CT angiography

CT angiographic images need to be analyzed in different MIP projections (maximum intensity projections), MPR (multiplanar reconstruction), or 3D VRT reconstruction (volumetric imaging method). In these processing modes, a resolution with a pixel length of 0.5 mm in cross section (X-Y plane) and a higher resolution along the body axis (Z axis) is used. As a result, anisotropic voxels of different lengths are formed. The introduction in 2001 of multidetector CT scanners with sixteen-slice technology allowed to investigate a larger volume of the body length of the patient with obtaining almost isotropic wok-selenium up to 1 mm and an acceptable scanning time. The following pages present the recommended protocols for the study of various vascular pools with illustrative examples of CT imaging.

Intracranial arteries

After examining the axial sections, it is additionally necessary to use MIP, sagittal MPR and VRT. For a better evaluation of the arteries of the brain, the study is performed in thin sections with partial overlapping - a thickness of 1.0 - 1.25 m, a reconstruction interval of 0.6 - 0.8 mm. In order to obtain a high degree of contrast enhancement of blood vessels, the scan should be started immediately after the first portions of KB enter the vilysia circle, i.e., with a delay after injection of approximately 20 seconds, until the contrast medication is filled with venous sinuses. If the automatic bolus tracking mode is not used, it is necessary to test the contrast preparation to determine the individual circulation time of the KV. The following protocols can be used as a basis for visualization of the vilic circle:

Subsequent reconstruction of the sections can display vessels as a bottom view in the transverse MIP or as a front view in the coronal MIP. On these sections, large branches of the anterior and middle cerebral arteries are clearly visible.

Venous sinuses

To visualize the venous system, the volume of the area of interest must be expanded and include the cranial vault. The scan start delay is increased to 100 seconds. For both the arterial and venous phases, scans are performed in the craniocaudal direction. Median sagittal reconstruction is ideal for examining the contrasted veins of Galen and the venous outflow pathways of the brain.

Venous sinus thrombosis

With normal venous blood flow through the brain sinuses, you will find hyperdense lumens of both transverse sinuses of both sigmoid sinuses without any filling defects with contrast enhancement. The construction of 3D reconstructions and reconstructions in the MIP projection may be difficult due to the presence of high-density skull bones in the neighborhood. Often these reconstructions do not provide additional information.

Sleepy arteries

The most important condition for identifying the stenotic process of carotid arteries is the precise definition of the degree of stenosis. To do this, the study is carried out in thin sections, for example, 4 x 1 mm or 16 x 0.75 mm, allowing planimetric evaluation of stenosis with a sufficient degree of accuracy for specific axial sections. In addition, when constructing a sagittal or coronal MIP (reconstruction interval 0.7 - 1.0 mm, overlapping of sections of 50%), the stepped contour of the structures is not expressed.

To reconstruct the carotid arteries was of the highest quality, the contrast of the jugular veins should be minimal. Therefore it is necessary to use the program of automatic tracking of the bolus of CS. If, in the preliminary Doppler study, pathology is suspected in the field of bifurcation of carotid arteries, scans should be made in the caudocranial direction; with pathology at the base of the skull - in the craniocaudal. It is often useful to use VRT to better navigate the location of the anatomical structures.

Aorta

As mentioned above, CT angiography of the aorta is performed to exclude aneurysms, narrowing and possible stratification, and to determine the extent of the lesion. It is advisable to use automatic bolus tracking, especially in patients with cardiac pathology and changing the time of circulation of contrast medium in a small circle of circulation. The window for determining the threshold density value is located on the aorta just above the study area. To reduce respiratory artifacts affecting diaphragmatic aortic divisions, scans of the thoracic aorta are performed in the caudocrani-al direction, as involuntary respiratory movements are more likely at the end of the study. In addition, when exploring in the caudocranial direction, the initial venous inflow of the contrast medium is masked through the subclavian and brachiocephalic veins and their superposition on the arteries of the aortic arch.

How the construction of reconstructions of MIP and MPR, and MOB allow a full assessment of the pathology of the vessels. This is clearly seen in the example of the infrarenal aneurysm of the abdominal aorta. Aneurysmal enlargement begins immediately distal to the renal arteries, without affecting the superior mesenteric and iliac arteries.

When planning surgical treatment, it is important to have an idea of the involvement of visceral and peripheral arteries in the process, as well as the possibility of stratification. In addition, with the aneurysm of the descending thoracic aorta, it is necessary to take into account the involvement of the Adamkiewicz artery located at this level and blood supplying the spinal cord in the region of the thoracolumbar junction.

Often, a layered examination of coronal or sagittal MPR helps to quickly and accurately determine the prevalence of pathological changes, as in the case of thrombosis of the aneurysm of the abdominal aorta shown here. Individual axial sections allow an accurate planimetric assessment of the degree of stenosis, and on the sagittal MPR the trunk of the superior mesenteric artery is clearly visualized.

Of course, the benefit of the 3D VRT image depends on the viewing angle. If you look at this angle, you can underestimate the prevalence of thrombosis and, in the presence of plaques without calcification, it is easy to make mistakes. It is much better to evaluate the distribution of the process from different angles. The last image illustrates the result of visual removal of overlapping bone structures that interfere with the examination. The high density of the lumbar spine interferes with the assessment of vessel changes in the original image. This possibility appears only after visual removal of the lumbar vertebrae.

