Registration of MR and CT images for skull base surgery using point-like anatomical features

Registration of 3-D images using weighted geometrical features

The authors present a weighted geometrical feature (WGF) registration algorithm. Its efficacy is demonstrated by combining points and a surface. The technique is an extension of Besl and McKay's (1992) iterative closest point (ICP) algorithm. The authors use the WGF algorithm to register X-ray computed tomography (CT) and T2-weighted magnetic resonance (MR) volume head images acquired from eleven patients that underwent craniotomies in a neurosurgical clinical trial. Each patient had five external markers attached to transcutaneous posts screwed into the outer table of the skull. The authors define registration error as the distance between positions of corresponding markers that are not used for registration. The CT and MR images are registered using fiducial paints (marker positions) only, a surface only, and various weighted combinations of points and a surface. The CT surface is derived from contours corresponding to the inner surface of the skull. The MR surface is derived from contours corresponding to the cerebrospinal fluid (CSF)-dura interface. Registration using points and a surface is found to be significantly more accurate then registration using only points or a surface.

Assessment of deposition of inhaled aerosol in the respiratory tract of man using three-dimensional multimodality imaging and mathematical modeling

Multimodality medical imaging enables measurement of the three-dimensional spatial distribution of a radiolabeled aerosol within the lung. Using a conceptual spatial morphological model these data may be transformed to provide information on deposition per airway generation. This methodology has been used to study the intrapulmonary deposition patterns of two formulations of a metered dose inhaler and two nebulizers in control subjects. The nebulizer study has also been stimulated using a computer model of deposition. The comparison between derived experimental results and those from computer modeling shows areas of agreement, although there are also areas of discrepancy. The new methodology has considerable potential value in the fields of inhalation therapy and deposition modeling, although more detailed validation is still required.

Effects of coregistration of MR to CT images on MR stereotactic accuracy

Coregistration of different modality imaging serves to increase the ease and accuracy of stereotactic procedures. In many cases, magnetic resonance (MR) stereotaxis is supplanting computerized tomography (CT). The advantages of increased anatomical detail and multiplanar imaging afforded by MR, however, are offset by its potential inaccuracy as well as the more cumbersome and less available nature of its hardware. A system has been developed by one of the authors by which MR imaging can be performed separately without a stereotactic fiducial headring. Then, immediately prior to surgery, a stereotactic CT scan is obtained and software is used to coregister CT and MR images anatomically by matching cranial landmarks in the two scans. The authors examined this system in six patients as well as with the use of a lucite phantom. After initially coregistering CT and MR images, six separate anatomical (for the patients) and eight artificial (for the phantom) targets were compared. With coregistration, in comparison to CT fiducial scans, errors in each axis are less than or equal to 1 mm using the Cosman-Roberts-Wells system. In fact, the coregistered images are more accurate than MR fiducial images, in the anteroposterior (p = 0.001), lateral (p < 0.05), and vertical (p < 0.03) planes. Three-dimensional error was significantly less in the coregistered scans than the MR fiducial images (p < 0.005). The coregistration procedure therefore not only increases the case of MR stereotaxis but also increases its accuracy.

Computer planning of stereotactic iodine-125 seed brachytherapy for recurrent malignant gliomas

At St Thomas' Hospital, we have developed a computer program on a Titan graphics supercomputer to plan the stereotactic implantation of iodine-125 seeds for the palliative treatment of recurrent malignant gliomas. Use of the Gill-Thomas-Cosman relocatable frame allows planning and surgery to be carried out at different hospitals on different days. Stereotactic computed tomography (CT) and positron emission tomography (PET) scans are performed and the images transferred to the planning computer. The head, tumour and frame fiducials are outlined on the relevant images, and a three-dimensional model generated. Structures which could interfere with the surgery or radiotherapy, such as major vessels, shunt tubing etc., can also be outlined and included in the display. Catheter target and entry points are set using a three-dimensional cursor controlled by a set of dials attached to the computer. The program calculates and displays the radiation dose distribution within the target volume for various catheter and seed arrangements. The CT co-ordinates of the fiducial rods are used to convert catheter co-ordinates from CT space to frame space and to calculate the catheter insertion angles and depths. The surgically implanted catheters are after-loaded the next day and the seeds left in place for between 4 and 6 days, giving a nominal dose of 50 Gy to the edge of the target volume. 25 patients have been treated so far.

