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A Fenster

Publications and source records attributed to A Fenster.

At least 55 records · Page 3Linked to original sources
PubMed1997

A real vessel phantom for imaging experimentation.

Vascular phantoms are used to evaluate imaging techniques such as ultrasound (US), CT, and angiography. They are expected to mimic the vasculature, surrounding tissue, and blood, and therefore must meet specific requirements on the mimicking materials, with respect to x-ray attenuation and acoustic properties (velocity, attenuation). In the past, researchers have used a variety of vessel models, including walled (typically latex tube) and wall-less phantoms (obtained by moulding a lumen in a block of agar). These models lacked the exact geometry of human vessels as well as pathologic features such as plaques and calcifications. To overcome these disadvantages, this paper describes a real vessel phantom for US and x-ray studies. The phantom consists of an agar-filled acrylic box containing a formaldehyde fixed section of a real human vessel (obtained at autopsy) cannulated onto two acrylic tubes. This phantom was evaluated by comparing the images obtained with x-ray angiography, CT, and 3-D B-mode US. The images show good overall correlation based on the location of the geometrical features within the phantom, such as lumen, plaques, and calcifications. Discrepancies, artifacts, and difficulties were minor, and are discussed. The use of a real vessel, with its natural geometry and pathology, makes this phantom attractive for evaluation of imaging techniques including projection radiography, CT and US, and for extending its use to MR and US based flow studies.

Angiography↗
PubMed1997

A quantum accounting and detective quantum efficiency analysis for video-based portal imaging.

The quality of images generated with radiographic imaging systems can be degraded if an inadequate number of secondary quanta are used at any stage before production of the final image. A theoretical technique known as a "quantum accounting diagram" (QAD) analysis has been developed recently to predict the detective quantum efficiency (DQE) of an imaging system as a function of spatial frequency based on an analysis of the propagation of quanta. It is used to determine the "quantum sink" stage(s) (stages which degrade the DQE of an imaging system due to quantum noise caused by a finite number of quanta), and to suggest design improvements to maximize image quality. We have used this QAD analysis to evaluate a video-based portal imaging system to determine where changes in design will have the most benefit. The system consists of a thick phosphor layer bonded to a 1 mm thick copper plate which is viewed by a T.V. camera. The imaging system has been modeled as ten cascaded stages, including: (i) conversion of x-ray quanta to light quanta; (ii) collection of light by a lens; (iii) detection of light quanta by a T.V. camera; (iv) the various blurring processes involved with each component of the imaging system; and, (v) addition of noise from the T.V. camera. The theoretical DQE obtained with the QAD analysis is in excellent agreement with the experimental DQE determined from previously published data. It is shown that the DQE is degraded at low spatial frequencies (< 0.25 cycles/mm) by quantum sinks both in the number of detected x rays and the number of detected optical quanta. At higher spatial frequencies, the optical quantum sink becomes the limiting factor in image quality. The secondary quantum sinks can be prevented, up to a spatial frequency of 0.5 cycles/mm, by increasing the overall system gain by a factor of 9 or more, or by improving the modulation transfer function (MTF) of components in the optical chain.

Biophysical Phenomena↗
PubMed1997

X-ray imaging technique for in vitro tissue composition measurements using saline/iodine displacement: experimental verification.

A novel in vitro radiographic technique using saline/iodine displacement, which can be used to study the bone-equivalent and soft-tissue-equivalent thicknesses within vessel walls, was applied to imaging of arterial specimens. Results concerning the accuracy and precision of the bone-equivalent and soft-tissue-equivalent thickness measurements obtained with this technique are reported and discussed. Planar radiographs of a phantom were obtained under two different conditions: (1) when it is immersed in an isotonic saline solution using a 45-kVp spectrum with no added filtration, and (2) when it is immersed in a concentrated iodine solution using a 100-kVp spectrum with 12.5-mm aluminum-added filtration. Calibration step wedges made out of bone-mimicking and soft-tissue-mimicking materials are imaged simultaneously to generate calibration curves that are used to convert the radiographs into bone-equivalent and total-thickness images. A soft-tissue-thickness image is obtained from the subtraction of the bone-equivalent image from the total-thickness image. Thickness measurements obtained from these images yielded average accuracies of +/- 110 microns for both the bone-equivalent and the soft-tissue-equivalent images. The precision (one standard deviation) of the thickness measurements was +/- 60 and +/- 90 microns for the bone-equivalent and the soft-tissue-equivalent images, respectively. In conclusion, since calcified plaque can become as thick as 3-4 mm, the saline/iodine displacement technique has the potential to be a very useful technique for ex vivo studies of the progression of atherosclerosis because of its high accuracy and precision.

