Local histogram correction of MRI spatially dependent image pixel intensity nonuniformity.

We describe a computationally straightforward post-hoc statistical method of correcting spatially dependent image pixel intensity nonuniformity based on differences in local tissue intensity distributions. Pixel intensity domains for the various tissues of the composite image are identified and compared to the distributions of local samples. The nonuniformity correction is calculated as the difference of the local sample median from the composite sample median for the tissue class most represented by the sample. The median was chosen to reduce the effecters on determining the sample statistic and to allow a sample size small enough to accurately estimate the spatial variance of the image intensity nonuniformity. The method was designed for application to two-dimensional images. Simulations were used to estimate optimal conditions of local histogram kernel size and to test the accuracy of the method under known spatially dependent nonuniformities. The method was also applied to correct a phantom image and cerebral MRIs from 15 healthy subjects. Results show that the method accurately models simulated spatially dependent image intensity differences. Further analysis of clinical MR data showed that the variance of pixel intensities within the cerebral MRI slices and the variance of slice volumes within individuals were significantly reduced after nonuniformity correction. Improved brain-cerebrospinal fluid segmentation was also obtained. The method significantly reduced the variance of slice volumes within individuals, whether it was applied to the native images or images edited to remove nonbrain tissues. This statistical method was well behaved under the assumptions and the images tested. The general utility of the method was not determined, but conditions for testing the method under a variety of imaging sequences is discussed. We believe that this algorithm can serve as a method for improving MR image segmentation for clinical and research applications.

Brain mapping with functional MR imaging: comparison of gradient-echo--based exogenous and endogenous contrast techniques.

PURPOSE: To compare directly the two most widely used methods of functional magnetic resonance (MR) imaging--dynamic contrast material-enhanced MR imaging and blood oxygenation level-dependent (BOLD) MR imaging. MATERIALS AND METHODS: Five healthy volunteers underwent dynamic contrast-enhanced and BOLD MR imaging with a conventional 1.5-T MR unit during visual stimulation and a dark control state. BOLD studies were performed with a gradient-echo sequence, and dynamic MR imaging was performed with an echo-shifted gradient-echo sequence after intravenous administration of a bolus of gadopentetate dimeglumine. RESULTS: A significantly greater percentage signal change was found with dynamic MR imaging than with the BOLD technique. The extent of area activated was also significantly greater. CONCLUSION: With standard clinical imagers and these gradient-echo-based techniques, greater percentage activation and area of activation can be achieved with dynamic MR imaging than with BOLD MR imaging.

3-dimensional functional imaging of human brain using echo-shifted FLASH MRI.

A 3-dimensional MRI method has been developed for functional mapping of the human brain, based on blood oxygenation level dependent (BOLD) contrast mechanisms. The method uses recently introduced principles of echo-shifted FLASH to acquire a single 3D data set in 20 s. The technique was tested on a conventional 1.5 Tesla clinical scanner with a standard head coil using visual stimulation with a 8 Hz flashing white light, or a varying checkerboard pattern. Areas of increased signal intensity were identified in the visual cortex, consistent with the known functional organization.

Functional brain MR imaging based on bolus tracking with a fast T2*-sensitized gradient-echo method.

Dynamic physiological scanning, based on temporary changes in local field homogeneity during the passage of a contrast agent bolus, has been performed hitherto with echo-planar imaging (EPI) or conventional gradient-recalled techniques (FLASH). Here, it is shown that the T2* sensitivity of conventional FLASH techniques can be improved drastically on a conventional whole body instrument by delaying the gradient-echo until the subsequent TR-period without increasing total imaging time. Examples are given for a full k-space matrix (128 x 256) obtained within 2 s with a TE of 25 ms, resulting in images free of artifacts. The method is applied to bolus tracking through the brain of healthy volunteers during visual stimulation and in the dark. An average increase of regional cerebral blood volume (rCBV) in the visual cortex of 10.9% (n = 9, p = .001) was found.

A functional MRI technique combining principles of echo-shifting with a train of observations (PRESTO).

We present a fast MRI technique sensitized to microscopic susceptibility effects. The method combines elements of echo-shifted gradient-recalled MR imaging (TE > TR) with the acquisition of multiple k-space lines within a single TR-period. The sequence results in a much reduced imaging time as compared with conventional gradient-echo MRI methods. The feasibility of the method is demonstrated for susceptibility bolus tracking in the cat brain using an imaging time of 153 ms. The relative cerebral blood volume maps created with this method are comparable with those obtained with conventional methods.

Multisection proton MR spectroscopic imaging of the brain.

The authors developed a hydrogen-1 proton magnetic resonance (MR) imaging method in which metabolic information is acquired by obtaining multiple sections through the brain. A spin-echo sequence is used for section selection, an octangular outer volume saturation pulse for lipid suppression, and a chemical-shift-selective saturation pulse for water suppression. High-quality maps of choline, creatine, and N-acetylaspartate were obtained in six studies performed in four volunteers. Water and lipid signal from the skull area was well suppressed by the pulse sequence used.

Fast echo-shifted gradient-recalled MRI: combining a short repetition time with variable T2* weighting.

The principles of a fast T2*-sensitized MR imaging method (Moonen et al., Magn. Reson. Med. 26, 184 (1992)) are extended to further increase T2* sensitivity. It is shown that the period of T2*-weighting can be lengthened by n TR-periods by appropriate gradient schemes without RF refocusing resulting in progressively delayed gradient-recalled echoes. This extension of the echo-shifting concept thus introduces large flexibility in the choice of T2*-weighting without changing total imaging time. The coherence pathway formalism is used to evaluate and describe the selection of the desired echo and the attenuation of unwanted coherences. The new techniques are demonstrated for tracking a bolus of susceptibility contrast agent in cat brain. Relative blood-volume maps are derived with expected contrast between white and gray matter.

Mapping of brain tumor metabolites with proton MR spectroscopic imaging: clinical relevance.

Brain tumor metabolism was studied with hydrogen-1 magnetic resonance spectroscopy and positron emission tomography with fluorine-18 fluorodeoxyglucose in 50 patients. N-acetylaspartate (NAA) was generally decreased in tumors and radiation necrosis but was somewhat preserved at neoplasm margins. Choline was increased in most solid tumors. Solid high-grade gliomas had higher normalized choline values than did solid low-grade gliomas (P < .02), but the normalized choline value was not a discriminator of tumor grade, since necrotic high-grade lesions had reduced choline values. Serial studies in one case showed an increase in choline as the glioma underwent malignant degeneration. Choline values were lower in chronic radiation necrosis than in solid anaplastic tumors (P < .001). In two cases studied before and after treatment, clinical improvement and a reduction in choline followed therapy. Lactate is more likely to be found in high-grade gliomas, but its presence is not a reliable indicator of malignancy.

A fast gradient-recalled MRI technique with increased sensitivity to dynamic susceptibility effects.

A fast imaging method that is based on gradient-recalled echoes of spins whose excitation and echo formation are separated by more than one TR period is presented. This method does not incorporate chemical-shift refocusing and thus results in drastically increased sensitivity to dynamic susceptibility effects, while maintaining a short total imaging time. The efficiency of the new technique is demonstrated in dynamic contrast-enhanced experiments (bolus tracking) in the cat brain using a duration of 600 ms for each image. Blood volume maps are derived with expected contrast between white and gray matter.