Registration of three-dimensional magnetic resonance and radionuclide skeletal images.

PURPOSE: To apply postprocessing techniques to register three-dimensional TI-201 bone SPECT datasets with MRI. This may provide more accurate anatomic-functional correlation when localizing active tumors. MATERIALS AND METHODS: Three-dimensional datasets were constructed from previously acquired MRIs using routine imaging protocols. Registration software was used to coregister the TI-201 SPECT studies and the MRIs in three dimensions. RESULTS: Adequate TI-201 uptake in muscles and soft tissues along with relatively low accumulation in tendons and joint spaces provided adequate landmarks for visually aligning SPECT and MRI datasets. MR abnormalities were more extensive because of surrounding reactive tissue, and more focal TI-201 uptake could be demonstrated within the region of MR signal abnormality, allowing the focal metabolically active tissue to be distinguished from adjacent edema. CONCLUSIONS: Image registration of SPECT and anatomic imaging (CT or MRI) is used routinely to evaluate functional abnormalities within the brain. This technique has now been applied to the combination of TI-201 SPECT and MR data for evaluating bone lesions and may provide additional anatomic information for localizing functional abnormalities. This may be valuable for defining targets for biopsy, planning surgical treatment, and using minimally invasive therapies.     

The clinical significance of anterior horn meniscal tears diagnosed on magnetic resonance images

We assessed the accuracy of magnetic resonance imaging in detecting clinically significant lesions of the anterior horn of the meniscus by reviewing 947 consecutive knee magnetic resonance imaging reports. Of these, 76 (8%) indicated a tear of the anterior horn of the medial or lateral meniscus. Thirty-one of these 76 patients underwent a subsequent arthroscopic examination, and their operative reports were reviewed. The 45 patients who were not examined arthroscopically were contacted and interviewed for clinical follow-up. Among the 31 patients who underwent arthroscopic examination, 8 anterior horn tears were noted in the predicted area (26% true-positive results), 23 patients had intact anterior horns (74% false-positive results), and 18 had normal intact menisci in all zones. Of the 45 patients who did not undergo arthroscopic surgery, 6 had isolated anterior horn tears reported on magnetic resonance imaging, and 5 of the 6 were asymptomatic at follow-up. The other 39 patients had multiple pathologic conditions noted on the magnetic resonance imaging report and continued to report knee pain at the follow-up interview. Increased signal intensity at the anterior horn of the meniscus seen on magnetic resonance imaging commonly does not represent a clinically significant lesion. We recommend correlation with the physical examination when interpreting this "positive" finding on knee magnetic resonance imaging examinations.     

Three-dimensional reconstruction of magnetic resonance images of the temporomandibular joint by I-DEAS.

Evaluation of the temporomandibular joint has been limited by the inability of current technology to image complex morphology and motion in three dimensions. An engineering design program, I-DEAS, has been used to construct solid models from magnetic resonance images. A dried skull with an acrylic resin temporomandibular disc replica, immersed in water, provided sagittal and coronal MR images. Linear dimensions and disc volumes obtained from the models were compared with the original and found to be consistent, within the limits imposed by the slice thickness. We have applied the method to the living joint in an asymptomatic volunteer, and report our initial experience in demonstrating the spatial relationships and motion of the joint components.     

True three-dimensional nuclear magnetic resonance zeugmatographic images of a human brain

Summary A three-dimensional image of a preserved human brain, resolved into cubical 0.03 cm³ volume elements, has been obtained by nuclear magnetic resonance (NMR) zeugmatography, using a twostage reconstruction technique. Intensities in such images represent concentrations of water and other liquids or liquid-like substances. The image has been displayed as computer-generated multiple transverse, coronal and sagittal sections, so as to display most clearly a number of anatomical features. The potential of this technique in physiological research and clinical practice is discussed.     

