Transcript for:
Understanding Temporal Resolution in Ultrasound

hi learners it's em from sano nerds and this video is on unit 13 our second discussion on resolution focusing on temporal resolution unit 13 resolution number two temporal recall that resolution is the machine's capability to display reflectors accurately we already discussed axial and lateral resolution as they pertain to reflectors that are parallel and perpendicular with the sound beam respectively this unit is going to add a third type of resolution temporal resolution temporal resolution is the machine's capability to accurately display moving objects the moving objects could be those that are physically moving within the body like the heart or the machine keeping up with the sonographer moving the transducer which is called real-time imaging section 13.1 real-time imaging let's start with what real-time imaging is by learning how ultrasound started now early sonographic images were created by the stenographer physically moving a transducer that was attached to an articulating arm each sweep created a scan line and then the scan lines were added together to make one frame or one picture this is called static scanning so static scanning is the name given to the process of creating one frame and then displaying it static scanning makes it impossible to see moving structures in this image we have the sonographer and the patient and this is that articulating arm and right at the end of it is a transducer and what the stenographer would do is sweep side to side with the transducer across the abdomen or across the area of interest each sweep that they created created one scan line so if they were making an image of 50 scan lines it'd be 50 times back and forth across the abdomen those 50 scan lines then would be added together to make just one picture so this makes sense why it would be impossible to see moving structures imagine how long it would take to sweep that arm back and forth back and forth back and forth enough times to get a wide enough view of anatomy if the sonographer wanted to create a second picture then they would have to restart the scanning process again moving that articulating arm back and forth to create all the scan lines to create a second picture now our current machines create the illusion of real-time imaging because now the generation of the scan lines is automated and happens within just tiny tiny tiny little fractions of seconds so for this unit we really need to remember that each scan line is created by at least one pulse and the machine is going to send that pulse out wait for the echoes to return from that pulse and then get ready to send out either a second pulse for that same scan line or get ready to send out a pulse for the next scan line once all the echoes return from all the scan lines the machine is going to take that information process it through the memory and then display one complete image so although our machine makes it look like we're watching a movie or just watching things in real time what truly is happening is individual images being displayed refreshed again and displayed refreshed again and displayed very very quickly to the point where it seems very seamless to us as stenographers if you think about going to a movie at a movie theater watching the movie play out it feels like you're just sitting there watching this happen in real time but in reality there is a camera in back that is showing you still frames very very quickly so it feels like it's all happening in real time imagine flipping through like a flip book where you drew little animations probably as a kid and you would flip the pages and as the pages went by you could tell that the objects were moving but you knew that that wasn't really in real time so that's kind of what we're talking about temporal resolution are those still image frames happening quick enough so it feels seamless so it feels like a movie so it feels like real life or are you flipping the pages on a flipbook getting to see a frame and then kind of seeing how that motion changes in the next frame and seeing that a little bit slower seeing it with a little bit more space in between the recreation of each frame so when a machine has good temporal resolution we're going to get more of that movie-like appearance when temporal resolution is reduced we're going to get more of a leggy jumpy appearance so section 13.2 temporal resolution kind of goes over that idea again now temporal resolution is the accuracy that moving objects are displayed in their correct position so again when we have a lot of frames produced per second the temporal resolution is good we see real realistically how the structures are moving in the body when we have fewer frames produced per second then we're not really seeing the true motion of the objects and maybe really only seeing fragments of that motion now in this example here we have a fetus and we can see the heart moving here we've got the walls of the ventricles moving we can see the valves opening and shutting this feels relatively real time like we're watching this actually happen but remember in reality the machine is scan scan scan scan scan make a picture scan scan scans can scan make a picture and it's doing it so quickly that our brains perceive it to be just constant real-time imaging now the frame rate on this machine is actually 52 hertz at the moment that means that there are 52 frames being produced every second to create what we are perceiving as real-time imaging if this starts if this number starts to drop below 30 we're actually going to be able to recognize those individual frames being created and it's going to feel much more like that flipbook where we are seeing individual frames and only getting part of the information of that moving object so here's a great example of those frames per second being created now we've got 10 frames per second 30 frames per second and 60 frames per second focus on the 10 frames per second image you can see how that block is turning you can almost recognize the individual frames as it turns now if you move your attention over to the 30 and 60 frames per second those feel really seamless the 30 frames per second looks like it's just spinning in space like you could see this actually moving and the 60 frames per second adds even this extra layer of kind of smoothness to it we cannot perceive that these are individual images being produced for our eyes and see