Biomedical subjects

Donald B Plewes

Publications and source records attributed to Donald B Plewes.

18 recordsLinked to original sources

PubMed2007

A positive contrast, tri-modality tissue marker for breast tumour localization.

A new interstitial breast localization marker is proposed which exhibits positive contrast in T1-weighted MRI, ultrasound and x-ray mammography. Unlike previous markers which provide MRI contrast on the basis of a susceptibility-induced signal void, this marker provides a clear positive contrast without any loss of signal or spatial distortion. The marker is composed of 400 microm diameter copper microspheres suspended in a Gd-DTPA-doped gel matrix. Optimal contrast in T1-weighted spoiled gradient recalled MRI was found to occur with the addition of 10 mM Gd-DTPA. Ultrasound contrast was generated on the basis of scattering from the copper microspheres. X-ray contrast was provided by the high x-ray attenuation properties of the copper microspheres. The study demonstrates potential suitability of the marker for use as a breast localization marker based on ex vivo studies of chicken breast.

Animals↗

PubMed2006

MR imaging-guided breast localization system with medial or lateral access.

PURPOSE: To evaluate the degree of error of the authors' magnetic resonance (MR) imaging-guided needle localization system for biopsy of suspicious lesions visualized only with MR imaging, by using both prospectively recorded and retrospectively reviewed data, including MR imaging lesion coordinates as the reference standard, and to determine whether any lesion or breast characteristics affect this error. MATERIALS AND METHODS: Institutional review board approval, along with informed consent, was obtained as directed by the board. In 31 patients (age range, 34-64 years; mean age, 54.5 years), 38 wires were placed for 35 lesions by means of an MR-guided needle localization system with medial or lateral access and computer software assistance for needle placement calculation. Needle and wire placement error measurements were calculated before and after necessary placement correction, accounting for tissue shift in the z plane. The error was statistically correlated with MR imaging lesion variables, breast density, and histopathologic findings by means of univariate and multivariate linear regression analyses or two-tailed paired t test. Procedure times and the frequency of medial or lateral approaches were recorded. RESULTS: Eleven of 35 localizations (31%) were medial, and 24 of 35 (69%) were lateral. The mean total magnet time was 61.6 minutes, and the mean needle deployment time was 9 minutes (range, 4-17 minutes). Sixteen of 35 lesions (46%) were malignant (seven ductal carcinoma in situ, six invasive ductal, two invasive lobular, and one lymphoma). The mean uncorrected needle placement error was 1.3 mm (range, 0-6 mm) for the x plane, 2.4 mm (range, 0-6.5 mm) for the y plane, and 5.6 mm (range, 0-15.6 mm) for the z plane. Fourteen of 38 needles (37%) required repositioning for z-plane error. The corrected z-plane error improved to 3.2 mm (range, 0-10.0 mm). Factors that significantly increased the uncorrected error included tissue shift in the z plane (R = 0.7), small lesion size (R = -0.59), and fatty breast density (P = .029). CONCLUSION: The authors' system is accurate for performing MR-guided needle localizations for both medial and lateral approaches. Factors that increased the uncorrected needle placement error included small lesion size, fatty breast density, and tissue shift in the z plane.

Adult↗

PubMed2005

A novel microbubble construct for intracardiac or intravascular MR manometry: a theoretical study.

It has been demonstrated that gas-filled microbubble contrast agents, based on their volume changes, can serve as pressure probes in an MR field. It was recently reported that such an MR-based pressure measurement with microbubbles at 1.5 T must make use of microbubbles that have a volumetric magnetic susceptibility difference with the blood of at least 34 ppm in SI units. In this work, we show through analytical approximations and numerical simulations that such a microbubble formulation can be achieved by coating typical lipid-shelled microbubbles with particles of high dipole moment. Through finite-element simulations we demonstrate that the effective volumetric magnetic susceptibility of a coated microbubble is dependent on the radius, the shell volume fraction and the magnetic susceptibility of the particulates on the shell. Our calculations suggest that a suitable microbubble formulation which will be MR-sensitive to small pressure changes at 1.5 T must be 2-3 microm in radius and be uniformly coated with single-domain magnetic nanoparticles, such as magnetite, at shell volume fractions below 5%.

Arteries↗

PubMed2005

Development of an MRI/x-ray/ultrasound compatible marker for pre-operative breast tumour localization.

