Convex ultrasound probe. Types of ultrasound sensors. Possible problems with each of them and repair of ultrasound sensors. What are the technical features of convex sensors

Types of ultrasound transducers have been developed for medical ultrasound for over 50 years. The sensors have different operating frequencies, the dimensions of the sensor itself and the scanning surface, display an image in different resolutions and formats. For example, sector phased sensors have a small (usually 20*15mm) working (contact) surface to fit between the ribs and be able to create sector images with wide coverage and depth at high frame rates (more than 100 frames/sec). One way or another, there is little information about why specific sensors are more suitable for certain manipulations, which was the reason for writing this article. In particular, here we discuss the relationship between transducer, image format, and clinical applications. Systematized selection criteria that match transducer performance to specific clinical needs are presented in a new framework that explains why specific transducer types are used in specific clinical trials and provides a basis for selecting transducers for new research areas. Criteria include access and area of ​​interest (ROI), maximum scan depth and image size, and cover the main diagnostic modes required for accurate diagnoses. For the sake of completeness, single crystal transducers, mainly used intraluminally or catheterically, will also be discussed below. As appropriate, we will review the historical experience of choosing a sensor, but mainly highlight new trends.

Scanning images

It is widely known that piezoelectric sensors, being located inside the body or on its surface, transmit ultrasonic pulses and receive their reflection from tissues and organs. To create images that can help in clinical research, an additional scanning element is needed. Typically, an acoustic wave generated by a single transducer travels in a predetermined direction or, when mechanically or electronically directed, creates a series of pulses and their reflections that define the image plane. For orientation, graph 1A demonstrates a system useful for explaining linear scanning on the xz plane. The image obtained as a result of two-dimensional scanning is built precisely along these axes. A simple scanning method is to gradually move the acoustic wave (defined as Δx) along the "x" axis. At each position, a sound wave is created, after which the set of waves is interpolated to create a rectangular image in which the lateral shift is displayed from beam a to beam b. An alternative approach to translation is to gradually shift the acoustic wave along an arc at a small angle (Δθ) in order to determine the image in the xz plane, as shown in graph 1B. Shown here is the rotation from the "c" axis to the "d" axis. Note that each axis represents an acoustic wave, as shown graphically in Diagram 2a. As before, the resulting set of waves is interpolated into a sector image. Another variation of linear shift is the curved geometry shown in Figure 1C. In this case, the wave constellation is reflected in a curved shape along the radius of curvature (R) and the line increment (Δs) occurs along the curved surface rather than a straight line. What is interesting about this geometry is that the increment along the curved surface goes from the "e" beam to the "f" beam, which is equivalent to the angular shift in the relation Δs = R × Δθ. Due to scanning along the arc, the lines diverge in the radial direction.

Merging Table Cells Vertically

The same principle is used for scanning in the yz plane. In this case, translation occurs along the y axis with a step Δy, and angular scanning is performed with a step Δθ in the yz plane. To achieve a 3D scan, or scan anywhere in the positive half-space defined by positive x, y, and z axes, scans in both the xz and yz planes can be combined to form a pyramid-shaped volumetric image, as shown in Figure 2B.

Image formats

Although early (single element) transducers performed mechanical scanning in 2D planes for ultrasound imaging, by the early 1980s, transducers were commonly used for scanning. The ultrasonic array consists of a collection of individual single transducers or elements that can be controlled by groups or clusters to create pulsed echo beams. For a linear array, groups of elements from the same row are gradually turned on and off, shifting the active group of elements along Δx, one at a time creating individual impulse echo beams that are combined into an image. The pulsed echo beams are interpolated to form the resulting rectangular aspect ratio and corresponding transducer shape, which are shown in diagram 1 of Figure 3 and the corresponding linear transducer in Figure 4A, respectively.

Picture Form Types:

Focusing can be done mechanically or electronically. For the linear format of Figure 4A, electronic focusing is achieved for each line of the scanned image by controlling the delay time at which the voltage of the individual elements is transferred to the group of active elements. At the height or yz plane (i.e. the plane perpendicular to the image plane, often referred to as slice thickness), fixed focus is achieved using a mechanical lens.

