchial – lining of the bronchi
• Atelectasis – collapsing of partial or whole lung
• Pleural thickening –  thickening of the tissue around the lungs
• Pulmonary granuloma – a lump of immune cells found in the lungs, in response to areas of infection or inflammation

Imaging techniques used in diagnosis

Chest X-ray

During a radiographic scan, an X-ray or gamma-ray is passed through the body to visualise the internal structure of the body. Depending on the structure’s density and composition, some of the rays are absorbed, while others are scattered. In dense tissue such as bones, more rays are absorbed so they appear white in the final image, less dense tissues, such as organs, appear in shades of grey. The rays that pass through the object are captured by a detector behind. The results are a two-dimensional image.

FM can appear as a subtle widening of the mediastinum and the distortion of the usual distinct mediastinal lines. Hilar calcification may also be apparent in the mediastinal, which is common in the localised form of FM.

Computed tomography (CT) scan

CT scans operate similarly to X-rays, however, the rays are in a rotating tube which allows for a three-dimensional imaging of the area. If you have FM, a CT scan may show a mass or lump near, and occasionally far from, the lungs indicating the disease. In more extensive forms, the normal fat around the heart might be replaced by softer tissue. This tissue can invade or wrap around the nearby structures. Other widely reported features are calcium deposits within the mass, or in the nearest lymph nodes; severe narrowing of the airways; "hazy" areas on the lung scan due to blocked air passages; and spots where the lung has collapsed because of blockage related to the lymphatics. FM may also induce thickening and calcification of the chest lining as well as scars or tiny lumps in the lungs from previous infections, such as tuberculosis or Histoplasmosis. Also, arteries can be enlarged, due to poor blood flow, and the patterns of lung tissue may be changed by compression of lymphatic pathways.²

Magnetic Resonance Imaging (MRI)

An MRI machine contains a powerful magnet that creates a strong magnetic field. The magnetic field causes protons, which are abundant in water and fat found in the body, to align with the direction of the magnetic field. The MRI machine then sends out a pulse of radio waves to the specific region being scanned, this temporarily disorientates the alignments of the protons in this region. After, the radio signal is switched off and the protons in this area return to their original orientation. As this happens they emit radio signals, which are detected by a coil placed around the body area being scanned. Different tissues realign at different rates, creating contrast in the image. MRI scans are generally considered safer than X-rays and CT scans as they don’t use radiation. It also allows for highly detailed images of soft tissue specifically. The characteristics of FM on an MRI are similar to those found on a CT scan.   

Positron emission tomography (PET) scan

Before undergoing a PET scan, a radioactive form of glucose is injected into the body. Glucose is necessary for the production of energy in the body. Cells that require greater energy consumption, such as cancer cells, will take up more of the radioactive glucose. The tracer emits particles called positrons as it’s taken up by cells. When the positrons collide with electrons found in the body, a gamma ray is emitted. The computer system then processes these signals and can detect where the tracer has accumulated in the body, based on the levels of gamma rays being emitted.

Although PET scans are primarily used for visualising and diagnosing tumours due to their high energy demand, there is some use in disorders with a high inflammatory content as these regions also have a high metabolic demand. In the diagnosis and tracking of disease development in FM, a PET scan is often combined with a CT scan.

Echocardiogram

An echocardiogram is a form of ultrasound that uses sound waves to take images of the heart and surrounding regions. This technique allows for structure and function visualisation in this region, in real time. The device emits high-frequency sound waves which are not audible to humans. As the sound waves travel through the body they bounce off structures and tissues, these returning echoes are captured by the transducer. This is converted to moving images of the region which appear on a screen. This tool can help clinicians visualise the impact of FM on the heart and blood vessels, for example, the narrowing of blood vessels to the heart that can be caused by FM.

Comparison of imaging techniques

The strengths and weaknesses of each imaging technique are assessed by doctors when determining which technique to use for FM. Some imaging options like CT scans provide detailed, cross-sectional images. They can show the amount of fibrous materials and their effects on local tissue. However, CT scans or any type of X-ray also expose patients to ionising radiation, which can be harmful to health over time. This is often a consideration when using CT scans regularly.

MRI provides excellent soft tissue contrast and does not use radiation. It is therefore a safer option for long-term monitoring. However, MRI is less available and more expensive. The addition of PET scans (a test looking for inflammation in the body using a radioactive label) can help determine the level of active inflammation. Furthermore, both availability and expense can limit the acceptance of PET imaging as a routine measure for myositis patients. There are also some differences in patient comfort. Some people feel uncomfortable as MRI machines are noisy and because patients need to enter head first and remain still inside the machine. CT scans are quicker and generally more comfortable, but radiation exposure remains an issue. The cost and availability of these imaging techniques can vary significantly, this influences the accessibility and affordability of the scan for the patient. The use of multiple imaging techniques is often used in FM. By combining the strengths of different imaging techniques, such as the detailed anatomical images from CT or MRI with the functional insights from PET, clinicians can get a comprehensive understanding of the disease. This approach allows for more accurate diagnosis, better monitoring of disease progression, and more informed treatment decisions.

Conclusion

In summary, imaging techniques are essential for diagnosing and managing FM. Several imaging modalities—chest X-ray, CT scan, MRI, PET scan, and echocardiogram—provide diverse information on the extent and impact of FM on neighbouring structures. Each method comes with its pros and cons. The selection of an imaging technique will depend on factors like the exact clinical situation, patient comfort and resource availability. Holistic evaluation of FM necessitates the combined use of advanced imaging techniques. This underscores that FM requires a multidisciplinary management approach.