This paper describes an in vitro investigation into the composition, structure and development of an magnetic resonance imaging (MRI), ultrasound (US) and x-ray imaging compatible marker for breast tumour localization. The marker is composed of 0.4-0.6 mm glass and iron-containing aluminium microspheres suspended in a gelatin matrix. The final form of the marker is a cylindrical shape 7 mm long with 2.05 mm diameter to facilitate delivery through a 12 gauge biopsy needle. To get optimal reflectivity for the US contrast, the glass microsphere concentration was found to be 40% by weight. US contrast is independent of marker orientation and the cylindrical shape made its US signal appearance distinctive thus ensuring confident identification. To control the MRI contrast, iron content was varied to generate a clear and local susceptibility signal void to reflect the marker position. Optimal iron content was found to be 52 microg iron which produced a clear signal void in spoiled gradient recalled MR images. The appearance of the susceptibility artefact is determined by the marker's shape, orientation and echo time. The final marker produces a dark artefact in MRI while appears as a clear hyperintense structure with acoustic shadowing in US images. The x-ray image showed the marker as a radio-opaque structure. This in vitro study demonstrates that the marker forms an alternative to traditional wire localization currently used for breast surgical procedures and creates new opportunities for US guided surgical procedures.

Aluminum↗

PubMed2005

Retrospective motion compensation using variable-density spiral trajectories.

PURPOSE: To develop a method of retrospectively correcting for motion artifacts using a variable-density spiral (VDS) trajectory. MATERIALS AND METHODS: Each VDS interleaf was designed to adequately sample the same center region of k-space. This central overlapping region can then be used to measure rigid body motion between the acquisition of each VDS interleaf. By applying appropriate phase shifts and rotations of the k-space data, rigid body motion artifacts can be removed, resulting in images with less motion corruption. RESULTS: Both phantom and volunteer experiments are shown, demonstrating the technique's ability to further reduce artifacts in images acquired with an already motion-resistant acquisition trajectory. Registration accuracy is highly dependent on the trajectory design parameters. This space was explored to find an optimal design of VDS trajectories for motion compensation. CONCLUSION: Using appropriately designed VDS trajectories, residual motion artifacts can be significantly reduced by retrospectively correcting for in-plane rigid body motion. An overlapping region of approximately 8% of the central region of k-space and approximately 70 interleaves were found to be near-optimal parameters for retrospective correction using VDS trajectories.

Artifacts↗

PubMed2005

Oxygen-sensitive contrast in blood for steady-state free precession imaging.

Steady-state free precession (SSFP) methods have gained widespread recognition for their ability to provide fast scans at high signal-to-noise ratio. This paper demonstrates that such methods are also capable of reflecting functional information, particularly blood oxygenation state. It is well known that SSFP signals show substantial sensitivity to small off-resonance frequency variations. However, that mechanism cannot explain the oxygen-sensitive contrast in blood that was observed with steady-state methods using phase-cycled radiofrequency pulses. From theoretical and experimental models it is demonstrated that the mechanism responsible for such contrast originates from the motion of spins through local field inhomogeneities in and around deoxygenated red blood cells. In addition, this work shows that it is critical to choose the scan parameters carefully for robust oxygen-sensitive contrast. Finally, it is demonstrated that it is possible to build a quantitative model that incorporates the Luz-Meiboom model, which had been used in the past to estimate quantitative measures of vascular blood oxygen levels. It is envisioned that this method could be instrumental in real-time imaging focused on detecting diseases where the oxygen state of blood is impaired.

Chi-Square Distribution↗

PubMed2005

An MRI/US/x-ray compatible breast localization marker: in vivo evaluation.

RATIONALE AND OBJECTIVES: An in vivo evaluation of a new trimodality breast localization marker was performed with magnetic resonance imaging (MRI), ultrasound (US), x-ray, and histopathology. The evaluation of the marker in animal tests should help define its utility for surgical biopsy localization in humans. MATERIALS AND METHODS: Five rabbits were used and sacrificed at 2 days, 1 week, 2 weeks, 4 weeks, and 7 weeks after marker implantation. The marker placement and tissue biopsies were performed under US guidance. MRI, US, and x-ray imaging were performed to monitor the contrast of the marker, track marker migration. The biologic compatibility of the marker was demonstrated by histopathologic analysis. RESULTS: The contrast of the marker was clear and stable on each imaging modality over the 7-week study period. Acute inflammation was visible by 2 days after marker injection, with evidence of granulation tissue and angiogenesis at 2 weeks after implantation. A modest degree of chronic inflammation and angiogenesis remained evident at 4 weeks after procedure, and fibrosis persisted 7 weeks after procedure with no further tissue changes. These results suggest that the new marker is biocompatible and can remain interstitial for up to 7 weeks. Furthermore, very little marker migration was observed. On removal, the marker was found to be mechanically stable. CONCLUSION: This in vivo animal study demonstrates that the new marker may be appropriate for in vivo human testing and as an alternative to traditional wire localization currently used for breast surgery.