them more as real-time movie-like appearance and that is because the human eye needs a frame rate of at least 30 frames per second to perceive the images occurring in real time anything under 30 frames per second and especially as we get down to like that 10 5 frames per second we're really going to recognize the change of each individual image if our temporal resolution is poor then we can't truly evaluate motion in the body it's going to feel like our machine is either lagging behind our movements or we're going to miss a lot of the motion in anatomical structures that are moving section 13.3 frame rate now you've already heard me kind of throw this term out here the frame rate we've talked about frames per second so let's go ahead and define it frame rate is the number of frames or still pictures that can be created per second now notice that this is an events per second so we know that the unit for frame rate is going to be hertz because hertz is in events per second frame rate is one of those true per second events so frame rate is not reported in kilohertz or megahertz even though we talk about like cycles per second and then say it's a megahertz thing frame rate is one of those things that actually is just a hurt so we can only produce so many frames per second so with the higher frame rate the more movie-like our images are going to be displayed as so think back to that 10 30 60. the 10 was kind of leggy 30 was pretty good and 60 was even a little bit better so as we increase our frames per second or increase our frame rate we are going to see improved temporal resolution more like a movie now if we were to lower the frame rate we're going to start seeing those individual frames being displayed slower so there's going to be less frames per second and when we can individually see each of those frames it appears like our image is becoming a little bit more jumpier or lags behind our movements now there are three things that are going to determine what the frame rate is the first one is sounds speed in the medium now remember as a stenographer you cannot change the speed in the medium that is built in to the material that we are ultrasounding since we are doing medical ultrasound we are talking about soft tissue so it'll always be that 1540 meters per second 1.54 millimeters per microsecond the sound's speed in the medium relates back to frame rate because remember once that pulse is sent it can only travel so fast through the body to the maximum imaging depth and then those echoes can only travel so fast back to the transducer before that next pulse is sent so the speed and the medium ends up being a fixed variable not adjustable by the sonographer and is related back to how fast the sound will be able to travel through the medium and back to the transducer before another pulse can be sent now the second factor is going to be the depth of the imaging and this is something that as a sonographer you can control so we have the option to image at five centimeters eight centimeters 20 centimeters whatever we need to see our anatomy so when we increase the depth of imaging we increase the time it's going to take for that pulse to leave the body get to the maximum imaging depth and then return back to the transducer the longer that takes the less frames that can be created the third factor is going to be the number of pulses per picture this is again something that can be controlled by the sonographer if one frame only needed 100 pulses to be created compared to a frame that needed 300 pulses to be created remember those pulses all have to send their information back to the machine and then the machine will process and display them so if it takes longer to get all those pulses out and then their information back it's going to take longer to display a frame and if it takes longer to display a frame there are fewer frames produced per second so again since speed of sound is that constant 1540 meters per second in soft tissue the two sonographer controlled factors are going to play a larger role in the frame rate we can't do anything about the medium but as a stenographer we can definitely change the depth that we're using and control how many pulses per picture are needed so in general when we are thinking about frame rate if the frame rate increases which means i have more frames per second then the time to create one frame must decrease and if the frame rate decreases that is because the time to make one frame has increased so if we are spending less time making one frame we can get more of those done per second compared to a lot of time to make one frame we're not going to be able to get as many out in that second and these general statements actually might sound really familiar to other hertz and time things that we have discussed already remember frequency is unit hertz and it is inversely related to the period the time it takes to make a cycle so frequency increases there are more cycles per second period must decrease because it's going to take less time to make those cycles if the frequency decreases then there are fewer cycles per second the period increases because it's a longer time frame per cycle so same idea with frame rate and the time it takes to make one frame and we actually have a name for the time it takes to make one frame it's called t frame t for time referring to the frame so frame rate and t frame are also reciprocals because frame rate is a hertz unit and t frame is a time unit so when we multiply them together they should equal one now if we take a look at this image on the bottom here we can kind of see just like frequency and period are related the frames per second and the time to make a frame are also related in that inverse and reciprocal relationship so in our 30 frames per second we have a decreased frame rate and that is because it takes longer to make a picture and display it so all this time goes by we make the picture and display it more time goes by make the picture and display it so 30 frames per second would be the same as saying 30 hertz and then the reciprocal of that is 1 30th of a second so each image takes 1 30th of a second to create that's the t frame that's the reciprocal of it same idea with 60 frames per second then we are producing 60 individual pictures per second that means 60 frames per second is the same as 60 hertz the t frame or the time it takes to create one frame is going to be one sixtieth of a second one sixtieth of a