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hi this is Pam Johnson and today I am going to be talking about the role of CT in the patient with suspected a pulmonary embolism pulmonary embolism is important diagnosis not to miss we know the incidence ranges from about 44 to 145 patients in 100,000 but of course the quandary is the variable presentation that often makes it difficult to make the diagnosis clinically it's a diagnosis that we don't want to miss because of the high risk of mortality if untreated and it's a diagnosis that we want to be accurate when we make because of the risks associated with anticoagulation what are the risk factors for pulmonary embolism when we learn these in medical school they include long airplane trips the use of oral contraceptives or other hormone replacement therapy patients with cancer or who have had recent surgery or are immobilized for other reasons have a higher risk pregnant patients have an increased risk of pulmonary embolism and these can be among those challenging exams to perform and then other risk factors that we don't commonly think about include pneumonia hypertension congestive heart failure smoking and obesity there are a number of clinical scoring scales that the clinicians use that help them to determine what is the positive pretest probability that a patient has a pulmonary embolism and as we can see here from the revised Geneva score system older age previous history of PE or DVT tachycardia recent surgery recent fracture presence of hemoptysis patient who has a known malignancy a patient who has unilateral lower limb limb pain these are among the many things that are considered in determining whether they think the patient has a high or low probability of pulmonary embolism another scoring system is the well system includes similar variables such as a previous history of PE tachycardia recent surgery hemoptysis clinical signs of a DVT so these are considered in the decision-making as to whether the patient should be scanned with CT or other type of imaging modality and we're going to discuss that in a little bit but with the Welles model we see that based on how many points the patient scores based on these clinical indicators the risk can be defined clinically as low moderate or high so what is the role of CT and how does it compare to other imaging modalities for identification of pulmonary embolism when I was a resident we didn't have CT for pulmonary embolism we only had vq imaging and if that was equivocal then the patient had to undergo pulmonary arteriography the the utility of CT is has been an incredibly valuable addition to our imaging tools and at this point it's really the preferred test especially in patients with a high clinical suspicion or a low to moderate suspicion in a positive d-dimer the problem with VQ is that it's not diagnostic in all patients and ultrasound is helpful they DVT is present in 70% of the patients with an acute pulmonary embolism but that is not 100% sensitive for making the diagnosis so according to the American College of Radiology appropriateness criteria where they've rated different imaging modalities well a chest radiograph is definitely considered and and a good choice for for frontline imaging primarily to exclude other causes of acute chest pain but to make the diagnosis of pulmonary embolism the current standard of care is CT angiography with contrast in pregnant patients it's a little bit more controversial according to the ACR appropriateness criteria both CT angiography and VQ scan were given the same rating of course it's always good to consider starting with a lower extremity ultrasound in a pregnant patient because if you can make the diagnosis without radiation and contrast administration that would be optimal so d-dimer is a laboratory test that has gained a lot of popularity in recent years for trying to determine clinically whether the patient has some type of acute thrombosis the problem with the d-dimer test is that it can be positive for a number of different reasons and so it has a high sensitivity but a low specificity there are two methods by which a d-dimer test can be performed and the iliza test is much more accurate so that's one thing to consider the accuracy of the test that's used in your institution there are a range of other processes and diseases that can elevate the d-dimer so the differential diagnosis includes presence of any kind of infection or inflammation cancer surgery or trauma burns or bruises and then ischemic heart disease or stroke a ruptured aneurysm or dissection and it can also be elevated in pregnant patients compounding the problem of the d-dimer test is that it increases with age so you can exclude a PE by a negative d-dimer in about 60% of patients under the age of 40 but only 5% over the age of 80 because it may be elevated in elderly patients and new age dependent cut-offs are being defined false negative d-dimer's might be may be identified in patients who have symptoms II this lasted more than two weeks if their anticoagulated or the heparin is initiated before the d-dimer test so what is the advantage of CT well as high diagnostic accuracy and it's a definitive in a majority of cases of course we all know that that is technique and image quality