Biomedical subjects

G B Pike

Publications and source records attributed to G B Pike.

45 records · Page 3Linked to original sources

PubMed1990

Three-dimensional isodose distributions in stereotactic radiosurgery.

A personal computer based three-dimensional treatment planning system, which may be used for planning any linear accelerator (Linac) based radiosurgical technique, is presented. The system is used to calculate dose distributions for most of the Linac-based techniques currently in use as well as the theoretically optimum 4 pi geometry. The maximum and minimum dose falloffs are used to compare the various Linac-based radiosurgical techniques. The dynamic rotation technique developed at McGill University is shown to produce distributions with dose falloffs similar to the multiple converging arc techniques used elsewhere and those obtained for the Gamma Unit. Also considered are the effects of beam energy, in the range of 4-25 MV, and beam profiles on the dose distribution.

Brain Neoplasms↗

PubMed1989

Stereotactic neurosurgery planning on a personal-computer-based work station.

Stereotactic surgery requires knowledge of cerebral structures derived from more than one image source. We have developed a PC-AT-based workstation that accepts patient images, made with the stereotactic frame in place, from CT, MRI, and DSA modalities. Reference markers on the frame are identified in the images to establish the coordinate geometry for each modality. Target points may be identified on each image type and trajectories of probe paths to these points defined. Targets identified on one set of images may be transferred automatically to other images of the same patient in order to guarantee a vessel-free path of approach to a target point deep within the brain. To date several hundred patients have had stereotactic surgery performed on the basis of plans using this system. Procedures included biopsy and aspiration of lesions, implantation of electrodes for the recording of deep EEG signals, and radiosurgical techniques. We present clinical examples of the use of this system in typical stereotactic neurosurgery procedures, address stereoscopic applications, and discuss the results of intermodality tests to establish the accuracy of the technique.

Angiography, Digital Subtraction↗

PubMed1989

Radiosurgery with high energy photon beams: a comparison among techniques.

The presently known radiosurgical techniques with high energy photon beams are based either on the commercially available Gamma unit utilizing 201 stationary cobalt beams or on isocentric linear accelerators. The techniques using linear accelerators are divided into the single plane rotation, the multiple non-coplanar arcs, and the dynamic rotation. A brief description of these techniques is given, and their physical characteristics, such as precision of dose delivery, dose fall-off outside the target volume, and isodose distributions are discussed. It is shown that the multiple non-coplanar arcs technique and the dynamic rotation give dose distributions similar to those of the Gamma unit, which makes these two linear accelerator based techniques attractive alternatives to radiosurgery with the Gamma unit.

Brain Diseases↗

PubMedDate not supplied

Pulsed magnetization transfer spin-echo MR imaging.

Cross relaxation between macromolecular protons and water protons is known to be important in biologic tissue. In magnetic resonance (MR) imaging sequences, selective saturation of the characteristically short T2 macromolecular proton pool can produce contrast called magnetization transfer contrast, based on the cross-relaxation process. Selective saturation can be achieved with continuous wave irradiation several kilohertz off resonance or short, intense 0 degree pulses on resonance. The authors analyze 0 degree binomial pulses for T2 selective saturation, present design guidelines, and demonstrate the use of these pulses in spin-echo imaging sequences in healthy volunteers and patients. Using the phenomenologic Bloch equations modified for two-site exchange, the authors derive the analytic expressions for water proton relaxation under periodic pulsed saturation of the macromolecular protons. This relaxation is shown to be monoexponential, with a rate constant dependent on the saturation pulse repetition rate and the individual and cross-relaxation rates.

Brain↗

PubMedDate not supplied

MRI in varicella-zoster virus leukoencephalitis in the immunocompromised host.

Immunocompromised patients are at increased risk for CNS and disseminated varicella zoster virus (VZV) infection. In this report we present the MR findings in a leukemic patient with active, biopsy-proven VZV leukoencephalitis. The characteristic MR features of this infection were clustered subcortical plaque-like lesions demonstrating rapid demyelination. Active lesions enhanced with intravenous contrast medium administration. Edema and hemorrhage were not prominent early findings but developed as the infection evolved. These findings were strikingly similar to those reported in prior autopsy studies of immunocompromised patients with VZV leukoencephalitis.

Adolescent↗

PubMedDate not supplied

Dose distributions in radiosurgery.

A PC-based, three-dimensional treatment planning system, which may be used for planning of radiosurgical treatments with the Gamma unit or with any of the radiosurgical techniques based on isocentric linear accelerators (linacs), is described and used to calculate isodose distributions for various linac-based radiosurgical techniques ranging from the single plane rotation to a 4-pi geometry. The latter gives an isotropic dose falloff outside the target volume but cannot be used for practical radiosurgery, while the single plane rotation is simple to use but gives unacceptably shallow dose falloffs in the transverse plane. Dose falloffs for several other techniques of varying degrees of complexity are shown and discussed. Also discussed is the effect of beam energy and beam profiles on radiosurgical dose distributions.

