‘Physics: how to get the best from the system’
Assist. Professor of Medical Physics, Faculty of Medicine, University of Crete
Medical ultrasonography is based on the pulse-echo principle. A short ultrasound pulse is emitted into the human body. A part of the sound beam is reflected back and the scanner converts it to a distance. The ultrasound scanner shows the reflector as a dot on the display. If one pulse of ultrasound is emitted, one series of dots, in other words one scan line, is displayed. This is due to the fact that not all of the pulse is reflected back from any reflector. Most of the pulse emitted from the scanner will continue on to be reflected back from surfaces located deeper in the body. The brightness of each dot corresponds to the echo strength. A single image is made up of many parallel scan lines, producing what is known as a gray-scale image.
Ultrasound is a series of repeating pressure waves. It is convenient to illustrate them as sine waves forms. Ultrasound imaging uses pulses of ultrasound. These pulses consist of several cycles. The frequency is the number of times the cycle is repeated per second. The wavelength depends on the speed of ultrasound in the medium c and the frequency f, i.e. ë=c/f. The length of the pulse is given by the formula length = (nxc)/f where n=number of cycles per pulse, c the frequency of ultrasound and f the frequency of ultrasound. Obviously, if the frequency increases, the pulse length decreases.
As an ultrasonic pulse traverses the soft tissues of the body, it undergoes continuous modification. Attenuation is a general term, which describes the progressive weakening of the beam as it travels through tissue. Higher frequencies are selectively attenuated much more rapidly than low frequencies. Attenuation occurs through 3 main processes: absorption, reflection and scattering. Reflection is the redirection of a portion of the beam back toward its source whenever the beam meets a reflecting surface i.e. an interface. An interface occurs whenever two tissues with different acoustic impedance Z are in contact with each other. The acoustic impedance of a tissue is the product of the density of the tissue ñ and the sound propagation speed c, i.e. Z = ñ x c. The strength of the reflected ultrasound beam is greater when Z1-Z2, the difference of the two acoustic impedances is large. The difference in acoustic impedance between soft tissue and gas or bone is large, so most of the beam is reflected. For this reason, ultrasound scanners cannot see through gas or bone.
Production and detection of ultrasound is possible by utilizing the piezoelectric principle. When a voltage is applied to a piezoelectric crystal, the crystal vibrates. If the crystal is compressed it will generate a voltage. In modern ultrasound instruments, transducer arrays are used rather than single-element transducers. Linear arrays consist of groups of piezoelectric elements lined up side by side. In linear arrays, piezoelectric crystals are activated as groups to produce an ultrasound pulse. In linear phased arrays, piezoelectric crystals are activated with time differences. A phased array transducer produces sector-type images. Linear arrays produce a rectangular field. In linear arrays, the width of the image is approximately equal to the length of the array.
Resolution is the ability to distinguish two echoes. Axial resolution refers to the minimum separation of two interfaces along the beam required to generate two separate echoes. If two reflectors are separated by more than one pulse length, the echoes do not overlap and the transducer sees two echoes.
Obviously, we can increase axial resolution by decreasing pulse length. Lateral resolution refers to the minimum separation of two interfaces perpendicular to the beam required to generate separate echoes.
If two reflectors are separated by more than one pulse width, the echoes do not overlap and the transducer sees two echoes. Degradation of the information of ultrasound images is possible due to artifacts. Some artifacts are introduced by the imaging system. Others are inherent in the pulse-echo ultrasound technique.
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