To alleviate the limitation of fixed focus somewhat, some imaging system manufacturers offer gratings with multiple rows in the height direction. However, fully controllable elevation-plane focusing requires 2D sensor converters that are capable of providing not only improved height focusing, but also 3D and 4D (4D) images. On fig. 2B shows the simultaneous electronic focusing of a 2D array for both elevation planes and xz and yz azimuths.

For example, formats 1 and 4 in Fig. 3 are associated with a type A linear matrix converter in Fig. 4. For an example of a sector or angle scan, the image format is shaped like a piece of cake, as shown in image 2 of Fig. 3 and the corresponding phased array converter shown in Fig. 4B.

Choosing the Right Sensor Type

With the help of Figures 3 and 4, it is possible to create a systematic organization of image formats and group them by sensor type, with an emphasis on taking into account scan types, modes and planes. To categorize formats and converters, abbreviations can be combined to describe specific converter-image relationships. In particular, to indicate the type of scanning, "M" means mechanical scanning; "E", electronic scan, and "F" (fixed), no scan. Scanning direction linear (L) along the x-axis, angular (

As described above, each transducer can be associated with scan types and planes. For example, the line probe "L" in Figure 4A refers to an electronic line scan, "E" in the xz plane and fixed focus, and "F" in the yz plane; Therefore, the resulting designations are abbreviated "ELxz" and "Fyz" and their associated formats are "1" and "4" in Figure 3. The combined representation is the first example shown in Figure 4A. The trapezoidal format, labeled "4" in Figure 2, can be viewed as a rectangular format with two partial sectors at each end for the line array in Figure 4A. Similarly, the phased array in Fig. 4B is related to the sector format 2 in Fig. 3 and the same planes as in the previous examples.

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

Merging Table Cells Vertically

Figure 5

Sensor family:
Upper left square: three upper transducers are transesophageal; the two lower ones are endovaginal.
Upper right square: microconvex probe in the center and two phased transducers on each side.
Lower right square, from left to right: convex probe, three line probes, curved line probe, phased array probe.
Lower left square, from left to right: two surgical probes and two intraoperative probes.

A curved or convex probe (Figure 4C) is similar to a linear probe, except that the features are on a curved rather than linear surface as described in Scan Method "C" in Figure 4C. 1C, resulting in aspect ratio 3 in Fig. 3. This format, similar in shape to a sector or piece of pie that has had its top bitten off, is often described as a field of view (FOV) that defines its lateral angular extent. This example uses an electronic line scan "E" in the xz plane and a fixed focus "F" in the yz plane; Therefore, the resulting designations are abbreviated as "ECxz" and "Fyz" and correspond to the format "3" as shown in Figure 4B.

Since the importance of 3D visualization is steadily growing, it is appropriate to discuss it in more detail. For 3D imaging, a volume is scanned instead of a plane, as shown by the outer contour depicted in Figure 2B. For a two-dimensional or matrix grating (Fig. 4F), scanning can be electronic and usually angular in both directions, so that the scanned volume is pyramidal (image 7, Fig. 3). In this case, electronic focusing is achieved in both planes with angular scanning, so the corresponding notation and format are "E

Alternatively, linear or convex arrays can be mechanically scanned around the x-axis in the yz-plane to achieve cost-effective 3D imaging. In these cases, the arrays move in liquid-filled acoustically transparent chambers. For example, a line array (typically type A) is rotated about the z-axis to create a series of flat images (typically format 1 or 4), so that the result is a mechanically scanned type F transducer in Figure 4 and a scanned volumetric image of 5 in Figure 3. Similarly , a curvilinear or convex grating (usually type C) is rotated about an axis to create a series of flat images (usually format 3), so that the result is a type G mechanical sensor in Fig. 4 and a 3D image 6 in Fig. 3.

