Yan Wenming, Department of Radiotherapy, Affiliated Hospital of Inner Mongolia Medical University
Reviewed by An Yongping and Sun Lihua Yan Wenming (reviewer)
Chinese Journal of Practical Medicine, 2007, Vol. 1, No. 1: 33-36
[Abstract] The use of spiral CT for continuous uninterrupted thin layer stereo volume scanning of the examined level including the target vessels, and then the use of computer image post-processing, and finally make the target vessels three-dimensional display of the vascular imaging technology.CTA is a special application of spiral CT, clinical practice shows that the rational application of CTA can provide similar diagnostic information as conventional angiography, and has the advantages of short scanning time, low complication incidence, etc. It has the advantages of short scanning time and low incidence of complications.
[Keywords]; Spiral CT; Angiography; X-ray computer
[I.C. Classification] R814.3, R814.42 [Article ID] B [Paper ID]
CT angiography (abbreviated as CTA) is a vascular imaging technique that uses spiral CT to continuously and uninterruptedly scan a thin layer of stereoscopic volume at the examined level, including the target vessel, after intravenous injection of contrast, and then uses a computer for image post-processing to finally display the target vessel in three dimensions [1].CTA is a special application of spiral CT, and clinical practice shows that the rational application of CTA can provide a diagnosis similar to that of The clinical practice shows that the rational application of CTA can provide diagnostic information similar to that of conventional angiography, and has the advantages of short scanning time and low complication incidence. However, to apply CTA flexibly to the diagnosis of various clinical diseases, a more comprehensive understanding of the basics of spiral CT, the best way to apply iodine contrast and computerized three-dimensional imaging technology is required [2], which is reviewed as follows.
I. Application techniques of CTA
1, Selection of imaging range The imaging range of CTA depends on the anatomical site we want to observe and the clinical diagnostic problem we want to solve. First, we pre-set the imaging range for axial scanning to determine the starting point and end point of CTA, however, as far as diagnosis is concerned, the larger the range covered by CTA, the better. In order to better respond to the lesion site and improve the clinical diagnosis rate, it is also necessary to select the appropriate imaging parameters.
2, Selection of imaging parameters The scan volume (V) of spiral CT = scan layer thickness (SW) × pitch (pitch) × continuous scan time (ST) [3], so layer thickness and pitch are two important parameters for our selection. Pitch is the ratio of the distance of bed movement to the width of the collimator when the spherical tube rotates 3600. The larger the layer thickness, the larger the volume covered in the same scan time, but the spatial resolution decreases; the smaller the layer thickness, the higher the spatial resolution and the smaller the volume covered in the same scan time. Intracranial CTA should show the smaller blood vessels, so the higher the spatial resolution, the better, with a layer thickness of 1mm is more appropriate. The abdominal vessels are thicker, and the volume covered is as large as possible, so the spatial resolution of 5 mm layer thickness can meet the requirements, and can cover a larger volume. The larger the pitch, the larger the volume covered by the scan at the same time, but the total amount of information is reduced, so the image resolution decreases; conversely, the smaller the pitch, the total amount of information increases significantly, and the image resolution also increases. CTA mostly takes 30 seconds as a scan interval for the following two reasons: (1) the thoracoabdominal CTA requires breath-holding scanning, and most patients can hardly hold their breath for more than 30 seconds; (2) the rated thermal capacity of the bulb does not allow too long continuous exposure under certain bulb current. exposure. Under certain scanning conditions, the smaller the field of view (FOV), the greater the spatial resolution, so the CTA should control the FOV within the minimum permissible range.
3, the use of contrast agents The appropriate use of intravenous contrast agents is another important factor in the success of CTA. A high-pressure syringe is a necessary piece of equipment because of the need to control the rate of administration, the amount of drug, and the delay time from the start of administration to the scan. For optimal enhancement of the target vessel at the time of scanning, with minimal impact of unrelated veins and parenchymal organs on the target vessel, it is crucial that the bolus enhancement process is matched to the hemodynamics of the subject. Due to individual differences, the circulation time must be measured first, as the approximation based on heart rate and/or blood pressure and different contrast agents is very imprecise and can cause about 40% of the vessels to fail to achieve optimal intensification. The procedure is as follows: 20 ml of iodine contrast is injected intravenously from the forearm at a rate of 5 ml/s, and an axial spiral scan is performed every two seconds at a predetermined test point between 8 and 30 seconds after bolus, from which a time density curve is drawn. This curve was used to determine the delayed scanning time after bolus injection of contrast agent during the spiral scan.
