Chest infections are a frequent cause of morbidity and mortality both in the community and hospital-acquired settings. These infections are generally self-limiting or mild; however, they can become severe in immunocompromised individuals. The most common site of infection is lung parenchyma, with extrapulmonary sites including nodes pleura and chest wall. Tracheobronchial tree is not only the most common route of spread of infection but is the most frequently involved site.
ROLE OF IMAGING
- To confirm the diagnosis of pneumonia
- To detect complications and to look for treatment response
- Help in diagnosing specific pathogen in case of characteristic imaging finding
- For identifying underlying lung disease which may predispose to superadded infection.
APPROACH TO CHEST INFECTIONS
- The diagnostic approach to lower respiratory tract infection requires combined evaluation of clinical, laboratory and imaging features.
- In the clinical evaluation, the background setting is important whether it is community-acquired pneumonia or a hospital-acquired pneumonia; the patient is immunocompetent or immunocompromised; and whether the patient is an adult or a child.
- Immunocompromised individuals are prone to opportunistic infections. Opportunistic infections are caused by microbes which are otherwise4 innocuous in healthy individuals. They are more frequent and severe in immunocompromised than in immunocompetent individuals. Immunosuppression can be secondary to various causes such as congenital, drugs, malignancy, acquired immunodeficiency syndrome (AIDS), etc.
- Proper evaluation based on the above factors allow in narrowing down the differential diagnosis and pinpoint a possible pathogen.
- The final diagnosis depends on isolating the organism. However, it is not always possible to isolate the organism. Hence, clinicoradiological evaluation regarding a possible pathogen allows the initiation of therapy prior to the availability of microbiological reports.
- Histopathology or cytology is generally employed in extrapulmonary sites and in nonresolving pneumonia.
- Imaging is best evaluated using a pattern approach, though overlap of patterns is often observed. Some infections present a pathognomonic feature on imaging allowing a confident prediction of a possible pathogen.
- This book hence, in addition to detailing imaging findings of chest infections resulting from specific organisms, also discusses imaging in various clinical scenarios as well as approach to radiological patterns. While taking the pattern-based approach on imaging chest infections, noninfectious causes of similar appearances should always be borne in mind.
IMAGING MODALITIES
Chest Radiograph
- Chest radiograph is the primary imaging modality used in the evaluation of chest infections, particularly in community-acquired pneumonia.1
- Frontal chest radiograph erect posteroanterior (PA) view being the standard technique, anteroposterior (AP) view is used in those unable to co-operate for a PA view such as in sick patients and small children (Figs 1.1A and B).
- The practice of addition of a lateral view as a part of standard protocol is variable across the world. Currently, lateral view for chest is grossly underutilized mainly because of the rampant usage of computed tomography (CT) scan. There are occasions where lateral view can provide information over and above what the PA view can provide (Figs 1.2A and B). Assessment of lung volume, evaluation of hila and detection of lesions in hidden areas are better performed with lateral view. However, compared with CT scan there is virtually no situation where lateral view outperforms CT; except being significantly low on radiation, the ease of performing it on bed side and less expensive. Role of ancillary chest radiographic views is enlisted in Table 1.1.
Figs. 1.1A and B: Chest radiograph PA view (A) showing multiple ill-defined opacities in bilateral lungs and bilateral hydropneumothorax (thin arrows). Chest tube is seen in the left side pleural cavity.
Figs. 1.2A and B: Chest radiograph lateral view (A) in a different patient showing ”spine sign” (arrow) which is evidenced by increased opacity over the lower dorsal spine suggestive of lower lobe consolidation. Middle lobe is also well visualised in this view
Computed Tomography
- CT scan is a revolutionary modality and the workhorse as far the chest imaging is concerned.
- The spatial resolution of CT scan for lungs is much superior than any other modality. The ability to show it in cross section, removing overlapping structures, fast acquisition enough to freeze motion (recent scanners);6 and ability to show contrast enhancement are some of the major points in its favor.
- One of the major downside of CT scan is the amount of radiation involved in the process, which necessitates its judicious use. There are various indications for performing a CT scan, some of which are shown in Box 1.1.
- In the immunocompromised setting where the interest is in the lung parenchyma evaluation, NCCT is optimal.
- In situations such as mediastinal lymphadenopathy, empyema and mass like lesions evaluation with CECT is necessary.
Figs. 1.3A to D: Standard set of images for chest CT interpretation in a patient with left upper lobe pneumonia. CECT mediastinal window (A), lung window (B), lung window in high resolution kernel (C), bone window (D) showing consolidation in the left upper lobe and a prevascular lymphadenopathy.
Figs. 1.4A and B: Coronal multiplanar reformation (MPR) in lung window (A) and thin minimum intensity projection image (MinIP) (B) of the same patient in Figure 3 showing left upper lobar consolidation. MPR and MinIP images provide information about the craniocaudal distribution of disease and highlighting areas of hyperinflation.
