Shifting of artifacts by reordering of k-space

Details Shifting of artifacts by reordering of k-space Shifting of artifacts by reordering of k-space

2002-03-28

2004-01-27

Gutierrez, Diego (Department: 2859)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

Reexamination Certificate

active

06683454

ABSTRACT:

BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method for shifting motion-based artifacts from the primary region of interest in the central field of view within an image field of view (FOV) to the side parts of the FOV by manipulating the order of a segmented k-space acquisition. It will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Magnetic resonance imaging is a diagnostic imaging modality that does not rely on ionizing radiation. Instead, it uses strong (ideally) static magnetic fields, radio-frequency (RF) pulses of energy and magnetic field gradient waveforms. More specifically, MR imaging is a non-invasive procedure that uses nuclear magnetization and radio waves for producing internal pictures of a subject. Three-dimensional diagnostic image data is acquired for respective “slices” of an area of the subject under investigation. These slices of data typically provide structural detail having a resolution of one (1) millimeter or better.
The data for each slice is acquired during respective excitations of the MR device. Ideally, there is little or no variation in the phase of the nuclear magnetization during the respective excitations. However, movement of the subject (caused, for example, by breathing, cardiac pulsation, blood pulsation, and/or voluntary movement) and/or fluctuations of the main magnetic field strength may change the nuclear magnetization phase from one excitation to the next. This change in the phase of the nuclear magnetization may degrade the quality of the MR data used to produce the images.
When utilizing MRI to produce images, a technique is employed to obtain MRI signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of MRI measurement cycles, which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to discriminate NMR signals from specific locations in the subject. This is accomplished by employing gradient magnetic fields denoted G
x
, G
y
, and G
z
. These gradient magnetic fields are static magnetic fields along the x, y, and z axes exhibiting a gradient along the respective x, y and z axis. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be altered and the location is encoded in the resulting NMR signals.
Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences, which have a very short repetition time (TR) and result in complete scans, which can be conducted in seconds rather than minutes. For example, when applied to cardiac imaging, a complete scan of a series of images showing the heart at different phases of its cycle or at different slice locations can be acquired in a single breath-hold.
There are two common techniques for acquiring cardiac MR images. The first is a prospectively gated, single-phase, multi-slice conventional spin echo sequence. In each cardiac cycle, data at different spatial locations are acquired with the same k-space phase encoding value. Images at the different spatial locations are then acquired at different temporal phase of the cardiac cycle.
In gated spin echo, data for each slice location is acquired at a fixed delay from the cardiac R-wave. With variations in the cardiac rhythm, the heart may be at a different phase of the cardiac cycle when data is acquired even though the cardiac delay time is the same. Normal variations of the cardiac cycle usually result in disproportionately larger changes in the diastolic portion of the cardiac cycle, and gated spin echo images acquired at the end of the cardiac cycle often exhibit blurring or ghosting artifacts.
Another disadvantage of gated spin echo is that images at different slice locations are acquired at different cardiac phases. Hence, it may be difficult to relate information from one spatial location to the next as the heart is pictured at different phases of the cardiac cycle. Furthermore, small structures may also be missed due to inadequate temporal and spatial coverage. Motion of the heart during the cardiac cycle can also lead to image contrast variations from slice to slice due to differential saturation or inter-slice cross talk.
A short TR gated gradient echo pulse sequence may be used to acquire (cine) images at multiple time frames of the cardiac cycle. As described in U.S. Pat. No. 4,710,717, conventional cine pulse sequences run asynchronously to the cardiac cycle with the phase encoding value stepped to a new value at each R-wave trigger. In CINE, each RF excitation pulse is applied at the same spatial location and repeated at intervals of TR in the cardiac cycle. Since the sequence runs asynchronously, the RF excitation pulses may occur at varying time delays from the R-wave from one cardiac cycle to the next. On detection of the next cardiac R-wave, the acquired data from the previous R—R interval are resorted and interpolated into evenly distributed time frames within the cardiac cycle. This method of gating is also known as retrospective gating as the data for the previous R—R interval is resorted only after the current R-wave trigger is detected.
The cardiac cycle is divided into equal time points or frames at which images of the heart are to be reconstructed. In order to reconstruct images at each of these time points, data acquired asynchronously is linearly interpolated to the pre-determined time points in the cardiac cycle. In order to account for variations in the cardiac R—R interval during the scan (from changing heart rate), the interpolation varies from cardiac cycle to cardiac cycle, depending on the R—R interval time. This method allows reconstruction of images at any phase of the cardiac cycle, independent of variations in heart rate. As in gated spin echo, only one k-space phase encoding view is acquired per heartbeat. The total image acquisition time is then on the order of 128 heart beats.
Faster scan times can be achieved by segmenting k-space and acquiring multiple phase encoding k-space views per R—R interval. The scan time is accelerated by a factor equal to that of the number of k-space views acquired per image per R—R interval. In this manner, a typical cine (e.g. movie or temporal series of images) acquisition with a matrix size of 128 pixels in the phase encoding direction can be completed in as little as 16 heart beats, when 8 k-space views per segment are acquired.
Multiple phases of the cardiac cycle can be visualized by repeated acquisition of the same k-space segment within each R—R interval but assigning the data acquired at different time points in the cardiac cycle to different cardiac phases. Thus, the cardiac cycle is sampled with a temporal resolution equal to the time needed to acquire data for a single segment, such that the temporal resolution equals the views per second, vps multiplied by the TR, where vps is the number of k-space lines per segment, the TR is the pulse sequence repetition time. The total scan time is then given as: the quantity y
res
divided by vps, times the R—R interval time, where y
res
is the number of phase encod

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