4, computer post-processing of the raw data after scanning (reconstruction of CTA) After scanning, the raw data are sent to the computer workstation for post-processing of CTA. In order to reduce the effect of partial volume effect, improve the detection rate of fine vessels, and make the 3D image smoother, the scanned data are to be reconstructed into cross-sectional images with 33% overlap. The volumetric data is obtained by spiral CT continuous volume scanning mode, and the original data can be reconstructed in various ways (MIP, SSD, VR, CPR) by 3D workstation. Vessels can be reconstructed in any level and direction.
At present, there are three clinical display methods of CTA: shaded surface disply (SSD), maximum intensity projection (MIP) and oblique or curve planar reformation (MIP). ). (1) For most cases, SSD can be synthesized directly from the reconstructed image without editing the original data, and the synthesis of SSD starts by setting a domain value, deleting those pixels above and below this domain value, and all the remaining pixels are processed by the computer to synthesize a manifestation image facing a certain direction of the light source. This surface reflection image is encoded in grayscale, and the SSD shows the tiny structures of the blood vessels and is valuable for the description of the overlapping areas of the blood vessels. Due to the selection of a single domain value, calcified spots cannot be distinguished from the contrast in the lumen of the vessel, and the stenosis of the vessel is incorrectly shown as a discontinuity when the CT value of the pixel at the stenosis site is lower than the domain value due to partial volume effect. (2) MIP images are similar to DSA and conventional angiography. Because the density of high-density structures such as bone and calcification is higher than that of contrast-containing vessels, it can be distinguished from vessels on MIP images. Contrast between contrast-filled vessels and surrounding tissues is obvious and can show the degree of stenosis, ulceration and plaque. MIP is a projection of each projection beam from a set direction of the volume data encoded by the maximum value to form a single projection image. . These images can also be displayed sequentially on the fluorescent screen in the form of a movie to dynamically observe the three-dimensional relationship. Since the image gray scale is coded according to the X-ray attenuation value , the limitations caused by domain value coding in SSD images do not exist in MIP images. The calcified spots on the vessel wall can be clearly distinguished from the intravascular contrast, and the limitation of MIP images is in the area of vessel overlap, where higher density vessels can obscure relatively low density vessels. Editing of the original image prior to MIP is helpful to improve the image quality of MIP. (3) Oblique (curved) composite image is a straight or curved line set on the tomographic image, and along this straight (curved) line, an oblique (curved) image perpendicular to the original tomographic image with one pixel thickness is synthesized. This image is mainly used to observe the morphology and travel of target vessels from the longitudinal axis. It is a complement to MIP and SSD techniques, which can show twisted vessels. However, oblique (curved) plane synthesis is largely dependent on the operator’s level, and there are human errors, such as inaccurate selection of oblique (curved) plane, or lesions being masked, or producing false-positive results.
II. Clinical application of CTA
1, Abdomen.
(1) Abdominal aorta: CTA is suitable for preoperative evaluation of aneurysms of the abdominal aorta, which can determine the size of the aneurysm and the extent of involvement, the branches of the involved artery and the degree of stenosis of the involved artery. cTA can do projection of different angles, so it is better than conventional angiography for observing the neck of pararenal and suprarenal aneurysms and the complex relationship with the surrounding area. For aortic coarctation, the involvement of the aortic branches and the compression of the true lumen by the false lumen can be outlined.
(2) Mesenteric artery: CTA has been proven by conventional angiography in showing stenosis of the celiac artery and superior mesenteric artery, and also clearly shows its collateral bypass vessels.
(3) Renal artery: CTA can accurately detect and show the stenosis of the renal artery, and its stenosis classification is basically consistent with conventional angiography, and MIP is superior to SSD in terms of reconstruction methods. dilatation of the renal artery after stenosis, and abnormalities in the size and density of the kidney after imaging increase the specificity of CTA, which fully indicates that the stenosis is greater than 70% and has produced meaningful hemodynamic changes.