- The routine postcontrast CT of chest is performed during the venous phase and depending on the indication, CT pulmonary angiographic and/or CT thoracic angiographic phase can be added on to it, for instance in patients presenting with hemoptysis.
Ultrasonography
Transthoracic ultrasonography has recently emerged as a useful adjunct to chest radiograph in specific clinical scenarios such as in children and sick patients admitted in intensive care units (ICUs).
Magnetic Resonance Imaging
- Magnetic resonance imaging (MRI) is one of the latest additions to chest imaging armamentarium. Traditionally, MRI was considered as an unsuitable modality, mainly because of its susceptibility to artifacts and low proton density in the lungs. However, with recent improvements in hardware and sequences lung MRI is emerging as an alternative modality, especially in children and pregnant women.
- Most of the lung pathology, especially infections, tends to increase water content in the parenchyma replacing the air leading to increased density of protons. Hence, these areas appear hyperintense on T2-weighted images.
- MRI can show areas of interstitial infiltration, septal thickening and consolidation.
- There are a variety of sequences and their modifications described in the literature. The basic sequences are described in Box 1.3.
- An abbreviated MRI lung study has also been proposed which can be performed in few minutes. This can reduce the MRI scanner time, sedation time and can be used as quick screening tool without radiation penalty. HASTE, TrueFISP, radially acquired T2 WI and volumetric interpolated breath-hold examination (VIBE) are the sequences generally performed as a part of “rapid lung MRI”.3
18F FDG-PET-CT
- 18F fluorodeoxyglucose (FDG)-positron emission tomography-computed tomography (PET-CT) has been in wide usage in oncological diseases and its role in the evaluation of lung infection or inflammation is emerging. Most of the times radiograph or CT scan is sufficient to characterize chest infections. However, in exceptional situations 18F PET-CT can play a supportive role.4
- It has been shown to demonstrate response after treatment in pneumonia such as pneumocystis pneumonia. However, the cost and amount of radiation involved generally discourages its utility as a primary modality to evaluate lung infections.
Figs. 1.5A to E: Lymphadenopathy, pneumonitis and pleural effusion on MRI in a patient with TB. T1W MRI (A), STIR (B) and TrueFISP images (C) showing enlarged lymph nodes (thin arrow) in right paratracheal and right hilar locations. TrueFISP image also shows areas of pneumonitis (thick arrow) and pleural effusion (outlined arrow).
Figs. 1.6A to C: Mediastinal and pleural collection on MRI. Axial T1W image (A), T2 fat saturated image (B) and postcontrast image (C) showing collections in the anterior mediastinum (thin arrows) and in the left pleural cavity (thick arrows). Postcontrast image showing thick enhancement in the walls of the collections.
(FFE: Fast field echo; FIESTA: Fast imaging employing steady-state acquisition; LAVA: Liver acquisition with volume acquisition); THRIVE: T1W high resolution isotropic volume examination; TrueFISP: True fast imaging with steady state precession; TSE: Turbo Spin echo; VIBE: volumetric interpolated breath-hold examination)
Figs. 1.7A and B: 18F FDG-PET in a patient with carcinoma lung. CECT and PET images (A and B) showing a central mass lesion involving the right main bronchus showing avid tracer uptake (thin arrows) and adjacent collapse-consolidation in the right lung (thick arrows) which is showing distinctly different pattern of tracer uptake. PET-CT can reliably differentiate tumor from the adjacent collapsed/consolidated parenchyma.
CONCLUSION
There are number of modalities and techniques available to evaluate chest infections and each of them has their own advantages and disadvantages. Still, radiographs are the primary imaging modality used for screening and preliminary evaluation.11
Figs. 1.8A and B: Mediastinal fibrosis on 18F FDG-PET. CECT and FDG-PET images showing plaque like ill-defined soft tissue seen in the right paratracheal location. There was minimal uptake of tracer in that area and biopsy from the lesion showed fibrosis.
CT is the modality of choice to evaluate complicated lung infections and where radiographs fail to provide desired information. MRI chest is in the process of evolution and currently it can be used in children and pregnant patients. Like anywhere else, diagnosis should be made based on the clinical, radiological and histopathological findings. On imaging, one should follow a systematic approach based on various imaging patterns.
REFERENCES
- Speets AM, van der Graaf Y, Hoes AW, et al. Chest radiography in general practice: indications, diagnostic yield and consequences for patient management. Br J Gen Pract. 2007;56(529):574–8.
- Whiting P, Singatullina N, Rosser JH. Computed tomography of the chest: I. Basic Principles. Contin Educ Anaesth Crit Care Pain. 2015;15(6):299–304.
- Sodhi KS, Khandelwal N, Saxena AK, et al. Rapid lung MRI in children with pulmonary infections: Time to change our diagnostic algorithms. J Magn Reson Imaging. 2016;43(5):1196–206
- Capitanio S, Nordin AJ, Noraini AR, et al. PET/CT in nononcological lung diseases: current applications and future perspectives. Eur Respir Rev. 2016;25(141):247–58.