Magnetic monitoring of a small Foucault pendulum

A three-axis magnetometer is used to measure the precession angle of a small Foucault pendulum of 65.4 cm length and period 1.623 s. The swinging brass bob (4 kg) contains a small neodymium magnet to detect and sustain its motion. A microcontroller is used to control electronics that drive the pendulum while sampling the time-dependent magnetic field from the swinging bob. These data are used to calculate the precession angle for each pendulum period. Long-term studies demonstrated a precession rate of 10.31°/h which is within 0.5% of that expected for the latitude of the experiment. Considerable short-term variation in the precession rate is observed (1.5°/h) which is correlated with the structure of the pendulum mechanics. The angle measurement noise is found to be 0.059° which enables a clear detection of the earth's rotation with only 26.7 s of observation.

Using surface markers for MRI guided breast conserving surgery: a feasibility survey

Breast MRI is frequently performed prior to breast conserving surgery in order to assess the location and extent of the lesion. Ideally, the surgeon should also be able to use the image information during surgery to guide the excision and this requires that the MR image is co-registered to conform to the patient's position on the operating table. Recent progress in MR imaging techniques has made it possible to obtain high quality images of the patient in the supine position which significantly reduces the complexity of the registration task. Surface markers placed on the breast during imaging can be located during surgery using an external tracking device and this information can be used to co-register the images to the patient. There remains the problem that in most clinical MR scanners the arm of the patient has to be placed parallel to the body whereas the arm is placed perpendicular to the patient during surgery. The aim of this study is to determine the accuracy of co-registration based on a surface marker approach and, in particular, to determine what effect the difference in a patient's arm position makes on the accuracy of tumour localization. Obtaining a second MRI of the patient where the patient's arm is perpendicular to body axes (operating room position) is not possible. Instead we obtain a secondary MRI scan where the patient's arm is above the patient's head to validate the registration. Five patients with enhancing lesions ranging from 1.5 to 80 cm(3) in size were imaged using contrast enhanced MRI with their arms in two positions. A thin-plate spline registration scheme was used to match these two configurations. The registration algorithm uses the surface markers only and does not employ the image intensities. Tumour outlines were segmented and centre of mass (COM) displacement and Dice measures of lesion overlap were calculated. The relationship between the number of markers used and the COM-displacement was also studied. The lesion COM-displacements ranged from 0.9  to 9.3 mm and the Dice overlap score ranged from 20% to 80%. The registration procedure took less than 1 min to run on a standard PC. Alignment of pre-surgical supine MR images to the patient using surface markers on the breast for co-registration therefore appears to be feasible.

Physics of MRI: a primer

This article is based on an introductory lecture given for the past many years during the "MR Physics and Techniques for Clinicians" course at the Annual Meeting of the ISMRM. This introduction is not intended to be a comprehensive overview of the field, as the subject of magnetic resonance imaging (MRI) physics is large and complex. Rather, it is intended to lay a conceptual foundation by which magnetic resonance image formation can be understood from an intuitive perspective. The presentation is nonmathematical, relying on simple models that take the reader progressively from the basic spin physics of nuclei, through descriptions of how the magnetic resonance signal is generated and detected in an MRI scanner, the foundations of nuclear magnetic resonance (NMR) relaxation, and a discussion of the Fourier transform and its relation to MR image formation. The article continues with a discussion of how magnetic field gradients are used to facilitate spatial encoding and concludes with a development of basic pulse sequences and the factors defining image contrast.

Long-term results of screening with magnetic resonance imaging in women with BRCA mutations

Background:

The addition of breast magnetic resonance imaging (MRI) to screening mammography for women with BRCA mutations significantly increases sensitivity, but there is little data on clinical outcomes. We report screening performance, cancer stage, distant recurrence rate, and breast cancer-specific mortality in our screening study.

Methods:

From 1997 to 2009, 496 women aged 25 to 65 years with a known BRCA1/2 mutation, of whom 380 had no previous cancer history, were enrolled in a prospective screening trial that included annual MRI and mammography.

