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Biomedical subjects

J F Schenck

Publications and source records attributed to J F Schenck.

At least 19 recordsLinked to original sources
PubMed2000

Safety of strong, static magnetic fields.

Issues associated with the exposure of patients to strong, static magnetic fields during magnetic resonance imaging (MRI) are reviewed and discussed. The history of human exposure to magnetic fields is reviewed, and the contradictory nature of the literature regarding effects on human health is described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with magnetic field exposure and no evidence for hazards associated with cumulative exposure to these fields. The very high degree of patient safety in strong magnetic fields is attributed to the small value of the magnetic susceptibility of human tissues and to the lack of ferromagnetic components in these tissues. The wide range of susceptibility values between magnetic materials and human tissues is shown to lead to qualitatively differing behaviors of these materials when they are exposed to magnetic fields. Mathematical expressions are provided for the calculation of forces and torques.

Electromagnetic Fields↗
PubMed1999

Dimensions of quality of life and self-monitoring therapy with oral anticoagulants--still research or everyday practice?

In correlation with increased life expectancy of patients, quality of life (QOL) has become a factor of increasing interest by the patient himself and also of importance in health-care planning and recruitment of financial resources. In this context, self-monitoring of long-term anticoagulant treatment might be a strategy that could mean a step forward in health-related as well as general life satisfaction for patients participating in self-monitoring programs. Also, the new strategy of increased home-control of anticoagulant treatment illustrates the complexity of multiple factors that can lead to changes in the subjective feeling and objective aspects of QOL. Our intention in a pilot study was to probe the feasibility of QOL research and relevant factors of influence by retrospectively evaluating data from two groups of outpatients seen in a large treatment center. The high frequency (n = 8 in sample 2) of disturbed sleep as a simple screening indicator stresses the probable importance of undetected depression, which might require treatment and could confound research as to QOL. Instruments to measure QOL in oral anticoagulation self-monitoring should therefore be adapted to the heterogeneous structure of factors in the target population, and include psychological parameters, especially in regard to health-related locus of control and mood.

Administration, Oral↗
PubMed1999

Very-high-field magnetic resonance imaging: instrumentation and safety issues.

Because of their advantage in terms of signal-to-noise ratio, high-field magnetic resonance imaging systems have become favored in the last few years for functional magnetic resonance imaging (fMRI) applications. In many ways the conceptual development of these high-field scanners has involved more-or-less straightforward extensions of practices at lower field strengths. However, in other ways specific engineering challenges have been encountered and largely overcome in the quest for scanners capable of realizing the advantages of high-field systems. An understanding of the technical trade-offs that can be made in terms of hardware performance is useful in deciding on the optimum system for a given fMRI application. In this article the technical issues surrounding high-field scanning are reviewed in the context of a typical brain mapping protocol. In addition there is a discussion of the safety issues related to the use of these systems.

Brain Mapping↗
PubMed1998

Experience with MR-guided therapy.

The use of magnetic resonance imaging (MRI) for the real time guidance of surgical procedures is now undergoing clinical trials. Among the many procedures explored, open craniotomy neurosurgery appears to be among the most promising. Over 50 such cases have been done at the Brigham and Women's Hospital (BWH) in Boston. We review the technical approach used in these and related procedures. We consider the way in which imaging is used to augment and improve the procedures. As well, the implications of these protocols for remote diagnosis and telesurgery are explored. Finally, the implications of this experience for the insertion of new technology into medicine are discussed.

Biopsy↗
PubMed1998

MR safety at high magnetic fields.

MR imaging requires the exposure of human patients to magnetic fields that are much more intense than ever occur naturally. Under most circumstances, however, these fields do not appear to pose a health or safety risk even at levels well above those currently in clinical use. This margin of safety results from the extreme weakness of the diamagnetic interaction between human tissues and magnetic fields. Ferromagnetic materials, however, present either as implants within the patient or inadvertently introduced into the scanner environment, pose significant risks and must be scrupulously avoided. At very high field strengths, there is evidence for mild sensory effects, such as vertigo and metallic tastes, which do not appear to be harmful.

Animals↗
PubMed1996

The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds.

