A comparison of similarity measures for use in 2-D-3-D medical image registration.

A comparison of six similarity measures for use in intensity-based two-dimensional-three-dimensional (2-D-3-D) image registration is presented. The accuracy of the similarity measures are compared to a "gold-standard" registration which has been accurately calculated using fiducial markers. The similarity measures are used to register a computed tomography (CT) scan of a spine phantom to a fluoroscopy image of the phantom. The registration is carried out within a region-of-interest in the fluoroscopy image which is user defined to contain a single vertebra. Many of the problems involved in this type of registration are caused by features which were not modeled by a phantom image alone. More realistic "gold-standard" data sets were simulated using the phantom image with clinical image features overlaid. Results show that the introduction of soft-tissue structures and interventional instruments into the phantom image can have a large effect on the performance of some similarity measures previously applied to 2-D-3-D image registration. Two measures were able to register accurately and robustly even when soft-tissue structures and interventional instruments were present as differences between the images. These measures were pattern intensity and gradient difference. Their registration accuracy, for all the rigid-body parameters except for the source to film translation, was within a root-mean-square (rms) error of 0.54 mm or degrees to the "gold-standard" values. No failures occurred while registering using these measures.

The EASI project--improving the effectiveness and quality of image-guided surgery.

In recent years, advances in computer technology and a significant increase in the accuracy of medical imaging have made it possible to develop systems that can assist the clinician in diagnosis, planning, and treatment. This paper deals with an area that is generally referred to as computer-assisted surgery, image-directed surgery, or image-guided surgery. We report the research, development, and clinical validation performed since January 1996 in the European Applications in Surgical Interventions (EASI) project, which is funded by the European Commission in their "4th Framework Telematics Applications for Health" program. The goal of this project is the improvement of the effectiveness and quality of image-guided neurosurgery of the brain and image-guided vascular surgery of abdominal aortic aneurysms, while at the same time reducing patient risks and overall cost. We have developed advanced prototype systems for preoperative surgical planning and intraoperative surgical navigation, and we have extensively clinically validated these systems. The prototype systems and the clinical validation results are described in this paper.

Voxel-based 2-D/3-D registration of fluoroscopy images and CT scans for image-guided surgery.

Registration of intraoperative fluoroscopy images with preoperative three-dimensional (3-D) CT images can be used for several purposes in image-guided surgery. On the one hand, it can be used to display the position of surgical instruments, which are being tracked by a localizer, in the preoperative CT scan. On the other hand, the registration result can be used to project preoperative planning information or important anatomical structures visible in the CT image onto the fluoroscopy image. For this registration task, a novel voxel-based method in combination with a new similarity measure (pattern intensity) has been developed. The basic concept of the method is explained at the example of two-dimensional (2-D)/3-D registration of a vertebra in an X-ray fluoroscopy image with a 3-D CT image. The registration method is described, and the results for a spine phantom are presented and discussed. Registration has been carried out repeatedly with different starting estimates to study the capture range. Information about registration accuracy has been obtained by comparing the registration results with a highly accurate "ground-truth" registration, which has been derived from fiducial markers attached to the phantom prior to imaging. In addition, registration results for different vertebrae have been compared. The results show that the rotation parameters and the shifts parallel to the projection plane can accurately be determined from a single projection. Because of the projection geometry, the accuracy of the height above the projection plane is significantly lower.

A randomized controlled trial of a computer-based physician workstation in an outpatient setting: implementation barriers to outcome evaluation.

OBJECTIVE: A research prototype Physician Workstation (PWS) incorporating a graphical user interface and a drug ordering module was compared with the existing hospital information system in an academic Veterans Administration General Medical Clinic. Physicians in the intervention group received recommendations for drug substitutions to reduce costs and were alerted to potential drug interactions. The objective was to evaluate the effect of the PWS on user satisfaction, on health-related outcomes, and on costs. DESIGN: A one-year, two-period, randomized controlled trial with 37 subjects. MEASUREMENTS: Differences in the reliance on noncomputer sources of information, in user satisfaction, in the cost of prescribed medications, and in the rate of clinically relevant drug interactions were assessed. RESULTS: The study subjects logged onto the workstation an average of 6.53 times per provider and used it to generate 2.8% of prescriptions during the intervention period. On a five-point scale (5 = very satisfied, 1 = very dissatisfied), user satisfaction declined in the PWS group (3.44 to 2.98 p = 0.008), and increased in the control group (3.23 to 3.72, p < 0.0001). CONCLUSION: The intervention physicians did not use the PWS frequently enough to influence information-seeking behavior, health outcomes, or cost. The study design did not determine whether the poor usage resulted from satisfaction with the control system, problems using the PWS intervention, or the functions provided by the PWS intervention. Evaluative studies should include provisions to improve the chance of successful implementation as well as to yield maximum information if a negative study occurs.

A randomized evaluation of a computer-based physician's workstation: design considerations and baseline results.

We are performing a randomized, controlled trial of a Physician's Workstation (PWS), an ambulatory care information system, developed for use in the General Medical Clinic (GMC) of the Palo Alto VA. Goals for the project include selecting appropriate outcome variables and developing a statistically powerful experimental design with a limited number of subjects. As PWS provides real-time drug-ordering advice, we retrospectively examined drug costs and drug-drug interactions in order to select outcome variables sensitive to our short-term intervention as well as to estimate the statistical efficiency of alternative design possibilities. Drug cost data revealed the mean daily cost per physician per patient was 99.3 cents +/- 13.4 cents, with a range from 0.77 cent to 1.37 cents. The rate of major interactions per prescription for each physician was 2.9% +/- 1%, with a range from 1.5% to 4.8%. Based on these baseline analyses, we selected a two-period parallel design for the evaluation, which maximized statistical power while minimizing sources of bias.

Measured attenuation correction using the Maximum Likelihood algorithm.

Quantitative determination of local radioactivity concentration in positron emission tomography (PET) requires a good attenuation correction procedure to reconstruct the emission image. Using a similar Maximum Likelihood (ML) algorithm as for the reconstruction of the emission image, a method is proposed to reconstruct a transmission image, i.e. a map of absorption coefficients. This reconstructed transmission image is then used to calculate the attenuation correction factors needed for the ML reconstruction of the emission image. This approach takes automatically into account the convolution step in the acquisition process (caused by various smoothing factors, e.g. the detector width). This results in appreciable noise suppression without loss of resolution due to filtering, thus making the reconstructed images easier to interpret. A comparison is made with other estimates for the measured attenuation correction using phantom studies.