Biomedical subjects

Ernest L Madsen

Publications and source records attributed to Ernest L Madsen.

9 recordsLinked to original sources

PubMed2005

Tissue-mimicking agar/gelatin materials for use in heterogeneous elastography phantoms.

Five 9 cm x 9 cm x 9 cm phantoms, each with a 2-cm-diameter cylindrical inclusion, were produced with various dry-weight concentrations of agar and gelatin. Elastic contrasts ranged from 1.5 to 4.6, and values of the storage modulus (real part of the complex Young's modulus) were all in the soft tissue range. Additives assured immunity from bacterial invasion and can produce tissue-mimicking ultrasound and NMR properties. Monitoring of strain ratios over a 7 to 10 month period indicated that the mechanical properties of the phantoms were stable, allowing about 1 month for the phantom to reach chemical equilibrium. The only dependable method for determining the storage moduli of the inclusions is to make measurements on samples excised from the phantoms. If it is desired to produce and accurately characterize a phantom with small inclusions with other shapes, such as an array of small spheres, an auxiliary phantom with the geometry of the cylindrical inclusion phantoms or the equivalent should be made at the same time using the same materials. The elastic contrast can then be determined using samples excised from the auxiliary phantom. A small increase of about 10% in volume of the cylindrical inclusions occurred-a tolerable increase. Interestingly, the smallest increase (about 5%) occurred in the phantom with the largest elastic contrast.

Adhesives↗

PubMed2005

Tissue-mimicking phantom materials for narrowband and ultrawideband microwave applications.

We propose and characterize oil-in-gelatin dispersions that approximate the dispersive dielectric properties of a variety of human soft tissues over the microwave frequency range from 500 MHz to 20 GHz. Different tissues are mimicked by selection of an appropriate concentration of oil. The materials possess long-term stability and can be employed in heterogeneous configurations without change in geometry or dielectric properties due to osmotic effects. Thus, these materials can be used to construct heterogeneous phantoms, including anthropomorphic types, for narrowband and ultrawideband microwave technologies, such as breast cancer detection and imaging systems.

Breast Neoplasms↗

PubMed2005

Imaging the shear modulus of the heel fat pads.

BACKGROUND: Steady state, dynamic MR elastography provides quantitative images of the shear modulus of tissues in vivo. MR elastography was evaluated for its ability to characterize the mechanical properties of the weight bearing plantar soft tissues in vivo. METHODS: MR elastography was used to image the heel fat pad and surrounding soft tissues when the subject applied a low pressure on the foot and again when the subject applied high pressure. The placement of the foot was identical for both sets of images. FINDINGS: The results agree well with expected trends. The shear modulus of the tissue under the calcaneus increased from 8 kPa to 12 kPa with increasing pressure while that of peripheral tissues remained constant at 8 kPa which is similar to the shear modulus of fat in breast tissue. INTERPRETATION: Preliminary results from the steady state MR elastography methods being developed to measure the shear modulus of plantar soft tissues are promising. MR elastography is sufficiently accurate to observe the change in shear modulus with changes in applied pressure and is capable of characterizing the mechanical properties of the plantar soft tissues. Detailed anatomic information can be combined with co-registered mechanical properties. MR elastography could play a significant role in understanding the weight bearing functions of the plantar soft tissues and in evaluating those structures for improved diagnosis and assessment of disease progression.

Adipose Tissue↗

PubMed2005

Association of automated and human observer lesion detecting ability using phantoms.

A set of tissue-mimicking phantoms containing spherical negative contrast simulated lesions was employed to associate an automated method for determining detectability with human observers. Six alternative methods for computing the lesion signal-to-noise ratio (LSNR) were employed for quantifying automated detecting ability. The six methods differ regarding effective lesion area and whether or not gradients in local mean background echo levels were accounted for. The two-alternative-forced-choice (TAFC) technique was used to associate detecting ability of human observers with LSNR values. Although the six methods gave similar results, one method exhibited the least dependency on lesion diameter and is recommended; that method accounts for gradients in local mean background echo levels and employs an effective sphere area of 2/pi times the projected sphere area. A reasonable LSNR detection threshold value of -2.0 was found to apply for nominal transducer frequencies from 4 through 6 MHz and for lesion diameters from 2 through 5 mm. This result allows rapid human-observer-calibrated automated determination of the depth range of detectability as a function of sphere diameter and contrast.

Humans↗

PubMed2005

Optimization of angular and frequency compounding in ultrasonic attenuation estimations.

Previous reports have shown that the variance in ultrasound attenuation measurements is reduced when spatial and frequency compounding were applied in data acquisition and analysis. This paper investigates factors affecting the efficiency of compound attenuation imaging methods. A theoretical expression is derived that predicts the correlation between attenuation versus frequency slope (beta) estimates as a function of the increment between measurement frequencies (deltaf ) and the angular separation between beam lines (Delta (theta)). Theoretical results are compared with those from attenuation measurements on tissue-mimicking phantoms and from simulation data. Both predictions and measurement results show that the correlation between beta estimates as a function of (Delta f ) is independent of the length of the radio frequency (rf) data segment over which beta is derived. However, it decreases with an increase in the length of the data segment used in power spectra estimates. In contrast, the correlation between beta estimates as a function of delta(theta) decreases when the rf data segment length is longer or the frequency of the signal is higher. O 2005 Acoustical Society of America.

