ABSTRACT

Idiopathic pulmonary fibrosis is a severe and progressive chronic fibrosing interstitial lung disease, a definitive diagnosis being established by specific combinations of clinical, radiological, and pathological findings. According to current international guidelines, HRCT plays a key role in establishing a diagnosis of usual interstitial pneumonia (UIP). Current guidelines describe three UIP patterns based on HRCT findings: a typical UIP pattern; a pattern designated "possible UIP"; and a pattern designated "inconsistent with UIP", each pattern having important diagnostic implications. A typical UIP pattern on HRCT is highly accurate for the presence of histopathological UIP, being currently considered to be diagnostic of UIP. The remaining patterns require further diagnostic investigation. Other known causes of a UIP pattern include drug-induced interstitial lung disease, chronic hypersensitivity pneumonitis, occupational diseases (e.g., asbestosis), and connective tissue diseases, all of which should be included in the clinical differential diagnosis. Given the importance of CT studies in establishing a diagnosis and the possibility of interobserver variability, the objective of this pictorial essay was to illustrate all three UIP patterns on HRCT.

Keywords: Tomography, X-ray computed; Lung diseases, interstitial; Pulmonary fibrosis.

RESUMO

A fibrose pulmonar idiopática é uma pneumopatia intersticial fibrosante crônica de curso grave e progressivo, e seu diagnóstico se dá em combinações específicas de correlações clínicas e radiológicas, ou ainda patológicas. A TCAR tem papel chave no diagnóstico morfológico do padrão de pneumonia intersticial usual (PIU), segundo as recomendações internacionais vigentes. Os níveis de certeza para a leitura do padrão tomográfico foram inseridos em diretrizes atuais, descritos como padrão PIU típico, padrão PIU possível e padrão inconsistente com PIU, cada qual com importantes implicações na cadeia diagnóstica. A presença do padrão PIU típico tem alta concordância com o padrão PIU histopatológico, e, nessa situação, a TCAR é tida como suficiente para a determinação do padrão morfológico. Nos demais padrões, investigações diagnósticas complementares são indicadas. O diagnóstico diferencial com outras entidades, incluindo pneumopatias intersticiais por exposição medicamentosa, exposições extrínsecas (pneumonite de hipersensibilidade crônica), doenças ocupacionais (asbestose) e doenças do tecido conjuntivo, deve ser considerado clinicamente. Dada a importância da abordagem tomográfica, a qual pode apresentar relevantes discordâncias na avaliação interobservador, nosso objetivo foi ilustrar os padrões tomográficos de PIU neste ensaio pictórico.

Palavras-chave: Tomografia computadorizada por raios X, Doenças pulmonares intersticiais, Fibrose pulmonar.

INTRODUCTION

Idiopathic pulmonary fibrosis (IPF) is a chronic fibrosing interstitial lung disease that is usually progressive. Recently defined diagnostic criteria include exclusion of other known causes of interstitial lung disease, the presence of a usual interstitial pneumonia (UIP) pattern on HRCT, and specific combinations of HRCT and surgical lung biopsy patterns.(1,2)

A typical UIP pattern on HRCT has been shown to be highly accurate for the presence of a UIP pattern on surgical lung biopsy (90-100%); reliable imaging features of UIP are currently considered to be diagnostic of a UIP pattern, without the need for a surgical lung biopsy.(1) Surgical lung biopsy is currently recommended when HRCT findings are not typical of UIP, a definitive diagnosis being established by specific combinations of imaging and histopathological findings.(1) Therefore, correct interpretation of HRCT findings is essential for a definitive diagnosis, particularly in view of the fact that antifibrotic agents have recently been approved for use in the treatment of IPF.(3)

Current guidelines describe three UIP patterns based on HRCT findings: a typical UIP pattern (which eliminates the need for surgical lung biopsy); a pattern designated "possible UIP"; and a pattern designated "inconsistent with UIP", surgical lung biopsy being required in patients presenting with either of the last two patterns.(1,2,4) The objective of the present study was to describe and illustrate the criteria for classifying patients as having a typical UIP pattern, a possible UIP pattern, or an inconsistent with UIP pattern.

