Ultrasound machines and their control panels vary widely in design depending on manufacturers, but the functions are essentially the same (Fig. 2.3a, b). The understanding of the different knob functions, or knobology, is necessary to operate an ultrasound machine. The basic functions are gain, time gain compensation (TGC), depth, zoom, freeze, measurements, and calculations. Advanced knobology includes M-mode, Doppler, color Doppler, power Doppler, focus, harmonics, optimization, and presets [7]. Most of the listed functions above will be discussed in the coming sections of this chapter.

Ultrasound probes, or transducers, have a wide range of frequencies, and one should choose the correct frequency range or bandwidth, to best image the body region of interest [5]. The most common transducers are linear array, convex or curved array, and phased array (Fig. 2.4a, b).

Linear array probes produce higher frequencies, designed to image superficial structures. Some linear probes are designed to produce lower frequencies, which allow imaging of deeper structures. For linear array probes, groups of in-line crystal elements are turned on and off in increments, creating individual echo lines as each group of elements is activated, creating a rectangular image [5]. These probes generally have a flat scanning surface, and the section of tissue being imaged, or sector, is exactly the surface area of the probe footprint (Fig. 2.5a, b). The frequency of linear probes is generally considered to be 10–5 MHz. However, modern broadband transducers often range from 5 or 6 to as high as 14 or even 16 MHz. There are linear probes that operate at a much higher frequency, significantly improving image quality of superficial structures. Breast and musculoskeletal ultrasound imaging have transitioned to using higher-frequency linear array transducers [5].

Curved array probes have a curved footprint and produce lower frequencies, so the sound waves penetrate deeper into the body. These transducers are similar to the linear array, except the crystal elements are arranged on a curved surface. This is typically used for imaging of the thorax, abdomen, and pelvis. For patients with a larger body habitus, the curved array or curvilinear probe can be used to image the buttocks or thighs. The image sector is wider than the footprint of the probe itself (Fig. 2.6a, b), similar to a pie slice with a bite taken out of the top [5].

Another curved array probe is the endocavitary probe. The crystals and scanning surface are at the end of a long handle. This is designed for intraoral, transvaginal, and transrectal imaging. The sector of imaging is quite wide, up to almost 180° (Fig. 2.7a, b). This probe sends out higher frequencies (13–8 MHz), producing high-resolution images of structures with little tissue between the probe and the structure of interest [7]. The endocavitary probe can be used intraorally to diagnose and drain peritonsillar abscesses. It can also be used transvaginally to better evaluate pelvic structures, such as evaluating for early pregnancy, ectopic pregnancy, torsion, tubo-ovarian abscesses, etc. Urologists use the endocavitary probe for prostate evaluation as well [5].

Phased array probes have a flat scanning surface. The crystals are grouped tightly, and each crystal element is activated with each ultrasound pulse [7]. The probe creates a wedge-shaped image. Phased array probes are mainly used for echocardiograms, but it can also be used for thoracic and intra-abdominal imaging. The small footprint fits well in between the ribs, increasing maneuverability (Fig. 2.8a, b).

Ultrasound machines have various examination presets for different probes (Fig. 2.9). Acoustic power, gain, focal zones, lines per sector, sector size, and other settings are optimized to the ideal level for that particular exam [7]. For example, obstetric presets lower the power output to FDA-approved levels [7, 8]. Cardiac settings increase frame rate at the expense of image quality so it can keep up with the cardiac activity. There are also calculation packages that have preset formulas. An example is calculating cardiac stroke volume. The user needs to activate the calculation package and make a few measurements, and the calculation package will give you the results after using its preprogrammed formula (Fig. 2.10). Presets can also be customized, depending on the machine manufacturer.

There are buttons on the machine’s control panel that allow you to adjust the displayed image field in one centimeter (or half centimeter) gradation increments. When increasing the depth , the structures in the image sector become smaller to accommodate imaging of the deeper structures and vice versa. It is important to remember to decrease the depth if you do not need deeper imaging, so the structures of interest are better visualized with higher resolution (Fig. 2.11a, b). The machine also monitors longer for reflected sound waves when the depth is increased, which reduces the frame rate, hence the temporal resolution. This means the stream of images will not be as smooth. This can be an issue with diagnostic accuracy and procedural guidance [7].

Another common adjustment to improve image quality is to increase gain . Increasing gain of an image means increasing the brightness of the image. The machine increases the amplitude of the signals after they have returned to the probe [9]. If the gain is increased above the optimum level, subtler findings may be obscured (Fig. 2.12a, b).

To adjust the gain of the various levels of an image, one can adjust the time gain compensation (TGC). Ultrasound beams are progressively attenuated as they travel through different tissues in the body. Therefore, strength of echoes returning from greater depths is weaker. TGC function allows to selectively amplify the signals returning from greater depths, so that equal reflectors at varying depths are displayed as structures of equal brightness on the screen [9]. At times, the machine may automatically overgain or undergain certain parts of the image depending on the type of tissue ultrasound beams go through, and the user can optimize the image manually by using TGC. Machines can vary on what type of buttons is used to adjust TGC. Some use knobs, allowing the user to turn the knobs to adjust the gain in the near field or far field. Other machines have sliders that correspond to different depths of the image, allowing the user to adjust the brightness of multiple levels more smoothly (Fig. 2.13a–d).

Ultrasound probes transmit sound waves in the shape of an hourglass, with the best resolution typically in the narrowest point (center) of the hourglass which is known as focal point [5]. The area just above and below where the ultrasound beam is still relatively narrow is the focal zone, and the sound waves converge to focal zone and then diverge from the focal zone. The broad converging beam above the focal zone near the footprint of the transducer is the near field, and the diverging broad beam beyond the focal zone is the far field. Machines allow the user to adjust the location of the focus or even add multiple foci to the region of interest (Fig. 2.14). However, although this may increase the lateral resolution, the temporal resolution will decrease, since the machine is taking more time to listen to returning signals.

Perhaps the most frequently used button on the control panel is the button that automatically optimizes the image by changing the acoustic power, gain, focus, and harmonics [7]. There are various names for this button, but the principle is the same. This is a good start to improving image quality, since it is simple and effective, but the user should also know how to change each of the above settings separately (Fig. 2.15a, b).

One of the key functions of the ultrasound machine is the ability to capture a still image with the “Freeze” button. Pressing the “Freeze” button will preserve a snapshot of whatever is on the screen. This still image will stay on the screen until you press the “Freeze” button a second time, and the image will be in real-time again. To save a frozen image to the machine or to an ultrasound image repository, the “Save” or “Clip” button should be pressed (Fig. 2.16). Typically, there is a button on the control panel that allows you to save the image. After pressing the “Freeze” button, the operator can also scroll through the previous frames on the internal memory of ultrasound machine hard drive to select the best image for saving. A benefit of ultrasound is the dynamic nature of image acquisition, so machines will also allow recording of a cine loop prospectively or retrospectively. The length of each clip can be adjusted as well.

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