My name is Petra Duran-Gehring and I will be talking to you today about the FAST exam and other applications for ultrasound in the setting of trauma
The goal of this lecture is to be able to identify free fluid in the abdomen and the pericardium using ultrasound in the setting of trauma. You will learn to distinguish between a pericardial effusion and pericardial tamponade. Evaluating the patient for hemothorax and pneumothorax using the extended FAST exam will also covered.
Ultrasound has been used to evaluate trauma patients for over 30 years. Its use began in Europe in the 1970 and spread to the US, replacing DPL during the 1980s. It finally got included in the Advanced Trauma Life Support Course in 1997. And FAST is now required training in all Emergency Medicine residencies.
There are several ways to evaluate the abdomen after trauma. CT scan gives the most detail, is non-invasive, but requires the patient to be hemodynamically stable. Diagnostic Peritoneal Lavage or DPL has been noted to be the most sensitive test, but is invasive. Ultrasound on the other hand, is non-invasive and is useful in detecting as little as 250 cc of free fluid.
FAST stands for the focused assessment with sonography in trauma. The goal of the FAST exam is to detect free fluid in the peritoneal cavity and pericardial effusions. Hemothorax and pneumothorax can also be identified by expanding the FAST exam to the chest cavity.
The FAST exam has both pros and cons. It can be performed quickly, is non invasive, inexpensive, and easily repeated if the clinical scenario should change. It is also highly specific for the need for laparotomy. However, the big drawback of the FAST exam is that it is non-specific, the fluid found could be blood or merely ascites. It is also limited by body habitus and any subcutaneous air.
The FAST exam is indicated chest or abdominal trauma to detect hemoperitoneum, pericardial effusion and pleural effusions. It is best used on hemodynamically unstable patients to determine if a trip to the OR is necessary.
The use of the FAST exam has been extensively studied in blunt abdominal trauma. It has been shown to identify intraperitoneal bleeding causes by solid organ injury. The use of FAST for penetrating injury is somewhat controversial due to its low sensitivity for determining the need for laparotomy and its inability in detecting bowel injury. However, some clinicians advocate its use in assessing for significant hemoperitoneum and in prioritizing management when multiple penetrating injuries are present.
Bedside ultrasound may significantly decreases mortality in patients with penetrating cardiac injuries. Early detection of pericardial effusions can prevent cardiac tamponade in cases where a wound to the heart bleeds into the pericardium, not the chest cavity. This may occur when the pericardium seals itself after penetrating injury. Thus, all patients with penetrating chest injury should be “screened” for a potential pericardial effusion. If an effusion is present cardiac injury is assumed until proven otherwise.The extended FAST exam can be used to detect hemothorax after penetrating injury and is actually more sensitive than the standard supine chest x-ray. US can detect as little as 20cc of fluid in the chest.On the other hand, significant blunt cardiac injury is relatively uncommon, as most patients who suffer severe cardiac injury die prior to arrive in the trauma center. Of those who make it to the trauma center with a cardiac injury, most will develop a pericardial effusion which can be identified on FAST exam.
The FAST exam works to identify free fluid using the basic anatomy of the abdomen and pelvis. Fluid pools in very specific areas based on the tethering of the bowel and intrabdominal organs. This tethering channels fluid into specific areas around the solid organs of the abdomen. The amount of fluid in an area can further be maximized by positioning the patient to take advantage of this drainage system. Trendelenburg increases fluid into the RUQ and LUQ. Whereas reverse t-burg increases fluid in the pelvis
This schematic illustrates the anatomic drainage which makes the FAST exam possible. The phrenicolic ligament traps fluid in the LUQ and the mesentery of the transverse colon funnels fluid to the RUQ. Fluid then can drain down the pericolic gutter from the RUQ to the pelvis.
To perform a FAST exam, you will need an ultrasound machine with a low frequency probe than will allow you to visualize deeper abdominal structures. Ideally the ultrasound machine should be portable or at least manuverable in a potentially crowded trauma bay.
I have already alluded to the components of the FAST exam, but lets review them again: RUQ, LUQ, Cardiac, and Suprapubic. The cardiac view may be obtained in the subxiphoid plane or the parasternal long axis. The extended FAST or E-FAST uses the traditionally 4 views, but then also looks into the chest cavity. Lets now look at each view in depth.
