Evaluation of targeting errors in ultrasound-assisted radiotherapy

Review of ultrasound image guidance in external beam radiotherapy: I. Treatment planning and inter-fraction motion management

In modern radiotherapy, verification of the treatment to ensure the target receives the prescribed dose and normal tissues are optimally spared has become essential. Several forms of image guidance are available for this purpose. The most commonly used forms of image guidance are based on kilovolt or megavolt x-ray imaging. Image guidance can also be performed with non-harmful ultrasound (US) waves. This increasingly used technique has the potential to offer both anatomical and functional information.This review presents an overview of the historical and current use of two-dimensional and three-dimensional US imaging for treatment verification in radiotherapy. The US technology and the implementation in the radiotherapy workflow are described. The use of US guidance in the treatment planning process is discussed. The role of US technology in inter-fraction motion monitoring and management is explained, and clinical studies of applications in areas such as the pelvis, abdomen and breast are reviewed. A companion review paper (O'Shea et al 2015 Phys. Med. Biol. submitted) will extensively discuss the use of US imaging for intra-fraction motion quantification and novel applications of US technology to RT.

Registration of CT to 3D ultrasound using near-field fiducial localization: A feasibility study

OBJECTIVE

Registration of ultrasound to computed tomography (CT) images is used in several image-guided procedures, including laparoscopic surgery and radiation therapy. Conventional approaches use an external tracker calibrated to the ultrasound transducer and CT system, but several calibration steps are required. Registration can also be performed by aligning image features between modalities, but differences in feature depiction make matching difficult and initial approximate alignment is often needed. Registration using fiducials is a simpler approach but is limited by the need to implant fiducials in the anatomical region of interest so they are visible to both ultrasound and CT. This paper investigates the feasibility of using fiducials near the skin surface, and whether such fiducials can be sufficiently localized in the very near field of a 3D ultrasound transducer without significantly degrading image quality. This approach can also be used as an initialization step for feature-based registration techniques.

MATERIALS AND METHODS

A stand-off pad containing fiducials (n > 3) was constructed using polyvinyl chloride and steel ball fiducials that are visible in both 3D ultrasound and CT images. Experiments on phantoms were performed to assess image quality and registration errors. Controlled variables included pad thickness and ultrasound imaging parameters. Initial tests were also conducted of a potential application in partial nephrectomy surgery.

RESULTS

Image quality was degraded by an average of 6-11-13% (elevational-axial-lateral) in resolution of point targets and 5% in lesion contrast. Average fiducial localization error was 1.34 mm (axial) to 2.38 mm (lateral and elevational); average fiducial registration error (FRE) was 0.46 mm (axial), 1.08 mm (lateral) and 0.90 mm (elevational); and average total registration error (TRE) was 1.84 mm (axial), 0.89 mm (lateral) and 3.31 mm (elevational). Clinical results showed a similar FRE to that in the phantom study, but with an average TRE of 14.04 mm (over three patients). Ultimate alignment of the organ boundaries was affected mainly by motion from respiration.

CONCLUSIONS

The small loss of image quality from the fiducial stand-off pad and the minimal inconvenience of using the pad at the time of the CT scan may be a worthwhile trade-off for purposes of registration since the pad provides a registration accuracy of several millimeters while still allowing subsequent feature-based registration. Future research will focus on using the registration from the fiducial stand-off pad for deformable feature-based registration of 3D ultrasound to CT for tumor localization in renal surgery.

Sonographic optic nerve sheath diameter as an estimate of intracranial pressure in adult trauma

BACKGROUND

Intracranial pressure (ICP) is currently measured with invasive monitoring. Sonographic optic nerve sheath diameter (ONSD) may provide a noninvasive estimate of ICP. Our hypothesis was that bedside ONSD accurately estimates ICP in acutely injured patients. The specific aims were (1) to determine the accuracy of ONSD in estimating elevated ICP, (2) to correlate ONSD and ICP in unilateral and bilateral head injuries, and (3) to determine the effect of ICP monitor placement on ONSD measurements.

MATERIALS AND METHODS

A blinded prospective study of adult trauma patients requiring ICP monitoring was performed at a University-based urban trauma center. The ONSD was measured by ultrasound pre- and post-placement of an ICP monitor (Camino Bolt or Ventriculostomy).

RESULTS

One-hundred fourteen measurements were obtained in 10 trauma patients requiring ICP monitoring. Pre- and post-ONSD were compared with side of injury in the presence of an ICP monitor. ROC analysis demonstrated ONSD poorly estimates elevated ICP (AUC = 0.36). Overall sensitivity, specificity, PPV, NPV, and accuracy for estimating ICP with ONSD were 36%, 38%, 40%, 16%, and 37%. Poor correlation of ONSD to ICP was observed with unilateral (R(2) = 0.45, P < 0.01) and bilateral (R(2) = 0.21, P = 0.01) injuries. ICP monitor placement did not affect ONSD measurements on the right (P = 0.5), left (P = 0.4), or right and left sides combined (P = 0.3).

CONCLUSIONS

Sonographic ONSD as a surrogate for elevated ICP in lieu of invasive monitoring is not reliable due to poor accuracy and correlation.