Basic Principles of Image Acquisition

Insights Into The Principles Of Ivus Imaging

In order to understand the physics that governs image acquisition by intravascular ultrasound several fundamental principles of ultrasound wave transmission needs to be understood. We know that when ultrasound waves travel they have a frequency ≥ 20000 cycles per second. This is clearly above the human audible range, therefore ultrasound waves are not detected by the operators or other bystanders. A unique feature of the ultrasound waves is that, the velocity at which sound travels through human soft tissue is fairly constant at approximately 1540 m/s. The production of these waves is created by transducers which are devices that convert one type of energy into another. In brief, the electrical current passes through a piezoelectric (pressure-electric) crystalline material (usually a ceramic) that expands and contracts to produce sound waves as it is excited by electrical current. Following emission of an ultrasound pulse, the ultrasound wave travels away from the transducer. When it encounters a boundary between two tissues - fat and muscle, for example - the beam will be partially reflected and partially transmitted. The degree of reflection depends on the difference between the mechanical impedance of the two materials. A characteristic illustration of the ultrasound wave reflection is the case of imaging of highly calcified structures where acoustic shadowing, a nearly complete reflection of the signal at the soft tissue/calcium interface, occurs. In situation where the wave passes through many tissue interfaces, its energy is attenuated (reduced). Thus, only a small percentage of the emitted signal returns to the transducer, which produces an electrical impulse that is converted into the image. Actually, the received signal is converted to electrical energy and sent to an external signal processing system for amplification, filtering, scan-conversion, user- controlled modification, and finally, graphic presentation. As we mentioned, ultrasound wave travels through all tissues at a fixed speed, thus the time that it takes for the transmitted ultrasound impulse to be backscattered and returned to the transducer is a measure of distance. Penetration depends on a number of factors including the power output of the transducer (which is in part related to transducer design and aperture) and imaging frequency. Penetration is inversely related to frequency – the higher the frequency the less the penetration. Larger transducers with lower frequencies are used for examination of large vessels because they create a deeper near field and have greater penetration.

The intensity of the backscattered signal depends on a number of factors. These include:

• 1.

  The distance from the transducer to the target (intensity is inversely related to distance);

• 2.

  The angle of the signal relative to the target (the closer the angle is to 90^o, the more intense is the reflected signal); and

• 3.

  The density (or reflectivity) of the tissue (which determines how much ultrasound energy passes through the tissue and how much is backscattered).

• 4.

  The intensity of the transmitted signal;

• 5.

  The attenuation (reduction) of the signal as it passes through tissue (all tissue attenuates ultrasound energy);

All these factors not only affect the overall appearance of an image, but also the relative appearance of different sectors of the image throughout its 360^o circumference.

Additionally, transmission of the ultrasound beam has some other limitation that we should keep in mind. The ultrasound beam remains fairly parallel for a certain distance (near field) and then it starts to diverge (far field). In the near field, as the beam is narrower and more parallel, the quality of ultrasound images and the resolution is greater, and a more accurate detection of the characteristic backscatter (reflection of ultrasound energy) from a given tissue can be achieved. The equation L 5 r² / Y, where L is the length of the near field, r is the radius of the transducer, and Y is the wave length can be used to define the length of the near field. Therefore, larger transducers with lower frequencies are used for examination of large vessels to extend the near field into the region of diagnostic interest. The consequences of increasing the distance of the imaging object, is that far field structures appear less distinct, their borders are less clear, and the interpretation is less certain.