Biomedical subjects
Publications and source records attributed to S E Derenzo.
The structure of dilutetium silicon pentaoxide, Lu2SiO5, has isolated ionic SiO4 tetrahedral units and non-Si-bonded O atoms in distorted OLu4 tetrahedra. The OLu4 tetrahedra form edge-sharing infinite chains and double O2Lu6 tetrahedra along the c axis. The edge-sharing chains are connected to the O2Lu6 double tetrahedra by isolated SiO4 units. The structure has been determined by neutron diffraction.
Development of PET instrumentation over the past 42 years has moved from simple dual-detector coincidence scanners, to proposed systems having 60,000 detectors and simultaneous coverage of 15-cm regions of the body with spatial resolutions better than 4 mm. The principal determinants of positron emission tomography (PET) instrumentation advances are positron range, noncollinearity of the annihilation photons, scattering, random event rates, detector size, efficiency, speed and light output; capability to correct for depth of crystal interaction, attenuation compensation, axial coverage, and rapid data analysis and presentation. While general-purpose systems with 2-mm resolution are expected, special-purpose PET devices are being built for breast and brain tumor studies with resolutions from 1.7 to 5 mm.
Explore the source record for details and available documents.
The intrinsic resolution of the Donner 600-crystal positron emission tomograph (PET 600) is 2.6 mm full width at half maximum (FWHM) in-plane and 6 mm FWHM axially. More than 100 patients with glioma, radiation necrosis, Alzheimer disease, or epilepsy have been studied with this system. Approximately 1 million events are acquired in 15 minutes, starting 1 hour after injection of 10 mCi (370 MBq) of fluorine-18-fluorodeoxyglucose. Normal structures as small as the superior colliculi and the external capsule have been resolved. Improved separation of the cortical ribbon from adjacent white matter has allowed more accurate determination of cortical metabolic rate. In two of 15 patients undergoing evaluation for recurrent glioma, the PET 600 images showed tumor uptake that was not apparent on a lower-resolution study. A high-activity orbiting transmission source with electronic collimation allows accurate, short-duration transmission measurements to be made after radiopharmaceutical administration. The anatomic detail seen on the transmission images can be used for reproducible patient positioning with an accuracy of 1-2 mm perpendicular to the image plane. These findings demonstrate the practicality and clinical effectiveness of high-resolution positron emission tomography.
Positron emission tomography with a spatial resolution of 2 mm full width at half maximum for quantitation in regions of interest 4 mm in diameter will become possible with the development of detectors that achieve ultrahigh resolution. Improved resolution will be possible using solid-state photodetectors for crystal identification or photomultiplier tubes with many small electron multipliers . Temporal resolution of 2 seconds and gating of cyclic events can be accomplished if statistical requirements are met. The major physical considerations in achieving high-resolution positron emission tomography are the degradation in resolution resulting from positron range, emission angle, parallax error, detector sampling density, the sensitivity of various detector materials and packing schemes, and the trade off between temporal resolution and statistical accuracy. The accuracy of data required for physiological models depends primarily on the fidelity of spatial sampling independent of statistical constraints.
Alzheimer disease is the most common cause of dementia in adults. Despite recent advances in our understanding of its anatomy and chemistry, we remain largely ignorant of its pathogenesis, physiology, diagnosis, and treatment. Dynamic positron emission tomography using [18F]fluorodeoxyglucose (FDG) was performed on the Donner 280-crystal ring in 10 subjects with dementia of the Alzheimer type and six healthy age-matched controls. Ratios comparing mean counts per resolution element in frontal, temporoparietal, and entire cortex regions in brain sections 10 mm thick obtained 40-70 min following FDG injection showed relatively less FDG uptake in the temporoparietal cortex bilaterally in all the Alzheimer subjects (p less than 0.01). Left-right alterations were less prominent than the anteroposterior changes. This diminished uptake was due to lowered rates of FDG use and suggests that the metabolic effects of Alzheimer disease are most concentrated in the temporoparietal cortex. Positron emission tomography is a most powerful tool for the noninvasive in vivo assessment of cerebral pathophysiology in dementia.
Explore the source record for details and available documents.
