Alexander Hartl

Towards intra-operative 3D nuclear imaging: reconstruction of 3D radioactive distributions using tracked gamma probes

Nuclear medicine imaging modalities assist commonly in surgical guidance given their functional nature. However, when used in the operating room they present limitations. Pre-operative tomographic 3D imaging can only serve as a vague guidance intra-operatively, due to movement, deformation and changes in anatomy since the time of imaging, while standard intra-operative nuclear measurements are limited to 1D or (in some cases) 2D images with no depth information. To resolve this problem we propose the synchronized acquisition of position, orientation and readings of gamma probes intra-operatively to reconstruct a 3D activity volume. In contrast to conventional emission tomography, here, in a first proof-of-concept, the reconstruction succeeds without requiring symmetry in the positions and angles of acquisition, which allows greater flexibility. We present our results in phantom experiments for sentinel node lymph node localization. The results indicate that 3D intra-operative nuclear images can be generated in such a setup up to an accuracy equivalent to conventional SPECT systems. This technology has the potential to advance standard procedures towards intra-operative 3D nuclear imaging and offers a novel approach for robust and precise localization of functional information to facilitate less invasive, image-guided surgery.

Freehand SPECT reconstructions using look up tables

Nuclear imaging is a commonly used tool in todays diagnostics and therapy planning. For interventional use however it suffers from drawbacks which limit its application. Freehand SPECT was developed to overcome these limitations and to provide 3D functional imaging during an intervention. It combines a nuclear probe with an optical tracking system to obtain its position and orientation in space synchronized with its reading. This information can be used to compute a 3D tomographic reconstruction of an activity distribution. However, as there is no fixed geometry the system matrix has to be computed on-the-fly, using ad-hoc models of the detection process. One solution for such a model is a reference look up table of previously acquired measurements of a single source at different angles and distances. In this work two look up tables with a one and four millimeter step size between the entries were acquired. Twelve datasets of a phantom with two hollow spheres filled with a solution of Tc99wm were acquired with the Freehand SPECT system. Reconstructions with the look up tables and two analytical models currently in use were performed with these datasets and compared with each other. The finely sampled look up table achieved the qualitatively best reconstructions, while one of the analytical models showed the best positional accuracy.

Towards intra-operative PET for head and neck cancer: lymph node localization using high-energy probes

We present a novel approach for intra-operative localization of lymph nodes and metastases in the head and neck region using the radio-tracer [18F]FDG. By combining an optical tracking system with a high-energy gamma probe to detect 511keV annihilation gammas, we enable intra-operative PET to visualize activity distributions. Detection of these gammas is modeled ad-hoc analytically, taking into account several factors affecting the detection process. This allows us to iteratively reconstruct the radio-tracer distribution within a localized volume of interest. As a feasibility study we analyze clinical data of 7 patients with tumors in the head and neck region, and derive a realistic neck phantom configuration with [18F]FDG-filled lesions mimicking tumors and lymph nodes. We demonstrate the capabilities and limitations of our approach using that neck phantom. We also outline possible improvements to make our method clinically viable towards less invasive surgeries.

Evaluation of a 4D cone-beam CT reconstruction approach using a simulation framework

Current image-guided navigation systems for thoracic abdominal interventions utilize three dimensional (3D) images acquired at breath-hold. As a result they can only provide guidance at a specific point in the respiratory cycle. The intervention is thus performed in a gated manner, with the physician advancing only when the patient is at the same respiratory cycle in which the 3D image was acquired. To enable a more continuous workflow we propose to use 4D image data. We describe an approach to constructing a set of 4D images from a diagnostic CT acquired at breath-hold and a set of intraoperative cone-beam CT (CBCT) projection images acquired while the patient is freely breathing. Our approach is based on an initial reconstruction of a gated 4D CBCT data set. The 3D CBCT images for each respiratory phase are then non-rigidly registered to the diagnostic CT data. Finally the diagnostic CT is deformed based on the registration results, providing a 4D data set with sufficient quality for navigation purposes. In this work we evaluate the proposed reconstruction approach using a simulation framework. A 3D CBCT dataset of an anthropomorphic phantom is deformed using internal motion data acquired from an animal model to create a ground truth 4D CBCT image. Simulated projection images are then created from the 4D image and the known CBCT scan parameters. Finally, the original 3D CBCT and the simulated X-ray images are used as input to our reconstruction method. The resulting 4D data set is then compared to the known ground truth by normalized cross correlation(NCC). We show that the deformed diagnostic CTs are of better quality than the gated reconstructions with a mean NCC value of 0.94 versus a mean 0.81 for the reconstructions.

