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High frequency (20 MHz) ultrasonic devices: advantages and applications
European Journal of Ultrasound 10 (1999) 53 – 63 www.elsevier.com/locate/ejultrasou
Scientific paper
High frequency (20 MHz) ultrasonic devices: advantages and applications M. Berson a,*, J.M. Gre´goire b, F. Gens b, J. Rateau c, F. Jamet b, L. Vaillant d, F. Tranquart a, L. Pourcelot a,b a
Unite´ INSERM 316, Faculte´ de Me´decine, 2 bis boule6ard Tonnelle´, 37032 Tours Cedex, France b GIP Ultrasons, Faculte´ de Me´decine, 2 bis boule6ard Tonnelle´, 37032 Tours Cedex, France c Ser6ice d’Ophtalmologie, CHU Tours, Hoˆpital Bretonneau, 2 boule6ard Tonnelle´, 37032 Tours Cedex, France d Ser6ice de Dermatologie, CHU Tours, Hoˆpital Trousseau, F-37044 Tours Cedex, France Received 30 November 1998; received in revised form 4 June 1999; accepted 7 June 1999
Abstract Objecti6e: This paper investigates the problems, advantages and potential applications of 20 MHz ultrasonic devices. Method: Aqueous gel and a thin appropriate membrane to enclose the front tip were used with 20 MHz probes without obvious decrease in resolution and sensitivity compared to the results obtained without a membrane and this considerably facilitates their routine use. Results: Many applications with linear scanning were evaluated in dermatology, ophthalmology (investigations of the anterior chamber of the eye, checking of corneal grafts), stomatology (detection and evaluation of periodontal disease) and in the field of measurement of very low velocities in small vessels by means of a duplex probe comprising two 20 MHz transducers: an imaging transducer and an inclined blood flow measurement transducer. Velocity profiles (velocities less than 0.50 mm/s) were measured in 100–300 mm diameter vessels using a cross-correlation method. Conclusion: The use of 20 MHz frequency limits resolution but we have shown that this frequency allows the development of easy to handle probes. © 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: High frequency ultrasound; Membrane; Linear scanning; Duplex probe; Blood flow; Cross-correlation method; Low velocities
1. Introduction Many new clinical applications of B-mode ultrasound imaging at frequencies greater than 15– * Corresponding author. Tel.: +33-247-366052; fax: + 33247-366152. E-mail address: [email protected] (M. Berson)
20 MHz have been under development for some years. The use of these frequencies enables the transcutaneous exploration of superficial tissues in fields such as dermatology, ophthalmology and microsurgery and for investigations in small laboratory animals (mice, rats, etc.). The increase in range to higher frequencies has followed the notable progress in areas such as transducers
0929-8266/99/$ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 9 2 9 - 8 2 6 6 ( 9 9 ) 0 0 0 4 3 - 9
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M. Berson et al. / European Journal of Ultrasound 10 (1999) 53–63
(piezoelectric material, sensitivity and focusing), fast electronic circuits and digitalization of signals. Presently the field of high ultrasonic frequency ranges from 15 – 20 to 150 – 200 MHz, providing a very high spatial resolution between 100 mm (axially) ×200 – 300 mm (laterally) and 5–10× 10– 30 mm. However the increased signal losses in tissue (attenuation) associated with higher ultrasound frequencies naturally limit imaging depth: 1 – 2 cm (15 – 20 MHz) to about 1 mm (150–200 MHz). High ultrasonic frequencies (10 – 30 MHz) were first used to measure skin thickness (Alexander and Miller, 1979; Tan et al., 1982; Serup, 1984). Then a range of frequencies between 15 and 50 MHz was tested in B-mode imaging with encouraging results (Payne et al., 1982; Dines et al., 1984; Breitbart et al., 1985; Payne, 1985, 1987) but no efficient systems were usable routinely, generally due to poor probe performance. Indeed the probes often had a too narrow bandwidth and provided poor axial resolution. Querleux et al. (1988), De Rigal et al. (1989) (image acquisition time: about 2 min) and Berson et al. (1992) (real time: 15 images/s) obtained very good quality images of various structural elements of the human skin with frequencies around 20 MHz. The study and use of new piezoelectric materials such as plastics (PVDF, poly(vinylidene fluoride)) and small grain size (3 – 6 mm) piezoceramics (PZT) allowed Foster’s Canadian team to build small focused ultrasonic transducers in the 40–100 MHz frequency range (Sherar and Foster, 1989; Lockwood et al., 1991, 1994). With these high performance transducers and associated electronic devices they obtained remarkable images of cutaneous tissues, the anterior chamber of the eye and mouse embryos (Sherar et al., 1989; Pavlin et al., 1990, 1992, 1993; Foster et al., 1993; Turnbull et al., 1995; Lockwood et al., 1996). Passmann and Hermert (1996) recently proposed the B/D (or B/Z) scanning technique (scanning of the transducer in the Z or depth direction) and a synthetic aperture focusing technique (SAFT) and signal processing to minimize the disadvantages of using very strongly focussed high frequency transducers and to improve image quality (homogenization of resolution and sensitivity) through the whole
depth of exploration. They thus obtained very high-quality homogeneous images of the skin (cancer, eczema, etc.) and pig eye (cornea and anterior chamber area). Finally Knapik et al. (1997) proposed a real time 200 MHz B-scan imager using spherically focused lithium niobate transducers for the assessment of micro structures. Several studies have been performed in the field of dental diagnosis, to explore periodontal diseases at 20 MHz (Muraoka et al., 1982; Fukukita et al., 1985). These studies have shown the possibility of such investigations but, despite the need, this application which requires appropriately sized and shaped probes has not yet been fully developed. High frequencies are also suitable to detect and measure blood flow in microcirculation. Frequencies around 100 MHz were proposed first (Frew and Giblin, 1985; Berson et al., 1989) and then Foster’s team presented a duplex system with a duplex probe comprising a 60 MHz imaging transducer and a 40 MHz continuous wave Doppler transducer (Christopher et al., 1996). The same team has recently built a high frequency pulsed wave Doppler system using a 50 MHz PVDF transducer (Christopher et al., 1997) which can detect and measure blood velocities of less than 5 mm/s in arterioles and venules with diameters as small as 20 and 30 mm respectively. The use of very high frequencies (greater than 50 MHz) is very valuable to obtain high-quality images and information but it also has some important disadvantages: very short depth of exploration; considerable technological difficulties in the building of high performance ultrasonic transducers and probes and associated electronic systems; considerable difficulties in the manipulation of probes due to the impossibility of using gel and particularly a thin membrane as an acoustic window. Therefore, the highest frequency systems are generally difficult to use routinely and they remain laboratory systems. We therefore chose to explore all the routine possibilities and applications of the 20 MHz devices.
