Rogers C. Ritter

Magnetic manipulation instrumentation for medical physics research

The noncontact magnetic manipulation of probe masses within the body is an area of research that has received substantial attention from the medical physics community, especially during the past three decades. The therapeutic and diagnostic possibilities arising from such technology include site‐specific drug delivery within the central nervous system, advancement of techniques for navigation and selective catheterization of vessels within the cardiovascular and cerebrovascular systems, and the nonsurgical exploration of the alimentary and respiratory tracts. In this review, we examine the physical principles underlying in vivo magnetic manipulation systems, and catalog the various types of instrumentation used for such purposes to date. Thereafter, we evaluate the different methods of image‐based localization used to identify the position of the probe within the body. Finally, we appraise an emerging technology known as nonlinear magnetic stereotaxis, a technique that permits minimally invasive access to...

Nonlinear magnetic stereotaxis : three-dimensional, in vivo remote magnetic manipulation of a small object in canine brain

In a series of in vivo experiments on five adult canines, a small cylindrical permanent magnet (approximately 5-mm diameter x 5 mm long) was magnetically moved under fluoroscopic guidance from an occipital-lobe burr hole to a predetermined destination within the brain and then removed. On three of the animals, dorsal and temporal skull markers were used to establish a coordinate system against which the motions of the seed were referenced. These procedures were sufficiently accurate to permit the guided motion of the seed along nonlinear paths within the brain, including traversal of the midline through the corpus callosum. For removal, the seed could be steered either to a frontal lobe location for extraction through an auxiliary burr hole, or back to the same burr hole through which it had been inserted. This article discusses the way in which stereotactic motions were obtained, the performance limits of the instrumentation and the precision of motion achieved.

Characteristics of an improved magnetic-implant guidance system

The previous companion paper (see ibid., vol. 42, no. 8, p.793, 1995) described the motivation, design, and early experiments of a Magnetic Stereotaxis System. The part of the system considered in these papers is a helmet with a roughly cubic array of six superconducting coils used to apply force on small permanent magnet pellets in brain and in brain phantom material. This apparatus will be used to deliver drugs and other therapies directly into deep brain tissues, under control of a computer and fluoroscopic imaging system. Here, the authors analyze the general stability problems of controlling the currents in the coils for impulsive stepwise motion of the pellet, subject to quench avoidance in the superconducting coils, and in the face of Earnshaws theorem governing stability in static magnetic fields. The authors also describe solutions that have been found to the primary difficulties limiting controlled pellet motion in the studies presented in the companion paper.<<ETX>>

Role of inner urethral softness in urinary continence.

Given adequate external squeeze upon suitable urethral geometry, continence depends on the mechanical properties of the outflow tract wall. Evidence indicates that these mechanical properties include an ability to deform and behave in a fluid-like manner.

Torsion balances, torsion pendulums, and related devices

The torsion pendulum is not only a mainstay instrument in the world of precision measurement and gravitational physics, but is important in electrical science, biophysics, petrology, metallurgy, and various other fields of endeavor. Whether used in the ‘‘static’’ (deflection) mode, the ‘‘dynamic’’ (oscillating) mode, or in some more complex configuration, instrumentation of this kind enables one to isolate and measure weak effects that would otherwise be difficult if not impossible to observe against the background gravitational field of the earth. In this review, we present a brief history of fiber‐suspended apparatus and assess the fundamental limits of performance of the dumbbell pendulum. We then inventory the different versions of such systems presently used by gravitational physicists and discuss the various interrogation techniques used to monitor the movement of the suspended test mass. Next, we tabulate some of the applications for torsion instruments outside of gravitational physics, and close w...

Experimental determination of the force required for insertion of a thermoseed into deep brain tissues.

Our laboratories are developing a new technique for delivering localized hyperthermia to deep-seated brain tumors. In this technique, a spherical thermoseed is stereotactically navigated through the brain and tumor tissues via the noncontact application of an external magnetic force. The force required to produce motion of a 3 mm diameter sphere through in vitro brain tissues was measured to be 0.07 ± 0.03 N. This result was obtained from a series of experiments performed on whole brain specimens extracted from adult canines. Data were also taken with a 3 mm × 3 mm cylinder and a 5 mm sphere. An experimental procedure simulating physiological conditions was developed prior to testing. Evaluations of systematic effects included determinations of the calibration uncertainties, tests of the dependence of the measured force on temperature, and studies of the effects of method of storage of the tissue specimens. The results obtained are compared with (and confirmed by) two different series of experiments performed in vivo on adult canines and with another series of experiments using brain phantom gelatin.

Magnetic Stereotaxis: A Technique to Deliver Stereotactic Hyperthermia

Advances in imaging techniques and computer software over the past decade now define brain abnormalities such as tumors in precise, three-dimensional images. We have taken advantage of these technological improvements in designing a system capable of performing magnetic manipulation of an object in a nonlinear trajectory and able to deliver hyperthermia to highly specific targets within the brain. This device relies on external magnets to pull a small metal pellet (thermoceptor) through the brain, and on biplane fluoroscopy to localize the thermoceptor with respect to previously obtained magnetic resonance images. A radiofrequency tuned circuit serves as the hyperthermia applicator and selectively heats the thermoceptor. This paper describes experiments conducted in a series of dogs showing that all three components of the system (magnetic drive, stereotactic real time imaging, and hyperthermia) can be achieved. Integration of the system was accomplished in one animal. These encouraging results need further detailed substantiation in each of the components, yet demonstrate the feasibility of such a device.

The Physical Basis of Obstructive Uropathy

A majority of urologists probably would not agree on a precise definition of obstruction, although most would agree that a reduction in urinary flow rate and an elevation of detrusor pressure are early signs. Beyond these relatively clear indications, however, the concepts of obstruction get a bit fuzzy. Is obstruction always associated with a constriction of the urethra? Does a constriction of the urethra necessarily imply obstruction? What are the actual physical mechanisms associated with lower urinary tract obstruction? Is an understanding of the physical basis of obstruction really that important?

Precision Measurements and Fundamental Constants

Recent developments in the techniques for making precision measurements and their use in the determination of the fundamental constants are reviewed. Particularly noteworthy developments are clocks with high stability, the proposed redefinition of the meter in terms of the standard for time, and the increased precision with which electrical standards can be maintained. The relevance of precision measurements to tests of general relativity is briefly discussed.

Goniometric motion controller for the superconducting coil in a magnetic stereotaxis system

For the purpose of positioning the 80 kg, 2 T superconducting coil/cryostat used in the development version of a magnetic stereotaxis system, a four degree-of-freedom goniometer has been designed, built, and tested. Computer-controlled, stepping motor actuators enable movement of the coil via either joystick, keyboard, or translator-module keypad commands. An integral arrangement of counterweights and counterbalances minimizes the overall weight and size of the goniometer while maintaining static and dynamic stability during operation. As much of the structure as possible has been made of nonmagnetic materials (mostly aluminium) to minimize distortion of the superconducting coils field. The design principles for the goniometer, the essential features of its construction, and its performance characteristics and limitations are discussed, along with a strategy for performing precision magnetic stereotaxis procedures with an arrangement of static superconducting coils.<<ETX>>