The principle of ultrasound: Difference between revisions

'''Refraction''' is simply transmission of the ultrasound with a bend.  This occurs when we have an oblique incidence and different propagation speed from one media to the next.  The physics of the refraction is described by Snell’s law.  Sine (transmission angle)/sine (incident angle) = propagation speed 2/ propagation speed 1.   

More on image quality or resolution.  We have touched upon axial resolution (ability to differentiate objects that are located along the imaging beam axis) when we discussed spatial pulse length.  The smaller the axial resolution length, the better the system is and it can resolve structures that are closer together.  Thus the shorter the pulse length, the better picture quality.  Current transducers are designed with the minimum number of cycle per pulse to optimize image quality. The primary determinant of axial resolution is the transducer frequency.  Axial resolution (mm) = 0.77 x # cycles / frequency (MHz).  One must remember that attenuation is also dependent on the transducer frequency, thus a tradeoff must be reached.  Lateral resolution is the minimum distance that can be imaged between two objects that are located side to side or perpendicular to the beam axis.  Again, the smaller the number the more accurate is the image.  Since the beam diameter varies with depth, the lateral resolution will vary with depth as well.  The lateral resolution is best at the beam focus (near zone length) as will discuss later when will talk about the transducers.  Lateral resolution is usually worse than axial resolution because the pulse length is usually smaller compared to the pulse width.  Temporal resolution implies how fast the frame rate is. FR = 77000/(# cycles/sector x depth).  Thus frame rate is limited by the frequency of ultrasound and the imaging depth.  The larger the depth, the slower the FR is and worse temporal resolution.  The higher the frequency is, the higher is the FR and the temporal resolution improves.  Sonographer can do several things to improve the temporal resolution: images at shallow depth, decrease the #cycles by using multifocusing, decrease the sector size, lower the line density.  However one can realize quickly that some of these manipulations will degrade image quality.  And this is in fact correct: improving temporal resolution often degrades image quality.  M-mode is still the highest temporal resolution modality within ultrasound imaging to date.