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Enhancement of T1 MR images using registration for signal averaging
ABSTRACTS
I nhancement of T1 MR Images Using Registration for Signal Averaging Colin J. Holmes, Rick Hoge, Louis Collins and Alan C. Evans Montreal Neurological Institule, Montreal, Quebec, Canada
The resolution of magnetic resonance (MR,) images is of central concern in anatomical and functional studies of the human brain. Most attempts to enhance the resolution of MR involve modifications of the device itself, such as the use of higher strength magnets, enhanced coils, or specialized pulse sequences. Simply decreasing the voxel size requires increasing the number of signal averages, which significantly extends the scan time, inevitably resulting in movement and other artifacts. Here we present a simple method relying on registration to achieve significant increases in signal to noise ratios and therefore higher resolution images from standard MR instruments. Many sites involved in neuroimaging have developed rubust computational methods for registering intermodality and intersubject-intramodality 3D volumes. These tools can also be applied to scans from the same subject, in the same modality, with the result being posthoc enhancement of the signal to noise ratio through averaging. As each scan can be kept short, this gain comes without the cost of long individual scantimes. A series of 27 Tl-weighted MR scans was obtained from a single subject (CJH) using a Phillips 1.5 Tesla MR. The scan parameters were: 3D sagittal volume composed of 140 1ram slices, FOV 256mm (SI) x 204mm (AP), acquired with a spoiled GRASS ( T R / T E = 1 8 / 1 0 m s , flip angle 30, NSA(NEX) 1, flow compensation on) for a total scan time of 10:50 x 27 scans. Each scan was registered and tranformed into the MNI_305 average brain space (based on Talairach 1988), using an automatic registration tool, and resampled at up to 0.5ram 3 before normalization and intensity averaging. Theoretically, the effect of averaging 27 volumes is a gain of 5.2 on the signal to noise average. This effect is accentuated by the reduction of partial volume effects. Due to the unavoidable small motions of the head between scans each region of space is imaged in slightly different voxels in each scan. As the final point sample is drawn from many volumes, all slightly displaced with respect to one another, the final subsampled voxels reduce the overall partial volume effect. The combination of these effects upon the visibility of finer structure in the resultant MR average image is striking. Most immediately noticable are the divisions of the thalamus: the internal medullary lamina can be seen separating the medial from lateral nuclear groups and surrounding the centromedian nucleus. Ventrally, the subthalamic nucleus can be seen dividing the thalamus from midbrain regions. The red nucleus, substantia nigra and white fibre bundles within the mesencephalon itself are clearly visible. The finer structure of the brainstem is readily apparent, with a clearly evident periaqueductal grey and corpora quadragemina. The pontine t e g m e n t u m is clearly distinguishable from the rectum. Fine vessels can be seen along the cortical margin, as well as crainial nerves on the brainstem. Within the cerebrum, the fimbria of the hippocampus is visible along its length, as is the typical "jellyroll" appearance of the hippocampus itself. The grey bridges between the caudate and putamen stand out, as does the grey intensity difference between the putamen and the globus pallidus. The cortical/white boundary is sharp and major fibre fascicles, such as the optic radiations, are visible within the white matter in all planes of section. As dramatic as the improvement of the images is, its production is relatively simple. Indeed, one only needs a subject who will undergo a long series of scans. As many investigators in this field have already accumulated a number of scans of the same subject(s), there may exist the basis to compose a high resolution average at a number of MR sites. S28
I nhancement of T1 MR Images Using Registration for Signal Averaging Colin J. Holmes, Rick Hoge, Louis Collins and Alan C. Evans Montreal Neurological Institule, Montreal, Quebec, Canada
The resolution of magnetic resonance (MR,) images is of central concern in anatomical and functional studies of the human brain. Most attempts to enhance the resolution of MR involve modifications of the device itself, such as the use of higher strength magnets, enhanced coils, or specialized pulse sequences. Simply decreasing the voxel size requires increasing the number of signal averages, which significantly extends the scan time, inevitably resulting in movement and other artifacts. Here we present a simple method relying on registration to achieve significant increases in signal to noise ratios and therefore higher resolution images from standard MR instruments. Many sites involved in neuroimaging have developed rubust computational methods for registering intermodality and intersubject-intramodality 3D volumes. These tools can also be applied to scans from the same subject, in the same modality, with the result being posthoc enhancement of the signal to noise ratio through averaging. As each scan can be kept short, this gain comes without the cost of long individual scantimes. A series of 27 Tl-weighted MR scans was obtained from a single subject (CJH) using a Phillips 1.5 Tesla MR. The scan parameters were: 3D sagittal volume composed of 140 1ram slices, FOV 256mm (SI) x 204mm (AP), acquired with a spoiled GRASS ( T R / T E = 1 8 / 1 0 m s , flip angle 30, NSA(NEX) 1, flow compensation on) for a total scan time of 10:50 x 27 scans. Each scan was registered and tranformed into the MNI_305 average brain space (based on Talairach 1988), using an automatic registration tool, and resampled at up to 0.5ram 3 before normalization and intensity averaging. Theoretically, the effect of averaging 27 volumes is a gain of 5.2 on the signal to noise average. This effect is accentuated by the reduction of partial volume effects. Due to the unavoidable small motions of the head between scans each region of space is imaged in slightly different voxels in each scan. As the final point sample is drawn from many volumes, all slightly displaced with respect to one another, the final subsampled voxels reduce the overall partial volume effect. The combination of these effects upon the visibility of finer structure in the resultant MR average image is striking. Most immediately noticable are the divisions of the thalamus: the internal medullary lamina can be seen separating the medial from lateral nuclear groups and surrounding the centromedian nucleus. Ventrally, the subthalamic nucleus can be seen dividing the thalamus from midbrain regions. The red nucleus, substantia nigra and white fibre bundles within the mesencephalon itself are clearly visible. The finer structure of the brainstem is readily apparent, with a clearly evident periaqueductal grey and corpora quadragemina. The pontine t e g m e n t u m is clearly distinguishable from the rectum. Fine vessels can be seen along the cortical margin, as well as crainial nerves on the brainstem. Within the cerebrum, the fimbria of the hippocampus is visible along its length, as is the typical "jellyroll" appearance of the hippocampus itself. The grey bridges between the caudate and putamen stand out, as does the grey intensity difference between the putamen and the globus pallidus. The cortical/white boundary is sharp and major fibre fascicles, such as the optic radiations, are visible within the white matter in all planes of section. As dramatic as the improvement of the images is, its production is relatively simple. Indeed, one only needs a subject who will undergo a long series of scans. As many investigators in this field have already accumulated a number of scans of the same subject(s), there may exist the basis to compose a high resolution average at a number of MR sites. S28