P. Msaki

Initial results from the Sherbrooke avalanche photodiode positron tomograph

The design features and engineering constraints of a PET system based on avalanche photodiode (APD) detectors have been described in a previous report. Here, the authors present the initial results obtained with the Sherbrooke APD-PET scanner, a very high spatial resolution device designed for dynamic imaging of small and medium-sized laboratory animals such as rats, cats, rabbits and small monkeys. Its physical performance has been evaluated in terms of resolution, sensitivity, count rate, random and scatter fractions, contrast and relative activity recovery as a function of object size. The capabilities of the scanner for biomedical research applications have been demonstrated using phantom and animal studies.

Object and detector scatter-function dependence on energy and position in high resolution PET

The authors have shown in previous works that distinct non-stationary analytical scatter kernels can be extracted from line source measurements and used to independently subtract object scatter and subtract or restore detector scatter in high resolution PET. In this work, the dependence of the scatter components on energy threshold and source position was investigated. Line source measurements were acquired in multispectral mode using the Sherbrooke PET simulator. Scatter parameters were extracted from data summed in energy windows with a lower threshold varying from 129 keV to 516 keV in steps of 42 keV, and a fixed upper threshold of 644 keV. Decreasing the lower threshold from 344 keV to 129 keV increases the trues by only 25%, but increases object scatter by 136% and almost triples detector scatter. A gain in efficiency by a factor of 2 or more would result from recovering the latter by restoration in the broad window. The intensity and shape of the scatter functions for both object and detector are shown to have a significant dependence on energy and position. This dependence needs to be taken into account in the design of kernels for accurate scatter correction over a broad energy range. >

A PET camera simulator with multispectral data acquisition capabilities

The Sherbrooke positron emission tomography (PET) simulator was designed and built to investigate parameters which influence the performance of a high-resolution PET camera based on avalanche photodiode detectors. The simulator consists of a computer controlled scanning table with 32 detection channels shared between front-end cassettes and FASTBUS boards, and of a PC-based multichannel analyzer (MCA) used as a histogramming memory for multiparametric data acquisition. Tomographic data are collected by scanning one of two opposite arrays of detectors and by rotating the object in a predetermined sequence to simulate a complete ring of detectors with various sampling schemes. All acquisition parameters are programmable through digital-to-analog converters or onboard registers. Data can be acquired in several modes: calibration, where direct or coincident energy spectra from all detectors can be registered simultaneously; standard, where only energy-validated coincident events are histogrammed as lines-of-response addresses; and multispectral, where the LOR address is encoded with the energy information to provide a multiparameter histogram. Data samples obtained in these modes are presented. >

Normalization of multispectral data in positron emission tomography

Data acquisition using more than one energy window has been proposed principally for scatter correction in positron emission tomography (PET). However, prior to scatter correction, such data must be corrected for various distortions including detector efficiency and spectral nonuniformities. The aim of this work is to describe and validate a normalization technique that can minimize these undesirable effects. As with conventional methods using broad windows, the technique reduces the non-uniformity of data acquired in several narrow windows, while performing an additional task unique to multispectral acquisition, i.e. to restore symmetry in data acquired by mirror window pairs of individual detectors. Non-uniformity and asymmetry reductions have been achieved by averaging each pair of detectors over all detectors and averaging each mirror window pair, respectively. The method has been validated using data with different degrees of spatial and spectral distortion, and has been found to be capable of restoring the greatest spectral asymmetric acquisition possible with our animal PET system. Unlike the broad-window imaging, a match of spectral composition of photon fluence between the calibration and measurements is not a requirement. In conclusion, multispectral data acquisition in PET is possible with fairly non-uniform detector response. The success of the technique, however, depends on detectors with very stable characteristics.

