Biomedical subjects

D B Plewes

Publications and source records attributed to D B Plewes.

At least 19 recordsLinked to original sources

PubMed2005

A method to evaluate human spatial coordination interfaces for computer-assisted surgery.

Computer assistance for breast conserving surgery requires a guidance method to assist a surgeon in locating tumor margin accurately. A wide array of guidance methods can be considered ranging from various pictorial representations, symbolic graphical interfaces as well as those based on other sensory cues such as sound. In this study, we present an experimental framework for testing candidate guidance methods in isolation or in combination. A total of 22 guidance approaches, based on stereographic, non-stereographic, symbolic and auditory cues were tested in a simulation of breast conserving surgery. Observers were asked to circumscribe a virtual tumor with a magnetically tracked scalpel while measuring the spatial accuracy, time and the frequency with which the tumor margin was intersected. A total of 110 studies were performed with 5 volunteers. Based on these findings, we demonstrated that a single view of the tumor with a stereo presentation in conjunction with an auditory guidance cue provided the best balance of accuracy, speed and surgical integrity. This study demonstrates a practical and helpful framework for testing guidance methods in a context dependent manner.

Breast Neoplasms↗

PubMed2003

A hybrid breast biopsy system combining ultrasound and MRI.

System design and initial phantom accuracy results for a novel biopsy system integrating both magnetic resonance (MR) and ultrasound (US) imaging modalities are presented. A phantom experiment was performed to investigate the efficacy of this hybrid guidance biopsy technique in a breast tissue mimicking phantom. A comparison between MR-guided core biopsy verses MR/US-guided core biopsy of phantom targets was realized using a scoring system based on the consistency of the acquired core samples (14 gauge). It was determined that the addition of US to guide needle placement improved the accuracy from an average score of 7.4 out of 10 (MRI guidance alone), to 9.6 (MRI/US guidance) over 21 trials. The average amount of needle tip correction resulting from the additional US information was determined to be 3.7 mm. This correction value is substantial, equal to approximately one radius of the intended targets. Hybrid US/MRI guided biopsy appears to offer a simple means to ensure accurate breast tissue sampling without the need for repeat MRI scans for verification or the need for real-time imaging in open MRI geometries.

Biopsy, Needle↗

PubMed2001

Comparison of breast magnetic resonance imaging, mammography, and ultrasound for surveillance of women at high risk for hereditary breast cancer.

PURPOSE: Recommended surveillance for BRCA1 and BRCA2 mutation carriers includes regular mammography and clinical breast examination, although the effectiveness of these screening techniques in mutation carriers has not been established. The purpose of the present study was to compare breast magnetic resonance imaging (MRI) with ultrasound, mammography, and physical examination in women at high risk for hereditary breast cancer. PATIENTS AND METHODS: A total of 196 women, aged 26 to 59 years, with proven BRCA1 or BRCA2 mutations or strong family histories of breast or ovarian cancer underwent mammography, ultrasound, MRI, and clinical breast examination on a single day. A biopsy was performed when any of the four investigations was judged to be suspicious for malignancy. RESULTS: Six invasive breast cancers and one noninvasive breast cancer were detected among the 196 high-risk women. Five of the invasive cancers occurred in mutation carriers, and the sixth occurred in a woman with a previous history of breast cancer. The prevalence of invasive or noninvasive breast cancer in the 96 mutation carriers was 6.2%. All six invasive cancers were detected by MRI, all were 1.0 cm or less in diameter, and all were node-negative. In contrast, only three invasive cancers were detected by ultrasound, two by mammography, and two by physical examination. The addition of MRI to the more commonly available triad of mammography, ultrasound, and breast examination identified two additional invasive breast cancers that would otherwise have been missed. CONCLUSION: Breast MRI may be superior to mammography and ultrasound for the screening of women at high risk for hereditary breast cancer.

Adult↗

PubMed2001

New devices to deliver somatosensory stimuli during functional MRI.

