Since different agents are associated with the occurrence of pulmonary infections at different time points after LT, a subdivision in early (up to 1 month), primary late (1–4 months), and secondary late (after 4 months) infections, according also to the different etiologies, has been applied (Fig. 1) [1, 2].
Early infections (up to 1 month)
Bacterial infections are the most common postoperative infections in LT recipients, especially during the early phase (i.e., 1 month). Nevertheless, it should be considered that they may also occur later on during recipients’ life [8, 9].
Gram-negative bacilli, such as Klebsiella species and Pseudomonas aeruginosa, followed by Gram-positive, like Staphylococcus aureus, are the most frequent agents . Pulmonary Actinomycosis, even if rare, should be considered among the differential diagnoses especially in case of mediastinitis . It has also to be highlighted that recipients with cystic fibrosis carry a higher risk of infections due to Pseudomonas aeruginosa and Burkholderia cepacia complex.
Unfortunately, the CT features of bacterial infections are quite unspecific, since they may include atelectasis, bronchocentric opacities, airspace consolidations (i.e., sub-segmental, segmental, or lobar), branching nodular and linear opacities (i.e., tree-in-bud), interlobular septal thickening, and pleural effusions  (Figs. 2 and 3). Pulmonary Actinomycosis may cause areas of consolidation surrounded by ground-glass opacities (Fig. 4). Staphylococcal pneumonia is frequently complicated by the development of pneumatoceles (Fig. 5).
Candida species often colonize the airways of LT recipients. Very aggressive infections usually occur during the first month after the LT, either as postoperative complications due to a prolonged stay in the intensive care unit or because of a colonized donor organ [11,12,13].
CT patterns of fungal infections include patchy and confluent pulmonary infiltrates, nodules (i.e., occasionally miliary), mass-like foci or airspace consolidation, and interstitial involvement. In case of single LTs, fungal pneumonia mainly affects the transplanted lung (Fig. 6). Very aggressive infections may also cause candidemia, mediastinitis, or necrosis of the anastomosis .
Primary late infections (1–4 months)
Respiratory viral infections are a very important cause of morbidity and mortality in LT recipients .
Cytomegalovirus (CMV) is the second-most common cause of pneumonia and the most common opportunistic infection in LT patients [1, 15], even if it has to be considered that the current prophylaxis with antiviral drugs is reducing its occurrence . Overall, CMV pulmonary infections have a peak incidence at 1–4 months (i.e., range 1–12 months) rarely occurring in the first two weeks after the LT . Primary pulmonary infections due to CMV are usually very severe and may occur in seronegative recipients receiving a graft from a seropositive donor. On the contrary, secondary infections, commonly less severe, are mainly diagnosed in seropositive recipients either infected by a different CMV strain or undergoing a reactivation due to immunosuppression .
In addition to CMV, community-acquired viruses, such as respiratory syncytial, parainfluenza, influenza virus, and adenovirus, may infect LT recipients especially between 2 weeks and 2 years after the LT, with a rate of occurrence between 8 and 14% .
Other less common viral pathogens are herpes simplex (i.e., the incidence is constantly decreasing because of prophylaxis), varicella zoster (i.e., mainly causing muco-cutaneous involvement), and Epstein–Barr virus (i.e., especially important because it may then cause lymphoproliferative disorders) .
CT findings of viral infections include ground-glass, air-space consolidations, tree-in-bud opacities, airway dilatation, bronchial wall thickening, and pleural effusion (Fig. 7) [1, 8, 17]. Severe infections may present with a crazy paving pattern (i.e., ground-glass opacity with superimposed interlobular septal thickening and intra-lobular reticular thickening) (Fig. 8). The CMV pneumonia is also associated with a high risk of superimposed bacterial and fungal infections. Thus, it needs to be taken into consideration that findings due to the additional infection could overlap with the above-mentioned viral features at imaging (Fig. 9).
Aspergillosis is the most common fungal infection in LT recipients [7, 18] and it usually occurs 1–6 months after the LT (i.e., peak incidence within the first 3 months after the LT) .
Ulcerative tracheobronchitis is the earliest and most common expression of the disease, and it may cause anastomotic dehiscence. Its diagnosis is easily reached at CT with the detection of focal bronchial wall defects and perianastomotic air collections (Fig. 10) .
Aspergilloma, necrotizing pneumonia, invasive pulmonary disease, disseminated infection, or empyema may also be due to Aspergillus infection. Invasive pulmonary infections tend to occur in the later phase after LT and are mainly associated with the dissemination to other organs. Typical features at CT imaging of Aspergillus infection include focal nodular and mass-like consolidations, cavitation, nodules (i.e., solitary or multiple) surrounded by a rim of ground-glass opacity (i.e., halo sign), and pleural thickening (Figs. 11 and 12) [1, 7].
Secondary late infections (> 4 months)
Among the bacterial infections occurring in the secondary late period after LT, those due to Mycobacterium tuberculosis are quite frequent (i.e., estimated prevalence between 2 and 3.8%) [1, 8], and are usually caused by the reactivation of a focus in the native lung or by transmission via the allograft.
The colonization by atypical mycobacteria occurs also fairly frequently (0.46–8%) [19,20,21] with Mycobacterium abscessus complex being one of the main agents .
CT findings of typical and atypical species include clusters of multiple small nodules, ground-glass opacities or infiltrates, consolidations, cavitations, interlobular septal and/or pleural thickenings, unilateral or bilateral pleural effusions, and mediastinal enlarged lymph nodes (Figs. 13 and 14) .
Prior to the trimethoprim/sulfamethoxazole prophylaxis, Pneumocystis jirovecii pneumonia (PJP) was diagnosed in around 90% of the LT recipients and the first 6 months after the LT were at very high risk (i.e., highest peak between the second and sixth month). Nowadays, PJP is very uncommon during the first year after LT and it mainly occurs later on .
Typical PJP findings at CT are represented by bilateral ground-glass opacities with sub-pleural sparing and predilection for the upper lobes. Less frequently, focal areas of consolidation and thickening of the interlobular septa or diffuse micro-nodules are evident at imaging (Fig. 15). A cystic form of PJP may also occur and these patients carry a high risk of pneumothorax or pneumo-mediastinum .