Victor Zhou

Real-time 3D motion tracking for small animal brain PET.

High-resolution positron emission tomography (PET) imaging of conscious, unrestrained laboratory animals presents many challenges. Some form of motion correction will normally be necessary to avoid motion artefacts in the reconstruction. The aim of the current work was to develop and evaluate a motion tracking system potentially suitable for use in small animal PET. This system is based on the commercially available stereo-optical MicronTracker S60 which we have integrated with a Siemens Focus-220 microPET scanner. We present measured performance limits of the tracker and the technical details of our implementation, including calibration and synchronization of the system. A phantom study demonstrating motion tracking and correction was also performed. The system can be calibrated with sub-millimetre accuracy, and small lightweight markers can be constructed to provide accurate 3D motion data. A marked reduction in motion artefacts was demonstrated in the phantom study. The techniques and results described here represent a step towards a practical method for rigid-body motion correction in small animal PET. There is scope to achieve further improvements in the accuracy of synchronization and pose measurements in future work.

Optimised Motion Tracking for Positron Emission Tomography Studies of Brain Function in Awake Rats

Positron emission tomography (PET) is a non-invasive molecular imaging technique using positron-emitting radioisotopes to study functional processes within the body. High resolution PET scanners designed for imaging rodents and non-human primates are now commonplace in preclinical research. Brain imaging in this context, with motion compensation, can potentially enhance the usefulness of PET by avoiding confounds due to anaesthetic drugs and enabling freely moving animals to be imaged during normal and evoked behaviours. Due to the frequent and rapid motion exhibited by alert, awake animals, optimal motion correction requires frequently sampled pose information and precise synchronisation of these data with events in the PET coincidence data stream. Motion measurements should also be as accurate as possible to avoid degrading the excellent spatial resolution provided by state-of-the-art scanners. Here we describe and validate methods for optimised motion tracking suited to the correction of motion in awake rats. A hardware based synchronisation approach is used to achieve temporal alignment of tracker and scanner data to within 10 ms. We explored the impact of motion tracker synchronisation error, pose sampling rate, rate of motion, and marker size on motion correction accuracy. With accurate synchronisation (<100 ms error), a sampling rate of >20 Hz, and a small head marker suitable for awake animal studies, excellent motion correction results were obtained in phantom studies with a variety of continuous motion patterns, including realistic rat motion (<5% bias in mean concentration). Feasibility of the approach was also demonstrated in an awake rat study. We conclude that motion tracking parameters needed for effective motion correction in preclinical brain imaging of awake rats are achievable in the laboratory setting. This could broaden the scope of animal experiments currently possible with PET.

An Event-Driven Motion Correction Method for Neurological PET Studies of Awake Laboratory Animals

PurposeThe purpose of the study is to investigate the feasibility of an event driven motion correction method for neurological microPET imaging of small laboratory animals in the fully awake state.ProceduresA motion tracking technique was developed using an optical motion tracking system and light (<1g) printed targets. This was interfaced to a microPET scanner. Recorded spatial transformations were applied in software to list mode events to create a motion-corrected sinogram. Motion correction was evaluated in microPET studies, in which a conscious animal was simulated by a phantom that was moved during data acquisition.ResultsThe motion-affected scan was severely distorted compared with a reference scan of the stationary phantom. Motion correction yielded a nearly distortion-free reconstruction and a marked reduction in mean squared error.ConclusionsThis work is an important step towards motion tracking and motion correction in neurological studies of awake animals in the small animal PET imaging environment.

A motion adaptive animal chamber for PET imaging of freely moving animals

Small animal positron emission tomography (PET) is a potentially powerful tool for understanding the molecular origins of debilitating brain disease such as dementia, depression and schizophrenia. However, its full potential in such investigations has not yet been realized due to the use of anaesthesia to avoid motion artifacts. Anaesthesia alters biochemical pathways within the brain and precludes the study of animal behavior during the imaging study. Previously we have reported a motion correction approach for conscious animal PET imaging that employs motion tracking and line of response (LOR) rebinning. We are currently extending this technique to allow PET imaging of freely moving animals, enabling the non-invasive measurement of biochemical processes in the brain of a fully conscious rat while simultaneously observing its behavior. As a first step we report a robot-controlled motion adaptive animal chamber which translates in the horizontal plane based on the head position reported by a motion tracking system to compensate for gross animal movement and keep the head within the field of view (FOV) as long as possible during the scan. In a pilot animal study within a simulated microPET environment, the control algorithm increased the time the head spent centrally in the FOV from 38% to 83% without any apparent disturbance to the animals behaviour. We conclude that a robot-controlled motion adaptive chamber is a feasible approach and an important step towards imaging freely moving animals.

