Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-04-01
2003-12-23
Imam, Ali M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06666824
ABSTRACT:
TECHNICAL FIELD
This invention relates to the acquisition and display of medical ultrasonic image data. In particular, the present invention relates to altering the dynamic range used to process image data, as a function of noise actually detected in scanning particular regions of a patient's body, to achieve the best possible results in imaging the patient.
BACKGROUND OF THE INVENTION
Ultrasonic imaging technology has been a tremendous boon to the medical field. Ultrasonic imaging allows physicians to examine patients' internal tissues and organs without resorting to the use of ionizing radiation or invasive exploratory surgery. As a result, ultrasonic imaging is a very important diagnostic technology for, among many other applications, reviewing fetal development and recovery from injuries which require frequent internal examinations.
As is well known, in ultrasonic imaging, a series of high-frequency sonic pulses are generated, and these pulses “bounce” off various objects in their path. Specifically, different structures in a patient's body exhibit different levels of impedance, and ultrasonic echoes are generated when the ultrasonic signals contact impedance boundaries between these structures. The interval between the emission of the pulses and the receipt of the corresponding echoes is measured to determine the distance between the source of the pulse and the impedance boundary from which the echo resulted. In addition, the relative intensity of the echo conveys information regarding the nature of the tissues causing the echoes. Different tissues exhibit different levels of impedance to the ultrasonic signals. Therefore, varying impedance differentials exist, for example, at the boundary between muscle tissue and bone as opposed to the boundary between fatty tissue and bone. As a result, when an ultrasonic pulse strikes the impedance boundary between muscle tissue and bone, a more robust echo is generated than the echo generated when an ultrasonic pulse strikes the impedance boundary between fatty tissue and organ tissue. Ultimately, it is the mosaic assembled from each of these echoes received, reflecting the position and the nature of the objects causing the echoes, that constitutes the multi-dimensional images obtained through the use of ultrasonic imaging.
More specifically, ultrasonic images typically are generated from echoes of transmitted ultrasonic imaging pulses at a frequency of between 500,000 Hz to 15 MHz. The speed of ultrasonic waves in the body is on the order of 1,540 meters per second. The time between the generation of each pulse and its echo is used to determine the distance from the source of the pulse to the source of the echo. Rapid generation of these pulses permits the interrogation of an entire region to build a detailed image.
FIG. 1
displays a prior art ultrasonic imaging system
10
. At the front end of the system is a scanhead
20
having a linear array of transducer elements
30
coupled to associated time gain control amplifiers
40
and directed by a beamformer
50
. These devices are responsible for selectively generating the ultrasonic imaging pulses, transmitting them to the patient, receiving the echoes returned, amplifying the returned echo signals as appropriate, and combining signals corresponding to the echoes to effectively form beams focused to selected regions of the patient's body. Once the signals representing the echoes are received and amplified, the signals are processed by a signal/image processing unit
60
. The signal/image processing unit
60
receives the amplified echo signals and assembles them into an image of the patient's internal anatomy. Finally, the image formed by the signal/image processing unit
60
is presented on a display
70
. The system might include additional devices, such as storage devices and possibly other output devices (not shown). These supplemental devices allow the results of the scans to be stored and reviewed at a later time. The system
10
also includes a compression map select processor
80
, the function of which will be subsequently explained.
This is a simplified rendering of the ultrasonic imaging process; there are many problems that must be overcome for the ultrasonic system to generate a useful image having sufficient resolution to help a medical professional assess the portion of the patient's body being studied. Some of these problems can be addressed by actually manipulating the patient. For example, because ultrasonic waves do not penetrate gaseous regions well, it is difficult to image any structure behind or beneath a lung or an empty gastrointestinal tract. A physician can mitigate the problem of an empty gastrointestinal tract by requiring the patient to consume a significant amount of fluid without eliminating until after the imaging has taken place.
Other problems, however, are not quite so easily solved. Of these, perhaps the largest single problem is that of noise. In any system, the signal of interest subsists against a background of ambient signals. These other signals have nothing to do with the signal of interest, other than the fact that these unwanted signals interfere with the signal of interest. These ambient signals constitute a noise component. Furthermore, some external noise sources, such as electrical equipment used proximally to the ultrasonic imaging equipment, may introduce noise into the signal path. In addition, as a result of electrons moving through the components of the imaging system itself, the imaging system will exhibit thermal noise. For example, the noise level varies with the phasing of the transducer elements
30
of the scanhead
20
used to generate and direct ultrasonic pulses or receive the echoes of those pulses.
Both the noise and the desired signals are detected by the ultrasonic imaging system
10
. Unfortunately, if the noise were to be analyzed as though it were part of the desired signal, the resulting ultrasonic image would be compromised, and the image then would inaccurately portray structures in the patient's anatomy. To avoid the noise unduly compromising the integrity of the desired signals, the desired signals must be separated from the noise, or the noise must be suppressed as much as practical. There are several mechanisms for reducing noise present in the image, including filtering the received spectrum to consider only frequencies of interest. To the extent noise exists within the spectra of the frequencies of interest, however, frequency filtering does little or nothing to separate the noise from the signals of interest.
Another way to preserve the integrity of the signal, when the magnitude of the desired signals exceeds that of the noise, is to adjust the dynamic range used in mapping the signal to the display
70
of the system
10
through the use of a mapping function referred to as a compression map. As is well known, the dynamic range is an expression of the ratio of the received magnitudes of the largest signal to that of the smallest discernible signal. Dynamic range also commonly known as the signal-to-noise ratio. Dynamic range is typically expressed as a logarithm of a ratio, expressed in units of decibels (dB). Reducing or compressing the dynamic range, in effect, involves cutting off signals having a magnitude below a predetermined value. The dynamic range can be compressed by programming the signal/image processing unit
60
to disregard signals having a magnitude below a certain level so that these unwanted signals do not unduly compromise images shown on the display
70
. If the magnitude of the useful component of the signals conveying information about the patient's tissues is largely greater than the magnitude of the noise component, the noise component can be partially or completely suppressed, leaving a useful signal largely free of noise from which an image can be derived.
However, if the dynamic range is compressed too much, it detracts from the ability to process desired signals. The problem is relatively insignificant when there is relatively little
Roundhill David
Rust David
Dorsey & Whitney LLP
Imam Ali M.
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