HRCT FEATURES CHARACTERIZING UIP PATTERNS

Typical UIP pattern

A typical UIP pattern on HRCT consists of predominantly basal and peripheral reticular opacities and honeycombing, with or without traction bronchiolectasis. In addition, all of the findings that are considered to be inconsistent with UIP must be absent (Figure 1).(1) When all of the aforementioned criteria are met, the findings are considered to be pathognomonic for UIP, eliminating the need for a surgical lung biopsy.(1) There is good interobserver agreement among radiologists for typical UIP findings.(5,6) It is of note that UIP and IPF are not synonyms, known causes of a UIP pattern including drug-induced interstitial lung disease, occupational diseases (e.g., asbestosis), hypersensitivity pneumonitis, and connective tissue diseases.(5)
 



Possible UIP pattern

A possible UIP pattern consists of predominantly basal and peripheral reticular opacities and no honeycombing or any of the findings that are considered to be inconsistent with UIP (Figure 2).(1) A possible UIP pattern is less specific for UIP than is a typical UIP pattern, the main differential diagnosis being with fibrotic nonspecific interstitial pneumonia (NSIP).(5) No honeycombing, extensive ground-glass opacity, subpleural sparing, and lower lobe volume loss are suggestive of NSIP.(7-9) Honeycombing is rare in cases of NSIP, having been found in less than 5% of the patients with idiopathic NSIP investigated in one study.(7)
 



Inconsistent with UIP pattern

Findings that are considered to be inconsistent with UIP include a) longitudinal disease distribution in the middle and upper lung fields; b) peribronchovascular predominance of changes in the axial axis (Figure 3); c) extensive ground-glass opacity, the extent of which is greater than that of reticular opacities; d) bilateral scattered micronodules predominantly in the upper lung fields (Figure 4); e) cysts (multiple, bilateral, away from areas of fibrosis); f) a mosaic perfusion pattern/air trapping (bilateral, in three or more lobes; Figure 5); and g) consolidations. Several of the aforementioned findings are suggestive of chronic hypersensitivity pneumonitis (CHP), further investigation being required for a differential diagnosis.(5)
 

 


 




SPECIAL CONSIDERATIONS

Technical aspects

Technically satisfactory image acquisition is required for a correct diagnosis, minimum technical requirements including a) images acquired at full inhalation, without motion artifacts; b) thin, sequential or volumetric axial images with a reconstruction interval ≤ 2 cm; c) slice thickness ≤ 2 mm; d) use of a high-resolution algorithm; e) a field of view optimized to include only lung parenchyma; f) images acquired during exhalation are useful for defining air trapping; g) use of the prone position in case of uncertainty regarding position-dependent opacities; and h) use of multiplanar reconstructions of volume acquisition CT images.(10)

Inadequately performed inspiratory maneuvers can result in increased/heterogeneous lung attenuation and motion artifacts that can adversely affect CT studies (Figure 6). Suggestions for improving the quality of CT studies include the use of simple, clear instructions on how to perform inspiratory/expiratory maneuvers, patient training in different breathing levels before image acquisition, and the use of rest periods during sequential acquisitions.(10)
 



Diagnosis of honeycombing


A correct CT diagnosis of honeycombing is a crucial step in identifying a typical UIP pattern and establishing a clinical and imaging diagnosis of IPF.(1) However, even among evaluators with extensive experience in interstitial lung disease, there is significant interobserver variability in attempts to detect honeycombing and differentiate it from other findings, such as traction bronchiectasis, cystic disease, and pulmonary emphysema (Figure 7).(11) Diagnostic criteria for honeycombing include predominantly subpleural cysts of 3-10 mm in diameter, sharing relatively thick (1-3 mm) walls and grouped on layers, and the exclusion of emphysema.(11,12)
 



Interobserver agreement: CT criteria for UIP patterns

Walsh et al.(13) evaluated interobserver agreement for the current criteria for a UIP pattern on CT. Interobserver agreement was found to be only moderate for experienced general radiologists and thoracic radiologists, the difficulty in distinguishing among UIP patterns being attributed to discrepancies regarding the presence and distribution of honeycombing.

