Convex ultrasound sensor. Types of ultrasound sensors. Possible problems with each of them and repair ultrasound sensors. What technical features have convex sensors

Types of ultrasound sensors developed for medical ultrasound studies for more than 50 years. Sensors have different operating frequencies, the dimensions of the sensor itself and the scanning surface, output in different permissions and formats. For example, sectoral phased sensors have a small (usually 20 * 15mm) working (contact) surface to be placed between the ribs and be able to create sector images with a wide coverage and depth at a high frame rate (more than 100 kadras / s). One way or another, information about why specific sensors are more suitable for certain manipulations, quite a little, which was the reason for writing this article. In particular, here we will discuss the relationship of the sensor, image format and clinical use. Systematized selection criteria that make it possible to compare the characteristics of sensors with specific clinical needs, are presented in a new structure, which explains why specific types of sensors are used in specific clinical studies and provides grounds for selecting sensors for new research areas. Criteria include access and coverage of the area of \u200b\u200binterest (ROI), the maximum depth of scanning and image size, and also cover the main diagnostic modes necessary for the exact diagnosis. For completeness of the picture, single-crystal sensors, mainly used intrepreneurially or catheter, will also be considered below. As we appropriate, we will look at the historical experience of choosing a sensor, but mostly highlight new trends.

Image scanning

It is widely known that piezoelectric sensors, being located inside the body or on its surface, transmit ultrasound pulses and reflect their reflection from tissues and organs. To create images capable of helping with clinical studies, an additional scanning element is required. Typically, an acoustic wave created by a separate sensor moves in a given direction or, being mechanically or electronically directed, creates a series of pulses and their reflections that determine the plane of the image. For orientation on the schedule 1A, a system is detected, useful for explaining linear scanning on the XZ plane. The image obtained as a result of a two-dimensional scanning is based on these axes. A simple scanning method consists in gradual movement of an acoustic wave (defined as Δx) along the X axis. In each position, a sound wave is created, after which the waveset is interpolated to create an image of a rectangular shape, in which the side shift is displayed from the beam A to the beam b. An alternative approach to the broadcast is a gradual shear of the acoustic wave on an arc at a small angle (Δθ) in order to determine the image in the XZ plane, as is displayed on the 1B graph. Here shown the turn from the "C" axis is shown to the axis "D". Please note that each axis displays an acoustic wave, as shown graphically in Scheme 2A. As before, the resulting set of waves is interpolated to the sector image. Another variant of the linear shift is the curved geometry shown in the 1C graph. In this case, the combination of waves is reflected in the curved form along the curvature radius (R) and the increment of the string (ΔS) occurs along the curved surface, and not direct. What is interesting in this geometry is that the increase in the curved surface goes from the beam "E" to the beam "F", which is equivalent to an angular shift in relation ΔS \u003d R × Δθ. Due to the scanning along the arc lines diverge in the radial direction.

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In the same principle, scanning in the YZ plane is carried out. In this case, the broadcast occurs along the Y axis with a step ΔY, and the angular scanning is made in a step Δθ in the YZ plane. To achieve three-dimensional scanning or scanning anywhere in the positive half-space defined by positive values \u200b\u200bof x, y and z axes, scanning in both XZ and YZ planes can be combined to form a pyramid-shaped volumetric image, as shown in Figure 2b.

Image formats

Despite the fact that early (single-element) converters carried out mechanical scanning in 2D planes for ultrasound visualization, by the beginning of the 1980s, converters were usually used to scan. The ultrasonic matrix consists of a set of separate single converters or elements that can be controlled by groups or clusters to create pulsed echo rays. For a linear lattice of a group of elements from one row, they gradually turn on and off, shifting an active group of elements by Δx, one by one creating separate pulsed echoes that are combined into the image. Pulse echoes interpolate to form the resulting rectangular image format and the corresponding format of the converter, which is shown on the diagram 1 of Figure 3 and on the corresponding linear converter in Figure 4a, respectively.

Types of image form:

Focusing can be performed mechanically or electronically. For a linear format from Figure 4A, an electronic focus is achieved for each line of the scanned image by controlling the delay time at which the voltage of individual elements is transmitted to the group of active elements. In the height or plane YZ (i.e., the plane perpendicular to the plane of the image, a frequently called cut-off thickness), a fixed focus is achieved using a mechanical lens.

