Assessment of Hepatic Motion Secondary to Respiration for Computer Assisted Interventions

Helical tomotherapy for single and multiple liver tumours

Purpose

Dosimetric evaluations of single and multiple liver tumours performed using intensity-modulated helical tomotherapy (HT) were quantitatively investigated. Step-and-shoot intensity-modulated radiotherapy (SaS-IMRT) was used as a benchmark.

Methods

Sixteen patients separated into two groups with primary hepatocellular carcinomas or metastatic liver tumours previously treated using SaS-IMRT were examined and re-planned by HT. The dosimetric indices used included the conformity index (CI) and homogeneity index (HI) for the planned target volume (PTV), max/mean dose, quality index (QI), normal tissue complication probability (NTCP), V_{30 Gy}, and V_50% for the specified organs at risk (OARs). The monitor units per fraction (MU/fr) and delivery time were also analysed.

Results

For the single tumour group, both planning systems satisfied the required PTV prescription, but no statistical significance was shown by the indexes checking. A shorter delivery time and lower MU/fr value were achieved by the SaS-IMRT. For the group of multiple tumours, the average improvement in CI and HI was 14% and 4% for HT versus SaS-IMRT, respectively. Lower V_50%, V_{30 Gy} and QI values were found, indicating a significant dosimetric gain in HT. The NTCP value of the normal liver was 20.27 ± 13.29% for SaS-IMRT and 2.38 ± 2.25% for HT, indicating fewer tissue complications following HT. The latter also required a shorter delivery time.

Conclusions

Our study suggests dosimetric benefits of HT over SaS-IMRT plans in the case of multiple liver tumours, especially with regards sparing of OARs. No significant dosimetric difference was revealed in the case of single liver tumour, but SaS-IMRT showed better efficiency in terms of MU/fr and delivery time.

Real-time MR thermometry for monitoring HIFU ablations of the liver

A high-resolution and high-speed pulse sequence is presented for monitoring high-intensity focused ultrasound ablations in the liver in the presence of motion. The sequence utilizes polynomial-order phase saturation bands to perform outer volume suppression, followed by spatial-spectral excitation and three readout segmented echo-planar imaging interleaves. Images are processed with referenceless thermometry to create temperature-rise images every frame. The sequence and reconstruction were implemented in RTHawk and used to image stationary and moving sonications in a polyacrylamide gel phantom (62.4 acoustic W, 50 sec, 550 kHz). Temperature-rise images were compared between moving and stationary experiments. Heating spots and corresponding temperature-rise plots matched very well. The stationary sonication had a temperature standard deviation of 0.15 degrees C compared to values of 0.28 degrees C and 0.43 degrees C measured for two manually moved sonications at different velocities. Moving the phantom (while not heating) with respect to the transducer did not cause false temperature rises, despite susceptibility changes. The system was tested on nonheated livers of five normal volunteers. The mean temperature rise was -0.05 degrees C, with a standard deviation of 1.48 degrees C. This standard deviation is acceptable for monitoring high-intensity focused ultrasound ablations, suggesting real-time imaging of moving high-intensity focused ultrasound sonications can be clinically possible.

Role of image-guidance systems during NOTES

Natural orifice translumenal endoscopic surgery (NOTES) is a developing field with the potential to revolutionize our approach to abdominal surgery. Performing operations via a flexible endoscope introduced through a natural orifice presents several challenges to physicians. Orientation and interpretation of the endoscopic video image can be difficult. The surgeon must also learn to operate with the camera and instruments "in line." Advances in technology are currently addressing the challenges of NOTES. Image-guided navigation could potentially provide invaluable assistance during NOTES. Real-time information on spatial positioning and orientation as well as assistance with the identification of anatomy and localization of pathology are some of the possibilities. Image-guided surgery has become commonplace in disciplines such as neurosurgery where the anatomy is relatively rigid. To become widespread in intra-abdominal procedures and NOTES, advances that will allow systems to adapt to moving and deforming anatomy are needed. This article reviews the basics of image-guided surgery, the various image-guided systems, and their potential application to NOTES.

