Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-12-30
2002-12-10
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S419000, C600S420000, C382S128000, C382S130000, C382S154000, C382S254000, C324S307000, C324S309000, C324S310000
Reexamination Certificate
active
06493569
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates generally to an improved method for acquiring magnetic resonance images (MRI) of blood-carrying vessels, and more particularly to, a method and apparatus to acquire post-contrast images to visualize arterial and venous structures that is not time dependent on the acquisition of the images during the arterial or immediate post-injection period of a contrast bolus and uses steady-state free precession (SSFP) pulse sequences.
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
y
. 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. Magnetic resonance angiography (MRA) is an emerging technology for the non-invasive assessment of arterial and venous structures. Intravenously administered contrast agents increase the visualization of these structures for contrast-enhanced MRA, especially if performed during their initial passage (a.k.a. first pass or arterial phase) in the target vessel. The imaging of small vessels with contrast-enhanced MRA techniques however requires a compromise of image spatial resolution and imaging time. In general, higher spatial resolution requires extended imaging time. Extended imaging time, however, diminishes the ability to achieve high arterial signal-to-noise (S/N) since bolus duration must be extended to match the elongated imaging time. This is problematic since a slower bolus administration results in lower achieved concentrations of contrast media. In addition, by prolonging the acquisition period, the signal intensity of the venous structures is increased due to venous re-circulation. Therefore, extending or prolonging data acquisition after the initial contrast passage will lead to not only diminished arterial signal intensity but also increased venous signal intensity, the result of which is compromised ability to visualize diseased vessels.
In order to improve vascular depiction, background suppression is usually employed. Background suppression for MRA is obtained by either applying fat suppression pulses during the first pass acquisition, or subtracting out the background signals using a pre-contrast mask. The pre-contrast mask image has identical acquisition parameters as the first pass acquisition. Signal intensity for all structures in the pre-contrast mask image is identical to that of the first pass acquisition, except for the vascular structures. Subtracting the pre-contrast mask from the first pass image then yields only signal from vascular structures. This is particularly important in imaging the small vessels such as may exist in the calf region in a patient because of the amount of fat and soft tissue background which often obscures adequate visualization of the small vessels. However, there are problems with both techniques. For example, applying fat suppression pulses is dependent on good magnetic field homogeneity and tends to increase the overall scan time. This increased scan time also increases the possibility of venous signal contamination and suboptimal arterial signal. The technique of using a mask subtraction requires that the patient not move significantly between the mask acquisition and the contrast-enhanced acquisition. Failure to do so will cause mis-registration artifacts in the reconstructed image, thereby resulting in inadequate background suppression and artifacts from the subtraction of mis-registered objects.
Coherent steady-state free precession (SSFP) is a technique in which the free induction decay (FID) signal (i.e., S
+
signal) and the spin echo signal (S
−
signal) from a train of RF pulses are refocused within each repetition time (TR) interval. The zeroth gradient moments accumulate to zero at the end of each TR interval. This results in the same amount of transverse and longitudinal magnetization being generated after each radio frequency (rf) pulse and increases the available image signal-to-noise ratio (S/N). However, use of this contrast acquisition technique results in high signal intensities from fat which do not decrease significantly with increasing flip angles. Furthermore, as the tissue contrast is a function of the ratio of the spin-spin relaxation time (T
2
) and the spin-lattice relaxation time (T
1
) high signal intensity is also returned from fluid. Subsequently, high signal intensity is obtained from the internal abdominal organs. This significant drawback has prevented coherent SSFP acquisition from being used with first pass MRA since there is but one chance to acquire a good image. Furthermore, coherent SSFP acquisition results in high signal intensities from water as well. Therefore, the signal from the small bowel, bladder, etc., remains high in both the post and pre-contrast SSFP images which diminish its application for imaging vessels in the abdomen and pelvis. Since the motion from these structures cannot be predicted and is fairly random, subtraction cannot effectively suppress signals from these structures.
On the other hand, coherent SSFP images have higher image S/N than conventional gradient echo images as used for conventional contrast-enhanced MRA. These S/N improvements of SSFP can also improve visualization of vessels having slow or disrupted flow as seen in areas of stenosis or intimal pathology circumstances which often result in the over-estimation of disease using conventional contrast-enhanced MRA methods. It would therefore be desirable to have a technique, or a series of techniques, to maximize the use of SSFP for MRA applications that do not require the use of a pre-contrast mask image, and is therefore not as sensitive to the time relative to the administration of a contrast bolus, nor spatially limited by the same temporal considerations of bolus kinetics.
SUMMARY OF INVENTION
The present invention relates to a system and method for acquiring MR images using SSFP pulse sequences to produce images with high background suppression and significant contrast, such as between vessels and adjacent soft tissue, such as fat, muscle, bone marrow, fluid, and such, that solves the aforementioned problems.
Since it is known that the coherent SSFP technique results in an acquisition with high signal intensity for blood, water, and fat, the present invention takes advantage of this result by repeating such an acquisition with an additional S
−
SSFP acquisition in which structures with moving blood have relatively low signal intensity. The S
−
SSFP acquisition is an incoherent steady-state technique that acquires signal only from the FID that is refocused by the subsequent rf pulse. Thus, tissue contrast is a function of T
2
and it has similar signal intensity characteristics as the coherent SSFP signal. The exception is that by spoiling the FID, the refocused echo is highly sensitive to flow related dephasing, leading to dark signal in vascular structures. By subtracting the S
−
SSFP image from the SSFP image, a high image S/N of preferentially arterial and venous structures can be obtained. This is possible even where both images a
Foo Thomas K. F.
Ho Vincent B.
Della Penna Michael A.
GE Medical Systems Global Technology Company LLC
Horton Carl B.
Lateef Marvin M.
Lin Jeoyuh
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