To somewhat mitigate the fixed focus limit, some visualization system manufacturers offer lattices with several rows in the direction of height. However, for a fully controlled focusing in the elevation plane, 2D matrix converters are required, which are capable of providing not only improved focusing in height, but also three-dimensional and 4-dimensional (4D) images. In fig. 2B shows the simultaneous electronic focus of the 2D array for both the elevation planes and the azimuths of the XZ and YZ.

For example, formats 1 and 4 in Fig. 3 are associated with a converter of the linear matrix of type A in Fig.4. For an example of a sector or angular scanning, the image format has a piece of pie, as shown in the image 2 Fig. 3 and the corresponding phased matrix converter shown in Fig.4B.

Selection of a suitable type of sensor

With the help of Figures 3 and 4, it is possible to create a systematic organization of image formats and their association by sensor types with focus on accounting types of scanning, modes and planes. To classify formats and converters, abbreviations can be combined to describe specific converter and image relationships. In particular, to indicate the type of scanning, "M" means mechanical scanning; "E", electronic scanning, and "f" (fixed), without scanning. Scan Direction Linear (L) Along the X axis, angular (

In accordance with the above description, each converter may be associated with scanning types and planes. For example, the Linear Sensor "L" in Figure 4a refers to the electronic linear scanning, "E" in the XZ plane and fixed focus, and "f" in the YZ plane; Therefore, the resulting designations are reduced by both "ELXZ" and "FYZ", and the associated formats - "1" and "4" in Figure 3. The combined representation is the first example shown in Figure 4A. The trapezoid format marked as "4" in Figure 2 can be considered as a rectangular format with two partial sectors at each end for a linear array in Figure 4A. Similarly, the phased grille on Fig.4B is associated with sector format 2 in Fig. 3 and the same planes as in previous examples.

Other converters and formats are also collected in Figures 3 and 4. A variety of sensor types is shown in Figure 5.

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Figure 5.

Sensor family:
Left upper square: three upper sensors - emergency; Two lower-itendovaginal.
Right upper square: microconvex sensor in the center and two phased on each side.
Lower right square, from left to right: convex sensor, three linear sensors, curved linear sensor, phased sensor.
Lower left square, from left to right: two surgical probe and two intraoperative.

The curved, or convex sensor (Fig. 4c) is similar to linear, except that the elements are on the curved, and not a linear surface, as described in the scanning method "C" in Fig. 1C, which leads to the image format 3 in Fig. 3. This format, similar in shape with the sector or a piece of cake, which was bitten up, is often described as a field of view (FOV), which determines its lateral angular length. In this example, an electronic linear scanning "E" in the XZ plane and fixed focus "F" in the YZ plane are used; Therefore, the resulting designations are reduced as "ECXZ" and "FYZ", it corresponds to the format "3", as shown in Figure 4B.

Since the significance of 3D visualization is steadily growing, it is appropriate to discuss it in more detail. For three-dimensional visualization, instead of a plane, the volume is scanned as shown by the external circuit shown in Figure 2b. For a two-dimensional or matrix lattice (Fig. 4f), scanning can be electronic and usually angular in both directions, so that the scanned volume has a pyramidal form (image 7, Fig. 3). In this case, the electronic focus is achieved in both planes with angular scanning, so the corresponding designations and format have the form "e

Alternatively, to achieve a profitable 3D image, linear or convex arrays can be mechanically scanned around the x axis in the YZ plane. In these cases, arrays are moved into the liquid filled with an acoustically transparent chambers. For example, a linear array (usually type A) is rotated around the z axis to create a series of flat images (usually format 1 or 4), so that the result is a mechanically scanned converter type F in Fig.4 and the scanned three-dimensional image 5 in Figure 3. Similarly , curvilinear or convex grille (usually type C) turns around the axis to create a series of flat images (usually 3), so that the result will be a mechanical type G sensor in Fig.4 and the volumetric image 6 in Fig.3.