Potentials of multispectral acquisition in positron emission tomography

Multispectral acquisition (MSA) positron emission tomography (PET) imaging, whereby the energy information is encoded at the same time as the detector addresses, is proposed as a novel approach to recover useful information from the Compton spectrum and improve both sensitivity and contrast in high-resolution PET images. Preliminary experimental data are reported which demonstrate the potential of MSA for improving scatter correction methods and minimizing sensitivity loss associated with small detectors. MSA PET imaging has the following advantages over standard PET: event selection can be made during or after acquisition in terms of the energy and other suitable parameters derived from the spatial domain; and the increased degrees of freedom in energy space reduce averaging of scatter-dependent variables in the spatial domain and improve scope and flexibility to manipulate data more adequately in each energy range. These features can be exploited to extract useful events from the Compton spectrum and to develop efficient methods to reject undesirable events from all sources.<<ETX>>

The convolution scatter subtraction hypothesis and its validity domain in radioisotope imaging

The additive degradation model assumes that the measured projection data is the sum of scatter and primary distributions. Within this framework, the scatter projection can be estimated by convolving the primary projection with a point or line response function normalized to primary. The convolution scatter-subtraction was formulated to overcome the fact that the primary projection is not available for such estimation. This hypothesis assumes that accurate scatter projection data can be obtained by convolving the measured projection (which is readily available) with scatter response function normalized to primary plus scattered events. The aims of this work are (i) to investigate the truth of this assumption and (ii) to delineate its validity domain. The authors have found that the assumption is valid if relatively large matrices are used to accommodate all the values of the scatter response functions. This requirement is particularly critical when imaging large objects using large discrimination windows. The validity of the assumption rapidly deteriorates as the measured projections change from smooth to diffuse distributions. At this point, the shapes of scatter response functions become important.

Initial results from the Sherbrooke avalanche photodiode PET scanner

The design features and engineering constraints of a PET system based on avalanche photodiode (APD) detectors have been described in a previous report. Here, the authors present the initial results obtained with the Sherbrooke APD-PET scanner, a very high spatial resolution device designed for dynamic imaging of small and medium-sized laboratory animals such as rats, cats, rabbits and small monkeys. Its physical performance have been evaluated in terms of resolution, sensitivity, count rate, random and scatter fractions, and activity recovery as a function of object size. The capabilities of the scanner for biomedical research applications have been demonstrated using phantom and animal studies.

A normalization technique for multispectral acquisition in positron emission tomography

A novel normalization technique for multispectral acquisition (MSA) in positron emission tomography (PET) imaging is described. MSA data are affected by spectral nonuniformity due to variations in detector characteristics. As with conventional methods, the normalization must preserve counts as it reduces nonuniformity but spectral shapes must be restored and symmetry distortions between mirror windows must be compensated. The proposed method achieves these goals by averaging each window pair over all detectors and by balancing mirror windows. The method was tested with a variety of spectral distortions and was found to be successful in restoring data uniformity. It is concluded that MSA PET imaging is possible with relatively nonuniform detector response, but with stable detector characteristics are essential.<<ETX>>

Position-dependent scatter response functions: will they make a difference in SPECT conducted with homogeneous cylindrical phantoms?

This paper explains why it is possible to perform accurate quantitative SPECT when scatter correction is based on stationary and non-stationary scatter functions. This is achieved by comparing the variations of scatter parameters as a function of phantom thickness. The results show that the decrease of scatter fraction with phantom thickness and the decrease of values of scatter kernel inside the field of view are about equal. The deviation of the position-dependent slope from the average value is small for central distributions. These observations explain why estimations of scatter projection by non-stationary convolution and by stationary convolution are comparable when SPECT measurements are conducted with uniform cylindrical phantoms. It is concluded that investigations on the perceived superiority of non-stationary over stationary scatter subtraction in SPECT should be conducted with elliptic phantoms that deviate appreciably from cylindrical shape.

Assessment of scatter components in multispectral PET imaging

High resolution images in PET based on small individual detectors are obtained at the cost of poor sensitivity and significant scatter in the detectors. These drawbacks can be partially overcome by acquiring low-energy events and by using more efficient energy-dependent scatter correction methods. The feasibility of multispectral scatter correction was assessed by analyzing projection response functions measured in several energy windows to extract scatter components. While the fraction of well-positioned events drops from 90% in the photopeak region to less than 20% in the lower windows, the detector scatter fraction increases from 5% to 60% in the same range. By independent restoration of the latter component in every energy window, nearly 90% of all detected events can be recovered. Multispectral acquisition, therefore, is a promising approach for recovering sensitivity without loss of resolution in high resolution PET.<<ETX>>