Results:

In 1847 screening rounds, 57 cancers were identified (53 screen-detected, 1 interval, and 3 incidental at prophylactic mastectomy), of which 37 (65%) were invasive. Sensitivity of MRI vs mammography was 86% vs 19% over the entire study period (P<0.0001), but was 74% vs 35% from 1997 to 2002 (P=0.02) and 94% vs 9% from 2003 to 2009 (P<0.0001), respectively. The relative sensitivities of MRI and mammography did not differ by mutation, age, or invasive vs non-invasive disease. Of the incident cancers, 97% were Stage 0 or 1. Of 28 previously unaffected women diagnosed with invasive cancer, 1 BRCA1 mutation carrier died following relapse of a 3 cm, node-positive breast cancer diagnosed on her first screen at age 48 (annual breast cancer mortality rate=0.5%). Three patients died of other causes. None of the 24 survivors has had a distant recurrence at a median follow-up of 8.4 years since diagnosis.

Conclusion:

Magnetic resonance imaging surveillance of women with BRCA1/2 mutations will detect the majority of breast cancers at a very early stage. The absence of distant recurrences of incident cancers to date is encouraging. However, longer follow-up is needed to confirm the safety of breast surveillance.

Supine breast MRI

PURPOSE

To achieve high-quality unilateral supine breast magnetic resonance imaging (MRI) as a step to facilitate image aiding of clinical applications, which are often performed in the supine position. Contrast-enhanced breast MRI is a powerful tool for the diagnosis of cancer. However, prone patient positioning typically used for breast MRI hinders its use for image aiding.

MATERIALS AND METHODS

A fixture and a flexible four-element receive coil were designed for patient-specific shaping and placement of the coil in close conformity to the supine breast. A 3D spoiled gradient sequence was modified to incorporate compensation of respiratory motion. The entire setup was tested in volunteer experiments and in a pilot patient study.

RESULTS

The flexible coil design and the motion compensation produced supine breast MR images of high diagnostic value. Variations in breast shape and in tissue morphology within the breast were observed between a supine and a diagnostic prone MRI of a patient.

CONCLUSION

The presented supine breast MRI achieved an image quality comparable to diagnostic breast MRI. Since supine positioning is common in many clinical applications such as ultrasound-guided breast biopsy or breast-conserving surgery, the registration of the supine images will aid these applications.

Improvement in DCIS detection rates by MRI over time in a high-risk breast screening study

Although magnetic resonance imaging (MRI) is much more sensitive than mammography for detecting early invasive breast cancer, in many high-risk screening studies MRI was less sensitive than mammography for detecting ductal carcinoma in situ (DCIS). We reviewed our experience detecting DCIS in our single center study of annual MRI, mammography, ultrasound and clinical breast examination (CBE) for screening very high-risk women. All cases of DCIS±microinvasion and invasive cancer were compared in two time frames: before (period A) and after (period B) July 2001-when we acquired expertise in the detection of DCIS with MRI-with respect to patient demographics, method of detection, and rates of detection of invasive cancer and DCIS. In period A there were 15 cases (3.1% of 486 screens) in 223 women, of which 2 (13%) were DCIS-one with microinvasion-neither detected by MRI. In period B there were 29 cases (3.3% of 877 screens) in 391 women, of which 10 (34%) were DCIS±microinvasion (p=0.04), all 10 detected by MRI but only one by mammography. No DCIS cases were detected by ultrasound or CBE. Specificity was lower in period B than in period A but acceptable. The ability to detect DCIS with screening MRI improves significantly with experience. MRI-guided biopsy capability is essential for a high-risk screening program. In experienced centers the increased sensitivity of MRI relative to mammography is at least as high for DCIS as it is for invasive breast cancer.

Evaluation of multicoil breast arrays for parallel imaging

PURPOSE

To evaluate three multicoil breast arrays for both conventional and SENSE-accelerated imaging.

