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Biomedical subjects

G T Gullberg

Publications and source records attributed to G T Gullberg.

71 records · Page 4Linked to original sources
PubMed1982

Regional distribution of myocardial blood flow measured by single-photon emission tomography: comparison with in vitro counting.

An emission computed tomography system (SPECT), which uses a single large-field-of-view gamma camera, was evaluated for its ability to measure the relative distribution of myocardial blood flow and to assess the effect of attenuation, scatter, and cardiac motion on the tomographic images. Normalized regional myocardial counts from the SPECT images of the living dogs correlated closely with those from the anatomic slices and the samples counted at necropsy except for an over-estimate of tracer in the perfusion defect (SPECT) 57.7 compared to tissue count 32.1; p less than 0.05. The differences were less for the other imaging conditions. Heart and thorax motion, attenuation, and scatter contributed less than 25% to the over-estimate of defect counts. We conclude that the SPECT system accurately reflects regional distribution of myocardial blood flow except for overestimation of flow in regions of perfusion defects. Small perfusion defects might therefore be missed, but no artifactual defects are created.

Animals↗
PubMed1978

Quantitative potentials of dynamic emission computed tomography.

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.

Computers↗
PubMed1977

Emission computer assisted tomography with single-photon and positron annihilation photon emitters.

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.

Elementary Particles↗
PubMed1977

Three dimensional imaging of the myocardium with radionuclides.

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.

Animals↗
PubMedDate not supplied

Tuning and matching networks for MR imaging and spectroscopy.

The authors briefly discuss four tuning and matching networks and their application range in magnetic resonance (MR) imaging and MR spectroscopy. Analytic expressions for the network elements needed to tune and match an MR imaging or spectroscopy probe for a given coil impedance are given, and the connection to the "standard" two-element capacitive matching networks is shown. The power loss factor and the phase shift between the input current and the load current are explicitly calculated for each of the four networks. The merits of the different matching networks are then discussed in the context of a general-purpose license plate coil commonly used in MR imaging.

Computer Communication Networks↗
PubMedDate not supplied

A mathematical model for signal from spins flowing during the application of spin echo pulse sequences.

Models are presented for both laminar and plug flow that predict the signal from spins flowing during the application of slice-selective spin echo pulse sequences. The models permit calculation of the total signal from a cylindrical vessel lying perpendicular to the slice and incorporate the effect of the physical displacement of the spins between successive excitations. This time-of-flight effect gives a signal which is composed of contributions from a finite number of spin populations, with each population signal weighted by the fractional volume of that spin population within the cylindrical vessel segment. The signal and fractional volume from each spin population are derived analytically for ten different spin echo pulse sequences. The models for plug and laminar flow have important application for predicting and interpreting flow effects observed in clinical images. They are shown to be useful for selecting pairs of pulse sequences that can be used to obtain digitally subtracted MR images which provide optimum contrast for flowing blood with essentially complete suppression of stationary anatomy. These models provide a means for quantitatively comparing the expected signal from flowing spins for the many techniques presently being investigated for MR angiography.

Blood Circulation↗
PubMedDate not supplied

Experimental verification of technetium 99m-labeled teboroxime kinetic parameters in the myocardium with dynamic single-photon emission computed tomography: reproducibility, correlation to flow, and susceptibility to extravascular contamination.

BACKGROUND: This research was undertaken to investigate the ability of dynamic single-photon emission computed tomographic (SPECT) imaging to quantify myocardial perfusion in vivo with 99mTc-labeled teboroxime. The objectives of this experiment were (1) to determine the reproducibility of the model parameters fv, K1, and k2 (volume fraction of blood in the tissue and wash-in and wash-out of 99mTc-labeled teboroxime in the myocardium, respectively), (2) to determine if the kinetics of 99mTc-labeled teboroxime correlate with an independent measure of myocardial blood perfusion, and (3) to determine the amount of extravascular activity in the arterial blood time-activity curve generated from the dynamic SPECT images. METHODS AND RESULTS: Twenty-nine studies were performed on 10 dogs. The dogs were divided into three groups: reproducibility, occlusion, and vasodilation. In the reproducibility protocol, the physiology was kept as constant as possible. In the occlusion and vasodilation protocols, myocardial perfusion was either decreased by occluding the left anterior descending coronary artery or increased by continuous adenosine infusion, respectively. In all studies an independent measure of myocardial perfusion was obtained with either radiolabeled microspheres or a flow probe, and the activity of 99mTc-labeled teboroxime in the arterial blood was monitored continuously. The root-mean-square errors calculated with data from the reproducibility group were 19.9% +/- 8.5%, 14.0% +/- 5.7%, and 14.5% +/- 5.0% for fv, K1, and k2, respectively. The occlusion group data showed that K1 demonstrated a higher correlation (r = 0.887) with microsphere myocardial perfusion than did k2 (r = 0.689). The results from the vasodilation group showed that K1 demonstrated a stronger relationship to myocardial blood flow (MBF) measured with a flow probe (K1 = 0.237 + 0.629 MBF [r = 0.935] and k2 = 0.177 + 0.263 MBF [r = 0.904]). Finally, the amount of extravascular activity in the blood time-activity curve generated from the dynamic SPECT images was determined to be 45.5% +/- 16.4%. CONCLUSIONS: The wash-in parameter K1 correlates with myocardial perfusion in dogs; therefore dynamic SPECT imaging of 99mTc-labeled teboroxime has the potential for measuring regional myocardial perfusion in patients.

