The principle of ultrasound: Difference between revisions

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.     

'''Second Harmonic''' is an important concept that is used today for image production.  The basis for this is that fact that as ultrasound travels through tissue, it has a non-linear behavior and some of its energy is converted to frequency that is doubled (or second harmonic) from the initial frequency that is used (or fundamental frequency).  There are several parameters that make second harmonic imaging preferential.  Since it is produced by the tissue, the deeper the target the more second harmonic frequency is returned.  As the ultrasound beam travels through tissue, new frequencies appear that can be interrogated.  Second harmonic data gets less distortion, thus it produces better picture.  Also, the second harmonic is strongest in the center of the beam, thus it has less side lobe artifacts.  At the chest wall the fundamental frequency gets the worst hit due to issues that we have discussed (reflection, attenuation) – if one can eliminate the fundamental frequency data then these artifacts will not be processed.  One concept of eliminating fundamental frequency data is called pulse inversion technology.  The transducer sends out 2 fundamental frequency pulses of the same amplitude but of different phase.  As these pulses are reflected back to the transducer, because of the different phase they cancel each other out (destructive interference) and what is left is the second harmonic frequency data which is selectively amplified and used to generate an image.