second is a shorter amount of time than one thirtieth of a second and therefore we can produce more pictures per second as we keep increasing that frame rate that means that our time must decrease so 120 frames per second would be the same as saying 120 hertz and that means that the time it takes to create each individual frame is now one one twentieth of a second which is far less than one sixtieth of a second and one thirtieth of a second so frame rate and the time it takes to make one frame are reciprocals of one another so we call it t frame so again t frame is the time it takes to make one frame and we just learned that t frame and frame rate are inversely related and they are reciprocals of one another so it actually gives us our first formula related to t frame and frame rate when we multiply t frame by the frame rate we should get one now i kind of already showed you on that image how we are going to take the frames per second or the hertz for the frame rate and then change that into reciprocal however i do have a couple examples so we're going to head over to the board so i can show you step by step on converting frame rate to t frame and t frame to frame rate this first formula shows us that the frame rate and t frame are inversely related we have t frame and frame rate equal to one anytime you multiply two factors together and they equal one they are not only inversely related but reciprocals of one another so if a system can refresh the image at a rate of 20 hertz that means the frame rate is 20 hertz or that the system can make 20 frames per second so it's going to scan show an image scan again show an image and it can do that 20 times per second to figure out how long it takes then to make one frame is going to be the reciprocal of 20 hertz all we need to do is move that 20 hertz into the denominator position underneath a 1 and that gives us our t frame so it takes 1 20th of a second to generate a system that can refresh the frames at 20 hertz now if you're given the t frame let's say that you're told it takes 1 50th of a second to create one frame then we know that this is going to be a 50 hertz frame rate take the 50 and move it into the numerator position you could put it over one but anything divided by itself is itself so in the end we see that this is 50 hertz now as a little bit of a tip the t frame is always going to be some sort of decimal or fraction if you have a t frame that is three seconds then that means we can only create a frame rate of one-third of a hertz and that's just really not a thing we don't really do fractions of hertz because you can't have just part of an event an event is a whole thing so it's either a whole event or nothing if you are ever told to figure out the frame rate and your options are some sort of decimals or fractions you can eliminate that one pay attention to your units as well t frame is always going to be time typically seconds so again if you're told to calculate the t frame and one of your answers is in hertz then you know that's the wrong answer same idea with frame rate we should always see it as a unit of hertz we're going to head back to the lecture then to check out another formula that is going to be related to t frame and give us more detail on what factors determine the time it takes to make one image now we also have a second formula that's going to show us how the time to make one frame is directly related to the number of pulses to create a frame and the pulse repetition period remember the pulse repetition period is the time it takes for one pulse to travel to the maximum depth and back and because the way that this formula is set up we've already learned that any of the factors are going to be directly related to the product so if number of pulses increases the time it takes make one frame is going to increase if the pulse repetition period increases then the time to make that frame is also going to increase so of course i have some more examples and we're going to head over to the board again to look at those kind of step by step showing us how the number of pulses and the pulse repetition period are directly related to the t frame before we get into the example i want to show you really quickly how number of pulses and prp are related to the time now if we have our image that image is going to be made of scan lines and there are going to be so many scan lines to create this full image each of these scan lines is going to need at least one pulse and we'll learn later why there might actually be more than one pulse to create a scan line but for the example we're going to say that there is one pulse per scan line now when that pulse is sent out the machine is going to look at how deep the max imaging depth is and it's going to send the pulse out wait for echoes to return from that max imaging depth and then have that information for one scan line and then be able to send out another pulse and wait for that information to come back and then send out the next pulse and so on and so forth for each pulse needed to create the image so if we increase the number of pulses needed to make an image that makes sense that our t frame will increase if we increase the depth of our image making this deeper then that makes sense that it's going to take more time for sound to travel to that maximum depth and return back to the transducer let's take a look at some examples using the formula and numbers to show this concept now to set this example up we are going to start with some settings we're going to say that the depth is set at 10 centimeters and then we're going to say that we need 100 pulses to create one image now the first thing that we need to do is calculate that prp remember that prp is based on the maximum imaging depth which i have set as 10 centimeters and the 13 microsecond rule that tells us to multiply the depth by 13 microseconds and we will get a prp of 130 microseconds all we need to do now is plug our numbers into our formula so we have 100 pulses multiplied by 130 microseconds and that will give us a t frame of 13 000 microseconds so this is where we are kind of starting with the parameters that we have set for the machine now the second part of the example i want to show you what happens if we increase our prp the only way to increase your pulse repetition period is to increase your depth so let's say we've increased our depth now to 20 centimeters and we still need 100 pulses to create our image