M. Berson et al. / European Journal of Ultrasound 10 (1999) 53–63
This paper presents a 20 MHz device and its different applications in dermatology, ophthalmology and stomatology as a duplex system to measure blood flow in very small vessels by means of a cross-correlation method.
2. Materials and method
2.1. Scanning The success of array transducers for low frequency (B 10 MHz) applications is well known and the value of these arrays to explore the skin, for example, is very clear. However, the difficult technological problems to build these arrays increase with frequency and they are not yet commercially available in the high frequency field and we, therefore, have to use mechanically moved monotransducers to obtain B-mode images. The focusing of the acoustic beam can be achieved by using a curved transducer or an acoustic lens fixed onto a plane transducer or a combination of a
55
plane transducer with an appropriately curved mirror. We tested three types of mechanical scanning of the transducer for the different applications: Linear scanning (Fig. 1a) with movement of the transducer parallel to the skin contact area. This movement provides a simple rectangular echographic cross section. Sector scanning (Fig. 1b) with oscillating movement of the transducer around an axis of rotation providing an echographic cross section in the form of a truncated sector. Sector scanning obtained with a ‘toothbrush probe’ and rotation of the acoustic beam (Fig. 1c). The beam emitted by the transducer in the axis of the probe is reflected perpendicularly and focused by a curved rotating mirror. Due to the limited acoustic window on the side of the probe the echographic cross-section also has the shape of a truncated sector. This special system is very suitable for explorations in the mouth and especially for investigations of the gengiva.
Fig. 1. Different types of scanning of the ultrasonic transducer: (a) linear scanning, (b) sector scanning, (c) perpendicular sector scanning.
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2.2. Membrane
Fig. 2. Setup used to measure the influence of the membrane.
Linear and sector scanning of Fig. 1a and b are fairly similar but sector scanning allows the use of a smaller probe and particularly a smaller front tip in contact with the skin. This can be an advantage in investigating the anterior chamber of the eye or some areas of the body (nose, ears, etc.). On the other hand the need to use a scan converter to present the image is a complication and a disadvantage of sector scanning. The presentation of the rectangular image of linear scanning is very simple and in general such scanning is very suitable for exploration of large plane surfaces of the body but it is mechanically more complicated and difficult to perform. In all three cases a coupling liquid must be used to transmit the acoustic beam and frequency dependent attenuation means that this coupling liquid can act to a greater or lesser degree as an attenuating low pass filter which reduces image quality (resolution, sensitivity). In practice, in the high frequency range, water is the only possible coupling liquid.
It may be clear that the presence of a thin membrane as an acoustic window to close the front tip of the probe considerably facilitates explorations in routine use. However, as with the coupling liquid, the membrane can be an attenuating low pass filter which damages the impulse response of the system to a greater or lesser degree. This membrane can also create parasitic repetitive echoes which blur the image. The influence of the membrane depends on its thickness, attenuating properties and acoustic impedance. To study the effect of a thin membrane on the impulse response of a 20 MHz center frequency system, we built a small test setup (Fig. 2). The acoustic beam which crosses a membrane is reflected by a flat target in a small water tank. Studying the echo provides an evaluation of the influence of the membrane. Fig. 3 shows that the impulse response (echo) produced with a very inappropriate membrane (Kapton) (Fig. 3b) is
Fig. 3. (a) Reference echo obtained without a membrane, (b) attenuated delayed echo with an inappropriate membrane.
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MHz. This is due to the attenuation which considerably increases with frequency.