In addition to the electronically controlled motion, these one-dimensional (1D) lattice (type A, B or C) can also be mechanically moved manually in the 3D hand mode, in which the obtained images are usually collected in three-dimensional volumes. It is worth noting here that the reconstruction of the image for a 3D mode of a free hand implies either assumptions about a constant interval, or additional spatial information for each spatial visualization plane, which can be achieved using position sensors.

Finally, images obtained by single-element transducers are mainly used for intra-race or catheter applications (such as intravascular or intraconductural ultrasound examination) are also shown in images 8 and 9 in Figure 3. The sensor shown in Figure 4H can scan mechanical to obtain 2D or 3D images, as represented in images 8 and 9, Fig.3. For format 8, the sensor (Figure 4H) moves at an angle around the circle to get the image in the form of a donut. It appropriate to note here that there is also a matrix version of this endovascular ultrasound device. If this mechanical converter rotates and moves along the Y axis, a cylindrical volumetric image is obtained, 9 (Figure 3).

In conclusion, the types of converters depicted in Figure 4 can be mapped to image formats shown in Figure 3, by using the formats and scanning designations below the forms of the converter in Figure 4.

Converter characteristics for visualization

This section discusses the criteria in order to determine which properties of ultrasonic image converters and their formats must be identified for various clinical applications.

First of all, it applies to clinically used image sensors that operate in the frequency range from 1 to 20 MHz.

Converters operating above this frequency are used for special applications, such as endovascular examination (see Fig. 4, F and G) or a preclinical survey of small animals, but also included in the discussion wherever possible.

Acoustic windows

How good the sensor type is suitable for the "acoustic window" or a place where it contacts the body to visualize organs or tissues of interest? Standard acoustic windows provide a unimpeded review of the body or region; Many, by agreement, have specific names, such as "transabdominal" or "parastinal long axis", so that images can be compared and described sequentially. Typical windows are located inside or on the surface of the following main parts of the body: head, chest, belly, pelvis, limbs, vessels and various body openings. Sensors can be associated with certain regions using Latin prefixes: "Trans-", "intra-" "endo-", etc. An example is the "transom-worn", a category that includes transducers forming an image through the chest . The transcranial sensor scans the head through the skull.

As already mentioned, a phased grille for the transducer window would be the most suitable if the image processing task requires that the converter is located between the edges; This is provided for the room in the intercostal intervals and maximizing the scanned area (image 2 in Fig. 3). For most contact surfaces that are relatively flat and / or slightly deformed (for example, used for small parts or images of vessels), the most common and frequently used type of converter is a linear matrix intended for contact with flat surfaces with a decrease in surface area and increasing frequency . Here, rectangular and trapezoidal formats (1 and 4 in Fig. 3) provide the corresponding viewing areas.

When visualizing in the abdomen area to increase the viewing area with a minimum increase in the contact area, convex matrices (Fig. 4C) form an image format 3 (shown in Figure 3) and are designed to provide surface contact in the deformable soft areas of the body.

Specialized sensors

Specialized converters are designed to work inside the body. These include transzezofagal probes with phased lattices suitable for manually manipulation inside the esophagus (Image 2 and type B sensor in Figure 4). A number of other specialized probes were also designed for surgical or interventional use, such as laparoscopic and intracardial probes. These probes can be linear or phased, depending on the use and access windows. Some endo probes: endovaginal, endorectal and intrasaginal (type D-shaped) is functionally similar to phased torch sensors (Image 2 and Figure 4B) or convex sensors (format 3 and Figure 4C) at the end of the cylindrical handle of the small diameter, for the room in the opening and At the same time, the maximization of the field of view. Another example is an intravascular ultrasonic sensor (Fig. 4n), which is entered into veins to obtain a flat image of format 8 or volume in format 9.

Permission and penetration

The selected scan depth allows you to view the range of interest. The factors associated with the possibilities of visualization include the size of the active aperture, the depth of the transmitted focus and the frequency range. Penetration is the minimum scanning depth at which electron noises are visible, despite the optimization of the available control elements (as a rule, with the deepest transfer focus and maximum gain), and the electronic noise remains at a fixed depth, even when the array is shifted laterally. Penetration primarily is determined by the central frequency of the sensor: the higher the frequency, the less the penetration depth, since the absorption of the ultrasonic wave passing through the tissue is increased with the frequency.

