Transcript for:
Understanding Axial and Lateral Resolution

hi Learners it's M from soundon nerds and this video is on Unit 10 resolution number one axial and lateral so we're going to focus on axial and lateral resolution today in Unit 10 there are different types of resolutions so this is the first resolution lecture that we're going to cover resolution describes the way detail can be displayed when we say something has more or better higher resolution it means that it can create a more accurate display therefore higher resolutions are preferred for image accuracy there are many types of resolution related to Ultrasound you have spatial resolution which is going to be more dependent on the pixels of a monitor and scan lines in an image we have contrast resolution which is the ability to differentiate between the Shades of Gray we have temporal resolution that determines the accuracy of moving objects elevational resolution is going to determine if we are accurately seeing a thin slice of anatom in the ultrasound beam lateral resolution determines the accuracy of side by-side structures being displayed and lastly we have axial resolution which determines how accurately A system can display two reflectors that are parallel to the sound beam this lecture is going to focus on axial and lateral resolution throughout the first few units of this course you may recall hearing these terms axial and lateral resolution axial resolution is highly dependent on spatial pulse length where lateral resolution is dependent on the beam width two topics we've already covered let's start with section 10.1 axial resolution axial resolution is the machine's capability to accurately image reflectors that are parallel to the sound beam when sound beams are emitted from a transducer the pulse is sent directly from the face of the transducer now the sound beam is going to travel into the body and interact with reflectors and it's reflectors that sit within the length of pulse or run in the same direction of the pulse that we consider to be parallel to the beam another way we can think of this are reflectors that are sitting in front of or in back of one another rather than side by side in this example the orange reflectors are parallel with the sound beam and how accurately they are displayed will be determined by axial resolution an accurate display of these reflectors would show three separate reflectors when reflectors are displayed appropriately we call them resolved an inaccurate display would show the reflectors incorrectly showing too few and incorrect size when reflectors are displayed incorrectly we call them unresolved in this example we can see a transducer face here and these pink arrows are representing beams as they come out of the transducer face heading into the body the orange reflectors are parallel with the sound beam or sit in the length of the sound beam or sit in front of and in back of one another so these reflectors how accurately we can display these are going to be determined by the the axial resolution of the machine and the transducer if the machine accurately picks up on the reflectors in the path of the sound beam in this example we had three reflectors we have three discrete Echoes displayed on our machine in this bottom image though these two reflectors are kind of close together and the Machine might not be able to tell that they are indeed two separate reflectors and instead displays them as one this is where we get the unresolved reflector versus resolved where they're all accurately displayed axial resolution is determined by the spatial pulse length recall that spatial pulse length or SPL is the distance that a pulse takes up in space if we could see the spatial pulse length we could get out a tiny little ruler and measure it but we actually can't do that we can however calculate SPL and therefore calculate axial resolution so as a reminder this is the formula for SPL we have SPL in millimet is going to be equal to the number of Cycles in a pulse multiplied by the wavelength which is also millimet and here is our new formula for axial resolution axial resolution in millimet is equal to the SPL / 2 so I was kind of joking about the ruler thing but I thought it would show it nicely what we're talking about remember spatial pulse length is a distance measurement so we could use our ruler if we could see it and we can measure how much space this pulse takes up so in this example just as a visual we've got a pulse that takes up about a cenm and a half of space now knowing that the SPL the spatial pulse length is about a cenim and a half or 15 mm half of that is the axial resolution so half of 15 is 7 1/2 mm or 75 cm and it turns out that the average axial resolution of an Imaging ultrasound system is 05 mm to about5 mm it's important to remember that when axial resolution has a low numerical value that means the better the axial resolution is and that's because axial resolution is going to tell us the minimum distance two reflectors must be when they sit parallel to the sound beam to become resolved reflectors on the display so again we have our SPL here we measured it to be about 15 mm half of that is the axial resolution which would be 72 mm so by our definition if two reflectors are at minimum 7 1 12 mm apart they will be displayed as two reflectors otherwise they going to kind of be mushed together in a Big Blob and it makes sense on why a small numerical value means improved axial resolution if our SPL was only half a cimeter then our axial resolution would be 2.5 mm and that would mean that two reflectors would only have to be 2 1/2 mm apart as opposed to 7 1/2 mm to be displayed as two separate