Rapid three-dimensional surface reconstruction of magnetic resonance images of large arteries and veins: A preliminary evaluation of clinical utility

We have implemented a computerized system for relatively rapid (<1 h) three-dimensional (3D) surface rendering of flow-sensitive (gradient refocused) magnetic resonance images. The method has been applied in 8 patients with six different clinical problems and was found to enhance understanding of normal and abnormal aortic, caval, and portosystemic venous anatomy. 3D images are useful for communicating complex anatomic information and may help with difficult diagnoses. Advantages and limitations of the present system are discussed.     

Global optimization of mutual information: application to three-dimensional retrospective registration of magnetic resonance images.

A global optimization technique for image registration, based on mutual information, that can be used in conjunction with a multi-resolution paradigm is described. This technique combines genetic algorithm in continuous space, which is a stochastic method and is very efficient in large search space, with dividing rectangle, which is a deterministic method that theoretically guarantees global optimization and is efficient in small search space. Calculations were performed for determining the optimum parameters for implementing this method. This technique was applied to register magnetic resonance images of brain. For comparison, the registration results using AIR, a commonly employed software package, are presented.     

An unusual meniscal finding on magnetic resonance imaging

A 13-year-old male with a previous one-month history of a ‘locking’ painful knee underwent an MR investigation of his knee. The MR findings showed a discoid meniscus with a meniscal tear, which at arthroscopy proved to be an unusual Wrisberg III variant. The morphology of discoid meniscus is defined including the Wrisberg III variant. The MR factors are discussed including optimum planes and sequences used by MRI to investigate meniscal pathologies including tears. The treatment regimes are investigated including surgical options.     

Evaluation of lung volumetry using dynamic three-dimensional magnetic resonance imaging

RATIONALE AND OBJECTIVES: We sought to investigate lung volume and surface measurements during the breathing cycle using dynamic three-dimensional magnetic resonance imaging (3D MRI). MATERIALS AND METHODS: Breathing cycles of 20 healthy volunteers were examined using a 2D trueFISP sequence (3 images/second) in combination with a model and segmented 3D FLASH sequence (1 image/second) MR images using view sharing. Segmentation was performed semiautomatically using an interactive region growing technique. Vital capacity (VC) was calculated from MRI using the model (2D) and counting the voxels (3D) and was compared with spirometry. RESULTS: VC from spirometry was 4.9+/-0.9 L, 4.4+/-1.2 L from 2D MRI measurement, and 4.7+/-0.9 L for 3D MRI. Using the 3D technique, correlation to spirometry was higher than using the 2D technique (r>0.95 vs. r>0.83). Using the 3D technique, split lung volumes and lung surface could be calculated. There was a significant difference between the left and right lung volume in expiration (P<0.05). CONCLUSIONS: Dynamic 3D MRI is a noninvasive tool to evaluate split lung volumes and lung surfaces during the breathing cycle with a high correlation to spirometry.     

Reproducibility study of 3D geometrical reconstruction of the human carotid bifurcation from magnetic resonance images.

The combined magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) modeling approach is playing an increasingly important role in advancing our understanding of the relationship between hemodynamics and arterial disease. Nevertheless, such a modeling approach involves a number of uncertainties associated with various stages of the process. The present study is concerned with the reproducibility of geometry reconstruction, one of the most crucial steps in the modeling process. The reproducibility test was conducted on the right carotid bifurcation of eight normal human subjects, each of whom were scanned twice using the same MR protocol with an in-plane resolution of 0.625 mm. Models constructed from different scans of the same subject were compared and assessed using four quantitative measures: centerline distance, cross-sectional area, contour shape factors, and mean radius difference. The difference in the maximum carotid bulb area between the two scans was found to be <8.1% for all subjects. Shape factors (measuring the dissimilarity between two contours) of <10% were achieved in most of the common carotid arteries (CCAs) and internal carotid arteries (ICAs). The mean radius difference between the two scans was <0.4 mm for all subjects. Among the three vessels, the geometry of CCA was well reproduced by the reconstruction procedure in most of the cases, while the external carotid artery (ECA) showed the worst reproducibility. The impact of geometrical differences on CFD-predicted flow patterns and wall shear stress (WSS) will be investigated and discussed in a separate paper.     