In addition to electronically controlled movement, these one-dimensional (1D) gratings (types A, B, or C) can also be mechanically moved manually in free-hand 3D mode, in which the acquired images are typically assembled into three-dimensional volumes. It is worth noting here that image reconstruction for freehand 3D implies either constant spacing assumptions or additional spatial information for each spatial imaging plane, which can be achieved with position sensors.

Finally, images acquired with single element transducers, primarily used for intraluminal or catheter applications (such as intravascular or intracardiac ultrasound), are also shown in images 8 and 9 in Figure 3. The transducer shown in Figure 4H can be scanned mechanically to obtain 2D or 3D images, as shown in images 8 and 9, Fig.3. For format 8, the sensor (Figure 4H) is moved at an angle around the circle to produce a donut-shaped image. It is pertinent to note here that there is also a matrix version of this endovascular ultrasound device. If this mechanical transducer rotates and moves along the Y axis, a cylindrical three-dimensional image is obtained, format 9 (Figure 3).

Finally, the types of transducers depicted in Figure 4 can be mapped to the image formats shown in Figure 3 by using the formats and scan notation shown below the transducer shapes in Figure 4.

Characteristics of the imaging transducer

This section discusses criteria for determining which properties of ultrasound imaging transducers and their formats need to be identified for various clinical applications.

This is primarily applicable to clinically used image sensors that operate in the frequency range from 1 to 20 MHz.

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

acoustic windows

How well does the transducer type fit into the "acoustic window" or the place where it contacts the body in order to visualize the organs or tissues of interest? Standard acoustic windows provide an unobstructed view of the organ or area; many, by convention, have specific names such as "transabdominal" or "parasternal long axis" so that images can be compared and described consistently. Typical windows are located inside or on the surface of the following major parts of the body: head, chest, abdomen, pelvis, limbs, vessels, and various orifices of the body. Transducers can be associated with specific regions using Latin prefixes: "trans-", "intra-", "endo-", etc. An example is "transthoracic", a category that includes transducers that form images through the chest . The transcranial sensor scans the head through the skull.

As already mentioned, for a transthoracic window, a phased array would be most appropriate if the task of image processing requires that the transducer be positioned between the ribs; this is designed to fit into the intercostal spaces and maximize the scanned area (image 2 in Figure 3). For most contact surfaces that are relatively flat and/or slightly deformable (such as those used for fine detail or vessel imaging), the most common and commonly used transducer type is a linear array designed to contact flat surfaces with decreasing surface area and increasing frequency. . Here, the rectangular and trapezoidal formats (1 and 4 in Figure 3) provide the respective viewing areas.

When imaging in the abdomen, to increase the field of view with minimal increase in contact area, the convex matrices (Fig. 4C) form an image format 3 (shown in Fig. 3) and are designed to provide surface contact in deformable soft areas of the body.

Specialized sensors

Specialized transducers are designed to work inside the body. These include phased array transesophageal probes suitable for manual manipulation within the esophagus (image 2 and probe type B in figure 4). A number of other specialized probes have also been developed for surgical or interventional use such as laparoscopic and intracardiac probes. These probes can be linear or phased, depending on the application and access windows. Some endo-probes: endovaginal, endorectal and intracavitary (D-shape type) are functionally similar to phased torch probes (image 2 and figure 4B) or convex probes (format 3 and figure 4C) at the end of a small diameter cylindrical handle, for insertion into holes and while maximizing the field of view. Another example is an intravascular ultrasound transducer (Fig. 4H), which is inserted into a vein to obtain a flat 8 format image or a 3D format 9 image.

Permission and penetration

The selected scan depth allows you to view the range of interest. Factors related to imaging capabilities include the size of the active aperture, the depth of transmitted focus, and the frequency range. Penetration is the minimum scan depth at which electronic noise is visible despite optimization of available controls (typically at deepest transfer focus and maximum gain), and electronic noise remains at a fixed depth even when the array is shifted laterally. Penetration is primarily determined by the center frequency of the transducer: the higher the frequency, the shallower the penetration depth, since the absorption of an ultrasound wave passing through tissue increases with frequency.