MATERIALS AND METHODS

Two commercially available 8-element coils and a prototype 16-element coil were compared. One 8-element array had adjustable coils located next to the breast tissue and the other had a fixed coil arrangement; both were designed to allow parallel imaging in the left-right direction. The 16-element coil was designed to have coil sensitivity variation in both the left-right and superior-inferior directions, and also had adjustable coils. Their performance was assessed in terms of signal-to-noise ratio (SNR), g-factor, and uniformity with a custom-built phantom.

RESULTS

The 16-element array with adjustable coils provided the highest SNR, while the 8-element coil with a fixed coil arrangement had the best uniformity. All coils performed well for SENSE acceleration in the left-right direction. The 8-element coils did not have the capability for acceleration in the superior-inferior direction across the whole volume. The 16-element coil enabled acceleration in the superior-inferior direction in addition to the left-right direction.

CONCLUSION

Smaller, adjustable coil elements located next to breast tissue can provide greater SNR than larger, fixed coil elements. A multicoil breast array with high intrinsic SNR and low g-factors enables high-quality parallel imaging.

Flow-induced disturbances in balanced steady-state free precession images: means to reduce or exploit them

In this work computer simulations and phantom measurements are presented that show the effect of flow on in-plane balanced steady-state free precession images. The images were studied for various flow velocities, excitation regions, relaxation times, RF-pulse angles, and off-resonance frequencies. The work shows that flow-induced disturbances are present in the images, but can be reduced by the application of inhomogeneous excitation regions. Also, a velocity quantification method that utilizes the disturbances was developed and proved to quantify flow velocities accurately. The work concluded that the flow-induced disturbances can be reduced to improve image quality, but can also be exploited to quantify the flow velocity.

MR acoustic radiation force imaging: in vivo comparison to ultrasound motion tracking

MR acoustic radiation force (ARF) imaging was developed for measuring tiSsue elastic properties using focused ultrasound to deliver a localized tissue motion. In this study, an imaging ultrasound transducer was mounted on the focused ultrasound transducer and ultrasound motion tracking was performed simultaneously to MR ARF imaging to validate the measurement results. In vivo studies on rabbit thigh muscle were performed and results showed a general agreement between the two modalities (slope=0.96 and R2=0.67). The temporal information by the ultrasound measurement indicates that the parameters in MR ARF imaging should be optimized according to the tissue type, acoustic power, and envelope and frequency of the ARF modulation.

Removing undersampling artifacts in DCE-MRI studies using independent components analysis

In breast MRI mammography both high temporal resolution and high spatial resolution have been shown to be important in improving specificity. Adaptive methods such as projection reconstruction time-resolved imaging of contrast kinetics (PR-TRICKS) allow images to be reconstructed at various temporal and spatial resolutions from the same data set. The main disadvantage is that the undersampling, which is necessary to produce high temporal resolution images, leads to the presence of streak artifacts in the images. We present a novel method of removing these artifacts using independent components analysis (ICA) and demonstrate that this results in a significant improvement in image quality for both simulation studies and for patient dynamic contrast-enhanced (DCE)-MRI images. We also investigate the effect of artifacts on two quantitative measures of contrast enhancement. Using simulation studies we demonstrate that streak artifacts lead to pronounced periodic oscillations in pixel concentration curves which, in turn, lead to increased errors and introduce bias into heuristic measurements. ICA filtering significantly reduces this bias and improves accuracy. Pharmacokinetic modeling was more robust and there was no evidence of bias due to the presence of streak artifacts. ICA filtering did not significantly reduce the errors in the estimated pharmacokinetic parameters; however, the chi-squared error was greatly reduced after ICA filtering.

Evaluating an optical-flow-based registration algorithm for contrast-enhanced magnetic resonance imaging of the breast

Dynamic contrast-enhanced magnetic resonance imaging studies of the breast are frequently degraded by patient motion. In order to correct for this, any registration algorithm must overcome two major challenges: the highly deformable nature of the breast itself and the need to remove changes in signal intensity due to patient motion whilst leaving potentially significant changes in signal intensity due to changes in contrast agent concentration unchanged. In this paper, we evaluate the use of a non-rigid registration method that uses optical flow equations to drive the displacement of a grid of control points. With conventional optical flow techniques it is assumed that changes in image intensity are solely due to motion, making it unsuitable for use with contrast-enhanced studies. The registration algorithm evaluated in this paper overcomes this problem by including an additional term to account for changes in image intensity. Studies simulating physiologically plausible deformations of the breast together with realistic changes in contrast-enhancement derived from patient studies demonstrate that the algorithm is capable of registering images to sub-voxel accuracy within minutes. This technique has now been successfully incorporated into a breast cancer screening protocol allowing registered images to be provided routinely to the radiologist immediately after the scanning session.

