Evaluation of a novel therapeutic focused ultrasound transducer based on Fermat's spiral.

The purpose of this study was to evaluate a novel phased array transducer design rule for therapeutic focused ultrasound applications. This design rule uses the discretized Fermat's spiral to determine the positioning of the transducer elements for a given number of elements and f-number. Using this principle, three variations of Fermat's spiral were generated, aimed at (1) grating lobe minimization, (2) side lobe minimization, and (3) an optimized element packing efficiency. For each spiral, sparse layouts using identical circular elements and fully populated layouts based on additional Voronoi tessellation were evaluated numerically. Evaluation criteria included the element size distribution, beam steering capabilities, focal plane pressure distribution, prefocal pressure distribution, and practical considerations. Finally, one Voronoi-tessellated design with a focal length and aperture diameter of 16 cm and a natural frequency of 1.3 MHz was evaluated experimentally through hydrophone measurements. The numerical evaluation showed that while sparse arrays possess superior beam steering capabilities for a given number of elements, the focal point quality and prefocal pressure distribution is substantially more favorable when using the Voronoi-tessellated designs. Beam steering was shown to be feasible with the tessellated designs for lateral deflections up to 10 mm and axial deflections up to 20 mm. The experimental evaluation showed that such a transducer is capable of inducing 40.00 MPa rarefactional and 237.50 MPa compressional peak pressure levels at 800 W instantaneous acoustic output power under free-field conditions, making the system potentially relevant for thermal ablation therapy, histotripsy applications, and shockwave-enhanced heating.

Thermal ablation of a confluent lesion in the porcine kidney with a clinically available MR-HIFU system.

The incidence of small renal masses (SRMs) sized <4 cm has increased over the decades (as co-findings/or due to introduction of cross sectional imaging). Currently, partial nephrectomy (PN) or watchful waiting is advised in these patients. Ultimately, 80-90% of these SRMs require surgical treatment and PN is associated with a 15% complication rate. In this aging population, with possible comorbidities and poor health condition, both PN and watchful waiting are non-ideal treatment options. This resulted in an increased need for early, non-invasive treatment strategies such as MR-guided high intensity focused ultrasound (MR-HIFU). (i) To investigate the feasibility of creating a confluent lesion in the kidney using respiratory-gated MR-HIFU under clinical conditions in a pre-clinical study and (ii) to evaluate the reproducibility of the MR-HIFU ablation strategy. Healthy pigs (n = 10) under general anesthesia were positioned on a clinical MR-HIFU system with integrated cooling. A honeycomb pattern of seven overlapping ablation cells (4 × 4 × 10 mm3, 450 W, <30 s) was ablated successively in the cortex of the porcine kidney. Both MR thermometry and acoustic energy delivery were respiratory gated using a pencil beam navigator on the contralateral kidney. The non-perfused volume (NPV) was visualized after the last sonication by contrast-enhanced (CE) T 1-weighted MR (T 1 w) imaging. Cell viability staining was performed to visualize the extent of necrosis. RESULTS: a median NPV of 0.62 ml was observed on CE-T 1 w images (IQR 0.58-1.57 ml, range 0.33-2.75 ml). Cell viability staining showed a median damaged volume of 0.59 ml (IQR 0.24-1.35 ml, range 0-4.1 ml). Overlooking of the false rib, shivering of the pig, and too large depth combined with a large heat-sink effect resulted in insufficient heating in 4 cases. The NPV and necrosed volume were confluent in all cases in which an ablated volume could be observed. Our results demonstrated the feasibility of creating a confluent volume of ablated kidney cortical tissue in vivo with MR-HIFU on a clinically available system using respiratory gating and near-field cooling and showed its reproducibility.

Increasing the HIFU ablation rate through an MRI-guided sonication strategy using shock waves: feasibility in the in vivo porcine liver.

