let's take a closer look today at spectral Doppler ultrasound we've looked at the concept of the Doppler effect and Doppler shift within tissues and we've seen how we can use that Doppler shift value in order to measure the magnitude of movement within tissues and if we know the angle of our ultrasound pulse versus the angle of the movement within tissues we can use that Doppler angle in order to measure a specific velocity within tissues now spectral Doppler is the display of velocity change over time within a region of Interest now in our previous talk we looked at continuous wave and pulse wave Doppler ultrasound imaging both of which can create spectral Doppler ultrasound now today I want to First cover how we go about creating a spectral waveform and then touch briefly on some measurements that we can do within a spectral waveform and end things off with two very brief clinical examples now the focus here is going to be on the physics we're not going to be focusing on actual clinical outcomes and all of my schematics here diagrammatic representations of the spectral Doppler waveform nothing here is extremely accurate I want you to get the concepts over actually measuring specific values now in order to create a spectral Doppler waveform the first thing we generally do is create a b mode image a brightness mode image with a standard ultrasound transducer that allows us to appreciate the underlying Anatomy within the patient's tissues then often we place what is known as an active area over our region of interest and this is a color Doppler Zone Zone where we can see movement within the tissues this often helps us to identify our vessel of interest and helps us to place our Dopp gate within a very specific region in that vessel the next thing we do is choose an a line of data a single a line within this B mode image that we are going to use pulse wave Doppler in this example to create our spectral waveform once we've chosen that a line of data we can then place what is known as a gate now the gate spe specifies two things it specifies the depth in the tissue that we want to image and it specifies the volume of movement we want to image the distance between these two gates here will be the area that we're sampling for the Doppler shift now once we've placed the gate we need to tell the machine what angle our a line is in comparison to our vessel of interest and this setting of the angle means that we know what our Doppler angle is in the image now this line This angle calcul needs to be parallel with the blood flow within a vessel so now we've set up our pulse wave doler image we can go about creating these spectral waveform that will form from blood moving between this gate here now the spectral waveform is made up of two axes the first axis is time the spectral waveform is the change in velocity over a period of time and on our machine we can change the time scale known as The Sweep speed in which which we display that spectral waveform now in this example say this was representing 2 seconds of data we could see that this patient's heart rate is 120 beats per minute now we could change this to display 4 seconds of data or to display 1 second of data and that will stretch and compress our spectral waveform accordingly now the Y AIS represents the velocity of blood within the spectral gate we can see that the Doppler shift that we measure within that gate is proportional to the velocity of blood moving through that gate and we'll see later how we can change this velocity scale by changing the pulse repetition frequency of our pulse wave Doppler the next component of the spectral waveform is the Baseline the level which represents no flow of blood zero velocity now the Baseline can be moved on our screen the screen that's displaying the spectral waveform we can move that Baseline in order to better see the spectral waveform if all of our spectral waveform has positive velocities we can move that Baseline down slightly to see the spectral waveform better CU at some point we're going to be measuring specific points on the spectral waveform in order to calculate specific indices that have a clinical value now another thing that the spectral waveform displays is direction of blood flow by convention we say that a positive velocity represents blood flowing towards the transducer and a negative velocity blood flowing away from from the transducer now on a machine we can actually change the direction if we want to it's not changing the direction of blood flow it's changing the way we display that blood flow on the spectral waveform now the actual spectral waveform the line itself can be broken down into various different measurements and I've labeled the most important ones here now when we think about how this line is created there are multiple red blood cells thousands of red blood cells passing through this gate at any given period of time our incident ultrasound beam will be reflecting off all of these thousands of red blood cells they will each be sending back different Reflections back to our ultrasound transducer now all of those Reflections heading back will interfere with one another we'll be getting one continuous stream of data heading back towards our ultrasound transducer and that stream of data will be a combination of all of the hundreds and thousands of waveforms heading back from the various different red blood cells and we can use a process known as fast furier transform to take that one continuous data stream and it will break it down into multiple different individual waveforms heading back now those individual waveforms will represent specific Doppler shifts based on the red blood cells that have