Idiopathic Pulmonary Fibrosis Imaging 

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Idiopathic pulmonary fibrosis (IPF) is defined as a specific form of chronic, progressive fibrosing interstitial pneumonia of unknown etiology, occurring primarily in older adults, limited to the lungs, and associated with the histopathologic and/or radiologic pattern of usual interstitial pneumonia (UIP). It is characterized by progressive worsening of dyspnea and lung function and is associated with a poor prognosis; most patients die of respiratory failure. ^[1]  The mean survival is approximately 4 years.

According to the American Thoracic Society (ATS), European Respiratory Society (ERS), Japanese Respiratory Society (JRS), and Latin American Thoracic Association (ALAT) evidence-based guidelines for diagnosis and management of IPF,  the criteria for diagnosis are as follows ^[1] : 

Radiologic characteristics of pulmonary fibrosis appear in the image below.

The diagnosis is confirmed with a lung biopsy, but the histology shows striking variation from one region to the next (ie, the disease is characterized by histologic temporal and spatial heterogeneity). It is not unusual to find areas of normal lung next to areas with severe thickening of alveolar walls. Therefore, findings on bronchoscopic or percutaneous lung biopsy are difficult to interpret. Open lung biopsy and video-assisted thoracoscopic lung biopsy are the preferred methods.

IPF usually affects patients 50-70 years of age. Most series report a male preponderance, with a male-to-female ratio of 2:1. Clinical features consist of progressive exertional dyspnea; the presence of interstitial infiltrates, as evidenced on chest radiographs; and physiologic evidence of restriction and impaired gas exchange on pulmonary function testing.

Patients are generally treated with corticosteroids, other immunosuppressants, or both.

The diagnosis of IPF is made on the basis of the patient’s history, clinical findings, pulmonary physiology, and imaging results. The diagnosis is one of exclusion. Nonidiopathic causes must be excluded first because of the important therapeutic implications. After nonidiopathic causes are excluded, further investigation of patients with IPF typically reveals radiographic abnormalities and restrictive lung physiology with decreased diffusion capacity. ^{[2, 3, 4]}

Plain chest radiography is usually the first investigation performed for patients with suspected interstitial lung disease. However, the findings on conventional radiography are highly nonspecific.

High-resolution computed tomography (HRCT) scanning defines the underlying lung parenchymal abnormalities better than plain radiography. ^{[5, 6]}  Studies have shown that HRCT may obviate surgical lung biopsy in some patients. Raghu et al compared the diagnostic accuracy of clinical evaluation in combination with HRCT with the accuracy of histology of surgical lung-biopsy samples. ^[7] Clinical assessment in conjunction with careful review of HRCT scans was 60% sensitive and 97% specific for IPF. However, although HRCT may obviate the need for tissue diagnosis in 60% of patients, surgical lung biopsy is still needed in 40%.

For diagnoses other than IPF, a combination of clinical assessment and HRCT is neither sensitive nor specific enough to be relied on without surgical biopsy. Open lung biopsy remains the criterion standard. In immunocompetent patients, the benefit is relatively low because corticosteroid therapy is frequently administered after biopsy. In immunocompromised patients, approaches to therapy change substantially after tissue confirmation, but mortality is high. Therefore, open biopsy should be performed only in patients in whom the diagnosis is likely to change therapy and in patients who have a reasonable prognosis.

Radionuclide scanning with gallium-67 may depict interstitial fibrosis and may show changes early. This feature may be of therapeutic benefit, but the changes are nonspecific and do not remove the need for lung biopsy.

The radiographic pattern differs with the stage of the disease. Early in the disease, the most common radiographic changes are an interstitial shadowing of small (1 to 2 mm), irregular opacities, which are seen in about 75% of patients. Less common are small, round opacities, which are seen in 20% of patients. This finding is generally known as reticulonodular opacities. Septal lines are occasionally observed. The distribution is predominantly basal. (See the image below.)

Peripheral accentuation is also a common feature, but it is more easily appreciated on CT scans than on plain chest radiographs.

The pattern is usually symmetrical. Another common pattern is hazy, ground-glass opacification, which is either diffuse or patchy. Volume loss and a raised diaphragm are seen in up to 60% of patients. This may be accompanied by basal discoid atelectasis.

Pleural disease is not typical of IPF. Its presence should raise the possibility of other conditions, such as asbestosis, rheumatoid pulmonary disease, or systemic lupus. Pneumothorax, pneumomediastinum, or both have been reported in a few patients; these conditions have been associated with bullae in the lung parenchyma.

