Clinical evaluation of a speed of sound aberration correction algorithm in quantitative ultrasound-aided image guided radiotherapy (Conference Abstract)

To investigate the clinical impact of a recently introduced speed of sound (SOS) aberration correction algorithm in ultrasound (US) imaging systems for image guided radiotherapy (IGRT). The SOS algorithm was applied to four prostate cancer patients and two liver cancer patients to assess in a real clinical application the extent and spatial distribution of the correction

A CT based correction method for speed of sound aberration for ultrasound based image guided radiotherapy

Purpose To introduce a correction for speed of sound (SOS) aberrations in three dimensional (3D) ultrasound (US) imaging systems for small but systematic positioning errors in image guided radiotherapy (IGRT) applications. US waves travel at different speeds in different human tissues. Conventional US-based imaging systems assume that SOS is constant in all tissues at 1540 m/s which is an accepted average value for soft tissues. This assumption leads to errors of up to a few millimeters when converting echo times into distances and is a source of systematic errors and image distortion in quantitative US imaging. Methods At simulation, US applications for IGRT provide a computed tomography (CT) image coregistered to a US volume. The CT scan provides the physical density which can be used in an empirical relationship with SOS. This can be used to correct for different SOS in different tissues within the patient. For each US scan line each voxel's axial dimension is rescaled according to the SOS associated to it. This SOS correction method was applied to US scans of a PMMA container filled with either water, a 20% saline water solution or sunflower oil, and the results were compared to the CT. The correction was also applied to an US quality assurance (QA) phantom containing rods with high ultrasound contrast. This phantom was scanned with US through a container filled with the same three liquids. Finally, the algorithm was applied to two clinical cases: a prostate cancer patient and a breast cancer patient. Results After the correction was applied to the phantom images, spatial registration between the bottom of the phantom in the US scan and in the CT scan was improved; the difference was reduced from a few millimeters to less than one millimeter for all three different liquids. Reference structures in the QA phantom appeared at more closely corresponding depths in the three cases after the correction, within 0.5 mm. Both clinical cases showed small shifts, up to 3 mm, in the positions of anatomical structures after correction. Conclusions The SOS correction presented increases quantitative accuracy in US imaging which may lead to small but systematic improvements in patient positioning

Various approaches for pseudo-CT scan creation based on ultrasound to ultrasound deformable image registration between different treatment time points for radiotherapy treatment plan adaptation in prostate cancer patients

The purpose of this study was to evaluate eight possible approaches to create pseudo-CT images for radiotherapy (RT) treatment re-planning. These re-planning CT scans would normally require a separate CT scan session. If important changes occur in patient's anatomy between simulation (SIM) and treatment (TX) stages, 3D ultrasound (US) images acquired at the two stages, available in US guided RT workflows, can be used to produce a deformation field. Proof of concept research showed that the application of this deformation field to the SIM CT image yields a pseudo-CT which can be more representative of the patient at TX than SIM CT. Co-registered CT and US volumes acquired at five different time points during the RT course of a prostate cancer patient were combined into data pairs, providing ground truth CT images (CTtx). Eight different methods were explored to create the deformation field that was used to produce the pseudo-CT scan. Anatomical structure comparison and γ index calculations were used to compare the similarity of the pseudo-CT volumes and the reference TX CT volumes. In five out of ten data pairs, all the eight approaches resulted in the creation of a pseudo-CT equally or more similar to the TX CT than the SIM CT within the region of interest, with an average improvement of 54.1% (range: 5.1%–126.5%) in dice similarity coefficient (DSC) and 32.3% (range: 0.3%–52.6%) in γ index. For the remaining data pairs, four up to seven approaches resulted in an improvement in both DSC (range: 4.3%–54%) and γ index (range: 0.8%–41.3%). In conclusion, at least four out of eight explored approaches resulted in more representative pseudo-CT images in all the data pairs. In particular, the approaches in which an initial rigid alignment was combined with deformable registration performed best

Pseudo-CT scan creation using registration of transabdominal ultrasound volumes of a prostate cancer patient

A crucial step in radiotherapy workflows is patient setup. The patients’ position prior to each treatment fraction (TX) must be as similar as possible to the simulation CT scan (CTsim), as the treatment plan was based on this scan. Even when the patient seems correctly aligned externally, internal tissue distributions might have changed. Delivery of the initial treatment plan might then result in suboptimal dose depositions. As frequent CT scan (CTtx) acquisition for treatment plan verification is not feasible, we propose to create instead pseudo-CT scans using simulation ultrasound (USsim) to treatment ultrasound (UStx) registratio

Various approaches for pseudo-CT scan creation based on ultrasound to ultrasound deformable image registration between different treatment time points for radiotherapy treatment plan adaptation in prostate cancer patients

