Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-05-17
2001-07-24
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06265875
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus for efficient MRI tissue differentiation using an RF pulse designed to provide a frequency response combining a magnetization transfer contrast and fat suppression simultaneously.
Coronary artery disease is currently the leading cause of death in western hemisphere countries. Therefore, the visualization of the coronary arteries is an important step in preventing deaths due to coronary artery disease. Currently, coronary artery visualization is accomplished using x-ray contrast angiography, a highly invasive procedure. In x-ray angiography, a catheter is inserted into the artery through the groin area of a patient in order to accomplish x-ray angiography. It would be advantageous to acquire angiographic images of the coronary arteries without having to require such an invasive procedure.
Magnetic resonance angiography (MRA) is a non-invasive alternative to x-ray angiography that has been successfully used to image carotid arteries, the aorta, and other peripheral vessels. However, coronary arteries are small, tortuous vessels, usually no more than 1-5 mm. in diameter. Successful visualization of coronary arteries using conventional MRA is hampered because coronary arteries are often surrounded by pericardial fat and myocardium, especially the more distal segments. Under known methods, a high degree of fat suppression can be achieved using standard chemical saturation pulses. However, suppression of the myocardial signal, without an accompanying reduction in the blood signal, has heretofore been a challenging problem. Since the myocardial signal is a significant attribute to the contrast-to-noise ratio (CNR) of the coronary arteries, suppression of myocardium is essential for reliable visualization of more complete portions of the left anterior descending (LAD) coronary arteries, as well as the distal segments of the right coronary artery.
A technique known as magnetization transfer contrast (MTC) has been commonly employed in imaging sequences to improve image contrast but has heretofore been employed only in limited manner in MRA of the coronary arteries because of various limitations. The MTC technique relies on the fact that a fraction of the water in biological systems is bound to large macromolecules. This results in lowering of their tumbling rates and consequently, their relaxation times. The “bound pool” is in chemical exchange with the “free pool”, therefore resulting in transfer of magnetization. Saturation of the bound pool will then result in saturation of the free pool. MTC effects have been found to adequately suppress myocardial signals in human and animal hearts and can be produced by using either a binomial, zero-degree on-resonance excitation, or an off-resonance spectrally selective excitation. In the past, the off-resonance irradiation utilizes RF pulses of high B
1
amplitude and are set to at least 1 kHz from the water resonance frequency. The large resonance offset avoids undesired saturation of spins with long T
2
times in the imaged volume. However, the further off-resonance the irradiation is, the higher the RF power level is required for observable MTC effects. Continuous wave MTC results in too high of a specific absorption rate (SAR) to the patient, and requires an ancillary RF amplifier on conventional whole-body MR imaging scanners, and is therefore not widely used or desirable.
The off-resonant MTC technique is most effectively achieved using a train of Gaussian pulses of fixed bandwidth, for example, 500 Hz, at 1500 Hz off-resonance. The entire duration of the MTC pulse composite is typically about 200-300 ms. In MRA using a cardiac-gated 3D fast-gradient recalled echo sequence, several problems arise. First, the specific absorption rate of these pulses is very high, thereby limiting its use. It also prohibits use of continuous RF excitation, which is another mechanism for suppressing myocardial signals that can be used in conjunction with magnetization transfer based methods. However, when such RF pulses are applied at a time-delay from the start of the R-wave in the R—R interval, until mid-diastole, the resulting pulses are quite long and consequently not practical for use in interleaved or segmented 3D fast gradient recalled echo sequence, hybrid sequences, where multiple k-space lines are acquired in the same R—R interval, or any other fast image acquisition sequence.
It would therefore be desirable to have a method and apparatus capable of imaging coronary arteries with a lower effective specific absorption rate using the advantages of the MTC effect without the disadvantages while simultaneously suppressing fat tissue to provide enhanced visibility of tissue where the contrast between blood and the surrounding tissue is typically poor.
SUMMARY OF THE INVENTION
The present invention relates to a system and method for differentiating tissue types in MR imaging that can efficiently discriminate fat tissue from proteinated tissues, and each from water-based tissue, that solves the aforementioned problems.
In order to accomplish efficient MR angiography of coronary arteries, or MR images in any portion of the body where contrast between water-based tissue and the surrounding tissue is typically poor, the present invention combines fat suppression and magnetization transfer (MT) in an RF pulse to overcome the aforementioned problems. Two basic approaches are described. In one approach, by overdriving a fat saturation pulse that is off-resonant, the pulse can be designed to induce an MT effect by increasing the flip angle based on the amount of MT effect and fat suppression desired. However, increasing the flip angle, or the effective B
1
, of a fat saturation pulse that increases the MTC effect is done at the cost of degrading fat suppression at certain flip angles and care must be taken so as to not exceed the specific absorption rate desirable for a patient.
Another approach includes modulating an RF pulse with a sinusoidal function, such as that provided in a Hadamard excitation pulse. The frequency response of the resultant pulse is the frequency response of the original RF pulse shifted in frequency to both sides of the carrier. The spacing of the pulses is determined by the frequency of the modulation function. Using a Hadamard encoded pulse for fat saturation provides excitation on both sides of the transmitter frequency which provides an effective MTC effect equivalent to twice that of an equivalent single fat saturation pulse with MTC. Thus, using a Hadamard pulse, the same flip angle can be maintained for fat suppression and yet double the available MTC effect.
Therefore, in accordance with one aspect of the invention, a method of differentiating tissue in NMR imaging includes the steps of creating a spectrally selective suppression pulse having an RF pulse profile designed to produce a frequency response with high fat suppression and selecting a spectrally selective suppression amplitude to produce a magnetization transfer contrast between two tissue types. The method also includes applying the spectrally selective suppression pulse with a flip angle selected to optimize fat suppression and magnetization transfer contrast saturation simultaneously.
In accordance with another aspect of the invention, an MRI apparatus for MR angiography is disclosed that is capable of efficient tissue differentiation that includes an MRI system having a number of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system having an RF modulator controlled by a pulse control module to transmit RF signals to an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to construct a desired RF pulse based on input design criteria that includes desired bandwidth, desired percentage in-band (pass band) ripple, and desired percentage out-of-band (stop-band) ripple, and a desired flip angle so that the desired RF
Foo Thomas K. F.
Saranathan Manojkumar
Arana Louis
Gabou Christian G.
General Electric Company
Price Phyllis Y.
Ziolkowski Timothy J.
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