With progression of alveolitis to fibrosis, the initial fine lines become coarse, and small (2 mm) cysts appear. These cysts coalesce and increase to 5-7 mm in diameter; they appear as ring opacities within the honeycomb lung. As fibrosis worsens, the honeycombing becomes coarser with larger honeycomb cysts, and further volume loss occurs. In advanced stages, there is radiographic evidence of pulmonary arterial hypertension.

The radiographic findings are not correlated with the stage of the disease, the histology, the respiratory symptoms, the respiratory function tests, or the prognosis.

In the majority of patients with IPF, the chest radiograph is abnormal at presentation; previous radiographs often will have shown reticular shadowing, even before symptom development. ^[8] Chest radiography is frequently the first investigation employed for patients with IPF; physiologic testing and HRCT scanning follow.

For symptomatic patients in whom the diffusion capacity is abnormal, results of chest radiography may be normal. For other patients, the radiographic appearances are abnormal before clinical symptoms appear. Results of HRCT scanning are abnormal for most patients with IPF.

For patients with IPF, HRCT scan findings may be used to predict outcomes and to guide the treatment, because the findings are well correlated with the histologic pattern of IPF (see the images below). The accuracy of the diagnosis of IPF is significantly increased with HRCT, as compared with chest radiography. When a trained observer performs HRCT, the accuracy of the diagnosis is reported to be about 90%. ^[9] ​ One third of all cases of IPF are missed on HRCT; a confident diagnosis of IPF is made in about two thirds of cases. ^[9] On HRCT, end-stage lung disease is characterized by honeycombing without ground-glass attenuation in typical distribution; with such findings on HRCT, the diagnosis may be made with confidence. This spares patients the risk of invasive diagnostic processes, such as a lung biopsy. In the active stage, scans demonstrate ground-glass attenuations. The active stage of the disease, which is characterized by active alveolitis, is potentially reversible and potentially amenable to treatment, unlike end-stage disease, which is irreversible. ^{[10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]}

On HRCT, IPF is commonly characterized by patchy and predominantly peripheral, subpleural, and bibasilar reticular opacities. The distribution is predominantly posterior. It is often associated with traction bronchiectasis and subpleural honeycombing.

Evidence has been found to suggest that alveolar collapse may precede lung fibrosis in IPF, potentially aiding in earlier diagnosis. ^{[22, 23]}  

Ground-glass attenuations are relatively uncommon; they usually progress to the more common reticular attenuations and honeycombing. HRCT scans have been reported to show honeycombing in 90% of patients with IPF. In the absence of honeycombing, the extent of reticular and ground-glass densities can predict a diagnosis of IPF. The probability of IPF exceeds 80% in patients older than 60 years, with one third having reticular densities. ^[24]

In cases of suspected IPF in which lung HRCT shows more than 30% ground-glass attenuation, consideration should be given to other diagnoses; alternative diagnoses include desquamative interstitial pneumonitis, idiopathic bronchiolitis obliterans organizing pneumonia, respiratory bronchiolitis–associated interstitial lung disease, hypersensitivity pneumonitis, and nonspecific interstitial pneumonia.

Pulmonary artery size measured on HRCT has been studied as an outcome predictor in IPF.  In a study of 98 IPF patients, pulmonary artery and ascending aorta diameters were measured from chest HRCT with pulmonary artery:ascending aorta diameter (PA:A) ratio calculations. Patients with a PA:A ratio >1 had higher risk of death or transplant compared with patients with a PA:A ratio ≤1 (P< 0.001). A PA:A ratio >1 was also an independent predictor of outcomes in unadjusted and adjusted outcomes analyses (hazard ratio 3.99, P< 0.001, and hazard ratio 3.35, P=0.002, respectively). ^[25]

In cases of IPF, perfusion lung scintigraphy shows nonspecific, subsegmental mismatched perfusion defects. These are not correlated with clinical severity.