A new class of devices are described for improving investigation of somatosensory neuronal activation using fMRI. Dubbed magnetomechanical vibrotactile devices (MVDs), the principle of operation involves driving wire coils with small oscillatory currents in the large static magnetic field inherent to MRI scanners. The resulting Lorentz forces can be oriented to generate large vibrations that are easily converted to translational motions as large as several centimeters. Representative data demonstrate the flexibility of MVDs to generate different well-controlled vibratory and tactile stimuli to activate special proprioceptive and cutaneous somatosensory afferent pathways. The implications of these data are discussed with respect to the literature on existing devices for producing sensorimotor activation, as well as expanding the scope of current fMRI investigations.

Adult↗

PubMed2001

Biomechanical 3-D finite element modeling of the human breast using MRI data.

Breast tissue deformation modeling has recently gained considerable interest in various medical applications. A biomechanical model of the breast is presented using a finite element (FE) formulation. Emphasis is given to the modeling of breast tissue deformation which takes place in breast imaging procedures. The first step in implementing the FE modeling (FEM) procedure is mesh generation. For objects with irregular and complex geometries such as the breast, this step is one of the most difficult and tedious tasks. For FE mesh generation, two automated methods are presented which process MRI breast images to create a patient-specific mesh. The main components of the breast are adipose, fibroglandular and skin tissues. For modeling the adipose and fibroglandular tissues, we used eight noded hexahedral elements with hyperelastic properties, while for the skin, we chose four noded hyperelastic membrane elements. For model validation, an MR image of an agarose phantom was acquired and corresponding FE meshes were created. Based on assigned elasticity parameters, a numerical experiment was performed using the FE meshes, and good results were obtained. The model was also applied to a breast image registration problem of a volunteer's breast. Although qualitatively reasonable, further work is required to validate the results quantitatively.

Biomechanical Phenomena↗

PubMed2001

A constrained modulus reconstruction technique for breast cancer assessment.

A reconstruction technique for breast tissue elasticity modulus is described. This technique assumes that the geometry of normal and suspicious tissues is available from a contrast-enhanced magnetic resonance image. Furthermore, it is assumed that the modulus is constant throughout each tissue volume. The technique, which uses quasi-static strain data, is iterative where each iteration involves modulus updating followed by stress calculation. Breast mechanical stimulation is assumed to be done by two compressional rigid plates. As a result, stress is calculated using the finite element method based on the well-controlled boundary conditions of the compression plates. Using the calculated stress and the measured strain, modulus updating is done element-by-element based on Hooke's law. Breast tissue modulus reconstruction using simulated data and phantom modulus reconstruction using experimental data indicate that the technique is robust.

Breast Neoplasms↗

PubMed2001

A signal/noise analysis of quasi-static MR elastography.

In quasi-static magnetic resonance elastography, strain images of a tissue or material undergoing deformation are produced. In this paper, the signal/noise (S/N) ratio [SNR] of elastographic strain images, as measured by a phase-contrast technique, is analyzed. Experiments are conducted to illustrate how diffusion-mediated signal attenuation limits maximum strain SNR in small displacement cases, while the imaging point-spread function limits large displacement cases. A simple theoretical treatment agrees well with experiments and shows how an optimal displacement encoding moment can be predicted for a given experimental set of parameters to achieve a maximum strain SNR. A further experiment demonstrates how the limitation on strain SNR posed by the imaging point-spread function may potentially be overcome.

Diffusion↗

PubMed2000

Magnetic resonance imaging of ultrasound fields: gradient characteristics.

Phase-contrast magnetic resonance imaging (MRI) is used to image particle displacements arising from a 0.515-MHZ focused ultrasound (US) field. The technique used a phase-locked, self-resonant gradient matched to the US frequency in conjunction with a spin-echo sequence to generate phase images of US-induced displacement parallel to the US propagation direction. The gradient design was numerically optimized to provide maximum linearity and magnitude while minimizing gradient inductance. The windings were fabricated of Litz wire to minimize resistive losses and mounted in an oil-cooled imaging chamber. When driven by a resonance power supply, a peak magnetic field gradient of 0.40 T/m was attained with a peak current of 20 amp in a volume of 53 cm(3), achieving stable oscillation at the required US frequency. Clear detection of the nanometer scale particle motions of the US field was achieved and allowed quantitative, noninvasive visualization of the entire US field. While the required gradient slew rate for US detection is beyond that recommended for in vivo application, this imaging method opens new possibilities for in vitro or ex vivo research in the study of the interaction of US with tissue.