2.3. Probe and imaging de6ice Taking into account all the previous considerations, we first built the probe presented in Fig. 5 using linear scanning with a removable front tip closed by a thin membrane and a broadband 20 MHz center frequency transducer focused at a distance of 7.5 mm in water. The cylindrical device (31 mm in diameter, 170 mm in length) gives a 6 mm (width)× 5 mm (depth) echographic cross-section. The contact area with the skin (20× 12 mm) is minimal. This very easy to handle probe is part of an imaging system (Fig. 6) also developed at our laboratory and called Dermcup 2020 (marketed by 2MT, Toulouse, France). The high resolution (80 mm axially, 250 mm laterally) rectangular image is presented via
Fig. 4. (a) Reference echo without a membrane, (b) echo with an appropriate membrane, (c) echo with an inappropriate membrane.
attenuated and considerably delayed compared to the reference obtained without a membrane (Fig. 3a). Such a modified echo provides poor resolution and sensitivity. Fig. 4 shows the comparison of the three types of impulse response: reference without membrane (Fig. 4a), with an appropriate membrane (BPX 15, GIP Ultrasons, Tours) (Fig. 4b) and with an inappropriate membrane (Fig. 4c). It can be seen that the appropriate membrane provides an echo very similar to the reference. In addition, this appropriate thin (15 mm) membrane can be thermo-shaped on the front tip and its acoustic impedance is sufficiently well matched to avoid cumbersome repetitive echoes. The use of an appropriate membrane, possible in association with a broadband 20 MHz center frequency transducer, becomes impossible with frequencies higher than 30
Fig. 5. Diagram of the linear scanning probe.
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40° so that its acoustic beam crosses the central part of the echographic cross-section (Fig. 7b). The focal length ( f ) and the diameter (d) of the blood flow transducer are doubled as compared to those of the imaging transducer (the same f-number f/d is kept) to maintain good resolution and sensitivity because the distance of the area of measurement from the transducer is increased. The blood flow transducer is connected to an electronic module dedicated to the emission process and amplification (60 dB) of the received RF signal. The amplified RF signal is transmitted to the PC via a 12 bit-100 MHz sampling rate acquisition board. The duplex system thus comprises the duplex probe, the electronic imaging module, the electronic blood flow module and the PC. Software based on the acquisition of several tens of RF lines and on temporal shift evaluation by cross-correlation was developed to compute the blood velocity (Gens, 1998). This method is now widely used (Bonnefous and Pesque´, 1986; Ferrara et al., 1996). The software includes re-
Fig. 6. View of the imaging system.
an acquisition board on the screen of a personal computer (PC) at a rate of 10 images/s. The digitizer uses 8 bits and a 50, or 100 MHz sampling rate.
2.4. Duplex system In order to measure the blood flow in very small superficial vessels, we built a duplex system based on the imaging scanner presented above (Gens, 1998). The imaging probe was slightly modified at the level of the front tip to include an inclined 20 MHz pulsed wave transducer intended for blood flow measurement (Fig. 7a). The imaging transducer remains perpendicular and the blood flow transducer is inclined at an angle of
Fig. 7. (a) View of the duplex probe, (b) arrangement of the two ultrasonic transducers.
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Fig. 8. Skin exploration: (a) view of the probe in position for exploration, (b) image of normal skin on the back of the hand (d, dermis; v+ a, vein+agregates), (c) image of a small fibroma (f, fibroma), (d) image of a basal cell carcinoma (b, basal cell carcinoma).
alignment of RF lines and fixed echo canceling procedures. The value of the correlation coefficient is also used to improve detection in the case of low signal to noise ratio (SNR) (Gens et al., 1997).
3. Applications and results
3.1. Skin explorations The device is currently in routine use in several Departments of Dermatology and the probe de-
scribed above is very easy to handle to examine skin (Fig. 8a). The examination is conducted with a small quantity of appropriate ultrasonic gel (sufficiently liquid) to avoid a too high attenuation. We give some examples of images in Fig. 8 to illustrate the possibilities. Fig. 8b presents a classical image of normal skin on the back of the hand with the image of the membrane (top, slightly curved white line), the very bright line of the gel–skin interface, the echogenic layer of the dermis (d) (about 1 mm thick) and a vein. The walls of the vein and inside the aggregates of red cells (v + a) can be clearly
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seen. With a frequency of 20 MHz the epidermis is only visualized when its thickness is greater than the resolution of the system (about 80 mm, forehead, palms, etc.). An image of a small fibroma (f) (about 5 mm in width and 1 mm thick) is shown in Fig. 8c. The device is also very useful to visualize tumors (as illustrated in Fig. 8d with the image of a basal cell carcinoma (b)). Generally, the images of basal cell carcinoma are characterized by the presence of a few large bright echoes (arrow) inside the tumor. The very easy to handle probe and small front tip with a membrane considerably facilitate exploration of carcinomas located in areas with difficult access such as the nostrils.
3.2. Eye explorations Such explorations involve the anterior part of the eye, principally the anterior chamber. Due to the presence of the membrane the examination is performed easily, rapidly and safely (Fig. 9). This examination requires slight local anesthesia and the front tip can be placed on the eyelid or directly on the eye. Fig. 10a provides a classical image of a normal anterior chamber comprising sclera (s), cornea (c), iris (i), ciliary body (c b) and wide, normal open irido-corneal angle (i–c a). The tissue reflectivity of the different structures can be seen: weak for the cornea, high for the sclera and intermediate for the iris and ciliary
Fig. 9. View of the probe during an eye exploration.
body. For comparison, Fig. 10b shows an example of the very narrowed angle obtained in glaucoma. Cysts on the iris and ciliary body can also be clearly visualized (Fig. 10c). Rapid verification of corneal graft is another interesting application (Fig. 10d). The graft appears clearly on the image (circle) and the scar at the junction is slightly distended.
3.3. Dental explorations The image shown in Fig. 11 was acquired with the classical linear scanning probe because the special ‘toothbrush probe’ presented in Fig. 1c was not yet operational but this normal image demonstrates the potential for periodontal examinations. In this vertical (longitudinal cross-section of the tooth) cross-section, the gengiva, alveolar bone and the tooth can be seen. When periodontal disease is present, it is revealed by a cavity appearing between the gengiva and the tooth and in the case of extensive damage, by bone reduction. To evaluate this reduction it is necessary to measure the distance between the margin of the gengiva and the crest of the bone. The image presented shows that diagnosis can easily be made by ultrasound, particularly with a special ‘toothbrush probe’.