The purpose of this study was to evaluate eight possible approaches to create pseudo-CT images for radiotherapy (RT) treatment re-planning. These re-planning CT scans would normally require a separate CTscan session. If important changes occur in patient's anatomy between simulation (SIM) and treatment (TX) stages, 3D ultrasound (US) images acquired at the two stages, available in US guided RT workflows, can be used to produce a deformation field. Proof of concept research showed that the application of this deformation field to the SIM CT image yields a pseudo-CT which can be more representative of the patient at TX than SIM CT. Co-registered CT and US volumes acquired at five different time points during the RT course of a prostate cancer patient were combined into data pairs, providing ground truth CTimages (CTtx). Eight different methods were explored to create the deformation field that was used to produce the pseudo-CT scan. Anatomical structure comparison and gamma index calculations were used to compare the similarity of the pseudo-CT volumes and the reference TX CT volumes. In five out of ten data pairs, all the eight approaches resulted in the creation of a pseudo-CT equally or more similar to the TX CTthan the SIM CT within the region of interest, with an average improvement of 54.1% (range: 5.1%-126.5%) in dice similarity coefficient (DSC) and 32.3% (range: 0.3%-52.6%) in gamma index. For the remaining data pairs, four up to seven approaches resulted in an improvement in both DSC (range: 4.3%-54%) and gamma index (range: 0.8%-41.3%). In conclusion, at least four out of eight explored approaches resulted in more representative pseudo-CT images in all the data pairs. In particular, the approaches in which an initial rigid alignment was combined with deformable registration performed best

Integrating a MRI scanner with a 6 MV radiotherapy accelerator:impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field

\u3cp\u3eAt the UMC Utrecht, in collaboration with Elekta and Philips Research Hamburg, we are developing a radiotherapy accelerator with integrated MRI functionality. The radiation dose will be delivered in the presence of a lateral 1.5 T field. Although the photon beam is not affected by the magnetic field, the actual dose deposition is done by a cascade of secondary electrons and these electrons are affected by the Lorentz force. The magnetic field causes a reduced build-up distance: because the trajectory of the electrons between collisions is curved, the entrance depth in tissue decreases. Also, at tissue-air interfaces an increased dose occurs due to the so-called electron return effect (ERE): electrons leaving tissue will describe a circular path in air and re-enter the tissue yielding a local dose increase. In this paper the impact of a 1.5 T magnetic field on both the build-up distance and the dose increase due to the ERE will be investigated as a function of the angle between the surface and the incident beam. Monte Carlo simulations demonstrate that in the presence of a 1.5 T magnetic field, the surface dose, the build-up distance and the exit dose depend more heavily on the surface orientation than in the case without magnetic field. This is caused by the asymmetrical pointspread kernel in the presence of 1.5 T and the directional behaviour of the re-entering electrons. Simulations on geometrical phantoms show that ERE dose increase at air cavities can be avoided using opposing beams, also when the air-tissue boundary is not perpendicular to the beam. For the more general case in patient anatomies, more problems may arise. Future work will address the possibilities and limitations of opposing beams in combination with IMRT in a magnetic field.\u3c/p\u3

Consequences of intermodality registration errors for intramodality 3D ultrasound IGRT

Intramodality ultrasound image-guided radiotherapy systems compare daily ultrasound to reference ultrasound images. Nevertheless, because the actual treatment planning is based on a reference computed tomography image, and not on a reference ultrasound image, their accuracy depends partially on the correct intermodality registration of the reference ultrasound and computed tomography images for treatment planning. The error propagation in daily patient positioning due to potential registration errors at the planning stage was assessed in this work. Five different scenarios were simulated involving shifts or rotations of ultrasound or computed tomography images. The consequences of several workflow procedures were tested with a phantom setup. As long as the reference ultrasound and computed tomography images are made to match, the patient will be in the correct treatment position. In an example with a phantom measurement, the accuracy of the performed manual fusion was found to be ≤2 mm. In clinical practice, manual registration of patient images is expected to be more difficult. Uncorrected mismatches will lead to a systematically incorrect final patient position because there will be no indication that there was a misregistration between the computed tomography and reference ultrasound images. In the treatment room, the fusion with the computed tomography image will not be visible and based on the ultrasound images the patient position seems correct

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

Review of ultrasound image guidance in external beam radiotherapy part II: intra-fraction motion management and novel applications.

Imaging has become an essential tool in modern radiotherapy (RT), being used to plan dose delivery prior to treatment and verify target position before and during treatment. Ultrasound (US) imaging is cost-effective in providing excellent contrast at high resolution for depicting soft tissue targets apart from those shielded by the lungs or cranium. As a result, it is increasingly used in RT setup verification for the measurement of inter-fraction motion, the subject of Part I of this review (Fontanarosa et al 2015 Phys. Med. Biol. 60 R77–114). The combination of rapid imaging and zero ionising radiation dose makes US highly suitable for estimating intra-fraction motion. The current paper (Part II of the review) covers this topic. The basic technology for US motion estimation, and its current clinical application to the prostate, is described here, along with recent developments in robust motion-estimation algorithms, and three dimensional (3D) imaging. Together, these are likely to drive an increase in the number of future clinical studies and the range of cancer sites in which US motion management is applied. Also reviewed are selections of existing and proposed novel applications of US imaging to RT. These are driven by exciting developments in structural, functional and molecular US imaging and analytical techniques such as backscatter tissue analysis, elastography, photoacoustography, contrast-specific imaging, dynamic contrast analysis, microvascular and super-resolution imaging, and targeted microbubbles. Such techniques show promise for predicting and measuring the outcome of RT, quantifying normal tissue toxicity, improving tumour definition and defining a biological target volume that describes radiation sensitive regions of the tumour. US offers easy, low cost and efficient integration of these techniques into the RT workflow. US contrast technology also has potential to be used actively to assist RT by manipulating the tumour cell environment and by improving the delivery of radiosensitising agents. Finally, US imaging offers various ways to measure dose in 3D. If technical problems can be overcome, these hold potential for wide-dissemination of cost-effective pre-treatment dose verification and in vivo dose monitoring methods. It is concluded that US imaging could eventually contribute to all aspects of the RT workflow