Computed tomography (CT) plays a pivotal role in the assessment of lung pathology. It has been the mainstay of pulmonary diagnosis for many years, and several factors are at the heart of this position. CT has exquisite spatial and density resolution. With the introduction of more advanced multi-detector scanners (with up to 64 slices reconstructed per rotation), it is now possible to image the full chest with isotropic voxels on the order of 0.4 mm on a side in under 10 s. Reconstructed attenuation values (i.e. Houndsfield units or HU) are linearly related in the range of water and dense bone and, important to the quantitative assessment of function, are proportional to the amount of contrast agent delivered to a lung region. While there are new methods emerging to image regional ventilation and perfusion via use of hyperpolarized gas in conjunction with MRI (1–9), the ability to image detailed anatomy of the lung along with the well characterized nature of reconstructed attenuation values in CT serves to place CT as the current gold standard for the assessment of lung structure and function. CT is easily available in every hospital and very cost-competitive compared with (sometimes more invasive) other techniques. With a full chest CT scan being obtainable in under 10 s, the studies are well tolerated by most patients, even if they are dyspnoeic. Although CT has been used for morphologic assessment over many years, it is increasingly recognized that imaging needs to take functional assessment into account. This has resulted in development of methods for assessment of regional ventilation (10–13) and perfusion (14–16) in the lung periphery. This overview will focus on methods for assessing regional ventilation via CT. Ventilation CT assessment is important, because it combines information regarding both morphology and regional distribution of function. Regional ventilation can be assessed either through the evaluation of paired inspiratory and expiratory imaged thoracic volumes obtained during either static or dynamic manoeuvres, or via use of wash-in and or wash-out of radio-dense, stable xenon gas. Ventilation CT aims to quantitatively determine the delivery or elimination of inspired/expired air in a quantitative, and in the case of XeCT, a time-resolved regional fashion. With a change in lung volume, the amount of air delivered to or expelled from the peripheral lung will result in changes in regional density (per cent air content), allowing quantification of air distribution through the change in HU on a regional basis and thus allowing for the localized assessment of ventilation (10–13, 17, 18). Because regional lung function can change based upon whether one measures lung function via slowly or rapidly changing mechanical states (forced expiratory manoeuvres, static vs. dynamic compliance, etc.) so too will regional measures of the lung via CT change depending upon the manoeuvre used to obtain a given lung state at the time of imaging. Because, to date, commercial scanners have not been able to capture the 3D aspect of the lung dynamically, little work has been done to follow the lung during active respiratory manoeuvres. This will change in the near future now that MDCT scanners are able to cover broader z-axis ranges in a single rotation of the gantry, as gantry speeds are now on the order of 375 ms per rotation, and as newer methods emerge to gate the scanners to specific points in a respiratory cycle via lung volume monitoring through the use of pneumotachographs or other such devices. By mapping regional air content changes as a function of lung volume change, it will become possible to map regional specific compliance of the lung and thus to begin to map regional pathologic changes associated with, for instance, fibrotic or emphysematous processes or possibly early transformations in lung material properties associated with malignancies. Through assessment of regional density changes with lung volume change, it is also possible to establish an index of small airway diseases as inspired air becomes trapped at expiration. It is this additional information that makes imaging of ventilation distribution so valuable. M R I 0 3 0 B Dispatch: 26.10.04 Journal: MRI CE: Hari