Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-03-18
2001-06-26
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06252400
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In MRI, a substantially uniform temporally constant main magnetic field, B
0
, is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via an RF coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding RF magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
Different tissues of the body have different pairs of relaxation properties that are characterized by a pair of time constants: T1 which is the spin-lattice relaxation time, and T2 which is the spin-spin relaxation time. Therefore, different images and visualization of different anatomical structures are obtained depending upon the time constant most heavily relied upon. In this regard, a T1 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T1 relaxation properties of the tissue, and a T2 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T2 relaxation properties of the tissue.
In any event, the appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio &ggr; of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (
1
H) dipoles have a resonance frequency of approximately 64 MHZ. Generally in MRI, the hydrogen species is excited because of its abundance and because it yields a strong MR signal. As a result, typical MRI apparatus are equipped with built-in whole-body RF coils tuned to the resonant frequency for hydrogen.
For certain applications it is desirable to obtain a T
1
weighted image. Moreover, at times, having a spin echo (i.e., an echo derived from application of an RF pulse) image appearance is also advantageous. There are however obstacles to overcome, such as timeliness and specific absorption rate (SAR). With regard to the SAR, SAR=Joules of RF/Second/kg of body weight=Watts/kg. When the SAR is high, it leads to unwanted heating and potential burning of body tissue. That is to say, introduction of high levels of high energy RF pulses into the patient being imaged has the potential of burning the patient.
While prior MRI techniques have been adequate for their intended purposes, certain drawbacks make them less than ideal for the task at hand. For example, the typical spin echo (SE) technique has a relatively low SAR compared to other MRI sequences. However, the main disadvantage of the typical SE technique is that the acquisition time is lengthy relative to other techniques which collect multiple echos per TR (i.e., the time to repeat, or in other words, the time between excitations). While the conventional fast spin echo (FSE) technique results in improved timeliness over the SE technique, its introduction of high energy RF refocussing pulses leads to increased SAR issues. The SAR issues can be reduced through the use of a conventional gradient and spin echo (GSE) technique. However, the disadvantage of the typical GSE technique is that the TE (i.e., the time to echo, or in other words, the time from excitation to spin echo) is longer than desirable for a T1 weighted image due to the fact that one or more field echos or gradient echos (i.e., echos generated as a result of a magnetic gradient switching polarities) are acquired prior to the spin echo. Moreover, the typical GSE technique, through its particular use of field echos, reduces the true spin echo nature of the image and introduces artifacts typically associated with field echo imaging, such as susceptibility.
The present invention contemplates a new and improved MRI technique which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a method of MRI is provided. It includes inducing with an MRI apparatus a series of spin echos which issue from a subject being imaged without inducing intervening echos therebetween. The spin echos are received with the MRI apparatus in due course. Thereafter, the MRI apparatus is employed to induce a series of gradient echos which issue from the subject being imaged following the series of spin echos. Like the spin echos, the gradient echos are received with the MRI apparatus. Ultimately, an image representation of the subject is reconstructed from the received spin and gradient echos.
In accordance with a more limited aspect of the present invention, the series of spin echos consists of three spin echos.
In accordance with a more limited aspect of the present invention, the series of gradient echos consists of one gradient echo.
In accordance with a more limited aspect of the present invention, the spin echos are induced with the MRI apparatus by subjecting the subject to an initial resonance exciting RF pulse and a subsequent series of refocusing RF pulses. Each spin echo is induced following its corresponding refocusing RF pulse.
In accordance with a more limited aspect of the present invention, the resonance exciting RF pulse has a 90°flip angle, and the refocusing RF pulses have a 180° flip angle.
In accordance with a more limited aspect of the present invention, the series of gradient echos are induced with the MRI apparatus by subjecting the subject to a series of magnetic gradient pulses.
In accordance with a more limited aspect of the present invention, the method further includes repeating the echo inducing and receiving steps a plurality of times such that with each iteration a multiple echo acquisition is received with the MRI apparatus.
In accordance with a more limited aspect of the present invention, more spin echos are induced and received than gradient echos.
In accordance with a more limited aspect of the present invention, the reconstruction step includes: sampling data from the received spin and gradient echos; mapping the sampled data into k-space with data from the gradient echos filling in outermost sections of k-space; and, transforming the data in k-space to reconstruct the image representation of the subject.
In accordance with another aspect of the present invention, a pulse sequence for use in an MRI apparatus is provided. It includes a set of RF pulses applied to an RF coil of the MRI apparatus. The set of RF pulses induces a set of multiple contiguous spin echos that issue from a subject being imaged. The pulse sequence also includes a set of gradient pulses applied to a gradient coil assembly of the MRI apparatus. The set of gradient pulses induces a set of gradient echos that issue from the subject being imaged following the set of multiple contiguous spin echos.
In accordance with a more limited aspect of the present invention, the set of multiple contiguous spin echos includes three spin echos.
In accordance with a more limited aspect of the present invention, the set of gradient echos includes one gradient echo.
In accordance with a more limited aspec
Loncar Mark J.
Orefice Thomas P.
Fay Sharpe Fagan Minnich & McKee LLP
Fetzner Tiffany A.
Oda Christine
Picker International Inc.
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