DL FIGURE 116.4 Cross section of a typical focused circular ultrasound source of aperture diameter D and focal length F showing the focused beam of lateral beam width at the focus D signals constructively interfere at the desired focal region. At a receiver the signals are combined with delays associated with various elements to provide reinforcement of the signals from a receiving focal region The 3 db lateral beam width D, is directly dependent upon the wavelength A and focal length Fand inversely related to the aperture diameter(diameter of the transducer)D[ Kino, 1987 F入 D,=102 (116.3) ecause fi =G, the higher the frequency the smaller is A and Da The smaller D,, the better the lateral resolution near the focus, but the beam spread is greater with distance from the focus. Thus, the strength of the focusing varies among transducers so that the user may choose very good resolution over a short region or somewhat poorer resolution that is maintained over a greater depth With phased array transducers, the focal region can be varied dynamically to optimize lateral resolution at all distances. Principles of Pulse-Echo Ultrasound Ultrasound imaging usually employs frequencies in the 2-to 10-MHz range, though some of the new intra- vascular probes use higher frequencies. Images are formed by using a transducer within a probe to generate a short pulse(typically on the order of 1 us in duration)of ultrasound which is propagated through the tissue. A portion of the energy in this pulse is reflected back toward the transducer from specular reflectors and from scatterers in the tissue. These acoustic echoes, with amplitudes much lower than the transmitted pulse, are everted by the transducer to electrical signals which are converted to a(rectified)video signal, amplified by a time gain controlled amplifier, and displayed. The A-mode display is rarely used but simply involves display of the received echoes as amplitude versus time of arrival. The time of arrival is related by the wave speed to the tissue depth from which the echo returns, i.e., d= ct/2. Figure 116.5 provides a very simple representation of this process where the A-mode display associated with specular reflection from three different interfaces is illustrated. For clinical imaging the interfaces would not necessarily be perpendicular to the axis of the sound beam,and there would be a continuum of echoes, a continuous received signal, due to energy backscattered from within the tissues. Since the ultrasound pulse is attenuated as it propagates, all ultrasonic imaging systems use a logarithmic variation of amplifier gain with time to compensate the exponential attenuation of the tissue. Thus, echoes from structures reflecting or backscattering the same fraction of the incident signal will have the same amplitude after passing through the time gain controlled amplifier. A B-mode display is typically used for ultrasound imaging. It involves display of the echoes at various brightness or gray levels corresponding to their amplitude. a two-dimensional B-mode display involves movement of the transducer(manually or automatically), movement of a mirror to change the direction of the field(automatically), or movement of the ultrasound beam directly(electrically) such that it scans a plane through the body. Figure 116.6 provides a simplified representation(again, echoes are shown as arising from interfaces only)of the formation of a B-mode image. The direction of the beam is monitored so that the received signals along each path are placed in their correct location on the display. Typically, the orientation information and echoes are processed by a digital scan converter for appropriate display of the two-dimensional image e 2000 by CRC Press LLC
© 2000 by CRC Press LLC signals constructively interfere at the desired focal region. At a receiver the signals are combined with delays associated with various elements to provide reinforcement of the signals from a receiving focal region. The 3 db lateral beam width DL is directly dependent upon the wavelength l and focal length F and inversely related to the aperture diameter (diameter of the transducer) D [Kino, 1987]: (116.3) Because f l = c, the higher the frequency the smaller is l and DL. The smaller DL, the better the lateral resolution near the focus, but the beam spread is greater with distance from the focus. Thus, the strength of the focusing varies among transducers so that the user may choose very good resolution over a short region or somewhat poorer resolution that is maintained over a greater depth. With phased array transducers, the focal region can be varied dynamically to optimize lateral resolution at all distances. Principles of Pulse-Echo Ultrasound Ultrasound imaging usually employs frequencies in the 2- to 10-MHz range, though some of the new intravascular probes use higher frequencies. Images are formed by using a transducer within a probe to generate a short pulse (typically on the order of 1 ms in duration) of ultrasound which is propagated through the tissue. A portion of the energy in this pulse is reflected back toward the transducer from specular reflectors and from scatterers in the tissue. These acoustic echoes, with amplitudes much lower than the transmitted pulse, are converted by the transducer to electrical signals which are converted to a (rectified) video signal, amplified by a time gain controlled amplifier, and displayed. The A-mode display is rarely used but simply involves display of the received echoes as amplitude versus time of arrival. The time of arrival is related by the wave speed to the tissue depth from which the echo returns, i.e., d = ct/2. Figure 116.5 provides a very simple representation of this process where the A-mode display associated with specular reflection from three different interfaces is illustrated. For clinical imaging the interfaces would not necessarily be perpendicular to the axis of the sound beam, and there would be a continuum of echoes, a continuous received signal, due to energy backscattered from within the tissues. Since the ultrasound pulse is attenuated as it propagates, all ultrasonic imaging systems use a logarithmic variation of amplifier gain with time to compensate the exponential attenuation of the tissue. Thus, echoes from structures reflecting or backscattering the same fraction of the incident signal will have the same amplitude after passing through the time gain controlled amplifier. A B-mode display is typically used for ultrasound imaging. It involves display of the echoes at various brightness or gray levels corresponding to their amplitude. A two-dimensional B-mode display involves movement of the transducer (manually or automatically), movement of a mirror to change the direction of the field (automatically), or movement of the ultrasound beam directly (electrically) such that it scans a plane through the body. Figure 116.6 provides a simplified representation (again, echoes are shown as arising from interfaces only) of the formation of a B-mode image. The direction of the beam is monitored so that the received signals along each path are placed in their correct location on the display. Typically, the orientation information and echoes are processed by a digital scan converter for appropriate display of the two-dimensional image on FIGURE 116.4 Cross section of a typical focused circular ultrasound source of aperture diameter D and focal length F, showing the focused beam of lateral beam width at the focus DL. D F D L = 1.02 l
Probe |大| Amplitude Time Distance into tissue) IGURE 116.5 (a) The transmitted pulse(heavy wave)and echoes from reflecting structures;(b) the resulting A-mode (b) FIGURE 116.6(a) The transmitted pulse paths for a rotating transducer probe;( b)the resulting two-dimensional B-mode display of echoes from the interfaces only. a cathode ray tube in the standard format used for television picture display. Most B-mode systems in use today create an image in 0. 1 s or less, so that the image is displayed in real-time for viewing of moving structures, such as structures in the heart or the fetus moving within the womb. This is not possible with the typical magnetic resonance or computed tomography syst c2000 by CRC Press LLC
© 2000 by CRC Press LLC a cathode ray tube in the standard format used for television picture display. Most B-mode systems in use today create an image in 0.1 s or less, so that the image is displayed in real-time for viewing of moving structures, such as structures in the heart or the fetus moving within the womb. This is not possible with the typical magnetic resonance or computed tomography system. FIGURE 116.5 (a) The transmitted pulse (heavy wave) and echoes from reflecting structures; (b) the resulting A-mode display. FIGURE 116.6 (a) The transmitted pulse paths for a rotating transducer probe; (b) the resulting two-dimensional B-mode display of echoes from the interfaces only