Vessel target location estimation during the TIPS procedure

Creation of a transjugular intrahepatic portosystemic shunt (TIPS) requires passage of a needle toward a moving target that is only seen transiently by X-ray prior to needle passage. Intraoperative, 3D target localization would facilitate target access and improve the safety of the procedure. The clinical assumption is that patients undergoing the TIPS procedure possess rigid, cirrhotic livers that undergo only intraoperative translation without significant deformation or rotation. Based upon this assumption, we hypothesize that the position of any unseen, 3D target point within the liver can be determined intraoperatively by precalculation of the relative positions of the target point to a different 3D point that can be tracked intraoperatively. This paper examines this hypothesis using intraoperatively acquired, biplane, X-ray images of seven patients. In six, we tracked the effects of cardiac and respiratory motion, and in three the effects of needle pressure. Methods involved reconstruction of 3D vessel bifurcation and other trackable intrahepatic points from biplane angiograms, measurement of liver deformation by examining changing distances between these 3D points over time, and comparison of expected to actual displacements of these points with respect to a fixed reference point in the liver. We conclude that, for the rigid livers associated with patients undergoing TIPS, that there is less intraoperative deformation than previously reported by other groups addressing healthy liver deformation, and that the location of an unseen target can be predicted within 3mm accuracy.

On combining internal and external fiducials for liver motion compensation

This paper presents an in-vivo accuracy study on combining skin markers (external fiducials) and fiducial needles (internal fiducials) for motion compensation during liver interventions. We compared the target registration error (TRE) for different numbers of skin markers n(s) and fiducial needles n(f), as well as for different transformation types, in two swine using the tip of an additional tracked needle as the target. During continuous breathing, n(f) had the greatest effect on the accuracy, yielding mean root mean square (RMS) errors of 4.8 +/- 1.1 mm (n(f) = 0), 2.0 +/- 0.9 mm (n(f) = 1) and 1.7 +/- 0.8 mm (n(f) = 2) when averaged over multiple tool arrangements (n = 18, 36, 18) with n(s) = 4. These values correspond to error reductions of 11%, 64% and 70%, respectively, compared to the case when no motion compensation is performed, i.e., when the target position is assumed to be constant. At expiration, the mean RMS error ranged from 1.1 mm (n(f) = 0) to 0.8 mm (n(f) = 2), which is of the order of magnitude of the target displacement. Our study further indicates that the fiducial registration error (FRE) of a rigid transformation reflecting tissue motion generally correlates strongly with the TRE. Our findings could be used in practice to (1) decide on a suitable combination of fiducials for a given intervention, considering the trade-off between high accuracy and low invasiveness, and (2) provide an intra-interventional measure of confidence for the accuracy of the system based on the FRE.

Quantification of missing and overlapping data in multiple breath hold abdominal imaging

Magnetic resonance imaging (MRI) of the abdomen is often performed in multiple breath holds which are designed to contiguously cover the region of interest. This technique may result in a failure to image all the appropriate area, and the extent of this failure is difficult to appreciate on a set of 2D slices. With reference to three patient cases, we present a method to quantify the extent of this problem and suggest a solution. First, we manually delineate the region of interest on a single breath hold fast spoiled gradient echo (FSPGR) sequence. Subsequently, we align images acquired in separate breath holds to this reference volume. A coloured 3D presentation makes the extent of unimaged and repeatedly imaged areas clearly visible to the clinician. The alignment also helps radiologists to accurately determine the location of individual slices. The described method can easily be automated and is ideally implemented at the scanner console, ensuring the availability of contiguously sampled datasets to radiologists with minimum user interaction from the radiographer. Such datasets enable the deployment of robust 3D analysis algorithms.

Respiratory biofeedback during CT-guided procedures

PURPOSE

Respiratory motion can be a complicating factor during image-guided interventions. The ability to reproduce breath-holds may facilitate safer needle-based procedures. The purpose of this study was to evaluate if respiratory biofeedback decreased variability among breath-holds and if the signals from the respiratory bellows belt can be used to measure target motion.

MATERIALS AND METHODS

In phase 1 of the study, a respiratory bellows belt was applied to patients before image-guided interventional procedures. Belt stretch from respiratory motion was converted into voltage readings and displayed on a monitor as biofeedback. Patients were asked to perform inspiratory, expiratory, and midcycle breath-holds with and without the biofeedback. The variability in voltage readings between breath-holds with and without biofeedback was compared. In phase 2, the respiratory bellows belt was used during computed tomography (CT)-guided procedures with the patients blinded to the biofeedback. Voltage readings and CT series numbers were recorded as patients were asked to hold their breath during scans. The variability of CT z-axis targets was compared with the variability of voltage readings.

RESULTS

A significant decrease in variability was found during expiratory breath-holds (P = .0083) with trends toward significance with midcycle and inspiratory breath-holds. A positive correlation (Kendall tau = 0.5; P = .024) was shown between CT z-axis and belt stretch variability in subjects who received smaller doses of moderate sedation compared with those who received larger doses or general anesthesia.

