Detection of brachytherapy seeds at varying angles with a singular-spectrum-analysis algorithm

Abstract

A singular‐spectrum‐analysis (SSA) algorithm previously was shown to be successful in detecting brachytherapy seeds in B‐mode images when seeds were orthogonal to the ultrasound beam. In this study, the algorithm was extended to detect seeds at angles to the beam. The SSA algorithm derives P‐values from selected eigenvalue pairs of the autocorrelation matrix of seed echo signals. The P‐value indicates the probability of presence of a seed. A seed inserted in a gel pad and another inserted in beef tissue were scanned with a 5‐MHz transducer. The seed angle was varied from orthogonal to the beam to 23 deg from orthogonal, using 1‐degree increments. Simulations of seed echoes based on empirical data were generated and used to test the algorithm. Success in seed detection was expressed by a score computed from P‐values. Scores for simulations decreased from 0.8 to 0.45 as the angle increased from 0 to 12 deg. Scores for experimental data varied from 0.74 at 0 deg to 0.62 at 23 deg; the lowest value was 0.1 at 7 deg. Accordingly, the SSA algorithm was successful in detecting the seed with clinically relevant angles between seed and beam.