Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-02-06
2004-09-28
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06798199
ABSTRACT:
TECHNICAL FIELD
The invention relates to magnetic resonance (MR) imaging, and more particularly to synchronization of MR imaging data to motion of a patient.
BACKGROUND
Synchronizing MR images to the motion of a patient, e.g., to the beating of the heart, respiration of the lungs, or motion of a limb, gives the diagnostician images that have a known correspondence to the motion of interest, e.g., to the phase of the cardiac cycle. Such synchronization can be useful for both cine and still MR images. To achieve synchronization, it is necessary to have a timing signal that is indicative of the phase or position of the body. E.g., in cardiac imaging the timing signal might indicate the start of each cardiac cycle.
One technique for providing a timing signal in cardiac imaging is to connect appropriate electrodes to the patient for monitoring the patient's ECG while MR imaging data is collected. However, the magnetic fields and pulsed magnetic field gradients used in MR can interfere with the collection of the ECG signal. Special algorithms have been developed in an effort to overcome these difficulties. Chia et al., “Performance of QRS Detection for Cardiac Magnetic Resonance imaging with a Novel Vectorcardiographic Triggering Method,” Journal of Magnetic Resonance Imaging, 12:678-688 (2000). Fischer et al., “Novel Real-Time R-Wave Detection Algorithm Based on the Vectorcardiogram for Accurate Gated Magnetic Resonance Acquisitions,” Magnetic Resonance in Medicine, 42:361-370 (1999). Applying the ECG electrodes to the patient, and setting up to acquire the ECG data, is relatively complex and time consuming. And it can become necessary during imaging to relocate the electrodes for a viable ECG signal, and this typically requires that MR image acquisition be stopped and the patient withdrawn from the bore of the MR unit.
Synchronizing MR images is of particular value in segmented or interleaved cine imaging, in which the data for each image is derived from different cycles of the motion of interest. For the images to be meaningful, only data from corresponding phases of different cycles should be combined, hence the need to synchronize the imaging data with the motion. For example, in segmented cardiac cine imaging, the k-space lines for each image may come from 14 or more different cardiac cycles occurring during a single breath hold.
Synchronization of segmented or interleaved cine imaging can be done either prospectively or retrospectively. When done prospectively, imaging data acquisition begins in response to the timing signal derived from ECG electrodes, or alternatively from a finger pulse oximeter or other device designed to monitor a physiological signal that is synchronous with the cardiac cycle. Data acquisition typically continues for a fixed time interval, long enough to cover the systolic phase and the beginning of the diastolic phase. Then, there is typically a quiescent period until the next timing signal. Alternatively, the synchronization can be done retrospectively by continuously acquiring imaging data a synchronously with the ECG-based timing signal, while the time each line of data is acquired relative to the last trigger signal is recorded. After the acquisition, the data are assigned to the appropriate phase of the cardiac cycle based on the recorded timing data.
Attempts have been made in the prior art to derive timing information directly from MR data, in order to eliminate the need for an ECG or other additional timing measurement. But these efforts have relied on collecting additional MR data that is not used in producing the MR images.
For example, Spraggins U.S. Pat. No. 4,961,426 and Spraggins, “Wireless Retrospective Gating: Application to Cine Cardiac Imaging,” Magnetic Resonance Imaging, 8:675-681 (1990) teach acquiring additional “timing slices” from which a timing signal can be derived. The timing data, in the form of an echo without phase encoding, is interleaved with imaging data acquisition (every other acquisition is timing data), and can be acquired from a different area of the heart than that being imaged (e.g., an area where motion is more visible). Kim et al., “Extraction of Cardiac and Respiratory Motion Cycles by Use of Projection Data and Its Application to NMR Imaging,” Magnetic Resonance in Medicine, 13:25 (1990) uses a similar approach, except that the additional data is transformed into the spatial domain (Spraggins had used the frequency domain data directly) to provide a signal representative of a projection of an image slice onto a line oriented along the direction of time data acquisition.
Another approach is found in Vasanwala et al., “Prospective MR Signal-Based Cardiac Triggering,” Magnetic Resonance in Medicine, 42:82-86 (1999), wherein a special “triggering sequence” is used to acquire velocity encoded data representative of aortic blood velocity. When a triggering event is found, the system switches from the triggering sequence to an imaging sequence.
In the area of respiratory gating, a technique known as navigator gating or navigator echo derives a timing signal from extra, non-imaging data. Ehman & Felmlee, “Adaptive technique for high-definition MRI of moving structures.” Radiology, 173:255-263 (1988). Typically a projection perpendicular to the diaphragm is acquired while an edge detection algorithm is used to determine respiratory cycle position.
SUMMARY
In general, in a first aspect, the invention features synchronizing MR imaging data with motion of a patient (e.g., the beating of the heart) by extracting timing information from the MR imaging data, itself, rather than relying solely on additional data acquired solely for timing. By deriving the timing information from the MR imaging data, superior image quality is possible. For example, in cardiac imaging, where images are based on data collected during a single breath hold, more of the available time during the breath hold is available for collecting imaging data.
Superior image quality may also result from direct synchronization of the MR data with the motion affecting it, rather than some indirect measure in the form of external physiological signals, or MR signals not used in the image generation.
Clinical productivity is increased because data collection time is reduced, and because less time is required to prepare a patient for the MR study (e.g., because it is not necessary to attach ECG electrodes).
The invention solves the problem of acquiring an ECG signal in the hostile environment of an MR unit. Inability to acquire a reliable ECG signal is a common cause for failed cardiac MRI exams. Costly and complex equipment (e.g., ECG monitoring equipment) is not needed to produce the timing information.
A wide range of body motions can be synchronized, including voluntary motions like muscular contraction or chewing, as well as involuntary movements like respiration. Respiratory motion is a major cause of artifacts and poor image quality in MR scans of the chest and abdomen. Respiratory motion information can be extracted directly from the MR data and, used to synchronize the image data with the quiescent period of the respiratory cycle, avoiding motion artifacts. This allows patients with difficulty controlling their breath (e.g., elderly patients, infants) to have data collected during free breathing, avoiding the common requirement of breath holding for MRI of the chest or abdomen.
Timing signals can be derived from fetal MR data to avoid the complexity of measuring a fetal ECG, enabling the acquisition of high temporal and spatial resolution MR images of the fetal heart, something which to date has not been possible.
Preferred implementations of the first aspect of the invention may incorporate one or more of the following:
Imaging data may be acquired along radial or spiral k-space trajectories, so that timing information may be extracted from frequently collected k-space points at or near the origin. Depending on the method of extraction, the timing information may be acquired from the raw k-space data or from k-space data transformed into the spatia
Larson Andrew C.
Laub Gerhard
Simonetti Orlando P.
White Richard D.
Fish & Richardson P.C.
Shrivastav Brij B.
Siemens Medical Solutions USA , Inc.
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