CT-based ventilation imaging

Radiotherapy treatment of lung cancer patients can lead to radiation-induced toxicity in healthy lung tissue. It is therefore necessary to incorporate functional lung imaging into radiotherapy planning, to avoid irradiation of healthy lung areas. The primary function of the lungs is 'ventilation,' which on a mechanical level refers to the rate of air flow and / or gas exchange. At present, the `gold-standard' for ventilation imaging is based on nuclear-medicine (NM). In contrast, ventilation imaging based on four dimensional computed tomography (4D-CT) has the potential to provide functional imaging at reduced cost, higher accessibility and better resolution, but remains to be validated against NM. This is the primary focus of our work.

A 4D-CT ventilation image is acquired in 3 stages. (1) Paired 3D-CT images at the peak-exhale and peak-inhale phases are obtained from the CT image sets; (2) Deformable image registration (DIR) software is used to conduct spatial (voxel-wise) mapping of the peak-exhale image to the peak-inhale image, deriving a displacement vector field (DVF); (3) The final step is the creation of a CT ventilation image through quantitative analysis of the displacement vector fields quantifying ventilation metrics.

Image demonstrating how a pair of 4D-CT images (left) can be combined to produced a ventilation image (right). Regions of high ventilation are indicated in red; regions of moderate and low ventilation are indicated by yellow and blue, respectively.

The use of 4D-CT ventilation imaging in lung cancer radiotherapy

We are conducting a clincal study at Royal North Shore Hospital testing the physiological accuracy of CT ventilation imaging for use in lung cancer radiotherapy. The physiological accuracy of CT ventilation imaging will be assessed via a correlation study against the current gold standard for ventilation imaging, nuclear medicine imaging.

We will also quantify the dosimetric impact of CT ventilation imaging in functional based treatment planning which tries to avoid irradiating high-functional lung regions. Anatomic-based plans, currently standard practice, will be compared to functional-based plans.

Comparison of anatomical and functional based planning

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4D CBCT ventilation imaging

The efficacy of functional avoidance is diminished if there are temporal changes in the ventilation pattern throughout a course of radiotherapy. In order to account for temporal changes we are currently developing ventilation imaging using 4D cone-beam CT (4D-CBCT). 4D-CBCT is typically implemented as a low-dose, kilovoltage X-ray beam aligned orthogonally to a treatment beam. This hardware is already present on many treatment systems, and 4D-CBCT scans are being increasingly used for image guided radiation therapy (IGRT). As a result 4D-CBCT ventilation imaging could provide `free’ information about lung function at the time and place of treatment.

A challenge for 4D-CBCT ventilation imaging is that the underlying reconstructed image quality can suffer from lower soft-tissue contrast, noise and reconstruction artefacts (see animation below). Researchers at the Radiation Physics Laboratory are working to implement cutting edge, iterative reconstruction methods to improve the quality of 4D-CBCT ventilation images.

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Respiratory motion in a 4D-cone beam scan. Note the reduced image quality compared to the 4D-CT scan (Image courtesy Virginia Commonwealth University, VA USA.)

Respiratory motion in 4D-CT scan (Image courtesy Virginia Commonwealth University, VA USA.)