Frameless stereotaxy for pre-treatment planning and post-treatment evaluation of radiosurgery

In our centre, 111 patients have been treated with linear accelerator stereotactic radiosurgery. Angiographic, CT and MRI images are generated and the target coordinates calculated in 3 dimensions. For CT scanning, cross sections of perpendicular and oblique fiducial markers are seen. For follow-up CT scans done without the frame, a virtual frame is generated by means of a computer program that places fiducial markers on each CT scan cut, as if the patient had been wearing the OBT frame and the scan produced with the gantry parallel to the base of the frame. The position of the oblique marker may be calculated by knowing the thickness and position of each CT cut. Various natural fiducial markers (bony landmarks) are identified by coordinates in the scan with the patient wearing the real frame and in the scan with the virtual frame applied. A transformation matrix is utilized to establish the equivalence between the original CT scan with the real frame applied and subsequent scans without the real frame but with the virtual frame applied. In effect, the virtual frame is re-applied in exactly the same position as the real frame. Lesion measurements may then be duplicated and growth or regression accurately established. The uncertainty in this system of re-application residues in possible patient movement, CT scan slice thickness and inter-observer error in the identification of natural fiducial markers.

Combined and three-dimensional rendered multimodal data for planning cranial base surgery: a prospective evaluation

Magnetic resonance (MR), X-ray computed tomography (CT), and angiographic images best depict soft tissue, bone, and blood vessels respectively. No one on its own is sufficient in the preoperative assessment of cranial base lesions. We have developed and evaluated a computational technique for the three-dimensional (3D) combination and display of multimodality images for planning cranial base surgery. This evaluation was prospective and performed in such a way that the results could be quantified. Eight patients (three acoustic neuromas, four subfrontal and suprasellar meningiomas, and one petrous apex meningioma) underwent MR, CT, and MR angiographic investigations. These images were registered with anatomical landmarks rather than an external frame. Two techniques were used to display the resulting combined images: multiple slices in which bone from CT was overlaid on soft tissue from registered MR and pseudo-3D-rendered movie sequences showing bone from CT, lesions and optic nerves from MR, and blood vessels from MR angiography. The advantages of the combined displays compared with those of conventional methods of viewing were assessed prospectively by the operating surgeon and by an independent surgeon, and the results were compared with operative findings. The preoperative assessment showed a significant improvement (P < 0.05, sign test) in the depiction of both individual structures (lesion and bone from overlaid slices and lesion and vasculature from 3D-rendered displays) and structural relationships (tumor-bone relationships from overlaid slices and of tumor-vasculature relationships from 3D-rendered displays). The operative findings indicated that a more accurate interpretation of this information was possible from the combined images.

Accurate combination of CT and MR data of the head: validation and applications in surgical and therapy planning

A method is presented for the accurate combination of magnetic resonance (MR) and computed tomographic (CT) images of the head. Our technique is based on user identified 3D landmarks followed by data combination and display as adjacent slices, a single fused slice representation, colour overlay and three-dimensional (3D) rendered scenes. Validation with a point phantom and computer simulation has established the relationship of registration accuracy with point location accuracy, the number of points used and their spatial distribution. The technique is in clinical use in the planning of skull base surgery, transferring MR images acquired without a stereotaxic frame to stereotaxic space, and staging and planning therapy of nasopharyngeal tumours.

Significance of MRI changes after surgery of the skull base

Magnetic resonance (MR) imaging is commonly used in the followup of patients who have undergone surgical removal of tumors from the cranial base to assess the possibility of tumor recurrence, persistence, or surgical complications. Interpretations of this study must be cautious because postoperative enhanced signals are encountered frequently. Although technological improvements continue to enhance the usefulness of MR images for followup, problems remain in differentiating between fibrotic, reconstructive changes, and tumor recurrence. In this study, the hospital records and MR findings of 215 patients who had undergone skull base surgery were reviewed. The study was prompted by negative operative exploration in three patients in whom postoperative MR images strongly suggested tumor recurrence. One-year follow-up images were available in 174 patients. Of these, 94% showed signal enhancement on the 1-year follow-up study. Progressive changes were observed in 14% of patients. The correlation of preoperative MR findings and surgery, with regard to dural involvement, vascular involvement, cranial nerve infiltration, and extent of intracranial infiltration, was analyzed in 28 patients. Methods to facilitate the detection of tumor recurrence using MR images are reviewed.

Integrated presentation of multimodal brain images

This article discusses the fusion of brain images from multiple modalities as well as the presentation of the integrated image information. The paper has three parts. First, individual brain imaging modalities are compared as regards clinical appreciation, invasiveness, dimensionality, spatial resolution, temporal resolution, and cost. Next, methods to combine multiple images are briefly surveyed and collated by characteristics as accuracy, patient-friendliness, reproducibility, labour-extensiveness, feasibility of retrospective matching, and general applicability. Finally, techniques to display multimodal image information are outlined and examples of the various options for integrated presentation are shown.