Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2001-11-16
2003-12-23
Gutierrez, Diego (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06667618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance apparatus, of the type wherein a positional change of a region of an examination subject to be imaged relative to an imaging volume of the apparatus is acquired with orbital navigator echos.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for acquiring images of the inside of the body of a subject to be examined. In a magnetic resonance apparatus, rapidly switched gradient fields are superimposed on a static basic magnetic field. For triggering magnetic resonance signals, radio-frequency signals are emitted into the examination subject, the magnetic resonance signals that are triggered are being detected, and image data sets and magnetic resonance images being produced on the basis thereof. The magnetic resonance signals are detected by a radio-frequency system, are demodulated in phase-sensitive fashion and converted into complex quantities by sampling and analog-to-digital conversion. These complex quantities are stored as data points in a k-space dataset from which an appertaining image dataset, and thus a magnetic resonance image, can be reconstructed with a multi-dimensional Fourier transformation.
Functional imaging in medicine refers to all methods that utilize a repeated scanning of a structure of organs and tissues in order to image temporally changing processes such as physiological functions or pathological events. In the narrower sense, in magnetic resonance technology functional imaging refers to measuring methods that make it possible to identify and image sensory stimuli and/or areolae in the nervous system stimulated by a motor, sensory or cognitive task, particularly the cerebral areolae of a patient.
The BOLD effect (Blood Oxygen Level Dependent) is the basis of functional magnetic resonance imaging. The BOLD effect is based on different magnetic properties of oxygenated and de-oxygenated hemoglobin in the blood. An intensified neural activity in the brain is assumed to be locally connected with an increased delivery of oxygenated blood, which causes a corresponding intensity boost at a corresponding location in a magnetic resonance image generated with a gradient echo sequence.
In functional magnetic resonance imaging, for example, three-dimensional image datasets of the brain are registered every two through four seconds, for example with an echo planar method. After many image datasets have been registered at various points in time, the image datasets can be subtracted from one another, for example for forming images referred to as activation images, i.e. they can be compared to one another in view of signal differences for the identification of active brain areas. Even the slightest positional change of the brain during the overall exposure time span of the functional magnetic resonance imaging leads to undesirable signal differences that mask the brain activation that is being sought.
In one embodiment of a functional magnetic resonance imaging, image datasets of a region to be imaged are generated with an identical location coding in a time sequence. A retrospective motion correction of the image datasets is implemented following thereupon. Differences between the image datasets that are a result of a positional change of the imaged region with respect to the apparatus during the time sequence thus can be identified and corrected. To that end, a global difference between two image datasets is minimized, with a positional change between the two image datasets, that can be described by motion parameters, being linearized by a Taylor development of the first order, with the assumption of a uniform body motion. The minimization ensues iteratively by the motion parameters being repeatedly estimated with the linearization and applied to one of the two image datasets with interpolations. Such methods are known as Gauss-Newton method in the literature. For a more detailed description, the book by R. S. J. Frackowiak et al.,
Human Brain Function
, Academic press, 1996, particularly Chapter 3, pages 43 through 58 is referenced as an example.
In another embodiment of a functional magnetic resonance imaging, a prospective motion correction is implemented during the execution of the functional magnetic resonance imaging. To that end, possible positional changes, i.e. rotations and translation of the region to be imaged, are acquired from image dataset to image dataset by, for example, orbital navigator echos and a location coding is correspondingly adapted during the execution.
An orbital navigator echo is a magnetic resonance signal that is characterized by a circular k-space path and that is generated by a specific navigator sequence. A navigator echo is registered just like a magnetic resonance signal employed for image generation and is correspondingly stored in a navigator echo dataset as complex quantities for data points of k-space that form the circular k-space path. A positional change between the points in time can be determined on the basis of orbital navigator echos that are generated at different points in time. To that end, for example, the navigator sequence is implemented before each generation of an image dataset, a navigator echo is registered, and an appertaining navigator echo dataset is compared to a reference navigator echo dataset for acquiring positional changes.
As is known, a relationship between the image space and k-space exists via a multi-dimensional Fourier transformation. According to the shift theorem of the Fourier transformation, a translation of the region to be imaged in the image space is expressed as a modified phase of complex quantities of data points of k-space. A rotation of the imaged region in the image space effects the same rotation of appertaining data points in k-space. In order to decouple (distinguish) a rotation from a translation in k-space, only amounts of the complex quantities are considered for rotations. A rotation of the imaged region relative to a reference point in time thus can be identified by a comparison of amount values of the navigator echo dataset to those of the reference navigator echo dataset. The phase values are compared for a translation.
For acquiring arbitrary positional changes in three-dimensional space, a respective orbital navigator echo is generated in three planes that are orthogonal to one another. Given positional changes with rotations up to ±8° and translation up to ±8 mm, an imprecision of up to approximately ±1.5° and ±1.5 mm is to be expected for an arbitrary positional change in this contest. Such imprecision can be improved by a repetition of the orbital navigator echos for specifically directed positional changes. After a comparison of a first navigator echo dataset to the reference navigator echo dataset corresponding to an identified positional change, a location coding is adapted for this purpose, and a second navigator echo dataset is registered with the adapted location coding. This dataset in turn is compared to the reference navigator echo dataset, which again leads to the adaptation of the location coding when a positional change is found. Particularly given positional change with a rotational component, however, no improved precision can be achieved in view of the rotational component. The above-discussed use of the orbital navigator echos is explained in greater detail, for example, in the article by H. A. Ward et al., “Prospective Multiaxial Motion Correction for fMRI”, Magnetic Resonance in Medicine 43 (2000), pages 459 through 469.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved method of the type initially described which allows positional changes to be acquired with, among other things, a high precision.
This object is achieved in accordance with the invention in a method for the operation of a magnetic resonance apparatus, which allows a positional change of a region of an examination subject to be imaged relative to an imaging
Gutierrez Diego
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Vargas Dixomara
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