Patrick D. Lester

Magnetic resonance imaging of an orbital varix with CT and ultrasound correlation

A case of orbital varix demonstrated with computed tomography, ultrasound, and magnetic resonance imaging is reported. The clinical and radiographic findings are discussed.

Electromagnetically Induced Focused Heat in the Treatment of Surgically Created Aneurysm Models

A hand-held radiofrequency (rf) probe of a novel design based on the principle of the induced current convergence was used to treat aneurysm models using focused hyperthermia. Aneurysms were created surgically in rats by a side-to-side anastomosis between the inferior vena cava and the abdominal aorta or by grafting a donor abdominal aorta from one rat onto the abdominal aorta of another rat. Aneurysms were treated by inserting the 0.3-mm diameter probe tip into the fundus and applying the power for brief periods (0.5-1.5 sec) using a foot pedal. Collapse of the fundus was observed as the result of the heat-induced thrombosis. Thermal distribution in the immediate vicinity of the probe as well as the heating rate were measured in a uniformly dielectric phantom and in rat vessels. The aneurysms were histologically examined immediately, three days, and three weeks after the treatment. Complete obliteration of the aneurysms and patency of the parent arteries were confirmed. Partial integrity of the vessels around the lesion was also confirmed.

Properties of electromagnetic field focusing probe

The electromagnetic field focusing (EFF) apparatus consists of a radio fre quency generator, solenoidal coil, and a hand-held or catheter probe. Applica tions such as aneurysm treatment, angioplasty, and neurosurgery in various models have been reported. The probe is operated in the near field (within one wavelength of an electromagnetic field source) of a coil inducing eddy currents in biological tissues, producing maximal convergence of the induced current at the probe tip. The probe produces very high temperatures depending on the wattage se lected for the given radio frequency of output power. The high temperature can be used in cutting, cauterizing, or vaporizing. The EFF probe is comparable to different types of lasers and to bipolar and mono polar cautery. The EFF probe can be used with catheters or endoscopes. Objec tives of this study were to determine what the thermal properties of the EFF probe are and how instrument parameters can be varied to obtain different temperatures in the tissue near the probe tip. In this study an F2 catheter was used as an insulated sheath and the tip of the guide wire was used as the probe tip. Different powers, wave forms, coil-to- probe distances, and probe-tip lengths were tested on a phantom that simulates tissue electrical properties. Some of the experiments were conducted under nor mal saline to simulate treatment of tissue with body fluids such as blood vessels or brain tissue under normal physiologic conditions. It is concluded that the EFF probe has the advantages of easy manipulation, relative safety, cost effectiveness, and a high degree of spatial control. Only at the point of contact do extremely high temperatures occur. Small volumes of heat dissipation occur around the tip, especially when the sinusoidal wave form is used. Temperatures delivered are related to power applied, coil-to-probe-tip distance, and probe-tip length.

Electromagnetic Field Focusing Probe (EFFP) — A New Angioplasty Tool:

An electromagnetic field focusing probe (EFFP) consists of a radiofrequency generator, solenoidal coil, and a hand-held or catheter probe. The probe is oper ated in the near field (distance within one wave length of an electromagnetic field source) of a coil, which induces eddy current in a biological tissue. The induced eddy current is converged maximally at the tip of the probe upon con tact of the tip with the tissue. The probe produces very high temperatures de pending on the wattage selected. In this study, the EFFP was used to evaporate atheromatous plaques in hu man cadaver abdominal aorta specimens, which were then studied histologi cally. Gas produced by this technique was analyzed and the volume found to be related to power delivered, but in such small amounts as to be of no embolic significance. While temperature varied with wattage and time of application, it was maximal at the probe tip and easily controlled, resulting in clean oblitera tion of plaque.

Magnetic resonance imaging in childhood intracranial masses

Magnetic Resonance Imaging (MRI) was compared with computerized tomography (CT) in 40 pediatric and adolescent patients with intracranial mass lesions as part of an ongoing project to determine the potential of MRI as a primary and definitive imaging modality. Multiplanar, multisequence MRI surpassed CT in providing mass localization, extent of involvement, and delineating the relationship to adjacent vital structures. Further development of MRI is encouraged to overcome current inconsistencies in pathologic characterization in order for this new technology to be employed not only as a primary screening modality but also as a definitive diagnostic test.

Computer-assisted design of surface coils used in magnetic resonance imaging. III. The design and construction of two long twin axial antennae for imaging of the whole human spine

Two modified folded dipole MRI surface coils were designed, constructed and tested. These antenna which are long twin axial lines use the effective distributive capacitance resulting from the distance between two longitudinal elements to provide tuning. The principal advantage of this type of antenna is the ability to image longer objects such as vertebrae, spinal cord, and longer portions of the extremities. This type of antenna shows less localized high intensity in the image due to a more evenly distributed current pickup from the sample. The coils were designed by calculating theoretical magnetic field distribution for the twin axial coils. These were obtained by integrating the Biot-Savart equation. This gave excellent agreement with an MR image of a di-electrically uniform phantom. As antennae of this sort are nonlinear in response, giving rise to an image intensity nonuniformity, computer software for the MR image was developed to correct the image intensity profile over the experimental volume. The software significantly improved the image quality by reducing the saturated intensity of the region near the antenna, thereby revealing detailed structure of the tissue being imaged.