Donna Magid

Volumetric rendering of computed tomography data: principles and techniques

The methods and algorithms used for volumetric rendering of medical computed tomography data are described in detail. Volumetric rendering allows for the use of a mixture paradigm for representation of the volume to be rendered and uses mathematical techniques to reduce or eliminate aliasing. A step-by-step description of the process used to generate two types of images (unshaded and shaded surfaces) is included. The technique generates three-dimensional images of computed tomography data with unprecedented image quality. Images generated with this technique are in routine clinical use.<<ETX>>

The anatomy of the pelvis in the exstrophy complex

We compared computerized tomography scans of the pelvis of twenty-four patients who had exstrophy of the bladder with scans of age-matched controls in order to analyze the pelvic deformity that accompanies the variably severe manifestations of this condition. The patients who had classic exstrophy of the bladder were found to have a mean of 12 degrees of external rotation of the posterior aspect of the pelvis on each side, retroversion of the acetabula, a mean additional 18 degrees of external rotation and 30 per cent shortening of the pubic rami, and progressive diastasis of the symphysis pubis. The foot-progression angle demonstrated 20 to 30 degrees of external rotation beyond the normal limits seen in early childhood, but this improved with age. The patients who had exstrophy of the cloaca and the bladder not only had all of these pelvic deformities to a greater degree but also had asymmetry of measured parameters between the right and left sides of the pelvis, malformation of the sacro-iliac joints, and occasional dislocation of the hip. An understanding of the pelvic anatomy that accompanies exstrophy is essential when corrective approaches are planned. Such an understanding will improve the rate of success of both closure of the bladder and control of urinary continence postoperatively.

Evaluation of CT techniques for reducing artifacts in the presence of metallic orthopedic implants

Metallic intramedullary orthopedic implants generate artifacts that can markedly degrade transaxial CT images. The artifacts, typically seen as starburst streaking, result primarily from reconstructions involving missing projection data. Two approaches are clinically available to reduce the artifacts around orthopedic implants. These are (a) the imaging of implants with lower attenuation coefficients or smaller path lengths (less attenuating objects) and (b) the planar reformatting of image data. The sizing accuracy of these two approaches was quantified using phantoms and the efficacy using cadaveric femoral specimens. Results demonstrated that metal artifacts may be reduced and accurate bony dimensional data obtained.

Examination of the pathologic anatomy of ankle fractures.

A prospective study of the translational and rotational displacement of the lateral malleolus in ankle fractures was carried out utilizing roentgenographic techniques. Twenty-six ankle fractures in 25 patients were studied using both routine plain films and CT scanning with two- and three-dimensional multiplanar reconstruction. Eighty-one percent were Lauge-Hansen supination-external rotation type injuries. Overall, 21 fractures did not involve the medial malleolus. Initial talar shift was less than or equal to 2 mm in 15 fractures. Although all patients exhibited external rotation deformities of the lateral malleolus on plain films, only one fracture was found to possess any degree of external rotation relative to the talus. The proximal fibula was seen on CT scans to have increased internal rotation with respect to the tibia in 19 cases. One patient had a slightly externally rotated proximal fibula; the remainder appeared normally aligned. The displacements measured by the CT scans at the talofibular articulation were compared with the standard plain film measurements. The displacements at the distal lateral malleolus were consistently overestimated by the plain roentgenograms, presumably because the capsular and ligamentous attachments to the distal fibula limit malleolar displacement. The talocrural angle, determined on both plain films and CT scans, was also not found to be a sensitive measure of fibular shortening nor of the severity of the fracture. The results of this study suggest that, in an isolated lateral malleolar ankle fracture, the apparent external rotation of the fracture fragment is relative only to the proximal fibula and is not associated with derangement of the talofibular articulation. Based on these mechanical considerations, surgical intervention for such fractures may not be necessary. This hypothesis is consistent with previous long-term clinical studies.

THE ACETABULUM IN CONGENITAL AND NEUROMUSCULAR HIP INSTABILITY

Summary: Acetabular morphology in congenital and neuromuscular hip instability was evaluated with computed tomography (CT) and plain radiography. Thirty-three unstable hips in 27 children with congenital hip dysplasia, cerebral palsy, or spina bifida were studied. Fourteen normal hips in 14 children served as controls. Unstable hips in the children with congenital hip dysplasia demonstrated anterior and superior acetabular deficiency with increased acetabular anteversion. Global deficiency was present in acetabuli of cerebral palsy and spina bifida patients with anterior, posterior, and superior deficiencies. The acetabuli in cerebral palsy patients had significant posterior deficiency and were the shallowest of the groups evaluated.

Pulmonary carcinoid tumors: CT assessment.