Animals↗

PubMed2004

Surveillance of BRCA1 and BRCA2 mutation carriers with magnetic resonance imaging, ultrasound, mammography, and clinical breast examination.

CONTEXT: Current recommendations for women who have a BRCA1 or BRCA2 mutation are to undergo breast surveillance from age 25 years onward with mammography annually and clinical breast examination (CBE) every 6 months; however, many tumors are detected at a relatively advanced stage. Magnetic resonance imaging (MRI) and ultrasound may improve the ability to detect breast cancer at an early stage. OBJECTIVE: To compare the sensitivity and specificity of 4 methods of breast cancer surveillance (mammography, ultrasound, MRI, and CBE) in women with hereditary susceptibility to breast cancer due to a BRCA1 or BRCA2 mutation. DESIGN, SETTING, AND PARTICIPANTS: A surveillance study of 236 Canadian women aged 25 to 65 years with BRCA1 or BRCA2 mutations who underwent 1 to 3 annual screening examinations, consisting of MRI, mammography, and ultrasound at a single tertiary care teaching hospital between November 3, 1997, and March 31, 2003. On the day of imaging and at 6-month intervals, CBE was performed. MAIN OUTCOME MEASURES: Sensitivity and specificity of each of the 4 surveillance modalities, and sensitivity of all 4 screening modalities vs mammography and CBE. RESULTS: Each imaging modality was read independently by a radiologist and scored on a 5-point Breast Imaging Reporting and Data System scale. All lesions with a score of 4 or 5 (suspicious or highly suspicious for malignancy) were biopsied. There were 22 cancers detected (16 invasive and 6 ductal carcinoma in situ). Of these, 17 (77%) were detected by MRI vs 8 (36%) by mammography, 7 (33%) by ultrasound, and 2 (9.1%) by CBE. The sensitivity and specificity (based on biopsy rates) were 77% and 95.4% for MRI, 36% and 99.8% for mammography, 33% and 96% for ultrasound, and 9.1% and 99.3% for CBE, respectively. There was 1 interval cancer. All 4 screening modalities combined had a sensitivity of 95% vs 45% for mammography and CBE combined. CONCLUSIONS: In BRCA1 and BRCA2 mutation carriers, MRI is more sensitive for detecting breast cancers than mammography, ultrasound, or CBE alone. Whether surveillance regimens that include MRI will reduce mortality from breast cancer in high-risk women requires further investigation.

Adult↗

PubMed2004

Usefulness of contrast kinetics for predicting and monitoring tissue changes in muscle following thermal therapy in long survival studies.

PURPOSE: To investigate Gd-DTPA kinetics as indicators of subacute and subchronic histopathological changes following focused ultrasound (FUS) thermal therapy for improved evaluation. MATERIALS AND METHODS: A total of 18 FUS lesions were created in the thigh muscle of five rabbits under magnetic resonance (MR) guidance at 1.5 Tesla. The rabbits were killed at different times: 40 hours, three days, and seven days. All lesions were analyzed histologically. An analysis of the uptake kinetics of Gd-DTPA, injected within two hours postheating and before sacrifice, was performed. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to T(2)-weighted MR and histology. RESULTS: Images of K(trans) and v(e) better differentiate subacute and subchronic changes not visible on conventional MR in the days following therapy and are consistent with the histopathology observed. In particular, the border between nonviable and viable tissue is well demarcated. The extent of damage is best indicated on v(e), whereas the borders of inflammation are shown on K(trans). The total lesion extent is relatively stable over the 7 days posttherapy and can be predicted by v(e) or T(2)-weighted MR at early times after heating. CONCLUSION: Our results suggest that Gd-DTPA kinetics can complement conventional MR for improved evaluation of FUS thermal therapy by providing finer differentiation of necrotic states, inflammation, and repair processes.

Animals↗

PubMed2004

Observation of nonlinear shear wave propagation using magnetic resonance elastography.

