John F. Schenck

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.

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. J. Magn. Reson. Imaging 2000;12:2–19.

MRI‐guided noninvasive ultrasound surgery

In this study, the feasibility of using magnetic resonance imaging (MRI) to detect tissue necrosis induced by focussed ultrasound beams was investigated. It was shown that lesions produced in dogs thigh muscle in vivo were clearly visible in T2-weighted images and that the lesion dimensions measured from the images correlated with the postmortem measurements of the visible tissue damage. It was also shown that the sonications can be done in the magnet and that the lesions are visible immediately after the sonications with increasing image contrast as a function of time. These results showed that MRI can be used to direct and monitor on-line noninvasive ultrasound surgery. This may have a major impact in future patient treatments.

MR-Guided Focused Ultrasound Surgery

Magnetic resonance guided focused ultrasound surgery provides a minimally invasive controlled method for selectively destroying deep-lying tissue. A thermal analysis of focused ultrasound provides an estimate of the time-dependent temperature distribution and thermal dose required for ultrasound surgery. The temperature distribution is estimated by accumulating heat sources, considering the effects of thermal conductivity, heat content, and perfusion. In this study, both gel phantoms and excised in vitro bovine muscle specimens were imaged in a 1.5 T MR system while heated with a 5 cm diameter, 10 cm focal length, 1.1 MHz transducer. During sonication, the thermal effects were observed with T1-weighted pulse sequences. Below a critical temperature, the heat zone appeared as a dark spot that moved with the focal spot. Above a critical thermal dose, the in vitro tissue was irreversibly altered and the focal lesion was observed on both the MR image and the specimen slice.

Layer-specific variation of iron content in cerebral cortex as a source of MRI contrast

Recent advances in high-field MRI have dramatically improved the visualization of human brain anatomy in vivo. Most notably, in cortical gray matter, strong contrast variations have been observed that appear to reflect the local laminar architecture. This contrast has been attributed to subtle variations in the magnetic properties of brain tissue, possibly reflecting varying iron and myelin content. To establish the origin of this contrast, MRI data from postmortem brain samples were compared with electron microscopy and histological staining for iron and myelin. The results show that iron is distributed over laminae in a pattern that is suggestive of each region’s myeloarchitecture and forms the dominant source of the observed MRI contrast.

Magnetic resonance imaging of the temporomandibular joint meniscus

This report describes early experience with magnetic resonance imaging (MRI) of the temporomandibular joint meniscus in which surface coil technology was used. The results suggest remarkable imaging capabilities and speed with noninvasive methods.

Human exposure to 4.0‐Tesla magnetic fields in a whole‐body scanner

Details are given for the design, construction, properties, and performance of a large, highly homogeneous magnet designed to permit whole-body magnetic resonance imaging and spectroscopy at 4 T. The magnet has an inductance of 1289 H and a stored energy of 33.4 MJ at rated field. The health of a group of 11 volunteers who had varying degrees of exposure to this field was followed over a 12-month period and no change that could be associated with this exposure was detected. A mild level of sensory experiences, apparently associated with motion within the field of the magnet, was reported by some of the volunteers during some of their exposures. A questionnaire regarding sensory effects associated with magnetic resonance scanners and possibly caused by the static magnetic field of these instruments, was given to nine respondents who had experience within both 1.5-T scanners and this 4-T scanner and to another group of 24 respondents who had experience only within 1.5-T scanners. For the sensations of vertigo, nausea, and metallic taste there was statistically significant (p less than 0.05) evidence for a field-dependent effect that was greater at 4 T. In addition, there was evidence for motion-induced magnetophosphenes caused by motion of the eyes within the static field. These results indicate the practicality of experimental whole-body body scanners operating at 4 T and the possibility of mild sensory effects in humans associated with motion within a static magnetic field. The results also indicate the likelihood of a wide margin of safety for the exposure of noncompromised patients to the static fields of conventional magnetic resonance scanners operated at 1.5 to 2 T and below.

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

Hynynen K, Darkazanli A, Damianou CA, Unger E, Schenck JF. 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) TI-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.

Magnetic resonance imaging of brain iron.

The development of effective staining and other tissue analysis techniques has long provided pathologists with a means of studying [1–4] and quantifying [5] the regional distribution of the non-heme iron stored in the brain. However, the nature of these techniques precluded premortem studies. Soon after the introduction of magnetic resonance (MR) imaging in the 1980s, it was recognized that some of the MR contrast present in brain images correlated closely with the previously established patterns of brain iron deposition [6]. The initial studies were performed using 1.5 tesla (T) magnets but it was quickly noted that this contrast increased rapidly with the use of higher field strength magnets [7–9]. In the early 1990s, only a handful of whole-body magnets operating at field strengths above 1.5 T were in use, and these were confined to research applications. However, fully featured clinical scanners operating at 3 T are now coming into widespread use, and whole-body research systems operating at 7 and 8 T are available at some research centers. As a result, the study of iron-dependent contrast at high magnetic field strengths is now much more feasible than was previously the case. The MR signal arises from mobile protons in the tissues. In brain, these protons are almost entirely present in the solvent water molecules as H2O. The image contrast arises from variations among brain regions of the density of solvent water and the longitudinal (T1) and transverse (T2) relaxation times of the associated protons [10]. Generally, regions with short values of T1 appear bright on T1-weighted images and regions of short T2 appear dark on T2-weighted images. The presence of magnetic ions can alter T1 and T2 and thereby the tissue contrast [11]. Many metallic ions essential to brain function (sodium, potassium, calcium, magnesium, and zinc) are nonmagnetic and have no impact on MR images. However, ions of the transition group (e.g., iron, manganese, and copper), and of the rare earth group (e.g., gadolinium and dysprosium) exhibit nonzero magnetic moments in many compounds and do have the potential of affecting MR contrast.

In vitro simulation : Early results of stereotaxy for pedicle screw placement

Study Design. Frameless stereotaxy with doppler ultrasound and three dimensional computer model registration is assessed in vitro for pedicle screw placement. Objective. To identify feasibility of pedicle screw navigation and placement using this technology. Summary of Background Data. Inaccurate pedicle screw placement can lead to neurovascular injury or suboptimal fixation. Present techniques in pedicle screw placement involve only confirmation of hole orientation. Method. Forty‐four pedicle screws were placed in lumbosacral models and cadaver specimens. Accuracy was assessed with a computed tomography scan and vertebral cross sectioning. Results. All screws were intrapedicular. Accuracy of anterior cortical fixation was 1.5 mm, with a range of 2.5 mm. Conclusion. In vitro frameless stereotaxy is accurate for pedicle screw placement. This technology adds a component of navigation to pedicle screw placement.