Pulmonary embolism, as a disease that seriously affects people’s health, has aroused the general concern of the medical profession in various countries. Data show that the incidence of pulmonary embolism is very high, is the third largest disease in cardiovascular disease, second only to coronary heart disease and hypertension; the death rate is second only to tumors and myocardial infarction, accounting for the third cause of death; only 30% of the patients with pulmonary embolism were correctly diagnosed before they were born. Due to the high morbidity, mortality and disability of PTE, it is an important academic hot topic in current clinical medicine. In the past, this disease was considered to be a rare disease, which was mainly due to the lack of awareness of this disease, insufficient popularization of science, doctors’ lack of diagnostic awareness and poor diagnostic skills. In fact, the incidence rate is not low, but the detection rate is low. Therefore, fully raising the awareness of this disease and diagnostic consciousness, and rationally utilizing diagnostic techniques can further improve the diagnostic accuracy of this disease and improve the prognosis. I. Basic knowledge Pulmonary embolism refers to the clinical and pathophysiological syndromes caused by endogenous or exogenous embolism of the pulmonary artery or its branches resulting in pulmonary circulatory disorders. It is a general term for a group of diseases or clinical syndromes that have various emboli obstructing the pulmonary artery system as the cause of the disease, including pulmonary thromboembolism, lipoembolism syndrome, amniotic fluid embolism, and air embolism, etc., among which, pulmonary thromboembolism is the most common and much-needed hotspot topic. Pulmonary thromboembolism is a disease caused by the obstruction of the pulmonary artery and its branches by blood clots from the venous system or the right heart, with pulmonary circulation and respiratory dysfunction as its main clinical and pathophysiological features. It is the most common type of PE, accounting for more than 90% of the cases, and the substance of PE is usually called PTE. Pulmonary infarction: after the embolism of pulmonary artery, if necrosis occurs in the lung tissues in the area of its innervation due to the obstruction or interruption of blood flow, it is called pulmonary infarction. Embolization of pulmonary segmental arteries generally does not cause pulmonary infarction because the well-established bronchial arterial circulation is sufficient to maintain the blood supply to the area of pulmonary embolism, as well as direct alveolar oxygenation and retrograde pulmonary venous blood supply. Therefore pulmonary infarction is relatively rare, accounting for about 10-15% of PET. Sometimes solid changes occur due to extravasation of blood and filling of the alveoli with edema fluid, which generally do not cause necrosis of the lung parenchyma, and can be absorbed within 3-10 days with no fibrotic changes. II. Etiology and Mechanism PTE emboli mainly come from the inferior vena cava system (lower limb veins, pelvic veins) accounting for about 93%, followed by the superior vena cava system accounting for 4% and the right heart accounting for 3%. Unlike arterial thrombosis, endothelial damage is not an important factor in venous thrombosis, but slow stagnation of blood flow, local trauma and infection, increased viscosity of blood and reduced thrombolytic capacity are important pathogenic mechanisms. Common predisposing factors for clinical peripheral venous embolism: 1. Inactivity, such as post-trauma, post-surgery, and prolonged bed rest.2. Varicose veins, thrombophlebitis.3. Cardiac and pulmonary diseases. III. Clinical manifestations Mild cases can be asymptomatic, while severe cases show hypotension, shock, and even sudden death. Common symptoms include dyspnea, chest pain, hemoptysis, and syncope. There are two types of chest pain: pleuritic chest pain and angina-like chest pain. Exertional dyspnea 84%-90%, true typical pulmonary infarction triad less than 1/3. embolic site: bilateral > unilateral, multiple > single, lower lung > upper lung, right lung > left lung. Deep vein thrombosis of the lower limbs is a sign of pulmonary embolism. Physical examination reveals asymmetric swelling of both lower limbs, pressure and pain in the deep vein area, superficial vein dilatation, skin coloring, easy fatigue after walking and aggravation of swelling. Confirmation of the diagnosis mainly relies on imaging tests, and the imaging methods to confirm the diagnosis include SCTPA, MRPA, V/Q, PAA, ultrasound and so on. CT examination technology PTE has attracted more and more attention from the medical profession, and timely and accurate diagnosis and treatment can significantly improve the prognosis and reduce the mortality rate. Pulmonary arteriography has been regarded as the “gold standard” in clinical practice, with high sensitivity and specificity, but its wide application is limited by