Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess correction.

Ice-core measurements of carbon dioxide and the deuterium palaeothermometer reveal significant covariation of temperature and atmospheric CO2 concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO2 is an important forcing factor for climate. But this interpretation is challenged by some substantial mismatches of the CO2 and deuterium records, especially during the onset of the last glaciation, about 120 kyr ago. Here we incorporate measurements of deuterium excess from Vostok in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO2 and temperature, of r2 = 0.89 for the past 150 kyr and r2 = 0.84 for the period 350-150 kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO2 may be more responsive to temperature than previously thought.

On-line correction and visualization of motion during MRI-controlled hyperthermia.

Displacement of tissue during MRI-controlled hyperthermia therapy can cause significant problems. Errors in calculated temperature may result from motion-related image artifacts and inter-image object displacement, leading to incorrect spatial temperature reference. Here, cyclic navigator echoes were incorporated in rapid gradient-echo MRI sequences, used for temperature mapping based on the proton resonance frequency. On-line evaluation of navigator information was used in three ways. First, motion artifacts were minimized in echo-shifted (TE > TR) gradient-echo images using the phase information of the navigator echo. Second, navigator profiles were matched for a quantitative evaluation of displacement. Together with a novel processing method, this information was employed to correct the reference temperature maps, thereby avoiding persistence of motion-related temperature errors throughout the hyperthermic period. Third, on-line visualization of displacement, together with temperature maps and thermal dose images, was developed, allowing physician intervention at all times. Examples are given of on-line corrections during hyperthermia procedures with focused ultrasound and radiofrequency heat sources. Magn Reson Med 45:128-137, 2001.

Thermal therapies in interventional MR imaging. Focused ultrasound.

MR image-guided focused ultrasound (FUS) provides an entirely noninvasive approach for local thermal therapies. MR imaging allows target definition and continuous temperature mapping. Therefore, the heating procedure can be controlled spatially and temporally based on automatic feedback to the FUS apparatus. Phased-array ultrasound technology will further help the development. MR imaging/FUS may be applied not only for tissue ablation, but also for local drug delivery, gene therapy, and drug activation.

Local hyperthermia with MR-guided focused ultrasound: spiral trajectory of the focal point optimized for temperature uniformity in the target region.

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the region's boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T(2)-weighted images.

Hyperthermia by MR-guided focused ultrasound: accurate temperature control based on fast MRI and a physical model of local energy deposition and heat conduction.

Temperature regulation in MR-guided focused ultrasound requires rapid MR temperature mapping and automatic feedback control of the ultrasound output. Here, a regulation method is proposed based on a physical model of local energy deposition and heat conduction. The real-time evaluation of local temperature gradients from temperature maps is an essential element of the control system. Each time a new image is available, ultrasound power is adjusted on-the-fly in order to obtain the desired evolution of the focal point temperature. In vitro and in vivo performance indicated fast and accurate control of temperature and a large tolerance of errors in initial estimates of ultrasound absorption and heat conduction. When using correct estimates for the physical parameters of the model, focal point temperature was controlled within the measurement noise limit. Initial errors in absorption and diffusion parameters are compensated for exponentially with a user-defined response time, which is suggested to be on the order of 10 sec.

More than 200 meters of lake ice above subglacial Lake Vostok, Antarctica.

Isotope studies show that the Vostok ice core consists of ice refrozen from Lake Vostok water, from 3539 meters below the surface of the Antarctic ice sheet to its bottom at about 3750 meters. Additional evidence comes from the total gas content, crystal size, and electrical conductivity of the ice. The Vostok site is a likely place for water freezing at the lake-ice interface, because this interface occurs at a higher level here than anywhere else above the lake. Isotopic data suggest that subglacial Lake Vostok is an open system with an efficient circulation of water that was formed during periods that were slightly warmer than those of the past 420,000 years. Lake ice recovered by deep drilling is of interest for preliminary investigations of lake chemistry and bedrock properties and for the search for indigenous lake microorganisms. This latter aspect is of potential importance for the exploration of icy planets and moons.

Fast lipid-suppressed MR temperature mapping with echo-shifted gradient-echo imaging and spectral-spatial excitation.

The water proton resonance frequency (PRF) is temperature dependent and can thus be used for magnetic resonance (MR) thermometry. Since lipid proton resonance frequencies do not depend on temperature, fat suppression is essential for PRF-based temperature mapping. The efficacy of echo-shifted (TE > TR) gradient-echo imaging with spectral-spatial excitation is demonstrated, resulting in accurate and rapid, lipid-suppressed, MR thermometry. The method was validated on phantoms, fatty duck liver, and rat thigh, demonstrating improvements in both the speed and precision of temperature mapping. Heating of a rat thigh with focused ultrasound was monitored in vivo with an accuracy of 0.37 degree C and a time resolution of 438 msec.

Real-time control of focused ultrasound heating based on rapid MR thermometry.

RATIONALE AND OBJECTIVES: Real-time control of the heating procedure is essential for hyperthermia applications of focused ultrasound (FUS). The objective of this study is to demonstrate the feasibility of MRI-controlled FUS. METHODS: An automatic control system was developed using a dedicated interface between the MR system control computer and the FUS wave generator. Two algorithms were used to regulate FUS power to maintain the focal point temperature at a desired level. RESULTS: Automatic control of FUS power level was demonstrated ex vivo at three target temperature levels (increase of 5 degrees C, 10 degrees C, and 30 degrees C above room temperature) during 30-minute hyperthermic periods. Preliminary in vivo results on rat leg muscle confirm that necrosis estimate, calculated on-line during FUS sonication, allows prediction of tissue damage. CONCLUSIONS. The feasibility of fully automatic FUS control based on MRI thermometry has been demonstrated.