Angiography↗
PubMed1996

Evaluation of an automated real-time spectral analysis technique.

An adaptive real-time Doppler peak-frequency tracing algorithm was evaluated in vitro and compared to manual peak-frequency traces. A computer-controlled pump was used to generate physiological flow waveforms in a vasculature-mimicking phantom. Spectral waveforms were obtained on an ATL HDI along with real-time estimates of diagnostic parameters, including maximum systolic, minimum diastolic, time-averaged peak frequencies and pulsatility and resistance indices. The effect of the signal-to-noise ratio on the measured parameters was investigated. The imprecision in the measured parameters was found to depend somewhat on the waveform shape; e.g., the imprecision in PI was 4.1% for a normal renal waveform and 8.5% for a waveform having reverse diastolic flow. The peak frequency envelopes of the same waveform data were traced manually by nine operators, and the resulting diagnostic parameters were compared to ones obtained from automated peak-frequency traces of the same waveform data. The agreement between parameters measured by the automated routine and those measured manually was found to depend somewhat on the waveform shape; e.g., the bias in the PI was 1.3% for a renal waveform lacking diastolic flow, and 12% for a waveform with reverse diastolic flow. The between-observer variations in the manual measurements ranged from 0.8% up to 9.4%. The overall variations associated with the automated traces were found to be smaller than or equal to those of the manual traces.

Algorithms↗
PubMed1996

A Doppler ultrasound clutter phantom.

We describe two variations of a phantom designed to evaluate the wall filters implemented on colour and spectral Doppler instruments. Both variations use an acoustic beam splitter to place the same Doppler sample volume within a motor-driven clutter belt and a flow source, which is either a second belt (dual-belt phantom) or a vascular phantom (belt/vascular phantom). We used the dual-belt phantom to evaluate the effects of the clutter belt velocity, flow belt velocity and clutter-to-flow power ratio on the reported colour Doppler shifts. The results show that the choice of wall filter, as well as the amplitudes and velocities of the clutter and flow components, affect the measured Doppler shifts. Results obtained with the belt/vascular phantom show that colour Doppler shifts due to the moving fluid depend strongly on the clutter velocity and choice of wall filter. However, only a small dependence on Doppler signal strength was observed.

Blood Flow Velocity↗
PubMed1996

A three-dimensional ultrasound prostate imaging system.

We have developed a three-dimensional (3D) transrectal ultrasound imaging system, based on using a motorized 5 MHz transducer assembly, rotated under microcomputer control, to collect a series of 100 two-dimensional (2D) images, digitized by a video frame-grabber. These are then reconstructed into a 3D image on a computer workstation, permitting the prostate anatomy to be visualized in three dimensions, and distance and volume measurements to be performed. The accuracy of the distance measurements was assessed with a string test phantom, and that of the volume measurements with balloons of known sizes. Also, the resolution degradation engendered by the reconstruction algorithm was assessed by comparing the full-width at half-maximum (FWHM) of string cross-sectional images in the 3D image to their 2D counterparts. The results show that distance and volume measurements are both accurate to about +/- 1%, and that the reconstruction algorithm increases the mean FWHM by 8 +/- 3% axially and 3 +/- 3% laterally.

Endosonography↗
PubMed1996

Three-dimensional power Doppler imaging: a phantom study to quantify vessel stenosis.

