Image quality can be partially described by spatial resolution and contrast resolution. The spatial resolution is the capacity to differentiate two objects within the ultrasound image and has two principle directions: axial (parallel to the beam) and lateral (perpendicular to both the beam and the catheter). The axial resolution is the ability that the ultrasound technique has to separate the spatial position of two consecutive scatters through its corresponding echoes. The axial resolution (d _r) depends on ultrasound speed (c) and pulse duration (d _t) and is calculated as d _r = cd _t = cT = c/f = 1.54/f, where d _t is the pulse width, T is the period of ultrasound wave, and f is the ultrasound frequency. The lateral resolution is the capacity to discern two objects located in the tangential direction and depends on the beam width. The lateral resolution is calculated as d _θ  = 1.22λ/D, where D is the aperture diameter. For a typical transducer of 40 MHz with aperture diameter 0.6 mm, the axial resolution is approximately ≈39 μm, the lateral resolution is d _θ  ≈ 0.8°, and the focal length is L = 2.3 mm. Contrast resolution is the distribution of the gray scale of the reflected signal and is often referred to as dynamic range . The greater the dynamic range, the broader the range of reflected signal (form weakest to strongest) that can be detected, displayed, and differentiated. An image with low dynamic range appears black and white with only a few in-between gray-scale levels. High-dynamic-range images have more shades of gray and can discriminate more different tissue types and more structural elements.

The interaction of ultrasound waves with the tissues of the body can be described in terms of reflection, scattering, refraction, and attenuation (Fig. 1.3). When ultrasound waves encounter a boundary between two tissues such as fat and muscle, the beam will be partially reflected and partially transmitted. The amount of ultrasound reflected depends on the relative change in acoustic impedance between the two tissues and the angle of reflection. For example, imaging of highly calcified structures is associated with acoustic shadowing because of nearly complete reflection of the beam at the soft tissue/calcium interface. Scattering of the ultrasound signal occurs with small structures, such as red blood cells, because the radius of the cell (about 4 μm) is smaller than the wavelength of the ultrasound signal. The extent of scattering depends on particle size (red blood cells), number of particles (hematocrit), ultrasound transducer frequency, and compressibility of blood cells and plasma. Scattering results in a pattern of speckles. The intensity of blood speckle increases exponentially with the frequency of the transducer. Blood stasis resulted from the catheter crossing a tight stenosis increases blood speckle because of red cell clumping or rouleaux formation. Actually, static blood can be more echodense than plaque. Ultrasound waves can be refracted as they pass through a medium with a different acoustic impedance, which can result in ultrasound artifacts including double-image artifact. Attenuation is the loss of signal strength as ultrasound interacts with tissue. As ultrasound penetrates into tissues, signal strength is progressively attenuated (reduced) due to absorption of the ultrasound energy by conversion to heat, as well as by reflection and scattering. Therefore, only a small percentage of the emitted signal returns to the transducer. 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 [3].