Andersons pediatric cardiology 466

or time can be achieved by changing controls on the machine, which correct for the automated attenuation at specific depths of image Current ultrasound machines provide tools for automated image optimization that makes the need for manual adjustments less frequent Production of Images Each ultrasonic pulse, encountering numerous interfaces, gives rise to a series of reflected echoes returning at different time intervals corresponding to their depths In this way, each pulse from the ultrasonic crystal generates a radiofrequency signal that represents the amplitude of the reflected ultrasound wave as a function of time The signal detected by the transducer is typically electronically amplified The amount of amplification has a preset value but can be modified on an ultrasound system by using the “gain” button (typically the largest button on the operating panel) Importantly, the overall gain will amplify both the signal and potential measurement noise and will thus not affect the signal-to-noise ratio The returned signal is processed so that the radiofrequency signal is converted into an image The envelope of the radiofrequency signal is detected (demodulation) and is subdivided as a function of time in small intervals (pixels) Each pixel is attributed a gray-scale number, defined by the local amplitude of the signal, ranging between 0 and 255 (8-bit image) “0” represents “black,” and “255” represents “white.” Encoding can be changed to color maps (e.g., bronze) by changing from grayscale and selecting different color maps This can improve image visualization in the case of poor imaging windows Current scanners allow 12 and 16 bit encoding 4096 or 65,536 gray/color levels The contrast of the images can further be changed by adjusting the compression or dynamic range, which results in changing the representation of the images but does not affect the acquisition A low dynamic range or compression will result in more black and white images, whereas high compression or dynamic range adds a lot of gray values to the images For M-mode (motion mode) acquisition, the ultrasound beam is transmitted into the same direction for each transmitted pulse M-mode represents one line of information displayed on the vertical axis, with time on the horizontal axis Its advantage is the very high temporal resolution The frequency of repetition of the pulse producing a typical M-mode trace is more than 1000 frames per second Conventional cross-sectional echocardiography or B-mode imaging (“brightness mode”) depends on the construction of an image using multiple individual lines of information sent out in slightly tilted directions Typically 64 or 128 lines of information are required to produce one image or frame Multiple frames are constructed in real time each second, the limiting factor being the time necessary for the echoes from each pulse to return to the transducer Consequently, at depths of 5 to 15 cm, it is possible to achieve frame rates of 28 to 50 per second Modern systems are capable of manipulating the frame rate by sending out different pulses at different times, but in general increasing the frame rate will reduce the quality of the image, and hence its spatial resolution Frame rates and thus spatial resolution can be increased by reducing the imaging sector width (less lines and thus less time for construction of each frame) For pediatric imaging, optimizing both spatial and temporal resolution is a trade-off, and depending on the information required, priority can be given to either More recently, high-frame rate or ultrafast ultrasound has been introduced with frame rates up to 5000 to 10,000 frames/s Modern phased-array probes are built based on an array of piezoelectric crystals and not on mechanical motion of one single piezoelectric element By introducing specific time delays between the excitation of different crystals in the array, the ultrasound wave can be sent in a specific direction without mechanical motion of the transducer The received RF signal for a transmission in a particular direction is then simply the sum of the signals received by the individual elements These individual contributions can be filtered, scaled, and time-delayed separately before summing This process is referred to as beam-forming and is a crucial element for obtaining high-quality images There same principle is applied in three-dimensional arrays Second Harmonic Imaging When the amplitude of the transmitted wave becomes significant, the shape of the ultrasound wave will distort during propagation This wave distortion typically generates harmonic frequencies (i.e., integer multiples of the transmitted frequency) So, for example, transmitting a 1.7-MHz ultrasound pulse will result in the generation of frequency components of 3.4, 5.1, 6.8, 8.5 MHz, and so on These harmonic components will grow stronger with propagation distance The ultrasound scanner can be set up to receive only the second harmonic component through filtering of the received radiofrequency signal Such an image typically has a better signal-to-noise ratio by avoiding clutter noise due to (rib) reverberation artifacts and increases image resolution deeper in the tissue Thus harmonic image can improve image quality in patients with poor acoustic windows and limited penetration However, it has intrinsically poorer axial resolution Harmonic imaging has become the default cardiac imaging mode for adult scanning on many systems In younger infants, it is unnecessary to use harmonic imaging and it needs to be avoided because it reduces image resolution Switching between conventional and harmonic imaging is done by changing the transmit frequency of the system Quality of Images Image quality refers to the resolution of the imaging system Spatial resolution refers to the capacity of the system to resolve small structures Contrast resolution is the ability of the system to distinguish differences in the density of the soft tissues Temporal resolution refers to the capacity of the system to resolve differences in time Spatial Resolution This can be defined as the combination of axial and lateral resolution Axial resolution is the capacity of the ultrasonic system to distinguish how close together two objects can be along the axis of the beam, yet still be distinguished as two separate objects As mentioned previously, wavelength affects axial resolution and is improved by increasing the ultrasound frequency Axial resolution is much higher compared with lateral resolution, which is the capacity of the system to resolve two adjacent objects that are perpendicular to the axis of the beam as separate entities The width of the beam affects the lateral resolution: the wider the beam, the lower the lateral resolution This is influenced by the focal zone, which is the depth of the smallest beam width The near field is the zone between the transducer and the focal zone, and the far field is the region beyond the focal zone Optimizing the focus at a certain depth improves lateral resolution The limits of the focal zone are determined by the size and frequency of the transducer Small transducers focus well in the near field, whereas large transducers perform better in the far field The width of the beam is also influenced by the frequency of the transducer, with higherfrequency probes having a better lateral resolution compared with those that have a low frequency However, probes with higher frequency suffer from their limited ability to penetrate into the tissue Line density can also be improved by decreasing the sector width, resulting in better lateral resolution but more limited ... Frame rates and thus spatial resolution can be increased by reducing the imaging sector width (less lines and thus less time for construction of each frame) For pediatric imaging, optimizing both spatial and temporal resolution is a trade-off, and depending on the information required, priority can be given to either