reflectors on our image so again just to reiterate the spatial pulse length determines the axial resolution because axial resolution is SPL / 2 and axial resolution will tell us how far apart do these reflectors need to be so they are displayed as they truly are in the body in this example that we have on the bottom here we have the reflectors oh we'll say about half a centimeter apart so if the SPL is 1 cm the axial resolution would be half a centimeter that half a centimet would be appropriate for these two reflectors it would show two reflectors but if the SPL was 2 cm making the axal resolution 1 cm then the these reflectors are too close to one another for the machine to be able to tell that they are two separate reflectors and it'll display kind of a Big Blob instead of these two discrete reflectors so let's take a look at what's happening when this occurs this is a very oversimplified look at what's happening during the pulse but I think it will explain fairly well as to what's going on why we get these unresolved reflectors so the pulse is created by the transducer and it starts propagating through the body it's going to interact with a reflector and when it interacts with a reflector some of that sound is going to go back to the transducer and some is going to keep propagating forward now that sound that went back to the transducer is going to be processed and recognized as a reflector to be displayed that sound then is going to keep traveling through the body remember SPL is going to remain the same length throughout the propagation so another space of SPL or pulse length propagates through the body and it interacts with another reflector now that reflector too is going to send sound back towards the transducer and send sound forward transmit that sound forward so Echo goes back more sound goes forward now we have another reflector that sits within half the length of the SPL which is our axial resolution so that sound that propagated forward is already interacting with another reflector and what ends up happening is that reflector is also going to send sound back to the transducer and then of course keep propagating forward but here's the important part that sound from the second reflector is going to overlap with the sound from the first reflector because it came in turned around really quickly and now it's overlapping with that Echo so both of these Echoes are going to travel back to the transducer and they're going to arrive basically at the same time with one another so these sound waves are going to start traveling back towards the transducer at the same rate still overlapped with one another and they are going to strike the transducer and the transducer is not going to be able to tell that those are two separate Echoes it's going to think it's just one big long Echo and therefore the machine is going to display one long reflector on the monitor so axial resolution is half of the SPL length or the pulse length so it' be half of one of these if reflectors sit too close together with one another the Echoes from these reflectors are going to overlap and cannot be processed as separate Echoes by the machine therefore we get our unresolved display an elongated reflection on our image let's take another look at an example using some numbers so if the SPL of this one on the left is 4 mm that would make axial resolution 2 mm because axial resolution is half of the SPL this means that there needs to be at least 2 mm in between each of these reflectors so that the machine can receive their separate Echoes but look what happens here we have a pulse that overlaps two reflectors and The Echoes from those reflectors are going to overlap thus creating our unresolved reflection let's compare that to the other side then we have a SPL of 2 mm so that means our axial resolution is half of that or 1 mm now with this transducer these reflectors only need to be 1 millimeter apart from one another so we can see that we can sample much quicker and while this isn't the perfect example we at least have kind of a waveform that ends up going in between these two therefore these Echoes are going to come back separately the machine will recognize that and be able to display them as two separate reflectors looking at these examples we can see that the longer spatial pulse lengths result in longer axial resolutions longer axle resolutions do not provide good resolution we're going to get incorrect displays of reflectors low numerical value axial resolutions are created by short spatial pulse links and those are going to improve our overall resolution so to improve the detail in our image we want to optimize that axial resolution but this can't be done with a simple knob on the machine and that's because axial resolution is not adjustable instead we really need to take a closer look at the physics behind axial resolution so we can make better choices about our equipment recall that spatial pulse length and wavelength were both parameters that were dependent on not only the machine but also the medium through which the sound beam travels in so for ultrasound that means that it's tied to the frequency in the manufacturing of the transducer as well as the soft tissue through which it travels and if if it's true for spatial pulse length then it's also going to be true for axial resolution so if we take our spatial pulse length formula and our wave length formula and kind of mush them together with the axial resolution formula we actually get a new formula for axial resolution and that it is equal to 0.77 * the number of Cycles divided by the frequency now this is not the axal resolution formula that I want you to remember the big one is