reflected that ultrasound wave back now we've got a range of velocities each red blood cell will have slightly different velocities now the thickness of our spectral waveform line represents the differences in those velocities within the red blood C cells at that given point in time so you can see that if there was turbulent blood flow through the gate here we would have a very thick spectral line there will be a wide range of velocities at any given point in time some red blood cells actually may be flowing in the opposite direction while some might be flowing quickly towards the ultrasound transducer and this spectral waveform will spread over a wider region the narrower or the thinner our spectral waveform line the more uniform the velocity of those red blood cells heading through the spectral gate here now in terms of labeling the various components of the spectral waveform the first thing we need to know is the peak systolic velocity the highest velocity within our blood flow at any given period of time then we look at what's known as the enddiastolic velocity the velocity of blood flowing through our gates at the end of diast just before the next syst now this is an arterial waveform that we're looking at and the waveform shapes will vary depending on the type vessel that we're Imaging and that's out of the scope of this talk now the window below this waveform during syy is what's known as the spectral window and there are various reasons why that spectral window might be obliterated might be filled in by signal one of which as we've mentioned is turbulent flow we can also increase the gain on our machine too high where we start getting noise back and filling this spectral window falsely here now what we're representing with the spectral waveform is the change in velocity over time now what is the change in velocity over time that's the acceleration of an object now we can see this line here which is known as our systolic upstroke and this represents the acceleration of blood during syst now we can see that after we've had that contraction that systolic contraction and the Heart relaxes now that blood then decelerates the velocity changes over time so any movement in the spectral waveform that represents increasing velocities shows Accel a of blood and the opposite is also true decreasing velocities shows deceleration of blood now importantly blood that is decelerating doesn't mean it's reversing here it's slowing down if we're in a car and we break we start to slow down we start to decelerate but the car is still moving forward this Blood on the downstroke here is still moving forward and this spectral waveform actually represents blood that is continually moving forward moving towards our ultrasound transducer even at the end of diid there is still a positive velocity here now the last component that I want to mention is what is known as time to Peak the period of time it takes from the beginning of systo to get to our Peak systolic velocity now that time to Peak is related to the acceleration of blood and the faster the acceleration of blood the shorter that time to Peak is now importantly this spectral waveform that we are creating here is specific to the G gate that we have set and we can change the size of that gate now we know that blood flowing through a vessel exhibits a specific type of flow blood flowing in the center of that vessel will exhibit what's known as lamina flow it will have the most consistent flow at the highest velocities and the more peripheral we go out on that vessel the slower the velocities and the less consistent those velocities are now if we were to increase our gate size here we can see that our spectral wave thickness would increase the range in velocities that we detect here will increase our Peak systolic velocity won't change and our n diastolic velocity won't change but the thickness of that spectral waveform has changed because we are sampling a greater range of frequencies in a wider gate now I mentioned earlier that the velocity scale here is determined by our pulse repetition frequency when we change the velocity scale the look of our waveform would change now here we've only changed the scale here the maximum velocity that we are now displaying is 250 cm/s where previously it was 120 cm/s now nothing has changed in the velocity of the blood here only our scale has changed now we are going to use the peak systolic velocity and the N diastolic velocity to calculate what is known as resistive index and we need to be really accurate when setting those measurements and using a scale that is incorrect for our waveform means we're going to be less accurate here we want our waveform to take take up the entire real estate here so we can be accurate with those measurements now how do we go about actually calculating the scale here when we set our a line and set the gate here what we are doing is inadvertently setting our pulse repetition period the period of time between the first pulse and the next pulse we've set a specific depth here and that depth will allow us to determine how much time we have to wait in this receive time in order for the pulse to come back from our gate the round trip distance here so say for example this was 4 cm deep in the tissue we can then calculate how long it would take our pulse to go 4 cm in and return 4 cm back before then releasing the next pulse into the tissue that's what's known as our pulse repetition period now here our pulse repetition period has been set because of the depth that we are sampling within tissues now the pulse repetition period is inversely proportional to the pulse repetition frequency the number of pulses sent into