MR elastography (MRE) is an MRI modality that is increasingly being used to image tissue elasticity throughout the body. One MRE technique that has received a great deal of attention is based on visualizing shear waves, which reveal stiffness by virtue of their local wavelength. However, the shape of propagating shear waves can also provide valuable information about the nonlinear stress-strain behavior of tissue. Here an experiment is proposed that allows the observation of nonlinear wave propagation based on spatial-temporal phase contrast images. A theoretical description of the wave propagation was developed that reflects typical MRE excitation, which involves excitation modes both parallel and perpendicular to B0. Based on this model, it is shown that both odd and even higher harmonics are produced with their amplitudes dependent on the details of the actuator, imaging geometry, and the nonlinear tissue properties. With appropriate motion encoding, harmonic vibrations arising from nonlinear tissue response can be detected. The effect is demonstrated on an agarose gel phantom using a sinusoidal shear vibration of 150 Hz, and clearly shows the presence of harmonics at 600 and 750 Hz. Using an estimate of the strain energy of the phantom, we were able to determine the nonlinear tissue properties.

Elasticity↗

PubMed2003

Bilateral symmetry analysis of breast MRI.

Mammographic interpretation often uses symmetry between left and right breasts to indicate the site of potential tumour masses. This approach has not been applied to breast images obtained from MRI. We present an automatic technique for breast symmetry detection based on feature extraction techniques which does not require any efforts to co-register breast MRI data. The approach applies computer-vision techniques to detect natural biological symmetries in breast MR scans based on three objective measures of similarity: multiresolution non-orthogonal wavelet representation, three-dimensional intensity distributions and co-occurrence matrices. Statistical distributions that are invariant to feature localization are computed for each of the extracted image features. These distributions are later compared against each other to account for perceptual similarity. Studies based on 51 normal MRI scans of randomly selected patients showed that the sensitivity of symmetry detection rate approached 94%. The symmetry analysis procedure presented in this paper can be applied as an aid in detecting breast tissue changes arising from disease.

Breast↗

PubMed2003

Contrast-enhanced digital mammography: initial clinical experience.

PURPOSE: To investigate the potential of using intravenous contrast material with full-field digital mammography to facilitate the detection and characterization of lesions in the breast. MATERIALS AND METHODS: Twenty-two women scheduled for biopsy because they were suspected of having abnormalities at breast imaging underwent imaging with contrast material-enhanced digital mammography. Six sequential images of the affected breast were obtained, with a contrast agent injected intravenously between the time the first and second images were obtained. Image processing included registration and logarithmic subtraction. Lesions were evaluated for the presence, morphology, and kinetics of enhancement. Lesion type, size, and pathologic findings were correlated with the findings at contrast-enhanced digital mammography. RESULTS: At contrast-enhanced digital mammography, enhancement was observed in eight of 10 patients with biopsy-proved cancers. In one case of ductal carcinoma in situ and one case of invasive ductal carcinoma, enhancement was not observed. No enhancement was seen in seven of 12 cases in which lesions were suspected of being malignant at initial imaging but were benign. Morphology generally correlated with the pathologic diagnosis. The kinetics of lesion enhancement showed similarity to that seen with gadolinium-enhanced magnetic resonance imaging but was not consistent. CONCLUSION: The results of this preliminary study suggest that contrast-enhanced digital mammography potentially may be useful in identification of lesions in the mammographically dense breast. Further investigation of contrast-enhanced digital mammography as a diagnostic tool for breast cancer is warranted.

Adult↗

PubMed2003

Measuring the elastic modulus of ex vivo small tissue samples.

Over the past decade, several methods have been proposed to image tissue elasticity based on imaging methods collectively called elastography. While progress in developing these systems has been rapid, the basic understanding of tissue properties to interpret elastography images is generally lacking. To address this limitation, we developed a system to measure the Young's modulus of small soft tissue specimens. This system was designed to accommodate biological soft tissue constraints such as sample size, geometry imperfection and heterogeneity. The measurement technique consists of indenting an unconfined small block of tissue while measuring the resulting force. We show that the measured force-displacement slope of such a geometry can be transformed to the tissue Young's modulus via a conversion factor related to the sample's geometry and boundary conditions using finite element analysis. We also demonstrate another measurement technique for tissue elasticity based on quasi-static magnetic resonance elastography in which a tissue specimen encased in a gelatine-agarose block undergoes cyclical compression with resulting displacements measured using a phase contrast MRI technique. The tissue Young's modulus is then reconstructed from the measured displacements using an inversion technique. Finally, preliminary elasticity measurement results of various breast tissues are presented and discussed.

