Ulwasound in Med. &Bio/. Vol. 17, No. 5, pp. 519-528, 1991 Printed in the U.S.A.

0301-5629/91 $3.043 + .OO 0 1991 Pergamon Press plc

@Original Contribution

TWO-DIMENSIONAL AIRBORNE ULTRASOUND REAL-TIME LINEAR ARRAY SCANNER-APPLIED TO SCREENING FOR SCOLIOSIS

LARS MAURITZSON,~ JAN ILVER,+ G&AN BENONI,* KJELL LINDSTR~M* and STIG WILLNER*

+Department of Biomedical Engineering,~Department of Orthopaedic Surgery, University Hospital, S-2 14 01 Malmo, Sweden, and *Department of Electrical Measurements, Lund Institute of Technology, Lund, Sweden

Abstract-Diagnostic ultrasound is an established, noninvasive and harmless method for imaging the shape and appearance of organs and other tissues inside the body, and it has been used in many clinical applications for more than three decades. We have now applied some of this well-known technique together with the use of airborne ultrasound in medical applications, to build an equipment for anthropometrical investigation outside the body, e.g., measuring and registration of the shape and form of the human back. This is mostly done for screening purposes of young people in an attempt to find patients developing scoliosis, and in order to circumvent some of the disadvan- tages with the traditional screening method in this field of medical application.

Key Words: Airborne ultrasound, Anthropometry, Detector, Linear array, Scanner, Scoliosis.

INTRODUCTION

Medical imaging is the gathered conception of diag- nostic methods where the organs inside the body are displayed in pictures, showing the contour of the or- gans, rendering information on the tissue in the or- gans and the surrounding area or a series of images showing movements of an organ during a time pe- riod. These images are normally achieved from medi- cal equipment using ionising or nonionising radiation methods such as x-ray, NMR or ultrasound. The use of ultrasound has long been an area of special interest at Lund University as a noninvasive and harmless method for medical diagnosis, ever since the pioneer- ing work in echocardiography was presented by Edler and Hertz (1954). They created a research environ- ment in this field which has been preserved at the University through the years by a number of promi- nent researchers.

The normal propagation medium for ultrasound in medical imaging is soft tissues inside the body, such as fat, muscle, blood etc. However, our research group has experimented with the use of diagnostic ultra- sound techniques in different anthropometrical mea- surements, in order to generate image recordings of the body surface for subsequent investigation of its shape and form. For this purpose, airborne ultra-

Address all correspondence to Lars Mauritzson.

sound has often turned out to be the method of choice.

One medical state where the deformity of the body plays an important role is structural scoliosis. This deformity of the body is characterized by a lat- eral deviation and rotation of the spine, flattening of the sag&al curve and deformity of the trunk. This is mostly documented with x-ray technique. However, the scoliosis patients are often young, growing girls in their early teens and therefore should not be exposed to high x-ray doses unless absolutely necessary. In some cases, the scoliosis may progress very rapidly. Therefore, it is very important to find these patients early to be able to prescribe an efficient therapy. To achieve a good result in this area of preventive health care, it would be advantageous to routinely screen all school pupils in their young teens to save them from suffering from scoliosis.

Except for the use of x-ray and a few other more or less complex techniques, such as laser light, normal light, video, etc., there is only one established, easy, fast and harmless method to document the deforma- tion of the back and that is the Moire topography (Willner 1979). However, this is not a quantitative method, and there is an obvious need for an equip ment that can register the shape of the body, and espe- cially the back, using a fast, noncontact, nonionising and harmless method in a quantitative way. Airborne ultrasound (Lindstrom et al. 1982), implemented in

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520 Ultrasound in Medicine and Biology Volume 17, Number 5, 1991

much the same way as an ordinary diagnostic ultra- sound linear array scanner, seems to be a practicable method of registering the contours of the body sur- face.

thorough derivation will also show a temperature de- pendence for the velocity of sound proportional to the square root of the absolute temperature, T, in “K. This gives the velocity of sound, c, at an arbitrary tem- perature, t, in “C, from the equation:

PHYSICAL BACKGROUND OF AIRBORNE ULTRASOUND c = c,,( T/273)“2 = cO[ 1 + (t/273)]“2

Some kind of acoustic coupling medium, such as ultrasound transmission gel, is always used in diag- nostic ultrasound to ensure that air will not leak in between the ultrasound transducer and the skin of the patient, which would deteriorate the sound transmis- sion. Experienced ultrasound operators sometimes claim that the worsened transmission of ultrasound in the presence of air is a question of excessive absorp- tion of the ultrasound wave which cannot be transmit- ted through air. This is not the case. Instead, the prac- tical problems are caused mainly by the very large mismatch in acoustic impedance at the interface air/ tissue, which brings about an almost total reflection of the incident ultrasound wave. Much information on airborne ultrasound can actually be gained from the far-reaching similarities in the fundamental physical laws that govern the propagation of ultrasound waves in fluids, i.e., gases and liquids.

At normal room temperature, where the value of the temperature, t, is small in comparison to 273, this equation can be rewritten as:

c = c,(l + OS(t/273)] = c,, + c&/546)

Substitution of c,, with 33 1.6 me s-’ gives

c = 331.6 + 0.6t m-s-’

Velocity of sound

The velocity of sound exhibits a temperature de- pendence of about 0.2% per degree temperature change, which for a measurement distance of 200 mm produces an error of 0.4 mm. To produce accurate absolute distance measurements, the possible temper- ature changes along the sound transmission path should be compensated for. The relative humidity of air has only a minor, often negligent, influence on the velocity of sound.

Many persons use the time delay between light- Ultrasound is mechanical wave motion and ning and the subsequent thunder-clap to judge the needs some form of physical media to propagate. distance to a threatening thunderstorm. A common When this transmission medium changes, so will its figure for the velocity of sound in air used for this velocity of sound. Table 1 shows a comparison be- purpose is 340 m.s-‘. A simplified derivation of the tween the sound velocity of some different materials. velocity of sound, c,,, in air at normal air pressure Fluid media, such as gases and liquids, can mainly 1 .O 13 X 10’ Pa and 0°C will give a calculated value of support so-called longitudinal sound waves, while 33 1.6 me s-’ which is in excellent agreement with solid materials can contain different kinds of wave measured values (Kinsler and Frey 1962). A more propagation, each with a sound velocity of its own.

Table 1. Approximate values of density, velocity of sound and characteristic acoustic impedance for some gaseous, “liquid” and

solid materials at room temperature.

Material

Air Carbon dioxide Hydrogen Oxygen

Rubber Saline Soft tissue Water

Aluminum Bone Silver Tungsten

Density Velocity of sound Characteristic impedance (kg. m-‘) (m-s-‘) (kg a m-%-‘) x IO6

1.21 343 0.000 42 1.98 258 0.000 51

0.087 I 329 0.000 12 1.38 327 0.000 45

900 1 480 1.33 1 025 1531 1.57 1013 1 540 1.56

998 1 483 1.48

2 800 6 250 17.50 I 200-l 800 2700-4 100 3.20-7.40

10 500 3 600 37.80 19 300 5 170 99.80

Real-time linear array scanner 0 L. MAURITZSON et a/. 521

The sound velocities stated in Table 1 are given for longitudinal waves. Gases show, in general, much lower velocities of sound than for example human soft tissue (about one-fourth) and varies as a function of temperature as well as pressure.

Characteristic acoustic impedance The ratio of acoustic pressure in a medium to the

associated particle velocity is defined as the specific acoustic impedance. It is, for plane travelling waves, a real quantity, 2, of magnitude

z = pc

where p is the density of the material and c its velocity of sound. The product pc is often called the character- istic impedance of the medium. An echo is produced when a progressive, plane wave impinges on the boundary of a contiguous second medium with al- tered characteristic acoustic impedance. The relative size of the echo can be estimated from the reflection factor, R, which expresses the ratio of the acoustic pressure of the reflected wave to that of the incident wave, and can for a plane wave with normal incidence to the boundary, be expressed as:

R = K-G - Z,)/G + ZJI

where Z, is the characteristic acoustic impedance proximal to the boundary, and Z, the impedance dis- tal to the boundary. Other important factors when judging the actual reflection properties from different targets are size, form and direction of the boundary.