The RUQ is the most likely area to see free fluid in the abdomen. The probe is placed in a coronal plane with the probe indicator toward the patients head at the mid axillary line at the costal margin. Slide the probe along the mid axillary line to find Morrison’s pouch and to get around rib shadows. It is important to be able to see the entire kidney to ensure that you are missing any fluid that may pool at the inferior pole of the kidney.
Here is the classic RUQ view on ultrasound. Let’s take a moment to orient ourselves. The patient’s head is to the left of the screen and their feet are to the right of the screen. The top of the screen in what lies closest to the probe and the bottom of the screen is what is farthest from the probe. This view encompasses the liver, right kidney, and diaphragm. Morrison’s pouch lies in the hepatorenal interface, visible now only by the crisp white line of gerotta’s fascia. This is a normal FAST.
Now turn your attention to the diaphragm and superior to the diaphragm is the pleural space. Since we cannot see air on ultrasound as it scatters sound waves, we look for the mirror artifact to give us information about the lung. Mirror artifact is a phenomenon that occurs when the ultrasound bean hits a highly reflective surface such as the diaphragm, making an image on one side of the mirror appear on the other as well. Notice that there appears to be liver on both sides of the diaphragm, this is normal. If we saw black on the other side of the diaphragm, then it would indicate that there was fluid there taking up space where lung normally should be. This will become evident in further slides.
Now that we have seen a normal RUQ, lets take a look at some abnormal ones. Fluid pooling in the RUQ will typically be found in Morrison’s pouch, however, it may also pool inferior to the liver, superior to the liver, or at the inferior pole of the right kidney. Remember that fluid appears on ultrasound as anechoic or black. Both images illustrates fluid around the liver. We can also see fluid in the right pleural space in the lower picture here. Notice the lack of mirror artifact superior to the diaphragm, as well as the white structure within the fluid representing a collapsed lung.
Here is a classic view of of a positive FAST. Notice the anechoic or black stripe in Morrison’s pouch
Here is another image of a positive RUQ FAST. This one is more subtle than the previous slide. Again note that fluid appears as a black line around the superior liver dome.
The LUQ is our next stop during the FAST exam. Just like in the RUQ, the probe is placed in a coronal plane with the probe indicator toward the patients head at the mid axillary line at the costal margin. It is important to remember that the left kidney lies more superior in the abdomen than the right kidney. Therefore a good trick is to start slightly more cranial on the left side than you did on the right. The LUQ is always more difficult to image than the right due to the fact that the structures ae usually under the ribcage. Consequently, you may need to angle or fan your probe to get in between ribs to get the optimal view. If you are having difficulty getting your image due to the ribs, having the patient take a deep breath may help to inferiorly displace the diaphragm and resultant organs to get a better picture.
Here is the classic LUQ view. Recall again that the patient’s head is to the left of the screen and their feet are to the right, top of the screen is closest to the probe and the bottom of the screen is farthest from the probe. Note that this view includes the spleen, left kidney, and diaphragm. Notice that crisp hyperechoic or white line of gerotta’s fascia. This is a normal FAST.
It is important in the LUQ to find the inferior pole of the spleen as it is a common location for fluid to collect. Again we must also evaluate what is superior to the diaphragm as part of the extended FAST.
So here is an image of a positive LUQ. Notice the anechoic or black area anterior to the spleen. This patient also has fluid in the pleural cavity. Notice the lack of mirror artifact and the hyperechoic or bright lung floating.
Now that we have interrogated the upper quadrants, we will turn our attention to the cardiac view. The heart may be imaged in several planes, but for the FAST exam, the subxiphoid approach is the more common. The parasternal long axis view is also useful when the heart is not visible with the subxiphoid approach. We look at each approach in further detail shortly. Whichever view you obtain, the goal is still the same: Identify cardiac activity and look for pericardial effusion. If an effusion is present, then demonstrating right ventricular collapse will make the diagnosis of pericardial tamponade.
There are many factors that can affect which of the two cardiac views will be most successful. Much depends on the patient’s body habitus and clinical condition. Patients with severe abdominal pain may not tolerate the subxiphoid view. An enlarged abdomen due to pregnancy or obesity can make subxiphoid viewing more difficult as well, since the diaphragm and subsequently the adjacent heart, will be farther from the probe. Conversely, patients with COPD have elongated lung fields and lowing lying diaphragms, making subxiphoid viewing much easier.
