hi learners m from sano nerds and this video is going to be on unit 5 that sounds intense section 5.1 intensity again in unit 3 we discuss the seven parameters that can describe continuous and pulsed waves and intensity was one of those seven parameters so we learned that it is one of the ways to describe a wave's strength we also learned that it's proportional to power and then amplitude squared based on the formula we can see that it is inversely related to area but directly related to power and due to the units used for power and area intensity ends up with a unit of watts over an area squared for ultrasound it's typically centimeters squared and then finally that it has a range between 0.1 to 100 in our diagnostic ultrasound application the ultrasound beam is actually a lot like a flashlight beam so if you were to take a flashlight shine it at a wall you would see this really strong intense light at the center and then as you look towards the outside you'll start to see the light get dimmer which means that it's weakening or less intense and the ultrasound beam is going to do a very similar thing it's very intense in the center and then starts to weaken as you go out towards the edges and that's because we're getting more area that the light is being spread over therefore it's weakening inversely related to the area that it's covering continuing the flashlight analogy if we were to hold the flashlight beam on the wall for hours and hours on end that would be continuous exposure to the light energy very similar to continuous energy that we get from a continuous wave ultrasound the alternative to that would be turning the flashlight on and off just like we would with a pulsed ultrasound so there are two directions in which we need to consider the intensity we need to consider it as a spatial consideration that's looking at the area that the beam is covering that very intense center weaker outside and we need to look at it over time are we looking at the intensity when the flashlight is off are we looking at it when it is on and how long is it on for section 5.2 intensity and area as i mentioned intensity is inversely related to area if the beam is very concentrated in a small area the intensity is going to increase if it's spread out over a larger area the intensity will decrease we use a flashlight analogy as an excellent visual for imagining the strong concentrated center area versus the dimmer outer area but another way you can think about it is a campfire if you're too far away from the campfire you're going to feel very little if any heat that's because you're very far away from the fire there is a lot of area between you and the source now if we move a little bit closer you're going to start to feel that warmth and you might find a good comfortable area in there there's enough heat but it's not too intense however if you were to get super close to that fire even put a hand up to it that heat is going to be so much stronger and more concentrated on your hand it's going to get way too intense and you're going to pull away so the area that the energy needs to cover really matters if you're in a small area it's going to be very intense larger areas become weaker or less intense when an ultrasound beam leaves the transducer it has about the width of the transducer it then focuses towards the center becoming smaller and then begins to diverge beyond the focus becoming wider again so the area ends up changing from the transducer to the focus and in the distant portion of the sound beam so this diagram helps to show us that the energy in the sound beam is going to be strongest and most intense at its center if we look in the circle cross sections of the sound beam we can see that the orange is representing the more intense area and that is going to be more centered in the beam we can see then that the purple is representing the weakening or the less intense area of the cross section and that's going to be more towards the outside as we get further away from the center and start to increase the area that the beam is covering and then not only is the beam the most intense at the center of the cross section of the beam but it's also the most intense at the focus again the focus is where the beam becomes narrowest it's where the area is reduced the most which is why we have the most intensity when we consider how a beam's intensity looks in space we just need to remember the beam's intensity will vary be it through the cross section or down the ultrasound beam and because of this three things are true the beam spatial peak intensity is going to be at the very center of the beam the beam spatial peak average is a mathematical center of the intensities of the beam the spatial peak intensity is stronger than the spatial average intensity and lastly the beam has the most intensity at the focus let's take a look at this picture a little bit more the orange in this image is representing the spatial peak we can see in the cross-section image here that at the very center of the cross section is the most pure orange area and that is the spatial peak that is where we're going to find the most intensity in the cross-sectional area is at the center of the beam if we were able to kind of flip this beam on its side and graph the intensity through the edges to the center we would get something similar to this graph this is on the outer edge and this is on the outer edge as we move towards the center we'll reach the spatial peak so again spatial peak is center of your cross section center of your beam and we can visually see that as the peak of graft intensities now we also have the spatial average and the spatial average in this image is depicted by the more purple area now the spatial average is not the very edge of the beam and it's not the very center of the beam but it's the mathematical middle of the intensities found in this beam let's say that the center has an intensity of 100 milliwatts per centimeter squared and that the very outside has an intensity of 30 milliwatts per centimeter squared to get the average we would take 100 plus 30 which is 130 and then divide that by two because we have two data points to divide so 130 divided by 2 is 65. so we would find the intensity that corresponds with 65 centimeters squared and that would be our spatial average so it's not really towards the center it's not towards the edge but kind of somewhere in the middle depending on the intensities found within the cross section looking at our cross section of the beam graph then we could again figure out the intensities if this was 30 centimeters squared and this was representing 100 centimeters squared then we would do that math and we would be able to correspond on the graph where the spatial average is so biggest takeaways from this image spatial peak is going to be found at the very center of the beam spatial average is the mathematical middle of the cross section through the beam when we are looking at the beam in a cross section and we take the value