Charles Hayden

Monitoring Energy Calibration Drift Using the Scintillator Background Radiation

Scintillating materials commonly used in nuclear medicine can contain traces of isotopes that naturally emit gamma or beta radiation. Examples of these are 138La contained in LaBr3 and other Lanthanum based scintillators, and 176Lu contained in LSO, LYSO, LuYAP and other Lutetium based scintillators. In particular,176Lu decays into 176Hf and emits a beta particle with maximum energy 589 keV, and a cascade of gamma rays of energies 307 keV, 202 keV and 88 keV. We propose to use the background radiation for monitoring of detector calibration drift and for self-calibration of detectors in complex detector systems. A calibration drift due to random or systematic changes in photomultiplier tube (PMT) gain was studied in a Siemens PET scanner, based on LSO blocks. Both a conventional radioactive source (68Ge, 511 keV photons from electron-positron annihilation) and the LSO background radiation were used for calibration. The difference in the calibration peak shift at 511 keV estimated with the two methods was less than 10%.

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.

Evaluation of MLACF based calculated attenuation brain PET imaging for FDG patient studies

Calculating attenuation correction for brain PET imaging rather than using CT presents opportunities for low radiation dose applications such as pediatric imaging and serial scans to monitor disease progression. Our goal is to evaluate the iterative time-of-flight based maximum-likelihood activity and attenuation correction factors estimation (MLACF) method for clinical FDG brain PET imaging. FDG PET/CT brain studies were performed in 57 patients using the Biograph mCT (Siemens) four-ring scanner. The time-of-flight PET sinograms were acquired using the standard clinical protocol consisting of a CT scan followed by 10 min of single-bed PET acquisition. Images were reconstructed using CT-based attenuation correction (CTAC) and used as a gold standard for comparison. Two methods were compared with respect to CTAC: a calculated brain attenuation correction (CBAC) and MLACF based PET reconstruction. Plane-by-plane scaling was performed for MLACF images in order to fix the variable axial scaling observed. The noise structure of the MLACF images was different compared to those obtained using CTAC and the reconstruction required a higher number of iterations to obtain comparable image quality. To analyze the pooled data, each dataset was registered to a standard template and standard regions of interest were extracted. An SUVr analysis of the brain regions of interest showed that CBAC and MLACF were each well correlated with CTAC SUVrs. A plane-by-plane error analysis indicated that there were local differences for both CBAC and MLACF images with respect to CTAC. Mean relative error in the standard regions of interest was less than 5% for both methods and the mean absolute relative errors for both methods were similar (3.4%  ±  3.1% for CBAC and 3.5%  ±  3.1% for MLACF). However, the MLACF method recovered activity adjoining the frontal sinus regions more accurately than CBAC method. The use of plane-by-plane scaling of MLACF images was found to be a crucial step in order to obtain improved activity estimates. Presence of local errors in both MLACF and CBAC based reconstructions would require the use of a normal database for clinical assessment. However, further work is required in order to assess the clinical advantage of MLACF over CBAC based method.

Transmission-less brain TOF PET imaging using MLACF

Recent theoretical investigations concluded that both activity and attenuation distributions can be obtained from PET emission TOF data alone up to the knowledge of the sinogram scaling parameter. Attenuation is recovered in the form of attenuation factors. This allows for development of a fast iterative algorithm, referred to as ML-ACF, where iterative reconstruction is performed with respect to the activity image. Attenuation is a by-product of the estimation of activity. In general, the estimation of attenuation from PET data alone is not currently necessary due to the diagnostic value of PET-CT scanning. Nevertheless, it is desirable to limit patient CT doses. CT-less PET brain imaging is currently being pursued by manufacturers. This transmission-less brain imaging uses the assumption of a uniform attenuation map. The boundary can be derived from the processing of the activity image and reconstructed without attenuation correction. In this paper we enrich this approach by exploiting this uniform attenuation map as a starting condition for MLACF iterations. The scatter correction and the proper scaling of the activity image are based on this information. The head, especially the slices close to the top of the head, is relatively small compared to account the current scanners time resolution. Therefore it is not obvious a priori that TOF information can be exploited for brain imaging. In this paper we demonstrate that the brain image attenuation can be reconstructed by exploiting just two TOF bins, with the TOF bin size currently used on the Siemens mCT scanner. Preliminary investigations showed that the head activity image will be improved by MLACF compared to the standard CT-less processing based on a uniform attenuation.

Monitoring energy calibration drift using the scintillator background radiation

Scintillating materials commonly used in nuclear medicine can contain traces of isotopes that naturally emit gamma or beta radiation. Examples of these are 138La contained in LaBr3 and other Lanthanum based scintillators, and 176Lu contained in LSO, LYSO, LuYAP and other Lutetium based scintillators. In particular, 176Lu decays into 176Hf and emits a beta particle with maximum energy 589 keV, and a cascade of gamma rays of energies 307 keV, 202 keV and 88 keV. We propose to use the background radiation for monitoring of detector calibration drift and for self-calibration of detectors in complex detector systems. A calibration drift due to random or systematic changes in PMT gain was studied in a Siemens PET scanner, based on LSO blocks. Both a conventional radioactive source (68Ge, 511 keV photons from electron-position annihilation) and the LSO background radiation were used for calibration. The difference in the calibration peak shift at 511 keV estimated with the two methods was less than 10%.