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Computed tomography: traditional, spiral
Last reviewed: 23.04.2024
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Computed tomography is a special type of X-ray examination, which is carried out by indirect measurement of attenuation or attenuation, X-rays from various positions, determined around the patient being examined. In essence, all we know is:
- that leaves the x-ray tube,
- what reaches the detector and
- what is the place of the x-ray tube and detector in each position.
Everything else follows from this information. Most CT cross sections are oriented vertically with respect to the axis of the body. They are usually called axial or cross sections. For each slice, the X-ray tube rotates around the patient, the slice thickness is preselected. Most CT scanners work on the principle of constant rotation with fan-shaped divergence of the rays. In this case, the X-ray tube and the detector are rigidly paired, and their rotational movements around the scanned area occur simultaneously with the emission and trapping of X-rays. Thus, X-rays, passing through the patient, reach the detectors located on the opposite side. The fan-shaped divergence occurs in the range from 40 ° to 60 °, depending on the apparatus, and is determined by the angle starting from the focal spot of the x-ray tube and expanding in the form of a sector to the outer borders of a series of detectors. Usually, an image is formed at each 360 ° rotation; the data obtained are sufficient for this. In the scanning process, attenuation coefficients are measured at many points, forming a attenuation profile. In fact, the attenuation profiles are nothing more than a set of received signals from all detector channels from a given angle of the tube-detector system. Modern CT scanners are capable of emitting and collecting data from approximately 1,400 positions of the detector-tube system on a 360 ° circle, or about 4 positions in degrees. Each attenuation profile includes measurements from 1500 detector channels, i.e. Approximately 30 channels in degrees, subject to a beam diverging angle of 50 °. At the beginning of the study, while advancing the patient's table at a constant speed inside the gantry, a digital X-ray image (“scan image” or “topogram”) is obtained, on which the desired sections can be planned later. With CT examination of the spine or head, the gantry is turned at the right angle, thereby achieving the optimal orientation of the sections.
Computed tomography uses complex X-ray sensor readings, which rotate around the patient in order to obtain a large number of different images of a certain depth (tomograms), which are digitized and converted into cross-images. CT provides 2- and 3-dimensional information that cannot be obtained with a simple X-ray and with much higher contrast resolution. As a result, CT has become a new standard for imaging most of the intracranial, head and neck, intrathoracic and intra-abdominal structures.
Early samples of CT scanners used only one X-ray sensor, and the patient passed through the scanner incrementally, stopping for each shot. This method was largely replaced by a helical CT scan: the patient moves continuously through a scanner that rotates continuously and takes pictures. Screw CT greatly reduces display time and reduces plate thickness. Using scanners with multiple sensors (4-64 rows of x-ray sensors) further reduces the display time and provides a plate thickness of less than 1 mm.
With so many displayed data, images can be recovered from almost any angle (as is done in MRI) and can be used to create 3D images while maintaining a diagnostic image solution. Clinical applications include CT angiography (for example, for assessment of pulmonary embolism) and cardiovascularization (for example, coronary angiography, assessment of coronary artery hardening). Electron-beam CT, another type of rapid CT, can also be used to evaluate coronary hardening of the artery.
CT scans can be taken with or without contrast. Non-contrast CT scan can detect acute hemorrhage (which appears bright white) and characterize bone fractures. Contrast CT uses IV or oral contrast, or both. IV contrast, similar to that used in simple X-rays, is used to display tumors, infections, inflammation and injuries in soft tissues and to assess the state of the vascular system, as in cases of suspected pulmonary embolism, aortic aneurysm or aortic dissection. Excretion of contrast through the kidneys allows an assessment of the urinary system. For information on contrast reactions and their interpretation.
Oral contrast is used to display the abdominal area; it helps to separate the intestinal structure from others. Standard oral contrast - a contrast based on barium iodine, can be used when intestinal perforation is suspected (for example, in the event of injury); low osmolar contrast should be used when the risk of aspiration is high.
Radiation exposure is an important issue when using CT. The radiation dose from a conventional abdominal CT scan is 200 to 300 times higher than the radiation dose received with a typical x-ray of the thoracic region. CT today is the most common source of artificial exposure for the majority of the population and accounts for more than 2/3 of the total medical exposure. This degree of human exposure to radiation is not trivial, the risk of exposure of children today exposed to radiation from CT, for their entire life, is estimated to be much higher than the degree of exposure to adults. Therefore, the need for CT examination should be carefully weighed, taking into account the possible risk for each individual patient.