CONCLUSIONS

Biofeedback may help the patient to have a more consistent breath-hold. The belt could decrease the error and unpredictability from craniocaudal motion of targets during image-guided interventions.

Concepts and preliminary data toward the realization of image-guided liver surgery

Image-guided surgery provides navigational assistance to the surgeon by displaying the surgical probe position on a set of preoperative tomograms in real time. In this study, the feasibility of implementing image-guided surgery concepts into liver surgery was examined during eight hepatic resection procedures. Preoperative tomographic image data were acquired and processed. Accompanying intraoperative data on liver shape and position were obtained through optically tracked probes and laser range scanning technology. The preoperative and intraoperative representations of the liver surface were aligned using the iterative closest point surface matching algorithm. Surface registrations resulted in mean residual errors from 2 to 6 mm, with errors of target surface regions being below a stated goal of 1 cm. Issues affecting registration accuracy include liver motion due to respiration, the quality of the intraoperative surface data, and intraoperative organ deformation. Respiratory motion was quantified during the procedures as cyclical, primarily along the cranial-caudal direction. The resulting registrations were more robust and accurate when using laser range scanning to rapidly acquire thousands of points on the liver surface and when capturing unique geometric regions on the liver surface, such as the inferior edge. Finally, finite element models recovered much of the observed intraoperative deformation, further decreasing errors in the registration. Image-guided liver surgery has shown the potential to provide surgeons with important navigation aids that could increase the accuracy of targeting lesions and the number of patients eligible for surgical resection.

A new registration/visualization paradigm for CT-fluoroscopy guided RF liver ablation

2D-3D slice-to-volume registration for abdominal organs like liver is difficult due to the breathing motion and tissue deformation. The purpose of our approach is to ease CT-fluoroscopy (CT-fluoro) based needle insertion for the Radiofrequency Liver Ablation procedure using high resolution contrasted preoperative data. In this case, low signal-to-noise ratio, absence of contrast and additional presence of needle in CT-fluoro makes it difficult to guarantee the solution of any deformable slice-to-volume registration algorithm. In this paper, we first propose a method for creating a set of ground truth (GT) simulation data based on a non-linear deformation of the CT-fluoro volume obtained from real patients. Second, we split the CT-fluoro image and apply intensity based rigid and affine registration to each section. We then propose a novel solution, which consists of intuitive visualization sequences of optimal sub-volumes of preinterventional data based on the registration results. Experiments on synthetic and real patient data and direct feedback of two interventionalists validate our alternative approach.

Electromagnetic tracking for abdominal interventions in computer aided surgery

Electromagnetic tracking has great potential for assisting physicians in precision placement of instruments during minimally invasive interventions in the abdomen, since electromagnetic tracking is not limited by the line-of-sight restrictions of optical tracking. A new generation of electromagnetic tracking has recently become available, with sensors small enough to be included in the tips of instruments. To fully exploit the potential of this technology, our research group has been developing a computer aided, image-guided system that uses electromagnetic tracking for visualization of the internal anatomy during abdominal interventions. As registration is a critical component in developing an accurate image-guided system, we present three registration techniques: 1) enhanced paired-point registration (time-stamp match registration and dynamic registration); 2) orientation-based registration; and 3) needle shape-based registration. Respiration compensation is another important issue, particularly in the abdomen, where respiratory motion can make precise targeting difficult. To address this problem, we propose reference tracking and affine transformation methods. Finally, we present our prototype navigation system, which integrates the registration, segmentation, path-planning and navigation functions to provide real-time image guidance in the clinical environment. The methods presented here have been tested with a respiratory phantom specially designed by our group and in swine animal studies under approved protocols. Based on these tests, we conclude that our system can provide quick and accurate localization of tracked instruments in abdominal interventions, and that it offers a user-friendly display for the physician.

An Augmented Reality System for MR Image–guided Needle Biopsy: Initial Results in a Swine Model

PURPOSE

To evaluate an augmented reality (AR) system in combination with a 1.5-T closed-bore magnetic resonance (MR) imager as a navigation tool for needle biopsies.

MATERIALS AND METHODS

The experimental protocol had institutional animal care and use committee approval. Seventy biopsies were performed in phantoms by using 20 tube targets, each with a diameter of 6 mm, and 50 virtual targets. The position of the needle tip in AR and MR space was compared in multiple imaging planes, and virtual and real needle tip localization errors were calculated. Ten AR-guided biopsies were performed in three pigs, and the duration of each procedure was determined. After successful puncture, the distance to the target was measured on MR images. The confidence limits for the achieved in-plane hit rate and for lateral deviation were calculated. A repeated measures analysis of variance was used to determine whether the placement error in a particular dimension (x, y, or z) differed from the others.