CT angiography (heart)

Coronary arteries

The visualization of the coronary arteries is a difficult task because of the contractions of the heart. This study requires a short scan time and accurate calculation. If the patient's heart rate exceeds 70 beats per minute, in the absence of contraindications premedication with beta-blockers should be prescribed. Even a shorter rotation time (0.42 s for a sixteen device at the time of publication of this book) requires an additional ECG interface. To ensure the quality of the diagnostic image, the dimensions of the visualization area are reduced to the size of the heart, and scanning in the craniocaudal direction should begin from the bifurcation of the trachea and continue to the diaphragm. The oblique MIP parallel to the trunk of the left coronary artery are special projections for the examination of PMA, PKA, and the study of three-dimensional reconstruction. Contrast substance should be administered biphasic, first a bolus of 40 ml at a rate of 4 ml / s, and after a pause of 10 s - a second bolus of 80 ml at a rate of 2 ml / s. It is necessary to use the automatic bolus tracking mode KB with the location of the density control window on the ascending aorta.

Search for calcification of coronary arteries

Comparison with traditional coronary angiography is illustrated on the previous page. The search for calcification of coronary arteries is carried out without the introduction of a contrast medium and with some increase in the thickness of the sections. Scan without amplification is carried out in the craniocaudal direction.

Determination of the amount of calcifications in the coronary arteries is best done on a special workstation, but it can be performed on a conventional workstation after preliminary image processing. Non-amplified images are used, for example, for the Agatston scale, which is used to determine the risk of coronary pathology.

Agatston scale
0	Sites of calcifications
	Are not determined
1-10	The minimum areas of calcifications
11-100	Clear lacunae of calcifications
101-400	Clearly expressed moderate areas of calcifications
> 400	Common areas of calcifications

Clinical significance

• There is no risk of coronary pathology in 90-95%
• Stenosis is unlikely
• Possible signs of coronary insufficiency
• Signs of coronary insufficiency due to possible stenosis
• High probability of coronary insufficiency due to possible stenosis

Pulmonary embolism

The topogram establishes the area of interest and the scan volume, which starts slightly above the aortic arch with visualization of the vessels of the roots of the lungs and the heart with the right atrium (a possible source of embolism). Lateral and apical parts of the lungs need not be examined. The total scan time should not exceed 15 seconds, so that the entire study can be carried out at one breathing delay of the patient and avoid the appearance of artifacts. The research direction is caudocranial, with the most mobile zones near the diaphragm to the last stage already completely scanned, and the artifacts of the venous inflow of contrast medium through the brachiocephalic veins and the upper vena cava are reduced. It is necessary to strictly observe the timekeeping of bolus tracking (the window of density control is installed above the pulmonary trunk). The reconstructed sections should be at least 3 mm wide, and the sections for MIP should be about 1 mm, so as not to miss even small, barely discernible PE.

Against the background of the lung tissue, the contrast in the lumen of the vessels is clearly visible, which is well visualized all the way to the periphery.

Vessels of the abdominal cavity

Most pathological changes in large vessels are localized in the area of their mouths. Therefore, on the topogram, the area under study can be confined to two-thirds of the central space of the abdominal cavity. The mouths of the main arteries of the abdominal aorta are well visualized on axial sections, as well as on MIP and MPR images. If a large length of sections along the Z axis is required, a 4 x 2.5 mm collimation is established for a four-slice tomograph, which provides an acceptable scan time for one breathing delay of the patient. However, if suspicion of stenosis of the renal arteries is necessary to reduce the amount of research to the area of the kidneys. To ensure adequate visualization of possible stenosis in the thin renal arteries, the study should be performed with a small cut thickness, for example 4 x 1 mm, and a reconstruction index of only 0.5 mm.

Since the time of the blood flow is individual and often varies, it is not recommended to prescribe a fixed injection delay of the contrast medium. In return, it is better to use a trial injection of contrast medium or automatic bolus tracking. The density control window (inflow of contrast medium = beginning of scanning) should be better located at the level of the lumen of the upper segment of the descending aorta.

With occlusion of the superior mesenteric artery, the lumen of the vessel is interrupted and the network of collateral vessels is determined , which is clearly visible on VRT and MIP images.

The ileum and femoral vessels

In CT angiography of the vessels of the ileum-femoral segment, the patient is placed with his feet forward (feet first). Determine the necessary length of the investigated region along the Z-axis. To accelerate the progress of the table, collimation is used 4 x 2.5 mm or 16 x 1.5 mm (instead of 4 x 1 mm or 16 x 0.75 mm). The minimum overlapping of slices guarantees a qualitative reconstruction of the obtained images.

There may be a problem with choosing a scan delay after injection of contrast medium, especially in the case of unilateral severe stenosis, due to a decrease in the rate of blood flow through the altered vessels. If automatic bolus tracking is used, the window for monitoring the density of passage of contrast medium with high concentration is located in the thoracic part of the descending aorta or in the abdominal aorta. In many cases it is good to inspect the vessels from aortic bifurcation to the ankles allowing VRT.

With obliterating lesions of peripheral arteries, both atherosclerotic plaques and the narrowing of the lumen of the vessels with a clear delay in the distal blood flow are determined as compared to the usual speed in the tibial vessels. In patients with a high degree of occlusive damage to peripheral vessels, the test is carried out at a table moving speed of not> 3 cm / s. Moreover, during craniocaudal scanning, the speed can be further slowed down, given the delay in the bolus approach of the contrast medium.

Vascular prosthesis visualization

CT angiography is also used to monitor implantable stents or vascular prostheses. With color duplex sonography, the acoustic shadow of the calcification of the vessel walls hinders the assessment of the available changes.

Prospects of CT angiography

CT angiography is subject to rapid changes due to the development of technology - primarily detectors and computers. It is already possible to predict the appearance of visualization workstations, with fully automated programs for accelerated reconstruction of VRT. The reconstructed images of the descending aorta or large vessels of the thoracic cavity of VRT and MIP shown here are even more common. All this will force the user of CT systems to keep up with technological progress and bring their clinical protocols of KTA research to the level of modern requirements.

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