This study investigated whether three-dimensional (3D) power Doppler imaging can be used to quantify arterial stenosis and its potential as an alternative to x-ray angiography. Three-dimensional power Doppler images of in vitro stenotic vessels were generated under different hemodynamic conditions with a 3D power Doppler imaging system. This system includes: a Macintosh Quadra 840AV computer used to perform 3D imaging acquisition, reconstruction and display; a computer-controlled motor-driven translation assembly used to move the transducer; and an ATL Ultramark 9 HDI ultrasound system. Three vascular- and tissue-mimicking phantoms containing three wall-less stenotic vessels with area reduction of 80%, 50% and 30% were imaged with different flow rates under both steady and pulsatile flow conditions and with different Doppler angles under steady flow condition. With the use of the blood mimic, experimental results demonstrated that power Doppler imaging is nearly independent on flow velocity and Doppler angle. It was also demonstrated that 3D power Doppler imaging can produce nonpulsatile angiographic-like 3D images of the flow field. The stenotic vessels were quantified with an overall accuracy of 8.3% of the vessel area and an overall precision of 7% of the vessel area under the conditions described in this paper. It is believed that 3D power Doppler imaging can be used to quantify arterial stenosis, and in some applications it could be an alternative to x-ray angiography.

Blood Flow Velocity↗
PubMed1996

Accuracy of prostate volume measurements in vitro using three-dimensional ultrasound.

RATIONALE AND OBJECTIVES: We assessed the ability of a three-dimensional (3D) ultrasound imaging system to measure accurately prostate volume. METHODS: Multiple two-dimensional ultrasound images of cadaver prostates scanned in a water bath were reconstructed into three-dimensional (3D) images. The volumes of the prostates were calculated from these 3D images and compared with the actual volumes. Multiple 3D ultrasound volume readings were evaluated for precision. RESULTS: The slope of the best-fit line correlating 3D ultrasound estimated volume and true volume was 1.006 +/- 0.007. The average error was 0.36 +/- 1.17 cm3; the coefficient of determination (r2), which is the measure of the straight-line relationship, was .9997; and the standard error was 1.15 cm3. CONCLUSION: Three-dimensional ultrasound images accurately reflect true prostate volumes measured in vitro.

Cadaver↗
PubMed1996

Three-dimensional ultrasound imaging of the eye.

We assessed whether an inexpensive, three-dimensional (3D) ultrasound (US) imaging system could produce clinically useful 3D images, without causing patient discomfort. Five patients were examined. The 3D US system consisted of a transducer holder containing a mechanical motor, and a microcomputer. During data acquisition the transducer was mechanically rotated for 22 seconds, while 200 two-dimensional (2D) US images were collected and formed into a 3D image by the computer. The 3D image was viewed on the computer monitor. The 3D images correlated with the clinical and radiological findings. The new perspectives were helpful in diagnosing eye abnormalities and no patient discomfort occurred. The device was easy to use. It is concluded that, as good-quality 3D and 2D US images were produced quickly, with no patient discomfort, and the device is inexpensive, uncomplicated, and easily attached to existing ultrasound machines, it will probably be useful in clinical practice.

Adult↗
PubMed1996

Group therapy as an effective treatment modality for people of color.

The unique benefits for group therapy are examined with special emphasis on the treatment of blacks and Latinos. Because of racial prejudice, economic exploitation, and negative stereotypes, group forces have been especially detrimental to the personality development of people of color. The group therapist attempts to harness these powerful group forces and use them therapeutically, thus enabling people of color to relate better to others while retaining their own autonomy. Racial differences can intensely affect diagnosis, transference, countertransference, and the "real relationship." Distinctions between cultural and functional paranoia are particularly relevant in interracial groups. Implications for training group therapists are also explored.

Black or African American↗
PubMed1996

The variability of manual and computer assisted quantification of multiple sclerosis lesion volumes.