Multispiral computed tomography
Spiral computed tomography with multi-row detector arrangement (multispiral computed tomography)
Computer tomographs with a multi-row detector arrangement belong to the latest generation of scanners. Opposite the x-ray tube there is not one, but several rows of detectors. This makes it possible to significantly shorten the study time and improve the contrast resolution, which allows, for example, to more clearly visualize the contrasted blood vessels. The rows of Z-axis detectors opposite the X-ray tube are different in width: the outer row is wider than the inner one. This provides the best conditions for image reconstruction after data collection.
Comparison of traditional and spiral computed tomography
With traditional computed tomography, a series of consecutive equally spaced images are obtained through a specific part of the body, for example, the abdominal cavity or the head. Mandatory short pause after each slice to move the table with the patient to the next predetermined position. Thickness and overlap / intercut spacing are preselected. The raw data for each level is saved separately. A short pause between the cuts allows the patient, who is conscious, to take a breath and thus avoid gross respiratory artifacts in the image. However, the study may take several minutes, depending on the scan area and the size of the patient. It is necessary to choose the right time to obtain the image after IV injection of the CS, which is especially important for evaluating the perfusion effects. Computed tomography is the method of choice for obtaining a full-fledged two-dimensional axial image of the body without interference created by imposition of bone tissue and / or air, as is the case on an ordinary radiograph.
With spiral computed tomography with a single-row and multi-row detector arrangement (MSCT), patient research data is collected continuously during the table advancing inside the gantry. The x-ray tube then describes the screw trajectory around the patient. The table advancement is coordinated with the time required for 360 ° tube rotation (helix pitch) - data collection continues continuously in full. Such a modern technique significantly improves tomography, because respiratory artifacts and interruptions do not affect a single data set as significantly as with traditional computed tomography. A single raw data base is used to recover slices of various thickness and different intervals. Partial overlapping of sections improves the possibilities of reconstruction.
Data collection in the study of the entire abdominal cavity takes 1 - 2 minutes: 2 or 3 spirals, each lasting 10-20 seconds. The time limit is due to the patient's ability to hold his breath and the need to cool the x-ray tube. Some more time is needed to recreate the image. When evaluating the function of the kidneys, a short pause is required after the injection of the contrast agent to wait for the excretion of the contrast agent.
Another important advantage of the spiral method is the ability to identify pathological formations smaller than the thickness of the slice. Small metastases in the liver can be missed if, as a result of the unequal depth of the patient's breathing, they do not fall into a section during the scan. Metastases are well identified from the raw data of the spiral method in the recovery of sections obtained with the imposition of sections.
[8]
Spatial resolution
Image restoration is based on differences in the contrast of individual structures. Based on this, an image matrix of the imaging area of 512 x 512 or more image elements (pixels) is created. Pixels appear on the monitor screen as areas of different shades of gray depending on their attenuation coefficient. In fact, these are not even squares, but cubes (voxels = volume elements), having a length along the body axis, according to the thickness of the slice.
The image quality increases with the reduction of voxels, but this only applies to spatial resolution, further thinning of the slice reduces the signal-to-noise ratio. Another drawback of thin sections is an increase in the patient's dose. However, small voxels with the same dimensions in all three dimensions (isotropic voxel) offer significant advantages: multiplanar reconstruction (MPR) in coronal, sagittal or other projections is shown in the image without a stepped contour). The use of voxels of different sizes (anisotropic voxels) for MPR leads to the appearance of jaggedness of the reconstructed image. For example, it may be difficult to rule out a fracture.
Spiral pitch
The pitch of the helix characterizes the degree of movement of the table in mm per rotation and the thickness of the slice. Slow progress of the table forms a compressed spiral. Accelerating the movement of the table without changing the slice thickness or rotational speed creates a space between the cuts on the resulting helix.
Most often, the pitch of the helix is understood as the ratio of the displacement (supply) of the table with the turnover of the gantry, expressed in mm, to collimation, also expressed in mm.
