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. 2020 Apr;93(1108):20190303.
doi: 10.1259/bjr.20190303. Epub 2020 Jan 20.

A new respiratory monitor system for four-dimensional computed tomography by measuring the pressure change on the back of body

Xianwen Zhang  1 , Jintian Tang  2 , Gregory C Sharp  3 , Lei Xiao  4 , Shouping Xu  5 , Hsiao-Ming Lu  3
Affiliations

A new respiratory monitor system for four-dimensional computed tomography by measuring the pressure change on the back of body

Xianwen Zhang et al. Br J Radiol. 2020 Apr.

Abstract

Objective: A novel respiratory monitoring method based on the periodical pressure change on the patient's back was proposed and assessed by applying to four-dimensional CT (4DCT) scanning.

Methods: A pressure-based respiratory monitoring system is developed and validated by comparing to real-time position management (RPM) system. The pressure change and the RPM signal are compared with phase differences and correlations calculated. The 4DCT images are reconstructed by these two signals. Internal and skin artifacts due to mismatch between CT slices and respiratory phases are evaluated.

Results: The pressure and RPM signals shows strong consistency (R = 0.68±0.19 (1SD)). The time shift is 0.26 ± 0.51 (1SD) s and the difference of breath cycle is 0.02 ± 0.17 (1SD) s. The quality of 4DCT images reconstructed by two signals is similar. For both methods, the number of patients with artifacts is eight and the maximum magnitudes of artifacts are 20 mm (internal) and 10 mm (skin). The average magnitudes are 8.8 mm (pressure) and 8.2 mm (RPM) for internal artifacts, and 5.2 mm (pressure) and 4.6 mm (RPM) for skin artifacts. The mean square gray value difference shows no significant difference (p = 0.52).

Conclusion: The pressure signal provides qualified results for respiratory monitoring in 4DCT scanning, demonstrating its potential application for respiration monitoring in radiotherapy.

Advances in knowledge: Pressure change on the back of body is a novel and promising method to monitor respiration in radiotherapy, which may improve treatment comfort and provide more information about respiration and body movement.

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Figures

Figure 1.
Figure 1.
(a) The raw signal recorded by the pressure sensor which consists of respiratory motion and BCG. (b) The respiratory signal filtered from the raw signal. (c) BCG signal. BCG, ballistocardiogram.
Figure 2.
Figure 2.
Photo of the pressure based respiratory monitoring system. PVDF, polyvinylidene fluoride.
Figure 3.
Figure 3.
Block diagram for the synchronization of two respiratory signals. The “X-ray on” signal is recorded by two systems and used for synchronization. 4DCT, four-dimensional CT; PC, personal computer; RPM, real-time position management.
Figure 4.
Figure 4.
Illustration of the method to obtain the time shift and maximum of GCC. By moving pressure signal back and forth and computing GCC at each step, the time shift is the time value where the GCC was maximized. GCC, global correlation coefficient; PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 5.
Figure 5.
Illustration of the time shift at different phase angle in each breath cycle. PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 6.
Figure 6.
(a) A flowchart illustrates the method to evaluate artifacts in 4DCT images. The same procedure was used for evaluation of skin artifacts in sagittal views. (b, c) Illustration of measurement of skin and internal artifacts. 4DCT, four-dimensional CT.
Figure 7.
Figure 7.
Comparison of respiratory signals. (a) regular breathing; (b) irregular breathing with two breathing cycle longer than others; (c) the example of large excursion recorded by pressure sensor; (d) the situation where RPM signal is noisy but pressure signal is normal. PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 8.
Figure 8.
(a) Plot illustrates the value and trend of GCC and time shift of each patient. (b) Plot compares the respiratory cycle length computed from RPM and pressure signals. The bar represents standard deviation over all breathing cycles. The gray background indicates the 11 patients included in the evaluation of 4DCT images. 4DCT, four-dimensional CT; GCC, global correlation coefficient; PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 9.
Figure 9.
Plot of the time shift at different phase angles across all patients. The bar represents tandard deviation over all breathing cycles. The gray background indicates the 11 patients included in the evaluation of 4DCT images. 4DCT, four-dimensional CT.
Figure 10.
Figure 10.
Examples of 4DCT images reconstructed by RPM and pressure signals. Internal mushroom artifact can be observed in the diaphragm of images reconstructed by RPM for Patient No. 1. Skin artifacts can be observed in the upper abdomen of images reconstructed by PRMS for patient No. 6. PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 11.
Figure 11.
Bar charts illustrate the average number of artifacts and total phases with artifacts evaluated by three physicists. PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
Figure 12.
Figure 12.
Box plots illustrate the magnitudes of artifacts assessed by three physicists. PRMS, pressure-based respiratory monitoring system; RPM, real-time position management.
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