A framework for alignment of on-board imagers of medical linear accelerators.

PURPOSE: To develop an assumption-free methodology for aligning the geometry of on-board imagers with the geometry of medical linear accelerators applied in image-guided radiotherapy (IGRT). MATERIAL: Alignment of the on-board imaging (OBI) system with respect to the accelerator system is achieved using a multi-modular phantom described elsewhere (Tabor et al., 2017), enabling the geometry of the linear accelerator to be specified without any pre-assumptions. METHODS: The placement of two isocentres (of the on-board imager and of the therapeutic system) and of three frames of reference (those of the on-board imager, of the therapeutic system, and of the treatment table) is formulated as an optimization problem. It is solved by analysing the images of fiducial points positioned in 3D space in phantom modules attached to the treatment table and to the collimator of the accelerator. Fiducials are projected onto an imaging plane of unknown characteristics from a virtual source of unknown coordinates. CONCLUSIONS: An analytical framework exploiting projection images of the proposed multi-modular phantom has been developed, enabling precise alignment of the reference frame related to the on-board imager with the reference frame related to the therapeutic system. Within the proposed framework, the necessary corrections of treatment table positioning prior to patient irradiation, are delivered in the treatment table coordinates.

Microvolt T-wave alternans predicts cardiac events after acute myocardial infarction in patients treated with primary percutaneous coronary intervention.

BACKGROUND: Current risk stratification after acute myocardial infarction (MI) depends on left ventricular ejection fraction. Microvolt T-wave alternans (MTWA) is one of promising markers to predict cardiac events in patients after acute MI treated according to current guidelines. METHODS: In this single center study, 112 consecutive patients with the first anterior ST-elevation MI undergoing PCI <12 hours from symptom onset, were enrolled prospectively. Demographics, established risk factors, myocardial contrast echocardiography (MCE) perfusion, index event data and MTWA were assessed. Composite cardiac events (CCE) defined as: death, recurrent MI, sustained ventricular tachycardia (sVT) or readmission for acute heart failure (HF) were recorded during follow-up. RESULTS: MTWA test was negative in 76, positive in 18 and undetermined in 7 patients. MTWA negative patients had significantly higher LVEF at 30 days. At 4 years, 26 patients experienced CCE (10 died, 2 reinfarcted and 14 HF events). In multivariate Cox proportional hazard model maximum CKMB, non-negative MTWA and reduced LVEF made the best model to predict CCE. Four year CCE free survival was 77% and was significantly lower for non-negative MTWA (94% vs 50%, p<0.003). CONCLUSIONS: Non-negative MTWA with infarct size index and reduced LVEF could predict cardiac events in patients with anterior STEMI treated with primary PCI. MTWA non-negative patients have significantly worse outcome.

From simple to complex networks: inherent structures, barriers, and valleys in the context of spin glasses.

Given discrete degrees of freedom (spins) on a graph interacting via an energy function, what can be said about the energy local minima and associated inherent structures? Using the lid algorithm in the context of a spin glass energy function, we investigate the properties of the energy landscape for a variety of graph topologies. First, we find that the multiplicity N(s) of the inherent structures generically has a log-normal distribution. In addition, the large volume limit of ln / differs from unity, except for the Sherrington-Kirkpatrick model. Second, we find simple scaling laws for the growth of the height of the energy barrier between the two degenerate ground states and the size of the associated valleys. For finite connectivity models, changing the topology of the underlying graph does not modify qualitatively the energy landscape, but at the quantitative level the models can differ substantially.

Stochastic simulations of remodeling applied to a two-dimensional trabecular bone structure.

In this investigation the stochastic dynamics of trabecular bone remodeling (which is a key concept in a range of remodeling models) is verified. For this purpose, two-dimensional binary images of horizontal histological sections of lumbar vertebral bone (L-3) were collected from young and old subjects. Then the stochastic algorithm of remodeling was applied to the images of young trabecular bone. The resulting images of "simulated" old structures and those of "real" old structures were analyzed to evaluate the following architectural parameters: bone volume/trabecular volume; mean area of marrow cavities; mean trabeculae length; marrow space star volume; Euler number density; trabecular thickness; trabecular number; trabecular separation; mean two-point distance along the skeleton of trabecular structure; probability of disconnection; the ratio of marrow space star volume to geometrical volume and dimensionless ratio of mean area of marrow cavities; and mean trabecular length squared (form factor). Using the parametric t-test, the groups of simulated and real old bone images were compared. It is found that the p value of the t-test is never less than 0.20. For eight parameters the p value is >0.45. It is concluded that, as long as the horizontal sections of lumbar vertebrae are considered (sections perpendicular to the direction of the main spine load), the stochastic algorithm of bone remodeling will properly reproduce the architectural properties of trabecular bone.

Optimizing the assessment of age-related changes in trabecular bone.

The goal of this study was to develop an optimal procedure to determine age-related changes in trabecular bone. The investigations were based on two-dimensional images of the human vertebral trabecular bone specimens. The following indices of trabecular structure were considered: bone volume/total volume, star volume of the marrow cavity, Euler number and the probability of disconnection (straightforwardly connected with the number of separated parts of the network). To follow precisely the changes in the trabecular structure with age, a computer simulation model was used. Up to 35 years of physiological remodelling were simulated. The validation of the model calculations was based on a quantitative comparison with the data measured for older individuals. The simulations confirmed that the description of the age-related changes in the trabecular bone by means of the architectural parameter (star volume) constitutes a promising tool for subjects older than approximately 50 years. For individuals younger than approximately 50 years bone mineral density (bone volume/total volume) seems to be the best suited descriptor. The results suggest that the optimal diagnostic procedure is age-dependent and should not be limited to the bone mineral density measurement. The clinical usefulness of the procedure has been validated by examination of the CT images.

Comparison of trabecular bone architecture in young and old bones.

In this study, different parameters currently applied for the description of trabecular bone architecture in young and old subjects are compared. Moreover, new parameters are proposed and assessed. For the investigations, the two-dimensional images of vertebral body sections are acquired with the use of a low-magnification digital camera. The parameters were calculated both for the skeleton and for the bone/marrow interface distinguished in the images of the trabecular network. The following methods of the descriptions of the trabecular bone architecture were considered: histomorphometric analysis (BV/TV, asymmetry, mean trabeculae length), fractal geometry technique, Euler characteristic, star volume of the marrow cavity, the mean distance between two points of the trabecular network, and the probability of disconnection (is straightforwardly connected with the number of separated parts of the network). Moreover, bone mineral density (BMD) was determined for comparative purposes. The quantities directly extracted form the images are also combined to produce new indexes, which better differentiate young and old bones. It was found that the BMD, the BV/TV, the star volume, the Euler number, and the probability of disconnection might be used as indicators of the age-related changes of trabecular bone. The parameters could be measured with the precision comparable to that of the BMD. The only exception is the probability of disconnection. Highly significant correlations were demonstrated between bone density (BMD, BV/TV) and trabecular architecture (SV/GV, probability of disconnection).