Adult↗

PubMed2003

Prediction of subtle thermal histopathological change using a novel analysis of Gd-DTPA kinetics.

PURPOSE: To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment. MATERIALS AND METHODS: Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MR-guidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to peak temperatures, T(2)-weighted MR, and histopathology. RESULTS: Images of K(trans) and v(e) reveal regions of histopathological change not visible on conventional post-therapy MR. At early times after heating, v(e) predicts the area of injury more accurately than T(2) (7 +/- 2% vs. 25 +/- 6% underestimation). A circular region of extensive structural/vascular disruption is indicated only on K(trans) maps. The sharp decrease in K(trans) at the boundary of this region occurs at 47.5 +/- 0.5 degrees C, and may be a better estimate of cell death than the conventional method of temperature threshold (55 degrees C for coagulation) used in therapy planning. CONCLUSION: Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

Animals↗

PubMed2002

Tissue thermal conductivity by magnetic resonance thermometry and focused ultrasound heating.

PURPOSE: To investigate the combined use of magnetic resonance (MR) temperature imaging and focused ultrasound (FUS) for the noninvasive determination of tissue thermal properties. MATERIALS AND METHODS: Brief, spatial impulses of temperature elevation were created in tissue using a spherical, air-backed transducer operating at 1.68 MHz and measured using MR temperature imaging in a 1.5-Tesla clinical scanner. A novel technique based on thermal washout is applied in an analysis of the acquired MR temperature images to estimate tissue thermal conductivity and perfusion. RESULTS: Numerical simulations and experiments in vitro and in vivo demonstrate that thermal conductivity can be measured to within 10% of the true value with MR thermometry at 1.5 Tesla. With the temperature precision available at 1.5 Tesla, however, robust perfusion estimation is feasible only in highly perfused organs or tumors. CONCLUSION: This study has developed a method for determining tissue thermal properties specific to the patient and organ at the site of interest, and allows repeated application. This capability is relevant in thermal therapy planning of tumor ablation using MR-guided FUS systems.

Animals↗

PubMed2002

On the parameters affecting the sensitivity of MR measures of pressure with microbubbles.

Recently, it has been suggested that gas encapsulated distensible microbubbles may serve as pressure probes in the MR field through the relationship between bubble size and 1/T(2) or 1/T(*)(2). Currently, in vivo application of this technique is hindered by the ability of T(2) or T(*)(2) to detect pressure changes that are clinically relevant. This work identifies and characterizes, through numerical simulations, the set of parameters which optimize the ability of this technique to detect small pressure changes. Results show that when the bubbles do not interact magnetically, the T(2)- and T(*)(2)-based measurements of pressure are strongly influenced by the bubble size at atmospheric pressure, static magnetic field strength, magnitude of the susceptibility difference between the encapsulated gas and plasma, bubble volume fraction, and the refocusing interval. In particular, to detect clinically relevant pressure changes, microbubbles need to be approximately 2-3 microm in radius, distributed at a volume fraction of 0.15%, and have a volumetric magnetic susceptibility difference of at least 34 ppm.

Blood Pressure↗

PubMed2002

MR validation of soft tissue mimicing phantom deformation as modeled by nonlinear finite element analysis.

A study of the applicability of nonlinear finite element analysis (FEA) to predict soft tissue deformation was validated with phase contrast magnetic resonance velocity imaging. A phantom of varying stiffness was placed in a special purpose, computer controlled MR compatible compression apparatus which provided precise, time varying compression with surface deformations on the order of 11%. The resulting motion was measured with MR velocity images acquired throughout the cycle of compression. The phantom geometry was modeled with a finite element mesh and the mechanical properties of the phantom material were measured and incorporated in the FEA model. The motion as calculated by the FEA model was compared to the motion measured with MRI and the results were found to vary with the material's Poisson's ratio and the coefficient of friction. A minimum difference was reached when the Poisson's ratio and coefficient of friction were set to 0.485 and 0.3, respectively. Under these conditions, the root mean square difference was found to be 14.4%.

Agar↗

PubMed2002

Comparison of MR imaging breast coils.

In one volunteer, five breast coils were evaluated for signal-to-noise ratio (SNR), uniformity, comfort, subject orientation, access to the breast, and unilateral imaging options. The four-coil arrays provided superior SNR, imaging flexibility, and access. Uniformity and comfort were issues with all coils. Substantial design differences exist between coils; purchasers should ensure that their specific requirements are met.

Adult↗