This study investigated whether an MR-guided pulsed HIFU ablation strategy could be implemented under clinical conditions, using a transducer designed for uterine fibroid ablation, to obtain an ablation rate that is sufficiently high for clinical abdominal HIFU therapy in highly perfused organs. A pulsed HIFU ablation strategy, aimed at increasing the energy absorption in the HIFU focal area by local shock wave formation in the non-linear pressure regime, was compared to an energy-equivalent continuous wave sonication strategy in the linear pressure regime. Both ablation strategies were used for transcutaneous sonication of pre-defined treatment cells in the livers of 5 pigs in vivo. Temperature evolution in both the target area as well as the pre-focal muscle layer was monitored simultaneously using MR thermometry. Local energy absorption and thermal dose volumes were shown to be increased using the pulsed ablation strategy, while preserving healthy tissue in the near field of the acoustic beam. Respiratory motion compensation of both acoustic energy delivery and MR thermometry was applied through gating based on MR navigator echoes. Histopathology showed that confluent vacuolated thermal lesions were created when the pulsed ablation strategy was used. Additionally, it was shown that the heat sink effect caused by the presence of larger vessels could be overcome. The pulsed HIFU ablation strategy achieved an ablation rate of approximately 4 ml per hour in the in vivo porcine liver, without causing undesired damage to healthy tissues in the near field.

Correction for photobleaching in dynamic fluorescence microscopy: application in the assessment of pharmacokinetic parameters in ultrasound-mediated drug delivery.

We have previously demonstrated the feasibility of monitoring ultrasound-mediated uptake of a hydrophilic model drug in real time with dynamic confocal fluorescence microscopy. In this study, we evaluate and correct the impact of photobleaching to improve the accuracy of pharmacokinetic parameter estimates. To model photobleaching of the fluorescent model drug SYTOX Green, a photobleaching process was added to the current two-compartment model describing cell uptake. After collection of the uptake profile, a second acquisition was performed when SYTOX Green was equilibrated, to evaluate the photobleaching rate experimentally. Photobleaching rates up to 5.0 10(-3) s(-1) were measured when applying power densities up to 0.2 W.cm(-2). By applying the three-compartment model, the model drug uptake rate of 6.0 10(-3) s(-1) was measured independent of the applied laser power. The impact of photobleaching on uptake rate estimates measured by dynamic fluorescence microscopy was evaluated. Subsequent compensation improved the accuracy of pharmacokinetic parameter estimates in the cell population subjected to sonopermeabilization.

Intercostal high intensity focused ultrasound for liver ablation: The influence of beam shaping on sonication efficacy and near-field risks.

PURPOSE: One of the major issues in high intensity focused ultrasound ablation of abdominal lesions is obstruction of the ultrasound beam by the thoracic cage. Beam shaping strategies have been shown by several authors to increase focal point intensity while limiting rib exposure. However, as rib obstruction leaves only part of the aperture available for energy transmission, conserving total emitted acoustic power, the intensity in the near-field tissues inherently increases after beam shaping. Despite of effective rib sparing, those tissues are therefore subjected to increased risk of thermal damage. In this study, for a number of clinically representative intercostal sonication geometries, modeling clinically available hardware, the effect of beam shaping on both the exposure of the ribs and near-field to acoustic energy was evaluated and the implications for the volumetric ablation rate were addressed. METHODS: A relationship between rib temperature rise and acoustic energy density was established by means of in vivo MR thermometry and simulations of the incident acoustic energy for the corresponding anatomies. This relationship was used for interpretation of rib exposure in subsequent numerical simulations in which rib spacing, focal point placement, and the focal point trajectory were varied. The time required to heat a targeted region to 65 °C was determined without and with the application of beam shaping. The required sonication time was used to calculate the acoustic energy density at the fat-muscle interface and at the surface of the ribs. At the fat-muscle interface, exposure was compared to available literature data and rib exposure was interpreted based on the earlier obtained relation between measured temperature rise and simulated acoustic energy density. To estimate the volumetric ablation rate, the cool-down time between periods of energy exposure was estimated using a time-averaged power limit of 100 kJ/h. RESULTS: At the level of the ribs, the temperature rise-energy density proportionality constant was estimated to be 6.0-7.6 °C/(J/mm(2)). Beam shaping by the geometric shadow method typically reduces the acoustic intensity a factor of 2, considering the 1 cm(2) with the highest exposure. For a 4 mm diameter circular sonication trajectory, the near-field energy limit of 2.5 J/mm(2) was exceeded for all considered geometries. The estimated rib temperature was in all but one (sonication 50 mm behind the ribs, with 15 mm rib spacing and a 4 mm diameter circular sonication trajectory) of the considered scenarios within acceptable limits. For those sonication scenarios where a single sonication is considered safe both in terms of near-field as well as rib heating, volumetric ablation rates in the order of 1 ml/h are estimated. CONCLUSIONS: Intercostal sonication is associated with an increased risk of near-field overheating. This risk is strongly dependent on the considered rib spacing, the placement of the focus behind the ribs, and the selected sonication trajectory. For the hardware under simulation, obstruction by the thoracic cage renders ablations of clinically relevant volumes within a practical time-frame unfeasible in a large part of the liver. Improvements maybe expected from transducer designs with a larger active surface and/or nonlinear sonication strategies.