The subxiphoid view is achieved by placing the probe in the patient’s epigastrim, with the probe indicator to the patient’s right. The probe is then angled to the patient’s left shoulder. An important tip to remember is that the probe should almost be flat against the patient’s abdomen in order to achieve the angle necessary to see the heart. I usually recommend holding the probe from on top and pressing downward to achieve best results. We are imaging farther away from the probe than in the previous two views, so it is important to increase your depth setting on the machine as well in order to bring the heart into view
This is a schematic of the subxiphoid view. Notice that the liver lies at the top of the image corresponding to the structure closest to the probe. This make sense when you recall our probe position. We are using the liver to displace bowel and create a “window” for us to view into the chest cavity. Since we are viewing the heart from underneath, the left side of the heart is farthest from the probe, or at the bottom of the screen. Notice that this is a four chamber view of the heart.
This is an ultrasound image of the subxiphoid view . Just like in the schematic, the liver lies closest to our probe and at the top of the screen. The left side of the heart lies farthest from the probe at the bottom of the screen. The pericardium is outlined in white. The myocardium is gray and abuts the pericardium. It is important to note this relationship, as this will be where we will look for effusions during our FAST exam.
Here is a video of a normal FAST subxiphoid view
If you are unable to obtain a subxiphoid view of the heart, then you may use the parasternal window. The probe is placed adjacent to the left side of the sternum at the nipple line. Since the heart lies along a diagonal plane within the chest cavity, the probe should also be placed on diagonal to achieve a long axis view. Place the probe indicator to the patients right shoulder. Depending on the exact lie of the heart, you may have to slide up or down a rib space.
The parasternal long axis view is a three chamber view of the heart. The left side of the heart is visible but only the right ventricle appears in this view. This will be important when we look to determine if pericardial tamponade is present.
Here is an actual ultrasound of the heart in the parasternal long axis view. Notice that in this view the right ventricle is closest to our probe and the left side of the heart is farthest from us. The aortic root is also present in this view.
Here is video of the heart in motion using the parasternal long axis view
This image illustrates the heart in a parasternal long axis view with a large effusion surrounding the heart. Remember that fluid appear anechoic or black on ultrasound. Notice that there is a significant black ring around the heart indicating effusion.
If an effusion is identified on ultrasound during your FAST exam, it is important to note if pericardial tamponade is present. To do this, look at the right ventricle in motion. Normally the ventricle contracts symmetrically. With tamponade, the outer wall of the ventricle will depress centrally. It always looks to be as if someone took their finger and pushed the wall inward. This picture is a wonderful example of tamponade.
Now we see the heart in motion in the parasternal long axis. Notice that each time the heart beats, it appears as though something is depressing the mid portion of the right ventricle. This is right ventricular collapse and indicates pericardial tamponade
Now we will move to the suprapubic or pelvis region of the FAST exam. For this portion of the exam, the probe is placed over the bladder, midline superior to the pubic symphasis. The transverse or longitudinal view can be obtained. To view the area in transverse, place the probe with the indicator to the patient’s right. To view in a longitudinal or sagital plane, place the probe with the indicator to the patient’s head.
Regardless of the plane you choose, the same anatomic structures should be visible. The bladder should appear anterior or closest to our probe, at the top of the screen. Distal to the bladder will be the uterus in a women and the rectum in a man. When looking for free fluid, it is important to note the anatomic differences in the sexes. In men, you will find fluid retained in the rectovesicular space, the potential space between the rectum and the bladder. In women, you must look for fluid in two locations, the vesiculouterine space and the pouch of Douglas, the potential space between the uterus and rectum.
Here are two images of the pelvic portion of the FAST exam. The image on the left of the screen was taken in a sagital or longitudinal plane. Notice the bladder to the right of the image with the uterus directly underneath. The image on the right side of the screen was taken in a transverse plane. Notice the uterus lies distal to the bladder. Please note that the fluid contained within the bladder has smooth edges.
This is a positive suprapubic FAST. Notice the free fluid found in this image. The fluid contained within the bladder has smooth edges. Notice, however, that the free fluid has very sharp margins, indicating that the fluid is not contained within an organ, but floating freely in the potential spaces between organs.