Gallium-67 imaging has not proven to be of value in cases of established IPF. ^[26]

Technetium-99m diethylenetriamine penta-acetic acid (DTPA) is cleared more rapidly when capillary permeability is increased than when it is not, and the findings may provide an index of lung inflammation. ^[27] Fluorodeoxyglucose (FDG) positron-emission tomography (PET) may show FDG accumulation in the lung bases; such findings correlate with the honeycomb fibrosis seen on high-resolution HRCT. ^{[28, 29, 30, 31, 32]}

Win et al studied 13 patients with IPF recruited for 2 thoracic ¹⁸F FDG-PET studies performed within 2 weeks of each other. All patients were diagnosed with IPF in consensus at multidisciplinary meetings because of typical clinical, HRCT, and pulmonary function test features. The purpose of the study twas o investigate the reproducibility of pulmonary ¹⁸F FDG-PET in patients with IPF. This study demonstrated that there is excellent short-term reproducibility in pulmonary ¹⁸F FDG uptake in patients with IPF. ^[33]

Overview

What is idiopathic pulmonary fibrosis (IPF)?

What are the diagnostic criteria for idiopathic pulmonary fibrosis (IPF)?

How is the diagnosis of idiopathic pulmonary fibrosis (IPF) confirmed?

Which patient groups are at highest risk for idiopathic pulmonary fibrosis (IPF)?

Which medications are used to treat idiopathic pulmonary fibrosis (IPF)?

How is idiopathic pulmonary fibrosis (IPF) diagnosed?

What is the role of radiography in the workup of idiopathic pulmonary fibrosis (IPF)?

How accurate are radiographic findings in the diagnosis of idiopathic pulmonary fibrosis (IPF)?

What is the role of CT scan in the workup of idiopathic pulmonary fibrosis (IPF)?

What is the role of nuclear imaging in the workup of idiopathic pulmonary fibrosis (IPF)?

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Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR Consultant Radiologist and Honorary Professor, North Manchester General Hospital Pennine Acute NHS Trust, UK

Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR is a member of the following medical societies: American Association for the Advancement of Science, American Institute of Ultrasound in Medicine, British Medical Association, Royal College of Physicians and Surgeons of the United States, British Society of Interventional Radiology, Royal College of Physicians, Royal College of Radiologists, Royal College of Surgeons of England

Disclosure: Nothing to disclose.

Klaus L Irion, MD, PhD Consulting Staff, The Cardiothoracic Centre Liverpool NHS Trust, The Royal Liverpool University Hospital, UK

Klaus L Irion, MD, PhD is a member of the following medical societies: American Roentgen Ray Society, Radiological Society of North America

Disclosure: Nothing to disclose.

Anitha James, MBBS, DMRD, FRCR Specialist Registrar, Manchester Radiology Training Scheme, Hospitals NHS Trust, UK

Disclosure: Nothing to disclose.

Bernard D Coombs, MB, ChB, PhD Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand

Disclosure: Nothing to disclose.

Eugene C Lin, MD Attending Radiologist, Teaching Coordinator for Cardiac Imaging, Radiology Residency Program, Virginia Mason Medical Center; Clinical Assistant Professor of Radiology, University of Washington School of Medicine

Eugene C Lin, MD is a member of the following medical societies: American College of Nuclear Medicine, American College of Radiology, Radiological Society of North America, Society of Nuclear Medicine and Molecular Imaging

Disclosure: Nothing to disclose.

Jeffrey A Miller, MD Associate Adjunct Professor of Clinical Radiology, University of Medicine and Dentistry of New Jersey-New Jersey Medical School; Faculty, Department of Radiology, Veterans Affairs of New Jersey Health Care System

Jeffrey A Miller, MD is a member of the following medical societies: American Roentgen Ray Society, Radiology Alliance for Health Services Research, Society of Thoracic Radiology

Disclosure: Nothing to disclose.

Alberto Alonso, MD, MRCP Specialist Registrar in Radiology, Department of Radiology, Manchester Royal Infirmary, UK

Alberto Alonso, MD, MRCP is a member of the following medical societies: Radiological Society of North America, Royal College of Physicians, and Royal College of Radiologists

Disclosure: Nothing to disclose.

William Musda, MBBS Specialist Registrar, Diagnostic Radiology, Manchester Radiology Training Sceme, UK

Disclosure: Nothing to disclose.

Velauthan Rudralingam, MB, BCh, BAO, FRCS, FRCR Staff Physician, Gastrointestinal and Body Imaging Block, Hope Hospital and Wytenshawe Hospital, UK

Velauthan Rudralingam, MB, BCh, BAO, FRCS, FRCR is a member of the following medical societies: British Medical Association and Radiological Society of North America

Disclosure: Nothing to disclose.

Idiopathic Pulmonary Fibrosis Imaging 

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