Computed tomography assessment of 634 surgically proven solitary pulmonary nodules included 12 pulmonary carcinoid tumors. Five were central (involving or directly abutting the bronchial tree), and seven were peripheral (surrounded by parenchyma). Three central and one peripheral lesion had CT numbers indicative of focal calcifications, frequently in the periphery of the nodule. Nodule size ranged from 1.2 to 3 cm. Nine nodules were smooth and round, two were irregular and lobulated, and in one the entire peripheral contour was difficult to assess because of postobstructive atelectasis secondary to tumor occlusion of the bronchus. The calculated average CT number ranged from 80.5 to 179 HU.

Design of Custom Hip Stem Prostheses Using Three-dimensional Ct Modeling

Long life expectancy, demand for high activity levels, and bone loss at the time of revision motivate the search for reliable and successful nonce-mented hip stem designs. It is hypothesized that improved implant fit may increase the longevity of noncemented total joints. Quantitative X-ray CT has enabled the use of a computerized stem design program, which designs an optimal-fit hip stem for individual femurs. Computed tomography and interactive image processing methods are used to generate the individual three-dimensional femoral models, which are used by the stem design program. Optimal-fit design provides maximum stem-bone contact while satisfying the requirement of being surgically insertable. Previous methods of custom implant design, including those that use three-dimensional CT modeling, have not provided optimal stem-bone fit. Quantitative results of this new process are presented.

Reliability of radiographic evaluation for acromial morphology

Abstract Objetive. Bigliani’s classification system of acromial morphology utilizing the standard outlet radiograph has become in accepted method for evaluating patients with rotator cuff disease. This study evaluates the interobserver and intraobserver reliability of Bigliani’s classification system using observers at various levels of training. Patients and design. Supraspinatus outlet view radiographs of 40 patients (aged 18–78 years) with shoulder pain were reviewed twice, 4 months apart, in a masked protocol by six reviewers, including two attending (fellowship-trained) shoulder surgeons, an attending musculoskeletal radiologist, an orthopedic surgery sports fellow, and two orthopedic residents (PGY-2 and PGY-5). The reviewers were given standard diagrams of the Bigliani classification system and were asked to classify each film as a type I, II, or III acromion. Interobserver reliability and intraobserver repeatability values were calculated using kappa statistic analysis (0–0.2 slight, 0.21–0.4 fair, 0.41–0.6 moderate, 0.61–0.8 substantial, and 0.8–1.0 excellent). Results and conclusion. For each of the two readings, all six observers agreed only 18% of the time. Kappa values for pairwise comparison of interobserver reliability among the six observers ranged from 0.01 to 0.75 (mean 0.35), and intraobserver repeatability ranged from 0.26 (PGY-5 resident) to 0.80 (fellowship-trained surgeon), with a mean of 0.55. Intraobserver repeatability was not significantly different for the different levels of expertise. More definitive criteria are needed to distinguish and classify the acromion.

Clinical utility of a stability-based ankle fracture classification system

Objective: Neither of the ankle fracture classification systems (Lauge-Hansen or Weber) in widespread use today is prognostic. To test the hypothesis that ankle fracture prognosis is dependent on initial biomechanical stability, an alternative classification system created using stability-based treatment criteria was developed on the basis of a structured analysis of the ankle fracture literature. Data Sources: All English-language papers reporting on ankle fractures (searched using the terms “ankle + fracture”) published between 1966 and 2005 with available online abstracts via PubMed were screened. Study Selection: Abstracts were manually screened for inclusion using the following criteria: (1) there were at least 2 groups of patients categorized on the basis of either fracture configuration or treatment and (2) data was reported in sufficient detail to permit interstudy comparisons. Data Extraction: Each included paper was abstracted into a computerized database for consistent data capture. Data elements included the following: fracture classification, stability definitions, surgical indications, patient follow-up parameters, and outcome measures. Ankle fractures were also stratified into stable and unstable groups using predefined stability criteria, and the outcome measures were re-analyzed. Data Synthesis: Wilcoxon matched-pairs signed-rank test was used for statistical comparisons, assigning statistical significance to 2-tailed tests with P < 0.05. Conclusions: The results support the hypothesis that a stability-based ankle fracture classification system can be prognostic. For unstable ankle fractures, the radiographic outcomes were better after surgery, when the decision for surgery was made on the basis of stability (P = 0.0173). Overall, non-operative treatment results were also better with stability-based treatment (P = 0.0299).

Three-dimensional imaging and display of musculoskeletal anatomy

Conventional three-dimensional imaging uses thresholding, or surface rendering technique, which limits accuracy and detail, and hinders soft tissue definition. Volumetric image rendering preserves all CT data, not just surface boundaries, and therefore overcomes these limitations. Three-dimensional images thus generated can be used for muscular and vascular anatomy as well as skeletal structures, with preservation of subtle detail. Representative cases are used to illustrate this technique and its implications for therapy and plastic and reconstructive surgery.