Here is another image with free pelvic fluid. This image illustrates fluid in the pouch of Douglas, causing the uterus to appear to be floating.
We have already discussed the four standard views of the FAST exam and will now look at the additional views need for the extended FAST exam or E-FAST. The idea of the extended FAST is to briefly look for evidence of hemothorax and pneumothorax. We have already briefly talked about how to look for pleural fluid when we looked at our UQ views, but this is a key component of the extended FAST. So while you are interrogating the RUQ and LUQ, slide the probe slightly cranial to interrogate the pleural space. The lack of the mirror artifact means that pleural fluid is present. To interrogate the chest for pneumothorax does take an additional view, which we will discuss in detail shortly.
To look for a hemothorax or pleural fluid, we must review again our UQ views. You should place the probe in a coronal plane with the probe indicator to the patient’s head. Look to get the liver or spleen, depending on which side you are interrogating, the kidney and the diaphragm. Focus on the diaphragm and look for the mirror image on the cranial side. If this mirror is absent, then there is fluid in the pleural space. In more severe cases, the fluid will surround the lung and it will appear to float within this fluid collection with each inspiration.
Here is an image of the LUQ with pleural fluid. Notice the lack of the mirror artifact on the cranial side of the diaphragm. That anechoic or black area is fluid.
Here is another picture of fluid in the pleural space, more severe than the previous slide. Notice the anechoic or black area to the cranial side of the diaphragm which indicates pleural fluid or hemothoroax. Within that black fluid collection is the white hyperechoiclung tissue, which appears to be floating within.
Let’s move on to pneumothorax. Pneumothorax occurs in up to half of all trauma patients. Unfortunately, the standard supine chest x-ray misses about 30% of those. Ultrasound, on the other hand, is very sensitive in detecting small and anterior pneumothoaraces, equally the sensitivity of CAT scan.
To look for pneumothorax, you will need to change probes, using the high frequency linear probe. We change probes since we will be looking closer to the surface of the chest wall and need the increased detail the high frequency probe provides. The probe is place on the anterior chest wall in the sagital or longitudinal plane with the probe indicator toward the patient’s head. We will then slide down the patient’s chest wall, examining each interspace for pneumo.
When we look for a pneumothorax it is important to identify the anatomy properly. We are cutting across two ribs in this sagital plane, so we should see a transverse view of the ribs and their resultant shadow. You will also note the subcutaneous tissue and muscle. Finally more distal from the probe is the pleura itself. We are looking to watch the pleura slide.
Here is a normal lung picture on ultrasound. Please note, the ribs and their shadow and the hyperechoic pleural line. Some have described this picture as a “frog on a lily pad” with the frog’s eyes being the ribs and the pleura being the lily pad. I will leave that to your imagination.
It is important when evaluating someone with ultrasound for pneumothorax, to look at the pleura. In B-mode, the pleura should slide back and forth. If there is question as to whether the lung is sliding, then you can turn to M-mode for further confirmation. If the lung does not slide, then a pneumothorax is present
Here is a picture of a normal lung. Watch how the pleural slides.
Here is a video of a pnuemothorax. Notice how the subcutaneous tissue moves, but the pleura does not slide.
Here is a side by side comparison of a normal lung on the left and a pneumothorax on the right. Notice in both pictures the hyperechoic pleural line. You should be able to see the difference between the pleural slide and the stillness of the pneumothorax.
If you are having difficulty seeing the difference in the B-mode slide of the pleura, then M-mode can further clarify matters. M-mode is motion mode. It graphs every point along a line over time. Therefore, motion creates waves or curves, stillness creates straight horizontal lines. So place your M-mode curser on the pleura between the ribs and take a tracing. A normal lung will move and therefore create a lot of movement on the tracing, which has been classically describes as looking like a sandy beach. A pneumothorax, will lack motion creating a straight line across the tracing, denoting the classic “barcode” sign.
Although the FAST exam is the most widely used ultrasound in trauma, there are other uses for ultrasound in evaluating the trauma patient. A recent study from University of Georgia has shown that ultrasound can be used to identify diaphragmatic rupture. Using M-mode on the standard UQ views, the diaphragm can be interrogated for motion. A lack of motion indicates a rupture. Ultrasound can also be used to evaluation long bones for fracture. Using the high frequency linear probe the area of interest is visualized. Bone will appear as a bright hyperechoic white line on ultrasound. If there is a divot or break in that line, a fracture is present. The eye can also be evaluated via ultrasound.