3.4. Blood flow measurements Measurements were made with hospital in-patients, particularly those with angioma. Fig. 12a shows an image of an angiomatous area at the level of the calf of a child after laser treatment. Compared to normal skin, the dermis is very echogenic and there are many echoes within the hypodermis. We can distinguish (arrow) a small vessel (about 300 mm in diameter) in which we measured blood flow velocity. The velocity profile is clearly presented in Fig. 12b. The maximum velocity in the vessel was about 0.45 mm/s and several velocity levels can be differentiated between 0 and 0.25 mm/s, demonstrating the capacity of the system to measure very low velocities. Measurements were made with a pulse repetition frequency (PRF) of 1 kHz and a 40 mm measurement step. Other areas in the angioma were not
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Fig. 10. (a) Image of a normal anterior chamber of the eye (s, sclera; c, cornea; i, iris; c b, ciliary body; i-c a, irido-corneal angle), (b) very narrow irido-corneal angle, (c) images of cysts in iris and ciliary body, (d) investigation of a corneal graft.
Fig. 11. Dental exploration: vertical (or longitudinal) echographic cross-section of the bone – gengiva – tooth system.
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Fig. 12. Blood flow measurement: (a) image of an angiomatous area after laser treatment, with a small vessel (arrow), (b) velocity profile in the small vessel.
studied because the limited resolution (80 mm) of the system did not allow the detection of the numerous small capillaries.
4. Conclusion The use of 20 MHz ultrasonic frequency limits resolution (about 80 mm axially and 250 mm laterally) but, as we have demonstrated, allows the construction of easy to handle probes with removable front tips covered by an appropriate membrane, which considerably facilitates routine explorations. The examples provided of applications in dermatology, ophthalmology and stomatology and in the field of measurement of very low velocities (less than 0.50 mm/s) in very small vessels illustrate that 20 MHz systems can be very valuable investigation tools. It is evident that the special studied ‘toothbrush probe’ could be highly appropriate for routine use in the field of stomatology and dental explorations. High resolution is required and higher frequencies will be necessary to measure blood flow in small capillaries, however ultrasonic investigation of the microcirculation will remain difficult.
Acknowledgements The authors wish to thank R. Battault, L. Colin and C. Yvon for their help and essential contribution in performing the electronic and the software development.
References Alexander H, Miller DL. Determining skin thickness with pulsed ultrasound. J Invest Dermatol 1979;72:17 – 9. Berson M, Patat F, Wang ZQ, Besse D, Pourcelot L. Very high frequency pulsed Doppler apparatus. Ultrasound Med Biol 1989;15:121 – 32. Berson M, Vaillant L, Patat F, Pourcelot L. High-resolution real-time ultrasonic scanner. Ultrasound Med Biol 1992;18(5):471– 8. Bonnefous O, Pesque´ P. Time domain formulation of pulseDoppler ultrasound and blood velocity estimation by cross-correlation. Ultrason Imaging 1986;8:73 – 85. Breitbart EW, Rehpenning W, Hicks R, Dyson E. In vivo investigations of the skin, its structure and elasticity with high frequency ultrasound. Bioeng Skin 1985;1:231 –6. Christopher DA, Burns PN, Armstrong J, Foster FS. A high frequency continuous-wave Doppler ultrasound system for the detection of blood flow in the microcirculation. Ultrasound Med Biol 1996;22(9):1191– 203.
M. Berson et al. / European Journal of Ultrasound 10 (1999) 53–63 Christopher DA, Burns PN, Starkoski BG, Foster FS. A high frequency pulsed-wave Doppler ultrasound system for the detection and imaging of blood flow in the microcirculation. Ultrasound Med Biol 1997;23(7):997– 1015. De Rigal J, Agache P, Leveque JL. Assessment of aging of the human skin by in vivo ultrasonic imaging. J Invest Dermatol 1989;5:621–5. Dines KA, Sheets PW, Brink JA, et al. High frequency ultrasonic imaging of skin: experimental results. Ultrason Imaging 1984;6:408–36. Ferrara KW, Zagar BG, Sokil-Melgar JB, Silverman RH, Aslanidis IM. Estimation of blood velocity with high frequency ultrasound. IEEE Trans Ultrason Ferroelec Freq Contr 1996;43(1):149–157. Foster FS, Pavlin CJ, Lockwood GR, et al. Principles and applications of ultrasound backscatter microscopy. IEEE Trans Ultrason Ferroelec Freq Contr 1993;40(5):608– 16. Frew HS, Giblin RA. The choice of ultrasound frequency for skin blood flow investigation. Bioeng Skin 1985;1:193 –205. Fukukita H, Yano T, Fukumoto A, Sawada K, Fujimasa T, Sunada I. Development and application of an ultrasonic imaging system for dental diagnosis. J Clin Ultrasound 1985;13:597–600. Gens F, Remenieras JP, Patat F, Diridollou S, Gall Y, Berson M. Estimation of the correlation amplitude of RF signals. Application to microcirculation study. IEEE Ultrason Symp 1997:1251–1254. Gens F. Syste`me ultrasonore d’imagerie et d’estimation du flux sanguin par intercorrelation haute fre´quence: application a` l’e´tude de la microcirculation cutane´e. The`se en Sciences de la Vie et de la Sante´, Tours 1998. Knapik DA, Starkoski, Pavlin CJ, Foster FS. A real time 200 MHz ultrasound B-scan imager. IEEE Ultrason Symp 1997:1457–1460. Lockwood GR, Hunt JW, Foster FS. Design of protection circuitry for high frequency ultrasound systems. IEEE Trans Ultrason Ferroelec Freq Contr 1991;38:48 – 55. Lockwood GR, Turnbull DH, Foster FS. Fabrication of high frequency spherically shaped PZT transducers. IEEE Trans Ultrason Ferroelec Freq Contr 1994;41:231 – 5. Lockwood GR, Turnbull DH, Christopher DA, Foster FS. Beyond 30 MHz. Application of high-frequency ultra-
