Chest Imaging: Ultrasound

Although many studies have advocated a valuable role for thoracic MRI, it has currently limited clinical utilization with the exception of cardiovascular imaging. However, new technical developments and MRI sequences have continuously improved the quality and broadened the clinical indications for thoracic MRI. Furthermore, due to its high soft tissue contrast and the lack of radiation exposure, MRI allows for repeated measurements of the lung structures and, therefore, appears to be appropriate for functional investigation of lung. The purpose of this presentation is to review the currently available MR techniques useful in thoracic imaging and to provide an overview of present and emerging clinical applications of thoracic MRI.

Ultrasound has historically been used to determine the presence of pleural fluid prior to a decision on chest drain insertion. More recently it has been used to characterise pleural effusions, and is now being investigated as a potential tool to identify cardiac failure and pulmonary oedema. It is now also used to identify pneumothorax in patients in place of or more often as an adjunct to the CXR. Recent publications have also highlighted the ability to identify the intercostal vessels using colour Doppler to avoid potential laceration during biopsy or drain insertion.

The imaging of the chest in paediatrics needs a specific cultural approach planned to integrate and optimise the techniques available. The prevalence of air represents a particular challenge for the radiologist. It limits the intrathoracic ultrasound (interfaces between soft tissue and lung generate very strong echoes due to a large acoustic impedance gradient), is a problem for the MRI (low proton density and the susceptibility differences between tissue and air), determines an excellent contrast resolution for the conventional radiology and CT that remains the gold standard. Technically, CT is conditioned by a cost/benefit ratio that means dose/diagnosis relationship. Risk of chest PA and lateral is negligible. Classical indications for chest US (in the presence of an acoustic window) are opaque hemi-thorax, assessment of vascular abnormalities, evaluation of diaphragmatic motion and juxta-diaphragmatic processes, detection characterisation of a suspected mediastinal disease, evaluation of chest wall lesions, to confirm and characterise pleural effusions guiding for pleural drainage procedures. New extensive use of sonography as clinical portable tool takes information also from physical acoustic phenomena that are not directly convertible into images of the human body. These artefacts are non-anatomical images, which are at best a sensitive but, unfortunately, a very nonspecific sign of lung injury common to many conditions. Young paediatric radiologists can easily learn the traditional chest ultrasound and they should know strengths and weaknesses of “new” imaging.

In the last two decades, lung ultrasound (LUS) has become popular both in adult and children for the clinical evaluation of pulmonary diseases. In neonatal and paediatric age, LUS has been utilized by the clinician as point of care tool to address the decision making process. LUS features of the most common neonatal respiratory diseases (respiratory distress syndrome, transient tachypnoea of the newborn, meconium aspiration, pneumothorax, pleural effusion, pulmonary haemorrhage) as well as its functional application on the respiratory therapy will be presented.

Radiation dose in CT scan must be as low as reasonably achievable. High natural contrast of lung parenchyma makes it particularly suitable for radiation dose reduction. Various technical parameters have to be optimized, especially tube current, tube potential, and reconstruction algorithm, to obtain the better trade-off between radiation dose level and image quality. Iterative reconstruction algorithms take currently a dominant role in radiation reduction by lowering image noise induced by reduced-dose examinations. On the other hand, reduced-dose CT protocols should be adapted to the clinical situation. Their relevancy have been largely investigated for solid nodules detection on non-enhanced CT and for pulmonary emboli diagnosis on CT pulmonary angiography. Other potential applications include infectious pneumonia and pleural abnormalities detection (especially in asbestos-exposed workers) and neoplasia follow-up. However, radiation dose reduction is still limited for obese patients and in case of interstitial pneumonia.