Relationship between socioeconomic status and quality of life in older adults: a path analysis.

PURPOSE: The main objective of this study was to determine the relationship between quality of life, social functioning, depressive symptoms, self-efficacy, physical function, and socioeconomic status (SES) in community-dwelling older adults. METHODS: A cross-sectional design was used to examine the relationships. A sample of 193 community-dwelling older adults completed the measurements. Structural equation modeling with full information maximum likelihood in LISREL was used to evaluate the relationships between the latent variables (SES, social functioning, depressive symptoms, self-efficacy, physical function, and quality of life). RESULTS: The path analysis exhibited significant effects of SES on physical function, social functioning, depressive symptoms, and self-efficacy (γ = 0.42-0.73), and significant effects in regard to social functioning, depressive symptoms, and self-efficacy on quality of life (γ = 0.27-0.61). There was no direct effect of SES on the quality of life. The model fit indices demonstrated a reasonable fit (χ (2) = 98.3, df = 48, p < 0.001), matching the relative Chi-square criterion and the RMSEA criterion. The model explained 55.5 % of the variance of quality of life. CONCLUSIONS: The path analysis indicated an indirect effect of SES on the quality of life by social functioning, depressive symptoms, and self-efficacy in community-dwelling older adults. Physical function did not have a direct effect on the quality of life. To improve the quality of life in older adults, additional focus is required on the socioeconomic psychosocial differences in the community-dwelling older population.

Longitudinal measurement of physical activity following kidney transplantation.

The purpose of this longitudinal observational study was to (i) examine the change of daily physical activity in 28 adult kidney transplant recipients over the first 12 months following transplantation; and (ii) to examine the change in metabolic characteristics and renal function. Accelerometer-based daily physical activity and metabolic- and clinical characteristics were measured at six wk (T1), three months (T2), six months (T3) and 12 months (T4) following transplantation. Linear mixed effect analyses showed an increase in steps/d (T1 = 6326 ± 2906; T4 = 7562 ± 3785; F = 3.52; p = 0.02), but one yr after transplantation only 25% achieved the recommended 10 000 steps/d. There was no significant increase in minutes per day spent on moderate-to-vigorous intensity physical activity (T1 = 80.4 ± 63.6; T4 = 93.2 ± 55.1; F = 1.71; p = 0.17). Body mass index increased over time (T1 = 25.4 ± 3.2; T4 = 27.2 ± 3.8; F = 12.62; p < 0.001), mainly due to an increase in fat percentage (T1 = 30.3 ± 8.0; T4 = 34.0 ± 7.9; F = 14.63; p < 0.001). There was no significant change in renal function (F = 0.17; p = 0.92). Although the recipients increased physical activity, the majority did not meet the recommended levels of physical activity after one yr. In addition to the weight gain, this may result in negative health consequences. Therefore, it is important to develop strategies to support kidney transplant recipients to comply with healthy lifestyle recommendations, including regular physical activity.

Quantifying daily physical activity and determinants in sedentary patients with Parkinson's disease.

