Magnetic resonance imaging with selective phase encoding averaging

A gradient magnetic field control (20) and a transmitter (30) are operated under the control of a timing and control computer (40) to generate magnetic resonance excitation pulse sequences. Each sequence provides phase encoding with one of a plurality of phase angles to the resultant resonance signals. A receiver (34) receives the phase encoded magnetic resonance signals which are digitized by an analog to digital converter (50) to form a plurality of views which are stored in a view memory (52). A larger plurality of views are generated adjacent a central or zero phase angle, e.g. views -63 to +64 of FIG. 2, and only one or a smaller plurality of views are generated adjacent peripheral phase angles, e.g. views -127 to -64 and +65 to +128 of FIG. 2. The slower, low frequency motion artifacts, such as respiratory motion artifacts, manifest themselves in the low frequency phase encoded views adjacent the zero phase encode angle. Averaging a plurality of views encoded adjacent the zero phase angle attenuates low frequency motion artifacts. The high frequency views adjacent the .+-.90.degree. phase angles contribute little or nothing to the low frequency motion artifacts. Accordingly, the high frequency views are either uniquely collected or a smaller number are averaged.

BACKGROUND OF THE INVENTION 
The present invention relates to the art of magnetic resonance imaging. It 
finds particular application in conjunction with motion desensitization of 
magnetic resonance images and will be described with particular reference 
thereto. It is to be appreciated, however, that the present invention may 
also be applicable to other image enhancement, modification, and 
improvement techniques. 
Heretofore, medical diagnostic magnetic resonance imaging has included the 
sequential pulsing of radio frequency signals and magnetic field gradients 
across a region to be imaged. In two dimensional imaging, a patient is 
disposed with a region of interest in a substantially uniform main 
magnetic field. A slice select gradient is applied across the field to 
select a slice or other region of the patient to be imaged. A phase encode 
gradient is applied along one of the axes of the selected slice to encode 
the material with a selected phase angle along the phase encode axis. In 
each repetition of the pulse sequences, the phase encode gradient is 
commonly stepped in regular increments from a first peripheral phase 
encode angle of +90.degree. in increments through a central phase angle of 
zero to an opposite peripheral phase angle of about -90.degree.. Each 
repetition of the pulse sequence produces a corresponding set of sampled 
data points, generally termed a view or step. In this manner, each view is 
phase encoded with a corresponding one of the phase angle increments. The 
central or zero phase encoded views provide the contrast in the resultant 
image; whereas the views phase encoded near the peripheral angles 
contribute the fine detail or resolution. A frequency encode gradient 
pulse frequency encodes the material along another axis of the slice, 
conventionally perpendicular to the phase encode axis. 
Various motions during the acquisition of magnetic resonance data degrade 
the resultant images. The motions may be divided into two groups--rapid 
motions which transpire within the time to collect one view and slower 
motions which continue to occur over the collection of several views. The 
rapid motions tend to degrade fine resolution or detail, but have little 
effect on contrast. The effects of rapid motions can be reduced using 
prior art gradient rephasing techniques or the like. 
The slower, low frequency motions tend to cause ghosts and other contrast 
defects without degrading the fine detail. The effects of slower, low 
frequency motions, such as respiratory or body movement, are commonly 
reduced by averaging. That is, the set of pulse sequences that produce the 
set of view used to reconstruct an image is repeated a plurality of times 
to collect redundant data. The redundant views corresponding to the same 
phase angle are averaged and the averaged views are utilized to 
reconstruct the image representation. Heretofore, the same number of 
redundant views have been collected corresponding to every phase angle. 
Although the number of redundant views corresponding to each phase angle 
might be as low as two, larger numbers of views, such as eight, sixteen, 
or more, are not uncommon. 
One of the drawbacks with the prior art view averaging techniques has been 
the extended scan time. To produce and average two sets of redundant views 
requires twice the scanning time of collecting a single set. Similarly, 
averaging eight or sixteen sets of redundant views increases the scan time 
by a factor of eight or sixteen respectively. Thus, reducing image 
degradation attributable to slower motions causes a corresponding increase 
in scan time. Correspondingly, shortening the scan time can be achieved by 
reducing the number of views averaged, but at the cost of greater 
sensitivity to slower motion artifacts and degradation. 
