Pulmonary embolism, as a disease that seriously affects people’s health, has attracted widespread attention from medical circles in various countries. Some data show that the incidence of pulmonary embolism is high, and it is the third most common cardiovascular disease, after coronary heart disease and hypertension; the death rate is second only to tumor and myocardial infarction, accounting for the third cause of death; only 30% of the patients with pulmonary embolism have been correctly diagnosed before death. Because of the high incidence, mortality and disability of PTE, it is an important academic hot topic of concern in clinical medicine. In the past, this disease was considered to be rare, which is mainly due to the lack of awareness of this disease, insufficient scientific publicity, poor diagnostic awareness of physicians and poor diagnostic skills. In fact, the incidence is not low, but the detection rate is low. Therefore, fully raising awareness of the disease and diagnostic techniques can further increase the diagnostic accuracy and improve the prognosis of the disease. Xiaofeng Tang, Department of Radiology, Yantai Mountain Hospital I. Pulmonary thromboembolism is a clinical and pathophysiological syndrome in which endogenous or exogenous emboli embolize the pulmonary artery or its branches, causing pulmonary circulatory disorders, and is a general term for a group of diseases or clinical syndromes in which various emboli obstruct the pulmonary arterial system as the cause, including pulmonary thromboembolism, fat embolism syndrome, amniotic fluid embolism, air embolism, etc. Among them, pulmonary thromboembolism is the most common and hot topic of concern. Pulmonary thromboembolism is a disease caused by obstruction of the pulmonary artery and its branches by thrombus 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 cases. Pulmonary infarction: After embolism of the pulmonary artery, if necrosis occurs in the lung tissue in its innominate area due to obstruction or interruption of blood flow, it is called pulmonary infarction. Embolism of a segmental pulmonary artery generally does not cause pulmonary infarction because a well-established bronchial arterial circulation is sufficient to maintain the blood supply to the pulmonary embolic zone, along with direct alveolar oxygenation and pulmonary venous countercurrent 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 alveoli with edema fluid, which usually does not cause necrosis of the lung parenchyma and can be absorbed within 3-10 days and does not produce 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 injury 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 vein thrombosis: 1. Inactivity, such as after trauma, surgery and long-term bed-ridden. 2. Varicose veins, thrombophlebitis. 3. Heart and lung diseases. III. Clinical manifestations Mild cases can be asymptomatic, while severe cases show hypotension, shock, or even sudden death. Common symptoms include dyspnea, chest pain, hemoptysis, syncope, etc. There are two types of chest pain: pleuritic chest pain and angina-like chest pain. Exertional dyspnea 84%-90%, less than 1/3 of the true typical pulmonary infarction triad. embolism site: bilateral > unilateral, multiple > single, lower lung > upper lung, right lung > left lung. Lower extremity deep vein thrombosis is the hallmark of pulmonary embolism. Physical examination reveals asymmetric swelling of both lower extremities, pressure pain in the deep vein area, dilatation of superficial veins, skin staining, easy fatigue and increased swelling after walking. Confirmation of diagnosis mainly depends on imaging examination, and the imaging methods for confirming the diagnosis are SCTPA, MRPA, V/Q, PAA, ultrasound, etc. CT examination technique PTE is drawing more and more attention from the medical community, and timely and accurate diagnosis and treatment can significantly improve prognosis and reduce mortality. Pulmonary arteriography has been considered the “gold standard” in clinical practice, with high sensitivity and specificity, but its wide application is limited by its high price, invasiveness and complexity of operation techniques. In recent years, with the widespread use of multilayer spiral CT (MSCT), the advantages of CT angiography (CTPA) technology have become increasingly apparent, and it has become the imaging method of choice for clinical diagnosis of cardiopulmonary vascular diseases. The recently introduced Volume CT (VCT) is faster than the traditional MDCT, with a larger coverage area and clearer images. With the introduction of Volume CT machine, the screening method of “chest pain triad” was introduced, i.e., the use of cardiac gating technology on VCT, with a layer thickness of 0.625, the whole lung scan from the lung tip to the diaphragm can be completed usually in less than 10 seconds, allowing simultaneous screening of pulmonary embolism, coronary artery disease and thoracic aortic coarctation, which have chest pain as the main symptom. Highly lethal emergency lesions such as pulmonary embolism, coronary artery disease and thoracic aortic coarctation are screened simultaneously, gaining time for patient treatment. (i) . Selection of iodine contrast agent: CT imaging is based on the difference in tissue attenuation coefficient to X-rays, i.e., density. Usually, the density difference between flowing blood, thrombotic plaque and surrounding soft tissues is only little and difficult to recognize with the naked eye, therefore, contrast