 I'm going to demonstrate the use of the diagnostic A-scan probe. It's important to know that this is not a biometry only probe. This can be used for biometry, but also it's also used for what's called diagnostic A-scan of both the globe and the orbit. This is a separate probe from the B-scan probe. Some units have a combined A and B-scan probe in the same probe, and there's a vector display at the bottom of the screen. This is an independent display of the A-scan only. It's very helpful for quantitation, especially in the orbit, looking at muscles, looking at optic nerve, and in the globe for looking at tumors. So to major things, to analyze internal structure, this is a very useful technique. I will demonstrate the basic probe position. Compared to the B-scan, there is no mark on the A-scan probe, so orientation of the probe doesn't matter. It can be turned any direction, and it's the same display on the screen compared to that of the B-scan probe. The patient has had a drop of anesthetic placed in the eye, and I have the eye open. I'll demonstrate on the left eye, placing the probe at the 6 o'clock limbus, and then I aim superiorly going across the eye to display the superior part of the globe. So even though the probe is at 6 o'clock, I'm displaying the 12 o'clock position in the globe. And as I move the probe, I angle it down towards the inferior fornix, and I watch the display on the screen. And the display, the initial spike, is from the probe itself. I'm going through the vitreous here, which is usually a flat line because there's no reflection within the vitreous cavity unless there's vitreous pathology. And then I see the fundus spike, which is a high-rising spike in the back of the eye. So my goal is, as I move the probe, I try to maintain this in a steeply rising fashion. I want it to be steeply rising, high, and straight, which means I'm perpendicular. And perpendicularity is important for measuring purposes and internal structure analysis. And once I'm behind the initial retina spike, I'm in the orbit, which has many high spikes. So the contrast on the A scan is analogous to the B scan, where the B scan display in the globe is a lighter color grayscale because there's no reflection inside the globe. And correspondingly, the A scan display is a flat line. Once I get into high-reflective tissue on the B scan, which is contrasted to the A scan, I see high reflectivity from the orbital structure. The orbit has many interfaces in it. It has muscles and fat, septa, and these give high reflectivity, so that contrast on the A scan. So as I angle the probe, I'm scanning the globe. I start posteriorly towards the posterior segment. The probe is almost pointed directly posteriorly, and I angle it superiorly more and more as I scan the retina towards the peripheral retina where the signal starts to break down. That means I'm here at the auraserata area and I can't get a good perpendicular spike. So I scan the globe comparable to the B scan in six quadrants. This is the six o'clock position. I then move over here in the left eye. This would be at the four o'clock position, and I do the same thing. I aim the probe almost posteriorly, and I get a spike from the initial signal, and as I angle it, I try to maintain the fundus spike in a perpendicular manner until I break down as I approach the auraserata. I then go to the two o'clock position, do the same thing as I angle from the posterior segment more and more anteriorly to the peripheral retina. So by doing that, I scan the entire inner contents of the globe, and again, watching the vitreous for any spikes in the vitreous, and watching the fundus spike for any low reflectivity indicating a possible lesion, such as a tumor. In this patient, there's no pathology, so that's not demonstrated. I then can go to an orbit expansion, which simply has the same display in a more contracted view, allowing me to see the orbital tissue in a better display. And so as I go on the orbital expansion, I can do the same scanning technique. I place the probe inferiorly at the six o'clock position, and I am going superiorly, scanning the orbit. In this case, this is a superior orbit. So again, here's the initial spike from the probe, here's the vitreous display, which again is flat because of no reflectivity, and here's the orbit reflection with many interfaces because, again, there are many different structures with the orbital tissue to give reflectivity. And I scan the superior orbit, I then place the probe, again, at the four o'clock position, and I'm scanning the superior nasal orbit. Again, going back and forth, trying to maintain perpendicularity of that initial retinal spike. I then can concentrate on specific orbital structures, especially the extracurricular muscles and the optic nerve. And this allows quantitation of these structures, which is very important for following such pathologies as grave disease or myositis or other muscle conditions. And the optic nerve allows me to evaluate the nerve for swelling, as may be found in increased intracranial pressure with optic nerve fluid, or an example of a tumor of the nerve, such as glioma or meningioma with nerve thickening. So to display the muscles, I again place the probe inferiorly to display the superior rectus muscle. I'm going across the globe towards the superior orbit and I watch the display for a little deflection in the reflectivity pattern. Because most of the orbit is low reflective, high reflective, I want to get a low reflective pattern indicating I'm probably in the muscle. So example right here, the muscle would be from here to here. So the muscle sheath, I'm going across the sheath from the inferior muscle sheath to superior muscle sheath, and I would measure that distance. We have electronic calipers and measuring tools to allow us to do that as we measure different structures. On this unit I'm able to move the caliper and the cursor falls on the muscle sheath, and I then measure the other part of the muscle and I display this as a thickness of the muscle. There are various tables indicating muscle thickness and normal patients. I'd refer you to that. Also the textbook addresses that. But we