Recent advances in multi-detector-row CT (MDCT), 4D CT respiratory gating methods, and image processing techniques enable us to study pulmonary function at the regional level with high resolution anatomical information compared to other methods. As discussed in Chaps. 2 and 3, MDCT can be used to acquire multiple static breath-hold CT images of the lung taken at different lung volumes, or to acquire 4DCT images of the lungs with spiral scanning using a low pitch and retrospectively reconstructed at different respiratory phases with proper respiratory control [18, 20, 24]. Image registration can be applied to these data to estimate a deformation field that transforms the lung from one volume configuration to the other. This deformation field can be analyzed to estimate local lung tissue expansion, calculate voxel-by-voxel intensity change, and make biomechanical measurements. When combined with image segmentation algorithms [8, 9, 11, 25], functional and biomechanical measurements can be reported on a lung, lobe, and sublobar basis, and can be used to interpret regional lung function relative to specific segments of bronchial tree. Such measurements of pulmonary function have proven useful as a planning tool during RT planning [31] and may be useful for tracking the progression of toxicity to nearby normal tissue during RT and can be used to evaluate the effectiveness of a treatment post-therapy [6].

Early studies using CT to study regional air volume changes have proved to enhance our understanding of normal lung function. Several groups have proposed methods that couple image registration and CT imaging to study regional lung function. Guerrero et al. have used optical-flow based registration to compute lung ventilation from 4DCT [13, 14] with intensity-based ventilation measure. Christensen et al. used image registration to match images across cine-CT sequences and estimate rates of local tissue expansion and contraction [5] using a Jacobian-based ventilation measure. In addition, as shown in Sect. 13.2, the intensity-based and Jacobian-based ventilation measures are based on the assumption that the regional lung volume change is due solely to air content change, which may not always be a valid assumption [7]. Other factors, such as blood volume change, may also contribute to the regional lung volume change.

Radiation induced pulmonary diseases can change the tissue material properties of lung parenchyma and the mechanics of the respiratory system. In patients such as lymphoma, esophageal and breast cancer with reasonable functional reserve, this damage is often insignificant. However, lung cancer patients with marginal pulmonary function are at increased risk for developing symptomatic pulmonary dysfunction secondary to radiation [22]. The regional functional lung imaging offers the opportunity to optimize the radiation therapy planning that preferentially avoid the functioning lung regions, thus potentially reducing the physiological damage. 3D conformal radiotherapy (3D CRT), intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) have been previously studied for the functional avoidance and showed less damage to the highly functioning lung regions when appropriate avoidance structures derived from the functional imaging were assigned to the planning system [22, 29, 31]. Furthermore, the functional lung imaging can also be applied during the lung cancer radiation therapy to help the decision for the adaptive radiation therapy.

This chapter reviews the basic measures to estimate regional ventilation from image registration of CT images, the comparison of them to the existing golden standard and the application in radiation therapy.