SPL / 2 but what we are seeing here is how the wavelength formula and the SPL formula are applied to soft tissue cuz normally this would be 1.54 mm per microc but we had that divide by two piece of it so now it's 0.77 we still need the number of Cycles because we are referring to the spatial pulse length then we divide that all by the frequency so again kind of a mish mash of all of those formulas together give us a new axial resolution formula and this is important because now we can start to analyze the relation ships that determine axial resolution by analyzing this formula then we can see that frequency and axial resolution are inversely related so frequency is in our denominator position which is going to be inversely related to the quotient frequency increases numerical value of axial resolution decreases we also see then that the number of Cycles is directly related to axial resolution if we increase the number of Cycles we're also going to see an increase in the numerical value of axial resolution remember increased or higher numerical values are bad for axial resolution we want small numbers smaller numbers are good for axial resolution so while we don't have a knob that can improve our axial resolution we can choose high frequency transducers so to increase the frequency is going to give us short wavelengths short wavelengths give us short spls and short spls improve our axial resolution contrary to this then a low frequency transducer is going to create longer wavelengths which will degrade our axial resolution so as a stenographer you need to balance choosing the transducer with the highest frequency to improve resolution versus a low enough frequency to get to the anatomy that you need to see the other way we want to improve axial resolution is by reducing the number of Cycles within a pulse so now as sonographers we don't get to choose the number of Cycles in pulse cuz that's dependent on how the transducer is made but you should recall that when we introduce backing material into the transducer construction it's going to keep the PCT Crystal from ringing they're going to make that short little chirp it's going to have fewer Cycles in the pulse less Cycles in the pulse means short spls and again short spls mean improved axial resolution without that backing material we're going to see that the pctd crystals can continue to ring or make more Cycles in the pulse with which makes the pulse longer and again degrades our axial resolution so even though we can't choose the cycles that are in each pulse we do need to know that less ringing and fewer cycles per pulse make for better resolution so just a couple final thoughts on axial resolution to have the best axial resolution we want the highest frequency and the fewest Cycles so if you're presented with a test question that asks you which of these transducers is going to provide the best axial resolution this is what you're looking for high frequencies low number of cycles per pulse the other thing that I want to bring up is that there are actually a lot of synonyms or alternative names for axial resolution we've got axial resolution longitudinal resolution range resolution radial resolution and depth resolution and they all mean the same thing so it all means parallel to the soundbeam how the those reflectors are displayed now some of the newer texts omit all of these extra names stating that as sonographers we no longer need to know the alternative names however there is still a lot of material out there that does include these synonyms so I just want to present them to you in the event that you're using some other study material and come across these terms I don't want you to be in the dark about these terms but for the most part on your test you'll most likely see axial resolution being referred to all right so that brings us up to to our first practice of this unit and it's going to be on the axial resolution you're going to be given two scenarios and you just need to choose which scenario is going to provide Superior axial resolution go ahead and look through the practice questions unpause when you are ready to see the answers if you haven't already make sure that you are going through these practice problems because I am going to cover right now the answers to these and kind of go over the reasoning why these are the correct answers so the first question asks us which scenario has better ACC resolution a 5 MHz probe that produces pulses with three cycles per pulse or a 5 MHz probe that produces pulses with four cycles per pulse remember that when we are choosing the best axial resolution we want highest frequencies lowest cycles per pulse frequency is not an issue in the scenario so we are going to choose the lowest cycles per pulse second question asks us which scenario has better axial resolution a 5 MHz probe that produces pulses with three Cycles per pulse or a 12 MHz probe that produces pulses with three cycles per pulse again same idea but now we're focusing on the frequency both transducers provide a three cycle propulse beam so now we're going to focus on frequencies we want the highest frequency high frequencies provide better resolution so in this case it is our 12 mahz transducer the third question asks us which scenario has better axial resolution a pulse with an SPL of 2 mm or a pulse with an SPL of 1 mm remember that axial resolution is half of the SPL so we are not done calculating axial resolution by just being given these values of SPL but the same concept applies low numerical values are going to provide Superior axial resolution so it is our 1 mm pulse that will provide better axial resolution now the last question says which