(4) Intravascular metal stents: CTA can show intravascular stents and their grafts well. MIP-CTA can observe the relationship between stents and aortic branch vessels, but cannot show the inner lumen of stents, which is obscured by the high density of metal stents. The observation of metal stents with SSD-CTA is limited by the fact that SSD can only show the vessel shape that is thickened by the stent. Oblique (curved) tomography can be performed from the long axis of the stent, so it is considered to be a valuable method to evaluate the internal condition of metal stents, the presence of internal wall growth and stent deformation. Compared with conventional angiography, CTA can show the detachment of stent grafts and leakage of contrast around the grafts more clearly.
2, Pulmonary.
(1) Pulmonary embolism: As with conventional angiography, CTA of the pulmonary circulation can directly show thrombus in the lumen of the pulmonary vessels, manifesting as complete or partial filling defects, railroad track signs, etc. It has been confirmed in the literature that CTA is more accurate than MRI and conventional angiography in showing both central artery disease and pulmonary artery segments.
(2) Pulmonary aneurysm: CTA has a high diagnostic rate for this disease, which is manifested as a round pulmonary mass shadow with consistent central pulmonary artery enhancement. It can provide many preoperative and especially pre-embolization information that is very important before embolization, in addition to accurate localization, because the blood supply artery must be accurately shown before superselective embolization of pulmonary aneurysm [4], and both 3D SSD and MIP can help to show the size, morphology, number, presence or absence of aneurysm thrombosis and measurement of the internal diameter of the blood supply artery before embolization [5], making CTA an essential preoperative imaging modality for pulmonary aneurysms.
(3) Evaluation of peripheral pulmonary vessels by spiral CT: Spiral CT has been widely used for the evaluation of diffuse lung disease, but there are still limitations in the evaluation of microscopic nodules and lobular central lesions. Sliding thin block MIP is a more desirable technique ① It can display submillimeter vessels longer than the respective thin section. (ii) The MIP calculation method results in enhanced contrast resolution. ③The background mean value is maintained at a low level. ④No contrast injection and no background enhancement.
(4) Another clinical application of sliding thin block MIP is to show tiny intrapulmonary arteriovenous fistulas without enhancement [16].
(5) Congenital lung diseases: CTA also has some diagnostic value in showing congenital pulmonary vascular malformations and provides a reliable clinical basis for the treatment of congenital pulmonary vascular malformations.
(3) Liver: CTA of liver can clearly show the vascularity of liver tumors and provide morphological diagnostic basis for hepatic artery invasion. It can usually distinguish hepatocellular carcinoma from hepatic hemangioma.
4, Kidney: The main trunk of renal artery can be satisfactorily displayed by CTA, but the display of small arterial branches in the renal parenchyma is not satisfactory.
5, Cranial.
(1) Internal carotid artery: The accuracy of CTA depends on the compensation of calcified spots using image processing techniques, because in most arteries calcified spots are closely associated with stenosis. Automatic area growth techniques are used in SSD to eliminate calcified spots to show the site and extent of stenosis more clearly. It has been reported in the literature that the analysis of the original axial images in MIP-CTA without decomposition is meaningful to accurately determine the grade of stenosis. It has also been reported that the compliance rate between conventional angiography and CTA is only 50% if the amount of contrast and flow rate are not properly selected, the scan layer thickness is too wide (5 mm), and there is no calcification compensation method. The above results show that the use of CTA imaging technology plays a pivotal role in obtaining high-quality images and thus improving diagnostic accuracy.
AVA software reconstruction of blood vessels is an intelligent vascular analysis software, which is a reconstruction method combining MIP and multi-directional surface reconstruction. MIP images can observe the outline of the overall blood vessels, but cannot show the relationship between stenosis and surrounding bone and atherosclerotic plaque; turning tortuous vessels into images of straight vessels, while giving the corresponding inner diameter curve of vessels, can show the stenosis site, which can be selected on this image reference vessel, the beginning of the stenotic segment, and the software will automatically find out the most serious part of the stenosis. The multi-directional reconstructed image can observe the stenosis from all directions, and the area of the stenosis is automatically calculated on the image of the stenosis section perpendicular to the direction of vessel travel, which makes the calculation of the stenosis degree more accurate.