Echo-Planar Imaging↗

PubMed2000

Visualization and quantification of breast cancer biomechanical properties with magnetic resonance elastography.

A quasistatic magnetic resonance elastography (MRE) method for the evaluation of breast cancer is proposed. Using a phase contrast, stimulated echo MRI approach, strain imaging in phantoms and volunteers is presented. First-order assessment of tissue biomechanical properties based on inverse strain mapping is outlined and demonstrated. The accuracy of inverse strain imaging is studied through simulations in a two-dimensional model and in an anthropomorphic, three-dimensional finite-element model of the breast. To improve the accuracy of modulus assessment by elastography, inverse methods are discussed as an extension to strain imaging, and simulations quantify MRE in terms of displacement signal/noise required for robust inversion. A direct inversion strategy providing information on tissue modulus and pressure distribution is described along with a novel iterative method utilizing a priori knowledge of tissue geometry. It is shown that through the judicious choice of information from previous contrast-enhanced MRI breast images, MRE data acquisition requirements can be significantly reduced while maintaining robust modulus reconstruction in the presence of strain noise. An experimental apparatus for clinical breast MRE and preliminary images of a normal volunteer are presented.

Breast Neoplasms↗

PubMed2000

Two-dimensional MR elastography with linear inversion reconstruction: methodology and noise analysis.

A methodology for imposing approximate plane strain conditions in magnetic resonance elastography through physical constraint is described. Under plane strain conditions, data acquisition and analysis may be conducted in two dimensions, which reduces imaging and reconstruction time significantly compared with three-dimensional analysis. Simulations and experiments are performed to illustrate the constraint concept. A signal/noise analysis of a two-dimensional linear inversion technique for relative elastic modulus is undertaken, and modifications to the numerical method are described which can reduce the SNR requirements by a factor of two to four. Experimentally measured data are reconstructed to illustrate the performance of the method.

Breast Neoplasms↗

PubMed1999

Aorta wall motion monitoring by 1-d MRI of perpendicular diameters.

A method for monitoring the cross-sectional size of blood vessels rapidly is introduced. The method creates a one-dimensional (1-D) profile of a strip along the diameter of a vessel using magnetic resonance imaging (MRI). The strips can be much wider than pixels in a typical two-dimensional (2-D) image to increase the signal-to-noise ratio. A second strip perpendicular to the first is also imaged sequentially to allow the detection, estimation, and correction of errors in diameter measurements resulting from the strip inadvertently covering a chord, rather than a diameter. Diameter measurements derived from 1-D profiles agree with measurements derived from 2-D images. This method is nearly an order of magnitude faster than 2-D MRI and has the potential for real-time implementation. J. Magn. Reson. Imaging 1999;10:833-840.

Aorta↗

PubMed1999

Quantifying tissue damage due to focused ultrasound heating observed by MRI.

Focused ultrasound heating of ex vivo bovine kidney and liver was monitored using magnetic resonance imaging (MRI) to investigate the quantitative relationship between time-dependent temperature elevations and altered contrast in MR images due to thermal coagulation. Proton resonance frequency shift MR thermometry was performed during heating at 10 sec intervals (single-slice fast spoiled GRASS [FSPGR], theta/TE/TR 30 degrees/11/39 msec, field of view 8 cm, 256 x 256, 3 mm slice thickness, 1 NEX); post-heating MR images were T1-weighted (3D-FSPGR, theta/TE/TR 60 degrees/25/200 msec, 1 mm slice thickness, 3 NEX). Analysis of the resulting temperature versus time data using the Arrhenius relationship and a simple binary discrimination model showed that thermal coagulation occurred with heating at approximately 54 degrees C for 10 sec in both tissues and could be predicted with approximately 625 microm spatial resolution. These results suggest that quantitative MR guidance of thermal coagulation therapy is feasible, and they provide information useful for designing future investigations in vivo.