a tissue per second now the higher our pulse repetition frequency the shorter the pulse repetition period when we are sampling shallower tissues now as that pulse repetition frequency increases we can detect higher and higher Doppler shift values we looked at in the previous talk that the Doppler shift value the maximum Doppler shift value that we can detect is half of our pulse repetition frequency and that Doppler shift value corresp responds to a velocity value within tissues so the higher our pulse repetition frequency the higher the maximum velocity that we can SLE now if we want to change this velocity scale in order to change it back to the maximum velocity being 120 cm/s and getting the waveform filling the entire screen here allowing us to more accurately measure our Pak systolic and enddiastolic velocities what we need to do is lower our pulse repetition frequency and how do we lower the pulse repetition frequency we increase our receive time here we increase the pulse repetition period now we're sampling that tissue at a lower rate our pulse repetition frequency has decreased but because the Doppler shift in this blood at the set velocity is still less than half the pulse repetition frequency we can still accurately map those velocities over time so changing our pulse repetition frequency will change the scale that we use the velocity scale on our y- AIS in spectral Doppler now I've said earlier that we will use these values to calculate what is known as a resistive index now the resistive index takes the difference between our Peak systolic velocity and our end diastolic velocity and divides that by the peak systolic velocity now we can see here that if the N diastolic velocity is a positive value our resistive index will always be less than one if the N diastolic velocity ends up being zero if there's no flow at the end of diast our equation here will be our Peak systolic velocity over our Peak systolic velocity our resistive index will be one if there's reverse flow at the end of diast I.E the enddiastolic velocity is a negative value the numerator here will be more than our denominator our resistive index will be more than one our Peak systolic velocity subtracting a negative value is the same as adding that value we've got a larger numerator and a smaller denominator now the resistive index is a proxy for Downstream resistance we think of blood flowing through a vessel as the proximal portion of that vessel being Upstream as we head through that vessel we think of that is Downstream the resistive index shows us how much resistance there is Downstream so we can see that a low resistive index means low peripheral vascular resistance and the higher the resistive index the high High the peripheral vascular resistance now we use this resistance our body uses this resistance in order to titrate or shun blood to where it is needed the most now there are certain organs within the body that need a constant flow of blood they high metabolic demand organs we know these organs such as the brain it needs continuous blood flow the kidneys the liver they need a low peripheral vascular resistance in normal blood flow we will always have a positive enddiastolic value there is continual forward motion of blood to these high metabolic demand organs now there are certain tissues like our limbs or the bowel that doesn't need continual flow of blood we can have higher peripheral vascular resistance in these regions so that we not shunting blood towards our muscles when we're at rest now this can change if we start to exercise we start to run the capillary beds the vascular beds within our muscles are going to dilate they're going to reduce that peripheral vascular resistance and our resistive index will decrease we will shunt blood to where it's needed so our body can change the resistance of those peripheral vascular beds in order to move blood around the body we know that blood moves from higher pressure regions to lower pressure regions and that's a really important concept to remember now let's go ahead and look at a specific example if we take the common cored artery here that then splits into the external cored and internal cored artery out internal cored artery is going towards the brain it needs to supply a constant blood flow towards the brain our resistive index here is going to be less than one our Peak systolic velocity and and diastolic velocities are both positive values there's continual forward motion of blood the velocity of this Blood never crosses the Baseline now our external cored artery has multiple branches many to superficial structures on the head and neck now during diast the end end of dasle there is no flow within those vessels only during cyly and the first part of dle is there forward flow within the external corupted artery if the enddiastolic velocity value is zero our resistive index will be one if there's slight reverse flow our resistive index will be more than one now what happens when we scanning the internal cored artery looking for AOS scerotic disease we will see changes within the spectral waveform that will help us to realize whether there is is atherosclerosis within that vessel or not we're looking at if there's stenosis within a vessel and this is one of the most common applications of spectral ultrasound so let's zoom in on an internal cored artery here that has a thrombus within the wall now we can see that the blood flow heading in the proximal section the Upstream section of our internal cored artery has slightly changed here we've increased that peripheral vascular resistance we've seen that as we increase the peripheral vascular resistance we increase the resistive