VR technology is based on the domain value technique, in which the computer reconstructs images of voxels within the selected domain value range. When there is calcification and bone influence, VR cannot satisfactorily display the vessels, such as vascular calcification, the stenosis around it is not satisfactorily displayed, the bone density is high, and in the parts where the bone is close to the vessels, such as the internal carotid artery around the skull base and the vertebral artery around the transverse foramen, the bone cannot be separated from the vessels due to partial volume effect, and the display of the vessels in the corresponding parts is affected. another limitation of the VR technique is related to the reconstructors’ technique. Improper selection of domain values can delete small vessels, exaggerate the degree of stenosis or display unsatisfactory vessels with obvious stenosis.
(2) Intracranial vessels: Preliminary studies have shown that CTA can be used to evaluate the ring of Willis and the vertebrobasilar system to show aneurysms, stenoses and congenital anomalies. For the display of the finer intracranial arteries, a 1 mm scan layer thickness with submillimeter reconstruction intervals is applied. For certain arterial branches near the skull base, bone structures should be removed prior to 3D reconstruction. Sliding thin-layer MIP is an excellent method for visualizing intracranial vessels.
It has been reported in foreign literature that the entire intracranial and extracranial vertebrobasilar arteries have been successfully demonstrated with subsecond spiral CT, 3D-SSD reconstruction. This will provide a broader prospect for the imaging diagnosis of carotid and vertebral artery stenosis by spiral CTA.
III. Discussion
The results of the study show that CTA can clearly show the trunk of the aorta, vertebral artery, renal artery, inferior vena cava, common carotid artery, internal carotid artery, external carotid artery; the trunk of cerebral artery, pulmonary artery, pulmonary vein, celiac artery and femoral artery and their grade 1-3 branches; the portal venous system (including superior mesenteric vein, splenic vein, main trunk of portal vein and grade 1-3 branches and 3 hepatic veins). -The reliability of CTA is good, and the compliance rate with conventional angiography is high, and the visualization of vessels by CTA is mainly related to the luminal contrast concentration, scanning conditions and the method of revascularization, in addition to the performance of spiral CT itself. (1) Intraluminal contrast concentration is related to the type of contrast agent, iodine content, total amount injected, flow rate, scan delay time, site of contrast agent injection and patient’s circulatory status. (2) The scanning direction is consistent with the direction of blood flow, such as scanning the thoracoabdominal aorta should be scanned from the cephalad side to the pedicle side, and scanning the portal vein and hepatic vein should be scanned from the pedicle side to the cephalad side to maximize the peak of intravascular contrast enhancement.
With the introduction of spiral CT and the continuous development and improvement of CTA technology, CTA has been widely used in various clinical disciplines, especially in the carotid artery and vertebrobasilar artery, for the following reasons: cerebrovascular disease is one of the three leading causes of death in humans today. A large number of clinical reports have shown that carotid stenosis in the extracranial segment is closely related to ischemic cerebrovascular disease, and intimal thrombosis or sclerotic plaque dislodgement in the carotid artery can cause transient ischemic attack (TIA) and clinical symptoms of stroke. Carotid stenosis >70% is considered severe stenosis and prophylactic carotid endarterectomy should be considered [6-8]. In the past, the diagnosis of carotid stenosis mainly relied on DSA. In recent years, noninvasive or minimally invasive imaging examinations such as DUS, MRA, and CTA have pioneered new methods for carotid vascularization, which are increasingly used for early detection, monitoring of carotid stenosis, and postoperative follow-up.
However, CTA has its own advantages and disadvantages compared with conventional angiography. DSA has been undoubtedly the gold standard for stenosis evaluation, but it has a 2% to 3% risk [9], with possible complications such as vasospasm due to cannulation and dislodgement of atherosclerotic plaque. the CTA technique does not require transarterial cannulation and is more acceptable for patients with no obvious symptoms and for elderly patients. The advantages of SCTA are that it can show the calcification and thrombus of the vessel wall, and combine with cross-sectional and multiplanar reconstruction images to show the lumen, wall and adjacent tissues more clearly; it can also perform three-dimensional imaging to show the three-dimensional morphology of vascular lesions and their relationship with surrounding structures more realistically; the examination method is simple and easy, non-invasive, and the examination time is shorter than DSA, and the cost is relatively low. The disadvantage of SCTA is that when the blood vessels to be examined are large, the total amount of contrast agent and the capacity of the bulb have certain limitations, and the scan can only be performed by increasing the layer thickness and pitch, which results in a decrease in signal-to-noise ratio and resolution. The result is a decrease in signal-to-noise ratio and resolution. A large number of facts show that SCTA can be one of the preferred methods to replace DSA for lesions of large vessels.
