Target localization using scanner‐acquired SPECT data

Abstract

Target localization using single photon emission computed tomography (SPECT) and planar imaging is being investigated for guiding radiation therapy delivery. Previous studies on SPECT‐based localization have used computer‐simulated or hybrid images with simulated tumors embedded in disease‐free patient images where the tumor position is known and localization can be calculated directly. In the current study, localization was studied using scanner‐acquired images. Five fillable spheres were placed in a whole body phantom. Sphere‐to‐background 99mTc radioactivity was 6:1. Ten independent SPECT scans were acquired with a Trionix Triad scanner using three detector trajectories: left lateral 180°, 360°, and right lateral 180°. Scan time was equivalent to 4.5 min. Images were reconstructed with and without attenuation correction. True target locations were estimated from 12 hr SPECT and CT images. From the 12 hr SPECT scan, 45 sets of orthogonal planar images were used to assess target localization; total acquisition time per set was equivalent to 4.5 min. A numerical observer localized the center of the targets in the 4.5 min SPECT and planar images. SPECT‐based localization errors were compared for the different detector trajectories. Across the four peripheral spheres, and using optimal iteration numbers and postreconstruction smoothing, means and standard deviations in localization errors were 0.90±0.25 mm for proximal 180° trajectories, 1.31±0.51 mm for 360° orbits, and 3.93±1.48 mm for distal 180° trajectories. This rank order in localization performance is predicted by target attenuation and distance from the target to the collimator. For the targets with mean localization errors < 2 mm, attenuation correction reduced localization errors by 0.15 mm on average. The improvement from attenuation correction was 1.0 mm on average for the more poorly localized targets. Attenuation correction typically reduced localization errors, but for well‐localized targets, the detector trajectory generally had a larger effect. Localization performance was found to be robust to iteration number and smoothing. Localization was generally worse using planar images as compared with proximal 180° and 360° SPECT scans. Using a proximal detector trajectory and attenuation correction, localization errors were within 2 mm for the three superficial targets, thus supporting the current role in biopsy and surgery, and demonstrating the potential for SPECT imaging inside radiation therapy treatment rooms.PACS numbers: 87.55.Gh, 87.57.qp, 87.57.uh

Stereotactic body radiotherapy treatment of extracranial metastases

“…Recent development of MRI and single-photon emission computed tomography (SPECT) imaging integrated with the radiation treatment device can further enhance the verification accuracy using soft-tissue contrast and functional information of the targets. 33,43 To ensure that each metastasis receives the appropriate dose, real-time target verification is often performed, especially when respiratory-gated and dynamic tracking techniques are used. 44 Such target verification can be achieved with fluoro scopic imaging or megavoltage cine, which uses the treatment beam for imaging without exposing the patient to extra radiation dose.…”

Medical Imaging in Clinical Practice

“…Single photon emission computed tomography (SPECT) is a functional and molecular imaging modality, which is being used clinically to localize targets prior to biopsy and surgery (Roper et al, 2012). SPECT shows function by means of a three dimensional activity distribution of a radioactive tracer, which was injected prior to the measurement (Changizi et al, 2008).…”

A Monte Carlo study to investigate the feasibility of an on‐board SPECT/spectral‐CT/CBCT imager for medical linear accelerator

“…A group from Duke University has demonstrated the potential for on-board SPECT imaging used in the treatment room in simulation studies. [10][11][12] In these studies, the challenges remained, such as the high imaging localization errors for deep targets and difficulty to integrate the SPECT scanner into Linacs usually with on-board KV/MV imaging devices equipped for the parallel-hole collimator-based camera, alignment uncertainties from the co-registration of SPECT images and CBCT images and the trajectory error of the robots for the robotic multipinhole-based camera. These concerns might hamper the designs to be translated to practical implementation.…”