scenario has better axial resolution is it the wave with a 0.2 mm wavelength or is it the wave with a 0.4 mm wavelength I hope you looked at this question and thought to yourself I don't really have enough information to answer this question and you would be correct because a wave with 0.2 mm wavelength could be part of a pulse that has like 16 Cycles in it compared to the wave with the 0.4 mm wavelength might only have two cycles in it and in that case your axial res resolution is going to be better with your 0.4 mm wavelength so it all comes back to your spatial pulse length you need to know how many cycles are within the pulse and either what the wavelength is or what the frequency is so make sure you really understand what spatial pulse length is how we calculate spatial pulse length to better then characterize what scenarios produce better axial resolutions we're going to switch over then to section 10.2 lateral resolution now lateral resolution is the machine's capability to accurately image reflectors that are perpendicular to the sound beam so remember when we were learning about beam Anatomy we know that the beam has a width to it and when it leaves the transducer it starts to narrow at the focus and then it diverges again in the farfield the reflectors can sit parallel like we learned with axial resolution and they can sit perpendicular to the beam so with lateral resolution it is the reflectors that sit perpendicular to the beam or side by side or next to one another so it is the orange reflectors in this example that are perpendicular to the sound beam now depending on the width of the beam as it travels by those side by-side reflectors we are either going to get an accurate display showing resolved reflectors or we're going to get an inaccurate display showing unresolved reflectors when we are referring to lateral resolution and the unresolved reflectors we're going to see them widen and spread across the image this is also known as point spread artifact when we get a wider reflector than what it truly is now this can either be a combination of two reflectors or especially in the farfield what we'll see is kind of pinpoint reflectors start to get longer than they truly are so in this example again we have our transducer beam coming down from the transducer face and remember that there is a width to the beam usually starts out kind of wide at the top narrow and then winds again in the far field depending on where those reflectors are within the width of the beam if the beam intersects two at the same time the machine's not going to be able to tell them apart so if the beam was wide right here machine couldn't tell them apart it is going to combine them together just like it did with the axial resolution if the lateral resolution improves enough that a beam can fit in between them then the machine knows that there is a reflector here a beam goes through reflecting on nothing and then it strikes another reflector so it knows that there are going to be two reflectors in that area so axial resolution paid attention to reflectors sitting within the beam lateral resolution is going to look at reflectors that are side by side to one another and what the minimum distance they can be to be accurately displayed just like axial resolution we are able to calculate lateral resolution so remember when we were learning about the anatomy of the sound beam we saw that at different depths of the sound beam the width changed and it could be calculated we knew as it came out of the transducer it was the crystal width and we call that the diameter of the beam or D we also know that at one near Zone length it'll get to the smallest that it'll ever be which is half of the diameter or D / 2 and once the beam travels to near Zone lengths it'll again diverge to the width WID of the crystal which is the same width as D again so the good news is is that the formula for lateral resolution is also pretty easy lateral resolution in millimet is equal to the beam width in millimet and again the smaller the numerical value the better the lateral resolution is because this is the minimum distance that two structures need to be from one another to be represented as two separate structures on the monitor when we have a smaller numerical value it means the objects can be closer together and still be displayed appropriately now recall when we are making an image with our modern transducers a beam is sent out it creates one scan line it scans a new area creates a second scan line and so on and so on and so on until we create a full image and that's kind of what's happening in this diagram here so we are taking a small departure from our unfocused single Crystal transducers and looking what's actually happening in in our machines so again the beam is sent out interacts with reflectors and that information is sent back to the transducer to be displayed what I want to point out is that as that beam is traveling across the anatomy and those Reflections are coming back if the beam overlaps two reflectors at the same time then the machine will not know that they are indeed two reflectors in this example here it overlapped both so so we get an elongated structure then we had a Gap a new structure a bigger Gap hits another structure so it can be displayed as two separate items so with lateral resolution we need to have a moment where there is nothing coming back from that area otherwise the machine just thinks it's one long reflecting surface now improving lateral resolution is going to require just a little bit more of a Nuance discussion because up until that last diagram that I showed you we've really been talking about an unfocused beam which means