Ventilation Imaging: a New Emphasis on Chest Imaging

Abstract

Imaging of the chest is almost as old as the discovery of X-rays. Traditionally, imaging of the chest and lungs relied on projection imaging (with use of a fluorescent screen and radiographs). Since the introduction of body computed tomography (CT) in the 1970s, the lungs have been imaged using both standard and high-resolution CT. It has been shown that CT is much more sensitive for detection of lung diseases (1, 2). Although this has led to significant improvements in the diagnosis of lung diseases, the imaging was (and to a large extent remains) predominantly static and aimed at morphology rather than function. However, even CT is increasingly being applied to gain more information on functional parameters, such as ventilation and perfusion (and their interaction), as will be described in some of the other articles in this volume. Why is the focus returning to lung imaging, and the use of imaging to improve functional analysis? First, we would have to look at the epidemiology of lung diseases. Currently, chronic obstructive lung diseases are one of the four leading causes of morbidity and mortality and outpacing heart diseases and cancer in increasing prevalence (3). Recent data show that between 5 and 15% of adults in industrialized countries suffer from respiratory disorders, which are characterized by a gradually progressive reduction of maximum expiratory flow and delayed emptying of the lungs as a result of a combination of airways disease and parenchymal destruction (emphysema) (4). And it is not just COPD that is causing problems. A recent statistical report from the British Thoracic Society showed that lung disease now kills one in four people, which is more than coronary heart disease (5). It is also the most common cause of emergency admissions and costs the NHS more than any other disease (5). Many of these diseases are either related to smoking (as are emphysema and lung cancer) or are industry/environmental in aetiology (such as lung fibrosis, asthma and asbestosis). Another issue is how lung diseases are diagnosed and managed. Most commonly, lung diseases are diagnosed based on spirometry and, in a subgroup of patients, with the addition of (high-resolution) CT. However, both methods have their limitations. Spirometry is dependent on a number of factors, such as the cooperation of the patient, training and motivation of personnel and is only capable of giving an overall picture of the entire ventilatory system. Thus, it lacks spatial resolution and carries significant intraand inter-patient diversity. Computed tomography overcomes the issue of spatial resolution, but is virtually incapable of yielding functional information. he management of most lung diseases relies almost exclusively on spirometry, which has been used for many years and has resulted in a certain sense of understanding by respiratory physicians. Most recently, this has been emphasized in the GOLD guidelines (6). Thus, bronchodilators and immunosuppression have remained the mainstay of treatment for most lung diseases, such as COPD and asthma. Novel insights in drug delivery may have a significant role to play in the near future. For instance, the lungs are increasingly recognized as a potentially interesting route of drug delivery, such as inhaled insulin, because it avoids the hepatic metabolic pathway and may better simulate the physiological response to a food bolus. This could lead to a reduction in required dose to achieve similar effects, while the doses also become independent of liver metabolism, which may be changeable and is a source of drug interactions (enzyme induction). Furthermore, individualized drug delivery is another source requiring novel insights and assessment of lung ventilation. So how can ventilation be imaged? Several methods have been tested, and these will be discussed in this issue. Lung scintigraphy with a variety of agents has been possible for over 25 years. Aerosols with labelled compounds, such as 99m Technetium DTPA, have been used extensively. They have proven to be safe, albeit that the droplets have a propensity for central airway accumulation in patients with a significant inflammatory (mucus production) component. This problem was overcome by the M R I 0 2 8 B Dispatch: 26.10.04 Journal: MRI CE: Hari