Humans↗

PubMedDate not supplied

Magnetization transfer ratio is unable to lateralize epileptic foci in patients with temporal lobe epilepsy.

BACKGROUND AND PURPOSE: A preliminary report suggested that magnetization transfer ratio (MTR) was useful to lateralize epileptic foci in patients with refractory temporal lobe epilepsy (TLE). We attempted to confirm this finding in a larger group by investigating the relationship between MTR of mesial temporal structures and seizure lateralization in patients with refractory TLE. METHODS: We compared the MTR of amygdalae and hippocampi of 10 patients with unilateral TLE to values obtained from 10 healthy control participants. RESULTS: Three of 10 patients with TLE had MTR values that were 2 SD below the normal mean; the MTR abnormality was concordant with electroclinical lateralization in only one of the three. CONCLUSION: We conclude that MTR measurements of amygdalae and hippocampi are not useful for lateralization of TLE.

Adult↗

PubMedDate not supplied

Magnetization transfer MR of the normal adult brain.

PURPOSE: To establish a normal baseline of the percent magnetization transfer of gray (cortical and deep) and white matter structures in the brain in healthy adults and to determine whether there are adult age-related differences in these measurements. METHODS: Axial T1-weighted scans (800/20 [repetition time/echo time]) with and without magnetization transfer were prospectively performed on a 1.5-T MR imaging unit on 68 healthy patients (aged 20 to 76 years). Presaturation and postsaturation magnetization transfer images were obtained using an on-resonance binomial pulse. All patients had normal MR scans on all pulse sequences. A calculated "difference" image was used to calculate the percent magnetization transfer in multiple specific regions of the brain. In each hemisphere, 9 discrete areas of cortical and deep gray matter and 29 areas of white matter were measured in 68 patients to generate age-related changes in percent magnetization transfer in these anatomic regions. Ranges of normal percent magnetization transfer in each of the 38 measures were established. RESULTS: The percent magnetization transfer of the gray matter (28% +/- 2%) was lower than that of the white matter (36% +/- 2%). There was no statistically significant difference in the percent magnetization transfer in different areas of gray matter. Deep white matter in the different lobes (percent magnetization transfer, 31% to 38%) also showed no differences by age. Percent magnetization transfer was the highest in the genu of the corpus callosum (42%), and this was statistically significant compared with other white matter measurements. CONCLUSION: There were no statistically significant age-related variations in the percent magnetization transfer in healthy adults in gray or white matter. These percent magnetization transfer measurements provide baseline normative data, which can be used to measure the extent and severity of white matter changes in disease states.

Adult↗

PubMedDate not supplied

Measure of magnetization transfer in multiple sclerosis demyelinating plaques, white matter ischemic lesions, and edema.

PURPOSE: To define the percentage of magnetization transfer of multiple sclerosis (MS) plaques, ischemic white matter lesions, and vasogenic edema to determine whether this measurement can help differentiate these entities. METHODS: Findings were compared in 25 patients with proved MS, 20 patients with white matter ischemic lesions, and 72 patients with white matter edema (caused by tumors, infections, or acute/subacute infarctions) in the periventricular system, centrum semiovale, and subcortical white matter. Magnetization transfer was performed using an on-resonance binomial pulse. The percentage of magnetization transfer of the normal white matter was also calculated. RESULTS: Magnetization transfer was significantly higher in white matter ischemic lesions (range, 31% to 38%; mean, 34% +/- 0.6%) than in demyelinating plaques of MS (range, 19% to 28%; mean, 22.5% +/- 1%) and in edema (range, 29% to 37%; mean, 30.2% +/- 0.4%). No statistical difference in percentage of magnetization transfer was found among lesions in the periventricular system (34% +/- 0.6%), centrum semiovale (35% +/- 0.5%), or subcortical white matter (33% +/- 0.6%), or in vasogenic edema associated with tumors, infections, or infarction. CONCLUSION: Differences in magnetization transfer suggest less change of demyelination in white matter ischemic lesions than in MS plaques and are significantly different in this respect from similar MS plaques. Magnetization transfer of edema was less than that of normal white matter or fell between ischemic abnormalities and MS plaques. Percentages of magnetization transfer below the mid-20% range is highly suggestive of demyelination. Vasogenic edema, our surrogate for increased water content of white matter, caused a decrease in the percentage of magnetization transfer.

Adult↗