Self-adaptive tracking and phase encoding during data...

Details Self-adaptive tracking and phase encoding during data... Self-adaptive tracking and phase encoding during data...

2001-02-05

2003-07-22

Paschall, Mark (Department: 3742)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C600S410000, C324S306000, C324S309000

Reexamination Certificate

active

06597937

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to the art of diagnostic imaging. It finds particular application to contrast-enhanced magnetic resonance angiography (MRA) and dynamic agent uptake studies. Although the present invention is illustrated and described herein primarily with reference to magnetic resonance angiography, it will be appreciated that the present invention is also amenable to other magnetic resonance imaging techniques and to subjects other than the human body.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field (B
0
) is set up in an examination region in which a subject being imaged or examined is placed. Via magnetic resonance radio frequency (RF) excitation and manipulations, selected magnetic dipoles in the subject which are otherwise aligned with the main magnetic field are tipped to excite magnetic resonance. The resonance is typically manipulated to induce detectable magnetic resonance echoes from a selected region of the subject. In imaging, the echoes are spatially encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix, commonly known as k-space. By employing inverse Fourier, two-dimensional Fourier, three-dimensional Fourier, or other known transformations, an image representation of the subject is reconstructed from the k-space data.
Because the vascular system contains flowing blood, a moving entity, a number of magnetic resonance angiography (MRA) techniques have been developed for imaging the vascular system. Time-of-flight (TOF) techniques rely on the time interval between the transverse excitation of spins and the acquisition of the resulting magnetic resonance signal to distinguish between moving and stationary spins. During the time interval, fresh spins move into the region from which the magnetic resonance signal is acquired and excited spins move out of the region. In contrast, the stationary spins remain fixed during the interval between RF excitation and data acquisition, with the result that the magnetic resonance signal produced by stationary spins is substantially different in magnitude from that produced by moving spins. When an image is reconstructed from such magnetic resonance signals, the image pixels which correspond to moving spins are either much brighter or much darker than image pixels corresponding to stationary spins, depending on the sequence. In this manner, the vascular system that transports moving blood is made to appear brighter or darker than the surrounding stationary or slowly moving tissues in the resultant image.
Phase contrast (PC) techniques rely on the fact that the phase of the magnetic resonance signal produced by moving spins is different from the phase of magnetic resonance signals produced by stationary or slowly moving spins. Phase contrast methods employ magnetic field gradients during the magnetic resonance pulse sequence which cause the phase of the resulting magnetic resonance signals to be modulated as a function of spin velocity. The phase of the magnetic resonance signals can, therefore, be used to control the contrast, or brightness, of the pixels in the reconstructed image. Since blood moves relatively fast, the vascular system is made to appear brighter or darker in the resulting image.
Because the above-described TOF and PC techniques are subject to drawbacks or shortcomings, contrast-enhanced MRA techniques have been developed to enhance the diagnostic capability of MRA. In contrast-enhanced MRA, a contrast agent such as gadolinium is injected into the patient prior to the scan. Typically, the injection is carefully timed so that the central lines of k-space, which govern image contrast resolution, are acquired during peak arterial enhancement, i.e., when the contrast agent concentration reaches a peak or plateau in the vasculature of interest. Similarly, in a dynamic uptake study, an agent which targets one or more specific organs or tissues of the body is administered to the patient and region of interest is imaged in an magnetic resonance scanner to gauge the rate at which the agent is taken into the organ or tissue of interest.
Currently, two methods are widely used for timing the data acquisition to coincide with bolus arrival in MRA, namely, central phase encoding and time-resolved imaging of contrast kinetics (TRICKS). The centric phase encoding method collects the most important low-frequency encoded data at the projected center of the plateau. The TRICKS method samples the central region of k-space more frequently than the other regions of k-space to increase the odds that the centrally phase encoded data will be generated with the plateau.
The major drawback of the central phase encoding method is that it is based on assumed arrival characteristics of the bolus, i.e., the “best guess” of bolus arrival. The success of centric phase encoding also depends on the reliability of bolus detection, i.e., at which phase it is triggered.
FIG. 1A
is a graph of bolus concentration in the region of interest as a function of time, and shows the correct bolus detection time t
D
resulting in a central phase encoding data collection window
12
which captures the peak or plateau period of the concentration curve
10
. However, if the assumed arrival characteristics differ from the real curve
10
such that the detected time point t
D
is not sufficiently close to the plateau period, significant image artifacts may result or, in the worst case, the images may be clinically useless.
FIG. 1B
shows the effect of such a disparity between the real curve
10
and an assumed curve
20
. In the example shown, the assumed curve
20
results in an early detection time t
D
, resulting in centric phase encoding
22
which does not capture the plateau period.
The TRICKS method does not rely on assumed arrival characteristics of the bolus. By sampling the center of k-space at a higher frequency, the probability of catching the plateau period increases. However, this method can be understood as “blind” oversampling of the center of k-space and thus, lacks assured accuracy. No timing for triggering the acquisition is necessary for TRICKS, and the success of this method depends mainly on the arrangement or strategy of phase encoding sequences (different weighting factors for k-space) and whether the plateau period is actually captured during a central phase encoding segment. One drawback of TRICKS is that while the central (contrast governing) portion of k-space is sampled more, the other (spatial resolution governing) portions of k-space are sampled less, thus resulting in a loss of high and middle frequency spatial information and potentially causing blurring of the image.
FIG. 2A
illustrates an idealized k-space segmentation strategy wherein the high contrast resolution (C.PE) data lines are centrally located in k-space and the high frequency data lines (H.PE) are located at the periphery of k-space. In
FIG. 2A
, each segment H.PE (high frequency phase encoding), M.PE (middle frequency phase encoding), and C.PE (centric or low frequency phase encoding) is sampled only once during a time window of T
imaging
.
In the TRICKS implementations of
FIGS. 2B and 2C
, the low frequency central segment (C.PE) is sampled more than the other segments. While, this increases the likelihood capturing the plateau during central phase encoding, it does not necessarily do so.
FIG. 2B
shown an ideal TRICKS implementation wherein one of the C.PE segments has successfully captured the plateau period. In the TRICKS implementation of
FIG. 2C
, however, no C.PE segment captures the plateau ideally.
Accordingly, the present invention contemplates a new and improved contrast-enhanced MRA apparatus and method wherein phase encoding steps are dynamically adjusted according to time varying information and which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, a method of magnetic resonance imaging includes acquiring

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