Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-10-14
2001-05-29
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000, C324S312000
Reexamination Certificate
active
06239597
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to, a method and apparatus to rapidly acquire T
2
weighted MR images with a steady state pulse sequence.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
G
y
and G
z
) are employed. 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.
One of the problems with acquiring T
2
weighted images is that conventional techniques require a relatively long repetition time (TR) in order to achieve a pure T
2
(proton density) image contrast. In other words, T
2
weighted imaging is generally associated with long imaging times, as compared to T
1
weighted imaging. T
1
imaging is much faster since such imaging is achieved by shortening the repetition time.
Rapid imaging methods and sequences typically use short pulse repetition times and gradient reversal echoes. Often pulse repetition times are comparable with the transverse relaxation time T
2
, whereby the transverse magnetization is not destroyed between phase encoding cycles. The common factor between most of the prior art rapid imaging sequences is the application of a closely spaced train of excitation pulses with a flip angle, usually equal to or less than 90°. The signal is sampled between the pulses, following phase encoding dephasing/rephasing by a frequency encoding or view gradient. However, changes in the various gradient patterns will lead to significant differences in the resulting steady state signal, and image contrast. The different variants of the basic rapid acquisition concept may be categorized as either spoiling or refocusing sequences, depending on whether the transverse magnetization component contributes to the steady state magnetization. In spoiling, the transverse magnetization is destroyed between cycles, and only the longitudinal magnetization component contributes to this steady state.
To preserve the steady state, which includes the transverse magnetization, the effect of incrementing the phase encoding gradient must therefore be canceled. This can be done by applying a gradient of opposite area in each cycle, between data collection and the onset of the next pulse, which has led to the development of the refocusing sequences. In prior art spoiling sequences, the signal intensities are strongly T
1
weighted, and it is not possible to obtain such pure T
1
weighted contrast by using other rapid sequences. The refocusing sequences create contrast which are neither pure T
1
nor pure T
2
weighted, but are more of a ratio of T
1
and T
2
weighting. For similar reasons, it is difficult to obtain pure T
2
weighted contrast by the refocusing technique.
Other prior art rapid imaging schemes using steady state coherent imaging methods can acquire two images simultaneously. However, one image is a mixed T
1
and T
2
contrast image, and the other is similar to the first with an additional T
2
weighting. With appropriate sequence parameters, the ratio of the two images can approach a pure T
2
weighted image, but they are not pure T
2
weighted images, and therefore are less desirable. For good T
2
weighting, the flip angle must be high, preferably near 90°. However, since there is a distribution of flip angles throughout a slice, the contrast is quite variable throughout the slice with such methods. With 3-D imaging, this is theoretically less of a problem in central slices where the flip angles can be closer to 90°. However, even with 3-D imaging, the resulting images have been less than desirable.
It would therefore be desirable to have a method and apparatus capable of rapid T
2
weighting of MR images in times that approximate T
1
imaging that can function in steady state and provide pure T
2
weighted contrast images, and obtain pure T
1
contrast.
SUMMARY OF THE INVENTION
The present invention relates to magnetic resonance imaging (MRI) and includes a method and apparatus to rapidly acquire T
2
weighted MR images in a repetition time comparable to times usually associated with T
1
weighted imaging that solves the aforementioned problems. The invention also includes a pulse sequence for use with MR image acquisition to accomplish the foregoing. Two embodiments are disclosed, both of which have in common the use of a pulse sequence that has first and second RF pulses. Each RF pulse is separated by an interval &tgr; and the phase of the second and subsequent RF pulses are incremented linearly so as to spoil undesirable RF signal coherences so that the system can operate in steady state.
A method of acquiring T
2
weighted MR images with reduced repetition time TR is disclosed which includes acquiring a first MR image having pure T
1
weighted contrast, then acquiring a second MR image having both the pure T
1
weighted contrast from the first MR image, together with pure T
2
weighted contrast. The method next includes calculating a ratio image of the first and second MR images to acquire a T
2
weighted contrast image, and then repeating the acquisition steps in steady state.
The pulse sequence disclosed for use with the method and apparatus includes first and second RF pulses, wherein each of the RF pulses is separated by an interval &tgr; and have an amplitude and a phase, wherein the phase of the second and subsequent RF pulses in the pulse sequence are incremented linearly. The sequence includes at least two data acquisition periods in which data acquisition signals A and B are acquired. The data acquisition periods have centers offset from an RF pulse in the pulse sequence by an interval &egr;. In order to operate in steady state, the slice gradient areas located between RF pulses are set to have equal areas and any phase encoding and rewinder gradients are equally paired.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to rapidly acquire T
2
weighted MR images. The MRI apparatus has a magnetic resonance imaging system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch is controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The MRI apparatus also has a computer programmed to transmit an RF pulse, then to adjust a phase of subsequent RF pulses to spoil undesirable RF signal coherences. The computer is also programmed to acquire a first MR image having pure T
1
weighted contrast, and to acquire a second MR imaging having both pure T
1
weighted contrast and pure T
2
weighted contrast. The computer then calculates a ratio of the first and second MR images to acquire a pure T
2
weighted contrast image in a repetition time (TR) that is usually associated with T
1
imaging. A T
1
image is readily reconstructed as a beneficial by-product.
In order to reconstruct the T
2
weighted image, the method includes, and the computer is programmed to, create magnitude images for each of the first and second MR images and create a mask image from at le
Cabou Christian G.
Cook & Franke S.C.
General Electric Company
Oda Christine
Price Phyllis Y.
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