Ultrasound-based sensors for respiratory motion assessment in multimodality PET imaging

Breathing motion can displace internal organs by up to several cm; as such, it is a primary factor limiting image quality in medical imaging. Motion can also complicate matters when trying to fuse images from different modalities, acquired at different locations and/or on different days. Currently available devices for monitoring breathing motion often do so indirectly, by detecting changes in the outline of the torso rather than the internal motion itself, and these devices are often fixed to floors, ceilings or walls, and thus cannot accompany patients from one location to another. We have developed small ultrasound-based sensors, referred to as ‘organ configuration motion’ (OCM) sensors, that attach to the skin and provide rich motion-sensitive information. In the present work we tested the ability of OCM sensors to enable respiratory gating during in vivo PET imaging. A motion phantom involving an FDG solution was assembled, and two cancer patients scheduled for a clinical PET/CT exam were recruited for this study. OCM signals were used to help reconstruct phantom and in vivo data into time series of motion-resolved images. As expected, the motion-resolved images captured the underlying motion. In Patient #1, a single large lesion proved to be mostly stationary through the breathing cycle. However, in Patient #2, several small lesions were mobile during breathing, and our proposed new approach captured their breathing-related displacements. In summary, a relatively inexpensive hardware solution was developed here for respiration monitoring. Because the proposed sensors attach to the skin, as opposed to walls or ceilings, they can accompany patients from one procedure to the next, potentially allowing data gathered in different places and at different times to be combined and compared in ways that account for breathing motion.

Dosimetric study of the interplay effect using three-dimensional motion phantom in proton pencil beam scanning treatment of moving thoracic tumours

Aim: The dosimetric and clinical advantages offered by implementation of pencil beam scanning (PBS) proton therapy for moving thoracic tumours is hindered by interplay effect. The purpose of this study is to evaluate the impact of large proton beam spot size along with adaptive aperture (AA) and various motion mitigation techniques on the interplay effect for a range of motion amplitudes in a three-dimensional (3D) respiratory motion phantom. Materials and Methods: Point doses using ionisation chamber (IC) and planner dose distributions with radiochromic film were compared against the corresponding treatment planning system (TPS) information. A 3D respiratory motion phantom was scanned either for static or 4D computed tomographic (CT) technique for 6-, 10- and 14-mm motion amplitudes in SI direction. For free breathing (FB) treatment, a tumour was contoured on maximum intensity projection scan and an average scan was used for treatment planning. Each FB treatment was delivered with one, three and five volumetric repaintings (VRs). Three phases (CT40–60%) were extracted from the 4D-CT scans of each motion amplitude for the respiratory-gated treatment and were used for the treatment planning and delivery. All treatment plans were made using AA and robustly optimised with 5-mm set-up and 3·5% density uncertainty. A total of 26 treatment plans were delivered to IC and film using static, dynamic and respiratory-gated treatments combinations. A percent dose difference between IC and TPS for the point dose and gamma indices for film–TPS planner dose comparison was used. Results: The dose profile of film and TPS for the static phantom matched well, and percent dose difference between IC and TPS was 0·4%. The percent dose difference for all the gated treatments were below 3·0% except 14-mm motion amplitude-gated treatment. The gamma passing rate was more than 95% for film–TPS comparison for all gated treatment for the investigated gamma acceptance criteria. For FB treatments, the percent dose difference for 6-, 10- and 14-mm motion amplitude was 1·4%, −2·7% and −4·1%, respectively. As the number of VR increased, the percent difference between measured and calculated values decreased. The gamma passing rate met the required tolerance for different acceptance criteria except for the 14-mm motion amplitude FB treatment. Conclusion: The PBS technique for the FB thoracic treatments up to 10-mm motion amplitude can be implemented with an acceptable accuracy using large proton beam spot size, AA and robust optimisation. The impact of the interplay effect can be reduced with VR and respiratory-gated treatment and extend the treatable tumour motion amplitude.

Sub-millisecond 2D MRI of the vocal fold oscillation using single-point imaging with rapid encoding

Objective The slow spatial encoding of MRI has precluded its application to rapid physiologic motion in the past. The purpose of this study is to introduce a new fast acquisition method and to demonstrate feasibility of encoding rapid two-dimensional motion of human vocal folds with sub-millisecond resolution. Method In our previous work, we achieved high temporal resolution by applying a rapidly switched phase encoding gradient along the direction of motion. In this work, we extend phase encoding to the second image direction by using single-point imaging with rapid encoding (SPIRE) to image the two-dimensional vocal fold oscillation in the coronal view. Image data were gated using electroglottography (EGG) and motion corrected. An iterative reconstruction with a total variation (TV) constraint was used and the sequence was also simulated using a motion phantom. Results Dynamic images of the vocal folds during phonation at pitches of 150 and 165 Hz were acquired in two volunteers and the periodic motion of the vocal folds at a temporal resolution of about 600 µs was shown. The simulations emphasize the necessity of SPIRE for two-dimensional motion encoding. Discussion SPIRE is a new MRI method to image rapidly oscillating structures and for the first time provides dynamic images of the vocal folds oscillations in the coronal plane.

Comparison of virtual non-contrast dual-energy CT and a true non-contrast CT for contouring in radiotherapy of 3D printed lung tumour models in motion: a phantom study

Objectives: This work aims to investigate whether virtual non-contrast (VNC) dual-energy CT(DECT) of contrasted lung tumours can be used as an alternative for true non-contrast (TNC) images in radiotherapy. Two DECT techniques and a TNC CT were compared and influences on gross tumour volume (GTV) volume and CT number from motion artefacts in three-dimensional printed lung tumour models (LTM) in amotion phantom were examined. Methods: Two spherical LTMs (diameter 3.0 cm) with different inner shapes were created in a three-dimensional printer. The inner shapes contained water or iodine (concentration 5 mg ml−1) and were scanned with a dual-source DECT (ds-DECT), single-source sequential DECT (ss-DECT) and TNC CT in a respiratory motion phantom (15 breaths/min, amplitude 1.5 cm). CT number and volume of LTMs were measured. Therefore, two GTVs were contoured. Results: Deviations in GTV volume (outer shape) of LTMs in motion for contrast-enhanced ss-DECT and ds-DECT VNC images compared to TNC images are not significant (p > 0.05). Relative GTV volume and CT number deviations (inner shapes) of LTMs in motion were 6.6 ± 0.6% and 104.4 ± 71.2 HU between ss-DECT and TNC CT and −8.4 ± 10.6% and 25.5 ± 58.5 HU between ds-DECT and TNC, respectively. Conclusion: ss-DECT VNC images could not sufficiently subtract iodine from water in LTMs inmotion, whereas ds-DECT VNC images might be a valid alternative to a TNC CT. Advances in knowledge: ds-DECT provides a contrasted image for contouring and a non-contrasted image for radiotherapy treatment planning for LTM in motion.