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sound imaging. IEEE Eng Med Biol 1996;November– December:60 – 71. Muraoka Y, Sueda T, Kinoshita S. Examination of periodontal tissue with the ultrasonic apparatus. J Jpn Assoc Periodont 1982;24:601. Passmann C, Hermert H. A 100-MHz Ultrasound imaging system for dermatologic and ophtalmologic diagnosis. IEEE Trans Ultrason Ferroelec Freq Contr 1996;43 (4):545 – 52. Pavlin CJ, Sherar MD, Foster FS. Subsurface imaging of the eye by ultrasound backscatter microscopy. Ophtalmology 1990;97:244 – 50. Pavlin CJ, Harasiewicz KA, Foster FS. Ultrasound biomicroscopy of anterior structures in normal glaucomatous eyes. Am J Ophtalmol 1992;113:381 – 9. Pavlin CJ, Easterbrook M, Hurwitz JJ, Harasiewicz KA, Foster FS. Ultrasound biomicroscopy in the assessment of anterior scleral disease. Am J Ophtalmol 1993;116:628 – 35. Payne PA. Application of ultrasound in dermatology. Bioeng Skin 1985;1:293 – 320. Payne PA. Ultrasonic methods for skin characterization. Bioeng Skin 1987;3:347 – 57. Payne PA, Grove GL, Alexander H. Cross-sectional ultrasonic scanning of skin using plastic film transducers. Newslett Bioeng Skin 1982;3:241 – 6. Querleux B, Leveque JL, De Rigal J. In vivo cross sectional ultrasonic imaging of human skin. Dermatologica 1988;177:332 – 7. Serup J. Non invasive quantification of psoriasis plaques. Measurement of skin thickness with 15 MHz pulsed ultrasound. Clin Exp Dermatol 1984;9:502 – 8. Sherar MD, Foster FS. The design and fabrication of high frequency Poly(vinylidene fluoride) transducers. Ultrason Imaging 1989;11:75 – 94. Sherar MD, Starkoski BG, Foster FS. A 100 MHz B-scan ultrasound backscatter microscope. Ultrason Imaging 1989;11:95 – 105. Tan CY, Statham B, Marks R, Payne PA. Skin thickness measurement by pulsed ultrasound: Its reproducibility, validation and variability. Br J Dermatol 1982;106:657– 667. Turnbull DH, Starkoski BG, Harasiewicz KA, et al. A 40100 MHz B-scan ultrasound backscatter microscope for skin imaging. Ultrasound Med Biol 1995;21:79 – 88.
Scientific paper
High frequency (20 MHz) ultrasonic devices: advantages and applications M. Berson a,*, J.M. Gre´goire b, F. Gens b, J. Rateau c, F. Jamet b, L. Vaillant d, F. Tranquart a, L. Pourcelot a,b a
Unite´ INSERM 316, Faculte´ de Me´decine, 2 bis boule6ard Tonnelle´, 37032 Tours Cedex, France b GIP Ultrasons, Faculte´ de Me´decine, 2 bis boule6ard Tonnelle´, 37032 Tours Cedex, France c Ser6ice d’Ophtalmologie, CHU Tours, Hoˆpital Bretonneau, 2 boule6ard Tonnelle´, 37032 Tours Cedex, France d Ser6ice de Dermatologie, CHU Tours, Hoˆpital Trousseau, F-37044 Tours Cedex, France Received 30 November 1998; received in revised form 4 June 1999; accepted 7 June 1999
Abstract Objecti6e: This paper investigates the problems, advantages and potential applications of 20 MHz ultrasonic devices. Method: Aqueous gel and a thin appropriate membrane to enclose the front tip were used with 20 MHz probes without obvious decrease in resolution and sensitivity compared to the results obtained without a membrane and this considerably facilitates their routine use. Results: Many applications with linear scanning were evaluated in dermatology, ophthalmology (investigations of the anterior chamber of the eye, checking of corneal grafts), stomatology (detection and evaluation of periodontal disease) and in the field of measurement of very low velocities in small vessels by means of a duplex probe comprising two 20 MHz transducers: an imaging transducer and an inclined blood flow measurement transducer. Velocity profiles (velocities less than 0.50 mm/s) were measured in 100–300 mm diameter vessels using a cross-correlation method. Conclusion: The use of 20 MHz frequency limits resolution but we have shown that this frequency allows the development of easy to handle probes. © 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: High frequency ultrasound; Membrane; Linear scanning; Duplex probe; Blood flow; Cross-correlation method; Low velocities
1. Introduction Many new clinical applications of B-mode ultrasound imaging at frequencies greater than 15– * Corresponding author. Tel.: +33-247-366052; fax: + 33247-366152. E-mail address: [email protected] (M. Berson)
20 MHz have been under development for some years. The use of these frequencies enables the transcutaneous exploration of superficial tissues in fields such as dermatology, ophthalmology and microsurgery and for investigations in small laboratory animals (mice, rats, etc.). The increase in range to higher frequencies has followed the notable progress in areas such as transducers
0929-8266/99/$ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 9 2 9 - 8 2 6 6 ( 9 9 ) 0 0 0 4 3 - 9
54
M. Berson et al. / European Journal of Ultrasound 10 (1999) 53–63
