Quantitative prediction of respiratory tidal volume based on the external torso volume change: a potential volumetric surrogate

Abstract

An external respiratory surrogate that not only highly correlates with but also quantitatively predicts internal tidal volume should be useful in guiding four-dimensional computed tomography (4DCT), as well as 4D radiation therapy (4DRT). A volumetric surrogate should have advantages over external fiducial point(s) for monitoring respiration-induced motion of the torso, which deforms in synchronization with a patient-specific breathing pattern. This study establishes a linear relationship between the external torso volume change (TVC) and lung air volume change (AVC) by validating a proposed volume conservation hypothesis (TVC = AVC) throughout the respiratory cycle using 4DCT and spirometry. Fourteen patients' torso 4DCT images and corresponding spirometric tidal volumes were acquired to examine this hypothesis. The 4DCT images were acquired using dual surrogates in ciné mode and amplitude-based binning in 12 respiratory stages, minimizing residual motion artifacts. Torso and lung volumes were calculated using threshold-based segmentation algorithms and volume changes were calculated relative to the full-exhalation stage. The TVC and AVC, as functions of respiratory stages, were compared, showing a high correlation (r = 0.992 +/- 0.005, p < 0.0001) as well as a linear relationship (slope = 1.027 +/- 0.061, R(2) = 0.980) without phase shift. The AVC was also compared to the spirometric tidal volumes, showing a similar linearity (slope = 1.030 +/- 0.092, R(2) = 0.947). In contrast, the thoracic and abdominal heights measured from 4DCT showed relatively low correlation (0.28 +/- 0.44 and 0.82 +/- 0.30, respectively) and location-dependent phase shifts. This novel approach establishes the foundation for developing an external volumetric respiratory surrogate.

Respiratory motion models: A review

“…Simple scalar surrogate signals have been derived from skin surface data by tracking a single point on the skin surface (Hughes et al, 2009), or by calculating the volumes under the skin surface (Hughes et al, 2009;Li et al, 2009;McClelland et al, 2011). It has been shown that calculating the volume under the skin surface produces a signal similar to that obtained via spirometry, but without the drift often seen in spriometry signals Hughes et al, 2009).…”

Evaluation of automatic contour propagation in T2‐weighted 4DMRI for normal‐tissue motion assessment using internal organ‐at‐risk volume (IRV)

“…The volume‐conserving organs, such as the heart, liver, and stomach, do not change their volume with the respiratory motion, although they may deform, because non‐lung tissues are not compressible under respiratory pressure difference (3–6 mmHg) 48. Therefore, the delineated OAR volumes are expected to be constant.…”

A Novel Respiratory Motion Perturbation Model Adaptable to Patient Breathing Irregularities

“…The BP ( BP = ΔV thx /TV ), defined as the lung volume expansion change (AP) divided by the TV change, was used to quantify the involvement of the thorax (36, 37) and abdomen (1 – BP) in respiration. Therefore, the BP is a critical parameter that introduces directional information into the scalar spirometric TV data.…”

“…Because only motion variation caused by irregularity was calculated, the computation workload was significantly reduced (34, 35). Additionally, separating the superoinferior (SI) from anteroposterior (AP) motions using breathing patterns (36, 37) simplified RMP modeling computation. The baseline tumor motion can be obtained from 4-dimensional computed tomography (4DCT) at simulation, and the motion perturbation can be calculated using the respiratory changes in TV and BP fed by the OSI-based technique (20, 21) during treatment.…”