SU-E-T-125: High-Resolution Linear Diode Array for Use in Stereotactic Beam Commissioning.

PURPOSE: Task Group 106 discusses detector arrays and their usefulness in "soft-wedge" profile measurements. This study verifies an extension of their use; where a high-resolution linear diode array is used to measure profile scans for use in commissioning stereotactic beam. Small-field data is acquired with the LDA-99sc (IBA Dosimetry America, Memphis, TN), an array of 99 two-mm-diameter Hi-pSi diodes. These data are compared to data collected during serial scanning with an IBA SFD stereotactic diode. METHODS: Field measurements were taken at varying depths using an LDA-99sc in 1×1 cm2 , 2×2cm2 , and 3×3cm2 fields of a Trilogy accelerator (Varian Medical Systems, Palo Alto, CA). These scans were duplicated in resolution and sample rate with an SFD scanning diode. Both techniques acquired point data in 1.0mm increments. RESULTS: In identical conditions, the LDA-99sc provided readings with significantly less signal fluctuation than with traditional diode scanning; which is attributed to the reduction of water motion associated with detector travel during traditional scanning. This smoother data precluded the use of smoothing and filtration algorithms; allowing the beam to be modeled using raw data. The LDA-99sc measured an average penumbra of 0.1 mm wider than that which was measured with the diode. The LDA-99sc also measured an average field size of 0.3mm wider than that which was measured with the diode. With respect to time, a single profile scan (30cm-depth, 3×3cm2 field) using the LDA-99sc took 31 seconds, while a diode scan took 229 seconds. CONCLUSIONS: The LDA99 provided accurate commissioning-grade small-field beam profile data, as verified by comparison with diode scans, with a reduction of uncertainty in readings and a substantial reduction in time. Variation in penumbra and beam widths were deemed acceptable and can be attributed to the LDA-99sc data being raw, while the diode data was processed. Research supported in-part by IBA Dosimetry America.

SU-E-J-40: Lung Lesion Tracking Using Fixed-Spaced Non-Migrating Fiducial Markers in Robotic Radiosurgery.

PURPOSE: With lung lesion treatment being a major indication of the use of the CyberKnife (CK) robotic radiosurgery system (Accuray Inc, Sunnyvale, CA), the ability for the CK's stereoscopic kV imaging system to accurately track implanted fiducial markers becomes vital in the accurate delivery of therapeutic radiation. This study examines a novel fixed-space fiducial marker delivery system which is capable of delivering two VISICOIL (IBA Dosimetry America, Memphis, TN) non-migrating fiducial markers simultaneously at a fixed spacing through a single 20-gauge needle. More specifically, presented herein is a preliminary study which tests the CK'streatment localization system's (TLS) ability to track markers of varying size and spacing, comparing subsequent stereoscopic kV imaging to DRR's generated during the planning stages. METHODS: Three markers were placed in an XLT Lung Phantom (CIRS Inc, Norfolk, VA); two markers inserted along a diagonal line in a coronal plane, separated by biocompatible spacersof varying size, and a common third marker being placed in a non-varyinglocation in a coronal plane anterior to the marker pair. This third marker allows the calculation of rotational and translation corrections. Differentcombinations were scanned, planned, and simulated; 3.5mm- and 5.0mm- long markers, each 5mm in diameter, were separated by 15mm, 17mm, and 20mm spacers. RESULTS: The TLS system was able to track each of the aforementioned configurations with standard lung imaging parameters.Longer markers were not included in the study since earlier studies showed that without the natural deformation that would occur upon implantation, the length would induce false tracking. CONCLUSIONS: This is a necessary firststep in determining the minimum spacing with which the CK's TLS can track, a study which can now proceed with the use of phantom treatments elivered to orthogonally-overlapping radiochromic film, bisecting a tumor volume which is implanted with this fiducial marker system. Research supported in-part by IBA Dosimetry America.

How to measure the information gained from one symbol.

Information theory provides a powerful framework to analyse how neurons represent sensory stimuli or other behavioural variables. A recurring question regards the amount of information conveyed by a specific neuronal response. Here we show that the commonly used definition for this quantity has a serious flaw: the information accumulated during subsequent observations of neural activity fails to combine additively. Additivity is a highly desirable property, both on theoretical grounds and for the practical purpose of analysing population codes. We propose an alternative measure for the information per observation and prove that this is the only definition that satisfies additivity. The old and the new definitions measure very different aspects of the neural code, which is illustrated with visual responses from a motion-sensitive neuron in the primate cortex. Our analysis allows additional interpretation of several published results, which suggests that the neurons studied are operating far from their information capacity.

Efficient discrimination of temporal patterns by motion-sensitive neurons in primate visual cortex.

Although motion-sensitive neurons in macaque middle temporal (MT) area are conventionally characterized using stimuli whose velocity remains constant for 1-3 s, many ecologically relevant stimuli change on a shorter time scale (30-300 ms). We compared neuronal responses to conventional (constant-velocity) and time-varying stimuli in alert primates. The responses to both stimulus ensembles were well described as rate-modulated Poisson processes but with very high precision (approximately 3 ms) modulation functions underlying the time-varying responses. Information-theoretic analysis revealed that the responses encoded only approximately 1 bit/s about constant-velocity stimuli but up to 29 bits/s about the time-varying stimuli. Analysis of local field potentials revealed that part of the residual response variability arose from "noise" sources extrinsic to the neuron. Our results demonstrate that extrastriate neurons in alert primates can encode the fine temporal structure of visual stimuli.

Optimization principles for the neural code.

Recent experiments show that the neural codes at work in a wide range of creatures share some common features. At first sight, these observations seem unrelated. However, we show that these features arise naturally in a linear filtered threshold crossing model when we set the threshold to maximize the transmitted information. This maximization process requires neural adaptation to not only the DC signal level, as in conventional light and dark adaptation, but also to the statistical structure of the signal and noise distributions. We also present a new approach for calculating the mutual information between a neuron's output spike train and any aspect of its input signal which does not require reconstruction of the input signal. This formulation is valid provided the correlations in the spike train are small, and we provide a procedure for checking this assumption. This paper is based on joint work (DeWeese M 1995 Optimization principles for the neural code, Dissertation, Princeton University). Preliminary results from the linear filtered threshold crossing model appeared in a previous proceedings (DeWeese M and Bialek W 1995 Information flow in sensory neurons, Nuovo Cimento D 17 733-8), and the conclusions we reached at that time have been reaffirmed by further analysis of the model.