BACKGROUND: Although physical activity is beneficial for Parkinson's disease (PD) patients, many do not meet the recommended levels. The range of physical activity among sedentary PD patients is unknown, as are factors that determine this variability. Hence, we aimed to (1) assess daily physical activity in self-identified sedentary PD patients; (2) compare this with criteria of a daily physical activity guideline; and (3) identify determinants of daily physical activity. METHODS: Daily physical activity of 586 self-identified sedentary PD patients was measured with a tri-axial accelerometer for seven consecutive days. Physical fitness and demographic, disease-specific, and psychological characteristics were assessed. Daily physical activity was compared with the 30-min activity guideline. A linear mixed-effects model was estimated to identify determinants of daily physical activity. RESULTS: Accelerometer data of 467 patients who fulfilled all criteria revealed that >98% of their day was spent on sedentary to light-intensity activities. Eighty-two percent of the participants were 'physically inactive' (0 days/week of 30-min activity); 17% were 'semi-active' (1-4 days/week of 30-min activity). Age, gender, physical fitness, and scores on the Unified Parkinson's Disease Rating Scale explained 69% of the variability in daily physical activity. CONCLUSIONS: Performance-based measurements confirmed that most self-identified sedentary PD patients are 'physically inactive'. However, the variance in daily physical activity across subjects was considerable. Higher age, being female, and lower physical capacity were the most important determinants of reduced daily physical activity. Future therapeutic interventions should aim to improve daily physical activity in these high-risk patients, focusing specifically on modifiable risk factors.

Exercise and inflammation in pediatric Crohn's disease.

We examined inflammatory cells, cytokines and growth factors in response to acute bouts of moderate intensity continuous exercise and high intensity intermittent exercise in youth with Crohn's disease and in healthy matched-controls. 15 patients and 15 controls performed 30 min of cycling at 50% of peak mechanical power (PMP) and 6 bouts of 4×15-s of cycling at 100% PMP. Blood was collected at rest, at the mid-point, at the end of exercise and at 30 and 60 min into recovery. In patients with CD, both types of exercise increased immune cells and GH and decreased IGF-I. Moderate intensity exercise induced a greater increase in leukocytes (p<0.05), neutrophils (p<0.05), lymphocytes (p<0.001), monocytes (p<0.05), IL-6 (p<0.05), IL-17 (p<0.05) and GH (p<0.05) and a similar decrease in IGF-I, compared with high intensity exercise. TNF-α did not change significantly with either exercise. Responses in patients were similar compared with controls; however, in patients monocytes remained elevated significantly longer in response to MICE. Youth with Crohn's disease can engage in distinctly different types of exercise without a significant acute exacerbation of inflammation.

Uncertainty in hyperthermia treatment planning: the need for robust system design.

Hyperthermia treatment planning (HTP) is an important tool to improve the quality of hyperthermia treatment. It is a practical way of designing new hyperthermia systems and can be used to optimize the phase and amplitude settings to achieve optimal heating. One of the main challenges to be dealt with however is the uncertainty in the modeling parameters. The role of dielectric and combined dielectric and perfusion uncertainty on optimization was investigated by means of HTP for six different systems: the 70 MHz AMC-4 (AMC: Academic Medical Center) and AMC-8 system, a 130 MHz version of the AMC-8 system, a three-ring AMC-12 system operating at 130 MHz, the BSD SigmaEye applicator and a dipole applicator with three rings each containing six dipole pairs operated at 150 MHz. For five patients with cervix uteri carcinoma, a patient model was created based on a hyperthermia planning CT. Variation of tissue parameters resulted in 16 dielectric models for every patient. In addition, four thermal models were created to study the combined effect of perfusion and dielectric uncertainty. The impact of dielectric uncertainty on optimization is found to be clearly dependent on the number of channels and increased from 0.5 °C for four channels to 1.5 °C for the 18-channel system. As a result, the potential gain relative to the AMC-4 system for the 70 MHz AMC-8 system was found to be largely compromised, while for the remaining systems a robust improvement in T(90) was observed. The dipole applicator showed the best target heating for two out of five patients, while for three others heating efficacy was comparable to the 130 MHz AMC-12 system or the 130 MHz AMC-8 system (one patient). Considering the increase in complexity when the number of channels is increased from 12 to 18, the AMC-12 system is considered as a good compromise between heating efficacy and robustness while still being a manageable heating system in clinical practice.