(piezoelectric material, sensitivity and focusing), fast electronic circuits and digitalization of signals. Presently the field of high ultrasonic frequency ranges from 15 – 20 to 150 – 200 MHz, providing a very high spatial resolution between 100 mm (axially) ×200 – 300 mm (laterally) and 5–10× 10– 30 mm. However the increased signal losses in tissue (attenuation) associated with higher ultrasound frequencies naturally limit imaging depth: 1 – 2 cm (15 – 20 MHz) to about 1 mm (150–200 MHz). High ultrasonic frequencies (10 – 30 MHz) were first used to measure skin thickness (Alexander and Miller, 1979; Tan et al., 1982; Serup, 1984). Then a range of frequencies between 15 and 50 MHz was tested in B-mode imaging with encouraging results (Payne et al., 1982; Dines et al., 1984; Breitbart et al., 1985; Payne, 1985, 1987) but no efficient systems were usable routinely, generally due to poor probe performance. Indeed the probes often had a too narrow bandwidth and provided poor axial resolution. Querleux et al. (1988), De Rigal et al. (1989) (image acquisition time: about 2 min) and Berson et al. (1992) (real time: 15 images/s) obtained very good quality images of various structural elements of the human skin with frequencies around 20 MHz. The study and use of new piezoelectric materials such as plastics (PVDF, poly(vinylidene fluoride)) and small grain size (3 – 6 mm) piezoceramics (PZT) allowed Foster’s Canadian team to build small focused ultrasonic transducers in the 40–100 MHz frequency range (Sherar and Foster, 1989; Lockwood et al., 1991, 1994). With these high performance transducers and associated electronic devices they obtained remarkable images of cutaneous tissues, the anterior chamber of the eye and mouse embryos (Sherar et al., 1989; Pavlin et al., 1990, 1992, 1993; Foster et al., 1993; Turnbull et al., 1995; Lockwood et al., 1996). Passmann and Hermert (1996) recently proposed the B/D (or B/Z) scanning technique (scanning of the transducer in the Z or depth direction) and a synthetic aperture focusing technique (SAFT) and signal processing to minimize the disadvantages of using very strongly focussed high frequency transducers and to improve image quality (homogenization of resolution and sensitivity) through the whole
depth of exploration. They thus obtained very high-quality homogeneous images of the skin (cancer, eczema, etc.) and pig eye (cornea and anterior chamber area). Finally Knapik et al. (1997) proposed a real time 200 MHz B-scan imager using spherically focused lithium niobate transducers for the assessment of micro structures. Several studies have been performed in the field of dental diagnosis, to explore periodontal diseases at 20 MHz (Muraoka et al., 1982; Fukukita et al., 1985). These studies have shown the possibility of such investigations but, despite the need, this application which requires appropriately sized and shaped probes has not yet been fully developed. High frequencies are also suitable to detect and measure blood flow in microcirculation. Frequencies around 100 MHz were proposed first (Frew and Giblin, 1985; Berson et al., 1989) and then Foster’s team presented a duplex system with a duplex probe comprising a 60 MHz imaging transducer and a 40 MHz continuous wave Doppler transducer (Christopher et al., 1996). The same team has recently built a high frequency pulsed wave Doppler system using a 50 MHz PVDF transducer (Christopher et al., 1997) which can detect and measure blood velocities of less than 5 mm/s in arterioles and venules with diameters as small as 20 and 30 mm respectively. The use of very high frequencies (greater than 50 MHz) is very valuable to obtain high-quality images and information but it also has some important disadvantages: very short depth of exploration; considerable technological difficulties in the building of high performance ultrasonic transducers and probes and associated electronic systems; considerable difficulties in the manipulation of probes due to the impossibility of using gel and particularly a thin membrane as an acoustic window. Therefore, the highest frequency systems are generally difficult to use routinely and they remain laboratory systems. We therefore chose to explore all the routine possibilities and applications of the 20 MHz devices.
M. Berson et al. / European Journal of Ultrasound 10 (1999) 53–63
This paper presents a 20 MHz device and its different applications in dermatology, ophthalmology and stomatology as a duplex system to measure blood flow in very small vessels by means of a cross-correlation method.
2. Materials and method
2.1. Scanning The success of array transducers for low frequency (B 10 MHz) applications is well known and the value of these arrays to explore the skin, for example, is very clear. However, the difficult technological problems to build these arrays increase with frequency and they are not yet commercially available in the high frequency field and we, therefore, have to use mechanically moved monotransducers to obtain B-mode images. The focusing of the acoustic beam can be achieved by using a curved transducer or an acoustic lens fixed onto a plane transducer or a combination of a
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plane transducer with an appropriately curved mirror. We tested three types of mechanical scanning of the transducer for the different applications: Linear scanning (Fig. 1a) with movement of the transducer parallel to the skin contact area. This movement provides a simple rectangular echographic cross section. Sector scanning (Fig. 1b) with oscillating movement of the transducer around an axis of rotation providing an echographic cross section in the form of a truncated sector. Sector scanning obtained with a ‘toothbrush probe’ and rotation of the acoustic beam (Fig. 1c). The beam emitted by the transducer in the axis of the probe is reflected perpendicularly and focused by a curved rotating mirror. Due to the limited acoustic window on the side of the probe the echographic cross-section also has the shape of a truncated sector. This special system is very suitable for explorations in the mouth and especially for investigations of the gengiva.
