Virtual radiographs computed from TACT volume data as a gold standard for image registration prior to subtraction.

OBJECTIVE: To develop a three-dimensional (3D) model for quantitative analysis of image subtraction methods simulating clinical conditions and relevant to dental radiology. METHOD: A high-resolution volume representation of a formalin-preserved segment of a human maxilla was synthesized from a set of 51 digital radiographs equidistantly covering the entire sampling aperture by means of Tuned-Aperture Computed Tomography (TACT). Two-dimensional (2D) projection renderings of a 3D model were generated yielding arbitrary but well-known 2D projections with, and without, structured noise producing 'virtual radiographs'. RESULTS: Virtual radiographs were found to be similar to actual clinical images with respect to appearance, structure, and texture. Because the TACT reconstruction process allows all possible positions and orientations of source, specimen, and image plane to be simulated with negligible under sampling over a reasonable range of solid angles (sampling aperture), the resulting 3D model provided a rigorous method for establishing a truly objective gold standard (ground truth) for testing different registration techniques. CONCLUSIONS: TACT image registration can be assessed quantitatively by comparing actually observed vs theoretically professed parameters that presumably constrain the underlying projection geometries. Other attributes that vary from one method to the next, such as the use of nonlinear or region-specific techniques to facilitate registration, likewise, now can be rigorously measured by context-based methods such as quantitative determination of image similarity. Hence, a 3D model that renders idealized virtual radiographs from any desired projection geometry makes possible truly objective comparison of various digital subtraction techniques.

Emission tuned-aperture computed tomography: a novel approach to scintimammography.

UNLABELLED: Emission tuned-aperture computed tomography (ETACT) is a new approach to acquiring and processing scintimammography data. A gamma camera with a pinhole collimator is used to acquire projections of the radionuclide distribution within the breast. Fiducial markers are used to reconstruct these projections into tomographic slices. Simulation and phantom experiments were performed to evaluate the potential of the ETACT method. METHODS: In the simulation study, a hemispheric object of 15 cm in diameter was constructed to model a breast. A ray-tracing technique was used to generate ideal projections. These were blurred and noise was added to create images that resemble scintigraphic images. Tumor size, pinhole size, and target-to-nontarget radioactivity ratios (TNTs) were varied. The simulated projections were reconstructed into slices, and contrast and contrast-to-noise ratios were calculated to evaluate the effect of pinhole size. These results were compared with a simulated planar acquisition of the same object. A preliminary phantom evaluation was performed using an 8-mm "tumor" with a 10:1 TNT to validate the simulation results. RESULTS: A 3-mm pinhole was shown by the simulation study to be the optimal size. The ETACT images consistently yielded higher contrast than simulated planar images. The phantom study validated the simulation results and showed the feasibility of ETACT in a simulated clinical environment. CONCLUSION: ETACT is shown to be useful for imaging tumors <1 cm in diameter. Because ETACT requires only a gamma camera with a pinhole collimator, it has the potential to be applied in any hospital in a simple, flexible, and practical manner.

Stereotactic localization of breast lesions: how it works and methods to improve accuracy.

A computer simulation of stereotactic breast biopsy was developed that paralleled the geometric configuration of a currently available breast biopsy system. This model was developed to define and improve the targeting of breast lesions with stereotactic biopsy techniques. Lesions must be clearly identified and accurately targeted on both views for successful localization. Nonvisualization of a lesion may result from overlying tissue or from the geometric configuration of the imaging system. Familiarity with the geometric configuration of the biopsy unit, especially the location of the reference point and center of rotation, facilitates understanding of apparent changes in lesion position (parallax shift). Inaccuracy in lesion targeting on one or both views will manifest predominantly as an error in the calculated z value (depth). The magnitude and direction of this error are largely determined by the direction of the targeting error. Compensatory strategies include use of a long-throw core biopsy gun or directional vacuum-assisted biopsy device and additional sampling along the z axis and should be accompanied by critical evaluation of both pre- and postfire images. Understanding geometric considerations as well as how targeting accuracy affects accuracy in lesion localization should lead to greater success in sampling even challenging breast lesions at stereotactic biopsy.

Comparison and evaluation of retrospective intermodality brain image registration techniques.