Improved power steering with double and triple ring waveguide systems: the impact of the operating frequency.

INTRODUCTION: Regional hyperthermia systems with 3D power steering have been introduced to improve tumour temperatures. The 3D 70-MHz AMC-8 system has two rings of four waveguides. The aim of this study is to evaluate whether T(90) will improve by using a higher operating frequency and whether further improvement is possible by adding a third ring. METHODS: Optimised specific absorption rate (SAR) distributions were evaluated for a centrally located target in tissue-equivalent phantoms, and temperature optimisation was performed for five cervical carcinoma patients with constraints to normal tissue temperatures. The resulting T(90) and the thermal iso-effect dose (i.e. the number of equivalent min at 43°C) were evaluated and compared to the 2D 70-MHz AMC-4 system with a single ring of four waveguides. FDTD simulations were performed at 2.5 × 2.5 × 5 mm(3) resolution. The applied frequencies were 70, 100, 120, 130, 140 and 150 MHz. RESULTS: Optimised SAR distributions in phantoms showed an optimal SAR distribution at 140 MHz. For the patient simulations, an optimal increase in T(90) was observed at 130 MHz. For a two-ring system at 70 MHz the gain in T(90) was about 0.5°C compared to the AMC-4 system, averaged over the five patients. At 130 MHz the average gain in T(90) was ~1.5°C and ~2°C for a two and three-ring system, respectively. This implies an improvement of the thermal iso-effect dose with a factor ~12 and ~30, respectively. CONCLUSION: Simulations showed that a 130-MHz two-ring waveguide system yields significantly higher tumour temperatures compared to 70-MHz single-ring and double-ring waveguide systems. Temperatures were further improved with a 130-MHz triple-ring system.

3D versus 2D steering in patient anatomies: a comparison using hyperthermia treatment planning.

PURPOSE: In this study hyperthermia treatment planning is used to investigate whether the target temperature during hyperthermia treatment can be increased using the 3D AMC-8 instead of the 2D AMC-4 system (AMC: Academic Medical Center). METHODS AND MATERIALS: The heating ability of the AMC-4 and AMC-8 system was analysed for five patients with cervix uteri carcinoma. Dielectric and thermal models were generated, based on a hyperthermia planning computerised tomography (CT), at a resolution of 2.5 × 2.5 × 5.0 mm(3). Calculation of the electric fields with the finite-difference time-domain method was followed by SAR- and temperature-based optimisation. The ability to correct for axial shifts of the patient by phase/amplitude steering was investigated for both systems. Finally, it was investigated whether adjusting the ring-to-ring distance of the AMC-8 system can be used for further optimisation. RESULTS: An average increase in T(90) of ∼0.5°C (0.2°-0.8°C) was found for the AMC-8 system compared to the AMC-4 system. The gain in T(50) and T(10) was also 0.5°C on average. The additional power required to achieve this gain was 36% to 71% of the power required for the AMC-4 system. The AMC-8 system has the capability of correcting changes in axial position (-8 cm, +8 cm), contrary to the AMC-4 system. For both systems the axial position should be known within 1-2 cm. CONCLUSIONS: Hyperthermia treatment with the AMC-8 system can lead to a clinically relevant increase of the target temperature compared to treatment with the AMC-4 system. The AMC-8 system provides large freedom in the axial positioning of the patient.

Optimization in hyperthermia treatment planning: the impact of tissue perfusion uncertainty.

