Tomosynthesis in pulmonary cystic fibrosis with comparison to radiography and computed tomography: a pictorial review

Tomosynthesis images were acquired on a commercially available product (Definium 8000 and VolumeRAD; GE Healthcare, Chalfont St Giles, England). In chest tomosynthesis, tomographic images are generated from multiple low dose exposures directed towards a stationary digital amorphous silicon flat-panel detector, from tube angles of −17.5° to +17.5° of a moving X-ray tube. These exposures are used to reconstruct up to approximately 60 coronal sectional images with a nominal thickness of 3 mm for children and 4 mm for adults, without overlap. The structures included in each sectional image are sharply depicted, while structures located anteriorly or posteriorly to the section are blurred (Fig. 1) [13]. An antero-posterior (AP) chest radiograph is always automatically included in the tomosynthesis examination. Chest tomosynthesis is performed under breath hold in inspiration for a scan time of approximately 10 s. As small children unfortunately cannot cooperate adequately, tomosynthesis is currently used in children from about 8 years of age. Adults and teenagers are examined upright. Children between about 8 and 12 years are examined supine, which makes it easier for them to be still and concentrate on holding their breath, thus reducing motion artefacts.

Compared to CT, tomosynthesis has a higher spatial resolution in the imaging (coronal) plane since the full resolution of the detector can be utilised (2,022 × 2,022 pixels), while a coronal CT MPR section has a matrix size of 512 × at most 512 pixels. However, the depth resolution is inferior to CT, since the tomosynthesis sections have non-precise thickness, with a sharper image at the centre and superimposed adjacent anatomical structures that become increasingly blurred with increasing distance from the section. Consequently objects such as intravenous lines, tubes or other devices generate artefacts in the adjacent sections, which may obscure the lung parenchyma. In addition, tomosynthesis does not have CT’s inherent ability of multiplanar imaging.

All images were selected, after informed consent, from a prospective study population on CF patients, which was approved by the local ethics committee. At the time of this pictorial essay, the population included 36 children from 8 to 18 years with a total of 92 tomosynthesis exams and 39 adults (19 to 59 years) with a total of 43 tomosynthesis exams. When the patients were examined with radiography as part of a clinical control, the examination was supplemented with tomosynthesis. In 125 cases the reason was a yearly check-up and 10 patients were examined due to an exacerbation of the disease. During the study period seven HRCTs and three contrast-enhanced CTs of the chest were performed in seven of the paediatric patients for clinical reasons. In the adult group one HRCT and one contrast-enhanced CT of the chest were performed in two patients. From the clinical files a number of cases of cystic fibrosis with typical findings were selected. In all cases corresponding radiographs are shown, and in a number of cases sections from chest CT close in time have been selected for comparison.

The lungs of newborn children with CF are almost normal, but they are prone to bacterial infection, which triggers an inflammatory response. The “vicious circle” of infection, impaired mucociliary clearance, inflammation, bronchial obstruction and tissue damage is well documented [14]. Inflammation and squamous metaplasia lead to thickening of the bronchial walls, which reduces the size of the small airways causing airway obstruction. The excessive mucus production and airway impaction together with the weakening of airway walls, secondary to infection and inflammation, lead to the development of bronchiectases [15].

Common radiologic pulmonary changes seen in CF are mucus plugging, bronchial wall thickening and peribronchial changes, bronchiectases, overinflation and air trapping, atelectases, and consolidation of lung parenchyma [16, 17]. In particular mucus plugging, bronchial wall thickening and bronchiectases are better delineated with tomosynthesis compared to radiography (see Fig. 2 above).

In mild disease, bronchial wall thickening, discrete cylindrical bronchiectases and small mucus plugs can be seen. With advancing disease the bronchiectases increase in number and gradually become more dilated, and large mucus plugs can be seen in the dilated bronchi. Pneumonia, small abscesses and atelectases can also develop.

In the normal healthy lung, the bronchial walls are visible in the central lung with tomosynthesis but not in the periphery (Fig. 1). The vascular tree is depicted in more detail than with radiography, and vascular branches can be seen as far as approximately 1 cm from the pleura.

Small mucus plugs are usually not visible on radiographs but can be seen as a tree-in-bud pattern or as small clustered nodules in the periphery of the lobes with tomosynthesis (Figs. 2b and 3), with a similar appearance on coronal CT scans. Large mucus plugs can be seen as blurred nodular or cylindrical shadows on radiographs, but with tomosynthesis are sharply depicted inside the bronchi (Fig. 4). When the bronchi are thickened, they are clearly delineated with tomosynthesis, not only centrally, but also in the peripheral lung (Figs. 4 and 5). Bronchiectases may be clearly visualised with bronchography and CT, but are often difficult to assess with radiography. Radiographs depict suspicious bronchiectases as increased linear markings or blurred cystic lesions; with tomosynthesis it is possible to differentiate them in detail as cylindrical, varicose or cystic bronchiectases (Figs. 6, 7, 8 and 9). When the examination is performed upright air-fluid levels might be observed in the bronchiectases (Figs. 4c, 7b and 9b) or in abscesses, and small air-fluid levels seen on tomosynthesis are especially easy to miss on radiographs. In our patient group, no CF patient has yet presented with pneumothorax, but in other patients a small lateral or apical pneumothorax has been better visualised with tomosynthesis than with radiography. Localised air trapping and mosaic pattern, which often is well assessed with CT, cannot be adequately evaluated with tomosynthesis. These findings can be suspected with tomosynthesis when there is an absence of vascular markings or an area of hyperlucent lung. Bullae are more clearly delineated with tomosynthesis than with radiography, but can be seen in more detail with CT.

As tomosynthesis depicts the lungs in the coronal plane with contiguous slices, it gives a better overview of the bronchial tree (Fig. 10a–c) compared to traditional axial high resolution CT (HRCT) images with 10 mm interspace (Fig. 10d). However, most centres now have multidetector CT scanners, and traditional HRCT examinations have been replaced by multiplanar reconstructions of thin contiguous slices. In our opinion sequential studies are easier to compare when the whole volume of the lungs is included in the tomosynthesis examination or in the volume CT of the lungs.