Physics of Ultrasound: Image Artifacts Segment #4

Physics of Ultrasound: Image Artifacts Segment #4

now the third category of artifact due to refraction is ghosting this is pretty interesting it occurs that for example at the domino fashio plane which serves as a refractive medium leading to double-a Horta or in case of a uterus double gestational sac the yellow layers there represent the rectus abdominis muscle with the fascia layer and you have to Reno at a real abdominal aorta that notify the at a red circle as the autosound beam traverses beyond the abdominal muscle it reflects off of the aorta and the bounces back towards the surface of the muscle on underside and the beam is refracted through the de fashion plane and goes back at a translucent from a different at a different angle and so when you trace back the line of sight along the transducer beam line the image will now be relocated lateral to the actual a aorta location as an occupied gray circle so this is a ghost a order now since ultrabeat beams comes from both sides you end up having the same phenomenon happening on this right side of the rectus abdominis muscle giving you a double a orden so that was the ghosting phenomenon how do you minimize or eliminate this well you again you can vary the angle of insulation of the transducer or you can interrogate across a wider area so that you can see the artifact come in and out existence and I'll tell you that is an artifact and it's not real it's not anatomical anomaly now miss registration the focusing artifacts need to be considered typically they're not as serious but they are also the same type difficult to eliminate completely with the above techniques we just talked about reverberation artifacts in the previous section let's do a question on defin on and on the question is a metallic needle at 1 centimeter below the transducer surface generates multiple reparations what is the location of the second reparation the choices are as follows is it a one centimeter B 2 centimeters 3 centimeters or D 4 centimeters you may pause the tape to think about your response and continue when you're ready for your answer the correct answer for the preparation second preparation is C located at 3 centimeters from the surface since I'm count Lido it's generated at 1 centimeter below the surface note that the first echo or vibration echo is going to be at 2 centimeters 2 times the original distance and therefore stands reason that the second vibration we have 3 times the original distance which is 3 times 1 or 3 centimeters now let's talk about multipath propagation artifact and describes reflectors appearing at incorrect depths do – Oh Blake reflection of ultrasound waves following either a longer or shorter path length than the incident beam in this example we can talk about that bladder the posterior wall in this case is a very reflective surface as all size of a bladder as the incident beam hits the reflective part of the wall it will actually reflect the beam to the other side bladder and they'll reflect back finally returning to the transducer creating an impression that the length is longer so here your path length one described an incident beam reflecting off the outer edge paddling to laughing three describes a reflection back towards original surface and path length four describes the return path to transducer as a result the flatter fluid is perceived to be located distal to the actual location causing this multipath phenomenon in effect there is no fluid collection distal to the actual bladder potential solutions for multiple artifact is to change the angle in summation to avoid alike angle situation and also to interrogate an object of interest for example the bladder from multiple views and angles to ascertain what is real from what is might a factual next we can talk about mirror image mirror image that's very different from multipath as we'll talk about in a little bit essentially mirror image describes the generation of duplicate or virtual image of the objects of interest presenting on one side of a strong reflector showing up on the other side of this reflector examples are lesions and liver tumor hepatic vessels appearing on the other side diaphragm this case illustrates the liver bladder and gall bladder on one side the diaphragm which is abdomen but simultaneously having reflection an artefactual mirror image in the thorax which obviously is nonsensical arrow points to the diaphragm which acts as a protective surface the in admin side is the real liver and gall bladder for exercise the reflected liver and gallbladder now let's talk about the mechanism of action that makes it possible to have a mirror image you start first start off a reflective surface of a real object for which also beam going to path links one and two illustrates the incident and reflective beam passed path length three four five and six describes the path of the Assam beam as it reflects off at the diaphragm or reflective surface going back the reflective surface and final return to transducer creating the illusion that the real object is located on the other side of the reflective surface hence you have the end result of a virtual object located equidistant from the real object across in the reflective surface and in this example you have a liver lesion which is present in the abdominal side in the liver however due to the mirror image mechanism it also occurs on the other side of diaphragm equidistant to the reflective surface potential solutions for fixing mirror image artifacts is to change the angle of in summation to vary to reflectivity of the interface adjust a focal zone or the time gain constant at each level of the diaphragm to minimize the effect if ax t + to scan from multiple windows now let's move on talk about propagation speed air and orders on machine and this program assume that the velocity of sound through off softest shoes is 15 40 m/s if the beam passes through a structure consisting mainly of fat velocity is however is only 1450 therefore structures will appear further away from the transducer surface as a result well think about it if you assume a certain velocity in terms of B less obviously it's gonna take longer for it to come back and you think the structure is further away conversely if the reflector of interest is a high velocity higher than fifteen forty meters per second it will appear closer to the transducer surface than actually is so this phenomenon can also cause abnormal step off of the anatomic structures such as diaphragmatic shift that we'll talk about next here you have on the leading edge side of the sound field normal diaphragm and in the receding edge the diaphragm step off as shown by the hyper Kovach line on the distal to the liver now if you look at the yellow line this describes the ultrasound beam traveling through the liver which has some indication of being a little bit fatty and therefore a fatty liver will tend to have a slower velocity of sound and therefore the diaphragm will appear further away and actually is University look at path B which follows the O sound path through a the gallbladder which is a anechoic structure with fluid and the philosophy is higher as a result traveling through a region with high velocity will make the diaphragm be closer to the natural surface potential solutions of the propagation speed error is really just to be mindful of this possibility if you see a drop off it might not be a big deal in addition newer machines with multi beam features and improve signal processing capabilities usually will minimize this artifact next we're going to talk about range ambiguity range I'm acuity describes the use of a high pulse or a diction frequency or in the presence of shadow depth looking at shadow structures causing structures to appear closer to surface than they really are the key here is that the beam the next beam without a transducer outputs is sent before the auto return echoes are back from the preceding echo therefore you want to avoid using excessively high PRF when measuring deep structures therefore the solutions again is to not use a high pulse repetition frequency by measuring deep structures and we're measuring shallow structures use a high frequency linear probe or use a step off if you're an older machine that necessary to minimize the situation let's end a segment with a question a reflector is at a depth of 12 centimeters from the transducer surface but the ultrasound machine displays it as though it is at 10 centimeters so the question is most likely this is due to what phenomenon is that a it's the reason due to range ambiguity does it be that speed of sound is greater than 15 40 meters per second this is C did the posterior acoustic enhancement or is it D due to the fact that the speed of sound is less than 15 40 meters per second it may pause the CD to think about the answer and play it again when you're ready Deker our answer is B – the speed of sound through the structure of interest is greater than fifteen forty meters per second here's how you can think about it the velocity is greater than fifteen forty meters per second that is fast the object will appear closer tourists transducer service than it actually is on the other hand if velocity is less than fifteen forty or slow the object will appear further away from the transfer surface and actually is so you can use an acronym like saft saft slow away fast two words

2 thoughts on “Physics of Ultrasound: Image Artifacts Segment #4”

  1. In grey scale Pulse Echo sonography the operator has no controle on PRF as it is adjusted spontaneously by the ultrasound system according to the scanning depth while in case of Color Dopler the situation is different where the operator has control on incresing or decreasing of PRF.

    With regard to the solution of Range ambiguity artifact it is recommended in the lecture not to use High Pulse Repetition Frequency while imaging Deep structures.IT IS TO BE NOTED THAT AN OPERATOR OF ULTRASOUND MACHINE HAS NO CONTROL ON PRF, BEAM FORMER (PULSER ) OF MACHINE ITSELF ADJUST PRF AUTOMATICALLY WITH REGARD TO SCANNING DEPTH

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