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Biomedical subjects

C A Mistretta

Publications and source records attributed to C A Mistretta.

At least 91 records · Page 5Linked to original sources
PubMedDate not supplied

Effect of and correction for in-plane myocardial motion on estimates of coronary-volume flow rates.

The sensitivities of phase-difference (PD) and complex-difference (CD) processing strategies to in-plane motion were examined theoretically and experimentally. Errors in velocity and volume flow rate (VFR) estimates were attributed to (a) motion between different velocity encodings and, in the case of segmented k-space acquisition strategies, (b) motion over the segment duration. PD estimates were found to be insensitive to in-plane motion between velocity encodings, whereas CD VFR estimates were found to be sensitive to this motion. PD estimates, however, were affected by partial volume effects. A corrected CD (CD') scheme was developed that minimizes both partial-volume and in-plane motion effects. Segmented k-space acquisitions with sequential offset and sequential interleaved offset (or centric) phase-encoding schemes were studied. Images obtained using these techniques were found to include both blurring and replication artifacts. The amount of artifact generally increased with the number of views per segment (vps) and the in-plane velocity. PD, CD, and CD' VFR estimates were found to be degraded by these artifacts. The sequential offset phase-encoding scheme generally had acceptable VFR errors (at 4 vps. a CD' VFR error of 7.0%) when averaged over the physiologic range of myocardial motion (> 12 cm second-1); however, larger errors were observed outside this range. VFR estimates obtained using the sequential interleaved phase-encoding scheme at 4 vps were unacceptable. More accurate VFR measurements were obtained using a revised segmented PC strategy, which reversed the order in which the velocity and phase encodings were interleaved. The weighted average CD' VFR error obtained using the revised strategy was 24.5% (for 4 vps). Using displacement information obtained from the two velocity-encoded images, an estimate of the in-plane velocity was obtained and used to correct the acquired data. This decreased the VFR error (weighted average CD' error at 4 vps decreased from 24.5% to -6.3%); however, the implemented correction algorithm could potentially introduced other artifacts in the images.

Algorithms↗
PubMedDate not supplied

Real-time digital K-edge subtraction fluoroscopy.

We report in vitro and in vivo trials of K-edge fluoroscopy, by which iodine contrast concentration is displayed live, with tissue and bone images suppressed, free of patient-motion artifacts. Iodine and cerium, 125 and 225 mg/cm2 respectively, filter alternate TV fields of cine-pulsed 50 KVP x-rays. Weighted subtraction of successive TV fields isolates the iodine image and simultaneously minimizes artifacts. Digital techniques are used in real time. At our present x-ray tube limit, 500 mA instantaneous current, the patient exposure is 180 mR/sec and quantum mottle limits the image quality. Integrating four successive difference images provides a compromise between mottle and smoothly moving displays. Cardiovascular images of a 17-kg dog, using 1 ml/kg Renografin-60 injected into a foreleg vein, show that a 15-cm chest thickness is our present practical maximum. This method may be useful in diagnosing cardiovascular anomalies in infants without catheterization or suspension of breathing.

Animals↗
PubMedDate not supplied

Computerized fluoroscopy techniques for intravenous study of cardiac chamber dynamics.

A computerized fluoroscopy system which was recently developed in our laboratories permits image contrast increases of 8-16 relative to conventional image intensifer fluoroscopy and permits study of canine and human ventricular wall motion using peripheral intravenous injections. Two time-dependent image subtraction algorithms are illustrated in connection with observation of artificially infarcted dog hearts. The first algorithm produces a display analogous to direct ventriculography using catheterization. The second displays regions of dyskinetic motion as anomalous image grey shades.

Animals↗
PubMedDate not supplied

Spectral considerations for absorption-edge fluoroscopy.

In our previous reports on absorption-edge fluoroscopy, it was not possible to relate fully the subtleties involved in the selection of spectral parameters. This paper is intended as an overview of this important aspect of the technique. It is shown that, by using the 1-kVp, 2-filter technique, it is possible to image certain elements (e.g., iodine and xenon) in the presence of tissue variations of +/-2 cm about the thickness at which perfect tissue cancellation takes place. Use of logarithmic signal processing extends this range, but bone thickness variations may not be accomodated because only two x-ray energies are involved in the imaging process. Use of a 3-kVp, 3-filter technique with logarithmic signal processing is shown to solve this problem. Computer simulations show that 1-mg/cm2 iodine may be imaged in the presence of 10 cm or more tissue variations and 2000-mg/cm2 bone variations.