The present invention provides a new and improved magnetic resonance data 
processing technique which enables slow or low frequency motion 
degradation to be reduced without increasing imaging time or, conversely, 
to reduce imaging time without increasing artifacts from slower motion. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a set of views 
suitable for transformation into an image representation is produced. Each 
view in the set corresponds to each of a plurality of preselected phase 
encode angles. Redundant views corresponding to some, but not all, of the 
phase angles are further produced. The redundant views corresponding to 
the same phase angle are averaged before the views are transformed into an 
image representation. 
In accordance with a more limited aspect of the invention, a plurality of 
all views in the set are produced. However, a larger plurality of views 
are produced with the preselected phase angles. In this manner, a larger 
number of views corresponding to the preselected phase angles are 
averaged. 
In accordance with another more limited aspect of the present invention, 
imaging sequences with phase angles closest to a central or zero phase 
angle are repeated a larger number of times. Imaging sequences with phase 
angles closer to a maximum peripheral phase encode angle are performed 
only once or a smaller number of times. 
In accordance with another aspect of the present invention, slower motions, 
such as respiration and body movement, have a large effect on the views 
encoded near the central phase angle and little or no effect on the views 
encoded near the maximum peripheral phase angles. Redundant views encoded 
with phase angles near the central phase angle are averaged to reduce 
slower, low frequency motion degradation. Because the peripheral views are 
substantially unaffected by slower, low frequency motion, averaging 
multiples of the peripheral views has little or no effect on low frequency 
motion. 
One advantage of the present invention is that it attenuates slow, lower 
frequency motion artifacts without increasing the scan time of a standard 
scan sequence in which each view is redundantly collected the same number 
of times and averaged. 
Another advantage of the present invention is that it reduces image 
acquisition time with no reduction in motion artifact attenuation. 
Still further advantages of the present invention will become apparent to 
those of ordinary skill in the art upon reading and understanding the 
following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIG. 1, a uniform, main magnetic field is generated in a 
magnetic resonance imaging apparatus by a main magnetic field means 
including a main magnetic field controller 10 and a plurality of 
electromagnets 12. A gradient field control means 20 selectively controls 
the application of gradient fields across the main magnetic field by 
gradient field coils 22. By selectively applying current pulses to the 
appropriate gradient field coils, slice select, phase encode, and read 
gradients are selectively applied along mutually orthogonal axes to define 
an image slice or other region. A transmitter 30 selectively supplies 
radio frequency pulses to RF coils 32 to excite magnetic resonance and 
manipulate the magnetization of resonating dipoles. Magnetic resonance 
signals generated by the resonating dipoles are received by the RF coil 32 
and a receiver 34. A timing and control means 40 controls the timing and 
application of gradient and radio frequency pulses to perform imaging 
sequences as are well known in the art. 
With continuing reference to FIG. 1 and further reference to FIG. 2, each 
imaging sequence commonly includes an RF magnetic resonance excitation 
pulse, one or more slice select gradient pulses, a phase encode gradient 
pulse, a magnetic resonance inversion or other RF manipulation pulse, and 
one or more read gradient pulses. Thereafter, a magnetic resonance signal 
is received by the receiver 34 and digitized by an analog to digital 
converter 50 to form a view or line of digital data. The full set of views 
correspond to an image are accumulated and stored in a view memory means 
52. Commonly, each view is identified by the phase angle with which the 
resonance data was encoded by the selected phase encode gradient pulse. 
The digital data within or along each view or line is commonly frequency 
encoded by the read gradient pulse. 
In each repetition of the resonance imaging sequence, the phase angle is 
changed in increments. Commonly, the phase angle is stepped in increments 
from about -90.degree. through 0.degree. to +90.degree.. In a 256 view 
image data set, a central view with a zero phase encode angle view is 
generally denoted as the zeroth view. The progressively more peripheral or 
larger phase angle encoded views to one side of the central view are 
generally designated as +1 through +128. The views to the other side are 
generally designated as -1 adjacent the central view to -127 at the 
peripheral view. In this manner, a total of 256 views including the zeroth 
view, are produced. 
The zeroth or zero phase angle view contributes the lowest frequency 
components to the resultant image which in turn contribute primarily to 
the contrast. As the phase angle increases towards the .+-.90.degree. 
peripheral views, the corresponding views provide a progressively higher 
frequency component to the resultant image. The higher the frequency, the 
finer the resolution that is contributed. The lower the frequency, the 
greater the contribution to coarser resolution and contrast. Thus, the 
larger phase angles whether towards the plus or minus 90.degree. 
contribute primarily to the resolution, whereas the smallest phase angles 
contribute primarily to the contrast. 
