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Principles of Ultrasound Imaging and Signal Analysis

S. Lori BRIDAL

CNRS, INSERM, Laboratoire d'Imagerie Biomédicale (LIB), Sorbonne Université, Paris, France

1.1. Introduction: probing living systems with ultrasound

A primary objective of medical imaging is to reveal something about a patient that cannot be evaluated by other means, while imposing as few constraints as possible on the operator and the patient during the imaging process. Ultrasound has many qualities relative to other imaging modalities that help it to contribute towards this goal. In addition to practical advantages such as portability and relative affordability, the nature of interactions between biological media and mechanical waves, and the speed of these interactions give ultrasound a unique place in the biomedical imaging armamentarium.

Within the power and amplitude ranges authorized for diagnostic ultrasound imaging, its innocuous nature has made it the method of choice for prenatal screening. Diagnostic ultrasound is also widely applied for cardiovascular imaging and to examine the liver, kidney, muscles, prostate, thyroid and superficial vessels. Spatial resolution can be adjusted within a range from about a millimeter to hundreds of micrometers, depending on the selected transducer and the explored depth. Images are constantly refreshed on the monitor in real time to provide a continuous sequence of images from the scanned regions. Conventional systems offer imaging frame rates on the order of 50 images per second, and ultrafast systems can acquire data at frame rates reaching 20,000 images per second. Images are created from the sensitive detection of echoes returned from scattering structures of a tissue or organ and are presented on a grayscale where strongly reflecting interfaces or scattering structures are brighter. The resulting B-mode (brightness mode) images can be used to visualize the morphology of structures within the body - even those that are rapidly moving, like the beating heart. Doppler mode tracks the movement of red blood cells, and estimated values can be presented on a color scale overlaid on the morphological image to map out information about the blood flow speed, direction or concentration. Specially adapted ultrasound systems can deliver much higher acoustic intensity or power than that used for diagnostic imaging to a position that is selected by focusing. This can be applied, for example, to destroy a cancerous lesion or a kidney stone in zones that are difficult to reach with more conventional interventions.

This very versatile modality has a few notable limitations. Due to the relatively limited differences between the acoustic properties of soft biological tissues and the speckle pattern caused by the interference of echoes returned simultaneously to the transducer from sub-resolution-sized scattering structures, contrast between different types of tissues can be limited. This can hamper the detection of lesions or other anomalies. Compound imaging coherently adds echoes backscattered from the same imaged plane acquired from slightly different angles to reduce the effect of speckle. Image sequences are typically obtained in a single plane so that screening for anomalies relies on a sweep (most-often performed by hand) of the image plane through the volume of interest. Image placement and interpretation are very operator-dependent, so diagnosis can be influenced by the operator's choices and expertise. Growing capacities for three-dimensional ultrasound alleviate these limitations to some degree but, for full-body screening, ultrasound is still outranked by techniques like positron emission or X-ray tomography. Key ultrasound imaging advantages and limitations are summarized in Figure 1.1. Research is underway to curtail limitations and further empower ultrasound's ability to probe living systems.

Consider a typical ultrasound image (see Figure 1.2). The source of ultrasound, the ultrasonic probe, is positioned against the body (at the top of the image in Figure 1.2), and the cross-section of the body in the line of sight of the probe is mapped out in grayscale beyond this surface. Echoes from zones farther from the probe return to the probe later than echoes from more superficial regions so that, along the propagation axis, the echo time delay describing the lapse of time between the emission of the ultrasound pulse and echo reception, ?techo, is related to the average speed of sound propagation in the medium, c, and the depth, D, of the echogenic structure simply by relating the round-trip travel time to the depth:

Figure 1.1. Key advantages and limitations of ultrasound

The on-screen display identifies the type of ultrasound probe used to make the image (ultrasonic frequency and other salient characteristics). The mechanical index and thermal index can also be displayed to show that imaging safety limits have been respected and that the system is operating under conditions that will not create either cavitation or significant heating in the body. The number of images recorded per unit time, which is referred to as the frame rate, is typically presented in units of Hz or frames per second (fps). If we look more closely at the image, we may note that there are some bright and well-defined boundaries where the ultrasound wave has been reflected from surfaces between different types of tissues or organs. Zones in between such clear boundaries present a speckled brightness pattern instead of a uniform level of gray. Also, deeper regions can present a lower, average brightness than the more superficial regions of the same organ because the ultrasonic wave reaching these deeper zones and the echoes returned from them are more strongly attenuated.

This first chapter will present the basic concepts behind the formation of ultrasound images and, for each key aspect of ultrasound imaging, the subsequent chapters will provide deeper insight into specific ultrasonic techniques and a look at the most promising current innovations that are moving these techniques forward towards the next generation of ultrasound imaging.

Figure 1.2. Ultrasound B-mode image with an on-screen display of probe and imaging settings. In this case, the probe is a linear array transducer with a frequency range from 7 to 14 MHz (probe label, 14L7). The selected central transmit frequency in MHz (A), imaging depth in cm (B), focal position marked by arrowheads (C), gain in dB (D) and display dynamic range in dB (F) are shown on-screen. The white arrow designates a bright echo from a boundary, and the white box outlines a zone of speckle pattern. No decrease in the average brightness of the speckle pattern with depth is apparent in this image because time gain compensation (gain that increases as a function of depth) has been applied

1.2. Ultrasound probes

1.2.1. Piezoelectric materials: electro-acoustic conversion

Creating and detecting ultrasonic signals relies on the use of piezoelectric (PZT) materials. In 1880, Pierre and Jacques Curie discovered that applying a mechanical constraint along a selected axis of certain types of crystals led to the creation of an electrical potential across the crystal ("Piezo" comes from a Greek word for pressure). PZT materials are dielectric, which means that they are poor conductors of electric current. Thus, if the specifically arranged, electrical charges associated with the atoms within them are displaced minutely, a global charge distribution is created such that the crystal presents a net charge or electrical potential across its opposite faces. This is known as the direct piezoelectric effect (see Figure 1.3(a)). It works the other way too. The negative charges within the PZT material shift by a tiny amount to counter-act an electric field applied across the material, and this induces a tiny global deformation of the material's thickness in response to an applied electric potential. This is known as the inverse piezoelectric effect (see Figure 1.3(b)). By alternating positive and negative electric potentials across the crystal, the PZT can be made to mechanically vibrate. PZT materials provide the means for the electro-acoustic conversion used to generate and receive ultrasound signals.

Figure 1.3. Direct and indirect (or inverse) piezoelectric effects are used for electro-acoustic conversion

1.2.2. Transducers

The handheld ultrasonic probe generally contains an assemblage of PZT elements within its tip that are arranged geometrically to best adapt to the desired scanning conditions needed for the organ or zone to be examined. This ultrasonic transducer array of PZT elements converts electrical energy into acoustic energy and vice versa. For linear transducer arrays, the PZT elements are aligned in a row and the transducer will produce a rectangular set of scan lines. In a convex array, the PZT elements are arranged in a curvilinear fashion (along the arch of a curve) so that the scan lines can fan out from the ultrasound source to evaluate a zone that widens with depth. This can be very useful for the evaluation of large organs at greater depths within the body or for situations where there is only a limited space on the body's surface through which to transmit sound. A position on the body where the transducer can be placed to provide a pathway for wave propagation...