for the spatial peak and the value for the spatial average we can actually compare the two numbers to tell us how uniform that beam is from center to edge so the beam uniformity ratio or bur is a comparison of the spatial peak intensity and the spatial average intensity this is another unitless number and it's going to tell us how consistent or inconsistent the intensities are across the beam so the beam uniformity ratio or burr is equal to the spatial pulse divided by the spatial average the beam uniformity ratio actually comes with a lot of synonyms it's also known as the spsa factor the buc which stands for beam uniformity coefficient or buf beam uniformity factor typically the value is going to be over 1 that is because the spatial peak should always be stronger than the spatial average on the bottom i've got a couple examples that we have seen earlier of our cross-sectional beam the left one has a more orange hue to it all the way through that's going to be more of a consistent beam the center spatial peak is going to be much more closer to the spatial average because it's all kind of being represented by the same intensity that one's going to have a spsa factor much closer to one because again the spatial pulse is going to be much more similar to the spatial average compare that then to purple and orange circle on the right where we have kind of an intense center which would be our spatial peak then our spatial average is actually going to have to take into account a lot more of that purple weaker area and we're going to have a bigger discrepancy that circle's intensity is more inconsistent throughout the cross section of the beam and we'll see that spsa factor increase too much more than one to give kind of a real world example of the sba sa factor we can kind of think about microwaving a meal if you've ever microwaved something and had really inconsistent temperatures that's kind of what we're looking at with the spsa factor say your microwave a pot pie and the center of it is basically just lava and the outside of it is still frozen you had really inconsistent heating you have a very hot center compared to the frozen edges if you were to take an average temperature in there the lava temperature center would be much higher than the average because you got a factor in those frozen parts but if we microwave our pot pie and it's pretty hot in the middle and relatively hot on the edges we would see that we had more consistent heating because our center temperature would be much closer to the average temperature of the dish section 5.3 intensity and time when we're thinking about pulsed ultrasound we know that it typically has a very short burst of sound energy followed by a really long off period in which it's listening for echoes to return the amount of time that the sound energy is present compared to the off time is described by duty factor and duty factor can range from zero to one or zero percent to one hundred percent so when the machine is off we know that the duty factor is at zero percent typically when we're using our 2d imaging imaging and grayscale the duty factor is less than one percent so the sound is on for just a small fraction of time the rest of the time is waiting to receive echoes from that small burst of sound energy as we change to different ultrasound modes such as doppler the duty factor can increase by a little bit usually between one and ten percent but ninety percent of the time is still listening or off time lastly we do see one hundred percent when we are using continuous wave ultrasound and that means that there is constant ultrasound energy entering the body up until this point most of the graphical representations of pulsed ultrasound that you have seen has shown very uniform cycles within a pulse but in reality the cycles within the pulse vary in amplitude power and intensity typically the beginning and the end are a little bit weaker where the center of the pulse is strongest so at the center of the pulse is typically where we are going to find the temporal peak and that is going to be the strongest intensity within the pulse where the highest power is where the highest amplitude is because of the varying intensities within the pulse we also need to consider that there is a pulse average we have some lower intensity some higher intensities we will find the mathematical center of those intensities and call it the pulse average and lastly we have the temporal average now the temporal average is considered during the on time of the pulse and the off time of the pulse so we do have intensities found within the on time that we will consider but when we consider all the time in the off time remember that had an intensity of zero so the temporal average tends to be much lower than the temporal peak and the pulse average because it is considering all of that off time and we'd be able to calculate the temporal average based on multiplying the duty factor by the pulsed average when we look at the continuous waves temporal considerations remember that duty factor for continuous waves is one hundred percent or one if we were to plug that into our equation one multiplied by the pulse average we're going to see that the pulse average doesn't change therefore pulse average and temporal average are completely the same when using continuous wave ultrasound and that's completely because there is no off time in the continuous wave so knowing that the pulse is not uniformly strong knowing that there is on and off time we do need to consider that the intensity of the ultrasound beam when using pulse wave is going to change over time so we need to remember that temporal peak is the strongest intensity temporal average ends up being the weakest because it includes the on and off time and the pulse average is going to be weaker than temporal peak but stronger than temporal average because it is all of the on time and when the machine is listening or off the intensity is going to be at zero section 5.4 measuring intensity here i've got a couple examples of hydrophones which are also known as microprobes and these are tools that can measure the output intensity of a beam that's produced by one of our ultrasound transducers i have two types of hydrophones imaged here we have the needle variety which can be moved around through the beam and then we have the disc variety that kind of catches all of the information from the beam the hydrophone is attached to an oscilloscope and the hydrophone will be able to measure the pulse repetition period the pulse duration and the period of the wave and from these parameters as we've done in multiple examples in the past we can actually calculate most of the other parameters of the wave the hydrophone is capable of measuring amplitude power and intensity as well and we want to know the strengths of the beam because they have a large effect on how the beam will interact with human tissue that sound on human tissue interaction is known as bio effects and we always need to be mindful