PURPOSE: Hyperthermia treatment planning (HTP) potentially provides a valuable tool for monitoring and optimization of treatment. However, one of the major problems in HTP is that different sources of uncertainty degrade its reliability. Perfusion uncertainty is one of the largest uncertainties and hence there is an ongoing debate whether optimization should be limited to power-based strategies. In this study a systematic analysis is carried out addressing this question. METHODS: The influence of perfusion uncertainty on optimization was analyzed for five patients with cervix uteri carcinoma heated with the AMC-8 70 MHz phased-array waveguide system. The effect of variations (up to +/- 50%) in both the muscle and tumor perfusion level was investigated. For every patient, reference solutions were calculated using constrained temperature-based optimization for 25 different and known perfusion distributions. Reference solutions were compared to those found by temperature-based optimization using standard perfusion values and four SAR-based optimization methods. The effect of heterogeneity was investigated by creating 5 x 100 perfusion distributions for different levels of local variation (+/- 25% and +/- 50%) and scale (1 and 2 cm). Here the performance of the temperature-based optimization method was compared to a SAR-based method that showed good performance in the previous analysis. RESULTS: Solutions found with temperature-based optimization using a deviating perfusion distribution during optimization were found within 1.0 degrees C from the true optimum. For the SAR-based methods, deviations up to 2.9 degrees C were found. The spread found in these deviations was comparable, typically 0.5-1.0 degrees C. When applying intramuscle variation to the perfusion, temperature-based optimization proved to be the best strategy in 95% of the evaluated cases applying +/- 50% local variation. CONCLUSIONS: Temperature-based optimization proves to be superior to SAR-based optimization both under variation of perfusion level as well as under the application of intratissue variation. The spread in achieved temperatures is comparable. These results are valid under the assumption of constant perfusion at hyperthermic levels. Although similar results are expected from models including thermoregulation, additional analysis is required to confirm this. In view of uncertainty in tissue perfusion and other modeling uncertainties, the authors propose feedback guided temperature-based optimization as the best candidate to improve thermal dose delivery during hyperthermia treatment.

The impact of the waveguide aperture size of the 3D 70 MHz AMC-8 locoregional hyperthermia system on tumour coverage.

The 70 MHz AMC-4 system, with one ring of four waveguides, provides 2D power steering. The newly developed AMC-8 system enables 3D steering, using two rings of four 70 MHz waveguides. The current waveguide aperture size is 20.2 x 34.3 cm(2). Waveguides and water boluses cover a large area of the body, which is not ideal for short patients. The aim of this study is investigating the impact of smaller waveguides on tumour coverage, using treatment planning. Finite-difference time-domain simulations were performed at 2.5 x 2.5 x 5 mm(3) resolution. Virtual AMC-8 systems with waveguide aperture sizes of 20.5 x 34.25, 17.5 x 34.25, 14.5 x 34.25, 11.5 x 34.25, 8.5 x 34.25 cm(2) and the AMC-4 system were modelled. Simulations were performed for elliptical (36 x 24 x 100 cm(3)) tissue-equivalent phantoms and for five cervical cancer patients. For the phantoms S(ratio) (SAR(max_border)/SAR(target)) was evaluated for standard and optimized settings. For the patients, temperature distributions were evaluated after optimization of tumour temperature, while limiting normal tissue temperatures to 45 degrees C. Phantom simulations showed a favourable S(ratio) for all two-ring systems, compared to the AMC-4 system, for optimized phase-amplitude settings. Patient simulations demonstrated that the improvement in T(90) for the operational AMC-8 system was approximately 0.5 degrees C. This improvement was independent of the aperture size. The average number of imminent hot spots and their total volume was almost comparable for 8.5 and 20.5 cm wide apertures, but the locations were different. Two-ring waveguide systems with eight antennas and aperture sizes in the range from 20.5 x 34.25 cm(2) to 8.5 x 34.25 cm(2) showed a stable gain in tumour temperature compared to a single-ring four-antenna system with 20.5 x 34.25 cm(2) wide apertures.

Improving locoregional hyperthermia delivery using the 3-D controlled AMC-8 phased array hyperthermia system: a preclinical study.