Typical values of the characteristic acoustic im- pedance for a few gases, “liquids” and solid materials are given in Table 1. The very large difference be- tween gases and other materials is immediately obvi- ous. This means, e.g., that a sound wave impinging on a gas/water interface will be reflected to more than 99.9%! In practical terms, all airborne ultrasound en- ergy will be reflected at the first gas/liquid or gas/solid interface the propagating ultrasound wave will come across, and an ultrasound shadow will unfold be- hind it.

Acoustic attenuation in air The sound intensity of a progressive, plane ultra-

sound wave will diminish as the wave propagates. There are many reasons for this, such as absorption, reflection, scattering and beam divergence. In the spe- cial application of airborne ultrasound, the range- limiting factor of propagation is normally governed by the acoustic attenuation in air. Classical theory predicts the acoustic attenuation in dry air to follow a

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I --__-_--_~---___---+-_-_____~

I

/ Absorption in still air (dB/cm) /

I Relative humidities: O-100 % I I

Temperature: 20 “C

Fig. 1. Ultrasound absorption in still air as a function of the frequency in the range 1 kHz to 1 MHz for different relative

humidities.

square law dependence upon frequency. Measured values, presented in Fig. 1, show how the attenuation constant varies in air with frequency as a function of the relative humidity. These curves exceed the pre- dicted values and the additional attenuation is most significant in the audible range of frequencies at low relative humidities.

This excess attenuation is adequately explained in terms of thermal molecular relaxation of the nor- mally unexcited vibrational mode of atmospheric ox- ygen molecules (Kinsler and Frey 1962). Luckily, the complex relation between the measured attenuation and parameters, i.e., frequency, humidity and temper- ature for the lower ultrasound frequencies (20-100 kHz), is normally not of any great practical impedi- ment for anthropometrical measurements. So is, e.g., the half value length for propagation of lOO-kHz air- borne ultrasound, as a rule of thumb, in the order of meters. The influence of the molecular excess absorp- tion decreases for higher ultrasound frequencies and the attenuation is observed to be nearly independent

522 Ultrasound in Medicine and Biology Volume 17, Number 5, 199 1

of the relative humidity in air. The attenuation is here, in absolute terms, of about the same magnitude as attenuation handled by normal diagnostic ultra- sound: At 1 MHz, the attenuation in air is about 2 dB/cm vs. 0.5 dB/cm for human soft tissue.

Choice of ultrasound frequency The normally designed criteria for a diagnostic

ultrasound system is to use the highest ultrasound fre- quency that will enable echoes from the maximally specified measuring depth to return to the transducer with an acceptable signal-to-noise ratio. This will auto- matically ensure a high spatial resolution to the sys- tem. An ultrasound frequency of 3-5 MHz is today’s choice for abdominal work up to a depth of 20 cm, whereas eye scanning up to a depth of a few centime- ters is better performed in the frequency range of lo- 20 MHz.

It is possible to use data from Fig. 1 to make a similar estimation of the frequency of choice for air- borne ultrasound work. For ultrasound frequencies above 100 kHz, the attenuation, LY, can asymptoti- cally be described as a function of frequency as:

(Y = 1.6 X f * dB - cm-’ MHz-*

In a well-designed diagnostic ultrasound system, the echo signals seen at the transducer terminals will cover a large dynamic range-the echoes from a strongly reflecting plane interface should be at least 80 dB over noise (Hill 1986). As the echoes from air- borne ultrasound are strong indeed (total reflections), let us accept a total loss, atot, of 60 dB due to attenua- tion in the propagation medium. For pulsed-echo measurements at a distance x,,,,, from the transducer, this can be stated as:

atot = 60 I 2 X x,.,.,,, X 1.6 X fmax2 dB, or

f,,, _( 4.3 x (x,,,)-“2

where f,, is given in MHz, and x,, in centimeters. For a measurement distance of 20 cm, this will give a maximal ultrasound frequency of 0.96 MHz.

An extra advantage of airborne ultrasound is the low velocity of sound in air, which makes the wave- length, X, for 1 -MHz ultrasound to be about 0.34 mm, which is comparable to the wavelength of a 4.5MHz ultrasound scanner in soft tissue. Thus, it should be possible to design a scanning system for airborne ul- trasound with the same absolute resolution as today’s diagnostic ultrasound scanning systems demonstrate in human soft tissue.