Useful first approximation to estimate the penetration depth (DP) for this frequency is dp \u003d 60 / f cm-MHz, where F is given in megahertz. Thus, it would be possible to expect a 6 cm penetration from the central frequency converter by 10 MHz. As noted earlier, the absorption coefficient (the loss of acoustic power per unit of depth) is a frequency function and varies from tissue to tissue (values \u200b\u200bfor soft tissues range from 0.6 to 1.0 dB / cm-MHz4). A more general term describing acoustic losses is attenuation coefficient, which includes additional losses due to scattering and diffusion and, therefore, always more absorption coefficient. The attenuation coefficient is very dependent on the patient and the acoustic path.

To optimize image resolution, users and manufacturers have worked on an increase in the visualization frequency for various types of research. For example, about 30 years ago, people could visualize the abdominal cavity with a frequency of 2.25 MHz, whereas today this number is more often 3.5 MHz, and some obstetric and gynecological images reach 5 MHz.

Properties of sensors and visualization

Other criteria that should be included in the above-described selection process are the effectiveness of the converter, the design of the two-conducting system, the signal-to-noise ratio of the system and, as already noted, the absorption of the tissues studied. The main factor is the absorption - the composition and relative position of various types of tissues on the path of acoustic wave. For example, a thick layer of adipose tissue will reduce the penetration due to refraction or aberration errors on the acoustic path to the area of \u200b\u200binterest. Similarly, an enlarged amount of amniotic fluid with fetal visualization enhances penetration and can allow the use of frequencies higher than those commonly used in this area of \u200b\u200bscanning.

The frequency range, or the bandwidth of the sensor, is determined whether it can support visualization in two-dimensional mode at different central frequencies, as well as operate in doppler modes, harmonics and color flux. For the visualization modes based on Doppler mode, it is often necessary to work with lower frequencies than the frequency of the two-dimensional mode to minimize the overlay of the spectra. When the harmonic image, the reception frequency is used by definition, which is multiple (usually 2) transmitted frequency; Consequently, there is a need for wide bandwidth. Throughput and focus properties also affect image resolution. In clinical practice it is important to ensure that the resulting image can distinguish the smallest possible sizes in both lateral and axial directions.

Finally, the number of individual elements of the sensor is of interest, since the number of active elements (with the exception of phased gratings or scanned 2D arrays) determines the transverse length or width of the image. For phased lattices, an increasing number of elements are associated with improved resolution and penetration depth. For two-dimensional gratings (usually symmetrical), the number of elements along directions x and y determines the size of the volume for linearly scanned arrays. For a two-dimensional phased mesh, the resolution and penetration increase with a large number of elements along the x and y directions, but the angular form or fov remain unchanged regardless of the number of active elements used. Focusing in a fixed direction can indirectly affect the image, since focus is positioned only at one depth and much worse to another. For 3D images, mechanically scanned 2D arrays have the same fixed focal length depth limit, occurring in a 2D image. On the contrary, all elements of fully filled three-dimensional images or matrix lattices are focused by electronic paths at one point both in the azimuth plane and in the elevation plane to ensure much better resolution.

At the greatest depth, this is the maximum number of available active channels in the system, which determines the resolution (along with the focus and systemic noise). Spatial resolution is usually worse (usually 2 times) than a temporary resolution by scanning lines; In the discussion presented here, permission refers to a spatial resolution, unless otherwise indicated. For phased gratings, the number of channels usually corresponds to the maximum number of items. As a rule, since elements are usually located at a distance of half the wavelength, the more elements, the better the spatial resolution, which is inversely proportional to the active aperture in the wavelengths. For example, a 64-element matrix, a 32-fold aperture will have a maximum spatial resolution 2 times lower (wider beam) than a 128-element 64-wave grid. In the case of a linear grid that can have several hundred elements, the number of elements determines the lateral length of the image, but this is the number of active channels that controls the resolution. For these one-dimensional grilles, the resolution of the image plane (also known as the cut thickness) is bad, with the exception of almost fixed focal length. For 2D lattices, the spatial resolution is inversely proportional to active apertures that form the side 2D array. Two-dimensional arrays have an excellent resolution compared to 1D focusing array with fixed focusing in height, because the exact focus can be achieved simultaneously in azimuth and height for a three-dimensional image.

