Andrew P. Moor

LSO background radiation as a transmission source using time of flight.

LSO scintillators (Lu2Sio5:Ce) have a background radiation which originates from the isotope Lu-176 that is present in natural occurring lutetium. The decay that occurs in this isotope is a beta decay that is in coincidence with cascade gamma emissions with energies of 307, 202 and 88 keV. The coincidental nature of the beta decay with the gamma emissions allow for separation of the emission data originating from a positron annihilation event from transmission type data from the Lu-176 beta decay. By using the time of flight information, and information of the chord length between two LSO pixels in coincidence as a result of a beta emission and emitted gamma, a second time window can be set to observe transmission events simultaneously to emission events. Using the time when the PET scanner is not actively acquiring positron emission data, a continuous blank can be acquired and used to reconstruct a transmission image. With this blank and the measured transmission data, a transmission image can be reconstructed. This reconstructed transmission image can be used to perform emission data corrections such as attenuation correction and scatter corrections. It is observed that the flux of the background activity is high enough to create good transmission images with an acquisition time of 10 minutes.

LSO background radiation as a transmission source using time of flight information

LSO scintillators (Lu 2 Sio 5 :Ce) have a background radiation which originates from the isotope Lu-176 that is present in natural occurring lutetium. The decay that occurs in this isotope is a beta decay that is in coincidence with cascade gamma emissions with energies of 307, 202 and 88 keV. The coincidental nature of the beta decay with the gamma emissions allow for separation of the emission data originating from a positron annihilation event from transmission type data from the Lu-176 beta decay. By using the time of flight information, and information of the chord length between two LSO pixels in coincidence as a result of a beta emission and emitted gamma, a second time window can be set to observe transmission events simultaneously to emission events. Using the time when the PET scanner is not actively acquiring positron emission data, a continuous blank can be acquired and used to reconstruct a transmission image. With this blank and the measured transmission data, a transmission image can be reconstructed. This reconstructed transmission image can be used to perform emission data corrections such as attenuation correction and scatter corrections. It is observed that the flux of the background activity is high enough to create good transmission images with an acquisition time of 10 minutes.

Time alignment of time of flight positron emission tomography using the background activity of LSO

Time alignment of a positron emission tomography scanner (PET) is an important calibration as the nature of the data collected in PET is inherently time-correlated. This calibration is a key concern in scanners which employ time-of-flight (TOF) capability to localize events along a particular segment of a line of response. Emission data collected with TOF information requires the time difference measured by the scanner to be tightly coupled to the spatial information such that the true origin of the annihilation event of the radioisotope is colocated in both the time and spatial domains. There are many techniques to time align a PET scanner but all involve an external source. Our research shows that the background radiation of LSO provides an intrinsic source of radiation that can be used to measure and correct the time offsets for detector pixels. The physical distances of the static pixel locations can be compared to the measured time of flight of background events generated by LSO beta decay and cascade gamma traversing the PET field of view. The resulting information can be used to correct for the time offset between a pixel and all other pixels that the originating pixel is in coincidence with. This method can enable system operators to perform continuous time alignment whenever the system is idle, and can reduce personnel radiation exposure by reducing the need to handle sources during the calibration process.

Beyond list mode: On-line rebinning and histogramming for continuous bed motion in clinical whole-body TOF PET/CT

In this article, methods are described which help move the concept of continuous bed motion (CBM) for TOF PET (i.e. for PET/CT applications) out of the realm of list-mode-only research and into everyday clinical usage. Long proposed for PET, CBM moves the patient horizontally (at a more or less steady rate) through the PET FOV during the acquisition and offers several points of advantage over the more common patient-held-stationary-to-PET-FOV approach. One primary obstacle to frequent clinical application of CBM in PET has been the inability to provide on-line, real-time detector-pair-to-projection-space-bin-address rebinning calculations along with the associated on-line histogramming. Here are presented details for one approach to overcome just such obstacles — significantly allowing the on-line generation of single TOF projection data sets which may each represent an entire whole-body scan. A set of 5 real list-mode data files were collected from a TOF PET/CT — a system which has been outfitted with a patient handling system (PHS or “bed”) which is shown to be largely sufficient for CBM in PET. For an F-18 point source — placed for first a 1cm and then a 10cm transaxial offset, list-mode data was collected for both stationary and horizontal bed motion cases. In addition, a CBM list-mode data set was collected for a custom, F-18, 100cm-long, cylindrical phantom. These data sets were processed in a manner computationally equivalent to that proposed for the realtime, on-line processing case. By comparing stationary to CBM image quality, the resulting analysis strongly suggests these proposed methods will provide real-time CBM processing which is compatible with the needs of clinical-grade TOF PET. As perhaps expected from CBM, the Z-axis FWHM is shown to generally improve over the stationary case via finer axial sampling in the rebinning step. As may be atypical, one specific data point showed a Z-axis improvement of 6.5%. Other examples — which may be more typical — showed little or no Z-axis FWHM improvement when applying CBM.

A method for daily setup and quality checks of LSO:Ce based time of flight positron emission tomographs

The setup of PET scanners usually requires an external positron source. A correct setup procedure has to ensure that the detectors are calibrated for gain adjustments, photopeak locations, and positioning maps for individual detector crystals. In order to verify the quality of the setup on a daily basis, daily quality checks are performed where the variations are checked with an external source and flagged if a new setup should be performed. Performing equivalent checks and calibrations using the intrinsic radiation of LSO have successfully been studied previously. Using additional information given by the time of flight information, the intrinsic source originating from the isotope 176Lu can create very accurate setup data within time durations allowing daily scheduling of data collection. The addition of time of flight information also gives an intrinsic method to check the timing calibration essential to time of flight PET scanners. This intrinsic automated setup and quality checks are performed daily without the need of an operator or mechanical devices, creating a consistent condition that data is collected in.

Tracking coincidence events in pet even when count rates are extremely high: The Lost-Event Tally packet concept

We describe techniques that extend the usefulness of real-time data-handling architectures designed for clinical positron emission tomography (PET)-especially for instances of extremely high (>; 10 M events/s) count rate. As is widely known, Rubidium-82 (82Rb) with a 1.3-min half-life is often used in clinical PET. When used, 82Rb is more often applied for dynamic and/or gated studies-typically with little or no delay between tracer injection and start of acquisition. The use of 82Rb for short-duration-framed studies in clinical PET has its own set of challenges. For example, a “too large” dose may temporarily exceed the acquisition throughput. When such saturation occurs, coincidence events are lost. In the case of 82Rb, such loss is typically short-lived and limited to the count-rate peak. Such saturation also leads to a truncation of the count-rate profile-a truncation that often limits curve fitting essential for an accurate compartmental model across the entire study. The techniques offered here help to resolve this issue. By electronically keeping track of those events that are subject to saturation loss in the acquisition channel, the complete count-rate profile itself is preserved. Such tracking is done in real time with an accurate log of loss quantities added to the list-mode stream. This approach enables the clinician to raise the 82Rb dose without a specific concern over count-rate profile truncation. We describe a special 64-bit nonevent (i.e., “tag”) packet that is automatically inserted into the list-mode stream. The “Lost-Event Tally” tag packet stream enables a full recovery of the rate profile even when event packets are lost. During acquisition, this tag packet is repeatedly inserted into the stream to report the counted event-packet loss (up to 1 048 575) since the previous such tag packet was inserted.