agent must be introduced for enhancement scans to improve the density difference between flowing blood and thrombus. There are two main types of iodine contrast agents used clinically for CT-enhanced scans, including ionic (e.g., pantethine, ankigrafin, etc.) and nonionic (e.g., iophorol, iohexol, iopaprol, etc.), with the former having high osmotic pressure and certain toxic side effects. Therefore, CTPA examination for pulmonary embolism generally does not use ionic iodine contrast agent, and it is safest to use non-ionic iodine contrast agent to avoid toxic side effects caused by high osmotic pressure of ionic contrast agent, reduce complications of enhanced scanning, and improve the safety of examination. (ii) Contrast injection technique: Iodine contrast agent is required for CTPA imaging, which is injected through peripheral veins using a high-pressure syringe at a flow rate of 3-3.5 ml/s. It is recommended to use an intravenous needle to avoid leakage or extravasation of contrast agent due to high flow rate. Mostly the elbow vein is used for injection, but the observation of the right upper lobe pulmonary artery is affected by the interference of high-density contrast artifact in the superior vena cava, so the dorsal foot vein is also chosen for injection, but due to the individual differences in foot-pulmonary artery circulation time, which affects the scanning effect, especially for low-grade spiral CT. (iii) Contrast agent dosage: the classical scanning method for adults is routinely 1.5-2.0 ml/kg ( 300mgI/ml), and the total amount is usually 100~120ml, mainly due to its slow scanning speed and long scanning time. With the birth and popularization of MSCT, the high flow rate (4-5ml/s) short time protocol can meet the diagnostic needs even with a small dose of contrast agent, and the pulmonary arteriovenous contrast is obvious because the scan time has been greatly shortened. Iodine allergy testing should be done routinely, and caution should be exercised when there is a history of allergy to other drugs. A positive iodine test should be used as an absolute contraindication to prohibit iodine contrast, and MRI can be used to confirm the diagnosis at this time. Iodine contrast agent is 98% excreted by kidneys, about 38% in 1 hour, 45% in 3 hours, 83% in 6 hours and basically all in 24 hours after intravenous injection. Therefore, appropriate evaluation of the patient’s renal function should be done before the examination, and it should be used with caution in patients with low renal function. (iv) Delayed scan time: This refers to the time from the start of contrast injection to the start of CT scan data acquisition. Earlier single-row spiral CT generally does not have cycle time measurement and automatic trigger function, in which case the theoretical value can be referred to. According to our experience delaying the start of scanning by 12-14 seconds (the time should be appropriately extended for severe pulmonary hypertension or right heart insufficiency) gives more satisfactory results. The recent MDCT as well as the newer VCT devices support cycle time determination, and the delay time should depend on the measured cycle time. It is also possible to set the threshold directly to enable automatic trigger technology, but because there is a bed-walking time difference between the trigger area of interest and the scan start line that affects the image effect, it is recommended that those who have the conditions should routinely use the cycle time measurement. (v) Scanning technique: The patient is placed in the supine position, and the patient is instructed to hold his breath after inhaling as deeply as possible, and a CT scan of the chest is routinely performed from the lung tip to the diaphragm, followed by an enhancement scan. The enhancement scan is performed from the aortic arch to the level of the supra-diaphragm, including the sub-segmental pulmonary arteries. Advocating dual-temporal scanning is easy to achieve for MDCT and EBCT, while the single-spiral CT acquired earlier often has a long bulb cooling time, at which time the number of scanning layers can be appropriately reduced and the number of incoming beds increased. Layer thickness 2-4mm, pitch 1.5-2.0, 120-140KV, 200-250mA, FOV 28-35cm, matrix 512×512. for MDCT and VCT, mostly 0.625mm layer thickness, 0.2 pitch. To ensure image quality, it is important to hold the patient’s breath during scanning. In adult patients, it is tolerable to hold the breath for 18 seconds or a little longer, but in patients with severe pulmonary disease and respiratory distress, the breath-holding period is significantly shorter, and the imaging technical parameters should be adjusted accordingly. For single-helical CT, the scanning range can be reduced and the pitch increased appropriately; if patients really cannot achieve the required breath-holding time, they should be instructed to exhale slowly and ensure smooth breathing to keep the artificial artifacts caused by respiratory motion to a minimum. The latest MDCT and VCT scan times are 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 scanning direction from the cephalad side to the pedicle, and some authors have suggested that a scanning direction from the pedicle side to the cephalad side is appropriate, which can help reduce respiratory motion artifacts because of the relatively small respiratory motion at the lung tips. (vi) Image display technique: The main technique is windowing, including mediastinal and lung windows. For