Respiratory diseases are among the leading causes of death, according to the World Health Organization. With more than 3 million deaths in 2016, chronic obstructive pulmonary disease is the third leading cause of death worldwide . An important tool for diagnosing respiratory diseases is computed tomography of the lungs . The current state of the art approach for this is breath-hold CT, which requires patients to follow breathing cues and hold their breath on command . This procedure is not appropriate for certain groups of patients who are unable to follow instructions. Some of these patients are unconscious, mentally impaired, or are infants and young children . In these cases, medication must be administered to stop the breathing so that sufficient scans can be acquired. This not only carries risks for the patient, but also requires additional clinical staff [4, 5, 6].
To improve these issues, free-breathing computed tomography has been proposed. In this approach, scans are performed while the patient continues to breathe [4, 5, 7]. Because the lungs move during the scan, this method produces images with more artifacts, compared to the breath-hold approach. To optimize image quality and for comparability between scans, it would be beneficial to scan at times with little lung movement, such as during inhalation and exhalation. One way to achieve comparability is retrospective respiratory triggering, which uses the respiratory waveform to select the correct phase after CT images are acquired. However, this is not ideal for clinical use because of the high radiation dose involved. To achieve lower radiation exposure, prospective respiratory triggering utilizes a shorter duration scan triggered by a respiratory gating device. [8, 9]
To address the challenges associated with the continuous lung movement, two recording modes are further investigated.
- Sequential mode acquires images without CT couch motion, with image size limited by collimator width. Multiple images are stacked to cover a larger area. This is expected to work better for children, who tend to have a high respiratory rate and a small lung area to scan .
- The flash spiral mode utilizes a technique previously used for cardiac imaging with a high-pitch spiral CT . This makes it possible to scan the entire lung in one pass in less than one second. The advantages of this high pitch mode for lung imaging have already been demonstrated [4, 5, 8]. Promising results were achieved with regularly breathing patients, although the limitations of triggering with respect to irregular breathing were noted by Goo et al. 
For both of these approaches, triggering algorithms expand on phase space based respiratory triggering as presented by Werner et al.. A set of criteria is used to define target regions in the phase space representation of the respiratory signal data in order to emit a trigger depending on amplitude and velocity . The goal is to achieve the best possible robustness on patient data with a focus on especially challenging breathing patterns.
This thesis deals with the following work items:
- Customization of respiratory triggering algorithms to both acquisition modes
- Sequential respiratory triggering
- Flash spiral respiratory triggering
- Evaluation of robustness against other approaches
- Commercial respiratory gaiting
- Possibly: Reinforcement learning based triggering
- Image based validation of the reconstruction result
- With simulated breathing signals
- With breathing signals from real patients
 World Health Organization (WHO) et al. Global health estimates 2016: estimated deaths by age, sex and cause. Geneva: WHO, 2018.
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 Toshizo Katsuda, Shigeru Eiho, Chikazumi Kuroda, and Tsutomu Hashimoto. Analysis of breath holding for lung ct imaging. Radiography, 11(4):235–241, 2005.
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 Hyun Woo Goo. Combined prospectively electrocardiography-and respiratory-triggered sequential cardiac computed tomography in free-breathing children: success rate and image quality. Pediatric radiology, 48(7):923–931, 2018.
 Hyun Woo Goo and Thomas Allmendinger. Combined electrocardiography-and respiratory-triggered ct of the lung to reduce respiratory misregistration artifacts between imaging slabs in free-breathing children: initial experience. Korean journal of radiology, 18(5):860, 2017
 Cyrus Behzadi, Michael Groth, Frank Oliver Henes, Dorothee Schwarz, Andr´e Deibele, Philipp GC Begemann, Gerhard Adam, and Marc Regier. Intraindividual comparison of image quality using retrospective
and prospective respiratory gating for the acquisition of thin sliced four dimensional multidetector ct of the thorax in a porcine model. Experimental lung research, 41(9):489–498, 2015.
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 Stephan Achenbach, Mohamed Marwan, Tiziano Schepis, Tobias Pflederer, Herbert Bruder, Thomas Allmendinger, Martin Petersilka, Katharina Anders, Michael Lell, Axel Kuettner, et al. High-pitch spiral
acquisition: a new scan mode for coronary ct angiography. Journal of cardiovascular computed tomography, 3(2):117–121, 2009. Ren´e Werner, Thilo Sentker, Frederic Madesta, Tobias Gauer, and Christian Hofmann. Intelligent 4d ct sequence scanning (i4dct): concept and performance evaluation. Medical physics, 46(8):3462–3474, 2019.