Motion tracking of fully conscious small animals in PET

Pre-clinical positron emission tomography (PET) is becoming increasingly important in understanding brain physiology using animal models. One of the major challenges at present is being able to perform brain PET studies without the use of anesthesia. In most cases where the animal is minimally restrained this will require some form of motion tracking to provide the necessary temporal pose information for compensation before or during image reconstruction (eg. [1, 2]). In previous work we have demonstrated successful tracking of continuous movement in phantom studies and an anesthetized rat study in which the animal was moved manually [2]. Here we report on our first trials tracking the head of fully conscious rats moving continuously during emission and transmission PET acquisitions performed on a microPET Focus 220 scanner (Siemens Preclinical Solutions, Knoxville, USA). The motion tracking is based on a commercial stereo-optical motion tracking device called the MicronTracker Sx60 (Claron Tech. Inc., Toronto, Canada). We have previously reported in detail on this device and its suitability for small-scale motion tracking [3]. In this paper we (i) describe the motion tracking setup and marker considerations used for conscious animal head tracking, (ii) describe our animal handling methods, (iii) present data on motion tracker performance in the conscious animal trials, (iv) present data and observations on the character of rat movements in the imaging environment, and (v) show examples of the correction that can be obtained using these data for motion compensation.

A scheme for PET data normalization in event-based motion correction

Line of response (LOR) rebinning is an event-based motion-correction technique for positron emission tomography (PET) imaging that has been shown to compensate effectively for rigid motion. It involves the spatial transformation of LORs to compensate for motion during the scan, as measured by a motion tracking system. Each motion-corrected event is then recorded in the sinogram bin corresponding to the transformed LOR. It has been shown previously that the corrected event must be normalized using a normalization factor derived from the original LOR, that is, based on the pair of detectors involved in the original coincidence event. In general, due to data compression strategies (mashing), sinogram bins record events detected on multiple LORs. The number of LORs associated with a sinogram bin determines the relative contribution of each LOR. This paper provides a thorough treatment of event-based normalization during motion correction of PET data using LOR rebinning. We demonstrate theoretically and experimentally that normalization of the corrected event during LOR rebinning should account for the number of LORs contributing to the sinogram bin into which the motion-corrected event is binned. Failure to account for this factor may cause artifactual slice-to-slice count variations in the transverse slices and visible horizontal stripe artifacts in the coronal and sagittal slices of the reconstructed images. The theory and implementation of normalization in conjunction with the LOR rebinning technique is described in detail, and experimental verification of the proposed normalization method in phantom studies is presented.

Correction for continuous motion in small animal PET

In small animal PET imaging experiments, animals are generally required to be anaesthetized to avoid motion artifacts. However, anaesthesia can alter biochemical pathways within the brain, thus affecting the physiological parameters under investigation. The ability to image conscious animals would overcome this problem and open up the possibility of entirely new investigational paradigms.

Event-based motion correction for PET transmission measurements with a rotating point source.

Accurate attenuation correction is important for quantitative positron emission tomography (PET) imaging. In PET transmission measurement using external rotating radioactive sources, object motion during the transmission scan can affect measured attenuation correction factors (ACFs), causing incorrect radiotracer distribution or artefacts in reconstructed PET images. Therefore a motion correction method for PET transmission data could be very useful. In this paper we report a compensation method for rigid body motion in PET transmission measurement, in which transmission data are motion-corrected event-by-event, based on known motion, to ensure that events that traverse the same path through the object are recorded on the same LOR. After motion correction, events detected on different LORs may be recorded on the same transmission LOR. To ensure that the corresponding blank LOR records events from the same combination of contributing LORs, the list mode blank data are spatially transformed event-by-event based on the same motion information. The proposed method has been verified in phantom studies with continuous motion.

Event-by-event motion compensation for small animal PET

In small animal PET imaging anaesthesia is usually required to eliminate motion. However, both anaesthesia and the alternative of forcibly restraining the animal, can disturb the very biochemical pathways that are of most interest in the brain. The ability to image the awake animal would represent a major advance. To this end we have investigated a motion correction technique suitable for small animal imaging on the microPET scanner, based on previous successful work in human brain PET. The efficacy of our technique was assessed on the microPET system by applying movements to a micro hot rod phantom during list mode acquisition. Movements of the phantom in scanner coordinates were obtained using a commercially available optical motion tracking system operating in the visible spectrum. Motion correction, applied as corrective spatial transformations to individual lines of response, significantly reduced motion distortion. Our immediate goal is to perfect practical methods for marker attachment and motion tracking of small animals during microPET scans.

An optical tracking system for motion correction in small animal PET

Imaging conscious animals in small animal positron emission tomography (PET) presents a significant challenge, and some form of motion compensation will normally be necessary. In this work, a commercial optical motion tracker called the micron tracker has been adapted to the microPET system for this purpose. We evaluated marker size limits, performed a spatial calibration for the devices, developed a synchronization method, and carried out a phantom study involving multiple, discrete 3D movements to test key components of the motion tracking system. We have demonstrated that small and lightweight markers (approx. 15 mm x 18 mm) are feasible with this system for 3D motion tracking, calibration accuracy was 0.46 mm RMS. Synchronization of the data streams was achieved with a precision of approximately 20 ms. Moreover, a marked reduction in motion artifacts was demonstrated in the phantom study. The techniques and results presented here demonstrate the feasibility of adapting the micron tracker to the microPET environment for motion tracking of small laboratory animals. There is scope to improve on limitations in synchronization and further optimize marker design to achieve better pose accuracy and precision.