more typical PTE, a conventional mediastinal window with a window width of 300-400Hu and a window position of 40-50Hu can be used for observation. for early and smaller PTE, the conventional mediastinal window is not conducive to the observation of displaying emboli, and the window technique should be appropriately changed and adjusted to the best state. The application of the lung window mainly observes the changes of indirect signs such as lung texture, translucency and perfusion, and attention should be paid to the comparative observation of both sides and ipsilateral. (VII) Computerized post-processing CT acquires two-dimensional data in X-Y axis, and one reconstruction to obtain cross-sectional images 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, computer post-processing, which makes full use of the original data, especially the data acquired by MDCT isotropic volume scan, is gradually being widely used. Routine should be done mainly with cross-sectional images, supplemented by computer post-processing through image reorganization. Commonly used post-processing methods include multi-planar reorganization, surface reorganization, maximum density projection, VIP, volume reproduction, surface reproduction, simulation endoscopy, etc. MPR is a simple, practical and least time-consuming reorganization technique. The basic principle is to obtain a two-dimensional reorganized image of any profile by intercepting volume data with any cross-section. The key point of the technique is bi-directional adjustment, otherwise it affects the accuracy of diagnosis or causes misdiagnosis. For example, for bilateral coronal MRR images of the pulmonary arteries, the bilateral pulmonary arteries should be adjusted symmetrically in the cross-sectional plane, while the upper and lower directions of the pulmonary arteries should be adjusted in the sagittal plane, so as to obtain high-quality coronal images. CPR is a modified MPR, which first reconstructs a surface reconstructed image along the vessel axis by manually drawing the centerline of the vessel or automatically tracking the vessel trajectory within the volumetric data. It facilitates the observation of the lumen of curved vessels in all directions, and its biggest advantage is that curved or not in the same plane, such as the pulmonary artery, are shown in the same plane, which is most beneficial to the observation of the internal structure of the lumen; the key is the need for an isotropic volume data source and careful drawing of the centerline of the vessel. The basic principle of the MIP technique is to project the operator’s line of sight from an imaginary position along and through the volumetric data onto the screen; only the maximum CT value along the cotton field line of sight is retained by the computer during the projection process; this technique is often used to preferentially display contrast-filled vascular structures or the skeletal system, usually using multi-directional projections at certain angular intervals (5-15°) during the operation to obtain a multi-directional rotational view. It is also a commonly used and effective post-processing method for CTPA, which has the advantage of being reproducible and capable of comprehensively estimating the lesioned vessels within the scan area.Variations of this technique are Minimal Density Projection (Min MIP) showing airways and air-containing intestines.MIP has non-negligible limitations, one of which is that the intensity of the displayed pixel density represents only the maximum CT density on the projected line of sight, and there is bound to be overlap, and other High-density structures will obscure the vascular structure, especially the impact of the bone is most typical; the second is that MIP images can not resolve three-dimensional spatial relationships, can not show the surface and deep structural information, and even affect the display of thrombotic plaques; the third is to increase the average density intensity of the image background, etc. In view of the above shortcomings, MIP images also need to be combined with the original cross-sectional images or MPR images to have high diagnostic accuracy. VR is a new technology of data image post-processing introduced in recent years, which is also the result of rapid development of computer hardware and software, and the clinical applications are gradually increasing and achieving positive results. The basic principle is volume reproduction using a light projection model, where light is absorbed or reflected as it passes through the volume data, or the data itself emits additional light. The most characteristic feature is the non-selective use of the entire volume data, which integrates the projection of each solvent voxel along the operator’s line of sight, assigning different transparency to the CT values that make up the image, or being displayed as different luminance, or being displayed as different colors, thus representing the spatial characteristics of different tissue types and their interrelationships with high fidelity. The brightness and color determine the luminosity emitted by the volume data; the transparency determines the absorption and reflection of light by the object. 