BACKGROUND: The aim of this study is preclinical evaluation of our newly developed regional hyperthermia system providing 3-D SAR control: the AMC-8 phased array consisting of two rings, each with four 70 MHz waveguides. It was designed to achieve higher tumour temperatures and improve the clinical effectiveness of locoregional hyperthermia. METHODS: The performance of the AMC-8 system was evaluated with simulations and measurements aiming at heating a centrally located target region in rectangular (30 x 30 x 110 cm) and elliptical (36 x 24 x 80 cm) homogeneous tissue equivalent phantoms. Three properties were evaluated and compared to its predecessor, the 2-D AMC-4 single ring four waveguide array: (1) spatial control and (2) size of the SAR focus, (3) the ratio between maximum SAR outside the target region and SAR in the focus. Distance and phase difference between the two rings were varied. RESULTS: (1) Phase steering provides 3-D SAR control for the AMC-8 system. (2) The SAR focus is more elongated compared to the AMC-4 system, yielding a lower SAR level in the focus when using the same total power. This is counter-balanced by (3) a superficial SAR deposition which is half of that in the AMC-4 system, yielding a more favourable ratio between normal tissue and target SAR and allowing higher total power and up to 30% more SAR in the focus for 3 cm ring distance. CONCLUSION: The AMC-8 system is capable of 3-D SAR control and its SAR distribution is more favourable than for the 2-D AMC-4 system. This result promises improvement in clinical tumour temperatures.

Accelerated ray tracing for radiotherapy dose calculations on a GPU.

PURPOSE: The graphical processing unit (GPU) on modern graphics cards offers the possibility of accelerating arithmetically intensive tasks. By splitting the work into a large number of independent jobs, order-of-magnitude speedups are reported. In this article, the possible speedup of PLATO's ray tracing algorithm for dose calculations using a GPU is investigated. METHODS: A GPU version of the ray tracing algorithm was implemented using NVIDIA's CUDA, which extends the standard C language with functionality to program graphics cards. The developed algorithm was compared based on the accuracy and speed to a multithreaded version of the PLATO ray tracing algorithm. This comparison was performed for three test geometries, a phantom and two radiotherapy planning CT datasets (a pelvic and a head-and-neck case). For each geometry, four different source positions were evaluated. In addition to this, for the head-and-neck case also a vertex field was evaluated. RESULTS: The GPU algorithm was proven to be more accurate than the PLATO algorithm by elimination of the look-up table for z indices that introduces discretization errors in the reference algorithm. Speedups for ray tracing were found to be in the range of 2.1-10.1, relative to the multithreaded PLATO algorithm running four threads. For dose calculations the speedup measured was in the range of 1.5-6.2. For the speedup of both the ray tracing and the dose calculation, a strong dependency on the tested geometry was found. This dependency is related to the fraction of air within the patient's bounding box resulting in idle threads. CONCLUSIONS: With the use of a GPU, ray tracing for dose calculations can be performed accurately in considerably less time. Ray tracing was accelerated, on average, with a factor of 6 for the evaluated cases. Dose calculation for a single beam can typically be carried out in 0.6-0.9 s for clinically realistic datasets. These findings can be used in conventional planning to enable (nearly) real-time dose calculations. Also the importance for treatment optimization techniques is evident.

Acceleration of high resolution temperature based optimization for hyperthermia treatment planning using element grouping.