again we need to calculate our prp by multiplying 20 centimeters by 13 microseconds and that will give us a prp of 206 microseconds plug our numbers into the formula again 100 pulses multiplied by 260 microseconds equals a t frame of 26 000 microseconds so let's stop and look at what happened we increased our depth from 10 centimeters to 20 centimeters and we saw our t frame increase from 13 microseconds to 26 000 microseconds double your depth you'll double your t frame they are all directly related now we're going to return our depth back to 10 centimeters and we're going to see what happens when we increase our pulses let's go ahead and say that it now takes 200 pulses create an image again we need to calculate our pulse repetition period if we haven't just been given it so 10 centimeters multiplied by that 13 equals 830 microseconds and then if we take 130 and multiply it by 200 pulses we are again going to see a t frame of 26 000 microseconds if we stop and look at what happened this time we took 100 pulses doubled it to 200 pulses which again caused our t frame to increase in the same proportion it doubled so when you increase the number of pulses or increase the pulse repetition period we will see a longer t frame it takes more time to create that one image so from those two examples they show us that by increasing the prp which means to increase or have more depth or by increasing the number of pulses we are going to increase the time per frame or the t frame and when the t frame increases less frames can be made per second which means that we have decreased our frame rate and when we have a decreased frame rate our temporal resolution is also going to decrease or worsen so anything that makes our time per frame increase is going to result in poor temporal resolution recall that when we started our frame rate discussion i told you that there are three things that will change the frame rate the first one was the propagation speed and the medium which as stenographers we have no control over the other two things were the imaging depth and the number of pulses per frame now we have the discussion about t frame to talk about how we calculate the time it takes to make one frame and we saw that t frame is directly related to prp or depth and the number of pulses per frame so i've already introduced the idea that prp or depth is directly related to the t frame and since the t frame is inversely related to the frame rate that actually makes our prp inversely related to frame rate 2. so we've already kind of covered this topic in the purview of t frame but we also need to look at it a little bit closer in the purview of frame rate because this is one of the variables we have control over that affects the frame rate so knowing that the prp or depth is inversely related to frame rate we need to keep in mind that short prps mean shallow imaging and when we have shallow imaging we're going to improve our frame rate when we have long prps that means that we have deep imaging and we're going to have a poor frame rate so to improve our overall imaging scenario should be very aware of their depth and not using excessive depth first off it just kind of centers our region of interest it makes the image more appealing to the eye it makes the anatomy that we're interested front and center a little bit bigger in our image as opposed to when we use excessive depth we're imaging well beyond the area that we're interested in and now we've learned that it's always going to affect our frame rate so use the depth that's appropriate to see the anatomy that you need reduce depth whenever you can to focus on the area of interest and improve your frame rate now in the last example that we went to the board about t frame and prp we talked about how prp changes the t frame so we're going to head back to the board and use those examples again to look at how the prp now is going to affect the frame rate recall that when we were looking at the formula for t frame we had number of pulses multiplied by the prp and the example that we used was having depth set at 10 centimeters and 100 pulses for the image creation when we multiplied all that out using the prp and the 13 microsecond rule what we figured out was that our t frame was equal to 13 000 micro seconds so now what i want to show you is how we get from t frame to frame rate so remember that hertz is a second unit a per second unit so we need our t frame to also be in that second unit so we can't simply just take 13 000 do the inverse of it and have our hertz we have to take the inverse of a second so we need to first convert 13 000 microseconds to second so if we do 13 000 micro seconds to convert 13 000 micro seconds into seconds we're going to take our decimal place and move it six spots to the left one two three four five six and we're gonna fill in our gap with zero so thirteen thousand microseconds is equal to point zero one three seconds now we have a decimal that we can make the reciprocal of and turn into the hertz so we'll take 1 and divide it by 0.013 seconds and we will get if we round to a whole number 77 hertz so we went from a t frame of 0.013 seconds made the reciprocal of it and found the frame rate to be 77 hertz so that was based on a machine with a 10 centimeter depth and 100 pulses now let's go ahead and use that second example that we had where we increased our depth to 20 centimeters so depth goes to 20 centimeters we have 100 pulses and here's a really good question if we double we know that t frame also doubled so what do you think is going to happen to the frame rate if t frame goes up frame rate has to go down and what do you think frame rate is going to be reduced by let's go ahead and do some math and see if you're correct so by taking 20 centimeters figuring out the prp to be 260 microseconds we do the math and we get 26 000 microseconds but we do need to convert that 26 000 microseconds again by moving the decimal place six places to the left so one two three four five six and now we have our t frame being equal to .026 seconds we can divide that by one and that is going to give us a value of 38.5 hertz for our frame rate because frame rate is the reciprocal of the t frame so 38.5 hertz frame rate when we have a 20 centimeter depth remember that for 10 centimeters depth we had 77 hertz so when the depth doubled the t frame doubled and the frame rate halved so if you guessed that it would have you would have been correct so