3D stereoscopic images reconstructed by VR have a strong sense of stereo, display more intuitive, and can be observed from multiple angles, making them easy to understand and apply by clinicians. Attention should be paid to adjusting the window width and window center, transparency, brightness, shadows, and colors. Some post-processing methods should be selected appropriately according to the equipment condition. V. Pulmonary artery CT cross-sectional anatomy The main pulmonary artery emanates from the left and right pulmonary arteries, and after exiting the pulmonary hilum the pulmonary artery branches mostly coincide with the tracheal branches, so the naming of pulmonary artery branches adopts the bronchial branch nomenclature, and the right pulmonary artery is divided into upper, middle and lower lobar branches with a total of 10 segments; the left pulmonary artery is divided into upper, lingual and lower lobar branches with a total of 10 segments.SCTPA and EBCTPA chest continuous volume scan can be used to analyze pulmonary artery cross-sectional Anatomy, multi-layer continuous reading allows branch-by-branch analysis, and pulmonary arteries can be reorganized using MPR, CPR and SVR techniques. Numerous studies have shown that CTPA (including SCTPA, MDCTPA, and VCTPA) can be used to diagnose PTE to the level of pulmonary artery segments, and individual subsegments can also be analyzed, but most of them are fine and can only be used for anatomical studies but not for the diagnosis of PTE by its luminal contrast inhomogeneity, and have little practical clinical significance. The CT cross-sectional anatomical reading and analysis of the pulmonary artery is difficult, mainly because: 1. There are many variations in the opening and course of the pulmonary artery below the lobe, for example, the right upper pulmonary artery can be an opening, two openings (posterior apical branch and anterior branch opening) and three openings (apical, posterior and anterior segments opening respectively). 2. Pulmonary arteries and veins appear simultaneously on the enhanced CT film, especially at the level of lung segments. However, under normal circumstances, the vein wall is thin and the lumen is thicker than that of the artery at the same level, so the image is late and faint, which can be identified, or can be identified by layer-by-layer tracing. 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 pulmonary segment has two subsegments, which can be identified by enhanced CT, but the tube diameter is too small to identify the intraluminal filling defect. 5. The direction of the pulmonary artery branch travel that is parallel or oblique to the CT scan level will make the diagnosis very difficult, because the spatial volume effect will make us miss or misdiagnose the thrombus of the attached wall, the anterior segment of the upper lobe, the right middle lobe, the left lingual segment, etc. Multi-sectional reorganization can improve the identification of the pulmonary artery and improve the accuracy of the diagnosis. Careful observation of the absence of filling defects in the vessels can exclude pulmonary embolism, but it is still difficult to confirm the diagnosis in 9% of cases. VI. CT manifestations of pulmonary thromboembolism: From both direct and indirect signs. (i). Direct signs of pulmonary thromboembolism The direct demonstration of intra-pulmonary emboli is the most reliable direct sign for the diagnosis of PTE. On CTPA images, thrombus emboli show a significant density difference compared to contrast-containing blood in the enhanced pulmonary artery, which shows a hypointense filling defect. The CT presentation of pulmonary embolism itself varies due to its size and morphology and the length of disease. 1. Central thromboembolism: thromboemboli are free in the lumen of the vessel. The thrombus located in the center of the vessel shows a circular low-density filling defect on the transaxial CT image, surrounded by a band of blood flow containing high-density contrast in the form of “target sign”, or “double track” sign if parallel to the scan plane, and multiple “target signs”. If parallel to the scan plane, it is a “double-track” sign, and multiple “target signs” are clustered in a “honeycomb” appearance. The floating of the thrombus in the lumen can be seen in the film CT examination, which is called “floating sign” and is a sign of acute PTE. 2. 2. Complete thromboembolism: The thrombus basically completely obstructs the pulmonary artery in a cupped, irregular, round pestle shape. The vessel lumen is almost completely occupied by low-density thrombus, and there is no circumferential high-density shadow or “double-track” sign. It is difficult to determine the degree of old and new thrombus, but the diameter of the obstructing vessel is fuller than normal in new thrombus, and narrower than normal in chronic thrombus. 3. Partial or lateral filling defect: Intraluminal filling defects of varying degrees are located on one side of the pulmonary artery, mostly suggesting old embolism. 4. wall-attached filling defect: the embolus can be attached to the vessel wall when it reaches a certain degree of mechanization, which shows irregular thickening of the embolized vessel wall and low-density thrombus adhering to the wall of the pulmonary artery in a ring shape with central reinforced pulmonary artery blood flow, which is a sign of chronic pulmonary embolism. Of course, this acute and chronic are relative and sometimes it is impossible to distinguish; acute and chronic thrombus can exist simultaneously in the same patient. 5. Calcific embolism: Calcification can occur in mechanized thrombi, and calcified thrombi are shown on multilayer reconstructed images, which is also a sign of chronic pulmonary embolism, with a detection rate of about 10%. 