High resolution multiline ultrasonic beamformer

The present invention is a multiline beamformer for use in a medical ultrasound scanner using a phased array transducer. Lines of ultrasound energy are generated having a pressure at the array aperture corresponding to sin (.pi.x)/.pi.x, where x is the normalized distance of the array element measured from the center of the array. This transmitted energy creates a flat field in space which can then be received in multiple receive lines, using bundled, parallel time delays.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus used in medical ultrasound 
imaging. In particular, it relates to an apparatus which uses multiline 
beamforming. 
The term "multiline beamforming" refers to a method of image reconstruction 
wherein multiple receive lines are used for each transmit line. The 
obvious advantage of multiline beamforming is the potential for obtaining 
high frame rates with a given line density. In particular, by 
reconstructing multiple, simultaneous receive lines for each transmit 
line, it is possible to obtain frame rates equivalent to the multiple of 
receive lines generated. For example, by reconstructing three simultaneous 
receive lines for each transmit line, frame rates of approximately 120 
frames per second with 128 lines per scan at a range of 15 centimeters 
could be generated. This approximation is based upon a velocity of 
propagation of approximately 1.5 mm/.mu.sec. Cardiac scans obtained at 
this speed and viewed at a lower rate could reveal details of motion which 
would aid in the diagnosis of certain clinical problems, such as valvular 
disease and septal defects. 
Speckle reduction could be obtained at the same frame rate as is presently 
used. Averaging frames obtained under different conditions, such as 
aperture, frequency, and zone modulation, could be preformed, and the 
results could be displayed at different frame rates. Such averaging could 
improve image quality significantly. Also, it might be possible, at high 
frame rates, to view the motion of blood by following the speckle pattern 
produced by the scattering of red blood cells. Using this method, viewing 
would be performed on frames replayed at a slower frame rate than the rate 
at which they were obtained. Similarly, this method could be used to speed 
up the doppler flow imaging scan rates. Presently, it is necessary to slow 
the frame rate down to rates of 4 to 10 frames per second to acquire 
doppler data for a ninety degree sector. Multiline beamforming offers the 
potential for increasing this rate significantly. 
While the idea of multiline beamforming has been around for some time, 
heretofore, there has been no practical way to implement a device 
employing multiline beamforming without degrading image quality. 
During simulations performed previously to determine the effect on image 
quality of this method of imaging, it was discovered that image quality 
was degraded. It was postulated that the reason for the poor image quality 
was the narrow width of the transmit beam. Conventional methods of 
widening the beam, such as decreasing the transmit aperture and focusing 
at large distances resulted in loss of resolution with very little 
improvement in image texture. 
Skipping transmit lines introduces artifact into the image. The larger the 
angular spacing between transmit lines, the more apparent the artifact 
will be. The increased artifact results from the decreased number of 
samples as a function of angle. Accordingly, the image, which contains 
special frequencies in excess of those which can be reproduced without 
artifact at the given sample rate, is undersampled. Such undersampling of 
the space domain signal, as a function of angle, introduces artifact in a 
manner similar to undersampling a time domain signal in time. 
One way to limit the artifact would be to limit the spacial freqency 
content which can be processed. Unfortunately, the processing occurs in 
the array beam, and the only control of the array beam is via altering the 
acoustic beam. Widening the beam averages the spacial frequency content, 
so it would be expected to reduce artifact. Again, by analogy, widening 
the beam is equivalent to low pass filtering a time domain signal. 
Assuming that reciprocity holds in the space domain, the effect of widening 
the transmit beam without altering the receive beam should be the same as 
the effect of widening the receive beam without altering the transmit 
beam. In practice, however, the effect has been found to be somewhat 
different if the receive beam is dynamically focused. Two relatively 
simple, but crude, methods of widening the transmit beam are to decrease 
the aperture on transmit and to move the focal point far out so that the 
beam is wider than where the aperture is focused at the desired image 
plane. In practice, moving the focus point out is quite limited due to the 
natural focusing qualities of a finite aperture transducer. It has been 
shown experimentally that both moving the focus point out and decreasing 
the aperture size are effective in reducing artifact. However, reducing 
the aperture has a more predominant effect on artifact and produces less 
resolution degradation. Reducing the aperture also reduces the 
penetration. 