Adenosine↗
PubMedDate not supplied

Quantitative myocardial perfusion SPECT.

In recent years, there has been much interest in the clinical application of attenuation compensation to myocardial perfusion single photon emission computed tomography (SPECT) with the promise that accurate quantitative images can be obtained to improve clinical diagnoses. The different attenuation compensation methods that are available create confusion and some misconceptions. Also, attenuation-compensated images reveal other image-degrading effects including collimator-detector blurring and scatter that are not apparent in uncompensated images. This article presents basic concepts of the major factors that degrade the quality and quantitative accuracy of myocardial perfusion SPECT images, and includes a discussion of the various image reconstruction and compensation methods and misconceptions and pitfalls in implementation. The differences between the various compensation methods and their performance are demonstrated. Particular emphasis is directed to an approach that promises to provide quantitative myocardial perfusion SPECT images by accurately compensating for the 3-dimensional (3-D) attenuation, collimator-detector response, and scatter effects. With advances in the computer hardware and optimized implementation techniques, quantitatively accurate and high-quality myocardial perfusion SPECT images can be obtained in clinically acceptable processing time. Examples from simulation, phantom, and patient studies are used to demonstrate the various aspects of the investigation. We conclude that quantitative myocardial perfusion SPECT, which holds great promise to improve clinical diagnosis, is an achievable goal in the near future.

Algorithms↗
PubMedDate not supplied

Gibbs artifact removal in magnetic resonance imaging.

Gibbs artifact in magnetic resonance imaging results when band-limited interpolation is used. This is typically done when there are more reconstructed pixels in the phase encoding direction of the image than corresponding phase encoding measurements. Such sampling is effectively an ideal (in a noise sense) low-pass filter which provides a maximal improvement in contrast resolution at the expense of a decrease in spatial resolution. In this paper we demonstrate that an alternate low-pass filter can be used to improve contrast resolution with a loss in spatial resolution and yet not result in Gibbs artifact. We show that the noise performance of this filter can be made to approach that of an ideal filter by properly specifying the number of samples averaged for each phase encoding index.

Humans↗
PubMedDate not supplied

The solution of Bloch equations for flowing spins during a selective pulse using a finite difference method.

The movement of spins during periods of selective pulses result in a modulation of the signal intensity and phase of the received magnetic resonance imaging (MRI) signal, and is a major cause of signal loss from vessels imaged with slice-selective pulses. Methods are well developed for compensation of phase perturbations for spins flowing at constant velocity during the time of applied gradients. However, for spins flowing during selective pulses, the magnitude of the amplitude and phase perturbations has not been understood nor to this time has any method of flow compensation been proposed. This is due in part to the difficulty in using the Bloch equations to quantify the amplitude and phase modulation during radiofrequency (rf) excitation since solutions cannot be obtained analytically. In this paper a finite difference method is used to solve Bloch equations for flowing spins during a 90 degrees selective pulse. Compared with stationary spins, the magnetization distribution for flowing spins exhibits a shift of the slice profile in the direction of the flow, an expansion of the profile, phase shifts, and changes in profile shape. The profiles show residual phase errors which become more severe with higher flow velocities, with flow compensation schemes which apply in the case of spins flowing during applied gradients, and in the absence of an rf pulse. The measurement and understanding of the magnetization distribution is important to designing pulse sequences that compensate for flow. Flow compensated pulse sequences are necessary to reduce image flow artifacts and to increase signal of vessels in MR angiographic images.