its high price, traumatization and complicated operation techniques. In recent years, with the widespread use of multislice CT (MSCT), the advantages of CT angiography (CTPA) have become increasingly apparent, and it has become the imaging method of choice for clinical diagnosis of cardiopulmonary vascular diseases. The recently introduced volumetric CT (Volume CT ,VCT), compared with the traditional MDCT scanning speed is faster, the coverage is larger, the image is more clear. At the same time of the launch of the Volume CT machine, the “chest pain triad” screening method was proposed, that is, on the VCT, the use of electrocardiographic gating technology, a layer thickness of 0.625, usually less than 10 seconds to complete the whole lung scanning from the tip of the lungs to the diaphragm, and to be able to simultaneously screen for pulmonary embolism, coronary heart disease, and thoracic aortic coarctation and other chest pain as the main symptoms of emergency diseases with high mortality rates. Highly lethal emergency lesions, buying time for patient care. (i) . Choice of Iodine Contrast: CT imaging is based on the difference in tissue attenuation coefficients, i.e., densities, to X-rays. Normally, the density difference between flowing blood, thrombus plaque and surrounding soft tissues is only rarely enough to be recognized by the naked eye, therefore, contrast agent must be introduced for enhancement scanning to increase the density difference between flowing blood and thrombus. Currently, there are two main types of iodine contrast agents used clinically for CT enhancement scanning, including ionic (e.g., panagliflozin, ankylglafen, etc.) and nonionic (e.g., iophorol, iophealol, ioparol, etc.), with the former having a high osmolality and certain toxic side effects. Therefore, CTPA examination of pulmonary embolism generally does not use ionic iodine contrast agent, and it is safest to use non-ionic iodine contrast agent, in order to avoid the toxic side effects of high osmolality of ionic contrast agent, to reduce the complications of enhancement scanning, and to improve the safety of the examination. (ii) Contrast injection technique: CTPA imaging requires iodine contrast agent, which is injected with a high-pressure syringe through a peripheral vein at a flow rate of 3-3.5 ml/s. It is recommended that an indwelling needle be used to avoid leakage or extravasation of the contrast agent caused by the high-pressure and high-flow rate. Elbow vein injection is mostly used, but due to the interference of high-density contrast artifacts in the superior vena cava and affecting the observation of the right upper lobe pulmonary artery, so there is also a choice of dorsal pedicle vein injection, but due to the individual differences in the time of the foot-pulmonary artery circulation, which affects the effect of scanning, especially for the low-grade helical CT. (iii) Contrast dosage: The classic scanning method of the adult conventional dosage of 1.5-2.0 ml/kg ( 300mgI/ml), and the total amount is usually 100~120ml, mainly because of its slow scanning speed and long scanning time. With the birth and popularization of the application of MSCT, since the scanning time has been greatly shortened, the use of high flow rate (4-5 ml/s) short time program can meet the diagnostic needs even with a small dose of contrast agent, and the pulmonary arteriovenous contrast is obvious. Iodine allergy test should be done routinely, and caution should be taken when there is a history of allergy to other drugs, and iodine contrast should be banned as an absolute contraindication in those with positive iodine sensitization, when MRI can be used to confirm the diagnosis. 98% of iodine contrast agent is excreted by the kidneys, about 38% is excreted 1 hour after intravenous injection, about 45% is excreted in 3 hours, about 83% is excreted in 6 hours, and basically all is excreted in 24 hours. Therefore, proper evaluation of the patient’s renal function should be done before the examination, and it should be used with caution for those with low renal function. (d) Delayed scanning time: refers to the time from the beginning of the injection of contrast medium to the beginning of the acquisition of CT scan data. Earlier single-row spiral CT generally does not have cycle time measurement and automatic trigger function, in this case can refer to the theoretical value. According to our experience delaying the start of scanning by 12-14 seconds (the time should be extended appropriately for those with severe pulmonary hypertension or right heart insufficiency), the results are more satisfactory. Recent MDCT and newer VCT devices support cycle time measurement, and the delay time should depend on the measured cycle time. It is also possible to directly set the threshold to enable the automatic trigger technology, but due to the triggering of the area of interest and the scanning start line there is a time difference between the bed and affect the image effect, so