Animals↗

PubMed1999

An MRI calorimetry technique to measure tissue ultrasound absorption.

High-intensity focused ultrasound (US) surgery guided by magnetic resonance imaging (MRI) is a very promising form of minimally invasive thermal therapy. To apply this technique optimally, the interaction mechanisms of high-intensity US with tissue need to be better understood, in particular, the variation of ultrasound absorption with frequency and temperature. However, agreement on the value of measured tissue US absorption is poor, largely because of intrinsic experimental complications of prior investigations. A new approach toward measuring tissue US absorption, based on a form of MRI calorimetry, is proposed here, which allows non-invasive energy measurement through spatial temperature mapping with MRI. A modified two-dimensional spoiled gradient-echo sequence has been implemented to map temperature based on proton resonance frequency (PRF) shift. Validation experiments show excellent agreement of MRI measured energy with that delivered by a calibrated source. MRI calorimetry of US heating of tissue-mimicking polyethylene glycerol material has been performed. Using a hydrophone measurement of the incident US field, its US absorption coefficient was measured as 0.032 cm-1. As this approach can be applied over a range of frequencies, tissues, and temperatures, it should provide a much improved means of measuring absolute tissue US absorption coefficients to improve US therapy planning, future transducer design, and US dosimetry models.

Absorption↗

PubMed1998

Magnetic resonance imaging of ultrasonic fields.

A nuclear magnetic resonance imaging (MRI) method is described that allows noninvasive, quantitative mapping of medical ultrasound (US) fields in tissue. Application of a resonant magnetic field gradient operating at the US frequency permits detection of nanometer motions associated with ultrasound, and allows direct measurement of absolute pressure, intensity, and speed of sound. By altering gradient timing, the propagation of US fields in time and space can be observed; this enables tracking of US scattering phenomena in a tissue-equivalent medium. An experimental apparatus was constructed that combined a 515-kHz focused US transducer configured with its focus in the center of a small-bore oscillating gradient. This provided an oscillating gradient with a peak gradient strength of 0.40 T/m over a useable imaging volume of 61 cm3. When used in conjunction with 1.5-T clinical MRI system, this apparatus allowed the clear visualization of the focused US field within this volume and its propagation with time. Current limits of sensitivity indicate a noise equivalent sensitivity of 3.8 nm in displacement amplitude, 19 kPa in pressure amplitude and 12 mW/cm2 in acoustic intensity. These studies indicate that MRI can provide a new, noninvasive method for US exposimetry and the basic study of ultrasound biophysics in tissue.

Agar↗

PubMed1997

In vivo validation of MR pulse pressure measurement in an aortic flow model: preliminary results.

MR imaging experiments were conducted to investigate the feasibility of estimating vascular pulse pressure waveforms from measurements of blood flow rates and vessel cross-sectional area. Blood flow waveforms were measured in the aorta's of three 25-30-kg pigs at multiple imaging sections using phase-contrast velocity imaging. Estimates of pulse pressure were derived from these data by evaluating a model characterizing the relationship between pressure, flow, and the cross-sectional area of a vessel segment. Comparisons between the MR-derived estimates of pressure and those obtained from a micromanometer pressure catheter indicate that accurate measurements (mean error +/- SD = 8.2 +/- 3.4, n = 6) can be obtained using conventional velocity imaging techniques. Optimization of the method will require the application of rapid imaging techniques and the development of strategies for obtaining a more localized measurement. With these improvements, our results suggest that MR-based measurement of pulse pressure and related elastic parameters is feasible.

Animals↗

PubMed1996

Analytical description of the high and low contrast behavior of a scan-rotate geometry for equalization mammography.