SUMMARY OF THE INVENTION 
By generating a transmit beam optimized so as to give a flat response in 
space over the spacial angles associated with the corresponding multiple 
receive beams, the image quality is restored to the quality of the 
original image with no detectable loss in resolution. In view of the fact 
that an artifact free image is desired along a particular plane and the 
fact that merely skipping lines in the transmit mode fails to produce such 
an artifact free image, an examination of the space domain indicates that 
a mere line skipping approach fails to provide appropriate coverage in the 
transmit mode. The desired transmit coverage in a line skipping mode has 
been determined to require a pressure excitation at the array aperture 
equivalent to a normalized sin (.pi.x)/.pi.x function, where x represents 
the normalized horizontal distance along the array, measured from the 
center of the array. The use of the sin (.pi.x)/.pi.x function in the 
transmit mode has been found, when combined with a standard dynamic 
focused receive mode, to result in a multiline beamformer having a 
substantially reduced artifact, when compared to multiline beamformer 
using a standard transmit beam. 
The method of generating the optimal flat beam profile is to excite the 
individual elements in the phase array with an excitation chosen to have 
an amplitude which varies along the array. In particular, the voltage 
applied to the array elements which generate the pressure is chosen to 
have the function sin (.pi.x)/.pi.x. This requires a phase reversal on the 
outer elements of the array and amplitude weighting of all of the array 
elements. This method contrasts with conventional excitation methods which 
involve equal excitation of all elements. 
In the receive mode, each array element has a delay associated with it 
corresponding to the time it takes a signal to travel from a point in 
space to that array element. Accordingly, each array element has 
associated with it a delay corresponding to each point in space along the 
image line. 
In accordance with the present invention, the array elements are excited 
with a sin (.pi.x)/.pi.x weighting function and multiple receive lines are 
derived for each transmitted line. The multiple received lines result from 
a reconstruction of the receive signal after the various delays have been 
used to focus and steer the array.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, an n-element array 10 is shown. The array 10 is 
energized by a means for transmitting lines of ultrasound 12. The means 12 
provides the array 10 with voltages at each element of the array 10 such 
that the array elements generate ultrasound pressures whose amplitudes 
correspond to the formula sin (.pi.x)/.pi.x, where x is the normalized 
distance measured from the center of the array 10. In addition, a line in 
space 14 having a series of m points thereon is shown. The distance from 
point 1 on line 14 to array element 1 is illustrated by a distance 
d.sub.11. Similarly, the distance from point m on line 14 to the elements 
1 through n of the array 10 correspond to d.sub.m1 through d.sub.mn. In 
general, the distance from point i on line 14 to array element j 
corresponds to d.sub.ij. 
In order to focus the array 10 at a point k on line 14, time delays must be 
used with each array element. The purpose of the time delays is to insure 
that the signal which is processed by the electronics associated with each 
element of the array corresponds to the signal reflected by the point k in 
space at the same time. Accordingly, the time delay, T.sub.ik, associated 
with each array element k corresponds to the difference in time it would 
take a signal from point i to reach the closest array element and the 
amount it would take for the signal from point i to reach array element k. 
Accordingly, for each point i on the line 14 there is a delay T.sub.ik for 
array element k. 
Referring generally to FIG. 2, the various time delays which are associated 
with each point on the line 14, i.e., to associated the various array 
elements k with the various points i, may be thought of, for the purpose 
of understanding the present invention, as being bundled together into 
time delays T.sub.i, where each bundled time delay, T.sub.i, includes 
appropriate array element delays, T.sub.ik, of the type well known in the 
phase array art. 
With reference to FIG. 2, three bundled time delays 20, 22, 24 are used in 
parallel to generate three receive lines for each transmitted line. In 
general, however, two or more such bundled time delays may be used to 
generate a multiple number of receive lines for each transmit line in the 
multiline beamformer of the present invention. The outputs of the bundled 
delays 20, 22, 24 are fed into the memory of a standard digital scan 
converter of the type well known in the ultrasound art.