it is recommended that those who have the conditions should routinely use cycle time measurement. (e) Scanning technique: With the patient lying in the supine position, the patient was instructed to inhale as deeply as possible and then hold his/her breath, and a CT scan of the chest was performed routinely from the tip of the lungs to the diaphragm, followed by an enhancement scan. Enhanced scanning was performed from the aortic arch to the level of the upper diaphragm, including the subsegmental pulmonary arteries. Advocating dual time-phase scanning is easy to realize for MDCT and EBCT, while the earlier acquisition of single spiral CT, often the balloon cooling time is too long, at this time, it is appropriate to reduce the number of scanning layers and increase the number of bed feed. Layer thickness 2-4mm, pitch 1.5-2.0, 120-140KV, 200-250mA, FOV28-35cm, matrix 512×512. For MDCT and VCT, mostly 0.625mm layer thickness, 0.2 pitch. To ensure image quality, it is very important for the patient to hold his/her breath during scanning. In adult patients, holding their breath for 18 seconds or a little longer is tolerable, but patients with severe lung disease and dyspnea, the period of holding their breath is significantly shorter, and the imaging parameters should be adjusted accordingly. For single spiral CT can be appropriate to reduce the scanning range, increase the pitch; if the patient really can not achieve the required breath-holding time, they should be instructed to exhale slowly to ensure that the breathing is smooth, in order to minimize the artificial artifacts caused by respiratory movements. The latest MDCT and VCT scanning time is only 2-4 seconds, so even patients who cannot hold their breath on their own are able to obtain good quality images. It is customary to use a cephalad to pedunculopedic scanning direction, and some authors have suggested that a pedunculopedic to cephalad scanning direction is desirable to help minimize respiratory motion artifacts because the respiratory motion of the lung apices is relatively small. (vi) Image display techniques: Mainly window techniques, including mediastinal and pulmonary windows. For more typical PTE, conventional mediastinal window observation can be used, with a window width of 300-400 Hu and a window position of 40-50 Hu. For early and smaller PTE, conventional mediastinal window is not conducive to the observation of displaying emboli, and the window technique should be appropriately transformed and adjusted to the optimal state. The application of lung window mainly observes the changes of indirect signs such as lung texture, transmittance and perfusion, and attention should be paid to the contrast observation of both sides and the same side. (VII) Computerized post-processing The CT acquires two-dimensional data in the X-Y axis, and a single reconstruction gives a cross-sectional image, which has been able to meet the diagnostic needs of PTE. With the rapid development of CT hardware equipment and the continuous upgrading of computer software, make full use of the original data, especially MDCT isotropic volume scanning data, computer post-processing, and gradually become widely used. The routine should be based on cross-sectional images, supplemented by computer post-processing, which is accomplished by image reorganization. Commonly used post-processing methods include multiplanar reorganization, surface reorganization, maximum density projection, VIP, volume reproduction, surface reproduction, and simulated endoscopy. MPR is a simple, practical, and least time-consuming restructuring technique, and the basic principle is to use any cross-section to intercept the volume data and obtain a two-dimensional restructured image of any profile. The main point of the technique is bidirectional adjustment, otherwise it affects the diagnostic accuracy or causes misdiagnosis, such as bilateral pulmonary artery coronal MRR images, should be adjusted in the transverse plane of the bilateral pulmonary arteries symmetry, at the same time in the sagittal plane to adjust the upper and lower direction of the pulmonary arteries, so as to obtain a high-quality coronal image. CPR is an improved version of MPR, in which the centerline of the vessel is first manually drawn or the vessel trajectory is automatically tracked within the volumetric data to reconstruct a surface reorganization image along the vessel axis. It facilitates the observation of the lumen of curved vessels in all directions, and its greatest advantage is that the curved or non-planar vessels such as pulmonary arteries are shown in the same plane, which is the most favorable for the observation of the internal structure of the lumen of the vessel; the key is the need for isotropic volumetric data sources and careful drawing of the vessel centerline. The basic principle of the MIP technique is that the operator’s line of sight is projected onto the screen along the imagined position, through the volumetric data, and