dependent it's a fast test with high spatial and contrast resolution it's widely available and it often reveals an alternative diagnosis to account for the patient's symptoms when they don't have a pulmonary lizz 'm in this paper from AJR which was summarized the imaging modalities used in the detection of PE and DVT you can see that pulmonary mdc-t angiography had sensitivity of 96 to 100% specificity as high as 89% positive predictive values 92 to 96 percent and negative predictive values 94 to 100% so it's a it's a very good test for this clinical indication there are limitations these include the fact that I donated contrast is required so patients with renal insufficiency or anaphylaxis to iodine aided contrast cannot undergo the PE CT scan it involves radiation of course and our ability to make the diagnosis is both protocol dependent and patient dependent patients ability to cooperate in terms of suspending respiration and not moving because we know that respiratory motion artifact create pseudo emboli within the smaller arteries so the patient's ability to cooperate is really critical and often patients will be intubated or very ill so these are these can be difficult exams to perform and interpret as I mentioned respiratory motion artifact can be a problem if the contrast enhancement level is not adequate if the timing is not correct well then there's a lot of noise which limits our ability to make the diagnosis of an embolism in the segmental and sub segmental branches if the timing is wrong we may see mixing artifacts that can confound our ability to identify pulmonary embolism or to be confident that there is no pulmonary embolism if you do not administer a saline flush the superior vena cava often has dense contrast and causes streaked artifact across the pulmonary arteries which is another factor that can limit our ability to make the diagnosis unfortunately I think we all feel that sub-optimal exam do occur and they're unavoidable which can result in non diagnostic CT exams so one of the most important factors in performing these studies is getting the timing right and it's not something that you should be doing with a fixed delay we use bullish tracking knowing where to put the cursor and when to trigger the the timing based on what kind of a scanner you have is really critical and the principle is now if you watch the right arm you'll see that we are going to infuse contrast in the right arm into the axillary and subclavian veins into the innominate vein then into the superior vena cava and the right atrium and we really want to image this patient immediately after this as the contrast goes from the right ventricle into the pulmonary arteries that's when we take that is when we want to acquire the image and the timing can be difficult as you know it's usually less than 20 seconds once you get beyond that point you're in the arterial phase the systemic arterial phase and then you your exam your image quality begins to decrease so what's our technique we use between 80 and 120 MLS of contrast for larger patients we'll use a higher concentration to increase the enhancement level our infusion rate is at least 5 MLS per second if it's a large patient our technologist will actually infuse even above 5 MLS per second if they have a very good 18 gauge catheter and good venous access and in thin patients if you can perform flash mode if you have that type of scanner that really helps to to generate reliably good image quality what's the timing while depending on your the speed of your scanner if you're using a 64-slice scanner we put the cursor in the main pulmonary artery and trigger between 120 and 150 Hounsfield units with 128 slice scanner we trigger later because it's so fast that we can capture the peak of enhancement in just a few seconds so the trigger is closer to 200 pounds units so let's look at some some cases here's an example of excellent image quality this is what we're targeting here the contrast is in the pulmonary arteries only very little contrast in the aorta this was a 64-slice scanner the bullish tracker region of interest was placed in the main pulmonary artery and the scan was triggered at 143 Hounsfield units another case of excellent enhancement triggered 128 Hounsfield units and in this patient we can see that there is thrombus in both the right and left pulmonary arteries triggered at 155 pounds field units another example of excellent image quality with main right and left pulmonary artery clot and in this patient 64-slice scan triggered a little late 180 Hounsfield units still adequate but you can see that now the aortic enhancement is starting to approach the pulmonary artery enhancement and in this case this is a little bit later than ideally but it's still a good scan so one of the things that I tell the residents is it's really important to indicate the quality of the scan that should be the first line before they say whether there's thrombus or not because similar to a VQ scan where you may have low moderate or high probability if you have a low quality scan and you don't see a pulmonary embolism you know that your sensitivity is not as high as if you had an excellent quality scan so it's important for the clinicians to understand whether we have a really super bolus and