H.V. Ortega

Tissue Segmentation in MRI as an Informative Indicator of Disease Activity in the Brain

The presented tissue segmentation technique is based on a multispectral analysis approach. The input data were derived from high resolution MR images. Usually, only two inputs, proton density (PD) and T2-weighted images, are utilized to calculate the 2D feature map. In our method, we introduced a third input, T1-weighted MR image, for segmentation based on 3D feature map. k-Nearest Neighborhood segmentation algorithm was utilized. Tissue segmentation was performed in phantoms, normal humans and those with brain tumors and MS. Our technique utilizing all three inputs provided the best segmentation (p<0.001). The inclusion of T1 based images into segmentation produced dramatic improvement in tissue identification. Using our method, we identified the two distinctly different classes of tissue within the same MS plaque. We presume that these tissues represent the different stages involved in the evolution of the MS lesions. Further, our methodology for measuring MS lesion burden was also used to obtain its regional distribution as well as to follow its changes over time. The segmentation results were in full accord with neuropsychological findings.

Carotid magnetic resonance angiography : improved image quality with dual 3-inch surface coils

Magnetic resonance angiography (MRA) hs inherent artifacts due to variation in velocity and direction of flowing blood in the carotid bulb and regions of stenosis. We examined the efficiency of dual 3-inch surface coils to delineate carotid artery flow better. Carotid MRA was performed on ten healthy volunteers and six patients, on a 1.5 T system. A special adapter was constructed to use with 3-inch (receive-only) coils, which were placed over the carotid bifurcations. Routine anterior neck coils were also used. Contiguous axial two-dimensional (45/8.7, 1.5 mm, flip angle 60°) time-of-flight sequences were used. Image matrix was 256×256 with two signals averaged and acquisition time 6–10 min. These images were postprocessed and reformatted into angiographic views using a maximum intensity projection algorithm. Computer simulation of carotid artery blood flow through-out the cardiac cycle based on vessel contours derived from digital subtraction angiography was carried out by finite element analysis. Improved definition of vessel margin, particularly at the carotid bifurcation, and substantially increased signal-to-background ratio of flowing blood were obtained with 3-inchcoils. Apparent loss of signal in the carotid bulb was diminished. In one patient, contiguous flow throughout a high-grade stenosis was well defined, with the surface coil method, while drop-off of signal was observed with routine neck coil imaging.

Image nonuniformity correction in high field (1.5 T) MRI

The largest source of image nonuniformity in high field MRI systems arises from the specialized radiofrequency (RF) coils that are currently used. It degrades conspicuity of lesion(s), and reduces accuracy of image post-processing (e.g. MRA, tissue segmentation, etc.). In this investigation the authors devised a method to correct nonuniformity of MR images with correction matrices obtained from cylindrical uniform phantom. The phantom, filled with doped water and exactly fitted the volume of the head coil, was imaged using MRI/MRA clinical pulse sequences. The reference pixel intensity was defined using the most homogeneous region of the RF coil, and every voxel inside the phantom was normalized relative to the reference value. A correction matrix was obtained for each type of MRI contrast. MRI and MRA images of phantoms as well as normals and MS patients were obtained and processed by the correction matrices. Application of the correction matrices to phantom data resulted in up to 20 fold improvement in image uniformity. In humans, the corrected images improved sharpness and tissue contrast, leading to increased conspicuity of the lesions.

Computer simulation of blood flow using short te magnetic resonance angiography

Using computer simulation of pulsatile blood flow and magnetic resonance angiography (MRA), we investigated the hemodynamic factors leading to the formation and evolution of 1) atherosclerotic plaque in carotid arteries, and 2) aneurysms in the abdominal aorta. Phantom and patients were imaged by MRA, color Doppler and/or digital subtraction angiography (DSA). Computer modelling was carried out by finite element analysis to solve the Navier-Stokes equation. In the analyzed vessels, voxels representing pathology were digitally removed. Local wall shear stress and pressure were also calculated as a function of a cardiac cycle. There was general agreement between MRA, color Doppler, DSA and computer simulation in both phantom and in-vivo experiments. In MRA, the best results were achieved by short TE, thin slice 2D “time-of-flight” technique, which was least susceptible to the changes in velocity profiles, and best correlated with Doppler and computer simulation. The hemodynamic information obtained from analyzed carotid arteries predicted that during late systole, flow separation exists at the exact locations from where the plaque voxels were removed. We were also able to predict the location of abdominal aortic aneurysm and its evolution toward the distal vessel intima. In conclusion, MRA accurately depicted flow disturbances, and computer simulation of blood flow proved to be a good predictor of the development of vascular pathology.

Fast magnetization transfer MR imaging

We have implemented a new magnetization transfer (MT) technique, to decrease total imaging time and thus improve efficiency of MT based MRI. By judiciously rescaling different RF pulses, and utilizing maximum transmitter gain, we were able to generate very short and powerful MT pulse(s). MRI and MRA in normal and patients with MS were performed, with and without MT application. Phantom experiments indicated 1-2 kHz offset to be the optimal. MT based MRA demonstrated substantial improvement in conspicuity of small vessels with minimum increase in imaging time. MT based MRI showed 30% improvement in MT contrast as well. Importantly, both peak and average SAR values were within FDA approved safety limits.<<ETX>>

Fast 3D chemical shift magnetic resonance imaging

Two methods of water/fat suppression are investigated. The first, selective excitation, suppresses approximately 70% of fat or water signals. The second method, a phase-sensitive modification of the Dixon technique, achieves more than 95% fat or water suppression. Fat-suppressed data acquisition is applied to musculoskeletal and CNS imaging. In comparison to standard 3-D images, these sequences provide an enhanced demonstration of bone marrow and cartilage pathology in musculoskeletal imaging, and better delineation of normal morphology and tumors of the pituitary gland in CNS imaging. The technique is especially effective when used in conjunction with administration of a paramagnetic agent (gadopentetate dimeglumine).<<ETX>>

Multiple flip angle MRA: data acquisition and post-processing

We have implemented a MRA (magnetic resonance angiography) data acquisition and post processing method to improve the assessment of the carotid artery bifurcation needed for correct diagnosis. Axial high resolution MRA images were acquired using 3D time-of-flight (TOF) MRA technique with a multiple flip angle. Composite MRA images of these data sets with at least two different flip angles were created using a MIP (maximum intensity projection) algorithm and formatted into sagittal angiographic projections. Next, color coded MRA images were created using a MIP segmentation algorithm. The resulting images demonstrated improved image quality of the carotid bifurcation. These post-processed MRA images also demonstrate significant increase in the SNR within the carotid bifurcation (p<0.05).

Optimization Of The Excitation RF Pulse In Combined Inversion Recovery Gradient And Spin-echo (ir-crease) Technique

We designed a pulse sequence based on the simultaneous acquisition of gradient and spinecho inversion recovery images. The gradient echo image was obtained by a rapid readout gradient reversal immediately after the RF excitation pulse within a multi-echo inversion recovery technique. By analyzing the solution to the Bloch equations, we were able to maximize the image contrast and/or obtain images faster through short TR’s and optimization of the RF pulse excitation. The optimum excitation flip angle a will be small for an odd number of Inversion, Excitation and Refocusing RF pulses and conversely large (180°-a) for an even number. The in vivo results of the gradient and spin-echo inversion recovery technique were in full accordance with theoretical results. Clinical trials of this technique have begun. - ri