Computers↗
PubMedDate not supplied

An analytical approach to quantify uniformity artifacts for circular and noncircular detector motion in single photon emission computed tomography imaging.

Uniformity artifacts in rotating gamma camera tomography will result if there are errors in the correction factors which are routinely calculated from a static uniformity flood image. The accuracy of the correction factors is a function of the statistics in the collected flood image. Since the factors are applied to each projection view, an error in a correction factor will propagate as a projection error at the same pixel location for each view. For circular detector motion, the error in each projection is reconstructed as a ring whose maximum amplitude varies approximately inversely proportional to the square root of the distance of the projection error from the center of rotation. For noncircular detector motion the artifacts are not rings but are more complicated geometric curves. Simulations show that statistical fluctuations in the reconstructed image will mask the uniformity artifacts provided the correction flood satisfies minimum count requirements. An analytical expression is derived for the percent root-mean-square (% rms) error in the reconstruction and is compared with the percent relative amplitude error (% RAE) of the reconstructed artifacts in order to obtain expressions for uniformity flood counting statistics. For an elliptical source distribution with total counts equal to CT, the uniformity statistics required to reconstruct elliptical disks is inversely proportional to the square root of the area: U greater than or equal to KCT/area 1/2. The constant K depends on the filter function and type of detector motion.

Biophysical Phenomena↗
PubMedDate not supplied

Reconstruction for fan beam with an angular-dependent displaced center-of-rotation.

A convolutional backprojection algorithm is derived for a fan beam geometry that has an angular-dependent displacement in its center-of-rotation from the midline of the fan beam. In both x-ray computed tomography and single photon emission computed tomography, misalignment can occur when the mechanical center-of-rotation is not colinear with midline of the fan beam. In some cases the shift in the center-of-rotation is constant for every angle, whereas, in other cases it varies with angular position. Standard reconstruction algorithms, which directly filter and backproject the fan beam data without rebinning into parallel beam geometry, have been derived for a geometry having its center-of-rotation at the midline of the fan beam. However, in the case of any misalignment of the center-of-rotation, if these conventional reconstruction algorithms are used to reconstruct the fan beam projections, structured artifacts and a loss of resolution will result. Simulations are performed that illustrate these artifacts and demonstrate how the new algorithm corrects for this misalignment. A method for estimating the parameters of the fan beam geometry, including the angular-dependent shift in the center-of-rotation, is also described.

Algorithms↗
PubMedDate not supplied

Signal-to-noise efficiency in magnetic resonance imaging.

The dependence of signal-to-noise ratio (SNR) on the analog filter, the sampling rate and the number and dimensions of voxels is derived for magnetic resonance imaging (MRI). It is shown that the object signal-to-noise ratio scales directly with the voxel volume and the square root of the number of voxels. Defining an efficiency figure of merit as the SNR divided by the square root of the imaging time, it is shown that efficiency is always improved when imaging with the lowest possible resolution (largest voxel dimensions) consistent with viewing the desired anatomical detail. The results directly imply the relative efficiency of 3-D (volume), 2-D (plane), 1-D (line) and 0-D (point) imaging techniques. It is shown that spatial averaging is an inefficient method of noise reduction in MRI. As long as voxel size is maintained constant, one can image as many pixels in the readout direction as desired with no loss in SNR; that is, the number of pixels in the readout direction has no effect on the image SNR. Further, multiple sampling of each phase encoding value (to improve SNR) has no advantage over increasing the number of pixels in the phase encoding direction while leaving the voxel size constant. Some experimental observations are given.

Information Theory↗
PubMedDate not supplied

Estimation of geometrical parameters and collimator evaluation for cone beam tomography.

A method is presented for estimating the geometrical parameters for a cone beam detector geometry from the coordinates of the centroid of a projected point source sampled over 360 degrees. Nonlinear expressions are derived for the coordinates of the centroids in terms of the geometrical parameters which include: the two-dimensional coordinates of the projection of the center of rotation onto the detector image plane; the focal length; the distance from the focal point to the center of rotation; and the spatial coordinates of the point source itself. Experimental data were obtained using a rotating gamma camera with a symmetrically converging collimator. The Marquardt algorithm was used to estimate the parameters for this particular cone beam geometry. The method was able to estimate the geometrical parameters and evaluate the accuracy of the collimator construction.

Evaluation Studies as Topic↗