Many solutions have been proposed to overcome the problem of imaging the radiographically dense breast with high contrast mammographic film of limited dynamic range. In previous works, we have proposed utilizing a modulated fan-beam in a scan-rotate geometry RSER (Rotary Scanning Equalization Radiography), as an efficient method for producing exposure equalized mammograms. The image quality of RSER is similar to that attained with the inefficient single beam, raster scanning SER (Scanning Equalization Radiography) geometry. RSER has the potential to be a practical, efficient method for improving the detection of cancer in the dense breast. In this work, we present a theoretical analysis of the imaging properties of the RSER geometry in two regimes defined by the variation of x-ray transmission within the object. For low contrast objects, the imaging geometry was analyzed as a linear system, whereas the high contrast regime was studied by determining the contrast limit at which RSER requires nonphysical (negative) exposure modulation for a breast-like object. The low contrast transfer function of the RSER system is shown to be very similar to that of the SER geometry. We show that RSER enables the use of wide scanning beam of approximately 4 cm and thereby significantly reduces x-ray tube heat loading. Analysis of the high contrast behavior shows that a wide range of object contrasts and sizes can be equalized. For example, RSER can equalize a region of 100% glandular tissue within a 4.0 cm thick compressed breast composed of 100% adipose tissue. Thus, the RSER geometry produces images very similar to the more inefficient SER geometry, and is able to produce entrance exposure distributions appropriate for equalization of the range of contrast typically encountered in mammography.

Biometry↗

PubMed1996

Practical application of a scan-rotate equalization geometry to mammography.

The presence of dense fibroglandular tissue within the breast is the most significant cause of failure to detect breast cancer with mammography. The dense tissue often produces a range of exposure which exceeds the useful dynamic range of film-screen mammography. It has been shown that equalization radiography overcomes the latitude limitations of film-screen imaging. Equalization compensates for regional variations in x-ray transmission within the patient through spatial modulation of the entrance exposure. We have proposed rotary scanning equalization radiography (RSER), a scan-rotate geometry for efficient equalization radiography. In RSER the image receptor is exposed by repeated scans of a source-modulated fan beam. The fan beam is rotated with respect to the patient between scans. Numerical simulations and theoretical analysis have shown that the superposition of exposure from appropriately modulated fan beams at a variety of angles is an entrance exposure that effectively equalizes the film exposure. The design and characteristics of a prototype RSER imaging system are described. Anthropomorphic breast phantom images are used to determine the improvement in image contrast obtained with RSER, the expected tube loading, and the presence of artifacts. RSER increases the fraction of the breast imaged with high contrast (at least 90% of peak gradient) from 46% (conventional mammography) to 80%. Subjective examination of the phantom images show that RSER achieves image quality very similar to that of much less efficient equalization geometries with only 2.7 times greater tube loading than conventional mammography. As predicted by theoretical analysis of exposure artifacts in RSER, the prototype RSER system is relatively immune to artifacts. Exposure artifacts were demonstrated for extreme variations in x-ray transmission within the patient. These results show that RSER is an efficient, practical means of overcoming the latitude limitations of film-screen mammography, and improving the detection of breast cancer.

Biophysical Phenomena↗

PubMed1996

Magnetic resonance imaging in potential postsurgical recurrence of breast cancer: pitfalls and limitations.

OBJECTIVE: To determine the sensitivity and specificity of magnetic resonance imaging (MRI) of the breast for detecting recurrent carcinoma. PATIENTS AND METHODS: Thirteen patients ranging in age from 47 to 77 years who had undergone lumpectomy 5 months to 8 years earlier and who had mammographic findings suggestive of recurrence underwent contrast-enhanced dynamic MRI. Histologic confirmation was obtained in all cases. RESULTS: Of the eight lesions (in seven patients) for which biopsy proved recurrence, MRI correctly identified six; there were two false negative results. Of the six benign lesions, four were correctly identified by MRI. The two false positive results involved fat necrosis and a foreign-body reaction respectively. CONCLUSION: These results confirm previous reports of the poor specificity of MRI of focal breast lesions. The authors therefore recommend caution in the use of breast MRI in the assessment and management of suspected recurrent carcinoma.

Aged↗