A useful first approximation for estimating the penetration depth (dp) for a given frequency is dp = 60/f cm-MHz, where f is given in megahertz. Thus, one would expect 6 cm penetration from a 10 MHz central frequency converter. As noted earlier, the absorption coefficient (loss of acoustic power per unit depth) is a function of frequency and varies from tissue to tissue (values ​​for soft tissue are between 0.6 and 1.0 dB/cm-MHz4). A more general term to describe acoustic loss is the attenuation factor, which includes additional losses due to scattering and diffusion and is therefore always greater than the absorption factor. The attenuation coefficient is very dependent on the patient and the acoustic path.

To optimize image resolution, users and manufacturers have been working to increase imaging rates for different types of exams. For example, some 30 years ago people could image the abdomen at 2.25 MHz, whereas today the number is more commonly 3.5 MHz, with some obstetric and gynecological images as high as 5 MHz.

Sensor properties and visualization

Other criteria to be included in the selection process described above are the efficiency of the transducer, the design of the two-wire system, the signal-to-noise ratio of the system, and, as already noted, the absorbance of the tissues under study. The main factor is absorption - the composition and relative position of different tissue types in the path of an acoustic wave. For example, a thick layer of adipose tissue will reduce penetration due to refractive errors or aberrations in the acoustic path to the region of interest. Similarly, an increased amount of amniotic fluid with fetal imaging enhances penetration and may allow frequencies higher than those normally used in a given scan area.

The frequency range, or bandwidth, of a sensor determines whether it can support 2D imaging at different center frequencies, as well as Doppler, harmonics, and color flow. Imaging modes based on Doppler often require lower frequencies than 2D to minimize aliasing. Harmonic imaging, by definition, uses a receive frequency that is a multiple (usually 2) of the transmitted frequency; hence, there is a need for a wide bandwidth. Bandwidth and focus properties also affect image resolution. In clinical practice, it is important to ensure that the resulting image can distinguish the smallest possible dimensions in both the lateral and axial directions.

Finally, the number of individual sensor elements is of interest, since the number of active elements (with the exception of phased arrays or angle-scanned 2D arrays) determines the lateral extent or width of the image. For phased arrays, an increasing number of elements is associated with improved resolution and penetration depth. For two-dimensional arrays (generally symmetrical), the number of elements along the x and y directions determines the size of the volume for linearly scanned arrays. For a 2D phased grid, resolution and penetration increase with more elements along the x and y directions, but the angular shape or FOV remains the same regardless of the number of active elements used. Focusing in a fixed direction can affect the image indirectly, as focusing is only positioned at one depth and much worse at another. For 3D images, mechanically scanned 2D arrays have the same fixed focal length depth constraint encountered in a 2D image. In contrast, all elements of fully populated 3D images or array arrays are electronically focused to a single point in both the azimuth and elevation planes to provide much better resolution.

At the deepest depth, it is the maximum number of active channels available in the system that determines the resolution (along with focus strength and system noise). The spatial resolution is usually worse (usually by a factor of 2) than the temporal resolution along the scan lines; in the discussion presented here, resolution refers to spatial resolution unless otherwise noted. For phased arrays, the number of channels usually corresponds to the maximum number of elements. As a general rule, since the elements are usually half a wavelength apart, the more elements, the better the spatial resolution, which is inversely proportional to the active aperture in wavelengths. For example, a 64 element array, 32x aperture will have a maximum spatial resolution 2 times lower (wider beam) than a 128 element 64 wave array. In the case of a linear grid, which may have several hundred elements, the number of elements determines the lateral extent of the image, but it is the number of active channels that governs the resolution. For these one-dimensional gratings, resolution from the image plane (also known as slice thickness) is poor, except for a nearly fixed focal length. For 2D arrays, the spatial resolution is inversely proportional to the active apertures that form the sides of the 2D array. 2D arrays have superior resolution compared to a 1D height-fixed focus array because precise focusing can be achieved simultaneously in azimuth and height for a 3D image.