Another by considering permission is F #. The less F #, the better the resolution. A simple assessment of the beam width in millimeters, a general resolution measure, neglecting the absorption, is approximately equal to F # × λ, where λ is the wavelength (1.5 mm / μs / f [MHz]). For example, the resolution will be 0.3 mm at a frequency of 5 MHz for F # \u003d 1. Focus depths also depend on the active aperture. For example, for a 128-cell 64-wave mesh, the deepest focal depth achieved at maximum aperture and F # \u003d 1 is equal to F \u003d F # × L \u003d 64 wavelengths. The actual penetration depth or useful scanning depth, of course, will be deeper than the maximum focal depth.

Compliance of sensors and their clinical use

Now that we compared the types and properties of sensors with visualization windows and acoustic windows, we can use this information when choosing sensors for specific clinical applications. The suitability of certain converters for specific applications has historically developed with the help of special adapted structures. Primary considerations are the target area of \u200b\u200binterest, its length and accessible acoustic windows necessary for access.

Abdominal visualization

When the sensor matrices were first commercially presented for abdominal visualization (including obstetrics and gynecology), they were linear type (type A in Fig. 4 with an image format 1 in Fig. 3). In most cases, the contact area with the patient was not a critical problem, and some of these linear sensors were rather long (for example, 8 cm) to cover, say, the fetal head in the third trimester. However, it soon became clear that it was possible to achieve sufficiently large coverage due to the use of curvilinear or convex matrix lattices (type C in Fig. 4), without paying for the fact that it is necessary to manipulate rather bulky linear transducers.

Curved matrices (Fig. 4C) are tools of choice for most of the total 2D images during abdominal studies. The overall form factor associated with ergonomic factors and the correspondence of the shape of the sensor and FOV for use is still developing for abdominal 3D images. Three key handlers for these lattices are the base area (total aperture size), the field of view and the radius of curvature (Fig. 1c). The imprint depicts the contact area, usually in the form of a rectangle, a circle or ellipse. Although access is usually not a problem for abdominal visualization, when these types of converters are treated for new applications, access to the windows is of paramount importance. The radius of curvature and FOV (expressed in degrees of the maximum angular coverage) is associated with the scale and coverage of the image. To increase penetration into some systems, improved signal processing was added; However, this feature is usually available only on certain probes.

For mechanical 3D-probes, the form factor is currently a mechanically curved convex sensor (Figure 4G and format 6 in Figure 3); However, electronic convex 2D arrays are now fully available. In these cases, two fields of view are given for orthogonal (direct) scan directions. Alternatively, phased grilles, due to their small area and wide image format, are also used for abdominal visualization. Finally, two-dimensional or matrix grids are becoming increasingly common for these applications due to their excellent image quality, resolution and ease of use.

Intercostal visualization

The main applications of this visualization group are the scanning of the heart and the study of the liver between the ribs. Just because of the restrictive anatomy and limited acoustic windows created by ribs and often invading light, the choice of the converter here is limited to phased lattices (Fig. 4b). In this area, the first attempts to use linear lattices were made; However, they quickly disappeared due to the shading of the edges and the superiority of the phased lattice at the format 2 sensor (Fig. 4). For cardiological studies, probes, as a rule, have the size of the lattice of about 20 × 14 mm depending on the manufacturer. The area of \u200b\u200bcontact with the patient will be a little more. These figures developed over the past 40 years and depend on a number of factors, for example, such as the number of patients. Age is another consideration; The distance between the ribs and the penetration depth must be varied as children are growing up.

For negridiological intercostal studies, the size of the grilles is somewhat larger. As noted earlier, the existence of these anatomical restrictions creates the upper limit of performance for spatial permission, since the performance of the permission is inversely proportional to the size of the aperture, as explained above. In studies for the heart and general intercostal visualization, the depth of the image is large (depending on the size of the patient, it can reach 24 cm), which forces the use of lower frequencies (1-3.5 MHz) and leads to some additional losses of image processing.