by looking at these numbers a little bit closer we see then that as the depth increases we are going to see frame rate decrease because they are inversely related so if depth increases t frame increases frame rate decreases so our main takeaways from those examples is that if prp or depth increases our t frame is going to increase which makes our frame rate decrease and our temporal resolution worsen if the prp or depth decreases that means our t frame is also going to decrease which increases our frame rate and improves our temporal resolution so in general if prp or depth increases we're going to see more time per frame needed and when we take more time to make a frame we cannot make as many per second so our frame rate decreases and when we have a low frame rate we are going to see a worse temporal resolution so i have one more visual for you showing how depth is going to change our time per frame and therefore our frames per second and temporal resolution so here on the left we have a transducer that is set up to image down to five centimeters there are nine scan lines so the pulse goes out travels to the five centimeters and returns it's going to do that for each scan line and then display an image when the machine only needs to listen for echoes from five centimeters depth those echoes are going to return very quickly now compare that to the machine that is set up to listen for 10 centimeters it's still nine scan lines but it still has to wait for that pulse to go down and all the way back so it takes twice as long for the sound to go in and back and then display the image so when you have less depth you can make more images per second and more images per second are going to improve your temporal resolution so let's take a moment to watch how these frames are being created and how many the 5 centimeter depth transducer produces compared to the ten centimeter transducer so we just had one for five two for five one for ten three four five two for ten four four five five for five three for ten so you can see how that five centimeter depth is going to produce more frames per second compared to a deeper depth of 10 centimeters and how if we can produce more frames per second that is going to improve our temporal resolution now again we already talked about how number of pulses is related to the frame rate and we did the math and looked at the formula that tells us number of pulses is directly related to the time it takes to make one frame or the t frame and so then we know then that the number of pulses is directly related to t frame and t frame is inversely related to frame rate so that's also going to make the number of pulses inversely related to frame rate so anything that causes the number of pulses per image to increase is going to cause the time per frame to increase and when it takes more time to create one frame we see our frames per second decrease which worsens our temporal resolution so again anything that makes that time per frame increase is going to cause poor resolution so if we can create fewer pulses per frame we will see an improved frame rate when we have more pulses per frame we have a poor frame rate now there are three main things that are going to change the number of pulses per image those include the number of pulses per scan line and this is looking at are we doing a single focus or multi-focus what the sector size is do we have it opened really wide using all the scan lines or do we have a little bit narrow using fewer scan lines and then lastly the lines per angle of sector which is the same as the line density so remember we had three things that could affect frame rate we had propagation speed depth and pulses per frame now we have three things that can change that pulses per frame part of it and again that's number of pulses per scan line the sector size and the line density so let's take a look at each of these variables and show how they are going to affect the frame rate let's first take a look at number of pulses per scan line remember that each pulse can only have one focus so if a pulse is created when it comes out it can only have one focus so if the sonographer increases the number of foci from one to two or one to three then we need more pulses to create each scan line to account for each focus so if we increase the number of foci per scan line then we are increasing the number of pulses per image and when we increase the number of pulses per image we increase the time per frame and therefore that is going to show a reduction of frames per second and worsen our temporal resolution however when we add those more foci we do improve lateral resolution so as a sonographer you need to decide do i want better temporal resolution using the minimum amount of foci for my image or do i want really good lateral resolution and use more foci per scan line to create a superior still image so let's go over to the board so i can show you a little bit more step by step how the transducer is producing pulses for one focus each and how adding foci is going to really change our temporal resolution let's take a look at what it means to only have one focus per pulse and what it means then to use the multi-focus feature of your machine so let's review what our pulses look like in space the beam comes out of the transducer narrows out the focus and then widens in the far field so i'm going to be using more of a squiggly line feature to kind of show this we have our narrowing and then we get our widening out in the far field now when that pulse is sent down through a scan line wherever we set that focus is where that narrowing is going to be so if we've got it coming in like this it's going to get narrow through here and then start to widen if we move our focus up here then we're narrow through here and then it'll start to widen if we have our focus set somewhere in the middle again we'll get that narrowing right at that focus and then we'll see widening in the far field so no matter where that focus is the machine is going to electronically change the electrical pattern to change where the focus is based on where we set it as a sonographer now we also have the option to add in multiple foci so as a sonographer let's say we want to add in three foci so we'll have a near field a midfield and a far field foci now we cannot have a beam that comes in narrows out the first one narrows out the second one and then narrows again at the third one and continue on that's