Fig. 1. Different types of scanning of the ultrasonic transducer: (a) linear scanning, (b) sector scanning, (c) perpendicular sector scanning.
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2.2. Membrane
Fig. 2. Setup used to measure the influence of the membrane.
Linear and sector scanning of Fig. 1a and b are fairly similar but sector scanning allows the use of a smaller probe and particularly a smaller front tip in contact with the skin. This can be an advantage in investigating the anterior chamber of the eye or some areas of the body (nose, ears, etc.). On the other hand the need to use a scan converter to present the image is a complication and a disadvantage of sector scanning. The presentation of the rectangular image of linear scanning is very simple and in general such scanning is very suitable for exploration of large plane surfaces of the body but it is mechanically more complicated and difficult to perform. In all three cases a coupling liquid must be used to transmit the acoustic beam and frequency dependent attenuation means that this coupling liquid can act to a greater or lesser degree as an attenuating low pass filter which reduces image quality (resolution, sensitivity). In practice, in the high frequency range, water is the only possible coupling liquid.
It may be clear that the presence of a thin membrane as an acoustic window to close the front tip of the probe considerably facilitates explorations in routine use. However, as with the coupling liquid, the membrane can be an attenuating low pass filter which damages the impulse response of the system to a greater or lesser degree. This membrane can also create parasitic repetitive echoes which blur the image. The influence of the membrane depends on its thickness, attenuating properties and acoustic impedance. To study the effect of a thin membrane on the impulse response of a 20 MHz center frequency system, we built a small test setup (Fig. 2). The acoustic beam which crosses a membrane is reflected by a flat target in a small water tank. Studying the echo provides an evaluation of the influence of the membrane. Fig. 3 shows that the impulse response (echo) produced with a very inappropriate membrane (Kapton) (Fig. 3b) is
Fig. 3. (a) Reference echo obtained without a membrane, (b) attenuated delayed echo with an inappropriate membrane.
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MHz. This is due to the attenuation which considerably increases with frequency.
2.3. Probe and imaging de6ice Taking into account all the previous considerations, we first built the probe presented in Fig. 5 using linear scanning with a removable front tip closed by a thin membrane and a broadband 20 MHz center frequency transducer focused at a distance of 7.5 mm in water. The cylindrical device (31 mm in diameter, 170 mm in length) gives a 6 mm (width)× 5 mm (depth) echographic cross-section. The contact area with the skin (20× 12 mm) is minimal. This very easy to handle probe is part of an imaging system (Fig. 6) also developed at our laboratory and called Dermcup 2020 (marketed by 2MT, Toulouse, France). The high resolution (80 mm axially, 250 mm laterally) rectangular image is presented via
Fig. 4. (a) Reference echo without a membrane, (b) echo with an appropriate membrane, (c) echo with an inappropriate membrane.
attenuated and considerably delayed compared to the reference obtained without a membrane (Fig. 3a). Such a modified echo provides poor resolution and sensitivity. Fig. 4 shows the comparison of the three types of impulse response: reference without membrane (Fig. 4a), with an appropriate membrane (BPX 15, GIP Ultrasons, Tours) (Fig. 4b) and with an inappropriate membrane (Fig. 4c). It can be seen that the appropriate membrane provides an echo very similar to the reference. In addition, this appropriate thin (15 mm) membrane can be thermo-shaped on the front tip and its acoustic impedance is sufficiently well matched to avoid cumbersome repetitive echoes. The use of an appropriate membrane, possible in association with a broadband 20 MHz center frequency transducer, becomes impossible with frequencies higher than 30
Fig. 5. Diagram of the linear scanning probe.
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40° so that its acoustic beam crosses the central part of the echographic cross-section (Fig. 7b). The focal length ( f ) and the diameter (d) of the blood flow transducer are doubled as compared to those of the imaging transducer (the same f-number f/d is kept) to maintain good resolution and sensitivity because the distance of the area of measurement from the transducer is increased. The blood flow transducer is connected to an electronic module dedicated to the emission process and amplification (60 dB) of the received RF signal. The amplified RF signal is transmitted to the PC via a 12 bit-100 MHz sampling rate acquisition board. The duplex system thus comprises the duplex probe, the electronic imaging module, the electronic blood flow module and the PC. Software based on the acquisition of several tens of RF lines and on temporal shift evaluation by cross-correlation was developed to compute the blood velocity (Gens, 1998). This method is now widely used (Bonnefous and Pesque´, 1986; Ferrara et al., 1996). The software includes re-
Fig. 6. View of the imaging system.
an acquisition board on the screen of a personal computer (PC) at a rate of 10 images/s. The digitizer uses 8 bits and a 50, or 100 MHz sampling rate.
2.4. Duplex system In order to measure the blood flow in very small superficial vessels, we built a duplex system based on the imaging scanner presented above (Gens, 1998). The imaging probe was slightly modified at the level of the front tip to include an inclined 20 MHz pulsed wave transducer intended for blood flow measurement (Fig. 7a). The imaging transducer remains perpendicular and the blood flow transducer is inclined at an angle of
Fig. 7. (a) View of the duplex probe, (b) arrangement of the two ultrasonic transducers.
