Electromagnetic organ tracking allows for real-time compensation of tissue shift in image-guided laparoscopic rectal surgery: results of a phantom study.

BACKGROUND: Laparoscopic resection is a minimally invasive treatment option for rectal cancer but requires highly experienced surgeons. Computer-aided technologies could help to improve safety and efficiency by visualizing risk structures during the procedure. The prerequisite for such an image guidance system is reliable intraoperative information on iatrogenic tissue shift. This could be achieved by intraoperative imaging, which is rarely available. Thus, the aim of the present study was to develop and validate a method for real-time deformation compensation using preoperative imaging and intraoperative electromagnetic tracking (EMT) of the rectum. METHODS: Three models were compared and evaluated for the compensation of tissue deformation. For model A, no compensation was performed. Model B moved the corresponding points rigidly to the motion of the EMT sensor. Model C used five nested linear regressions with increasing level of complexity to compute the deformation (C1-C5). For evaluation, 14 targets and an EMT organ sensor were fit into a silicone-molded rectum of the OpenHELP phantom. Following a computed tomography, the image guidance was initiated and the rectum was deformed in the same way as during surgery in a total of 14 experimental runs. The target registration error (TRE) was measured for all targets in different positions of the rectum. RESULTS: The mean TRE without correction (model A) was 32.8 ± 20.8 mm, with only 19.6% of the measurements below 10 mm (80.4% above 10 mm). With correction, the mean TRE could be reduced using the rigid correction (model B) to 6.8 ± 4.8 mm with 78.7% of the measurements being <10 mm. Using the most complex linear regression correction (model C5), the error could be reduced to 2.9 ± 1.4 mm with 99.8% being below 10 mm. CONCLUSION: In laparoscopic rectal surgery, the combination of electromagnetic organ tracking and preoperative imaging is a promising approach to compensating for intraoperative tissue shift in real-time.

Comparative validation of single-shot optical techniques for laparoscopic 3-D surface reconstruction.

Intra-operative imaging techniques for obtaining the shape and morphology of soft-tissue surfaces in vivo are a key enabling technology for advanced surgical systems. Different optical techniques for 3-D surface reconstruction in laparoscopy have been proposed, however, so far no quantitative and comparative validation has been performed. Furthermore, robustness of the methods to clinically important factors like smoke or bleeding has not yet been assessed. To address these issues, we have formed a joint international initiative with the aim of validating different state-of-the-art passive and active reconstruction methods in a comparative manner. In this comprehensive in vitro study, we investigated reconstruction accuracy using different organs with various shape and texture and also tested reconstruction robustness with respect to a number of factors like the pose of the endoscope as well as the amount of blood or smoke present in the scene. The study suggests complementary advantages of the different techniques with respect to accuracy, robustness, point density, hardware complexity and computation time. While reconstruction accuracy under ideal conditions was generally high, robustness is a remaining issue to be addressed. Future work should include sensor fusion and in vivo validation studies in a specific clinical context. To trigger further research in surface reconstruction, stereoscopic data of the study will be made publically available at www.open-CAS.com upon publication of the paper.

Optical techniques for 3D surface reconstruction in computer-assisted laparoscopic surgery.

One of the main challenges for computer-assisted surgery (CAS) is to determine the intra-operative morphology and motion of soft-tissues. This information is prerequisite to the registration of multi-modal patient-specific data for enhancing the surgeon's navigation capabilities by observing beyond exposed tissue surfaces and for providing intelligent control of robotic-assisted instruments. In minimally invasive surgery (MIS), optical techniques are an increasingly attractive approach for in vivo 3D reconstruction of the soft-tissue surface geometry. This paper reviews the state-of-the-art methods for optical intra-operative 3D reconstruction in laparoscopic surgery and discusses the technical challenges and future perspectives towards clinical translation. With the recent paradigm shift of surgical practice towards MIS and new developments in 3D optical imaging, this is a timely discussion about technologies that could facilitate complex CAS procedures in dynamic and deformable anatomical regions.

Optimizing cuff width for noninvasive measurement of blood pressure.

BACKGROUND: It is well established that indirect measurements of blood pressure made with a standard 13cm-wide cuff are erroneously high for large arms and low for small arms. To correct for this error, the American Heart Association recommends adjusting cuff width to 40% of the arm's circumference. OBJECTIVE: To test the validity of this method of correction. DESIGN: This study was a prospective, nonblinded, paired Student's t-test analysis. METHODS: Blood pressures in 50 subjects were measured directly by using a radial artery line and indirectly by the Korotkov method. For each subject multiple indirect measurements of blood pressure were made with the cuff width:arm circumference ratio varied from 30-55% in 5% increments. Error was defined as indirect blood pressure minus direct blood pressure. RESULTS: A ratio of 40% resulted in overestimation of blood pressure for most arms and with particularly high errors for small arms. The ratio producing zero mean error for the pooled study group was 46.4+/-0.7% (mean+/-SEM). Using this ratio of 46.4%, the error varied inversely with arm circumference (P<0.02), resulting in overestimation of systolic blood pressure for small arms and underestimation of systolic blood pressure for large arms. This error is comparable in magnitude, but opposite in sign, to that which occurs with a standard 13cm-wide cuff for all arms. The optimum ratio was found to be closely approximated by the relationship, cuff width=9.34 log(10) arm circumference. Using this relationship, error in systolic blood pressure was insensitive to arm circumference (r=0.04, P>0.05) and near zero. CONCLUSION: The optimum cuff width for the indirect measurement of blood pressure is not directly proportional to arm circumference, but is proportional to the logarithm of the arm's circumference.