Ocular ultrasound is one of the easiest ultrasounds to perform and can easily diagnose several life threatening conditions. Increased intracranial pressure, pappilledema, retinal detachment can all be identified using ultrasound. Ultrasound can also located foreign bodies without the radiation exposure found in CT scans.
Ocular is one of the easiest scans because it does not matter if the probe is in longitudinal or transverse position since the eye is a sphere. Using the high frequency linear probe, have the patient close their eye and fill the patient’s orbit with gel. Gently place the probe on their eyelid and fan through the eye. The anterior chamber will appear at the top of the screen and the retina and optic nerve will appear distally. Notice the iris and lens as well. The optic nerve will appear as a shadow posterior to the globe.
The use of ultrasound to measure increased ICP is well described and validated in the literature. Once the optic nerve has been identified, freeze the image. Using the caliper function, measure 3 mm from the retina along the optic nerve. At this point you will use a second caliper to measure the diameter of the optic nerve. If the diameter is greater than 5 mm, than increased ICP is present. It is important to measure both eyes and take an average of the two measurements to determine increased ICP as there are reports of increased optic nerve diameters on a single side when a posterior orbotumor is present.
Pappilledema is easy to identify using ultrasound. In fact it is easier to see pappilledema on ultrasound many times than it is in a busy bright ED or trauma center, where the patient’s pupils are constricted. Pappilledema will appear as a bump or hump on the back of the retina in the region of the optic nerve.
Retinal detachment will appear as a thin white or hyperechoic line along the posterior wall if the eye. This thin white line floating in the globe is the retina itself.
FAST<br />Petra Duran-Gehring M.D.<br />Assistant Professor<br />Department of Emergency Medicine<br />University of Florida- Jacksonville<br />
Objectives<br />Evaluate the abdomen for free fluid, using the four ultrasonographic windows for FAST<br />Identify free peritoneal fluid, pericardial effusion and tamponade<br />Identify pneumothorax and hemothorax using extended FAST exam<br />
History<br />1970s: US first used to evaluate trauma patients in Europe<br />1980s: FAST replaces DPL in most trauma centers<br />1988: German Surgery Board requires US certification<br />1997: FAST included in ATLS course<br />2001: US training required in EM residencies<br />
Radiologic Evaluation in Trauma<br />CT: non-invasive, gives detail about organs<br />DPL: most sensitive, invasive<br />US: non-invasive, can detect 250cc of free fluid in Morrison’s Pouch<br />
What is FAST?<br />Focused Assessment with Sonography in Trauma<br />Objective:<br />Detection of intra-abdominal free fluid<br />Detection of pericardial fluid<br />E-FAST: detection of hemothorax and pneumothorax<br />
FAST<br />Benefits<br />Performed rapidly<br />Noninvasive<br />Inexpensive<br />Easily repeatable<br />Highly specific for therapeutic laparotomy<br />Limitations<br />Obesity<br />Subcutaneous emphysema<br />Non-specific<br />
Abdominal Trauma<br />Blunt trauma<br />Most widely studied<br />Intraperitoneal bleeding due to splenic or liver injury<br />Penetrating trauma<br /> 50% sensitivity for determining the need for laparotomy<br />Doesn’t detect bowel injury<br />Can still determine hemoperitoneum<br />
Chest Trauma<br />Penetrating trauma <br />Pericardial Effusion<br />Pericardium seals self creating effusion leading to tamponade<br />US identifies prior to Beck’s triad<br />Screening exam for effusion<br />Hemothorax<br />US more sensitive than CXR (20ml vs. 200ml for detection)<br />Extended FAST for pneumothorax, sensitive for supine pts<br />Blunt trauma<br />May determine cardiac rupture<br />