In magnetic resonance imaging, high and low frequency motions cause motion 
artifacts. The higher frequency, rapid motions which occur in a time 
period shorter than the duration between collecting subsequent images 
produce high frequency motion artifacts. The high frequency artifacts can 
be advantageously removed utilizing prior art gradient rephasing 
techniques. The slower, low frequency motions which continue during the 
collection of several views produce low frequency artifacts which are 
commonly removed by averaging. 
The low frequency motion affects the corresponding low frequency view(s) 
collected at the corresponding low phase encode angle(s). Quicker motions 
primarily affect the intermediate views encoded with the corresponding 
intermediate phase angles. The most rapid, high frequency motions affect 
primarily the largest phase angle encoded higher frequency peripheral 
views. By selectively averaging the affected views, the corresponding 
motion artifact in the image can be attenuated. 
With reference to FIG. 3, respiratory motion provides a low frequency 
motion artifact which affects primarily the central views, i.e. the views 
phase encoded with the smallest phase angles. By repeating the resonance 
excitation sequences and collecting a second set or array of views or 
lines which correspond only to the central phase angles, redundancy is 
provided for the lowest frequency, smallest phase angle views. That is, a 
larger number of views are collected corresponding to the phase angles 
that are most affected by low frequency motion. The redundant views are 
also stored in the view memory 52. A view averaging means 54 averages the 
corresponding stored redundant views. The averaged views and any unique 
peripheral views are transferred to an averaged view memory 56. That is, 
either an average or unique view corresponding to each preselected phase 
encode angle is transferred to the averaged view memory means 54. 
In the 256 view embodiment illustrated in FIG. 3, the 128 central views 
(views -63 to +64) are each the average of two corresponding views and the 
higher frequency 128 views (views -127 to -64 and +65 to 128) are uniquely 
collected. A larger plurality of redundant central views may, of course, 
be collected and averaged. The data set may include more or less than 256 
views. For a data set with 2n views, the views would range generally from 
-n to +n and the center views would range generally from -n/2 to +n/2. 
However, due to the presence of the zeroth view, either the upper or lower 
limit is shortened one view, i.e. from -(n-1) to +n or from -n to +(n-1). 
In the preferred embodiment, the 2n views range from (n-1) to +n and views 
-(n-1)/2 through +n/2 would be redundantly generated and averaged. 
A Fourier transforms means 60 performs a dimensional Fourier transform of 
the set of views in the averaged view memory 56 to form an image 
representation for storage in an image memory means 62. A typical image 
may be a 256.times.256 array of digital pixel values. The image 
representation may be improved with various known image enhancement 
techniques and stored or displayed on a video monitor 64 or other display. 
With reference to FIG. 4, one might collect each of the more central views 
eight times and the peripheral views twice. Each of the two corresponding 
peripheral views would be averaged and each of the corresponding eight 
central views would be averaged to produce one set or array of views for 
storage in the averaged view memory 56. It is to be appreciated that 
collecting the central half of the views eight times and the peripheral 
half of the view twice can be accomplished in the same duration as taking 
five complete view sets of data. However, because eight rather than five 
of the low frequency views are averaged, low frequency motion artifacts 
are attenuated significantly more. Conversely, collecting eight of the 
central half of the views and two of the peripheral half of the views can 
be accomplished in 621/2% of the time required to collect eight views and 
with no significant loss in low frequency motion artifact attenuation. 
Of course, the views need not be divided into only two groups. Rather, as 
illustrated diagrammatically in FIG. 5, the greatest redundancy may be 
provided in the central most views, significant but less redundancy in the 
next most central views, less redundancy yet in the next most central 
views, and the least redundancy in the peripheral most views. As yet 
another option as illustrated diagrmmatically in FIG. 6, the number of 
redundant views may vary along a bell curve having its apex at the central 
most view. 
If the motion artifact to be corrected does not correspond to the lowest 
frequency views, then the degree of redundancy view may be varied 
accordingly. As illustraed in FIG. 7, relatively little redundancy may be 
provided for the central and most peripheral views with the most 
redundancy being provided for intermediate views. Such an arrangement may 
correct for motion artifacts whose primary contribution comes at the more 
redundantly collected frequencies or phase encode angles. 
The invention has been described with reference to the preferred 
embodiment. Obviously, modifications and alterations will occur to others 
upon reading and understanding the preceding detailed description. It is 
intended that the invention be construed as including all such alterations 
and modifications insofar as they come within the scope of the appended 
claims or the equivalents thereof.