Multi-dimensional spatial NMR excitation

Details Multi-dimensional spatial NMR excitation Multi-dimensional spatial NMR excitation

2001-06-28

2003-04-01

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S307000, C324S318000

Reexamination Certificate

active

06541971

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with magnetic resonance imaging when exciting a region of interest or volume restricted in two or more dimensions and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to other applications using magnetic resonance imaging and is not limited to the aforementioned application.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. A series of radio frequency (RF) pulses are applied to the examination region to excite and manipulate magnetic resonance in hydrogen nuclei or other selected magnetic dipoles. Gradient magnetic fields are conventionally applied to encode information in the excited resonance.
Often, it is desired to excite a localized region of interest rather than whole slabs or slices that contain the region of interest. For example, it is often desired to examine blood flow at the heart. If a diagnostician is only interested in the region directly around the heart, collecting data from the regions of the lungs, spine, or muscular regions around the region of the heart increases image time. Moreover, collecting data in these regions can cause image artifacts that degrade the cardiac image.
Several methods exist for exciting resonance in a limited region. When applying fundamental one dimensional slice selective excitation principles multiple times, resonance is excited and manipulated in two intersecting slices or slabs within a patient to generate data from the intersecting region. For example, a 90° excitation pulse excites resonance in a first slice. A 180° inversion pulse in an intersecting slice causes a spin echo to form in the column of intersection. Of course, the magnetization in the non-intersecting portions of the slices or slabs remains perturbed until it decays. Other excitation pulse sequences have also been utilized to render the intersection of two slices or slabs with a different spin profile than the remainder of excited spins. For three dimensional localizing, an RF manipulation pulse is applied in another slice or slab intersecting the parallelepiped intersection of the first two to define a volume that is finite in three dimensions. Various excitation schemes have been used such as 90°-180°-180°, 90°-90°-90°, and the like.
Another method excites areas around the region of interest, and subsequently spoils them with gradient fields, leaving the region of interest unspoiled and excited or ready for excitation. K-space excitation concepts in two and three dimensions are also used. Various techniques use total tip angles that are not large, or apply constraints to mathematical analyses to define a linear or additive nature. Excitation and inversion pulses can be combined and applied to regions, as well as non-inverted pulses followed by the subtraction of 2, 4, or 8 excitations.
Depth pulses that use spatial characteristics of transmit fields are also used. Higher order gradients that are pulsed along with the RF excitation pulses are used. In some applications, signals are excited and scrambled or saturated outside the region of interest to dampen signal from anywhere but the region of interest. RF transmit coils that are very small, and close to the subject are used to excite small regions commensurate with their geometric size and shape.
These prior techniques all possess one or more of five basic limitations. One problem is that they take large amounts of time to perform, and tend to sacrifice scanning efficiency or patient throughput. A second problem is that they perturb signal outside the imaging region, limiting their practical use in interleaved or multiplexed applications. A third problem is that these excitation schemes may have poor spatial localization. A fourth problem is that signal to noise ratio decreases if performed in rapid repetition circumstances. A fifth problem is that additional hardware may be required beyond what is typically provided with an MRI device.
Many of these techniques fall short of desired quality in the images that they produce. Spatial definition of the excitation may not be sharp. Poor sharpness can be a result of broad or blurred excitation regions. Also, unwanted excitation outside the region of interest can cause poor spatial definition.
In certain applications, navigator scans are used to gather information to be used in determining factors in the imaging scan. For instance, navigators can be used to determine cardiac or pulmonary gating, or to determine boundaries of internal structures. A common side effect of navigator scans are dark crosses “burned” into images. These are an inconvenience at best, as the operator must worry about where to place the artifact in the final image so as not to obscure areas of interest. This becomes problematic when imaging larger regions, for instance, the liver, where it is more likely the artifact will obscure portions of the region of interest.
In addition to poor image quality, these methods often affect dipoles outside of the prescribed region such that they are saturated, inverted, or otherwise perturbed from desired conditions. If it is desired to also image these regions, the operator must wait for the perturbations to settle, else, the perturbed dipoles are not in a satisfactory state to precess together with the anticipated signal magnitude and phase. The down time in between regions further lengthens scan time and decreases patient throughput. This is problematic for any application that uses subsequent excitations wherein one excitation target s regions perturbed by one or more previous excitations.
If a subsequent sequence or sequence segment attempts to image through a perturbed slice or slab, the regions with perturbed magnetization give in appropriate responses, such as abnormally high or low signal strength. These inappropriate responses not only create artifacts in the perturbed region, but can cause artifacts elsewhere. Similarly, if one of the non-imaging resonance excitations or manipulations is applied in a slice that intersects a perturbed region, the perturbed region can generate additional echoes or other responses that contribute erroneous signal to the image.
Some of the aforementioned methods require additional hardware, such as specialized RF coil geometry, extra gradient field-producing hardware, and the like. These additions increase cost and system complexity, and are typically not implementable with software upgrades.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of magnetic resonance is provided. Magnetic resonance is excited in a subject by using temporally interleaved, non-commuting radio frequency (RF) pulses and their inverses. The RF pulses are applied in the presence of at least two different gradient fields.
In accordance with another aspect of the present invention, a magnetic resonance apparatus is provided. A main magnet assembly generates a main B
o
field in an imaging region. A gradient coil assembly and an RF coil assembly generate non-commuting pulse sequences that excite magnetic resonance in a region finite in two or more dimensions while leaving adjacent regions unperturbed. A gradient synthesizer synthesizes gradient pulses in accordance with a selected geometry. An RF pulse synthesizer synthesizes RF pulses in accordance with the selected geometry. A phase and timing circuit orders the gradient and RF pulses. A reconstruction processor processes received magnetic resonance signals into an image representation of a subject. Optionally, the phase and timing circuit spaces the gradient and RF pulses according to the B
o
field strength and the chemical shift between resonances.
In accordance with another aspect of the present invention, a magnetic resona

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