Since the dimensions (mm) in the numerator and denominator are balanced, the pitch of the helix is a dimensionless quantity. For MSCT for t. Volumetric spiral pitch is usually taken as the ratio of table feed to single slice, and not to the full set of slices along the Z axis. For the example that was used above, the volumetric spiral pitch is 16 (24 mm / 1.5 mm). However, there is a tendency to return to the first definition of the helix pitch.
New scanners provide an opportunity to choose the craniocaudal (Z axis) expansion of the study area according to the topogram. Also, the tube turnover time, collimation of the cut (thin or thick cut) and the time of the test (breath hold) are adjusted as necessary. Software, such as SureView, calculates the corresponding helix pitch, usually setting a value between 0.5 and 2.0.
Slice collimation: resolution along the Z axis
Image resolution (along the Z axis or the patient's body axis) can also be adapted to a specific diagnostic task using collimation. Sections from 5 to 8 mm thick fully comply with the standard examination of the abdominal cavity. However, the exact localization of small fragments of bone fractures or the assessment of subtle pulmonary changes require the use of thin sections (from 0.5 to 2 mm). What determines the thickness of the slice?
The term collimation is defined as obtaining a thin or thick slice along the longitudinal axis of the patient's body (Z axis). The doctor may limit the fan-shaped divergence of the radiation beam from the x-ray tube to a collimator. The hole size of the collimator controls the passage of the rays that fall on the detectors behind the patient in a wide or narrow stream. The narrowing of the radiation beam can improve the spatial resolution along the patient's Z axis. The collimator can be located not only immediately at the exit of the tube, but also directly in front of the detectors, that is, “behind” the patient, if viewed from the side of the x-ray source.
A collimator-dependent system with a single row of detectors behind the patient (single cut) can perform cuts 10 mm, 8 mm, 5 mm thick or even 1 mm thick. A CT scan with very thin cross sections is referred to as “High Resolution CT Scan” (VRKT). If the slice thickness is less than a millimeter, they say about “Ultra High Resolution CT” (SVRKT). The SURCT used to study the pyramid of the temporal bone with slices about 0.5 mm thick reveals fine fracture lines passing through the base of the skull or the auditory ossicles in the tympanic cavity. For the liver, high-contrast resolution is used to detect metastases, and slices of somewhat greater thickness are required.
Detection Arrangements
Further development of the single-slice spiral technology led to the introduction of a multislice (multislice) technique, in which not one but several rows of detectors are used, which are located perpendicular to the Z-axis opposite the x-ray source. This makes it possible to simultaneously collect data from several sections.
Due to the fan-shaped divergence of the radiation, the rows of detectors should have different widths. The layout of the detectors is that the width of the detectors increases from the center to the edge, which allows varying the thickness and number of sections obtained.
For example, a 16-slice study can be performed with 16 thin slices of high resolution (for Siemens Sensation 16 this is a 16 x 0.75 mm technique) or with 16 sections of twice the thickness. For ileo-femoral CT angiography, it is preferable to obtain a volumetric slice in one cycle along the Z axis. At the same time, the collimation width is 16 x 1.5 mm.
The development of CT scanners did not end with 16 slices. Data collection can be accelerated using scanners with 32 and 64 rows of detectors. However, the tendency to reduce the thickness of the sections leads to an increase in the patient's dose, which requires additional and already feasible measures to reduce the effects of radiation.
In the study of the liver and pancreas, many experts prefer to reduce the thickness of the sections from 10 to 3 mm to improve the sharpness of the image. However, this increases the interference level by approximately 80%. Therefore, in order to preserve the image quality, one must either additionally add the current strength on the tube, i.e., increase the current strength (mA) by 80%, or increase the scanning time (the product increases by mAs).
Image reconstruction algorithm
Spiral computed tomography has an additional advantage: in the process of image restoration, most data are not actually measured in a particular slice. Instead, measurements taken outside this slice interpolate with most of the values near the slice and become the data assigned to that slice. In other words: the results of data processing near the slice are more important for reconstructing the image of a specific section.