not how our beams and our pulses work you get one focus per pulse so when we turn this feature on in our machines what ends up happening our three pulses need to go out to accommodate for these three foci so the first one will come out take care of the near field focus and then widen in the far field those echoes return and the machine immediately will start to send the next pulse accommodates that midfield focus and then again widens in the far field once those echoes come back a third pulse is sent out accommodates for that far-field focus and then continues on once all three of those pulses have been sent echoes have been returned then it's going to start over again for the next scan line and again three pulses need to be sent out for that next scan line so you can see how this is very quickly going to cause a huge temporal resolution reduction because we are adding three pulses for every single scan line and while it does degrade temporal resolution quite a bit what we end up getting is a very narrow pulse almost all the way through our image and so lateral resolution wise this is an amazing picture temporal resolution horrible it's going to be very very slow so up until this point you have just been given in the examples how many pulses are needed to create the image now that we are looking at things that can change the value of the number of pulses in an image you're going to need to do some extra calculations to figure out how many pulses there are per scan line and then how many scan lines there are to understand how many pulses are needed to create a frame so our first example is going to tell us that we have 100 scan lines in our image now this is different than 100 pulses this is an image made of 100 scanlines in the scenario the stenographer has set only one focus so knowing that there's 100 scan lines and one focus we know that there is one pulse per scan line so we can take 100 scan lines multiply that by one pulse per scan line and we will know that this image is going to require 100 pulses to be completed now if a sonographer adds in three more foci for a total of four we now need four pulses down each scan line and so to calculate how many pulses we are going to need to complete this image we can take our 100 scan lines and multiply it by 4 and that's going to be our 4 pulses per scan line 100 times 4 is 400 pulses to create this image so we have increased the number of pulses per image which increases the t frame which decreases our frame rate which decreases our temporal resolution and again as a visual i have this image for you showing how we have to send one pulse per focus that has been added by the sonographer compared to the image that is produced with only one focus so the image on the left has three foci it has to send three pulses to account for each of those foci and it is going to do that over those nine scan lines compare that to the image on the right which only has one focus and only needs to send one pulse per scan line it can produce images very quickly so let's take a moment to look at this we've got one for the three foci one for the one foci two for the one foci oh two for the three foci three for the one foci so you can see that we've already produced three images to the two images being produced by the other setup so when we have less pulses per scan line which typically goes along with less foci per scan line that is going to improve our temporal resolution because it's going to take less time to create one frame let's take a look then at how sector size can affect the number of pulses in a frame so the field of view is the anatomy that is visible while we are scanning and it's the size of the field of view is going to depend on how wide or narrow the sector is so the sector is the image if we have it opened all the way up that's a wide sector and if we narrow it down we are going to start to lose views of our anatomy but we are going to have a narrow sector wider fields of view are going to require more scan lines to create that wider view and when we have more scan lines it means we have more pulses to create that image so when we increase the width of our sector we're increasing the number of pulses per image therefore increasing the time it takes to make a frame when we increase the time to make a frame we see our frames per second or frame rate decrease which decreases our temporal resolution so a wider field of view is going to improve the amount of anatomy you can see but we might not need that much anatomy and when we have a wider sector it's going to degrade our temporal resolution or make it worse let's head over to the board then to again discuss a little bit more about wide and narrow sectors and then show an example again of the math regarding numbers of pulses per frame and how it affects the t frame and the frame rate as a stenographer let's say that we have our field of view and we have it open all the way but let's say that we are just imaging something very small and that something very small has a thing moving in it well to improve our temporal resolution what we are going to want to do is narrow the sides of our sector in so what we end up getting is just the anatomy that we want within our field of view now to do that there is a knob on your machine and it'll usually say something like sector with and you can turn it one way or the other and it'll make your sector open up wide or it'll narrow it down now there are a couple benefits to narrowing your sector the biggest one and the reason that it comes up in this chapter is because it improves your temporal resolution some other things that might occur from it is that you'll get rid of kind of the junk that you don't need anymore say you're imaging through ribs and those ribs are going to cause a whole bunch of shadow on either side of the anatomy that you want so if you have a sector that's open wide like this you're getting all this information that you don't need so instead you could sector down and really just focus on that anatomy in between the pieces of information that you no longer need but the field of view is very important to the temporal resolution and that is because the number of scan lines it takes to make a wide field of view versus a narrow field of view so if our field of view is opened completely it might take 100 scan lines to complete this whole image when we narrow our sector down we might reduce that to 40 lines so if we still have one focus set 