Tracking errors related to cardiac cycle: a new approach

One-dimensional manual tracking was investigated in relation to cardiac activity. The task of the subjects was, by means of a joy-stick, to maintain a spotlight between two vertical lines moving horizontally across an oscilloscope screen. Error incidence was time-locked with respect to the cardiac cycle. Error rate was higher for faster (2-s trial periods) than for slower (3-s trial periods) target movement. Using linear-ramp and sinusoidal movements, it was demonstrated that error incidence is associated with positive and negative cardiac acceleration.     

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Comparison of inhomogeneity correction algorithms in small photon fields

Algorithms such as convolution superposition, Batho, and equivalent pathlength which were originally developed and validated for conventional treatments under conditions of electronic equilibrium using relatively large fields greater than 5 x 5 cm2 are routinely employed for inhomogeneity corrections. Modern day treatments using intensity modulated radiation therapy employ small beamlets characterized by the resolution of the multileaf collimator. These beamlets, in general, do not provide electronic equilibrium even in a homogeneous medium, and these effects are exaggerated in media with inhomogenieties. Monte Carlo simulations are becoming a tool of choice in understanding the dosimetry of small photon fields as they encounter low density media. In this study, depth dose data from the Monte Carlo simulations are compared to the results of the convolution superposition, Batho, and equivalent pathlength algorithms. The central axis dose within the low-density inhomogeneity as calculated by Monte Carlo simulation and convolution superposition decreases for small field sizes whereas it increases using the Batho and equivalent pathlength algorithms. The dose perturbation factor (DPF) is defined as the ratio of dose to a point within the inhomogeneity to the same point in a homogeneous phantom. The dose correction factor is defined as the ratio of dose calculated by an algorithm at a point to the Monte Carlo derived dose at the same point, respectively. DPF is noted to be significant for small fields and low density for all algorithms. Comparisons of the algorithms with Monte Carlo simulations is reflected in the DCF, which is close to 1.0 for the convolution-superposition algorithm. The Batho and equivalent pathlength algorithms differ significantly from Monte Carlo simulation for most field sizes and densities. Convolution superposition shows better agreement with Monte Carlo data versus the Batho or equivalent pathlength corrections. As the field size increases the DCF's for all algorithms converge toward 1.0. The largest differences in DCF are at the interface where changes in electron transport are greatest. For a 6 MV photon beam, electronic equilibrium is restored at field sizes above 3 cm diameter and all of the algorithms predict dose in and beyond the inhomogeneous region equally well. For accurate dosimetry of small fields within and near inhomogeneities, however, simple algorithms such as Batho and equivalent pathlength should be avoided.     

Visual perception: double pulse temporal resolution. (Lack of contingency upon the phase of resting cardiac and respiratory cycle)

Double light pulse resolution required longer inter-pulse time intervals of short duration of the pulse. The threshold of double pulse discrimination was not contingent upon the actual phase of cardiac or respiratory cycle.     

Cardiac frequency, phase and errors in one-dimensional sensorimotor tracking.

One dimensional manual tracking was investigated in relation to actual heart rate and phase of cardiac cycle. Higher heart rate was combined with increased tracking error incidence and decreased successful tracking time (light spot inside target). Error incidence was time-locked with respect to the phase of cardiac cycle.