There is an interesting aspect of the visualization of the heart, which had a deep influence on the nature of the probes. Due to the presence of ribs and other acoustic hostile fabrics on the beam trajectory, echocardiography suffers from visualization artifacts due to the reflective noise. The introduction of the harmonic image was very successful in reducing this noise. As a result, the importance of the bandwidth of the converter has become critical in the design of the heart sensor. Today, most cardiac systems operate at frequencies from 1.5 to 2.0 MHz and, of course, receive signals with frequencies, twice as large as this range.

The most important developing in the field of heart visualization was the implementation of the fully filled 2D or matrix lattices (type E) containing thousands (usually 50 × 50) elements. This makes it possible to display in real time (4D) pyramidal volumes (format 7, Figure 3), visualization of arbitrary sections of planes, four-dimensional heart visualization and color reproduction. In addition, the true electronic focusing in XZ and YZ planes provides excellent resolution compared to all other one-dimensional sensors.

Surface and breast visualization

This category refers to the "surface" visualization of carotid arteries, veins of legs, chest, thyroid gland, testicles, etc. and includes categories of small parts of the body, musculoskeletal system and images of peripheral vessels. This is the last bastion for the use of linear lattices (type A), which formed the initial type of construction for previously discussed studies. In this clinical category, access is usually not a problem, and the dimensions of the probes themselves may be small (due to the use of high frequencies from 7 to 15 MHz and the resulting small size elements). Muscular and skeletal studies for muscle visualization, ligaments and tendons also use lattices of this type. Over the past 10 years, the visualization of the mammary glands has moved to very high frequencies (for example, 14 MHz), while visualization of the peripheral vascular remained at lower (about 3-11 MHz) due to the need to include deeper veins and successful Doppler presentation . As a rule, the ability to add trapezoidal visualization (Format 4) is a significant advantage. As with abdominal visualization, a three-dimensional image with mechanically curved probes or electronic 2D lattices is now available for surface and infant application, which significantly improves affordable coverage and image quality. For applications related to vessel visualization, some probes have the advantages on the inclusion of modes that improve flow visualization.

obstetrics and gynecology

Currently, mechanical convex or linear lattices (types G and F) are widely used to provide 3D and 4D images of fruit in vivo (5-7 formats). Matrix or fully filled 2D arrays (Type E) are also available for this use (usually format 7).

For gynecology, special endo-matrix forms of the sensor are used (type D). As a rule, lattices are at the end of the sensor and are convex or curved lattices with wide view fields (format 3); However, phased lattices (type D) can also be used (format 2). The frequencies used usually make up 5 MHz and above. As in other applications, 2D lattices were designed for 3D images in these cases.

Neonatal and pediatric

Pediatric sensors tend to have smaller surfaces than sensors used for adults and operate at high frequencies (≥7 MHz) those used for adults. Depending on the body of the body, the types of converters are used, similar to those used for adults. Phased lattices (type B) and 3D converters (E and G Types) are suitable for visualization of the heart. Other matrices, which are also useful for these clinical needs, include static (2D), and for three-dimensional linear arrays, mechanically curved and convex lattices.

Domestic research

Intrapy sensors constitute a large group of specialized sensors, which are designed for visualizations within the body cavity. Transezofagal sensors are used to obtain the display of internal organs, especially the heart, from the inside of the esophagus (see Figure 5). They use higher frequencies (≥5 MHz) and implemented as phased grilles with manipulators and motors to adjust the sensor orientation. Miniature transzezofagal 2D sensors offer electronic scanning for 3D and 4D images.

Sensors can be highly specialized for viewing, as a rule, in body openings or vessels. Intricultural phased sensors are entered through the vessel to gain access to the inner chambers of the heart. Surgical specialized sensors include laparoscopic sensors inserted through small cuts for visualization and assistance in laparoscopic surgery (by analogy with endo probes); They are wonderful for their FOV despite small diameters. Intraoperative sensors have a special form for accommodation in vessels, organs and areas available during open operation (see Figure 5). Others in this class are surgical and interventional sensors with unique forms (see Figure 5).

As already noted, the sensors that are placed in the body are intended for installation through small holes and have a wide viewing field (90 ° -150 °). These sensors include transrectal (or endorectal) to visualize the pelvic region using the rear passage for access and already described endovaginal (also called transvaginal) for visualizing the female pelvis and reproductive organs using the vagina as an entry for gynecological and obstetric studies. These end sensors described earlier are made in the form of a cylinder to insert into small holes and have convex matrices (usually 3-9 MHz) at the ends with large field fields, biplanes or mechanically curved convex matrices. Probes for urological applications include biplane.