Motion estimation of tagged cardiac magnetic resonance images using variational techniques

This work presents a new method for motion estimation of tagged cardiac magnetic resonance sequences based on variational techniques. The variational method has been improved by adding a new term in the optical flow equation that incorporates tracking points with high stability of phase. Results were obtained through simulated and real data, and were validated by manual tracking and with respect to a reference state-of-the-art method: harmonic phase imaging (HARP). The error, measured in pixels per frame, obtained with the proposed variational method is one order of magnitude smaller than the one achieved by the reference method, and it requires a lower computational cost.     

Quantitative assessment of ventricular function using three-dimensional SSFP magnetic resonance angiography

PURPOSE: To evaluate three-dimensional (3D), free-breathing, steady-state free precession (SSFP) magnetic resonance angiography (MRA) for volumetric assessment of ventricular function. MATERIALS AND METHODS: In 18 subjects (mean age = 21.5 years) 3D datasets of the heart and great vessels were acquired using an ECG-triggered, free-breathing SSFP technique with a T2-preparation prepulse. Data were acquired during end-systole (ES) and end-diastole (ED) for assessment of stroke volumes (SVs). Through-plane flow measurements of the great arteries were performed as well as 2D-cine SSFP imaging for comparison. For image analysis of the 3D SSFP datasets a simplex mesh model was used. Papillary muscles were excluded from ventricular volumes using thresholds. Intra- and interobserver variability (Bland-Altman analysis) and correlations (Pearson's coefficient) between volumetric and flow measurements were assessed. RESULTS: ES and ED datasets were acquired successfully in all subjects. The best correlation was observed between flow vs. 3D SSFP SV for the LV (r = 0.85, mean difference = -1.0 mL) and the RV (r = 0.89, mean difference = -2.2 mL) with high intra- (LV: r = 0.93; RV: r = 0.94) and interobserver (LV: r = 0.91; RV: r = 0.93) reproducibility. CONCLUSION: 3D SSFP datasets combined with semiautomatic segmentation algorithms allow highly accurate and reproducible assessment of left (LV) and right ventricular (RV) SVs in free-breathing subjects.     

Multispectral analysis of magnetic resonance images

Magnetic resonance (MR) imaging systems produce spatial distribution estimates of proton density, relaxation time, and flow, in a two dimensional matrix form that is analogous to that of the image data obtained from multispectral imaging satellites. Advanced NASA satellite image processing offers sophisticated multispectral analysis of MR images. Spin echo and inversion recovery pulse sequence images were entered in a digital format compatible with satellite images and accurately registered pixel by pixel. Signatures of each tissue class were automatically determined using both supervised and unsupervised classification. Overall tissue classification was obtained in the form of a theme map. In MR images of the brain, for example, the classes included CSF, gray matter, white matter, subcutaneous fat, muscle, and bone. These methods provide an efficient means of identifying subtle relationships in a multi-image MR study.     

Prospective registration of human head magnetic resonance images for reproducible slice positioning using localizer images

PURPOSE: To facilitate assessing brain tumor growth and progression of stroke lesions by reproducible slice positioning in human head magnetic resonance (MR) images, a method for prospective registration is proposed that adjusts the image slice position without moving the patient and with no additional scans. MATERIALS AND METHODS: The gradient reference frame of follow-up examinations was adjusted to achieve the same image slice positioning relative to the patient as in the previous examination. The three-dimensional geometrical transformation parameters for the gradients were determined using two-dimensional image registration of three orthogonal localizer images. The method was developed and evaluated using a phantom with arbitrarily adjustable position. Feasibility for in vivo applications was demonstrated with brain MR imaging (MRI) of healthy volunteers. RESULTS: Standard retrospective registration was used for assessing the quality of the method. The accuracy of the realignment was 0.0 mm +/- 1.2 mm and -0.2 degrees +/- 0.9 degrees (mean +/- SD) in phantom experiments. In 10 examinations of volunteers, misalignments up to 49.2 mm and 21 degrees were corrected. The accuracy of the realignment after prospective registration was 0.1 mm +/- 1.5 mm and 0.2 degrees +/- 1.5 degrees. CONCLUSION: Image-based prospective registration using localizer images of the pre- and postexaminations is a robust method for reproducible slice positioning.     