RESULTS

For the 50 virtual targets, a mean error of 1.1 mm +/- 0.5 (standard deviation) was calculated. A repeated measures analysis of variance indicated no statistically significant difference (P > .99) in the errors in any particular orientation. For the real targets, all punctures were inside the 6-mm-diameter tube in the transverse plane. The needle depth was within the target plane in 11 biopsy procedures; the mean distance to the center of the target was 2.55 mm (95% confidence interval: 1.77 mm, 3.34 mm). For nine biopsy procedures, the needle tip was outside the target plane, with a mean distance to the edge of the target plane of 1.5 mm (range, 0.07-3.46 mm). In the animal experiments, the puncture was successful in all 10 cases, with a mean target-needle distance of 9.6 mm +/- 4.85. The average procedure time was 18 minutes per puncture.

CONCLUSION

Biopsy procedures performed with a combination of a closed-bore MR system and an AR system are feasible and accurate.

Robot-Assisted Biopsy Using Computed Tomography-Guidance

PURPOSE

We sought to develop a robotic system for computed tomography (CT)-guided biopsy to validate the feasibility, accuracy, and efficacy of the system using phantom tests.

MATERIALS AND METHODS

Ten peas (mean diameter 9.9+/-0.4 mm) embedded within a gel phantom were selected for biopsy. Once the best access was defined on CT images, the position of the phantom was recorded by an optical tracking system. Positional data about the phantom and the corresponding CT image was transferred to the robot planning system (Linux-based industrial PC equipped with video capture card). Once the appropriate position, angulation, and pitch were calculated, the robotic arm moved automatically with 7 degrees-of-freedom to the planned insertion path, aiming the needle-trajectory at the center of the target. Then, the biopsy was performed manually using a coaxial technique. The length of all harvested specimens was measured and short cut pieces of a guidewire were pushed into the target to evaluate the deviation of the actual needle track from the target.

RESULTS

In all targets, biopsy specimens (mean length 5.6+/-1.4 mm) were harvested with only 1 needle pass required. The mean deviation of the needle tip from the center of the target in the x and z axes was 1.2+/-0.9 mm and 0.6+/-0.4 mm, respectively.

CONCLUSIONS

Robotic-assisted biopsies in vitro, using CT guidance, were feasible and provided high accuracy.

A Complete Augmented Reality Guidance System for Liver Punctures: First Clinical Evaluation

We provided in an augmented reality guidance system for liver punctures, which has been validated on a static abdominal phantom. In this paper, we report the first in vivo experiments. We developed a strictly passive protocol to directly evaluate our system on patients. We show that the system algorithms work efficiently and we highlight the clinical constraints that we had to overcome (small operative field, weight and sterility of the tracked marker attached to the needle...). Finally, we investigate to what extent breathing motion can be neglected for free breathing patient. Results show that the guiding accuracy, close to 1 cm, is sufficient for large targets only (above 3 cm of diameter) when the breathing motion is neglected. In the near future, we aim at validating our system on smaller targets using a respiratory gating technique.

Robot-assisted biopsy using ultrasound guidance: initial results from in vitro tests

The purpose of this study was to develop a robotic system for ultrasound (US)-guided biopsy and to validate the feasibility, accuracy and efficacy using phantom tests. Twenty peas (mean diameter 9.3+/-0.1 mm) embedded within a gel-phantom were selected for biopsy. Once the best access was defined, the position of the US transducer was recorded by an optical tracking system. Positional data of the transducer and the corresponding US image were transferred to the roboter planning system (LINUX-based industrial PC equipped with video capture card). Once the appropriate position, angulation and pitch were calculated, the robotic arm moved automatically with seven degrees-of-freedom to the planned insertion path, aiming the needle-positioning unit at the center of the target. Then, the biopsy was performed manually using a coaxial technique. The length of all harvested specimens was measured, and the deviation of the actual needle tract from the center of the target was evaluated sonographically. In all targets, the biopsy specimen (mean length 5+/-1.2 mm) was harvested with only one needle pass required The mean deviation of the needle tip from the center of the target was 1.1+/-0.8 mm. Robotic assisted biopsies in-vitro using US-guidance were feasible and provided high accuracy.