Another way to look at permission is F#. The smaller the F#, the better the resolution. A simple measure of beamwidth in millimeters, a common measure of resolution, neglecting absorption, is approximately F# × λ, where λ is the wavelength (1.5 mm/µs/f [MHz]). For example, the resolution would be 0.3 mm at 5 MHz for F# = 1. Depths of focus also depend on the active aperture. For example, for a 128-element 64-wavelength grid, the deepest focal depth achievable at maximum aperture and F# = 1 is F = F# × L = 64 wavelengths. The actual penetration depth or useful scanning depth will of course be deeper than the maximum focal depth.

Compliance of sensors and their clinical application

Now that we have mapped the types and properties of transducers to imaging windows and acoustic windows, we can use this information when selecting transducers for specific clinical applications. The suitability of certain transducers for specific applications has historically evolved with the help of specially adapted designs. Primary considerations are the target area of ​​interest, its extent, and the available acoustic windows needed for access.

Abdominal Imaging

When transducer arrays were first commercialized for abdominal imaging (including obstetrics and gynecology) in the 1970s, they were of the linear type (type A in Figure 4 with aspect ratio 1 in Figure 3). In most cases, the contact area with the patient was not a critical issue, and some of these linear probes were quite long (eg 8 cm) to cover, say, the fetal head in the third trimester. However, it soon became clear that sufficiently large coverage could be achieved by using curvilinear or convex matrix arrays (type C in Fig. 4) without paying the price of having to manipulate rather bulky linear transducers.

Curved matrices (Fig. 4C) are the tools of choice for most general 2D imaging in abdominal examinations. The overall form factor, related to ergonomic factors and matching the sensor shape and application FOV, for abdominal 3D imaging is still evolving. The three key descriptors for these arrays are the base area (total aperture size), field of view, and radius of curvature (Fig. 1C). The imprint depicts the area of ​​contact, usually in the form of a rectangle, circle or ellipse. Although access is not usually a problem for abdominal imaging, when these types of transducers are considered for new applications, access to windows is paramount. The radius of curvature and FOV (expressed in degrees of maximum angular coverage) are related to the scale and coverage of the image. Advanced signal processing has been added to some systems to improve penetration; however, this feature is usually only available on certain probes.

For mechanical 3D probes, the currently preferred form factor is the mechanically curved convex probe (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 (straight) scan directions. Alternatively, phased arrays, due to their small area and wide image format, are also used for abdominal imaging. Finally, 2D or matrix grids are becoming more common for these applications due to their superior image quality, resolution, and ease of use.

Intercostal Imaging

The main applications of this imaging group are cardiac scanning and examination of the liver between the ribs. Simply because of the restrictive anatomy and the limited acoustic windows created by the ribs and often invading lungs, the choice of transducer here is limited to phased arrays (Figure 4B). It was in this area that the first attempts were made to use linear gratings; however, they were quickly eliminated due to shading of the ribs and the superiority of the phased array of the format 2 sensor (Fig. 4). For cardiac applications, probes typically have a array size on the order of 20 x 14 mm, depending on the manufacturer. The area of ​​contact with the patient will be slightly larger. These numbers have evolved over the past 40 years and depend on a number of factors such as the number of patients. Age is another consideration; the distance between the ribs and the depth of penetration must be varied as children grow older.

For non-cardiac intercostal studies, the grid sizes are slightly larger. As noted earlier, the existence of these anatomical limitations creates an upper performance limit for spatial resolution, since resolution performance is inversely proportional to aperture size, as explained above. In studies for cardiac and general intercostal imaging, the image depth is large (depending on the size of the patient, it can be up to 24 cm), which forces the use of lower frequencies (1-3.5 MHz) and leads to some additional losses in image processing performance.