Bone and Bones↗
PubMedDate not supplied

Selective iodine imaging using K-edge energies in computerized x-ray tomography.

Iodine is commonly used as a contrast material in computerized x-ray tomography. In some cases the determination of the iodine distribution in the image may be prevented by the presence of bone or tissue variations within the tomographic slice. This paper describes a method for quantitative selective imaging of the iodine concentration in the slice. The method employs scans using three heavily filtered x-ray beams, two having mean energies which straddle the iodine K edge (33 keV) and another at a slightly higher energy. The results are independent of tissue and bone over a broad range of projection path lengths. It is shown that, for separation of iodine from one other material, a two-beam K-edge approach requires less integral dose than a two-beam technique at conventional CT energies for slice diameters up to 30 cm. For selective iodine imaging in the presence of more than one other material, the three-spectrum K-edge technique is a necessity. Exposure requirements and beam-hardening corrections are discussed in detail and a computer-simulated CT image generated by the proposed scheme is presented.

Contrast Media↗
PubMedDate not supplied

Relative properties of tomography, K-edge imaging, and K-edge tomography.

The properties of tomography, K-edge imaging, and K-edge tomography are discussed in relation to the imaging of small concentrations of elements such as iodine and xenon and are compared by means of phantom images. It is demonstrated that the complementary selectivities provided by depth and energy subtraction are combined in K-edge tomography. Using a three-spectrum subtraction technique, the iodine difference signal predicted by computer calculations is on the order of 8000 times that of an equal concentration fo bone. The corresponding ratio in tomography without energy subtraction is 20:1. It is argued that K-edge tomography can successfully eliminate artifacts due to tissue inhomogeneities which presently enable 0.6% variations in tissue attenuation to mimic minimum detectable iodine signals in conventional computed tomography. Various instrumentation possibilities and energy subtraction techniques are discussed.

Models, Structural↗
PubMedDate not supplied

Limitations to iodine isolation using a dual beam non-K-edge approach.

In dual-beam selective iodine imaging, images of an object are made with each of two spectrally different x-ray beams. The mean beam energies may either straddle the 33 keV iodine K-edge or both lie above the K-edge. Both patient exposure considerations and the availability of sufficient x-ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone-to-iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x-ray spectra. Observed contrast ratios match the predicted value very well. In dual-beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum, energies for dual beam images are discussed.

Bone and Bones↗
PubMedDate not supplied

Three-beam K-edge imaging of iodine using differences between fluoroscopic video images: theoretical considerations.

Our lab has previously generated selective iodine images with an image intensifier fluoroscopic system using a three-beam K-edge approach. Logarithmically amplified video images Li were linearly combined to yield the final image k1L1 + k2L2 + k3L3. This paper discusses refinements of the K-edge technique. A study is made of the manner in which contrast-reducing effects such as x-ray scatter and image intensifier veiling glare enter into the final image. If such biases can be approximated as multiplicative and independent of the x-ray spectrum, and if the sum of the ki is zero, then the biases are canceled. Experimental data is presented which demonstrates that the inaccuracy due to such biases can be reduced by a factor as large as 10. The theorem that K1 + K2 + K3 is approximately equal to 0 is proven rigorously and discussed. Because the ki add to zero, the final image can be expressed as a linear combination of two of the differences between the Li. A difference-based algorithm which reduces biases and make allowance for nonlinearities such as beam hardening is proposed and discussed.

Fluoroscopy↗
PubMedDate not supplied

Three-beam K-edge imaging of iodine using differences between fluoroscopic video images: experimental results.