index of the spectral waveform so now we are still getting this peak systolic velocity coming from the heart here but in diast that enddiastolic velocity has now reduced it's coming towards zero here this resistance from the stenosis is causing that diastolic forward flow of blood to slow down we are reaching the Baseline here if we were to place a spectral gate right in the middle of this stenosis here we would see the velocity values that we calculated are way higher than this upstream or proximal section if you have a hose pipe that's got water coming out of it and you olude some of that hose pipe with your thumb you're not changing the velocity of water coming to your thumb that velocity suddenly becomes much faster as you narrow the Lumen or as you narrow the radius of where the blood is flowing through or where the water on our host pipe is flowing through now what you'll see here is that the velocity we measure is beyond the scale that we've set on our machine and we get a phenomenon known as aliasing here an ultrasound artifact that's specific to Doppler now in our next talk we're going to be looking specifically at aliasing and how we can go about reducing that aliasing artifact now the blood that makes it through the stenosis will now be turbulent it's going from a narrow Lumen to a wide Lumen and it's going round the thrombus so that Blood starts to become turbulent and we see now that we've reduced the peak systolic velocity and we've closed the spectral window there is turbulence within that blood there is a larger range of velocities within the blood heading out of the stenosis now further Downstream we get a very characteristic waveform that is a downstream effect of a proximal stenosis and this is what's known as Tardis parvis now what we have here is a lower Peak systolic velocity we are getting reduced velocities Downstream from the stenosis we also have a slower acceleration time here because only a set amount of blood can make it through this tight stenosis so this slower acceleration to Al Peak systolic velocity and a higher end diastolic velocity means the resistive index here has been decreased now why is this diastolic velocity so high well the brain has been starved of oxygen here we are not getting enough blood supply through this internal cored artery and as a compensatory mechanism we've reduced that peripheral vascular resistance and that reduction in resistance that reduction in vascular pressures means that we get forward flow in the low pressure diastolic State we are trying to increase that blood flow to the brain so we can see how a pathology leads to spectral waveform changes and how we can look at these spectral waveforms and try and explain them with an underlying physics principles now let's look at one more example this is the portal vein heading towards the liver it's taking blood from the gut and from the pancreas and from the spleen and taking it towards the liver now in a normal spectral waveform here we have a fairly thick spectral waveform multiple velocities heading towards the liver and they're generally at lower velocities 15 to 30 cm/s now that waveform has a slight phase to it and that phase is dependent on both the cardiac cycle as well as our respiratory rate as we breathe in and out we are changing intrathoracic pressures and that also changes our Venus pressures so we can see the slight f phase to the waveform now what happens if we get something like portal hypertension there's increased resistance to that blood flowing back to the liver we get a reduction in the velocity of blood within the portal vein we can see now that those velocities have decreased if there's no velocity within the portal vein we need to be worried about something like a portal vein thrombosis we've actually got a clusion and no flow of blood within the portal vein we can also get an increase in the variation within the blood flow in the portal vein and this occurs in something like tricuspid regurge where the heart goes through a systolic contraction and there's actually reverse flow of blood through that incompetent tricuspid valve down into our Venus system into the inferior vena and into the liver and we can see that there's some reversal of flow with those increased pressures within our inferior vnea so during syy there's reversal of flow and during diast we are getting that normal flow that we were seeing our initial waveform here now this talk is not meant to teach you the different types of waveforms in the different types of vessels I just want to show you how we can use the basic principles that underly spectral Doppler and apply them to clinical scenarios and in the future we're going to go through multiple different clinical scenarios and multiple different vessels throughout the body but that is for another time and if you're studying for an ultrasound Physics Exam focus on the initial part of this talk how we go about creating that spectral waveform and which parameter changes will lead to changes within the spectral waveform and in the next talk we're going to look at the concept of aliasing where our pulse repetition frequency is not sufficient for the velocities within the blood and we need to make different changes in order to compensate for that aliasing now the concept of spectral waveforms and aliasing comes up over and over again in exams and I've linked below a curated question bank for those of you that are studying for an exam so go and check that out if that is you otherwise I'll see you in the next talk where we're going to take a deep dive into what aliasing is and how we can go about reducing aliasing within the spectral waveform until then goodbye everybody