Positron emission tomography of the brain with 75-sec rubidium-82 obtained from a portable generator (25-day Sr-82 leads to Rb-82) was used to evaluate the integrity of the blood-brain barrier (BBB) in patients with brain tumors. Rubidium is normally excluded from the central nervous system by the intact BBB, but when the BBB is disrupted by a tumor. Rb enters and pools in the extravascular spaces of the central nervous system. Since Rb is also rapidly cleared from the blood, a high tissue-to-blood ratio of the Rb-82 tracer is achieved in regions of BBB disruption after intravenous injection. With dynamic positron emission tomographic imaging, the extravasation of the Rb tracer can be evaluated independent of the intravascular Rb concentration, and very small changes in the BBB permeability can be detected. The results of our studies in eight patients show that this technique is a promising method for evaluation of the BBB integrity in brain-tumor patients.U
This report presents analytical formulas for the image-forming and background event rates seen by circular positron-emission tomographs with parallel side shielding. These formulas include deadtime losses, detector efficiency, coincidence resolving time, amount of activity, patient port diameter, shielding gap, and shielding depth. A figure of merit, defined in terms of these quantitites, describes the signal-to-noise ratio in the reconstructed image of a 20-cm cylinder of water with uniformly dispersed activity. For 1-cm-wide nal (TI) detectors, a 50-cm patient port, an activity of 200 microCi per axial centimeter, and a shielding gap of 2 cm, the optimum shielding depth is 20 cm, which requires a detector circle diameter of 90 cm. For a 25-cm patient port and other conditions as above, the optimum shielding depth is 14 cm. Results are presented for the scintillators Nal(TI), bismuth germanate (BGO), CsF, and plastic; and for Ge(Li) and wire chambers with converters. In these examples, BGO provided the best signal-to-noise activity levels below 1000 microCi per cm, and CsF had the advantage for higher activity levels.
Statistical uncertainties in emission computed tomography were simulated in 60 computer studies involving various numbers of events and distributions of activity. Previous studies have shown that for a uniform disc of activity of rms percentage of uncertainty per resolution cell is: 120 X (number of resolution cells)1/4 X (number of events per resolution cell)- 1/2. In this work we examined the more general situation where one or two regions of uniform activity are surrounded by a uniform background, and found that for an equal number of recorded events the uncertainties were reduced when the activity was concentrated in a portion of the field. The empirical relation rms % uncertainty in nt = 120(N)1/4(nt)-3/4, where nt is the number of events in an average target (organ) resolution cell and N is the total number of events recorded, satisfactorily described the relationships between uncertainties, contrast, total number of detected events, and number of resolution cells for all 60 computer studies. By means of this relation, we show the theoretical possibility of gated cardiac imaging with 20% uncertainty in 1 cm X 1 cm regions, and of 1-sec cerebral blood-flow images with 20% uncertainty in 2 cm X 2 cm regions.
Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20-cm cylinder imaged in transverse section, only 20% of the positron annihilation events are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.
Transverse sections of the distribution of 129Cs and 201Ti in the human myocardium were obtained using 36 and 72 views of the thorax with a large field of view Anger camera. The cardiac cycle was divided into 100-msec intervals to obtain motion images of an average cycle of the beating heart. At least 300,000 events must be detected for each cardiac phase of each section for quantitative work. Over 8 million events from the upper thorax must be accumulated in gated studies if three or more sections are obtained for 8 intervals of 100 msec to 150 msec.
Explore the source record for details and available documents.
This paper presents an analytical study of a high-resolution positron ring detector system for transaxial reconstruction tomography. Our goal is a combination of good spatial resolution, high sensitivity, rejection of scattered photons, variable section thickness, and the minimization of the number of photomultipliers and coincidence circuits. A circular ring of 288 NaI(Tl) crystals 0.8 cm wide should provide a resolution of 4--7 mm FWHM over a circular region 30 cm in diameter. Coded light pipes permit readout using only 72 photomultipliers and 12 coincidence circuits. With properly designed shielding and an energy resolution of 30% FWHM, a positron activity of 200 muCi per axial centimeter in a 20-cm-diam cylinder of tissue should provide approximately 7,000 events/sex from a 2-cm-thick transaxial section (including a 5% accidental coincident background and a 26% scattered coincident background). This rate is adequate for both static and dynamic imaging. The device can operate at two to three times higher event rates with increased backgrounds.
Explore the source record for details and available documents.