In an earlier article we discussed the rationale for using differences between video images in three-beam selective iodine K-edge imaging. Rather than combining three initial images Li linearly to yield the final image k1L1 + k2L2 + k3L3, differences between the Li were first generated and then combined either to linear or quadratic order. This approach was motivated by the desire to suppress the large multiplicative biases of fluoroscopic imaging and justified by theoretically proving that k1 + k2 + k3 is approximately equal to 0. In this paper we discuss the instrumentation and experimental results obtained from this difference-based technique. A specially-constructed apparatus is described which automatically selects the optimum combination coefficients and combines the difference images up to quadratic order at realtime video rates. Three methods for generating K-edge subtraction images are compared: the former approach in which the Li are linearly combined and combination of differences to linear and quadratic order. In imaging phantoms in which the iodine distribution is known, the resultant subtraction images from all three methods appear similar. Inspection of signal sizes shows that the quadratic difference-based approach provides superior bone and tissue residual suppression by about a factor of 2. In imaging phantoms in which the iodine distribution is unknown, incomplete suppression of x-ray scatter and image intensifier veiling glare prevent a quantitative comparison of performance of the three algorithms. An experiment verification is provided of the theorem which states that k1 + k2 + k3 is approximately equal to 0.

Fluoroscopy↗
PubMedDate not supplied

Geometrical properties of a digital beam attenuator system.

A digital beam attenuator system has been developed to automatically generate patient-specific compensating filters for chest radiography. An initial low-dose test image is used to generate the attenuator, which is fabricated by overprinting multiple layers of a heavy-metal material onto a nonattenuating substrate. The attenuator is subsequently inserted into the x-ray beam for a final compensated radiograph. The effects of focal spot blurring and limited attenuator resolution result in the final compensated image containing only high-spatial frequency information. The frequency response of the process is not strictly describable by a modulation transfer function, but an approximation of the frequencies remaining in the compensated image is obtained for low-contrast conditions. It is found that a 4 X 4 blurring function on the original 64 X 64 test image is required for the attenuator to give appropriate compensated image appearance. A proposed attenuator printing scheme prints the attenuator in a 16 X 16 matrix, staggering successively printed layers to achieve the required 64 X 64 sampling with appropriate blurring. The resulting compensated image has good anatomical definition and contains a frequency response similar to that obtained by compensation techniques being investigated by Plewes and Sorenson.

Humans↗
PubMedDate not supplied

Scatter-glare correction using a convolution algorithm with variable weighting.

Several investigators have approximated the scatter component of digital radiographic images by applying a constant weighting factor to a convolved version of the detected image. The scatter approximation is then subtracted from the detected image. When a constant weighting factor is used, the scatter in areas of low transmission is underestimated. We have extended this technique by allowing for a spatially variable weighting factor based on the local image intensity. This technique improves the scatter estimate and should provide better results for general videodensitometric applications.

Algorithms↗
PubMedDate not supplied

Scatter-glare corrections in quantitative dual-energy fluoroscopy.

Previous attempts to use time subtraction intravenous digital subtraction angiography for ventricular imaging have been hampered by artifacts due to cardiac and respiratory motion. We have previously reported a motion-immune dual-energy technique in which kVp is switched between 60 and 120, at 300-500 mA, 30 times/s. In order to quantitate parameters such as ejection fraction and left ventricular volume, it is necessary to correct for scatter and veiling glare (SVG), which are the major sources of nonlinearities in videodensitometric digital subtraction angiography (DSA). In this report, a convolution filtering method has been investigated to estimate SVG in DSA images. In the first step, a grey level transformation of the detected image is utilized to get an estimated SVG image. In the second step this image is convolved to produce an image with appropriate spatial frequency content. Estimates of SVG in several Humanoid chest phantom images were obtained using Gaussian convolution kernels with a full width at half-maximum (FWHM) of 51-125 pixels. The root-mean-square (rms) percentage error of these estimates was obtained by comparison with direct SVG measurement. A convolution kernel with a FWHM of 75 pixels in each dimension applied to 16 Humanoid phantom images with various projections, thicknesses, and beam energies resulted in an average rms percentage error of 9.7% in the SVG estimate, for the 16 cases studied. The SVG estimation consisting of grey scale-to-SVG fraction lookup table (LUT) is made based on previous measurements. The x-ray settings required for each patient are utilized to alter the LUT in order to account for patient thickness variations.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiocardiography↗
PubMedDate not supplied

Quantification techniques for dual-energy cardiac imaging.