only the maximum CT value along the Cotton Field line of sight is retained by the computer during projection, and this technique is often used to prioritize the display of contrast-filled vascular structures or the skeletal system, and is usually operated using a multidirectional projection at certain intervals (5-15°) to obtain a multidirectional rotational view. to obtain a multidirectional rotational view. It is also a commonly used and effective post-processing method for CTPA, with the advantage of good reproducibility and comprehensive valuation of diseased vessels within the scanning range.Variations of this technique are the Minimum Density Projection for the Display of Airways and Gas-containing Intestines (MinMIP).The MIP has non-negligible limitations, one of which is that the density intensity of the displayed pixels represents only the maximal CT density in the projected line of sight, and overlap is inevitable, and other High-density structures will mask the vascular structure, especially the most typical effect of bone; second, MIP images can not resolve three-dimensional spatial relationships, can not display surface and deep structure information, and even affect the display of thrombus plaque; third, the increase in the average density intensity of the image background. In view of the above shortcomings, MIP images also need to be combined with original cross-sectional images or MPR images to have high diagnostic accuracy. VR is a new data image post-processing technology introduced in recent years, but also the result of the rapid development of computer hardware and software, the clinical application has gradually increased and achieved positive results. The basic principle is to utilize the light projection model for volumetric reproduction, where light is absorbed or reflected as it passes through the volumetric data, or the data itself emits additional light. The most important feature is the non-selective use of the entire volumetric data, which integrates the projection of each solvent voxel along the operator’s line of sight, and assigns the CT values that make up the image to different transparencies, or to be displayed as different luminance, or to be displayed as different colors, thus representing the spatial properties of different tissue types and their interrelationships with high fidelity. Brightness and color determine the luminance emitted by the volumetric data; transparency determines the absorption and reflection of light by the object.VR reconstructed 3D stereoscopic images with a strong sense of three-dimensionality are displayed more intuitively and can be observed from multiple angles, which makes it easy for clinicians to understand and apply. Attention should be paid to adjusting the window width and window center, transparency, brightness, shadow, and color. Some post-processing methods should be selected appropriately according to the condition of the equipment. V. CT cross-sectional anatomy of pulmonary artery The main pulmonary artery sends out the left and right pulmonary arteries, and after the pulmonary artery branches out of the lung door, the branches of the pulmonary arteries are mostly consistent with the branches of the trachea, so the naming of the pulmonary arteries adopts the naming of the branches of the bronchus, and the right pulmonary artery is divided into the upper, middle, and lower lobes of the branches of a total of 10 segments; and the left pulmonary artery is divided into the upper, lingual, and lower lobes of the branches of a total of 10 segments. Anatomy, multilayer serial readings can be analyzed branch by branch, and the pulmonary arteries can be reorganized using techniques such as MPR, CPR, and SVR. Numerous studies have shown that CTPA (including SCTPA, MDCTPA, VCTPA) can be analyzed up to the level of pulmonary artery segments for the diagnosis of PTE, and individual sub-segments can also be analyzed, but most of them are relatively fine, and can only be used for anatomical studies, but not for the diagnosis of PTE by the unevenness of its luminal development, and also have little practical clinical significance. Pulmonary artery CT cross-sectional anatomical reading and analysis is more difficult, mainly due to: 1. Lobes below the pulmonary artery openings and alignment variations, such as the right upper pulmonary artery can be an opening, two openings (cusp of the posterior segmental branch, the anterior segmental branch openings) and three openings (the cusp, the posterior, the anterior three segments are open). 2. The pulmonary arteries and veins appear simultaneously on enhanced CT, especially at the level of the lung segments, where the pulmonary arteries and veins are parallel. However, under normal circumstances, the vein wall is thin, the lumen is thicker than that of the artery at the same level, and the image is late and fainter, so it can be identified, and layer-by-layer tracing can also be used to recognize it. 3. The opening and distribution of pulmonary vessels on the left and right sides of the same level are asymmetrical, and there is no corresponding pattern of vascular variation on both sides. 