that there is no external apparati to help focus a beam like a lens a curved element or electronic focusing so the beam width and the focus that we've been talking about has been a natural Focus that occurs due to hen's principle again remember that's because it is going to naturally converge at the focus becoming half the diameter of the Crystal and then start to diverge again in the farfield so remember if you're ever asked to calculate lateral resolution that is the transducer that you're calculating for the single Crystal unfocused natural curvature of the beam however it's important to know that regardless of the transducer as long as it is focused the best the lateral resolution will ever be is at that Focus because a focused or unfocused beam is going to narrow that's going to have the lowest numerical Beam with which means it's going to have the lowest numerical value for lateral resolution so remember that at the focus is our best location for lateral resolution it's important to note too that lateral resolution does change with depth because the beam width also changes with depth so what we see is a relatively narrowish beam as it comes out so we've got okay lateral resolution up here as beam begins to narrow in the focal Zone that's really our good spot so this is really where we want to place a lot of our anatomies in that focal Zone this is going to provide relatively good image quality the best lateral resolution point is at the focus we really want this area at the level of our area of Interest remember two then as the beam diverges it doesn't immediately become so wide that we can't use it so we do get an okay region into the farfield and then once that beam just really begins to diverge that's where our worst lateral resolution is going to be and this is really true for all transducers if we apply some numbers to our example we can say that the diameter of the crystal is 10 mm so we know that as the beam comes out it is also 10 mm it's going to start to converge to the Natural focus and again the smallest it'll get is down to 5 mm or half of the diameter of the crystal after the focus it's going to widen again 210 mm when we are two near Zone lengths away after this point it's going to go to 11 12 13 20 25 as it just keeps diverging away from the transducer phase so we will see again okay lateral resolution up here getting better as we get to the focus best set the focus worsening then as we get into the far field and extremely bad in that further farfield it is the width of the beam then that tells us what is the minimum distance any of these reflectors need to be to be displayed appropriately so in this example beam fit right through them at 5 mm so we have two separate reflectors because the machine could tell that there was nothing in between them in this example the beam comes down and intersects both of these reflectors Echo are going to go back at the same time machine can't tell the difference between them and it's going to display them as one large reflector so by using the numbers we can see that as the beam diameter changes so does the lateral resolution because beam diameter equals lateral resolution and then we'll see in the far field after our focal Zone and especially after two near Zone lengths the beam will widen to the point where we really degrade our lateral resolution so a few final thoughts on lateral resolution when we compare axial resolution to lateral resolution the spatial pulse length is typically smaller than the width of the beam so axial resolution is better than lateral resolution again that goes back to the numerical value the small numerical value of the spatial pulse length is less than the numerical value of the width of the beam and just like axial resolution many texts do go over synonyms or alternative names for lateral resolution and those are going to include lateral angular transverse and as muthal so lateral angular transverse and as muthal all mean the same thing referring to the resolution of the ultrasound system in regards to reflectors that sit side by side or perpendicular to the sound beam and that's going to bring us to our next practice on lateral resolution again you will be presented with a few scenarios in which you need to decide where lateral resolution is superior and then we'll also give you a quick calculation problem as well go ahead and pause and work through the practice problems then unpause when you are ready to see the answers and here we are with those answers so the first question asks us which scenario has better lateral resolution at the focus the 20mm diameter Crystal or the 15 mm diameter Crystal remember we want a small numerical value and that at the focus it is half the diameter so half of 20 is 10 half of 15 is 7.5 so we know that the smaller diameter Crystal produces a smaller Focus therefore it is going to prove to be the better lateral resolution next question says which scenario has better lateral resolution in the far field is it our high frequency transducer or the low frequency transducer now this is going to take some knowledge going back to how the beam diverges in relationship to the frequency of the transducer remember we are think thinking about the Divergence or the spreading out of the sound beam in the far field we want the transducer that's going to provide less Divergence or a more narrow beam in that Firefield and because Divergence and frequency of the transducer are inversely related we know then that high frequency transducers diverge Less in the farfield which is going to provide better lateral resolution now the next question is kind of asking us the same thing except now we need to remember back