An interesting phenomenon follows from this. The patient dose (in mGr) is defined as mAs per rotation divided by the helix pitch, and the dose per image is equivalent to mAs per rotation without considering the helix pitch. If, for example, settings of 150 mAs per rotation with a pitch of 1.5 are set, then the patient dose is 100 mAs, and the dose per image is 150 mAs. Therefore, the use of spiral technology can improve the contrast resolution by choosing a high mAs value. In this case, it becomes possible to increase the image contrast, tissue resolution (image clarity) by reducing the slice thickness and select such a step and length of the helix interval so that the patient dose decreases! Thus, a large number of slices can be obtained without increasing the dose or the load on the X-ray tube.
This technology is especially important when converting received data into 2-dimensional (sagittal, curvilinear, coronal) or 3-dimensional reconstructions.
Measurement data from the detectors are passed, profile by profile, to the electronic part of the detector as electrical signals corresponding to the actual attenuation of x-rays. Electrical signals are digitized and then sent to the video processor. At this stage of image reconstruction, the “conveyor” method is used, consisting of preprocessing, filtering and reverse engineering.
The preprocessing includes all corrections made to prepare the obtained data for image recovery. For example, correction of dark current, output signal, calibration, track correction, increase in radiation rigidity, etc. These corrections are made to reduce variations in the operation of the tube and detectors.
Filtering uses negative values to correct image blur, inherent in reverse engineering. If, for example, a cylindrical water phantom is scanned, which is recreated without filtering, its edges will be extremely vague. What happens when the eight attenuation profiles overlap each other to restore the image? Since some part of the cylinder is measured by two combined profiles, instead of a real cylinder, a star-shaped image is obtained. By entering negative values outside the positive component of the attenuation profiles, it is possible to achieve that the edges of this cylinder become clear.
Reverse engineering redistributes the minimized scan data into a 2-dimensional image matrix, displaying broken sections. This is done, profile by profile, until the process of recreating the image is completed. The image matrix can be represented as a chessboard, but consisting of 512 x 512 or 1024 x 1024 elements, usually called "pixels". As a result of reverse engineering, each pixel exactly corresponds to a given density, which on the monitor screen has various shades of gray, from light to dark. The brighter part of the screen, the higher the density of the tissue within a pixel (for example, bone structures).
Effect of voltage (kV)
When the studied anatomical region is characterized by a high absorption capacity (for example, CT scan of the head, shoulder girdle, thoracic or lumbar spine, pelvis, or just a full patient), it is advisable to use increased voltage or, instead, higher mA values. When choosing a high voltage on the x-ray tube, you increase the rigidity of the x-ray radiation. Accordingly, X-rays are much easier to penetrate the anatomical region with a high absorption capacity. The positive side of this process is the reduction of low-energy radiation components that are absorbed by the patient’s tissues without affecting the image acquisition. It may be advisable to use a lower voltage for examining children and tracking a KB bolus than in standard installations.
[20], [21], [22], [23], [24], [25]
Tube current (mAs)
The current, measured in milliampere-seconds (mAc), also affects the patient's exposure dose. For a large patient to obtain a high-quality image, an increase in the tube current strength is required. Thus, a corpulent patient receives a greater dose of radiation than, for example, a child with noticeably smaller body sizes.
Areas with bone structures that more absorb and diffuse radiation, such as the shoulder girdle and pelvis, need more tube current than, for example, the neck, abdominal cavity of a thin person or leg. This dependence is actively used in radiation protection.
Scan time
The shortest scanning time should be chosen, especially when examining the abdominal cavity and chest, where contractions of the heart and intestinal peristalsis can degrade image quality. The quality of CT examination also improves as the likelihood of patient involuntary movements decreases. On the other hand, it may be necessary to scan longer to collect enough data and maximize spatial resolution. Sometimes the choice of an extended scan time with a decrease in amperage is deliberately used to prolong the life of the x-ray tube.
3D reconstruction
Due to the fact that the volume of data for the whole area of the patient's body is collected during spiral tomography, the visualization of fractures and blood vessels has improved markedly. Apply several different methods of three-dimensional reconstruction:
Maximum intensity projection (Maximal Intensity Projection), MIP
MIP is a mathematical method by which hyperintensive voxels are extracted from a two-dimensional or three-dimensional data set. Voxels are selected from a set of data obtained by iodine at various angles, and then projected as two-dimensional images. The three-dimensional effect is obtained by changing the projection angle with a small step, and then, visualizing the reconstructed image in quick succession (i.e., in the dynamic viewing mode). This method is often used in the study of blood vessels with contrast enhancement.
