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S R Cherry

Publications and source records attributed to S R Cherry.

9 recordsLinked to original sources
PubMed1992

Evaluation of a 3D reconstruction algorithm for multi-slice PET scanners.

A fully 3D reconstruction algorithm based on filtered backprojection was evaluated for the reconstruction of data obtained with multi-slice positron emission tomography (PET) scanners which have had the septa removed. This algorithm uses forward-projection through the reconstructed images of a 2D subset of the data to complete the 3D dataset thus satisfying the condition of shift invariance. This is followed by 3D filtered backprojection. Axial sampling was doubled by combining adjacent polar angles, thus improving reconstructed axial resolution. The algorithm was tested using real and simulated datasets and gave high quality reconstructions without artifacts over a wide range of imaging conditions. Events are placed accurately throughout the imaging volume as determined by measurements with a MRI/PET registration phantom. The forward-projection step leads to degradation in image resolution due to insufficient axial and transaxial sampling. This effect is amplified if multiple iterations of the algorithm are used, with little decrease in image noise. Changing the filter employed in the initial 2D reconstruction can be used to alter the noise and resolution characteristics of the 3D images. This algorithm has proved very robust at reconstructing 3D PET data and is relatively fast. Those small problems which exist can be attributed to detector sampling problems, especially in the axial direction, which is a consequence of the geometry of these scanners, which are designed primarily for 2D data acquisition.

Algorithms↗
PubMed1992

Design features and performance of a PET system for animal research.

The design features of a PET system designed for animal studies are described and its performance evaluated. The system employs a two-dimensional modular detector array consisting of bismuth germanate detector elements that are 3.5 mm (transaxially) by 6.25 mm (axially) by 30 mm (deep). These arrays are optically coupled to a pair of dual-photo-multiplier tubes (PMT). The detector ring is 64 cm in diameter with a field of view (FOV) of 40 cm by 5.4 cm axially, acquiring 15 slices at 3.4 mm spacing. These features include: (1) digitization of PMT signals from each block for improved position and energy discrimination of coincident events and (2) dual-window energy discrimination for simultaneous but separate acquisition of photopeak and scatter data. Intrinsic resolution averages 3.5 mm at the center of the FOV, while reconstructed resolution (ramp filter) ranges from 3.8 mm at the center of the FOV to 4.6 mm at an 8 cm radius. Axial resolution averages 4.4 and 4.9 mm and sensitivity averages 4.2 and 6.1 kcps/microCi/cc for cross planes and enhanced direct planes, respectively. Randoms fraction is high due to reduced interplane shielding, giving a peak true count rate of 103 kcps for a 10 cm cylinder. Scatter as a fraction of trues is 16% for a 10 cm cylinder at a lower energy threshold of 350 keV. All parameters are sensitive to energy threshold. Spatial resolution improves by 11% transaxially and 9% axially, scatter fraction drops to 10%, and overall sensitivity drops by 48% when the threshold value is increased from 350 keV to 450 keV.

Animals↗
PubMed1991

Measurements of blood-brain barrier permeability in patients undergoing radiotherapy and chemotherapy for primary cerebral lymphoma.

Positron emission tomography (PET) has been used to measure changes in regional blood-brain barrier (BBB) permeability in patients with primary cerebral lymphoma undergoing radiotherapy and chemotherapy. The method employed is to measure the rate of wash-out of a radioactive tracer (68Ga-EDTA) from blood into brain tissue using time-sequence PET imaging. Preliminary studies carried out on patients with more common primary cerebral tumours show that time-activity data are reproducible to approximately 10%. Measurements made in 2 patients with primary cerebral lymphoma treated with initial chemotherapy showed significant changes in permeability in the region of the tumour. Within 5 weeks of the start of treatment, permeability values reached the levels of normal brain. No changes in BBB permeability in normal brain were seen immediately after radiotherapy.

Adult↗
PubMed1990

Quantitative in vivo measurements of tumor perfusion using rubidium-81 and positron emission tomography.

Rubidium-81 (t1/2 = 4.58 hr) was investigated as a tumor perfusion tracer in the VX2 carcinoma implanted into rabbit thigh muscle using a large-area, multiwire proportional chamber positron emission tomography (PET) system. Perfusion was determined using the arterial reference sample method, and the results from PET imaging were compared with postmortem tissue sampling. Absolute quantitation of tumor perfusion was achieved using external probes to estimate local extraction fraction. Redistribution of rubidium-81 (81Rb) was investigated using a dual-tracer technique. Average perfusion was found to be 13.5 and 3.7 ml/min/100 g in tumor and normal muscle, respectively. The extraction fraction as estimated from a two-compartment model ranged from 0.94 to 1.00. No significant redistribution of 81Rb was observed in these tissues. Nine patients with malignancies were studied using 81Rb and PET. Tumor perfusion in four patients with carcinoma of the breast was elevated by a factor of 1.8 (range 1.2-2.3) compared to contralateral normal breast.