REFERENCES
1. Chia Jian-Fin, Xiao Ming, Wang Cheng-Yuan. Clinical application of spiral CT angiography. Radiology Practice 1997.12(4): 145-148
2. Kang YJ. An overview. CT angiography and its clinical application. Chinese Medical Imaging Technology, 1996.12(1):12
3. Stechling MK. lawrence JA. weintraub JL, et al. CT angiography: Expanded clinical applicationa. AJR, 1994, 163:947
4.Remy Jardin M, Remy J. Diagnosis of acute pulmonary embolism with spiral CT; Comparison with pulmonary angiography and Scintigraphy[J],Radiology: 1996.200:699-706
5. Johnson PT. Heath DG. CT angiography:thoracic Vasoular imaging with interactive Vloume rendering teehnique [J] J Comput Assist Tomogr,1997,21;110 -114
6. Esnol GF Fisher and the history of carotid artery desease. stroke, 1996.27:559
7. Polak GF, Kalina B. Donsldso MC. et al. Carotid endarterectomy; Preoperative evaluation of candidates with combined doppler sonography and MR angiography. Radiology. 1993, 186:333
8. Zhou D B, Cheng D Y, Xu B N, et al. Endarterectomy for carotid artery stenosis. Chinese Journal of Medicine. 1999. 79:816
9. Wang Peijun, Zhu Wenjiang, Tian Jianming, Lv Taozhen, Zuo Changjing, Wang Minjie, Yang Jijin, Xue Hong, Fan Yuelan. Spiral CT angiography study of aortic lesions. Chinese Medical Imaging Technology 1998 Vol. 14 No. 4: 305-306
10. Rubin GD. et al. CT angiography of the pelvis; aorts to femoral bifure ations. AJR, 1993.160:22
11.Napels. et al. CT angiography with spiral CT and maxinum intensity projection. Radiology.1992.185:607-610
12. Cline HE et al. 3D surface rendered MR images of brain and its vasculature. jACT. 1991,15:344-351
13.Marcus CD, ladam-Marcus VJ, Bigot JL, et al. Carotid arterial stenosis; evaluation at CT argiography with the volume-rendering technique. Radiology, 1999.211:775
14. Leclere X, Gauvrit YG. Pruvo JP. Usefulness of CT angiography with volume rendering after carotic angioplasty and stenting. aJR. 2000,174:820
15. Neri E. Cararnella D. Falaschi F,et al. Virtual CT intravascular endoscopy of the acata. pierced surface and floating shape thresholding artifats. Radiology. 1999.212:276
16. Lu X.Y., Zhang B.S., Wang D., Shi H.P., Xiong M.F., Song Y.L., Yu M., Fang H., Yang H. Spiral CT angiography and simulated endoscopic carotid artery. Chinese Journal of Medical Imaging 2001, Vol. 9, No. 3: 186-189
17. Tan Changlian, Li Detai, Liu Hui, Shen Shubin, Li Peng, Luo Jianguang. preliminary clinical application of CT angiography. Journal of Clinical Radiology 1997, Vol. 16, No. 6: 373-375
18. Liu Junxi. Overview. Clinical application of spiral CT angiography of pulmonary circulation. Chinese Medical Imaging Technology 2001, Vol. 17, No. 5: 477-479
19. Chia JF, Xiao M, Wang Cheng Yuan. Clinical application of spiral CT angiography. Radiology Practice 1997, 12(4): 145-148
20. Hong Z., Zhao D.F., Geng D.Y., Shen T.Z. Clinical application of electron beam CT angiography for evaluation of carotid artery stenosis. Chinese Journal of Medical Computer Imaging. 2000, Vol. 6, No. 2: 83-87
21. Pu YL, Luo DX, Li QF, Lu Y, Gao J, Zhang JL. Preliminary clinical application of double helix CT angiography. ct theory and application research 1995 Vol. 4 No. 3: 1-5