Table 2 Optimizing scan settings.38,39
From: State-of-the-art neonatal cerebral ultrasound: technique and reporting
Depth | The depth control changes the maximum scanning range on screen. The depth range button usually changes the displayed image field in 1-cm gradation increments. Increasing depth means a reduction of the image resolution (the signal needs to cover a longer distance), therefore the frame rate and the resolution are both lower. The optimal depth depends on beam penetration and therefore on transducer frequency |
Dynamic range (DR) | The DR controls the range of shades of gray displayed on the screen to be increased or decreased. It can make an image look either very black and white or very gray. It can remove low-level echoes and result in an image with more contrast |
Focus point(s) | The focus (point) determines the depth at which the ultrasound beam is focused and creates the best possible lateral resolution at that depth. It is often marked by an arrow on the display. The focal zone is ideally positioned at (or just below) the object the operator wants to study. More than one “focal zone” can be selected, but this can slow down the image frame rate and can induce artefacts |
Frequency | Adjusting the frequency allows the operator to improve the image resolution. Frequency is the number of cycles of acoustic waves per second. The unit is the Hertz (Hz) and one cycle per second is equal to 1 Hz; 106 cycles/second is equal to 1 MHz. Diagnostic ultrasound has a frequency of 2–20 MHz. One should consider using higher frequencies (10–20 MHz) when scanning superficial and low frequencies (5–10 MHz) when scanning deeper structures |
Gain (overall gain control) | The (overall) gain control will adjust the overall brightness of the real-time (B-mode) ultrasound image. Overall gain control amplifies all returning signals by a constant factor regardless of the depth (in contrast to time gain control). It has a similar effect to increasing the power. Gain is commonly expressed in decibels (dB). If the gain is increased too much, the noise will also be amplified leading to poor image quality |
Power | The power button regulates the “output power” to the transducer (the intensity of the ultrasound pulses). The operator can increase the amplitude of the electric signal to the transducer and make the returning echo signals brighter. The risk, however, is that the acoustic exposure of the patient increases |
Pulse repetition frequency (PRF) | The PRF controls the rate (per second) at which pulses of sound are transmitted by the transducer |
Time gain compensation (TGC) | Increasing TGC (i.e., by adjusting the TGC sliders) amplifies signals from deeper structures to compensate for attenuation causing the signals from deeper structures to be weaker than signals returning from more shallow structures. The goal of TGC is to make the entire image look evenly bright. TGC sliders are used to adjust the gain in specific areas of the image (near-, mid-, and far-field). The idea is to have lower gain in the near field and higher gain deeper in the image where image quality is weaker. Most manufacturers offer a software feature that automatically optimizes the gain and overall contrast of the image. This feature analyses the tissue in the image and attempts to provide you with the most optimized image, but correction by the operator remains necessary |