The concept of magnetic susceptibility is central to many current research and development activities in magnetic resonance imaging (MRI); for example, the development of MR-guided surgery has created a need for surgical instruments and other devices with susceptibility tailored to the MR environment; susceptibility effects can lead to position errors of up to several millimeters in MR-guided stereotactic surgery; and the variation of magnetic susceptibility on a microscopic scale within tissues contributes to MR contrast and is the basis of functional MRI. The magnetic aspects of MR compatibility are discussed in terms of two levels of acceptability: Materials with the first kind of magnetic field compatibility are such that magnetic forces and torques do not interfere significantly when the materials are used within the magnetic field of the scanner; materials with the second kind of magnetic field compatibility meet the more demanding requirement that they produce only negligible artifacts within the MR image and their effect on the positional accuracy of features within the image is negligible or can readily be corrected. Several materials exhibiting magnetic field compatibility of the second kind have been studied and a group of materials that produce essentially no image distortion, even when located directly within the imaging field of view, is identified. Because of demagnetizing effects, the shape and orientation, as well as the susceptibility, of objects within and adjacent to the imaging region is important in MRI. The quantitative use of susceptibility data is important to MRI, but the use of literature values for the susceptibility of materials is often difficult because of inconsistent traditions in the definitions and units used for magnetic parameters-particularly susceptibility. The uniform use of SI units for magnetic susceptibility and related quantities would help to achieve consistency and avoid confusion in MRI.

Biophysical Phenomena↗
PubMed1995

Imaging of brain iron by magnetic resonance: T2 relaxation at different field strengths.

Soon after the advent of magnetic resonance imaging (MRI) as a diagnostic modality in the 1980s, it was recognized that some of the contrast found in brain imaging correlated with patterns of iron deposition. The presence of non-heme iron had previously been established by pathological studies on post-mortem brains. The iron concentration is highest in specific nuclei of the basal ganglia and some associated structures. It is low at birth and increases with age until a relatively constant level is reached at an age of 20-30 years. There is evidence for further increases in very elderly persons. Although iron is ubiquitous in human tissues, only in a few situations is the concentration large enough to affect MRI. Because MRI has the ability to detect, in a noninvasive fashion, the naturally occurring iron in the basal ganglia and related nuclei, it may be used to study the physiology and pathology of these important structures. Magnetic resonance imaging has confirmed the results of earlier post mortem studies of the anatomical localization and age-dependence of brain iron. Initial steps have been toward the use of MRI to study disorders of thought, movement, and behavior that are believed to be related to brain iron. However, additional understanding is required of the physical details of the contrast mechanism, the physiology of the iron accumulation, and the significance of abnormal patterns of iron deposition. In this report, data are presented on the normal variation in MRI parameters and their dependence on magnetic field strength. The potential clinical and basic science applications are briefly reviewed. Information from widely differing fields is relevant to the study of the physical and pathological significance of brain iron, and for this reason, extensive, although not exhaustive, literature references are included.

Adult↗
PubMed1995

Focused US system for MR imaging-guided tumor ablation.

PURPOSE: To measure the performance characteristics of a focused ultrasound (US) system for magnetic resonance (MR) imaging-guided tumor ablation. MATERIALS AND METHODS: The authors constructed a focused US system for MR imaging-guided tumor ablation. The location of the heated region and thermal dose were monitored with temperature-sensitive MR images obtained in phantoms and rabbit skeletal muscle after application of each sonic pulse. RESULTS: The region heated by the focused ultrasound beam was within 1 mm of that observed on temperature-sensitive fast gradient-echo MR images of in vivo rabbit skeletal muscle. Analysis of heat flow and the rate of coagulation necrosis provided an estimate of the size of the ablated region that was in agreement with experimental findings. CONCLUSION: MR imaging provides target definition and control for thermal therapy in regions of variable perfusion or in tissues that are not well characterized.

Animals↗
PubMed1995

Superconducting open-configuration MR imaging system for image-guided therapy.

PURPOSE: To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. MATERIALS AND METHODS: A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, and nonmagnetic displays and with position-monitoring probes and device-tracking instrumentation. RESULTS: The magnet homogeneity was 12.3 ppm, and the gradient field was linear to within 1% over an imaging region 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences, were demonstrated at a rate of one image every 1.5 seconds. CONCLUSION: MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

Humans↗
PubMed1994

MR temperature mapping of focused ultrasound surgery.

Deep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherical shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70 degrees C. The procedure is monitored with a series of fast second gradient echo, T1-weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and near field.

Animals↗
PubMed1994

The usefulness of a contrast agent and gradient-recalled acquisition in a steady-state imaging sequence for magnetic resonance imaging-guided noninvasive ultrasound surgery.