Acoustics↗

PubMed2005

Tissue-mimicking liquid for use in exposimetry.

OBJECTIVE: Current determinations of diagnostic ultrasound exposure parameters (eg, peak rarefactional pressure and pulse intensity integral) are intended to correspond to propagation through soft tissue with a propagation speed of 1540 m/s and attenuation of 0.3 dB x cm(-1) x MHz(-1). These current measurements are made in water, which has very little attenuation, and a linear derating factor is applied to approximate 0.3 dB x cm(-1) x MHz(-1) attenuation. The fact that propagation through water as well as through soft tissue involves nonlinear propagation is not directly addressed. A better way to determine exposure parameters would be to use a liquid that has the desired tissue-mimicking properties, including a value of the nonlinearity parameter B/A representative of soft tissue. To be of practical use in the laboratory, the ultrasonic properties of this liquid must remain stable and spatially uniform for many months or years without need for periodic mixing by the user. METHODS: Fifty-two samples of fat-free milk that was concentrated to one third of its original volume by ultrafiltration were created. Each sample was preserved by a different method. The speed of sound, attenuation, and nonlinearity parameter B/A of each sample were periodically monitored by narrowband through-transmission techniques. RESULTS: Six of the 52 samples remained liquid and retained acceptably stable acoustic properties over 22 months of storage at room temperature. CONCLUSIONS: Fat-free milk, concentrated via ultrafiltration and preserved in 1 of 6 different methods, has been found to be a stable tissue-mimicking liquid with acoustic properties appropriate for use in exposimetry.

Acoustics↗

PubMed2005

Interlaboratory comparison of ultrasonic backscatter coefficient measurements from 2 to 9 MHz.

OBJECTIVE: As are the attenuation coefficient and sound speed, the backscatter coefficient is a fundamental ultrasonic property that has been used to characterize many tissues. Unfortunately, there is currently far less standardization for the ultrasonic backscatter measurement than for the other two, as evidenced by a previous American Institute of Ultrasound in Medicine (AIUM)-sponsored interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements (J Ultrasound Med 1999; 18:615-631). To explore reasons for these disparities, the AIUM Endowment for Education and Research recently supported this second interlaboratory comparison, which extends the upper limit of the frequency range from 7 to 9 MHz. METHODS: Eleven laboratories were provided with standard test objects designed and manufactured at the University of Wisconsin (Madison, WI). Each laboratory was asked to perform ultrasonic measurements of sound speed, attenuation coefficients, and backscatter coefficients. Each laboratory was blinded to the values of the ultrasonic properties of the test objects at the time the measurements were performed. RESULTS: Eight of the 11 laboratories submitted results. The range of variation of absolute magnitude of backscatter coefficient measurements was about 2 orders of magnitude. If the results of 1 outlier laboratory are excluded, then the range is reduced to about 1 order of magnitude. Agreement regarding frequency dependence of backscatter was better than reported in the previous interlaboratory comparison. For example, when scatterers were small compared with the ultrasonic wavelength, experimental frequency-dependent backscatter coefficient data obtained by the participating laboratories were usually consistent with the expected Rayleigh scattering behavior (proportional to frequency to the fourth power). CONCLUSIONS: Greater standardization of backscatter measurement methods is needed. Measurements of frequency dependence of backscatter are more consistent than measurements of absolute magnitude.

1-Propanol↗

PubMed2004

Noise reduction using spatial-angular compounding for elastography.

Ultrasound elastography has developed into an imaging modality suitable for detection and diagnosis of cancers in the breast, prostate, and thyroid and for monitoring ablative therapies in the liver, kidneys, and other sites. In this article, a new approach is described that enables the reduction of noise artifacts in elastography without a significant reduction in either the contrast or spatial resolution. The technique uses angular-weighted compounding of local angular strains estimated from echo signals scanned at different insonification angles. Strain estimated along angular insonification directions can be separated into strain tensor components along the axial (direction of compression) and lateral directions. The mechanical stimulus is applied only along one direction. Angular-weighting factors are derived from the relationship between the axial and lateral strains under the assumption of tissue incompressibility. Experimental results using a uniformly elastic, tissue-mimicking phantom demonstrate the improvement in the signal-to-noise ratio obtained with angular-weighted compounding. Variation in the signal-to-noise ratio obtained using different angular increments also is investigated. Elastograms obtained from an inclusion phantom also demonstrate the improvement in contrast detail resolution obtained using spatial-angular compounding.

Algorithms↗

PubMed2003

Ultrasound attenuation imaging using compound acquisition and processing.

A method that combines both spatial and frequency compounding is described for measuring attenuation in tissue. The technique applies a reference phantom to account for imaging system dependencies of echo signals. Emphasis is given to local attenuation estimates, to reduce the variance of the attenuation measurements over small regions of interest (ROI) and to enable coarse attenuation imaging. Experiments using a uniform phantom show that the standard deviation of local attenuation estimates within a ROI drops when greater degrees of compounding are applied. Attenuation images of a specially designed phantom containing inclusions with attenuation contrast illustrate the accuracy and precision of the technique.

Phantoms, Imaging↗