no motion artifact and we are a hundred percent confident that we do not see an embolism and we're not missing one or do we have a lower quality study which happens unfortunately and in which case it's important for them to understand that there's a chance that we might not be seeing the pulmonary embolism so some more examples here's a 128 slight scanner perfect timing triggered at 199 housing units you can see there's almost no contrast in the aorta and it is all entirely within the pulmonary arteries another patient imaged on 128 slice scanner triggered a little late 290 Hounsfield units but still even though we have contrast in the aorta we still have excellent contrast enhancement level in the pulmonary arteries so here are some of the pitfalls of a bullish tracking the patient can move or breathe and then we're not even targeting the correct area for the trigger as in this case you can see that the the ROI is between the pulmonary artery in the aorta and as a result the scan is of lower quality because it's a look it's late with contrast in the aorta equivalent to the enhancement level in the pulmonary arteries here's another one where the patient moves or breathes and the cursor is moving from the pulmonary artery to the heart and really confounding our ability to trigger at the right time you see it's so it's gone from the pulmonary artery to the heart back to the pulmonary artery and then again back to the heart with the patient moving and breathing so this is one of the problems that even when you're trying to trigger at the right time and you think you've put the cursor in the right location this will really prevent you from getting a high quality study so here's an example where everything was done right 145 pounds file unit trigger on a 64 slice scanner and yet we still have suboptimal pacification so what's happening well we know that there are a number of cases when the patient valsalva and then as a result we get a lower quality enhancement level in the pulmonary arteries which is why we no longer tell our patients to take a deep breath in we just tell them to take a breath in breathe out and just stop reading so expiratory imaging is really very important to avoid this pitfall so those two pitfalls again and there are a number of papers that have been published showing the role for expert ory imaging and how during inspiration there's a variable increase in unup Asif eyed venous blood from the IBC briefly diluting the contrast column entering from the SVC during inspiration so expiratory imaging critical in this paper paper in the journal of thoracic imaging another paper published in emergency radiology Waiting to Exhale with the conclusion that extra Tory imaging for non diagnostic CT pulmonary artery a graffiti Ullman Airy arterial enhancement and improves the diagnostic quality of the study by eliminating this transient attenuation artifact paper and clinical radiology again emphasizing the role for expiratory phase imaging and these expire Tory scans showed greater attenuation at the pulmonary trunk right and left pulmonary arteries lower and segmental branches and the lower incidence of the transient contrast medium interruption as well as a lower incidence of unsatisfactory pulmonary artery opacification so important to remember tell the patient breathe in breathe out and stop breathing another critical parameter in technique is your reconstruction section and ideally these are less than one millimeter the earlier literature suggested that under two millimeters was adequate as you can see in this paper in 2002 that was published in radiology the results showed that the use of one millimeter section width resulted in substantially higher detection rate and greater agreement between different readers compared to two and three millimeter sections subsequent papers here is one using a sixteenth light scanner where they compared point seven five millimeter sections to two four and six and only the two millimeter sections had results comparable to the 0.75 millimeters but at present this is still not really considered adequate as we can see here's a study performed with 64-slice published in actor radiology in 2011 where they concluded that you should be using less than one millimeter section thickness so our reconstruction sections for pulmonary embolism imaging we do a thicker section five millimeters but then we do a 0.75 millimeter by 0.5 millimeter high resolution volume and that's our gold stand that we review for every case and that is really what you should be doing in practice it should be less than one millimeter reconstruction section here's an example look at how much easier it is to make the diagnosis image on the right with a very thin section gives you a high level of confidence that there is actually a pulmonary embolism there as opposed to just some mixing artifact another case three millimeter sections compared to 0.75 with the 0.75 you really can capture the thrombus at this bifurcation and make the diagnosis which would be easily missed on the 3 millimeter sections okay so I think I'm going to stop there and when I resume we're going to start discussing the different CT findings that you will see in patients with pulmonary embolism so thank you very much and I'll be back soon