There is an interesting aspect of cardiac imaging that has had a profound effect on the nature of probes. Due to the presence of ribs and other acoustically hostile tissue in the beam path, echocardiography suffers from imaging artifacts due to reflected noise. The introduction of harmonic imaging has been very successful in reducing this noise. As a consequence, the importance of transducer bandwidth has become critical in heart transducer design. Today, most cardiac systems operate at frequencies between 1.5 and 2.0 MHz and, of course, receive signals at frequencies twice that range.

A major development in the field of cardiac imaging was the implementation of fully populated 2D or matrix arrays (type E) containing thousands (typically 50 × 50) of elements. This enables real-time (4D) display of pyramidal volumes (Format 7, Figure 3), visualization of arbitrary plane slices, 4D imaging of the heart, and color rendering. In addition, true electronic focusing in the xz and yz planes provides superior resolution compared to all other 1D sensors.

Superficial and thoracic imaging

This category refers to "surface" imaging of the carotid arteries, leg veins, chest, thyroid, testicles, etc., and includes the categories of small parts of the body, musculoskeletal system, and peripheral vascular imaging. This is the last bastion for the application of linear arrays (Type A) and formed the initial type of design for the studies discussed earlier. In this clinical category, access is generally not a problem, and the dimensions of the probes themselves can be small (due to the use of high frequencies between 7 and 15 MHz and the resulting small element sizes). Musculoskeletal studies also use this type of grating to visualize muscles, ligaments, and tendons. Over the past 10 years, breast imaging has shifted to very high frequencies (e.g., 14 MHz), while imaging of the peripheral vasculature has remained at lower frequencies (about 3-11 MHz) due to the need to include deeper veins and successful Doppler imaging. . Generally, the ability of the grid to add trapezoidal visualization (format 4) is a significant advantage. As with abdominal imaging, 3D imaging with mechanically curved probes or 2D electronic gratings is now available for superficial and thoracic applications, greatly improving the available coverage and image quality. For vascular imaging applications, some probes have the benefit of enabling modes that improve flow visualization.

obstetrics and gynecology

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

For gynecology special endo-matrix transducer forms (type D) are used. Typically, the gratings are at the end of the sensor and are convex or curved gratings with wide fields of view (format 3); however, phased arrays (type D) can also be used (format 2). The frequencies used are typically 5 MHz or higher. As with other applications, 2D gratings have been developed for 3D images in these cases.

Neonatal and pediatric

Pediatric transducers typically have smaller surfaces than adult transducers and operate at higher frequencies (≥7 MHz) than those used for adults. Depending on the area of ​​the body, types of transducers are applied, similar to those used for adults. Phased arrays (type B) and 3D transducers (types E and G) are suitable for cardiac imaging. Other arrays that are also useful for these clinical needs include static (2D) and, for 3D line arrays, mechanically curved and convex arrays.

Intracavitary studies

Intracavitary sensors are a large group of specialized sensors that are designed for imaging inside the body cavity. Transesophageal probes are used to get an image of the internal organs, especially the heart, from inside the esophagus (see Figure 5). They use higher frequencies (≥5 MHz) and are implemented as phased arrays with manipulators and motors to adjust the orientation of the sensor. Miniature transesophageal 2D transducers offer electronic scanning for 3D and 4D imaging.

Sensors can be highly specialized for viewing, typically in body orifices or vessels. Intracardiac phased probes are inserted through a vessel to access the internal chambers of the heart. Surgical specialized transducers include laparoscopic transducers inserted through small incisions to visualize and assist in laparoscopic surgery (similar to endo-probes); they are remarkable for their FOV despite their small diameters. Intraoperative probes are specially shaped for placement in vessels, organs, and areas accessible during open surgery (see Figure 5). Others in this class are surgical and interventional probes with unique shapes (see Figure 5).

As noted, sensors that fit into the body are designed to fit through small openings and have a wide field of view (90°-150°). These probes include transrectal (or endorectal) imaging of the pelvis using the anus for access, and the already described endovaginal (also referred to as transvaginal) imaging of the female pelvis and reproductive organs using the vagina as an entrance for gynecological and obstetric examinations. These endo-sensors described earlier are cylindrical to fit into small holes and have convex arrays (typically 3-9 MHz) at the ends with large fields of view, biplanes or mechanically curved convex arrays. Probes for urological applications include a biplane.

