Active ultrasound tracking method for prostate brachytherapy using acoustic needle prototype and sonomicrometry

Purpose:
This study examined whether sonomicrometry can measure displacements of active ultrasound-tracked needle prototype for prostate brachytherapy application with high linearity, accuracy, and precision as well as triangulate the needle’s motion relative to intra-rectal ultrasound probe.

Material and methods:
Sonomicrometry detects time-of-flight (ToF) of ultrasound signal transmitted from an active ultrasound-tracked needle (customized with a piezo-electric tracking sensor) to independent sensor. The transmission was tested in water for displacements parallel and perpendicular to the needle axis using distances of up to 35 mm. Three-dimensional (3D) tracking of the active ultrasound-tracked needle utilized a simulated intra-rectal ultrasound probe fitted with 5 piezo-electric sensors, and a sonomicrometry triangulation algorithm was used to measure the distances of the needle from the probe.

Results:
Incremental displacements of the active ultrasound-tracked needle were measured by sonomicrometry with strong linearity (R ≥ 0.999), high accuracy (mean difference ≤ 0.55 mm), and precision (1.96 × standard deviation of the mean difference ≤ 0.22 mm) compared with a reference. Sporadic false triggering of ToF detection was eliminated by signal inversion. Acoustic spatial tracking by the off-line triangulation algorithm produced virtual 3D wire meshes, which closely spatially overlapped with the corresponding (ground-truth) video clips of active ultrasound-tracked needle movement at several testing time points.

Conclusions:
We introduced an experimental active ultrasound-tracked needle and employed sonomicrometry to measure the needle’s displacements and triangulate its spatial position. The measurements and 3D spatial tracking yielded promising results for potential prostate brachytherapy applications. Clinical implementation will necessitate developments of automated triggering, real-time tracking, and a commercially viable acoustic needle design.