Animals↗
PubMed1989

Image quantification with a large area multiwire proportional chamber positron camera (MUP-PET).

A large area multi wire proportional chamber positron camera system is under evaluation for clinical Nuclear Medicine investigation using isotopes both from the house generators (68Ga) and from remote cyclotrons (18F, 124I, 64Cu and 81Rb). Images are reconstructed using a fully three dimensional (3D) algorithm and exhibit equal resolution in all three orthogonal directions. The axial field of view of 15 cm is large enough to allow coverage of the whole brain. This paper discusses the performance of the MUP-PET system with particular emphasis on quantification and the development of an attenuation correction scheme for a large area detector system. The distribution of scattered and accidental events is investigated and observed to contribute a fairly uniform background to the image. Dead time correction factors are calculated from an analytical expression obtained by considering the various sources of dead time present in the system. Following correction for attenuation, accidentals, scattered events and dead time, reconstructed phantom images show a strong linear correlation (r = 0.998) between count density and regional isotope concentration. The extension of the methods to clinical studies is discussed.

Image Processing, Computer-Assisted↗
PubMed1989

The performance of a multiwire proportional chamber positron camera for clinical use.

The Rutherford Appleton Laboratory clinical positron camera consists of two opposed multiwire proportional chambers (MWPCS) mounted on a rotating gantry capable of performing routine nuclear medicine studies. The system has operated since the end of 1986 with complete reliability. It has a sensitivity of 37 kcps MBq-1 cm3 per axial cm for a 20 cm diameter uniformly filled cylinder of activity. The best spatial resolution obtainable is about 6 mm, although in practice images are smoothed in order to reduce statistical noise with a corresponding decrease in resolution. Cross-plane rays are utilised during image reconstruction and the resulting three-dimensional images exhibit the same spatial resolution in three orthogonal directions over a large cylindrical field of view 15 cm high by 30 cm in diameter. The maximum data-taking rate is limited to 1.5 to 2 kcps at present due to deadtime in the read-out system. The performance of the system is described with particular reference to the problems of imaging with the large fractions of random and scattered events which are a consequence of using large-area detectors. Images of phantoms and patients are presented and proposed modifications to the camera are described.

Algorithms↗
PubMedDate not supplied

3D PET using a conventional multislice tomograph without septa.

A conventional multislice positron emission tomography scanner was modified to operate without interplane septa to evaluate its performance in collecting and reconstructing data in a three-dimensional (3D) format, thereby significantly increasing system sensitivity. A 3D filtered backprojection algorithm was implemented and tested, using both computer simulations and phantom measurements. No artifacts were apparent in the test images, although the algorithm was shown to lead to a 11% degradation in transaxial resolution in the outer planes. Following septa removal, sensitivity was found to increase by a factor of 7 with an increase in scatter fraction from 16 to 41%. Axial resolution degraded from 6.9 to 7.7 mm full width at half maximum at the center of the field of view. The maximum count rate without septa was 2.4 x 10(5) cps, at a concentration of 0.4 microCi/ml, compared with 1.3 x 10(5) cps at 1.5 microCi/ml with septa. Brain studies were performed with volunteers using 18F-fluorodeoxyglucose, 18F-fluorodopa, and H2 15O to compare noise-equivalent count rates and qualitatively assess image quality over a wide range of imaging conditions.

Algorithms↗
PubMedDate not supplied

Rapid automated algorithm for aligning and reslicing PET images.

A computer algorithm for the three-dimensional (3D) alignment of PET images is described. To align two images, the algorithm calculates the ratio of one image to the other on a voxel-by-voxel basis and then iteratively moves the images relative to one another to minimize the variance of this ratio across voxels. Since the method relies on anatomic information in the images rather than on external fiducial markers, it can be applied retrospectively. Validation studies using a 3D brain phantom show that the algorithm aligns images acquired at a wide variety of positions with maximum positional errors that are usually less than the width of a voxel (1.745 mm). Simulated cortical activation sites do not interfere with alignment. Global errors in quantitation from realignment are less than 2%. Regional errors due to partial volume effects are largest when the gantry is rotated by large angles or when the bed is translated axially by one-half the interplane distance. To minimize such partial volume effects, the algorithm can be used prospectively, during acquisition, to reposition the scanner gantry and bed to match an earlier study. Computation requires 3-6 min on a Sun SPARCstation 2.

Algorithms↗