Detection models for freehand SPECT reconstruction

Nuclear imaging modalities are commonly used tools in todays diagnostics and therapy planning. However for interventional use they suffer from drawbacks which limit their application. Freehand SPECT was developed to provide 3D functional imaging during interventions. It combines a nuclear detector with an optical tracking system to obtain its position and orientation in space and synchronizes this with the detector readings. This information can be used to compute a 3D tomographic reconstruction of an activity distribution of a nuclear tracer. As there is no fixed geometry, the system matrix has to be computed on the fly. This is done with models of the detection process for completely arbitrary freehand acquisitions. The accuracy of the reconstructions is highly dependent on the used models of the detection process. Different models of the detection process were developed and evaluated in this work, in particular two analytical models as well as lookup tables generated from either real measurements or Monte Carlo simulations. We showed that it is possible to perform acceptable reconstructions with a simple but efficient analytical model. The use of lookup tables to generate the system matrix in Freehand SPECT is a fast solution with good accuracy.

Two new ad-hoc models of detection physics and their evaluation for navigated beta probe surface imaging

Intra-operative surface imaging with navigated beta probes in conjunction with positron-emitting radiotracers like 18F-FDG has been shown to enable control of tumor resection borders. We showed previously that employing iterative reconstruction (MLEM) in conjunction with an ad-hoc model of the detection physics (based on solid-angle geometry, SA) improves the image quality. In this study, we sampled the beta probe readings of a point source using a precision step-motor to generate a look-up-table (LUT) model. We also generated a simplified geometrical model (SG) based on this data set. To see how these two models influence the image quality compared to the old SA model, we reconstructed images from sparsely sampled datasets of a phantom with three hotspots using each model. The images yielded 76% (SA), 81% (SG), and 81% (LUT) mean NCC compared to the ground truth. The SG and LUT models, however, could resolve the hotspots better in the datasets where the detector-to-phantom distance was larger. Additionally, we compared the deviations of the SA and SG analytical models to the measured LUT model, where we found that the SG model gives estimates substantially closer to the actual beta probe readings than the previous SA model.

Evaluation of an ad hoc model of detection physics for navigated beta-probe surface imaging

Intraoperative surface imaging with navigated beta-probes has been shown to be a possibility to enable control of tumor resection borders. By employing ad hoc models of the detection physics the image quality can be improved. Our model computes the amount of radiation from a single point source that reaches the detector, with the solid angle subtended by the detector on the source, assuming perfect shielding. The sensitivity of the detector to the source due to the angle between the detector axis and the source-to-detector vector is also considered. A set of experiments was performed with three sources (two 10x10mm2 and one 20x10mm2 pieces of cellulose saturated with FDG) on a plate as phantom. Five sets of measurements were taken, three of them at a distance of 10mm from the plate und two at 30mm. At both distances one measurement set was taken in a random manner and the other ones systematically covering the whole area. The same experiments were simulated with our model and the GATE simulation framework. The resulting measurements from the experiments and simulations were then used to perform a reconstruction of the sources. The real measurements were compared to those simulated with our model and GATE, with a mean NCC of 80.64% for our model and 70.14% for GATE. In the reconstructions of the real measurements the sources were visually quite well separated, however the reconstructions of the measurements simulated by the model show that there is still room for further improvement.

Freehand tomographic nuclear imaging using tracked high-energy gamma probes

Systems allowing freehand SPECT imaging inside the operating room have been introduced previously. In this work, we aim to take one step further and enable 3D freehand imaging using positron emitting radio-traces such as [18F]FDG. Our system combines a high-energy gamma probe with an optical tracking system. Detection of the 511 keV annihilation gammas from positron-emitting radio-tracers is modeled analytically. The algorithm iteratively reconstructs the radioactivity distribution within a localized volume of interest. Based on the PET/CT data of 7 patients with tumors and lymph node metastases in the head and neck region, we build a neck phantom with [18F]FDG-filled reservoirs representing tumors and lymph nodes. Using this phantom, we investigate the limitations and capabilities of our method. Finally, we discuss possible improvements and requirements needed so that our approach becomes clinically applicable.