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Fig. 8. Skin exploration: (a) view of the probe in position for exploration, (b) image of normal skin on the back of the hand (d, dermis; v+ a, vein+agregates), (c) image of a small fibroma (f, fibroma), (d) image of a basal cell carcinoma (b, basal cell carcinoma).
alignment of RF lines and fixed echo canceling procedures. The value of the correlation coefficient is also used to improve detection in the case of low signal to noise ratio (SNR) (Gens et al., 1997).
3. Applications and results
3.1. Skin explorations The device is currently in routine use in several Departments of Dermatology and the probe de-
scribed above is very easy to handle to examine skin (Fig. 8a). The examination is conducted with a small quantity of appropriate ultrasonic gel (sufficiently liquid) to avoid a too high attenuation. We give some examples of images in Fig. 8 to illustrate the possibilities. Fig. 8b presents a classical image of normal skin on the back of the hand with the image of the membrane (top, slightly curved white line), the very bright line of the gel–skin interface, the echogenic layer of the dermis (d) (about 1 mm thick) and a vein. The walls of the vein and inside the aggregates of red cells (v + a) can be clearly
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seen. With a frequency of 20 MHz the epidermis is only visualized when its thickness is greater than the resolution of the system (about 80 mm, forehead, palms, etc.). An image of a small fibroma (f) (about 5 mm in width and 1 mm thick) is shown in Fig. 8c. The device is also very useful to visualize tumors (as illustrated in Fig. 8d with the image of a basal cell carcinoma (b)). Generally, the images of basal cell carcinoma are characterized by the presence of a few large bright echoes (arrow) inside the tumor. The very easy to handle probe and small front tip with a membrane considerably facilitate exploration of carcinomas located in areas with difficult access such as the nostrils.
3.2. Eye explorations Such explorations involve the anterior part of the eye, principally the anterior chamber. Due to the presence of the membrane the examination is performed easily, rapidly and safely (Fig. 9). This examination requires slight local anesthesia and the front tip can be placed on the eyelid or directly on the eye. Fig. 10a provides a classical image of a normal anterior chamber comprising sclera (s), cornea (c), iris (i), ciliary body (c b) and wide, normal open irido-corneal angle (i–c a). The tissue reflectivity of the different structures can be seen: weak for the cornea, high for the sclera and intermediate for the iris and ciliary
Fig. 9. View of the probe during an eye exploration.
body. For comparison, Fig. 10b shows an example of the very narrowed angle obtained in glaucoma. Cysts on the iris and ciliary body can also be clearly visualized (Fig. 10c). Rapid verification of corneal graft is another interesting application (Fig. 10d). The graft appears clearly on the image (circle) and the scar at the junction is slightly distended.
3.3. Dental explorations The image shown in Fig. 11 was acquired with the classical linear scanning probe because the special ‘toothbrush probe’ presented in Fig. 1c was not yet operational but this normal image demonstrates the potential for periodontal examinations. In this vertical (longitudinal cross-section of the tooth) cross-section, the gengiva, alveolar bone and the tooth can be seen. When periodontal disease is present, it is revealed by a cavity appearing between the gengiva and the tooth and in the case of extensive damage, by bone reduction. To evaluate this reduction it is necessary to measure the distance between the margin of the gengiva and the crest of the bone. The image presented shows that diagnosis can easily be made by ultrasound, particularly with a special ‘toothbrush probe’.
3.4. Blood flow measurements Measurements were made with hospital in-patients, particularly those with angioma. Fig. 12a shows an image of an angiomatous area at the level of the calf of a child after laser treatment. Compared to normal skin, the dermis is very echogenic and there are many echoes within the hypodermis. We can distinguish (arrow) a small vessel (about 300 mm in diameter) in which we measured blood flow velocity. The velocity profile is clearly presented in Fig. 12b. The maximum velocity in the vessel was about 0.45 mm/s and several velocity levels can be differentiated between 0 and 0.25 mm/s, demonstrating the capacity of the system to measure very low velocities. Measurements were made with a pulse repetition frequency (PRF) of 1 kHz and a 40 mm measurement step. Other areas in the angioma were not
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Fig. 10. (a) Image of a normal anterior chamber of the eye (s, sclera; c, cornea; i, iris; c b, ciliary body; i-c a, irido-corneal angle), (b) very narrow irido-corneal angle, (c) images of cysts in iris and ciliary body, (d) investigation of a corneal graft.
Fig. 11. Dental exploration: vertical (or longitudinal) echographic cross-section of the bone – gengiva – tooth system.
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Fig. 12. Blood flow measurement: (a) image of an angiomatous area after laser treatment, with a small vessel (arrow), (b) velocity profile in the small vessel.
studied because the limited resolution (80 mm) of the system did not allow the detection of the numerous small capillaries.
4. Conclusion The use of 20 MHz ultrasonic frequency limits resolution (about 80 mm axially and 250 mm laterally) but, as we have demonstrated, allows the construction of easy to handle probes with removable front tips covered by an appropriate membrane, which considerably facilitates routine explorations. The examples provided of applications in dermatology, ophthalmology and stomatology and in the field of measurement of very low velocities (less than 0.50 mm/s) in very small vessels illustrate that 20 MHz systems can be very valuable investigation tools. It is evident that the special studied ‘toothbrush probe’ could be highly appropriate for routine use in the field of stomatology and dental explorations. High resolution is required and higher frequencies will be necessary to measure blood flow in small capillaries, however ultrasonic investigation of the microcirculation will remain difficult.
Acknowledgements The authors wish to thank R. Battault, L. Colin and C. Yvon for their help and essential contribution in performing the electronic and the software development.
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