High-pressure--high-temperature polymorphism in ta: resolving an ongoing experimental controversy.

Phase diagrams of refractory metals remain essentially unknown. Moreover, there is an ongoing controversy over the high-pressure melting temperatures of these metals: results of diamond anvil cell (DAC) and shock wave experiments differ by at least a factor of 2. From an extensive ab initio study on tantalum we discovered that the body-centered cubic phase, its physical phase at ambient conditions, transforms to another solid phase, possibly hexagonal omega phase, at high temperature. Hence the sample motion observed in DAC experiments is very likely not due to melting but internal stresses accompanying a solid-solid transformation, and thermal stresses associated with laser heating.

Comparison of thermal damage calculated using magnetic resonance thermometry, with magnetic resonance imaging post-treatment and histology, after interstitial microwave thermal therapy of rabbit brain.

Clinical application of high-temperature thermal therapy as a treatment for solid tumours requires an accurate and close to real-time method for assessing tissue damage. Imaging methods that detect structural changes during heating may underestimate the extent of thermal damage. This is due to the occurrence of delayed damage manifested at tissue locations exposed to temperatures lower than those required to cause immediate structural changes. An alternative approach is to measure temperature and then calculate the expected damage based on the temperature history at each tissue location. Magnetic resonance (MR) imaging methods now allow temperature maps of the target and surrounding tissues to be generated in almost real-time. The aim of this work was to evaluate whether thermal damage zones calculated on the basis of MR thermometry maps measured during heating correspond to actual tissue damage as measured after treatment by histological methods and MR imaging. Four male rabbits were treated with high-temperature thermal therapy delivered in the brain by a single microwave antenna operating at 915 MHz. MR scanning was performed before, during and after treatment in a 1.5 T whole-body scanner. Temperature maps were produced using the proton resonance frequency (PRF) shift method of MR thermometry. In addition, conventional T1-weighted and T2-weighted spin-echo images were acquired after treatment. Thermal damage zones corresponding to cell death, microvascular blood flow stasis and protein coagulation were calculated using an Arrhenius analysis of the MR temperature/time course data. The calculated zones were compared with the lesions seen on histopathological examination of the brains which were removed within 6-8 h of treatment. The results showed that calculated damage zones based on MR thermometry agreed well with areas of damage as assessed using histology after heating was completed. The data suggest that real-time calculations of final expected thermal damage based on an Arrhenius analysis of MR temperature data may provide a useful method of real-time monitoring of thermal therapy when combined with conventional T2-weighted images taken after treatment.

Prostate cancer: MR imaging and thermometry during microwave thermal ablation-initial experience.

Percutaneous interstitial microwave thermoablation of locally recurrent prostate carcinoma was continually guided with magnetic resonance (MR) imaging. Phase images and data were obtained with a rapid gradient-echo technique and were used to derive tissue temperature change on the basis of proton-resonance shift. Thermally devitalized regions correlated well with the phase image findings. MR imaging-derived temperatures were linearly related to the fluoroptic tissue temperatures. MR imaging can be used to guide thermoablation.

MRI monitoring of interstitial microwave-induced heating and thermal lesions in rabbit brain in vivo.

The purpose of this experiment was to use MRI to monitor microwave heating and thermal damage of brain tissue in vivo. Interstitial microwave antennas were implanted into the cerebral hemispheres of seven anesthetized rabbits. Variable power of 30 to 100 W was applied for periods of 5 to 15 minutes and tissue temperature was monitored continuously. MR images were obtained throughout the procedure at 20-second intervals, using a spoiled gradient-echo sequence, without significant artifact. Magnitude, phase, and complex difference images all demonstrated temperature-related signal changes during heating. The findings were better visualized on the phase and complex difference images. Phase difference image analysis revealed an approximately linear relationship between phase change and temperature. Post-treatment thermal lesions measured up to 2.0 cm in size on pathologic specimens and exhibited a zonal pattern on spin-echo MR images.

Magnetic resonance imaging of temperature changes during interstitial microwave heating: a phantom study.

Changes in magnetic resonance (MR) signals during interstitial microwave heating are reported, and correlated with simultaneously acquired temperature readings from three fiber-optic probes implanted in a polyacrylamide gel phantom. The heating by a MR-compatible microwave antenna did not interfere with simultaneous MR image data acquisition. MR phase-difference images were obtained using a fast two-dimensional-gradient echo sequence. From these images the temperature-sensitive resonant frequency of the 1H nuclei was found to decrease approximately by 0.008 ppm/ degree C. The method and results presented here demonstrate that noninvasive MR-temperature imaging can be performed simultaneously with interstitial microwave thermal treatment.

CT and MRI of mycotic aneurysms of the abdominal aorta.

In two of three patients with mycotic aneurysms of the abdominal aorta, diagnosis was delayed, with a fatal outcome in one patient. A combination of studies, such as indium white blood cell scanning, and anatomical imaging modalities, such as CT and MRI, may be necessary to arrive at the correct diagnosis.

Brain perfusion SPECT in a cerebellar infarction.

Brain perfusion single photon emission CT (SPECT) has been used effectively in the evaluation of neurological diseases including stroke. Recently the introduction of annular special purpose systems has resulted in substantially improved spatial resolution. High resolution SPECT allows us to look accurately at the functional abnormalities resulting from small structural defects. We present a case of infarction in the territory of the posterior inferior cerebellar artery which illustrates the utility of this method.