measure the superior rectus muscle in this situation. I then look at the medial rectus muscle by placing the probe on the lateral globe, aiming nasally towards the medial rectus muscle. I move the probe back and forth. I make little microscopic movements side to side up and down, and I watch for a dip in reflectivity and I freeze that. I can see the pattern here of the muscle compared to normal orbit, which is high reflective here, high reflective there, but here you see the dip in the pattern because the muscle is more homogeneous than adjacent orbital tissue. Again, normal orbit is quite high reflective because of many interfaces. The muscles are more homogeneous than surrounding orbit. They tend to be a little bit lower reflective, normally. So I can measure the muscle sheath here on the medial rectus muscle, and the cursor there and there, and I get a measurement of the muscle thickness, 4.42 millimeters. So I can do that for the other muscles. Look slightly down. I place the probes apparently, and I aim towards the inferior orbit to get the inferior rectus muscle, which is usually the smallest muscle of the extracurricular muscles. And I watch the pattern for a slight dip in reflectivity, and I will use that for my measuring purposes. So right here, I put the cursor on this sheath of the muscle, the other cursor on that sheath, and I measure the inferior rectus muscle to be 3.64 millimeters. We then measure the lateral rectus muscle by placing the probe nasally on the globe. And again, we aim towards the lateral rectus muscle on the globe. We watch the orbital pattern for a dip in reflectivity, and use that for our measuring purposes. So there to there would be the muscle sheath. Place the cursor on one side of it, and the other side, and measure the muscle thickness of 3.37 millimeters. So I measure the four rectus muscles. I can also measure the superior oblique muscle by placing the probe inferiorly temporally on the globe, and aiming towards the trochlea in this part. The superior oblique muscle tends to run along the medial wall of the orbit, so it usually is further back on the display on the screen than the other muscles. It is more towards the bone. So right here is the superior oblique muscle. So I place the cursor on one sheath of the muscle, the other cursor here, one oblique muscle, four rectus muscles. The inferior oblique muscle tends to run inferiorly, kind of in a hammock fashion across the inferior globe. It's very hard to display that muscle. You have to get the probe way up here, the brows in the way, and usually I can't display that muscle so I don't usually attempt to unless there is some question about pathology. Then I will attempt to do that. So that's the basic muscle measuring technique with the A-Scan. Again, this probe is not a biometry probe. You couldn't do that kind of examination with a biometry probe. You have to use a dedicated A-Scan probe to be able to do that. And also the size of the probe, as you can see, is much smaller than the B-Scan probe, which is a thicker dimension. And it would be hard to place the probe as I've done here in different parts of the globe for that purpose. Now to measure the optic nerve I usually place the probe on the temporal part of the globe and aim nasally. And I attempt to get a display of the nerve looking right next to the globe display. The muscles again, here's the meteorrectus muscle over here, a nice display showing the dip and reflectivity. But I'm looking here more anterior to that display to visualize the nerve she spikes. Now sometimes they jump right up, sometimes you have to work back and forth. My hand is moving little movements back and forth all the time, side to side to try to maximize that display. And I see two spikes right here. You want to try to get steeply rising spikes. This isn't perfect because of this little artifact here. But I see a spike there, I see a spike there. So in her, I would measure from this part of the nerve sheath, this is the anterior part, the medial part here and the other sheath I would measure here. So my nerve sheath is from there to there, measuring 2.36 millimeters which is in the normal range for optic nerve sheath measurements. You can also to measure the nerve from the medial approach. If you have a hard time temporally sometimes away the nerve kinks in the orbit, it's hard to get a good display. So I will then place the probe medially and aim temporally to try to capture the nerve there. And sometimes you can get a better display as you aim across the nerve going this way. So sometimes I'll go back and forth. I'll do on the temporal part first and then the nasal part attempting to display the nerve. So here it's more difficult, so I had a better view by placing the probe temporally. So this is called the trans-ocular approach. We're going across the globe to visualize orbital structures. You can also do a para-ocular approach where the probe is placed in a position to bypass the globe. Example here, I'm not going through the globe. I'm bypassing the globe to see the lacrimal gland and I'm placing the probe in the area of the lacrimal fossa and looking at the lacrimal gland signal. And here it's a normal gland so you see this kind of a step off in reflectivity as I go through the gland. If the gland is enlarged or reflectivity patterns are different I can pick that up by doing this technique. I'll also go around in the different quadrants, so here at 6 o'clock I will look bypassing the globe. So here's the globe pattern here. I want to bypass that so I'll go more inferiorly and I'll see the orbital display here. I'll go to 2 o'clock correction 4 o'clock and I'll show it here. I'll go to 2 o'clock and bypass the globe showing here. I'm going to 12 o'clock and not showing the globe pattern just the orbital pattern. I'm going to 10 o'clock the orbital pattern here and 8 o'clock the orbital pattern here. So by doing that I've done a para-ocular approach to show the orbit because sometimes orbital lesions are quite quite anterior and they can be displayed unless you show this kind of approach which would be missed on a trans-ocular approach by going further purely in the orbit.