Piecewise-quadrilateral registration by optical flow--applications in contrast-enhanced MR imaging of the breast

In this paper we propose a method for the nonrigid registration of contrast-enhanced dynamic sequences of magnetic resonance(MR) images. The algorithm has been developed with accuracy in mind, but also has a clinically viable execution time (i.e. a few minutes) as a goal. The algorithm is driven by multiresolution optical flow with the brightness consistency assumption relaxed, subject to a regularized best-fit within a family of transforms. The particular family of transforms we have employed uses a grid of control points and trilinear interpolation. We present validation results from a study simulating non-rigid deformation by a biomechanical model of the breast, with simulated uptake of a contrast agent. We further present results from applying the algorithm as part of a routine breast cancer screening protocol.

A quantitative evaluation of human coordination interfaces for computer assisted surgery

Computer assisted surgery (CAS) for tumor resection can assist the surgeon in locating the tumor margin accurately via some form of guidance method. A wide array of guidance methods can be considered, including model-based visual representations, symbolic graphical interfaces, and those based on other sensory cues such as sound. Given the variety of these guidance methods, it becomes increasingly important to test and analyze guidance methods for CAS in a quantitative and context-dependent manner to determine which is most suitable for a given surgical task. In this paper, we present a novel experimental methodology and analysis framework to test candidate guidance methods for CAS. Different viewpoints and stereographic, symbolic and auditory cues were tested in isolation or in combination in a set of virtual surgery experiments. A total of 28 participants were asked to circumscribe a virtual tumor with a magnetically tracked scalpel while measuring the surgical trajectory. This allowed measurement of surgical accuracy, speed, and the frequency with which the tumor margin was intersected, and enabled a quantitative comparison of guidance approaches. This study demonstrated that adding sound to pictorial guidance methods consistently improved accuracy, speed and margin intersection of the virtual surgery. However, the use of stereovision showed less benefit than expected. While guidance based on a combination of symbolic and pictorial cues enhanced accuracy, we found that speed could be substantially impaired. These studies demonstrate that optimal guidance combinations exist which would not be apparent by studying individual guidance methods in isolation. Our findings suggest that care is needed when using expensive and sometimes cumbersome virtual visualization technologies for CAS, and that simpler, non-stereo presentation may be sufficient for specific surgical tasks.

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.

Comparison of biphasic and reordered fat suppression for dynamic breast MRI

PURPOSE

To optimize a reordered k-space acquisition that applies intermittent fat saturation (FS) pulses to allow for a time-efficient reduction of fat signal in breast MR images, and compare it with an elliptic-centric biphasic FS method in terms of the degree of fat suppression and speed.

MATERIALS AND METHODS

The behavior of the fat and water signals under the influence of the reordered sequence was characterized. This allowed us to optimize the flip angle and visualize the expected artifacts by deriving the point spread function (PSF) of the fat signal. We compared the two sequences by acquiring images with a varying number of FS pulses, with a corresponding difference in scan time. The quality of the images was assessed by comparison with images obtained with full fat suppression as measured by a root-mean-square (RMS) error metric.

RESULTS

The reordered sequence allowed for an approximately twofold reduction in error compared to the biphasic sequence for the same scan time. With the reordered sequence and optimized scan parameters, we were able to reduce the time spent on fat suppression by as much as 99% with no discernible reduction in image quality.

CONCLUSION

This method will allow robust fat suppression with virtually no extension in imaging time for dynamic contrast-enhanced (DCE)-MRI.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.