In regional hyperthermia, optimization is useful to obtain adequate applicator settings. A speed-up of the previously published method for high resolution temperature based optimization is proposed. Element grouping as described in literature uses selected voxel sets instead of single voxels to reduce computation time. Elements which achieve their maximum heating potential for approximately the same phase/amplitude setting are grouped. To form groups, eigenvalues and eigenvectors of precomputed temperature matrices are used. At high resolution temperature matrices are unknown and temperatures are estimated using low resolution (1 cm) computations and the high resolution (2 mm) temperature distribution computed for low resolution optimized settings using zooming. This technique can be applied to estimate an upper bound for high resolution eigenvalues. The heating potential of elements was estimated using these upper bounds. Correlations between elements were estimated with low resolution eigenvalues and eigenvectors, since high resolution eigenvectors remain unknown. Four different grouping criteria were applied. Constraints were set to the average group temperatures. Element grouping was applied for five patients and optimal settings for the AMC-8 system were determined. Without element grouping the average computation times for five and ten runs were 7.1 and 14.4 h, respectively. Strict grouping criteria were necessary to prevent an unacceptable exceeding of the normal tissue constraints (up to approximately 2 degrees C), caused by constraining average instead of maximum temperatures. When strict criteria were applied, speed-up factors of 1.8-2.1 and 2.6-3.5 were achieved for five and ten runs, respectively, depending on the grouping criteria. When many runs are performed, the speed-up factor will converge to 4.3-8.5, which is the average reduction factor of the constraints and depends on the grouping criteria. Tumor temperatures were comparable. Maximum exceeding of the constraint in a hot spot was 0.24-0.34 degree C; average maximum exceeding over all five patients was 0.09-0.21 degree C, which is acceptable. High resolution temperature based optimization using element grouping can achieve a speed-up factor of 4-8, without large deviations from the conventional method.

Respiratory muscle activity and dyspnea during exercise in chronic obstructive pulmonary disease.

We aimed to determine by non-invasive EMG, whether during exercise: (1) COPD patients increase scalene and intercostal EMG activity, (2) increased EMG activity is associated with increased dyspnea, and (3) the ratio between EMG activity and volume displacement is increased in COPD compared to healthy subjects (HS). During a maximal incremental cycle test, scalene and intercostal EMG was derived transcutaneously in 17 COPD patients and 10 HS. Dyspnea was quantified using a Borg scale, ranging from zero to 10 (maximal dyspnea). For analyses the ratio between inspiratory muscle activity during exercise and activity during quiet breathing was used (logEMGAR). In COPD patients, scalene and intercostal activity increased at greater rate early in exercise compared to that of the HS. With a doubling of the logEMGAR, in COPD, dyspnea increased with 2.8/3.8 points, while in the HS, dyspnea increased less with 1.1/1.4 points. In COPD, there was a larger increase in EMG activity relatively to tidal volume increases.

[Pedometers for monitoring and improvement of the level of physical activity]. / Stappentellers voor het monitoren en bevorderen van het lichamelijke activiteitenniveau.

Pedometers are cheap and easy to handle devices that have proven their accuracy to measure the number of steps taken during a time period (day or week). However, at lower walking speeds and if used by obese persons the accuracy of several pedometers decreases significantly. Pedometers are not able to record the intensity of daily physical activity. In addition, the number of steps depends on age, gender, body weight and season. Nevertheless, monitoring of daily physical activity with a pedometer may be an effective method to enhance the daily physical activity of selected subgroups. For instance, subjects with an increased risk of obesity or hypertension, sedentary patients with diabetes mellitus type 2, patients with coronary heart disease and cancer patients can more easily be stimulated to become physically active and meet healthy living standards. Pedometers combined with existing effective counseling methods may be an effective strategy for many health care providers for adoption and enhancement of healthy lifestyles by sedentary patients.

Low bone mineral density in COPD patients related to worse lung function, low weight and decreased fat-free mass.

UNLABELLED: Low bone mineral density is frequently seen in COPD patients. Advanced COPD, low BMI and muscle depletion are risk factors for developing low bone mineral density (BMD). Low bone mineral density is seen in 75% of the GOLD stage IV patients. INTRODUCTION: We set out to investigate the prevalence of low bone mineral density (BMD) in chronic obstructive pulmonary disease (COPD) as well as the predictors of abnormal bone mineral density. METHODS: A cross-sectional design was used to evaluate 115 subjects with COPD (GOLD stages II-IV). Bone mineral density (BMD) was measured using an ultrasound densitometer. The forced expiratory volume in 1 s (FEV(1)) was assessed and fat-free mass was measured using bioelectrical impedance analysis. Chi-square tests and logistic regression were used for analysis. RESULTS: The prevalence of a T-score < -1.0 SD and > -2.5 SD was 28.6% in GOLD stage II, 40.3% in GOLD stage III and 57.1% in GOLD stage IV. The prevalence of a T-score