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Abstract. Automatic, non-rigid registration of blood vessels (and other tubular structures) within a timescale suitable for use in image-guided surgical applications remains a significant challenge. We describe a novel approach to this problem in which an extension to the coherent point drift (CPD) algorithm is developed to enable landmarks, such as vessel bifurcations, to improve the registration accuracy and speed of execution. The new method -referred to as landmark-guided CPD (LGCPD) -is validated using vessels extracted from brain MRA and liver MR images, and is shown to be robust to missing vessel segments and noise, commonly encountered in realworld applications.

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Abstract. Purpose: To quantify the effects of respiratory motion on high-intensity focused ultrasound heating of liver tissue by comparing the simulated ablation using a conventional respiratory gating versus a MR-model-based motion compensation approach.Methods: To measure liver motion, dynamic free-breathing abdominal MR scans were acquired for five volunteers. Deformable registration was used to calculate continuous motion models, and tissue heating at a moving single focus was computed in 3-D by solving the bioheat equation. Ablated volume ratios with respect to the static case, V ab , were determined for a range of exposure times t exp and heating rates r. Results: To achieve V ab > 90% required t exp < 0.5s and r > 120 • C/s when gating, whereas t exp < 1s and r > 60 • C/s for motion-compensation. Conclusions: Accurate compensation for respiratory motion is important for efficient tissue ablation. Model-based motion compensation allows substantially lower heating rates than gating, reducing the risk of skin burns and focal boiling.

We have discovered a Hz quasi-periodic oscillation (QPO) in the X-ray flux of the low-luminosity 7.06 ‫ע‬ 0.08 low-mass X-ray binary (LMXB) and atoll source 4U 1820Ϫ30. This QPO was only observable at the highest observed mass accretion rate, when the source was in the uppermost part of the banana branch, at a 2-25 keV luminosity of ergs s (for a distance of 6.4 kpc). The QPO had an FWHM of only Hz 37 Ϫ15.4 # 10 0.5 ‫ע‬ 0.2 during small time intervals (32 s of data) and showed erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms amplitude over the energy range 2-60 keV was . The amplitude increased with 5.6% ‫ע‬ 0.2% photon energy from between 2.8 and 5.3 keV to between 6.8 and 9.3 keV, above 3.7% ‫ע‬ 0.5% 7.3% ‫ע‬ 0.6% which it remained approximately constant at ∼7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was consistent with being zero (Ϫ ms). The properties of the QPO (i.e., its frequency and its presence 1.2 ‫ע‬ 3.4 only at the highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO observed in the highest luminosity LMXBs (the Z sources) when they are accreting near the Eddington mass accretion limit. If this is indeed the same phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which require the near-Eddington accretion rates, will not hold. Assuming isotropic emission, the 2-25 keV luminosity of 4U 1820Ϫ30 at the time of the 7 Hz QPOs is at maximum at only 40% (for a companion star with cosmic abundances), but most likely at ∼20% (for a helium companion star), of the Eddington accretion limit.

The Generalized Interacting Stellar Winds model has been very successful in explaining observed cylindrical and bipolar shapes of planetary nebulae. However, many nebulae have a multipolar or point-symmetric shape. Previous two-dimensional calculations showed that these seemingly enigmatic forms can be reproduced by a two-wind model in which the confining disk is warped, as is expected to occur in irradiated disks. In this paper we present the extension to fully three-dimensional Adaptive Mesh Refinement simulations using the publicly available hydrodynamics package Flash. We briefly describe the mechanism leading to a radiation driven warped disk, and give an equation for its shape. We derive time scales related to the disk and compare them to the radiative cooling time scale of the gas, thereby determining the relevant part of parameter space. By comparing two-dimensional calculations including realistic radiative cooling through a cooling curve, with ones employing a low value for the adiabatic index γ, we show that the latter, computationally less expensive approach, is a valid approximation for treating cooling in our nebulae. The results of the fully three-dimensional wind-disk simulations show our mechanism to be capable of producing a plethora of multipolar (and quadrupolar) morphologies, which can explain the observed shape of a number of (proto-)planetary nebulae.