PURPOSE: The primary objective of this study is to perform a blinded evaluation of a group of retrospective image registration techniques using as a gold standard a prospective, marker-based registration method. To ensure blindedness, all retrospective registrations were performed by participants who had no knowledge of the gold standard results until after their results had been submitted. A secondary goal of the project is to evaluate the importance of correcting geometrical distortion in MR images by comparing the retrospective registration error in the rectified images, i.e., those that have had the distortion correction applied, with that of the same images before rectification. METHOD: Image volumes of three modalities (CT, MR, and PET) were obtained from patients undergoing neurosurgery at Vanderbilt University Medical Center on whom bone-implanted fiducial markers were mounted. These volumes had all traces of the markers removed and were provided via the Internet to project collaborators outside Vanderbilt, who then performed retrospective registrations on the volumes, calculating transformations from CT to MR and/ or from PET to MR. These investigators communicated their transformations again via the Internet to Vanderbilt, where the accuracy of each registration was evaluated. In this evaluation, the accuracy is measured at multiple volumes of interest (VOIs), i.e., areas in the brain that would commonly be areas of neurological interest. A VOI is defined in the MR image and its centroid c is determined. Then, the prospective registration is used to obtain the corresponding point c' in CT or PET. To this point, the retrospective registration is then applied, producing c" in MR. Statistics are gathered on the target registration error (TRE), which is the distance between the original point c and its corresponding point c". RESULTS: This article presents statistics on the TRE calculated for each registration technique in this study and provides a brief description of each technique and an estimate of both preparation and execution time needed to perform the registration. CONCLUSION: Our results indicate that retrospective techniques have the potential to produce satisfactory results much of the time, but that visual inspection is necessary to guard against large errors.

MR geometric distortion correction for improved frame-based stereotaxic target localization accuracy.

We present a method to correct the geometric distortion caused by field inhomogeneity in MR images of patients wearing MR-compatible stereotaxic frames. Our previously published distortion correction method derives patient-dependent error maps by computing the phase-difference of 3D images acquired at different TEs. The time difference (delta TE = 4.9 ms at 1.5 T) is chosen such that the water and fat signals are in phase. However, delta TE is long enough to permit phase wraps in the difference images for frequency offsets greater than 205 Hz. Phase unwrapping techniques resolve these only for connected structures; therefore, the phase difference for fiducial rods may be off by multiples of 2 pi relative to the head. We remove this uncertainty by using an additional single 2D phase-different image with delta TE = 1 ms (during which time no phase-wraps are typically expected) to determine the correct multiple of 2 pi for each rod. We tested our method in a cadaver and in a patient using CT as the gold standard. Targets in the frame coordinates were chosen from CT and compared with their locations in MR. Localizing errors using MR compared with CT were as large as 3.7 mm before correction and were reduced to less than 1.11 mm after correction.

Registration error quantification of a surface-based multimodality image fusion system.

This paper presents a new reference data set and associated quantification methodology to assess the accuracy of registration of computerized tomography (CT) and magnetic-resonance (MR) images. Also described is a new semiautomatic surface-based system for registering and visualizing CT and MR images. The registration error of the system was determined using a reference data set that was obtained from a cadaver in which rigid fiducial tubes were inserted prior to imaging. Registration error was measured as the distance between an analytic expression for each fiducial tube in one image set and transformed samples of the corresponding tube obtained from the other. Registration was accomplished by first identifying surfaces of similar anatomic structures in each image set. A transformation that best registered these structures was determined using a nonlinear optimization procedure. Even though the root-mean-square (rms) distance at the registered surfaces was similar to that reported by other groups, it was found that rms distances for the tubes were significantly larger than the final rms distances between the registered surfaces. It was also found that minimizing rms distance at the surface did not minimize rms distance for the tubes.

A versatile system for multimodality image fusion.

This paper presents a versatile system for registering and visualizing computed tomography and magnetic resonance images. The system utilizes a semi-automatic, surface-based registration strategy which has proven useful for registering a number of different anatomical structures. A triangular mesh approximates surfaces in one image set while a set of surface points is used as a surface approximation in the other set. A non-linear optimization procedure determines the transformation that minimizes the total sum-squared perpendicular distance between triangles of the mesh and surface points. This system has been used without modification to successfully register images of the brain, spine and calcaneus.

Method for correcting magnetic resonance image distortion for frame-based stereotactic surgery, with preliminary results.

We previously described a technique for correcting patient-specific magnetic field inhomogeneity spatial distortion in magnetic resonance images (MRI), which was not applicable to patients fitted with MRI-compatible stereotactic fiducial frames. Here we describe an improvement to the technique that permits application for these patients. Measurements with a cadaver head show that this method achieves MRI stereotactic localization accuracy of 1 mm.