to how diameter and Divergence are related so in which scenario has better lateral resolution again in the farfield is it the large diameter transducer or the narrow diameter transducer and if you recall back to our beam Anatomy hopefully you remember that large diameter transducers are going to provide a beam that diverges Less in the far field so increasing your diameter means less Divergence in the farfield which will improve our lateral resolution now the last question is asking what is the minimum two reflectors must be from one another to be displayed correctly when imaged with an unfocused single element transducer that is 8 mm in diameter at the start of the near field the focus and two near Zone lengths lateral resolution is equal to the beam width and lateral resolution tells us the minimum distance we need to remember how the start of the near fi the focus and two near Zone lengths are related to one another and to the diameter of the crystal so at the start of the near field if you will recall it is going to start at 8 mm so to be imaged appropriately and that very very very near field as the sound comes out of the transducer reflectors need to be 8 mm apart that beam is going to converge down to 4 mm at minimum so at the focus reflectors need to be at minimum 4 mm apart and at two near Zone lengths we know that we return to our original width so that will be 8 mm past the point of two near Zone lengths then the Divergence just keeps getting larger and larger which keeps degrading our lateral resolution section 10.3 a clinical discussion so now that we've talked about axial and lateral resolution we have discovered how the beam acts in two planes parallel to the sound beam per axial resolution and perpendicular to the sound Beam for lateral resolution and this knowledge is going to help us choose which equipment we want to use for our exams the ultimate goal is to produce the most detailed picture that we can with our equipment so to improve our axial resolution we know that increasing the frequency or using a transducer that produces few cycles per pulse is going to improve our whole image because axial resolution being related to SPL doesn't change based on where we are at in the picture it just matters that those reflectors are parallel with the sound beam we also know that axial resolution is typically better in general than lateral resolution because the pulse is usually very small compared to the beam with and lateral resolution is still important though too because we need that side to side detail as well so now we need to counteract our highfrequency fuse cycle pulse transducer with the transducer that also helps in lat Al resolution so in the near field we know that we'll have better lateral resolution if we start with a smaller diameter transducer we also know that the focus is better with a smaller diameter transducer but in the far field we find that large diameter transducers are going to be better because they don't diverge as much and typically that high frequency is not only going to help the farfield but also help the near field so again we're seeing that those high frequency transducers are really what we need for better exams and we know that because we've now learned that high frequencies are going to improve both axial and lateral resolution we know that they have deeper focal depths they have less Divergence but they still attenuate very quickly and therefore they're going to provide very little Imaging depth which isn't helpful especially if we're trying to image into a chest or into an abdomen so we got a balance that with those low frequencies which we know are going to degrade both axial and lateral resolution they're going to have sh focal depths and they're going to diverge more in the far field but because they don't attenuate we can get a lot more imaging depth out of them to optimize the image in both axial and lateral resolution the sonographer is going to need to always choose the highest frequency that will allow you to see all the anatomy that you need and we're going to use modern tools to adjust the focus appropriately so let's talk about those modern tools in section 10.4 for focusing so remember in our unfocused beams there was still a natural Focus that occurs that natural focus is really inadequate though for what we need in the clinical setting we can't adjust it and it actually doesn't get all that narrow so therefore there are three types of focusing that have been utilized in transducer construction to improve the focus of a beam we're going to see lenses being used those are going to provide an external fixed Focus using curved elements provide an an internal fixed focus and lastly we've got electronic focusing this is going to be more of our modern machine tools it's adjustable and is really a Cornerstone of ultrasound imaging so let's take a look at all three of these in just a little bit more detail let's go ahead and start with lenses so we can use an acoustic lens very similar to how people use eyeglasses eyeglasses take the light that is around them refracts or bends that light so it hits the eye in the correct way so that person can see better same idea with an acoustic lens we put an acoustic lens in front of the PCT Crystal it's going to refract the sound beam and that path of the little wavelets and it's going to be able to create a more narrow focus when you see a lens on a transducer it's typically only seen with single element transducers so we had single element unfocused transducers using a lens makes it a single element focused transducer the lens is placed in between the PCT Crystal and the matching material now the kind of difficult part