The high resolution and excellent soft tissue contrast of Magnetic Resonance Imaging (MRI) have enabled direct, noninvasive visualization of Multiple Sclerosis (MS) lesions in vivo. This has allowed the quantification of changes in the appearance of lesions in MR exams to be used as a measure of disease state. Nevertheless, accurate quantification techniques are subject to inter- and intra-operator variability, which may hinder monitoring of disease progression. We have developed a computer program to assist an experienced operator in the quantification of MS lesions in standard spin-echo MR exams. The accuracy of assisted and manual quantification under known conditions was studied using exams of a test phantom, while inter- and intra-operator reliability and variability were studied using exams of a MS patient. Results from the phantom study show that accuracy is improved by assisted quantification. The patient exam results indicate that assisted quantification reduced inter-operator variability from 0.34 to 0.17 cm3, and reduced intra-operator variability from 0.23 to 0.15 cm3. In addition, the minimum significant change between two successive measurements of lesion volume by the same operator was 0.64 cm3 for manual quantification and 0.42 cm3 for assisted quantification. For two different operators making successive measurements, the minimum significant change was 0.94 cm3 for manual quantification, but only 0.47 cm3 for assisted quantification. Finally, the number of lesions to be monitored for an average change in volume at a given power and significance level was reduced by a factor of 2-4 by assisted quantification. These results suggest that assisted quantification may have practical applications in clinical trials, especially those that are large, multicenter, or extended over time, and therefore require lesion measurements by one or more operators.

Analysis of Variance↗
PubMed1996

Quantification of multiple sclerosis lesion volumes in 1.5 and 0.5 T anisotropically filtered and unfiltered MR exams.

Recently, guidelines for the use of MRI in the monitoring of MS have recommended the use of imaging systems with mid-field (0.5-1.0 T) or high-field (greater than 1.0 T) strengths. Higher field strengths provide many advantages, including increased signal-to-noise ratios (SNR). SNR also may be increased by post-processing algorithms that reduce noise. In this paper we evaluate the impact on operator variability of (a) lesion quantification in high-field (1.5 T) versus mid-field (0.5 T) exams; and (b) an anisotropic diffusion filter algorithm that reduces image noise without blurring or moving object boundaries. Inter- and intra-operator reliability and variability were studied using repeated quantification of lesions in 1.5 and 0.5 T filtered and unfiltered MR exams of a MS patient. Results indicate that inter-operator variability in 1.5 T unfiltered exams was 0.34 cm3 and was significantly larger than that in 1.5 T filtered (0.27 cm3), 0.5 T unfiltered (0.26 cm3), and 0.5 T filtered (0.24 cm3) exams. Similarly, intra-operator variability in 1.5 T unfiltered exams was 0.23 cm3 and was significantly larger than that in 1.5 T filtered (0.19 cm3), 0.5 T unfiltered (0.19 cm3), and 0.5 T filtered (0.18 cm3) exams. In addition, the minimum significant change between two successive measurements of lesion volume by the same operator, was 0.64 cm3 in 1.5 T unfiltered exams, but 0.53 cm3 or less in other exams. For two different operators making successive measurements, the minimum significant change was 0.94 cm3 in 1.5 T unfiltered exams, but only 0.75 cm3 or less in other exams. Finally, the number of lesions to be monitored for an average change in volume at a given power and significance level was greater by 30%-60% for quantification in 1.5 T unfiltered exams. These results suggest that inter- and intra-operator variability are reduced by anisotropic filtering, and by quantification in 0.5 T exams. Reduced operator variabilities may result from higher detail signal-to-noise ratios (dSNRs) in 0.5 T and filtered exams.

Analysis of Variance↗
PubMed1996

Angular-dependent coherent scatter measured with a diagnostic x-ray image intensifier-based imaging system.

Low-angle scatter of x rays at diagnostic energies is primarily coherent. This coherence gives rise to interference effects resulting in x-ray diffraction patterns that are characteristic of the scattering material. A method is described of imaging these low-angle (0 degree-10 degrees) x-ray diffraction properties of tissue specimens using a diagnostic x-ray beam and image intensifier-based system. The coherent-scatter cross sections of several materials measured this way are presented. It is shown theoretically that the measurements made with this system can be expressed as the mono-energetic cross section "blurred" by the x-ray spectrum using a linear superposition integral. Experimental results using aluminum powder confirm this. Using a 70 kVp x-ray beam filtered with gadolinium to reduce the spectral width, materials such as water, Lucite, and hydroxyapatite all have significantly different diffraction patterns. The cross sections determined from this analysis from the basis of a unique method of characterizing and identifying tissue samples according to their atomic structure rather than x-ray attenuation properties.

