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1.
Eur Radiol ; 14(10): 1781-6, 2004 Oct.
Article in English | MEDLINE | ID: mdl-15103501

ABSTRACT

The purpose of this study was to investigate the modified three-point Dixon technique as a method for obtaining fat-saturated T1-weighted sequences before and after intravenous gadolinium administration using an open MR imaging scanner. A preliminary experiment using an oil/gadolinium phantom was performed on a 0.35-T open magnet and an advanced 1.5-T unit. Fat saturation was achieved at 1.5 T using a frequency selective presaturation technique and a modified three-point Dixon technique on the low-field scanner. The modified three-point Dixon sequence was then evaluated in ten patients undergoing MRI examinations of the spine with gadolinium enhancement to determine image characteristics and diagnostic potential. The phantom study demonstrated a homogenous suppression of signal from oil and a good distinction between fat and a gadolinium chelate on the 0.35-T unit comparable to that on the 1.5-T scanner. By applying the modified three-point Dixon technique on the open-magnet, the distinction between fat and gadolinium dimeglumine was rated as very good in 139 and good in 17 axial slices in a total of 156 images. No image was rated as difficult or not possible. Motion artifacts that hampered the reading were detected in the lower cervical spine due to respiratory movement in four (3% of all) images. The modified three-point Dixon technique provides the combination of gadolinium enhancement with fat saturation on an open magnet. Early clinical applications appear promising.


Subject(s)
Contrast Media , Image Enhancement/methods , Image Processing, Computer-Assisted/methods , Magnetic Resonance Imaging/methods , Adipose Tissue , Adult , Artifacts , Female , Gadolinium DTPA , Humans , Magnetic Resonance Imaging/instrumentation , Male , Middle Aged , Phantoms, Imaging , Spinal Diseases/diagnosis
2.
Phys Med Biol ; 48(16): 2665-79, 2003 Aug 21.
Article in English | MEDLINE | ID: mdl-12974581

ABSTRACT

3D/2D patient-to-computed-tomography (CT) registration is a method to determine a transformation that maps two coordinate systems by comparing a projection image rendered from CT to a real projection image. Iterative variation of the CT's position between rendering steps finally leads to exact registration. Applications include exact patient positioning in radiation therapy, calibration of surgical robots, and pose estimation in computer-aided surgery. One of the problems associated with 3D/2D registration is the fact that finding a registration includes solving a minimization problem in six degrees of freedom (dof) in motion. This results in considerable time requirements since for each iteration step at least one volume rendering has to be computed. We show that by choosing an appropriate world coordinate system and by applying a 2D/2D registration method in each iteration step, the number of iterations can be grossly reduced from n6 to n5. Here, n is the number of discrete variations around a given coordinate. Depending on the configuration of the optimization algorithm, this reduces the total number of iterations necessary to at least 1/3 of it's original value. The method was implemented and extensively tested on simulated x-ray images of a tibia, a pelvis and a skull base. When using one projective image and a discrete full parameter space search for solving the optimization problem, average accuracy was found to be 1.0 +/- 0.6(degrees) and 4.1 +/- 1.9 (mm) for a registration in six parameters, and 1.0 +/- 0.7(degrees) and 4.2 +/- 1.6 (mm) when using the 5 + 1 dof method described in this paper. Time requirements were reduced by a factor 3.1. We conclude that this hardware-independent optimization of 3D/2D registration is a step towards increasing the acceptance of this promising method for a wide number of clinical applications.


Subject(s)
Bone and Bones/diagnostic imaging , Imaging, Three-Dimensional/methods , Radiographic Image Enhancement/methods , Radiographic Image Interpretation, Computer-Assisted/methods , Subtraction Technique , Tomography, X-Ray Computed/methods , Algorithms , Humans , Pelvis/diagnostic imaging , Quality Control , Reproducibility of Results , Sensitivity and Specificity , Skull/diagnostic imaging , Tibia/diagnostic imaging
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