Multiplanar Reconstruction, MPR
This technique makes it possible to reconstruct the image in any projection, be it coronal, sagittal or curvilinear. MPR is a valuable tool in fracture diagnosis and orthopedics. For example, traditional axial slices do not always provide complete information about fractures. The subtlest fracture without displacing the fragments and disturbing the cortical plate can be more effectively detected with the help of MPR.
Three-dimensional reconstruction of shaded surfaces (Surface Shaded Display), SSD
This method recreates the surface of an organ or bone defined above a given threshold in Hounsfield units. Selecting the angle of the image, as well as the location of the hypothetical light source, is a key factor for obtaining optimal reconstruction (the computer calculates and removes shadowing areas from the image). A fracture of the distal part of the radial bone, demonstrated by MPR, is clearly visible on the surface of the bone.
Three-dimensional SSD is also used when planning a surgical procedure, as in the case of a traumatic spinal fracture. Changing the angle of the image, it is easy to detect a compression fracture of the thoracic spine and assess the condition of the intervertebral holes. The latter can be explored in several different projections. On the sagittal MND, a bone fragment is visible, which is displaced into the spinal canal.
Basic rules for reading computed tomograms
- Anatomical orientation
The image on the monitor is not just a 2-dimensional display of anatomical structures, it contains data on the average amount of x-ray absorption by the tissues, represented by a matrix consisting of 512 x 512 elements (pixels). The slice has a certain thickness (d S ) and is a sum of cubical elements (voxels) of the same size, combined into a matrix. This technical feature underlies the private volume effect, explained below. The resulting images are usually a bottom view (from the caudal side). Therefore, the right side of the patient is on the image on the left and vice versa. For example, a liver located in the right half of the abdominal cavity is represented on the left side of the image. And the organs on the left, such as the stomach and spleen, are visible in the picture on the right. The anterior surface of the body, in this case represented by the anterior abdominal wall, is defined in the upper part of the image, and the posterior surface with the spine is defined below. The same principle of imaging is used in traditional radiography.
- Effects of private volume
The radiologist himself sets the slice thickness (d S ). For examinations of the thoracic and abdominal cavities, 8–10 mm are usually chosen, and 2–5 mm for the skull, spine, orbits, and pyramids of the temporal bones. Therefore, structures can occupy the entire thickness of the slice or only a part of it. The color intensity of a voxel on a gray scale depends on the average attenuation coefficient for all its components. If the structure has the same shape throughout the entire thickness of the slice, it will look clearly delineated, as in the case of the abdominal aorta and inferior vena cava.
The effect of private volume occurs when the structure does not occupy the entire thickness of the slice. For example, if the section includes only a part of the vertebral body and a part of the disc, then their contours turn out to be fuzzy. The same is observed when the organ narrows inside the slice. This is the reason for the poor definition of the poles of the kidney, the contours of the gall and bladder.
- The difference between the nodal and tubular structures
It is important to be able to distinguish enlarged and pathologically altered LN from vessels and muscles trapped in cross section. It can be very difficult to do this only in one section, because these structures have the same density (and the same shade of gray). Therefore, one should always analyze adjacent sections located cranially and caudally. Having specified how many sections this structure is visible, one can solve the dilemma, whether we see an enlarged node or a more or less long tubular structure: the lymph node will be detected only in one or two sections and is not visualized in the neighboring ones. The aorta, the inferior vena cava and the muscle, for example, the lumbar-iliac, are visible throughout the series of cranio-caudal images.
If there is a suspicion of an enlarged nodular formation in one section, then the doctor should immediately compare adjacent sections to clearly determine if this “formation” is simply a vessel or muscle in cross section. This tactic is also good in that it gives the opportunity to quickly establish the effect of a private volume.
- Densitometry (measurement of tissue density)
If it is not known, for example, whether a fluid found in the pleural cavity is effusion or blood, measuring its density facilitates differential diagnosis. Similarly, densitometry can be applied to focal lesions in the liver or kidney parenchyma. However, it is not recommended to make a conclusion based on the assessment of a single voxel, since such measurements are not very reliable. For greater reliability, the “region of interest” should be expanded, consisting of several voxels in a focal formation, some structure or volume of fluid. The computer calculates the average density and standard deviation.
