CT angiography

CT angiographic images must be analyzed in different projections MIP (maximum intensity projection), MPR (multiplanar reconstruction) or VRT (volume rendering method) 3D reconstruction. These processing modes use a resolution with a pixel length in the cross-section of 0.5 mm (X-Y plane) and a higher resolution along the body axis (Z axis). This results in the formation of anisotropic voxels of different lengths. The introduction of multidetector CT scanners with 16-slice technology in 2001 made it possible to examine a larger volume of the patient's body length with almost isotropic voxels up to 1 mm and acceptable scanning times. The following pages present recommended protocols for examinations of various vascular territories with illustrative examples of CT images.

Intracranial arteries

After examining the axial sections, it is necessary to additionally use MIP, sagittal MPR and VRT. For a better assessment of the cerebral arteries, the study is performed using thin sections with partial overlap - thickness of 1.0 - 1.25 mm, reconstruction interval of 0.6 - 0.8 mm. To obtain a high degree of contrast enhancement of the vessels, scanning should be started immediately after the first portions of CB enter the circle of Willis, i.e. with a delay after injection of approximately 20 s, until the venous sinuses are filled with contrast agent. If the automatic bolus tracking mode is not used, a test injection of the contrast agent should be performed to determine the individual circulation time of the CB. The protocols presented below can be used as a basis for visualizing the circle of Willis:

Subsequent section reconstruction can display the vessels as a ventral view in transverse MIP or as an anterior view in coronal MIP. In these sections, the major branches of the anterior and middle cerebral arteries are clearly visible.

Venous sinuses

To visualize the venous system, the region of interest must be expanded to include the cranial vault. The delay in starting the scan is increased to 100 seconds. For both the arterial and venous phases, scanning is performed in the craniocaudal direction. Midsagittal reconstruction is ideal for examining the contrast-enhanced vein of Galen and the cerebral venous outflow tract.

Venous sinus thrombosis

With normal venous blood flow through the cerebral sinuses, you will find hyperdense lumens of both transverse sinuses and both sigmoid sinuses without any filling defects with contrast enhancement. Three-dimensional reconstructions and reconstructions in the MIP projection can be difficult to construct due to the presence of high-density skull bones nearby. Often these reconstructions do not provide additional information.

Carotid arteries

The most important condition for identifying the stenotic process of the carotid arteries is the precise determination of the degree of stenosis. For this purpose, the study is carried out using thin sections, for example, 4 x 1 mm or 16 x 0.75 mm, which allow planimetrically to clearly assess stenosis with a sufficient degree of accuracy for specific axial sections. In addition, when constructing sagittal or coronal MIP (reconstruction interval 0.7 - 1.0 mm, overlap of sections 50%), the stepped contour of the structures is not expressed.

To achieve the highest quality reconstruction of the carotid arteries, jugular vein contrast should be kept to a minimum. Therefore, it is essential to use the automatic bolus tracking program for the CS. If a pathology in the area of the carotid bifurcation is suspected during the preliminary Doppler examination, scanning should be performed in the caudocranial direction; in case of pathology at the base of the skull - in the craniocaudal direction. It is often useful to use VRT to better orientate oneself in the location of anatomical structures.

Aorta

As mentioned above, CT angiography of the aorta is performed to exclude aneurysms, stenosis, and possible dissection, as well as to determine the extent of the lesion. It is advisable to use automatic bolus tracking, especially in patients with cardiac pathology and changes in the circulation time of the contrast agent in the pulmonary circulation. The window for determining the threshold density value is located on the aorta just above the section being examined. To reduce respiratory artifacts affecting the peridiaphragmatic sections of the aorta, scanning of the thoracic aorta is performed in the caudocranial direction, since involuntary respiratory movements are more likely at the end of the examination. In addition, when examining in the caudocranial direction, the initial venous influx of the contrast agent through the subclavian and brachiocephalic veins and their imposition on the arteries of the aortic arch are masked.

Both MIP and MPR reconstructions and MOB allow for a full assessment of vascular pathology. This is clearly seen in the example of an infrarenal aneurysm of the abdominal aorta. The aneurysmal expansion 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, as well as the possibility of dissection. In addition, in case of aneurysm of the descending thoracic aorta, it is necessary to take into account the involvement of the artery of Adamkiewicz, located at this level and supplying the spinal cord in the thoracolumbar junction.

Often, layered examination of coronal or sagittal MPRs can quickly and accurately determine the extent of pathological changes, as in the case of the thrombosed abdominal aortic aneurysm shown here. Individual axial slices allow precise planimetric assessment of the degree of stenosis, and the sagittal MPR clearly visualizes the trunk of the superior mesenteric artery.

Of course, the usefulness of the 3D VRT image depends on the viewing angle. When viewed from this angle, the extent of thrombosis can be underestimated and, in the presence of plaques without calcification, it is easy to make a mistake. It is much better to evaluate the spread of the process from different angles. The last image illustrates the result of visually removing the overlapping bone structures that interfere with the examination. The high density of the lumbar spine makes it difficult to evaluate the vascular changes in the original image. This becomes possible only after visually removing the lumbar vertebrae.