4. Each lung segment has two sub-segments, which can be recognized by enhanced CT, but the diameter of the tubes is too small and it is difficult to recognize the intraluminal filling defects. 5. If the direction of pulmonary artery branching is parallel or diagonally intersected with the CT scanning level, it will bring great difficulties to the diagnosis, because the spatial volume effect will cause us to miss or misdiagnose the thrombus attached to the wall, the anterior segment of the upper lobe, the right middle lobe, and the left lingual segment. Multi sectional reorganization improves the recognition of pulmonary arteries and improves diagnostic accuracy, and careful observation of the absence of intravascular filling defects can exclude pulmonary embolism, but the diagnosis is still difficult to confirm in 9% of cases. VI. CT manifestations of pulmonary thromboembolism: From both direct and indirect signs. (i). Direct signs of pulmonary thromboembolism Direct visualization of emboli in the pulmonary arteries is the most reliable direct sign for the diagnosis of PTE. On CTPA images, the thromboembolus shows a marked difference in density compared to the contrast-containing blood in the enhanced pulmonary artery, which is manifested as a low-density filling defect. The CT manifestations of pulmonary embolism itself vary due to its variable size and shape and the varying duration of the disease. 1. Central thromboembolism: the thromboembolus is free in the lumen of the vessel. The embolus located in the center of the vessel shows a rounded low-density filling defect on the transverse-axial CT image, surrounded by a band of blood flow containing high-density contrast as the “target sign”; if it is parallel to the scanning plane, it shows the “double-track” sign, and multiple “target signs” are gathered as the “target sign”. If parallel to the scanning plane, there is a “double-track” sign, and multiple “target signs” are clustered in a “honeycomb” appearance. Movie CT examination can see the thrombus floating in the cavity, called “floating sign”, for acute PTE signs. 2. Complete thromboembolism: the thrombus basically completely blocks the pulmonary artery in the shape of a cup and irregularly rounded pestle. The lumen of the vessel is almost completely occupied by a low-density thrombus, with no surrounding ring-shaped high-density shadow or “double-track” sign. It is difficult to determine the degree of the old or new thrombus, but the diameter of the obstructing vessel of a new thrombus is fuller than normal, while the diameter of the obstructing vessel of a chronic thrombus is narrower than normal. 3. Partial or lateral filling defects: the filling defects of different degrees are located on one side of the pulmonary artery, which suggests old embolism. 4. wall-attached filling defect: embolus mechanization to a certain extent can be attached to the vessel wall, manifested as irregular thickening of the embolized vessel wall, low-density thrombus in a ring-shaped adherence to the wall of the pulmonary artery, the center of the strengthened pulmonary arterial blood flow, which is a sign of chronic pulmonary embolism. Of course, this acute and chronic is relative, sometimes it is impossible to distinguish; the same patient acute and chronic thrombus can exist at the same time. 5. Calcified embolism: calcification of organic thrombus can occur, calcified thrombus is shown on multilayer reorganization image, also a sign of chronic pulmonary embolism, the detection rate is about 10%. 6. Heart wall thrombosis: manifested as thrombus plaques in the atrial or ventricular wall and septum. (ii). Indirect signs: 1. “Mosaic” sign: enhancement scan lung parenchyma perfusion distribution is not uniform, embolism caused by regional vascular perfusion reduction and increased transmittance, and the formation of normal or over-perfused areas of significant density differences, constituting the lung field “black and white” phenomenon, called “mosaic” phenomenon. This is called the “mosaic” sign. Scanning shows that the embolized area has thinned vascular branches, sparse vascular texture, and increased transmittance of the lung field, which should be observed carefully by comparing the two sides. 2. Lung infarction: Lung infarction as a direct consequence of PTE is not common, typical for lung segmental infarction, with the two lower lungs as the most common site. The infarct foci appear as wedge-shaped solid shadows on CT, with the base close to the pleural or diaphragmatic surface and the tip pointing to the hilar, often accompanied by pleural reaction. In the acute stage, the edge of the lesion is blurred, and the follow-up observation shows that the lesion is absorbed from the hilar side, gradually absorbed to the pleural side, and finally completely absorbed or formed scarred cord shadow and pleural hypertrophy. 