You should be especially careful not to miss the artifacts of increased radiation rigidity or the effects of private volume. If the formation does not extend to the entire thickness of the slice, then the density measurement includes the structures adjacent to it. The density of education will be measured correctly only if it fills the entire thickness of the slice (d S ). In this case, it is more likely that the measurements will affect education itself, rather than neighboring structures. If ds is larger than the diameter of the formation, for example, a focus of small size, this will lead to the manifestation of the effect of a particular volume at any scanning level.
- Density levels of various types of tissue
Modern devices are able to cover 4096 shades of gray scale, which represent different levels of density in Hounsfield units (HU). The density of water was arbitrarily taken as 0 HU, and air as 1000 HU. A monitor screen can display a maximum of 256 shades of gray. However, the human eye is able to distinguish only about 20. Since the spectrum of human tissue densities extends wider than these rather narrow frames, it is possible to select and adjust the image window so that only tissues of the required density range are visible.
The average density level of the window should be set as close as possible to the density level of the tissues under study. Light, due to increased airiness, it is better to explore in the window with the settings of low HU, whereas for bone tissue the window level should be significantly increased. The contrast of the image depends on the width of the window: the narrowed window is more contrasting, since the 20 shades of gray cover only a small part of the density scale.
It is important to note that the density level of almost all parenchymal organs lies within the narrow boundaries between 10 and 90 HU. The exceptions are easy, therefore, as mentioned above, it is necessary to set special window parameters. With regard to hemorrhages, it should be taken into account that the density level of newly coagulated blood is about 30 HU higher than that of fresh blood. Then the level of density falls again in the areas of old hemorrhage and in zones of blood clot lysis. Exudate with a protein content of more than 30 g / l is not easy to distinguish from transudate (with a protein content below 30 g / l) with the standard settings of the window. In addition, it should be noted that the high degree of coincidence of densities, for example, in the lymph nodes, spleen, muscles and pancreas, makes it impossible to establish the belonging of a tissue only on the basis of density estimation.
In conclusion, it should be noted that the usual values of tissue density are also individual for different people and vary under the influence of contrast agents in the circulating blood and in the organ. The latter aspect is of particular importance for the study of the genitourinary system and relates to the / in the introduction of CV. At the same time, the contrast agent quickly begins to be excreted by the kidneys, which leads to an increase in the density of the renal parenchyma during scanning. This effect can be used to assess kidney function.
- Documenting studies in various windows
When the image is received, to document the study, you must transfer the image to film (make a hard copy). For example, when assessing the condition of the mediastinum and soft tissues of the chest, a window is established so that muscles and adipose tissue are clearly visualized with shades of gray. It uses a soft-woven window with a center at 50 HU and a width of 350 HU. As a result, fabrics with a density from -125 HU (50-350 / 2) to +225 HU (50 + 350/2) are represented in gray. All fabrics with a density lower than -125 HU, such as lung, look black. Fabrics with a density above +225 HU are white, and their internal structure is not differentiated.
If it is necessary to examine the lung parenchyma, for example, when nodules are excluded, the center of the window should be reduced to -200 HU and the width increased (2000 HU). When using this window (pulmonary window), the structures of the lung with low density are better differentiated.
To achieve maximum contrast between the gray and white matter of the brain, a special brain window should be chosen. Since the densities of gray and white matter differ slightly, the soft-tissue window should be very narrow (80 - 100 HU) and high-contrast, and its center should be in the middle of the brain tissue density values (35 HU). With such installations, it is impossible to examine the bones of the skull, since all structures denser than 75-85 HU appear white. Therefore, the center and width of the bone window should be significantly higher - about +300 HU and 1500 HU, respectively. Metastases in the occipital bone are visualized only when bone is used. But not a brain window. On the other hand, the brain is almost invisible in the bone window, so small metastases in the brain substance will be invisible. We must always remember these technical details, because on the film in most cases do not transfer images in all windows. The physician conducting the study, looks at the images on the screen in all windows, so as not to miss the important signs of pathology.