6. Wall thrombosis: It shows as thrombotic 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 translucency, and normal or over-perfusion area formed a significant density difference, constituting a “black and white” phenomenon in the lung field. This is called the “mosaic” sign. Plain scan shows thinning of vascular branches, sparse vascular texture and increased translucency in the lung field in the embolized area, which should be carefully observed by comparing both sides. 2. Pulmonary infarction: Pulmonary infarction as a direct consequence of PTE is not common, but typically it is a segmental infarction, with the two lower lungs as the preferred site. In CT cross-section, the infarct foci are wedge-shaped solid shadows with the base close to the pleural or diaphragmatic surface and the tip pointing to the hilum, often with pleural reaction. In the acute stage, the edges of the lesion are blurred, and follow-up observation shows that the lesion is absorbed from the hilar side of the lung and gradually absorbed to the pleural side, and finally completely absorbed or formed a scar and pleural hypertrophy. 3. Pleural effusion: mostly occurs on the same side of the infarction. Pleural effusion in right heart insufficiency mostly occurs first in the right side of the chest. The lung window shows the gross irregularity of the pleural surface of the lung tissue in the infarcted area. 4. Signs of pulmonary hypertension: manifestation of dilatation of the main pulmonary artery or/and right and left pulmonary arteries, diameter of the main pulmonary artery > 1.5 times the diameter of the ascending aorta, compared to the thinning of the vessels below the pulmonary segment, and enlargement of the right ventricle. (iii) Pulmonary embolism-reperfusion injury: Pulmonary embolism-reperfusion injury is mainly manifested by 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, and its pathophysiological 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 pulmonary interstitium, leading to the formation of interstitial and alveolar edema. (D) Clinical typing: According to the number of embolized arteries and clinical manifestations, there are two types of large PTE and non-large PTE Large PTE: (1). Those with embolization of 2 or/and more lobar arteries, 7 or/and more segmental arteries, with or without associated blood pressure drop. (2). Less than 2 lobar arteries or 7 segmental arteries with blood pressure drop (body circulation systolic blood pressure of 40 mmHg for more than 15 minutes, subject to exclusion of new onset arrhythmia, hypovolemia or drop in blood pressure in infectious toxicity). Non-major PTE: Patients who do not meet the diagnostic criteria for major PTE. In this type of patients, those who present with right heart insufficiency without hemodynamic disturbances are classified as submassive PTE. large PTE and submassive PTE are critical and severe PTE, and generally require reasonable therapeutic regimens for treatment. According to the location of the thrombus, there are three types: central, peripheral and mixed. Central type: Pulmonary artery thrombosis is located in the main pulmonary artery, right and left pulmonary arteries and lobar arteries. Peripheral type: Pulmonary artery thrombosis is located in the pulmonary segment and the pulmonary arteries below the pulmonary segment. Mixed type: Pulmonary artery thrombosis is located in the central and peripheral pulmonary arteries. VII. Differential diagnosis: 1. Respiratory motion artifact: rapid change in the position of the pulmonary artery due to respiratory motion at the continuous level, with hypointense shadowing within the vessel due to partial volume effect, similar to pulmonary embolism. 2. Flow-related artifact: Uneven mixing of contrast and blood in the pulmonary artery due to various reasons, resulting in a striped hypointense shadow, similar to pulmonary embolism. 3. Hard bundle artifact: Rays of different energies can produce shadow bundle artifacts when passing through the superior vena cava containing high concentration of contrast, i.e., radiolucent hypointense shadow, which will obscure and affect the display of the right upper pulmonary artery, so pay attention to the differentiation. 4. Lymph nodes between the pulmonary hilum and segments: familiarity with the location of lymph nodes and attention to the analysis of the direction of pulmonary vessels can help identify them. 5. Perivascular round-like hypointense shadow is seen in patients with heart failure, which may be perivascular edema and should not be mistaken for chronic PTE. 6. Aortitis involving the pulmonary arteries, causing stenosis or occlusion of the involved pulmonary arteries with sparse distal branches, but without altered aortic involvement or filling defect of thrombus formation in the pulmonary arteries. Many studies have demonstrated that CTA has high sensitivity and specificity for the diagnosis of PTE, and high diagnostic accuracy. However, single-layer spiral CT scan time is long, motion artifacts also affect image clarity, and there is still the problem of not being able to diagnose subsegmental PTE. With the popularization of MDCT, VCT and EBCT, the images are clearer because of shorter acquisition time, thus reducing motion artifacts, and the ability to acquire images when the vessels are most fully developed. As a result, it has become the main method for confirming the diagnosis of PTE or an important tool for guiding treatment and evaluating the efficacy of treatment.