about this is that it does add another impedance through which the sound must travel before getting to the skin to overcome some of that energy that is lost with the impedance mismatch these tend to have to use very high power sound energy which then heats up the face of the transducer it's also worth noting that very steep curved lenses produce very shallow narrow focuses where flatter curves provided deeper but slightly wider focuses when lenses are used they're built right into the transducer construction so it's classified as an external because it's outside of the pz and it's also fixed meaning it can't be changed the only way you could change it is by switching transducers when ultrasound transducers were originally made they had these single element PCT crystals with the lens in front of them and you knew that that transducer created a beam with like a 5 cm focal depth and if you needed a 10 cm focal depth then you had to go and get a new transducer it's important to note that when we add in that lens we can only make the focus shallow compared to the Natural focus of the beam and essentially the degree to which it's going to focus is mostly determined by the frequency the aperture and the focal Zone length in our image here we have two transducers both of them have lenses this one has a very steep curved lens that's going to make a very narrow very shallow focal point where the little bit flatter lens causes the focal depth to go a little bit deeper and just a little bit wider but it's still better than the natural Focus so lenses are on the outside of the PCT crystal in between the PCT and the matching layer as the material that we used to make PCT elements became more pliable or more flexible we figured out that if we bend the PCT Crystal then we can introduce another type of fixed focusing with with the curved element so this was the more popular form of fix focusing as it removed that extra impedance between the pzt and the matching layer so now again the sound is going to leave directly from the PCT surface and because of that the wavelets end up interacting with one another in a different way than they would have from a flat PCT and because of that then we get a different Focus so just like with our lens if we have a steep curve to our PCT crystals we're going to see shallow narrow f focuses if we have flatter curves we're going to see deeper and wider focuses again the curved element is built into the transducer construction but this time we call it internal so lenses were external curved elements are internal because they involve the actual PCT Crystal it is still a fixed focus and we can't change it so again if you wanted to go from a 5 cm focal up to a 10 cm you had to go get a completely new transducer in our example then we've removed the green lens and we're just looking at a curved PCT again we have that steeper curve to the PCT producing a very shallow and narrow focal depth compared to the flatter curve where we get a little bit deeper and a little bit wider but again still better than the natural Focus we still find that curved elements can be used in our modern transducers but what most modern transducers are going to use are electronic focusing electronic focusing is the be's needs that's one of the best things about our array transducers because electronic focusing is adjustable by the sonographer you've got a knob or a toggle or a button on your machine that allows you to change where the focus is of that beam and because of that we can adjust the image to suit the anatomy that we're looking at despite the transducer frequency that we are using so now we can use lower frequencies adjust that focus and still get relatively detailed pictures and the depth that we need from that low frequency the important thing to remember is that electronic focusing can only be used on multi-element transducers and those are also known as arrays we're going to dive much deeper into how the transducers do what they do in another unit but for the meantime basically what happens with electronic focusing is that each element of the transducer face remember each element has a wire and all those wires connect back to the machine and the Machine is going to send voltages down those wires to activate the PC crystals with electronic focusing as a sonographer you're going to choose where you want the focus to be the machine is going to recognize that and send a pattern that matches that Focus you have chosen typically as the pattern comes down it needs to come down in a curved pattern meaning that we're going to activate our outer elements first and slowly activate towards the center so we're going to see a curved pattern to the voltages and because these are activated first and then these and these and these as we work towards the center those wavelets again are going to interact with one another to create a focus in the ultrasound machine as you change where that focus is by using your knob then the machine is going to recognize that and send a different pattern to make the focus maybe more shallow or deeper or possibly even add in more folky or make the focal zone bigger so there's a lot of options we can do with electronic focusing when you're looking at your image you will typically see a symbol next to your sector in this image here we've got kind of this I bar this is representing the focal Zone and this little Double Arrow is representing the focal point which you'll recall is in the center of the focal Zone we also have some that just represent focal points so this is a little triangle carrot here that's where the focal point is of of the beam creating the image in this example here we've got again