Steven W. Smith

Simulation studies of dual‐energy x‐ray absorptiometry

Computer simulations were performed to predict the performance characteristics of dual-energy x-ray absorptiometry. K-edge filter techniques were analyzed in detail and compared to 153Gd sources in terms of output intensity, precision, patient dose, image contrast, beam hardening, and marrow fat effects. Similar analyses were performed for two dual-kV techniques that have been reported in the literature. The simulations indicate that K-edge filter techniques, or a dual-kV technique combined with K-edge filtering, can provide performance capabilities that equal or exceed those achievable with 153Gd systems. A (70/140) dual-kV technique with conventional (Al or Cu) filtration also has advantages in terms of source output intensity, but is 2-3 X inferior to the K-edge techniques and 153Gd in terms of patient dose and beam-hardening effects.

Reconstruction of blood vessels from x-ray subtraction projections: limited angle geometry

Several algorithms have been investigated for reconstructing blood vessels from a limited number of x-ray subtraction projections, distributed over a limited range of angles. Both computer simulations and an in vivo animal study were carried out. The best reconstruction performance was achieved using an algorithm that folded in two pieces of a priori knowledge of the vascular density distributions: (1) the object is dilute, consisting mainly of a void; and (2) the density distribution in the reconstructions is most likely to be non-negative. Both the signal-to-noise ratio (SNR) and the signal to out-of-focus blur were quantitated. Compared to tomosynthetic reconstruction (backprojection), the amount of residual blur from out-of-focus planes was significantly reduced with only a small penalty in diminished SNR. The combined effect resulted in significant qualitative image improvement for real arterial distributions as demonstrated in a canine arterial imaging example.

Current noise in sodium β‘ alumina ceramic

Experimental bulk current noise spectra for Na β‘ alumina ceramics are observed over the frequency range f=3×10−3 Hz to 104 Hz and with low contact noise, amalgam electrodes. The noise is proportional to the square of the dc current and varies as f−3/2, suggesting bulk conductivity fluctuations arising from diffusion of the conducting ions. The results indicate that individual grains in the ceramic are not stochastically independent and that only a small fraction of the conducting ions present participate in the diffusion noise process. The number of such ions is thermally activated with an activation energy of 0.75 eV. Chemical and electrochemical contact noise indicates a reaction at the ceramic‐amalgam interface characterized by a reaction rate of 6.3×102 sec−1, even for low‐noise contacts.

A signal processing model of diagnostic x‐ray scatter

A model of scatter is developed from a signal processing approach. The scattering process is viewed as a nonlinear filter (NLF), which transforms a two-dimensional signal representing phantom thickness into a two-dimensional signal of scattered radiation. The NLF point spread function (PSF) is derived from a single scattering model, using the principles of Compton scattering and x-ray attenuation. The PSF is characterized by three approximations: a constant geometric shape, a volume that depends on the phantom thickness, and a width that depends on the phantom-to-detector distance. This leads to a closed form expression for the scatter-to-primary ratio as a function of phantom thickness, field size, photon energy, source-to-phantom distance, and phantom-to-detector distance. The NLF model is compared with previously reported measurements using constant thickness phantoms, and discrepancies are discussed. The good agreement found between the NLF model and measured data shows that the functional dependence of scatter on the above parameters, previously only explained in terms of empirical models or Monte Carlo simulations, can be incorporated into a signal processing model.

Spectral averaging and low‐frequency sampling modifications for the IQS 401 fast Fourier transform spectrum analyzer

The IQS 401 spectrum analyzer, a commercial addition to the Apple II and III personal computers, can be modified for low‐cost power spectrum measurements of random noise signals and similar waveforms. The IQS basic control program is changed to perform low‐frequency sampling, spectral averaging, and spectrum scaling. External analog filtering must be provided on the lowest sampling rates; however, no change is made to the original IQS hardware. Resolution of 0.5 dB is obtainable from 10−4 Hz to 20 kHz.

Electrode noise in beta aluminas

Experimental contact noise spectra of Na β″ alumina and Ag β″ alumina ceramics at room temperature exceed low‐frequency thermal noise for both blocking and ohmic electrodes. Contact noise spectra above 102 Hz for ohmic electrodes of sodium amalgam and NaI in propylene carbonate agree with Nyquist noise levels determined from ac impedance measurements, and the low‐frequency excess noise is shown to be associated with nonequilibrium electrochemical processes. Similar effects are found for ohmic silver amalgam and aqueous AgNO3 electrodes on Ag β″ alumina. Greatly increased voltage fluctuations are observed to accompany current in the sample, and both contact current noise and bulk granular current noise have similar characteristics. A total dc charge flow fo 25 C/cm2 at sodium amalgam electrodes to Na β″ alumina increases both contact and current noise many orders of magnitude and can be returned to initial levels by equivalent reverse charge flow. No corresponding changes in conventional experimental measu...

Fast Circular Tomography Device for Cardiac Imaging: Image Deflection Mechanism and Evaluation

A circular tomography system for cardiac imaging is described and measurements of the detector system are presented. Fast tomographic motion is obtained with a custom-made rotating focal spot X-ray tube combined with servo-operated scanning mirrors that deflect the optical image between a large field of view image intensifier and television camera. The motion of the deflection mirrors is locked in frequency and phase to the position of the focal spot with precision analog electronics. Initial measurements on the system components indicate that circular tomograms of the beating heart can be obtained in 1/60 s at rates as high as 30/s. Image degradation due to focal spot tracking error is minimal; however, the optical temporal lag of a P-20 type image intensifier output phosphor results in a factor of three contrast loss for millimeter-sized objects. A linear system analysis in conjunction with the measured temporal step response of the phosphor is in good agreement with the magnitude of this contrast loss.

Internal noise of low‐frequency preamplifiers

Experimental measurements of low‐frequency preamplifiers show that a type LM394 bipolar input stage has less internal input noise than the popular PAR‐113 commercial amplifier for source resistances under 10 kΩ. A type 2N6483 JFET design shows similar input noise to the PAR‐113. The input current noise of the JFET design is insignificant compared with the Nyquist noise of the source resistance.

Low‐noise Hall probe preamplifier

A low‐noise preamplifier circuit for use with Halltron HR‐66 Hall effect probes in the frequency range dc to 100 kHz, shows a wideband noise of 6‐ and 2‐mG rms for bandwidths of dc to 100 kHz and dc to 100 Hz, respectively. Below 100 Hz the noise is due to the 1/f noise from the Hall probe, while white preamplifier noise dominates above 100 Hz. Above 1 kHz, preamplifier noise exceeds the noise of the Hall probe alone by a factor of five. dc drift of the system is approximately 10 mG in 24 h.

NOISE IN SILVER β″ ALUMINA CERAMICS

Voltage fluctuations at ohmic electrodes to silver β″ alumina ceramic samples are observed both at contacts and in the bulk over the frequency interval 10- 3 to 10 4 Hz. Contact noise power in the absence of current varies as f −2 at low frequencies and is dominated by Nyquist noise of the sample at frequencies greater than 100 Hz. Bulk current noise measured at transverse contacts has a f −3/2 power spectrum and increases with the square of the current. Low-frequency contact noise, sample Nyquist noise and bulk current noise are all thermally activated with activation energies of -0.96, 0.18 and 0.47 eV, respectively. These experimental results are very similar to those previously reported for sodium β″ alumina ceramics.