RATIONALE AND OBJECTIVES: The ability of magnetic resonance imaging to detect small temperature elevations from focused ultrasound surgery beams was studied. In addition, the value of a contrast agent in delineating the necrosed tissue volume was investigated. MATERIALS AND METHODS: Gradient-recalled acquisition in a steady state (GRASS) T1-weighted images were used to follow the temperature elevation and tissue changes during 2-minute sonications in the thigh muscles of 10 rabbits. The effects of the treatment on the vascular network was investigated by injecting a contrast agent bolus before or after the sonication. RESULTS: The signal intensity decreased during the sonication, and the reduction was directly proportional to the applied power and increase in temperature. The signal intensity returned gradually back to baseline after the ultrasound was turned off. Injection of the contrast agent increased the signal intensity in muscle, but not in the necrosed tissue. The dimensions of the delineated tissue volume were the same as measured from the T2-weighted fast-spin-echo images and postmortem tissue examination. CONCLUSIONS: These results indicate that magnetic resonance imaging can be used to detect temperature elevations that do not cause tissue damage and that contrast agent can be used to delineate the necrosed tissue volume.

Animals↗
PubMed1993

Magnetic resonance-guided thermal surgery.

A demonstration of MR guided thermal surgery involved experiments with imaging of focused ultrasound in an MRI system, measurements of the thermal transients and a thermal analysis of the resulting images. Both the heat distribution and the creation of focused ultrasound lesions in gel phantoms, in vitro bovine muscle and in vivo rabbit muscle were monitored with magnetic resonance imaging. Thermal surgical procedures were modeled by an elongated gaussian heat source where heat flow is controlled by tissue thermal properties and tissue perfusion. Temperature profiles were measured with thermocouples or calculated from magnetic resonance imaging in agreement with the model. A 2-s T1-weighted gradient-refocused acquisition provided thermal profiles needed to localize the heat distribution produced by a 4-s focused ultrasound pulse. Thermal analysis of the images give an effective thermal diffusion coefficient of 0.0015 cm2/s in gel and 0.0033 cm2/s in muscle. The lesions were detected using a T2-weighted spin-echo or fast spin-echo pulse sequence in agreement with muscle tissue sections. Potential thermal surgery applications are in the prostate, liver, kidney, bladder, breast, eye and brain.

Acoustics↗
PubMed1993

Tissue thermometry during ultrasound exposure.

In order to quantify ultrasound therapy it is important to measure the tissue temperature during the treatment. Invasive probes induce several artifacts in ultrasound fields. The magnitude of these artifacts is probe dependent. Several different probes were evaluated for hyperthermia purposes in this study. An alternative noninvasive method to evaluate the temperature elevations and tissue damage is to use magnetic resonance imaging. The fast imaging sequences used in this study are marginally useful for monitoring hyperthermia. However, these imaging sequences can be utilized to guide and monitor ultrasound surgery.

Animals↗
PubMed1992

High-resolution MR imaging of the wrist and eye with short TR, short TE, and partial-echo acquisition.

High-resolution spin-echo and two- and three-dimensional gradient-recalled acquisition in the steady state (GRASS) and spoiled GRASS images of the wrist and eye were obtained with a whole-body magnetic resonance imager by using trapezoidal phase-encoding gradient waveforms and by lowering receiver bandwidth to +/- 8 kHz, which results in a minimum field of view (FOV) of 4 cm and pixel resolution of 156 microns for a 256 x 256 matrix. Short echo times were achieved by using partial-echo acquisition. Poor signal-to-noise ratio resulting from smaller voxel sizes was overcome by using prototype receive-only eye and wrist coils. Images were obtained in reduced time and were superior to those obtained in studies performed with an 8-12-cm FOV and commercially available coils.

Adult↗
PubMed1988

Noninvasive imaging of the normal temporal bone. Comparison of sagittal surface coil magnetic imaging and high-resolution computed tomography.

High-resolution computed tomography (HRCT) is a noninvasive technique for evaluating the middle ear for primary and recurrent cholesteatoma. However, a limitation of HRCT is that it cannot differentiate between cholesteatoma and granulation tissue. Magnetic resonance imaging (MRI) is a noninvasive, nonradiologic technique that has been effective in demonstrating histochemical differences between various soft tissues. We present images from a normal living subject's temporal bone in the sagittal plane obtained with both HRCT and MRI. Anatomic correlates in the same cut planes are presented. The HRCT provided excellent detail of the bony landmarks within the temporal bone and was used as the reference for the MRI. The soft-tissue structures such as cranial nerves, cochlea, vestibule, and semicircular canals were identified.

Humans↗