Atypical patterns and differential diagnosis

Although typical CT findings of UIP can predict a histopathological diagnosis of UIP, they are absent in up to 30% of patients.(14) Sverzellati et al.(14) studied histopathologically confirmed cases of UIP and found that radiologists made an alternative diagnosis in 62% of the cases. The aforementioned study(14) shows that, although CT is highly accurate in diagnosing UIP in typical situations, CT studies should not be used in order to exclude the possibility of UIP. In atypical cases, first-choice diagnoses include NSIP, CHP, sarcoidosis, and chronic organizing pneumonia.

Temporal evolution

The clinical course of IPF is variable and unpredictable at the time of diagnosis; although most patients experience a slow progressive decline, some remain stable, whereas others experience a rapid decline.(1,15) With regard to the severity of HRCT findings, areas of ground-glass attenuation usually progress to reticular opacities, which in turn progress to honeycombing, the extent of which increases over time.(16) It is of note that CT interpretation changes over time, meaning that a possible UIP pattern can progress to a typical UIP pattern (Figure 8).
 



Future directions

The risks of performing a surgical lung biopsy in patients with interstitial lung disease should be taken into consideration; in many cases, diagnosis and treatment are delayed because patient clinical status is a contraindication to biopsy.(17,18) Therefore, there is a growing interest in the noninvasive diagnosis of IPF, particularly in cases of patients presenting with CT findings of possible UIP. Recent studies comparing IPF patients with a typical UIP pattern and those with a possible UIP pattern have shown clinical and functional similarities between the two groups of patients, as well as showing evidence of a comparable response to antifibrotic treatment with nintedanib. (19-21) Several studies investigating patients with IPF have shown high rates of biopsy-proven UIP in those with a possible UIP pattern.(22-25) Age at disease onset and the extent of fibrosis on initial HRCT scans have been significantly related to a high probability of IPF, a possible UIP pattern being suggestive of a clinical and radiological diagnosis of IPF in the following cases: a) typical clinical and demographic presentation (i.e., patients over 60 years of age presenting with dyspnea on exertion and pulmonary fibrosis of indeterminate etiology), as determined by a specialist in interstitial lung diseases; and b) imaging findings of possible UIP, according to a specialist in interstitial imaging.(24,26)

It is of note that some of the studies suggesting that CT findings of possible UIP are sufficient for a diagnosis of IPF derived from clinical trials in which the prevalence of IPF was high, meaning that the results might have been overestimated.(23,24) In a study conducted by Brownell et al.,(25) it was found that a possible UIP pattern is highly specific for UIP on biopsy; however, the positive predictive value of that pattern is directly related to the prevalence of IPF in the study population. Therefore, according to the authors, a possible UIP pattern on HRCT should not be regarded as confirmatory of histopathological UIP in populations in whom the prevalence of IPF is low or indeterminate.(25) Given that the prevalence of CHP is high (i.e., as high as 15%) in Brazil, studies are needed in order to determine the prevalence of IPF in patients with possible UIP before a decision can be made regarding the need for biopsy in such patients.(27)

Noninvasive diagnostic algorithms for CHP have been proposed, including a typical CT pattern, lymphocytosis in BAL fluid (lymphocyte count > 20-30%), and identification of a causal relationship; such algorithms are extremely useful in the diagnosis of fibrotic interstitial lung diseases, given that the differential diagnosis between CHP and IPF is often difficult.(27,28)

FINAL CONSIDERATIONS

Of all idiopathic interstitial lung diseases, IPF is the most common; it has a poor prognosis in most cases, and the histopathological substrate of IPF is UIP.(29) In a recent review of the diagnostic algorithm for IPF, HRCT was shown to play an indispensable role in characterizing UIP, typical findings being diagnostic of UIP and atypical findings requiring histopathological analysis.(1) For a definitive diagnosis, radiologists must be familiar with all UIP patterns and must be able to describe them accurately when writing radiological reports or participating in multidisciplinary meetings, particularly in view of current perspectives on the treatment of IPF, with the use of antifibrotic agents.

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