The principle of ultrasound: Difference between revisions

Briefly, I would like to touch upon '''real time 3D imaging'''.  In order to accomplish this, the PZT elements need to be arranged in a 2D matrix.  Each PZT element represents a scan line, by combining all the data, a 3D set is reconstructed.  For example, if we have a matrix of 128 by 128 PZT elements, one can generate over 16 thousand scan lines.  With careful timing for individual excitation, a pyramidal volumetric data set is created.  When imaged several times per minute (>20), a real time image is achieved.   

Image production is a complex process.  Echo instrumentation must generate and transmit the ultrasound and receive the data.  Then the data needs to be amplified, filtered and processed.  Eventually the final result needs to be displayed for the clinician to view the ultrasound information.  As the first step in data processing, the returning ultrasound signals need to be converted to voltage.  Since their amplitude is usually low, they need to be amplified.  The ultrasound signal usually is out of phase so it needs to be realigned in time.  At this point one has the raw frequency (RF) data, which is usually high frequency with larger variability in amplitudes and it has background noise.  The next step is filtering and mathematical manipulations (logarithmic compression, etc) to render this data for further processing.  At this stage one has sinusoidal data in polar coordinates with distance and an angle attached to each data point.  This information needs to be converted to Cartesian coordinate data using fast Fourier transform functions.  Once at this stage, the ultrasound data can be converted to analog signal for video display and interpretation.   

Image display has evolved substantially in clinical ultrasound.  Currently, 2D and real time 3D display of ultrasound date is utilized.  Without going into complexities of physics that are involved in translating RF data into what we see every day when one reads echo, the following section will provide the basic knowledge of image display.  If one can imagine a rod that is imaged and displayed on an oscilloscope, it would look like a bright spot.  Displaying it as a function of amplitude (how high is the return signal) is called A-mode.  If one converts the amplitude signal into brightness (the higher the amplitude the brighter the dot is), then this imaging display is called B-mode.  Using B mode data, once can scan the rod multiple times and then display the intensity and the location of the rod with respect to time.  This is called M-mode display.  Using B-mode scanning in a sector created a 2D representation of anatomical structures in motion.