3. Pleural effusion: it mostly occurs on the same side of the infarction. In right cardiac insufficiency, pleural effusion mostly occurs in the right side of the chest first. The pleural surface of the lung tissue in the infarcted area can be seen in the lung window. 4. Signs of pulmonary hypertension: dilatation of the main pulmonary artery or/and the right and left pulmonary arteries, diameter of the main pulmonary artery > 1.5 times the diameter of the ascending aorta, compared with the thinning of blood vessels below the pulmonary segment, and enlargement of the right ventricle. (iii) Pulmonary Embolism-Reperfusion Injury: Pulmonary Embolism-Reperfusion Injury manifests itself primarily as reperfusion pulmonary edema, which increases the water content of the lung tissue. Reperfusion pulmonary edema is an acute, mixed, noncardiogenic pulmonary edema characterized by osmotic edema with diffuse alveolar injury.The pathophysiologic changes are mainly inflammatory response, pulmonary edema, and pulmonary dysfunction, including significant vascular dysfunction, especially disorders of the microvascular system and increased pulmonary arterial resistance. Inflammatory cells, inflammatory mediators and factors, and oxygen free radicals all cause damage to the vascular endothelium, alveolar epithelium, and interstitium, leading to the formation of interstitial edema and alveolar edema. （According to the number of embolized arteries and clinical manifestations, it is divided into two types: large-area PTE and non-large-area PTE Large-area PTE: (1). Embolization of 2 pulmonary lobar arteries or/and above, 7 pulmonary segmental arteries or/and above, whether or not accompanied by a drop in blood pressure. (2) Those with less than 2 pulmonary lobar arteries or 7 pulmonary segmental arteries accompanied by a drop in blood pressure (systolic blood pressure of 40 mmHg in the corporal circulation for more than 15 minutes, subject to the exclusion of new-onset arrhythmias, hypovolemia, or infectious toxicities with a drop in blood pressure). Non-massive PTE: Patients who do not meet the diagnostic criteria for massive PTE. Patients in this category who develop right heart insufficiency without hemodynamic disturbances are classified as submassive PTE.Massive PTE and submassive PTE are considered critical and severe PTE and generally require treatment with a rational therapeutic regimen. According to the location of the thrombus, it can be divided into three types: central, peripheral and mixed. Central type: pulmonary artery thrombus is located in the main pulmonary artery, right and left pulmonary arteries, and lobar arteries. Peripheral type: Pulmonary artery thrombus is located in the pulmonary segment and the pulmonary arteries below the pulmonary segment. Mixed type: pulmonary artery thrombus is located within the central and peripheral pulmonary arteries. VII. Differential Diagnosis: 1. Respiratory motion artifact: Rapid change in the position of the pulmonary arteries due to respiratory motion at the continuous level, which results in a hypodense shadow within the vessel due to partial volume effect, similar to a pulmonary embolism. 2. Flow-related artifacts: uneven mixing of contrast agent with blood in the pulmonary artery due to various reasons, resulting in the formation of a strip-shaped low-density shadow, similar to a pulmonary embolism. 3. Hard-beam artifacts: Radiation of different energies can produce beam artifacts, i.e., radiating low-density shadows, when passing through the superior vena cava containing a high concentration of contrast, which can cover and affect the display of the right upper pulmonary artery, so it is important to identify them. 4. Lymph nodes between pulmonary hilar and segmental: familiarize with the location of lymph nodes, and pay attention to analyze the direction of pulmonary vessels to help identify. 5. Circular low-density shadow can be seen around blood vessels in patients with heart failure, which may be perivascular edema, do not mistake it for chronic PTE. 6. Aortitis involving pulmonary artery, causing stenosis or occlusion of affected pulmonary artery, with sparse distal branching, but there is no change of aortic involvement, and there is no filling defect of thrombus formation in the pulmonary artery. Many studies have proved that CTA has high sensitivity and specificity for the diagnosis of PTE and high diagnostic accuracy. However, single-slice spiral CT scanning time is long, motion artifacts also affect the clarity of the image, and there is still the problem of not being able to diagnose subsegmental PTE. With the popularization and application of MDCT, VCT and EBCT, the image acquisition time is shorter, thus reducing motion artifacts, and the image can be taken at the time of the fullest vascular development, so that the image is clearer. Therefore, it has become the main diagnostic method of PTE or an important means to guide the treatment and evaluate the efficacy.