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

Long-lived States to Sustain SABRE Hyperpolarised Magnetisation

The applicability of the magnetic resonance (MR) technique in the liquid phase is limited by poor sensitivity and short nuclear spin coherence times which are insufficient for many potential applications. Here we illustrate how it is possible to address both of these issues simultaneously by harnessing long-lived hyperpolarised spin states that are formed by adapting the Signal Amplification by Reversible Exchange (SABRE) technique. We achieve more than 4 % net 1H-polarisation in a long-lived form that remains detectable for over ninety seconds by reference to proton pairs in the biologically important molecule nicotinamide and a pyrazine derivative whose in vivo imaging will offer a new route to probe disease in the futur

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

Accurate attenuation correction is important for quantitative positron emission tomography (PET) studies. When performing transmission measurements using an external rotating radioactive source, object motion during the transmission scan can distort the attenuation correction factors computed as the ratio of the blank to transmission counts, and cause errors and artefacts in reconstructed PET images. In this paper we report a compensation method for rigid body motion during PET transmission measurements, in which list mode transmission data are motion corrected event-by-event, based on known motion, to ensure that all events which traverse the same path through the object are recorded on a common line of response (LOR). As a result, the motion-corrected transmission LOR may record a combination of events originally detected on different LORs. 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 number of counts recorded on the resulting blank LOR is then equivalent to the number of counts that would have been recorded on the corresponding motion-corrected transmission LOR in the absence of any attenuating object. The proposed method has been verified in phantom studies with both stepwise movements and continuous motion. We found that attenuation maps derived from motion-corrected transmission and blank data agree well with those of the stationary phantom and are significantly better than uncorrected attenuation data

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 phantomduring 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. 15mm x 18mm) are feasible with this system for 3D motion tracking, Calibration accuracy was 0.46mm RMS.Synchronization of the data streams was achieved with a precision of approximately 20ms. 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

An event-driven motion correction method for neurological PET studies of awake laboratory animals

Purpose: The 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. Procedures: A 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. Results: The 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. Conclusions: This work is an important step towards motion tracking and motion correction in neurological studies of awake animals in the small animal PET imaging environment

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

Event-based motion correction in 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 radio tracer 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 phantomduring 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