PubMed Health⌕ Search

Biomedical subjects

M Defrise

Publications and source records attributed to M Defrise.

6 recordsLinked to original sources
PubMed1991

A normalization technique for 3D PET data.

Prior to reconstruction, emission data from a multi-ring PET camera must be corrected (normalized) for variations in detector sensitivity. The appropriate correction coefficients are obtained by measuring the response of all coincidence lines to a calibrated source of activity (a blank scan). State-of-the-art cameras may contain up to a million such lines of response (LORs), and therefore around 400 million counts will be required to calibrate each LOR to a statistical accuracy of 5%. Alternatively, by modelling the LOR sensitivity as the product of the individual detector efficiencies and a geometrical factor, a calibration procedure has been proposed which requires the determination of only 6000 parameters from this same data set. A significant improvement in the statistical accuracy of the coefficients can therefore be expected. Recently, multi-ring scanners have been operated with the septa retracted, increasing the number of measured LORs by a factor of eight. The acquisition of the calibration data necessary to achieve adequate statistical accuracy then becomes prohibitive. We show that, by modelling the LOR sensitivity, it is possible, with certain approximations, to normalize a septa-retracted emission data set with good accuracy. The input to the model is a high statistics blank scan acquired with the septa extended, which offers a number of practical advantages.

Calibration↗
PubMed1991

Normalisation and reconstruction of PET data acquired by a multi-ring camera with septa retracted.

Emission scan data acquired by a multi-ring PET camera operated with septa retracted must be corrected for (1) geometrical and detector sensitivity variations between the different lines of response (normalisation), (2) photon attenuation, and (3) mispositioned events due to photon scattering. These corrections must be applied to the full 3-D set of lines of response before reconstruction. The standard normalisation and attenuation correction procedures for 2-D scans increase the statistical noise in the emission scan, a problem which becomes even more serious in 3-D because of the large number of LORs involved (approximately 8 million). This paper will describe a fully 3-D reconstruction algorithm for multi-angle PET data incorporating a practical normalisation and attenuation correction procedure which minimises the increase in emission scan statistical noise. The correction factors are derived from 2-D, septa extended scans. The algorithm is currently used to reconstruct 3-D emission data from an ECAT 953B, a sixteen-ring PET camera with retractable septa.

Algorithms↗
PubMed1990

Implementation of three-dimensional image reconstruction for multi-ring positron tomographs.

In view of the number of PET studies involving low count rate acquisitions, there has been increasing interest recently in the development of positron cameras capable of fully three-dimensional acquisition and reconstruction. This interest has given impetus to the study of algorithms for 3D reconstruction, including those algorithms suitable for application to multi-ring PET scanners. While 2D reconstruction methods can often be generalised to 3D, a number of implementation problems arise which are unique to the 3D approach. This paper examines some of the difficulties associated with the generalisation of the filtered backprojection algorithm to 3D, paying particular attention to the approximations and variable transformations required for application to data from a multi-ring scanner.

Humans↗
PubMed1989

Aspects of three dimensional reconstruction for a multi ring positron tomograph.

An important feature of multi ring positron tomographs is the inter plane septa, the purpose of which is to reduce random and scattered coincidences. In general, such septa also eliminate the coincidence lines of response between pairs of detectors more than one ring apart. The operation of a camera without septa must result in an increase not only in the true coincidence rate, but also in the singles, and therefore in the dead time and randoms rate, and in the scattered coincidences. A configuration option in the coincidence hardware of the 8 ring, 15 slice ECAT 931/08-12 enables a full set of 64 sinograms to be acquired when the septa are removed. The detector normalisation and transmission data for studies with the septa out can be obtained using a rotating pin source. To take maximum advantage of the additional signal, the emission data must be reconstructed using a fully three dimensional reconstruction algorithm. This paper presents an analysis of some phantom studies acquired without septa and reconstructed in three dimensions. The results are compared with data acquired with septa for the same phantoms imaged under similar conditions. It is found that, with the septa removed, the signal to noise for a uniform, 20 cm diameter cylinder improves by a factor of 2.8 in the centre of the field of view, whereas in regions distant from the centre in the axial direction, the signal to noise decreases due to the increase in scatter and randoms. An improvement in signal to noise is observed in 6 cm of the 10 cm axial length of the tomograph.

Image Processing, Computer-Assisted↗
PubMed1989

Three-dimensional image reconstruction from complete projections.

Three-dimensional medical image reconstruction for both transmission and emission tomography has traditionally decomposed the problem into a set of two-dimensional reconstructions on parallel transverse sections. There is, however, increasing interest in reconstructing projection data directly in three dimensions. For emission tomography in particular, such a reconstruction procedure would clearly make more efficient use of the available photon flux. In the past few years, a number of authors have studied the problems associated with full three-dimensional reconstruction, especially in the case of positron tomography where three-dimensional reconstruction is likely to offer the greatest benefits. While most approaches follow that of filtered backprojection, the relationship between the various filters that have been proposed is far from evident. This paper clarifies this relationship by analysing and generalising the different classes of published filters and establishes the properties and characteristics of a general solution to the three-dimensional reconstruction problem. Some guidelines are suggested for the choice of an appropriate filter in a given situation.

Algorithms↗