Durapatite↗
PubMed1996

Medical physics.

Explore the source record for details and available documents.

Biophysics↗
PubMed1996

Three-dimensional prostate ultrasound and its application to cryosurgery.

Transrectal ultrasound (TRUS), despite its undeniable utility in prostate imaging, employs a spatially flexible and variable 2-dimensional (2-D) imaging technique to visualize a 3-dimensional (3-D) disease process and anatomy. To circumvent some of the limitations of 2-D TRUS, 3-D TRUS was developed. The system consists of a motorized assembly (to hold a standard ultrasound transducer) and a computer with a video frame grabber. The microcomputer, using software developed in our laboratory, rapidly reconstructs a series of linear ultrasound images into a 3-D image. The 3-D TRUS technology has been incorporated into our program for cryoablation of the prostate. With intraoperative 3-D TRUS providing a unique coronal view, cryoprobe placement is facilitated to detect malalignment and avoid subsequent suboptimal cryoablation. In our use of 3-D TRUS guidance, in all 21 initial patients, probe malalignment, otherwise not apparent on standard 2-D TRUS, was detected and corrected before cryosurgery. With median follow-up of just < 1 year, only 2 of 44 biopsy cores (from the same site of 1 patient) were positive at 3 and 6 months. 3-D TRUS appears to be a valuable adjunctive tool in prostate imaging, especially in providing guidance for cryoblation of prostate cancer.

Cryosurgery↗
PubMed1995

Quantitative investigation of in vitro flow using three-dimensional colour Doppler ultrasound.

A quantitative in vitro flow study was performed by using a three-dimensional colour Doppler imaging system. This system was based on a clinical ultrasound instrument with its transducer mounted on a motor-driven translation stage. A vascular and tissue-mimicking phantom containing two wall-less vessels, one normal and another stenotic, was used to quantify the measurement accuracy of the flow velocity and the flow field. Steady state flows, having Reynolds numbers ranging between 460 and 1300, were generated by a computer-controlled positive displacement pump. Effects of the parameter settings of the ultrasound instrument on results of the estimation of flow field were also studied. Experimental results show that our three-dimensional colour Doppler system's velocity accuracy was better than 7% of the Nyquist velocity and its spatial accuracy was better than 0.5 mm. The system showed a good correlation (r = 0.999) between the estimated and the true mean flow velocity, and a good correlation (r = 0.998) between the estimated maximum and the true mean flow velocity. This study is our first step toward validating the measurement of the three-dimensional velocity and wall shear stress distributions by using three-dimensional colour Doppler ultrasound

Blood Flow Velocity↗
PubMed1995

A wall-less vessel phantom for Doppler ultrasound studies.

Doppler ultrasound flow measurement techniques are often validated using phantoms that simulate the vasculature, surrounding tissue and blood. Many researchers use rubber tubing to mimic blood vessels because of the realistic acoustic impedance, robust physical properties and wide range of available sizes. However, rubber tubing has a very high acoustic attenuation, which may introduce artefacts into the Doppler measurements. We describe the construction of a wall-less vessel phantom that eliminates the highly attenuating wall and reduces impedance mismatches between the vessel lumen and tissue mimic. An agar-based tissue mimic and a blood mimic are described and their acoustic attenuation coefficients and velocities are characterised. The high attenuation of the latex rubber tubing resulted in pronounced shadowing in B-mode images; however, an image of a wall-less vessel phantom did not show any shadowing. We show that the effects of the highly attenuating latex rubber vessels on Doppler amplitude spectra depend on the vessel diameter and ultrasound beam width. In this study, only small differences were observed in spectra obtained from 0.6 cm inside diameter thin-wall latex, thick-wall latex and wall-less vessel phantoms. However, a computer model predicted that the spectrum obtained from a 0.3-cm inside diameter latex-wall vessel would be significantly different than the spectrum obtained from a wall-less vessel phantom, thus resulting in an overestimation of the average fluid velocity. These results suggest that care must be taken to ensure that the Doppler measurements are not distorted by the highly attenuating wall material. In addition, the results show that a wall-less vessel phantom is preferable when measuring flow in small vessels.

Acoustics↗