Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-10-10
2003-12-30
Imam, Ali M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06669638
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of imaging ultrasonic medical transducer assemblies, and, specifically, to an apparatus and method for cooling the transducer.
2. Description of the Related Art
Imaging ultrasonic medical transducers are used to observe the internal organs of a patient. The ultrasonic range is described essentially by its lower limit: 20kHz, roughly the highest frequency a human can hear. The medical transducers emit ultrasonic pulses which echo (i.e., reflect), refract, or are absorbed by structures in the body. The reflected echoes are received by the transducer and these received signals are translated into images. Such translation is possible because the reflections from the internal organs vary in intensity according to the “acoustic impedance” between adjacent structures. The acoustic impedance of a tissue is related to its density; the greater the difference in acoustic impedance between two adjacent tissues the more reflective their boundary will be.
The frequency of the ultrasonic beams has an effect on both the image resolution and the penetration ability of the ultrasonic device. Higher frequency ultrasound waves have a longer near field (i.e., the region in the sound beam's path where the beam diameter decreases as the distance from the transducer increases) and less divergence in the far field (i.e., the region in the sound beam's path where the beam diameter increases as the distance from the transducer increases): higher frequency ultrasonic waves thus permit greater resolution of small structures. However, high frequency ultrasonic waves have less penetrating ability because their energy is absorbed and scattered by soft tissues. On the other hand, lower frequency ultrasonic waves have a greater depth of penetration, but the received images are much less well defined. The conventional frequency range for imaging human internal organs (using sound waves) is typically from about 3 MHz to about 5 MHz.
Two types of resolution generally apply to ultrasound imaging transducers: lateral resolution and axial resolution. Lateral resolution is the ability to resolve objects side by side and, as discussed above, is proportionally affected by the frequency (the higher the frequency, the higher the lateral resolution). Higher frequency transducers are used for infants and children because there is less need for deep penetration and the smaller structures can be viewed with greater lateral resolution. Lower frequencies are used for adults where the internal structures are larger and there is a greater need for depth penetration. Of course, when determining the appropriate frequency to be used, the structure, tissue, or organ to be viewed (and the exact purpose of the imaging) can matter more than the age of the subject. For example, diagnostic breast imaging on an adult may require a frequency of about 7 MHz or higher.
Axial resolution is the ability to resolve objects that lie one above the other. Because this is related to depth penetration, axial resolution is inversely proportional to the frequency of the transducer (depending on the size of the patient). In large patients, higher frequency beams are rapidly absorbed by the objects closest to the transducer, thus reducing depth penetration and axial resolution.
The focusing of an ultrasonic transducer can be implemented in one of two ways: mechanical or electronic. Mechanical focusing consists of placing an acoustic lens on the surface of the transducer or using a transducer with a concave face. One or several piezoelectric elements are used. In order to create a sweeping beam for 2D imaging, a single element may be oscillated back and forth, several elements may be rotated, or a single element may be used with a set of acoustic mirrors. This last transducer type (with the acoustic mirrors) is sometimes called the “wobbler” because of the vibration created as the mirrors rotate or oscillate inside the housing.
Electronic focusing uses a process called phased array, where multiple piezoelectric elements in an array are stimulated (or “fired”) sequentially in order to form and focus the beam. In an annular array, circular or ringlike elements and/or arrays are used. In a linear array, a row of elements is used to form and focus the beam. A transducer contains an array of transmitting elements and a similar array of receiving elements. An example of how a linear array forms and focuses a sound beam is shown in FIG.
1
. In order to focus at point X, the outer elements
101
and
107
fire first, then elements
102
and
106
, then elements
103
and
105
, and finally element
104
. As shown in
FIG. 1
, the resulting wavefronts combine to form a semicircular ultrasonic pulse whose focal point is X. By varying the sequential pattern of firing, the distance of focal point X from the transducer can be changed. Furthermore, varying the sequential pattern of firing can also be used to steer the beam. Steering is used to move focal point X left and right in FIG.
1
. By rapidly steering a series of beams from left to right, a 2D cross-sectional image may be formed.
In 2D mode, one sweep from left to right is a frame, and the number of sweeps in a second is the frame rate (or fps—frames per second). Conventional frame rates ranges from about 12 fps to about 30 fps. The number of beams formed over time is the Pulse Repetition Frequency (PRF), measured in pulses per second. The range of PRFs for most commercial echocardiographs is between about 200 and about 5000 pulses per second. PRF varies with the type of imaging being performed. Most of the time spent in each second is used waiting for the echoes to return to the receiving elements in the transducer. In other words, after a beam is formed, the transmitting elements lie dormant while the beam travels to the various objects and then some of that sound energy returns (as echoes) to the transducer's receiving elements. The amount of time that the transmitting elements are transmitting sound energy is called the duty factor. Most transducers are acting as a receiver about 99% of the time, in which case the duty factor is 1 (%).
Aperture is the size of the active transmitting and receiving portion of a transducer array. Aperture is measured in square centimeters and is a function of the number of transducer elements used simultaneously to form an image. A common measurement of aperture size is F-number or F#, which is defined as the ratio of depth to aperture. These values are related to the lateral resolution (LR) by the following function:
LR
=
λ
*
F
#
=
λ
*
D
A
where &lgr;=wavelength of sound pulse
D=depth of the scan
A=aperture of the scan
As can be seen from the above equation, for a fixed frequency, the aperture size must increase as the scanning depth increases in order to maintain uniform lateral resolution throughout the image. Many ultrasonic systems select a transmit aperture based on the scan depth setting and continuously vary the reception aperture. It is desirable to achieve low F#s, which, because the scanning depth is limited by the position of the desired subject, is identical to seeking larger aperture sizes. It is also desirable to seek small wavelengths, which is equivalent to seeking higher frequencies.
There are a number of modes in which an ultrasonic transducer operates. The basic modes are A Mode, B Mode, M Mode, and 2D Mode. The A Mode is amplitude mode, where signals are displayed as spikes that are dependent on the amplitude of the returning sound energy. The B Mode is brightness mode, where the signals are displayed as various points whose brightness depends on the amplitude of the returning sound energy. The M Mode is motion mode, where B Mode is applied and a strip chart recorder allows visualization of the structures as a function of depth and time; The 2D Mode is two-dimensional (imaging) mode, where B Mode is spatially applied by sweeping the beam (as described above) so that structures are seen as a fun
Miller David
Peszynski Michael
Salgo Ivan
Imam Ali M.
Koninklijke Philips Electronics , N.V.
Vodopia John
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