The unique sensor is a biplane-probe consisting of two orthogonal matrices, creating images in XZ and YZ planes. Usually matrices are small (8-12 mm) and convex type. Each shape and sensor must comply with the format of a single-layer converter, such as format 3 in Fig. 3 and convex in Fig.4C. However, depending on the design of the converter, sectoral or linear lattices are also possible, so in practice several combinations can be used. Alternatively, a subset of the 2D array visualization capabilities is a simultaneous representation of two orthogonal 2D images.

Intravascular transducers are introduced into the blood vessels to visualize the walls of the vessels at various pathological conditions (type H and formats 8 and 9). Most often, they are mechanically rotated single converters with frequencies of more than 20 MHz and specialized image formation systems, although it also has tiny (with a diameter of about 2 mm) matrix.

Head research

Transcranial visualization of the brain and its vascular network is carried out through limited acoustic windows in a skull, such as whiskey or eyes. Transcrutal lattices are high-frequency (usually\u003e 20 MHz) ophthalmic converters and are used to visualize the eyes or use of the eye as an acoustic window. Transcranial probes are usually low-frequency (1-4 MHz) phased lattices used to visualize blood vessels skull through whiskey as windows.

conclusions

Many ultrasound transducers are designed to work in certain areas of the body for specific applications. The main purpose of this article is to provide a systematic approach that will help to agree on a clinical application converter, starting with an acoustic window, areas and depths that will be displayed. To this end, the checklist for selecting the converter is shown in Table 1.

Table 1. Converter selection checklist

As shown earlier, the main thing in the discussion of the visualization of the target area or bodies is access: the estimated acoustic window.

The type of the converter must provide access through the selected acoustic window. The type of the converter is associated with the image format, and the previously considered common collections include linear, phased, convex and 2D matrices. The size or contact area of \u200b\u200bthe converter must match the size of the window, and in extreme cases, when the converter window is a hole, the form of the converter must match the available opening. As noted above, there are special probes in some studies, such as endorectal sensors, which are sufficiently small in diameter (size) and have an extended shape suitable for entering the body opening.

Secondly, the size or fov and the image format is selected to obtain the desired coating in the area of \u200b\u200binterest. It is important here both the depth of scanning and the width of the image or FOV. For linear matrices, the presence of trapezoid visualization may be required for adequate coating. For 3D, or volumetric image, the length of the image can be specified as a set of maximum scanning angles in orthogonal directions or field of view and angle. A slightly more hidden parameter for 2D images to determine the coating zone for the area of \u200b\u200binterest is the focal depth depth, which describes the area with the finest cutting thickness.

Thirdly, the maximum selected scanning depth determines the highest achievable frequency through the penetration ratio given above in the "Resolution and Penetration" section. For example, if the scanning depth is 10 cm, then, as already discussed in the "Resolution and Penetration" section, the frequency from the penetration depth D is equal to 60 / d \u003d 60/10 \u003d 6 MHz. This frequency gives an estimate of the best lateral resolution of about 1 wavelength for F # \u003d 1, or, for this example, the resolution λ \u003d c / f \u003d 0.25 mm (from the "Properties of Sensor and Visualization" section). An exception to this rule is systems that use advanced signals processing to increase sensitivity and improve penetration. In addition, the use of piezoelectric materials can increase sensitivity and, accordingly, the depth of penetration.

Fourth, it is possible to determine the coverage of the main regimes of diagnostic visualization. Of the data provided by the manufacturer, an effective bandwidth required to support various modes, or for the system under consideration, you may be listed as actual modes, such as the Doppler Pulse Signal, the presence of several displayed frequencies or elastographic mode. Converters with piezoelectric materials can significantly increase bandwidth.

In conclusion, sensors and graphic formats evolved to better comply with specific clinical applications. The classification and organization given in this article serve as a prerequisite for selecting a converter for a specific purpose. In addition, the presented understanding can help in determining the characteristics of the sensor necessary for new cases, thereby expanding the range of use of the sensor.