CT angiography (heart)

Coronary arteries

Visualization of the coronary arteries is challenging due to the contraction of the heart. This examination requires short scanning times and accurate timing. If the patient's heart rate exceeds 70 bpm, premedication with beta blockers should be administered unless contraindicated. Even the shortened rotation time (0.42 s for a 16-slice device at the time of publication of this book) requires additional ECG coupling. To ensure the quality of the diagnostic image, the imaging volume is reduced to the size of the heart, and scanning in the craniocaudal direction should begin from the tracheal bifurcation and continue to the diaphragm. Oblique MIPs parallel to the left main coronary artery are special projections for examining the LAD, RCA and studying the 3D reconstruction. The contrast agent should be administered biphasically, 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 density control window positioned on the ascending aorta.

Search for coronary artery calcifications

A comparison with conventional coronary angiography is illustrated on the previous page. The search for coronary artery calcifications is performed without the introduction of a contrast agent and with some increase in the thickness of the sections. Scanning without amplification is performed in the craniocaudal direction.

Determination of the amount of calcification in the coronary arteries is best performed on a dedicated workstation, but can also be performed on a regular workstation after preliminary image processing. Unenhanced images are used, for example, for the Agatston scale, which determines the risk of coronary pathology.

Agatston scale
0	Calcification areas
	Not determined
1-10	Minimal areas of calcification are determined
11-100	Clearly expressed areas of loose calcification
101-400	Moderate areas of calcification are clearly visible
> 400	Common areas of calcification

Clinical significance

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

Pulmonary embolism

The area of interest and the scanning volume are determined based on the topogram, which starts slightly above the aortic arch with visualization of the vessels of the lung roots and the heart with the right atrium (a possible source of embolism). It is not necessary to examine the lateral and apical parts of the lungs. The total scanning time should not exceed 15 s, so that the entire examination can be performed during one breath hold of the patient and to avoid the appearance of artifacts. The direction of the examination is caudocranial, with the most mobile zones near the diaphragm already completely scanned by the last stage, and artifacts of the venous inflow of the contrast agent through the brachiocephalic veins and the superior vena cava are reduced. It is necessary to strictly adhere to the timing of bolus tracking (the density control window is installed above the pulmonary trunk). The reconstructed sections should be at least 3 mm wide, and the slices for MIP - about 1 mm, so as not to miss even small, barely visible 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, the area under study on the topogram can be limited 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 slices, as well as on MIP and MPR images. If a large length of slices along the Z-axis is required, a collimation of 4 x 2.5 mm is set for a four-slice tomograph, which ensures an acceptable scanning time for one breath-hold of the patient. However, if renal artery stenosis is suspected, it is necessary to reduce the examination volume to the kidney area. To ensure adequate visualization of possible stenosis in thin renal arteries, the examination should be performed with a small slice thickness, for example, 4 x 1 mm, and a reconstruction index of only 0.5 mm.

Since the blood flow time is individual and often varies, a fixed delay of contrast injection cannot be recommended. Instead, it is better to use a test injection of contrast or automatic bolus tracking. The density control window (contrast inflow = start of scanning) is best positioned at the level of the lumen of the upper descending aorta.

When the superior mesenteric artery is occluded, the lumen of the vessel is interrupted and a network of collateral vessels is identified , which is clearly visible on VRT and MIP images.

Iliac and femoral vessels

For CT angiography of the iliofemoral segment vessels, the patient is positioned feet first. The required length of the area to be examined along the Z-axis is determined. To speed up the table advancement, a 4 x 2.5 mm or 16 x 1.5 mm collimation is used (instead of 4 x 1 mm or 16 x 0.75 mm). Minimal overlapping of slices guarantees high-quality reconstruction of the resulting images.

The timing of the scan delay after contrast injection may be problematic, especially in cases of unilateral severe stenosis, due to decreased blood flow velocity through the affected vessels. If automatic bolus tracking is used, the density control window for high-concentration contrast is placed in the thoracic descending aorta or in the abdominal aorta. In many cases, VRT provides good visualization of the vessels from the aortic bifurcation to the ankles.

In obliterating peripheral arterial disease, both atherosclerotic plaques and narrowing of the vessel lumen are determined with a clear slowdown of distal blood flow compared to the normal speed in the tibial vessels. In patients with a high degree of occlusive peripheral vascular disease, the study is performed with a table advancement speed of no more than 3 cm/s. Moreover, during craniocaudal scanning, the speed can be further slowed down, taking into account the delay in the arrival of the contrast agent bolus.

Visualization of vascular prostheses

CT angiography is also used to monitor implanted stents or vascular prostheses. In color duplex sonography, the acoustic shadow of calcification of the vessel walls interferes with the assessment of existing changes.

Prospects of CT angiography

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

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