A unique sensor is the biplane probe, which consists of two orthogonal matrices that create images in the xz and yz planes. Usually matrices are small (8-12 mm) and convex type. Each shape and gauge must conform to the format of a single plane transducer, such as format 3 in Figure 3 and convex in Figure 4C. However, depending on the transducer design, sector or linear arrays are also possible, so several combinations can be used in practice. Alternatively, a subset of the rendering capabilities of a 2D array is the simultaneous presentation of two orthogonal 2D images.

Intravascular transducers are inserted into blood vessels to visualize vessel walls in various pathological conditions (type H and formats 8 and 9). Most often they are mechanically rotated single transducers with frequencies above 20 MHz and specialized imaging systems, although tiny (about 2 mm in diameter) arrays are also available for this.

head studies

Transcranial imaging of the brain and its vasculature is performed through limited acoustic windows in the skull, such as the temples or eyes. Transrobital arrays are high frequency (typically >20 MHz) ophthalmic transducers and are used to visualize the eye or use the eye as an acoustic window. Transcranial probes are typically low frequency (1-4 MHz) phased arrays used to visualize the blood vessels of the skull through the temples as windows.

conclusions

Many ultrasound transducers are designed to target specific areas of the body for specific applications. The main purpose of this article is to provide a systematic approach to help match the transducer to clinical use, starting with the acoustic window, area, and depth to be displayed. To this end, a checklist for transducer selection is provided in Table 1.

Table 1. Transducer Selection Checklist

As stated earlier, central to the discussion of imaging a target area or organs is access: the intended acoustic window.

The transducer type must provide access through the selected acoustic window. The transducer type is related to the aspect ratio of the image, and the common selections previously discussed include linear, phased, convex, and 2D matrices. The size or contact area of ​​the transducer must match the size of the window, and in extreme cases where the transducer window is a hole, the shape of the transducer must match the available hole. As noted above, some studies require special probes, such as endorectal probes, that are sufficiently small in diameter (size) and have an elongated shape suitable for entering a body opening.

Second, the size or FOV and aspect ratio are chosen to obtain the desired coverage in the region of interest. Both the depth of the scan and the width of the image, or FOV, are important here. For linear matrices, trapezoidal imaging may be required for adequate coverage. For a 3D, or volumetric image, the extent of the image can be specified as a set of maximum scan angles in orthogonal directions, or a field of view and an angle. A slightly more hidden parameter for 2D imaging to determine the area of ​​coverage for an area of ​​interest is the focal elevation depth, which describes the area with the thinnest slice thickness.

Third, the maximum scan depth selected determines the highest achievable frequency through the penetration ratio given above in the "Resolution and Penetration" section. For example, if the scan depth is 10 cm, then, as already discussed in the section "Resolution and penetration", the frequency from the penetration depth d is 60 / d = 60/10 = 6 MHz. This frequency gives an estimate of the best lateral resolution of about 1 wavelength for F#=1, or, for this example, a resolution of λ=c/f=0.25mm (from the Probe Properties and Imaging section). The exception to this rule are systems that use advanced signal processing to increase sensitivity and improve penetration. In addition, the use of piezoelectric materials can increase the sensitivity and thus the penetration depth.

Fourth, the coverage of the main modes of diagnostic imaging can be determined. From the data provided by the manufacturer, the effective bandwidth needed to support the various modes can be extracted, or the actual modes of interest can be listed for the system in question, such as pulsed Doppler, multiple displayed frequencies, or elastographic mode. Transducers with piezoelectric materials can significantly increase the bandwidth.

In conclusion, transducers and graphic formats have evolved to better suit specific clinical applications. The classification and organization given in this article is a prerequisite for choosing a transducer for a particular purpose. In addition, the provided insight can help determine the sensor characteristics required for new cases, thereby expanding the range of sensor use.