We have previously reported a motion immune dual-energy subtraction technique in which x-ray tube voltage and x-ray beam filtration are switched at 30 Hz between 60 kVp (2.0-mm Al filter) and 120 kVp (2.0-mm Al + 2.5-mm Cu filtration). In this paper we consider the suitability of these dual-energy images for quantitative measurements of iodine thickness and volume. Optimized iodine signal-to-noise ratio (S/N) was measured as a function of phantom thickness. Using a fixed mAs, the S/N of the dual-energy images was found to decrease by sevenfold as lucite thickness increased from 10 to 25 cm. For the same increase in lucite thickness S/N for time subtraction images decreased by fivefold. Image quality in two human volunteers was subjectively judged to be good. In order to quantitate physiological parameters such as ejection fraction and left ventricular volume, energy dependent corrections for scatter and veiling glare, beam hardening, detector nonuniformity, heel effect, and uncanceled bone signals were developed. Since the dual-energy technique does not completely cancel bone, a preinjection dual-energy subtraction image was used to estimate integrated bone contributions to iodine volume measurements. In a phantom measurement simulating exercise ventriculography, the known (Vk) and videodensitometrically measured (Vm) volumes of 19 mg/cm3 solution of iodine were related by Vm = 0.95 Vk + 1.50 cm3 (r greater than 0.99).

Contrast Media↗
PubMedDate not supplied

A correlated noise reduction algorithm for dual-energy digital subtraction angiography.

It has long been recognized that the problems of motion artifacts in conventional time subtraction digital subtraction angiography (DSA) may be overcome using energy subtraction techniques. Of the variety of energy subtraction techniques investigated, non-k-edge dual-energy subtraction offers the best signal-to-noise ratio (SNR). However, this technique achieves only 55% of the temporal DSA SNR. Noise reduction techniques that average the noisier high-energy image produce various degrees of noise improvement while minimally affecting iodine contrast and resolution. A more significant improvement in dual-energy DSA iodine SNR, however, results when the correlated noise that exists in material specific images is appropriately cancelled. The correlated noise reduction (CNR) algorithm presented here follows directly from the dual-energy computed tomography work of Kalender who made explicit use of noise correlations in material specific images to reduce noise. The results are identical to those achieved using a linear version of the two-stage filtering process described by Macovski in which the selective image is filtered to reduce high-frequency noise and added to a weighted, high SNR, nonselective image which has been processed with a high-frequency bandpass filter. The dual-energy DSA CNR algorithm presented here combines selective tissue and iodine images to produce a significant increase in the iodine SNR while fully preserving iodine spatial resolution. Theoretical calculations predict a factor of 2-4 improvement in SNR compared to conventional dual-energy images. The improvement factor achieved is dependent upon the x-ray beam spectra and the size of blurring kernel used in the algorithm.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗
PubMedDate not supplied

A K alpha dual energy x-ray source for coronary angiography.

The use of characteristic-line radiation from rare-earth targets bombarded by high-energy (up to 1 MeV) electron beams has been evaluated as an x-ray source for dual energy K-edge subtraction imaging of the human coronary arteries. Two characteristic-line x-ray sources, one using the split K alpha 1 and K alpha 2 lines of lanthanum excited by a high-energy electron beam and the other using the K alpha lines of barium and cerium, were studied. A Monte Carlo electron-photon simulation was used to calculate x-ray spectra and energy deposition profiles from targets of these elements bombarded by electrons in the energy range 140 keV to 1 MeV. A general dual-energy imaging model was developed that used these calculated source spectra to numerically investigate the dependence of the subtraction image signal-to-noise ratio on such factors as the ratio of K-line to x-ray continuum yield, continuum spectral shape, x-ray filtering, and detector response. A signal averaging technique for enhancing the signal-to-noise ratio was also evaluated. The results of these calculations were used to identify an optimum electron beam, target, filter, and detector configuration. A compact electron accelerator capable of providing the required electron beam parameters was designed. Calculations indicate that under ideal conditions the optimized system would be capable of imaging 2 mg/cm2 of iodine contrast agent in 20 g/cm2 of tissue with a signal-to-noise ratio of 5, a detector pixel size of 0.25 mm2, and a total image acquisition time of 10 ms. These parameters are consistent with those needed to image the human coronary arteries after an intravenous injection of iodine contrast agent. These capabilities, along with the relatively modest hardware requirements of this system, make it attractive as an x-ray source for dual energy transvenous coronary angiography.

Computer Simulation↗