100 scan lines for the whole image or 40 scan lines for the narrowed sector width 100 times 1 or 40 times 1 we get 100 pulses or 40 pulses to create the whole image so 40 pulses is fewer pulses and when we have a reduced number of pulses we reduce the time it takes to create one frame which improves our frame rate and therefore improves our temporal resolution so whenever possible you want a narrow field of view if you're trying to capture the true motion of a structure and again we have our side by side comparison we have the image on the left showing one focus 10 centimeters of depth but a narrow sector it only is using five scan lines to look at the anatomy compare that to the one on the right we are using a wide sector width it has nine scan lines to complete that entire sector and therefore it's going to take longer to create those images so again let's just take a moment to kind of look at how those images are being created we've got one for the five scan lines two for the five scan lines one for the nine scan lines three for the five scan lines two for the ten four for the five so again we're seeing that there's almost a doubling of the frames that can be created with the narrow sector compared to the wider sector so whenever possible reduce your sector to reduce the pulses per frame to improve your temporal resolution the last variable that we had then was line density and this is the number of scan lines that create an image is called the line density and this can be adjusted by the sonographer now this used to be probably a little bit more of a prominent feature on older machines but now the machines have gotten really really fast the line density piece of it is not as important for the temporal resolution because we have been able to make up with it with more technology and how the scan lines are being produced so remember that line density is a factor in the temporal resolution but might not be as available of an option for you to change as a sonographer however when we increase our line density we are increasing the number of scan lines and therefore we're increasing the number of pulses needed per image again when we increase that number of pulses per image we're going to see an increase in the t frame increased t frame means decreased frame rate which means poor temporal resolution line density though is also related to spatial resolution so if there are fewer scan lines this is called low line density and this is going to degrade or worsen our spatial resolution which is related to the detail that we can see in the image so with low line density we are going to have degraded spatial resolution but we're going to see improved temporal resolution because it doesn't take as long to create those images now the flip side to that if there are a lot of scan lines this is called high line density this is going to improve our spatial resolution because we can get more detailed information from the anatomy but when we use so many scan lines we are increasing the number of pulses that we need to create the image and therefore we're going to degrade our temporal resolution i have one last example then for you over on the board where we'll take a little bit closer look at how line density is calculated and referred to in mathematical problems and kind of show you how that's going to affect our t frame and frame rate line density is also going to kind of refer back to some of the knowledge that we learned about sector with so when we have an image we are looking at an angle at the top here draw another one and you can see that this angle is wider than this angle and we have a wider field of view versus a narrow field of view well we talk about this angle on top as our sector angle so we might have a 90 degree sector angle and a 30 degree sector angle and when we talk about line density we are talking about how many scan lines there are per degree of the angle so for this one let's say there's one scan line per degree and we'll say the same for the 30 degrees for this example so we have one scan line per degree for each of them so if there are 90 degrees and one scan line each that means that this image is created with 90 scan lines it would mean then that this image is created with 30 scan lines so whatever number scan line per the degree that's there you just multiply them together and you will figure out how many scan lines create the image so now this shows us another way of how the sector width varies the amount of scan lines therefore varies the amount of pulses but we are talking about line density so what then happens if we change to two scan lines per degree for each of these well now the machine is going to send another pulse in between each of those previous scan lines so we take two multiply it by 90 and now this image is being created with 180 scan lines and this one is being created with 60 scan lines now the size of the sector has not changed the only thing that has changed is that we are packing more scan lines in there therefore we are getting more detailed information and the spot where this really matters is in our far field so if we have scan lines coming down at one scan line per degree and then we change it to a high density and make it maybe three scan lines per degree we are going to be getting more information out of that far field instead of having to guess as to what those pixels look like we are legitimately scanning more of that anatomy versus trying to fill in and guess as the machine would do with just one scan line per degree now unlike sector width when we are changing the line density we are changing the amount of scan lines typically within the same sector width we are adding scan lines to the same shape so for this example let's say that we have a 90 degree sector window this is a 90 degree angle up here and we're going to start with lines that are spaced one line per two degrees so we are seeing one scan line per two degrees so we have one scan line per two degrees which is going to be half multiplied by 90. so in this example we are starting out with 45 scan lines if we have one focus set that means that we are one pulse per scan line so this whole image is going to take 45 pulses to create but we are missing a lot of information in our spatial resolution so let's say then that we increase our line density to have one scan line per one degree now we can take that one multiply it by 90 and we are going to have 90 scan lines when we add in