of this when we are using ultrasound equipment for the patient's safety here is an image of a disc hydrophone being used to measure the output from a transducer all of the equipment is placed into a water that's why it's a hydrophone it's a water microphone and it's going to receive that sound energy from the transducer display the information on an oscilloscope and then we'll be able to take data out of there to calculate the parameters that we've discussed when we are calculating those parameters though we need to consider a couple things where was the beam measured and when was the beam measured was it measured during a spatial peak or the spatial average and was it taking information when the wave was at its temporal peak during its temporal average or at its post average and because we can measure intensity at any two of the spatial considerations in any three of the temporal considerations we can actually combine them together to get a better picture of what is happening on the next slide we're going to see a little video that puts all the combinations together so sp is spatial peak sa is spatial average those are going to be combined with the temporal considerations tp for temporal peak ta for temporal average and pa for pulse average so when we add spatial peak and temporal peak we get sptp which is the strongest intensity next we have spatial average and temporal peak spatial peak and pulse average spatial average and pulse average spatial peak and temporal average which is what we want to consider for bio effects and lastly spatial average and temporal average which is our weakest intensity i mentioned on the spta image that we are concerned about bio effects when we are looking at the spta intensity the spatial peak temporal average intensity ultrasound really has not shown any severe adverse side effects in the application of diagnostic medical sonography however we do know that computer studies do suggest that in very strong pulses and for very long durations we might actually start to see some bio effects occur the first type of bio effect that we are concerned about are mechanical bio effects these are typically going to arise when the pulse is very strong what it ends up doing is causing liquid to form small bubbles and this is called cavitation there's a image on the side a little video and you can see that it is liquid pressure within the liquid causes a bubble and as that pressure oscillates through the bubble it can cause the bubble to collapse or burst and when it does that it actually has a lot of energy going with it so the concern is is that ultrasound will cause liquid in the body to form tiny little gas bubbles those tiny little gas bubbles will rupture causing cell damage when they do so 2d ultrasound or grayscale ultrasound is typically going to run the higher risk of causing cavitation and the sppa intensity is closely related to the amount of cavitation that will occur with a beam the other type of biofact that we are concerned about are thermal concerns now these are going to arise from a duration of a pulse that interacts with the body so if we were to hold a beam on one spot for a really long time there's a bigger chance that the tissue is going to heat up since the sound energy is going to be partially transferred into heat energy doppler is going to be more likely to cause heating in tissue and that's because the sample area has longer pulses and repeatedly samples the same area we're putting stronger sound energy directly in the same spot multiple times studies have shown that if we increase the area that ultrasound is located in by two degrees celsius we actually can cause some harm to the tissue for thermal considerations we watch the spatial peak temporal average that's why you need to know that spatial peak temporal average is closely related to bio effects the thermal bio effect is a little bit more concerning than the mechanical as there's a lot more factors that go into the mechanical cavitation part the thermal vial effects are just 100 percent the intensity of the beam interacting with the tissue and how much heat they can cause now the reason that we use facial peak and temporal average is because it is a truer representation of how the sound beam is interacting with the human body they are not just getting the temporal peak they're not just getting the pulse average they are getting the on and off time so we want to make sure that we are considering that there's on and off time and they're going to be getting all of that central beam intensity so that doesn't change ultrasound machines are not available for public use or purchase you do need to be a medical provider school or veterinarian to purchase a machine and that is because they are fda regulated and with the fda regulating them they make sure that the ultrasound machines do not produce an spta intensity more than 720 milliwatts per centimeter squared we'll be covering much more about bio effects towards the end of this course but in the meantime if you're curious about where you can monitor the mechanical and thermal bio effect we actually have it right on the screen typically up in the corner you'll see a number preceded by mi which stands for mechanical indices and ti which stands for thermal indices the higher the mechanical indice is the more likely cavitation is going to occur and the rupture of bubbles and the ti indicates the degree of temperature in which the ultrasound beam could possibly raise the temperature so in this example we could raise the temperature by 0.2 degrees celsius using the ultrasound beam that we have activated and that brings us to the end of unit 5. now ultrasound intensities wasn't a very long unit and that's because a lot of these concepts are a little bit more in the abstract the biggest things that you need to take away from the intensity unit is that intensity is measured in a watts per centimeter squared which means that is directly related to power and inversely related to area so as the area increases the intensity should decrease the ultrasound beam changes in relationship to space and it changes in relationship to time so we have two spatial considerations spatial peak which is at the center of the beam and is the strongest and the special average which is the mathematical middle of the intensities in space looking at the temporal ones we have temporal peak which is the strongest point of the pulse we have the temporal average which includes all intensities during the on and off time and then we have the post average which includes the mathematical average of the intensities found just within the pulse taking those five intensity measurements we combine them together to make six combination measured intensities the strongest is the spatial peak temporal peak the weakest is a spatial average temporal average we are most concerned with spatial peak temporal average for bio effects because it is the truest representation of what our patients experience in relationship to the ultrasound energy