Method For Free Breathing 3D Ventilation Measurements
SUMMARY
UCLA researchers have developed a method for free breathing 3D ventilation measurements using fast-helical CT scan.
BACKGROUND
Lung cancer patients referred to radiotherapy (RT) often present with reduced lung function due to regional lung function deficits, such as reduced ventilation and diffusion capabilities. Ventilation measurements using CT and other techniques are complex and of limited quantitative value. Some of this is due to the fact that ventilation measurements are taken with breath hold and that daily variations in breathing depth are not considered. Competing technologies include breath hold CT and SPECT.
The spatial pattern of lung function can be measured using ventilation/ perfusion (V/Q) single photon emission computed tomography (CT) combined with x-ray CT (SPECT-CT). The accessibility of SPECT-CT, however, is often limited and its spatial resolution is poor due to blurring during long acquisition times and low directionality of the signal. There are groups trying to use 4DCT (the forth dimension is time) for ventilation measurements for radiation therapy patient dose response assessments. They are ridden with image artifacts and a lack of quantitation.
INNOVATION
UCLA researchers have developed a method for free breathing 3D ventilation measurements. This invention utilizes an existing fast-helical CT protocol that scans patient's lungs while they are breathing. The scans are acquired fast enough that the motion artifacts are minimal. The patient is scanned multiple times so multiple images are acquired. At the same time, the patient's breathing is monitored using a real-time breathing surrogate, which can be spirometry, an abdominal bellows, or other technique.
The CT scans are deformably mapped to determine the locations of lung tissues (including tumors if applicable) as they were imaged. One of the images (or a separate image, for example breath hold) is typically selected as a reference image and is used to identify the mapped tissues in the other images. The CT Hounsfield Units are examined in the reference image and the corresponding positions of the reference image voxels on the other CT scans. The air content is calculated from the HU values and graphed against the surrogate's measurement. If the surrogate is tidal volume, the total sum of the slopes between the air content and the surrogate should equal 1 or 1.11 if the tidal volume is calibrated to intra-lung air temperature and humidity or room air, respectively. The local slopes can be considered dynamic measurements of ventilation.
APPLICATIONS
The primary application for this invention is to quantitatively measure ventilation while the patient is breathing freely. This data can then be used in clinical settings as an indicator for lung function.
ADVANTAGES
- Patients do not need to hold their breath during CT scans.
- Normalization to tidal volume minimizes the variations in breathing amplitude.