little carrots there's one here and one here saying that this is going to be the narrowest part of the beam this is where we're going to get the best lateral resolution we're going to have Okay resolution up here best resolution Okay resolution and then worsening resolution as we get through most likely they are trying to evaluate the valves of the heart here that is why the focus is set at that level and you can see we're just seeing more detail at this level compared to the near field and the far field just getting a lot more definition to our borders so with our electronic focusing the sonographer can adjust it and when you adjust it to optimize your image you should always place it at or just below your area of interest and I found this image from a study that was done called the influence of ultrasound equipment noology in abdominal sonography and it really exemplified how placing the focal point in the correct area is really going to help your image in this example they call it a focal Zone I think it' probably be more appropriate to call it the focal point because remember uh usually when you just have this like one little triangle the little carrot that's the focal point and then the Zone spreads out on either side of it but regardless we can see that the focal point is very high in this image that means we are narrowing to the point right here and then it's just Divergence after that we are widening the beam down here me we're going to increase the beam width which increases the lateral resolution numerical value and with that increase in numerical value that means we worsen our resolution or make it poorer compare that now to where the focal Zone has been moved down so now our beam is coming in narrowing at this level in the image and then diverging what we are seeing is a lot more detail in the vessels that we can see so we can actually see these walls we can see the parena much better instead of just kind of spread out blobs we're seeing a little bit more definition to the pra we've also brought down a more powerful part of the transducer beam so we're able to get better Reflections from this area because we have more sound energy coming to this area and this is why in general we tell you put your focus at or just below the area of interest because you want to improve not only the sound energy getting to that area but mostly improve the lateral resolution Now using all these focusing techniques does affect how the beam changes in space there are four effects that we're going to see so let's go over those in section 10.5 the effects of focusing to cover these four effects of focusing I have two examples of transducers down here we have a single element flat PCT Crystal this is going to be our unfocused beam coming in at its natural Focus but no external focus is available we're going to compare it to our Focus beam which has a curved PCT element and is causing the beam to focus in a different way than the natural Focus the first effect that we will see then is that the beam diameter in the near field is smaller than the element so here we can see that the beam is smaller than the PCT Crystal and that's because these wavelets are already interacting with one another inside the transducer before they ever get out so we're already starting to narrow the beam before it even leaves the transducer compare that to our unfocused beam which is going to start as the same size our next effect is that the focus moves closer to the transducer shortening the near Zone length so if we start focusing sooner we're going to see a shorter focal depth or near Zone length we'll see that the focus is typically moved closer to the transducer compared to the unfocused beam the natural focus is typically deeper into the sound pulse third we're going to see that the beam diverges very quickly after the focal Zone we have a shorter near Zone length which means we're going to have a shorter two near Zone length and because of that the focal Zone actually shortens up as well so after that focal Zone we're going to see a lot of Divergence occur compare that then to the unfocused beam past the focal Zone we will see Divergence but not nearly at the same angle as it does with the focus beam lastly then we're going to see that the focal Zone size is reduced in length and diameter as well so the focal zone is thinner in our focused beam but it's also not quite as long in the focused beam so we have a very thin area which improves the lateral resolution but it's not a very big area compare that to our unfocused beam we're going to have a longer focal Zone but is definitely wider by comparison to the focused beam our Focus transducer beam then smaller than the crystal shallow Focus thinner shorter focal Zone and a lot of Divergence after that focal Zone but this is still more desirable to our Imaging purposes because we are reducing the beam width therefore improving the lateral resolution and that brings us to the end of Unit 10 our first discussion on resolution focusing on axial and lateral resolution we're going to have a few more discussions about resolution because we still need to talk about spatial resolution temporal resolution elevational resolution and contrast resolution so look for those in future units but in the meantime make sure that you are going back to your workbook looking through those activities you'll have an opportunity to sort through some definitions and do some more calculations in regards to axial and lateral resolution then don't forget to go through your nerd check questions those are open-ended questions great for flashcards that you can use to self assess your knowledge of the material presented in this unit