Why FAST works<br />Fluid pools in predicable locations<br />Subhepatic<br />Perinephric<br />Perisplenic<br />Pelvic<br />Subpleural<br />Position patient to best locate fluid<br />Trendelenburg for upper quadrants<br />Reverse trendelenburg for pelvis<br />
FAST Components<br />Right Upper Quadrant<br />Left Upper Quadrant<br />Cardiac<br />Subxiphoid<br />Parasternal long axis<br />Suprapubic<br />Extended FAST<br />Lung fields<br />
Right Upper Quadrant<br />Position<br />Probe placed with indicator to pt’s head<br />Probe placed around 8-11th rib space, mid axillary line<br />May need to slide probe around rib shadow<br />Do not forget to interrogate inferior pole of kidney<br />
RUQ<br />Includes<br />Liver<br />Kidney<br />Morrison’s pouch<br />Diaphragm (E-FAST)<br />Look for: <br />Fluid in Morrison’s pouch<br />Lack of mirror artifact<br />Normally diaphragm acts as mirror, liver appears to be on either side<br />Indicates pleural fluid<br />
RUQ Free Fluid<br />Intraperitoneal fluid<br />Morrison’s Pouch<br />Along the lower edge of the liver<br />Superior to liver<br />Pleural fluid<br />Lack of mirror artifact<br />Lung appears to float with inhalation<br />
Left Upper Quadrant<br />Position<br />Probe placed with indicator to pt’s head<br />Probe placed around 6-9th rib space, mid axillary line<br />May be difficult to achieve due to rib shadow<br />Ask pt to inhale deeply to displace anatomy inferiorly<br />
LUQ<br />Includes<br />Spleen<br />Kidney<br />Diaphragm (E-FAST)<br />Look for: <br />Fluid in spleenorenal fossa<br />Fluid inferior to spleen<br />Lack of mirror artifact<br />
LUQ Free Fluid<br />Intraperitoneal Fluid<br />Splenorenalfossa<br />Inferior to inferior pole of spleen<br />Superior to spleen<br />Pleural Fluid<br />Lack of mirror artifact<br />Lung floating with inspiration<br />
Cardiac View<br />May use either:<br />Subxiphoid<br />Parasternal long axis<br />Look for<br />Cardiac activity<br />Pericardial effusion<br />RV collapse<br />
Suprapubic<br />Position<br />Transverse<br />Probe in the midline just cephalad to the pubic bone<br />Probe indicator pointed to pt’s right<br />Longitudinal<br />Probe in the midline just cephalad to the pubic bone<br />Probe indicator pointed to pt’s head<br />
Suprapubic<br />Identify<br />Bladder<br />Uterus in women<br />Pouch of Douglas<br />Look for Fluid:<br />Men: posterior to bladder<br />Women<br />Vesicouterine space<br />Posterior to uterus (Pouch of Douglas)<br />Sharp edges indicate fluid outside of bladder<br />
Suprapubic<br />Bladder<br />Bladder<br />Pouch of Douglas<br />Pouch of Douglas<br />Uterus<br />
Pelvic Free Fluid<br />Posterior to bladder<br />
Extended FAST<br />Addition to 4 view FAST exam<br />Includes evaluation for hemothorax and pneumothorax<br />Two additional components<br />Expand UQ views to visualize diaphragm<br />Lack of mirror artifact indicates fluid/hemothorax<br />Scan anterior chest<br />Lack of lung slide indicates pneumothorax<br />
Hemothorax<br />Includes<br />Liver or spleen<br />Kidney<br />Diaphragm<br />Look for:<br />Lack of mirror artifact<br />Normally diaphragm acts as mirror, liver appears to be on either side<br />Indicates pleural effusion<br />
Pneumothorax (ptx)<br />Occurs in 15-50% of pts with chest trauma<br />Supine CXR<br />Misses up to 1/3 of all pneumothoraces<br />Only 50-70% sensitive at detection ptx<br />Inaccurate for anterior ptx due to air layering<br />Ultrasound<br />Detects small or anterior ptx<br />Sensitivity 92-100% (equal to CT scan)<br />Negative predictive value 99-100%<br />
Pneumothorax<br />Use high frequency linear probe <br />(5-10 MHz)<br />Place probe on anterior chest wall, indicator to pt’s head<br />Slide down chest wall to interrogate each rib interspace<br />
Lung Fields<br />Includes<br />Rib with shadow<br />Subcutaneous tissue and muscle<br />Pleura<br />Look for: <br />Slide of the pleura<br />Lack of sliding indicates pneumothorax<br />
Pneumothorax<br />Watch for slide of the pleura<br />Lack of sliding indicates pneumothorax<br />B-Mode: direct visualization<br />M-Mode: “sandy beach”<br />May see the leading edge of pneumothorax<br />