Preprocessing programs of CT images for three-dimensional reconstruction

Up to now three-dimensional reconstructions in computed tomography have been applied mainly to the skeleton. This limitation is due to technical difficulties when dealing with images of the soft tissues. In order to overcome these limitations three new softwares were developed, thus enabling the operator to pre-elaborate and modify the axial CT images. The first program allows the operator to isolate the lesion and/or the anatomical structures of interest from the surrounding areas. The second program eliminates the areas of isodensity with the lesion it is obtained by means of a trackball. The third program enables the operator to write the modified axial images on the original file. The lesion and/or the structures of interest can thus be processed more easily by the 3D reconstructive program. The new softwares have been applied to 3D reconstructions of brain lesions, and two significant cases are presented. Furthermore, since the new softwares can enhance the quality of 3D images of the skeleton, the programs were also tested in cases of acetabular traumas.     

Gadolinium enhanced three-dimensional magnetic resonance portography with subtraction.

The aim was to evaluate a subtraction technique for breath-hold gadolinium enhanced three-dimensional magnetic resonance portography (3D-MRP). 26 patients with gastric and/or duodenal varices related to portal hypertension were investigated by 3D-MRP with two phase acquisitions. A partial volume maximum intensity projection (MIP) image after subtracting selective arterial phase images from subsequent portal venous phase images (subtraction 3D-MRP) was compared with the partial volume MIP without subtraction (non-subtraction 3D-MRP) to assess visualization of the portal vein and its collaterals. Subtraction 3D-MRP depicted excellent visualization of the portal vein, although this was not significantly better than non-subtraction 3D-MRP. However, subtraction 3D-MRP gave superior visualization of portal collaterals, with effective suppression of arterial and renal signal intensities, compared with non-subtraction 3D-MRP (p<0.001).     

Estimating the spatial resolution of in vivo magnetic resonance images using radiofrequency tagging pulses.

The spatial resolution of magnetic resonance (MR) images is usually specified by using nominal spatial resolution, the width of the simulated point-spread function (PSF), or measurement from a resolution phantom. The accuracy of these measures is limited because they do not take into account the effects of in vivo image degradation. In this work, tag lines were used to estimate the spatial resolution of in vivo MR images. The idea of using tag lines to measure resolution was originally proposed by Wayte and Redpath (Magn Reson Imaging 1998;16:37-44), who used sinusoidal tag profiles and calculated an indirect measure of resolution called the modulation percentage. This work uses widely separated thin tag lines to directly measure the shape of the PSF and produce estimates of the PSF width in millimeters. The feasibility of estimating the spatial resolution via tag lines was evaluated using phantom images. The results show that an accurate estimate can be obtained when the tag modulation width is equal to or less than one-half of the best possible resolution. The tag lines are then used to demonstrate the asymmetry and spatial variation of spatial resolution of in vivo images acquired by using a turbo spin-echo (TSE) and a spiral sequence.     

Making magnetic resonance images and beyond

Magnetic resonance (MR), a comparatively new imaging method in Canada, is based on physical principles that are different from all current imaging methods. An understanding of these principles is therefore essential to appreciate the clinical capabilities and limitations of MR. Because MR is a multiparameter imaging method, the challenge of choosing appropriate imaging sequences must be addressed. The technical development of anatomical image formation is now almost complete, so future research will focus on the use of MR to obtain more than just spatial information. The quantitative measurement of relaxation times, chemical composition, flow and spectra hold great promise in extending the capabilities of MR beyond those of other imaging methods.