another scan line per degree so our spatial resolution is improving but because we have increased the number of pulses to create this image we are effectively degrading our temporal resolution we can bring it another step further and say there is three scan lines per one degree and so now we are going to take 3 multiply it by 90 and we are going to see 270 scan lines to create this picture so there's a lot more scan lines in here which means a lot more pulses whenever we increase those pulses we are going to see an increase in the time it takes to make that frame and lastly we'll end with another side-by-side comparison the image on the left is showing us a wide sector but it's only being created by five scanlines so this is a low line density this is going to have worse spatial resolution but it's going to have better temporal resolution compared to the picture on the right on the right side we are creating the same size sector but using nine scan lines so when you improve your temporal resolution by reducing your scan lines you are reducing your spatial resolution as a consequence of that however if your focus is temporal resolution then you want fewer scan lines to reduce the pulses per frame to improve the temporal resolution section 13.4 image quality as a sonographer the choices that you make for the parameters of your machine are going to have a cascade of events following your decisions for example if we choose to image shallow we are creating a short pulse repetition period which causes a short t frame which improves our frame rate and improves our temporal resolution but if you're looking at your picture and not able to see all of the anatomy that you need then you're going to need more depth so when you increase that depth you're going to increase the pulse repetition period which increases the t frame which causes a worse frame rate and worse temporal resolution so you need to balance can i actually see all the anatomy that i want versus am i getting enough movement and real time imaging from the settings that i have same idea with multi-focus versus one focus if you have one focus per scan line that means you only are using one pulse per scan line and that's going to be a short t frame that's going to improve your frame rate which improves your temporal resolution but then you have to look at your picture am i creating a picture with enough lateral resolution am i getting enough detail out of this image if not then you're going to add more foci and when you flip into that multi-foci feature on the machine or add more foci per scan line then you're increasing the number of pulses per scan line which increases the time it takes to make a frame and because of that we get a worse frame rate and worse temporal resolution so again you need to evaluate am i creating a really good still image or am i trying to create a really good movie like clip so you'll need to decide do i want one focus per scan line or many to improve what i'm trying to achieve again same idea when we're looking at the sector with if you narrow your sector in you know you're going to improve your temporal resolution you're using fewer pulses per image reducing your t frame which improves your frame rate but again are you seeing all the anatomy that you need if not then you need a wider field of view and so when you open up that wide sector you're going to be using more pulses per image which increases the t frame decreases the frame rate and decreases your temporal resolution and same idea with line density now again this isn't an option that we mess around with too much as sonographers but it is still an option for you so if we want to improve our temporal resolution maybe we try to reduce the line density knowing that's going to cause fewer pulses per image and improve our frame rate but if we're not getting enough spatial resolution out of that low line density then we need to increase the line density making it more of a high line density and when we increase that high line density we need more pulses therefore worsening our frame rate and we're seeing our temporal resolution so it's going to be really important to look at these factors and try to decide do i want a better photograph or am i trying to make a better movie and maybe you're going to do some trade-offs with it one of the ways that i try to come at temporal resolution especially when i'm imaging like a fetal heart is looking at is there some trade-offs that i can make maybe you want that really detailed lateral resolution so i'm going to add in multifoci but then i'm going to decrease my field of view or decrease my sector so i'm using less pulses while i'm creating that multifoci so now i get pretty detailed views of that fetal heart but i can still see it moving with pretty good temporal resolution because i've decreased how many pulses are being used to make that frame i'm going to try to make sure that i'm not using too much depth when i'm looking at anything that's moving and if something that's moving is kind of deep in my image then maybe again i want to make some trade-offs narrow that sector in decrease my line density decrease the foci so i can have more depth but offset its effect on temporal resolution by changing other things so as a sonographer you really need to decide how you're going to improve your image and what your ultimate goal is do i want a really good still picture or do i want a better moving image so clinically and for your boards and for your tests it's going to be important that you remember what can you do to improve frame rate which improves temporal resolution and what you can do that worsens frame rate and might degrade temporal resolution but improve other types of resolution so higher frame rate we're going to see that with shallow imaging single focus narrow sector and low line density where low frame rates are more associated with deep imaging multifoci wider sectors and high line density remember too then that the multifocus is going to improve our lateral resolution because we are making that beam narrower over a longer portion of the image and that that high line density is going to improve the spatial resolution because we are getting more scan lines to get more detail from the anatomy and that is the end of unit 13 temporal resolution make sure to go through the activities in your workbook and check out the nerd check questions those open-ended questions to review the content that's been presented