Transducers for airborne ultrasound Ultrasound is generated and detected with trans-

ducers, i.e., elements which transform electrical en- ergy to ultrasound (transmitter), or ultrasound energy to electrical signals (receiver). The same type of pi- ezoelectric material that is used for diagnostic ultra- sound transducers can be used to generate airborne ultrasound. The very large mismatch between the pi- ezoelectric ceramic and air makes direct use inexpe- dient. The transducer can be matched to air by the use of metal resonators or specially matching layers using so-called quarter-wave plates. The price to be paid is a very frequency-dependent matching, which gives gen- erated ultrasound pulses a high Q-value, i.e., rather bad damping. There are signs that this may change in the near future, which will enable design of airborne ultrasound transducers in the megahertz range, with means to improve the shape of the sound beam and the resulting resolution (HW Persson, personal com- munication).

A capacitor microphone consists of a thin metal membrane, mounted close to a solid metal electrode, separated by a narrow air gap. The capacitance so cre- ated is charged to a few hundred volts through a high- value resistor. Every pressure change on the mem- brane resulting from an incident sound wave will change the magnitude of the capacitance. As the large resistor will prevent rapid variations in the charge of the capacitor, its voltage will vary in pace with the incident sound. After amplification, this voltage alter- ation will become the received ultrasound signal.

Such electrostatic transducers can also be used to generate sound. It is shown that the efficiency of such a transducer is proportional to the square of the area of the membrane, proportional to the square of the polarisation voltage, inversely proportional to the fourth power of the spacing in the air gap and in- versely proportional to the mechanical impedance of the vibrating system (Olsson 1957). To obtain good efficiency, the air gap should be as narrow as possible. This will limit the possible amplitude, which makes this transducer less suited for the low-frequency audio range, but excellent for part of the ultrasound range.

To generate broad-band ultrasound pulses, the transducers should work below the natural resonance frequency of the membrane. With the availability of soft, metal-plated plastic membranes, only a few mi- crons thick, it is now possible to manufacture trans- ducers with excellent transient properties. They can be used both as transmitters to generate the outgoing ultrasound waves, and as receivers for detection of the reflected signal (echo). For use in airborne ultrasound pulsed-echo systems up to a few hundred kilohertz, these transducers seem to be an excellent choice to-

Real-time linear array scanner 0 L. MALJRITZSON et al. 523

day: well damped and with an excellent bandwidth. A widely available and relatively inexpensive ultra- sound transducer of this kind, the Polaroid Instru- ment Grade Transducer #604 142, has been used in the following work. When the transducer is driven by a current mode interface (Lindstrom et al. 1982), it transmits a very short ultrasound pulse with a centre frequency of 50 kHz, and without noticeable preoscil- lations. The 35-mm diameter of the transducer im- plies an ultrasound beam of about the same diameter up to the near/far field transition at a distance of 45 mm, where the beam starts to diffract with an angle of about 13”.

METHODS

One way to create a topographical image of a curved surface, like the bottom of the sea or the back of a human body, is to perform a number of measure- ments of the distance between the surface and a fixed plane, serving as a reference. The result can be pre- sented graphically, either like a map with isometric contour curves or a number of profile images in various directions of the surface, or simply be pre- sented as a list of figures, representing the distance to the curved surface in the different measuring points.

A special optical image of the curved surface of the back of a patient with scoliosis, produced with the Moire technique, is shown in Fig. 2. Between two ad- jacent dark isometric curve lines is a difference in height of about 5 mm. This method is widely used in clinical routine for screening purposes, to search for the asymmetric topography of the back that could be the first sign of a scoliosis. Unfortunately, the method is basically qualitative and the recording will not di- rectly present numeric information about the defor- mity. A further disadvantage with the Moire method might occur when the patient’s back is pressed against the screen when positioned in front of the Moire appa- ratus. It is then possible that patients with a small deformity can disguise this by standing in an abnor- mal, unrelaxed position, thereby producing false nega- tive recordings.

To avoid these shortcomings, we have used methods of diagnostic ultrasound to develop a quanti- tative imaging instrument, applicable for rapid screening of scoliosis. Using the well-known pulsed- echo method, adapted to the physical limitations of airborne ultrasound, an airborne ultrasound trans- ducer is positioned 5- 15 cm behind the patient, and a short ultrasound pulse is transmitted toward the pa- tient’s back. The propagation time, i.e., the time for the ultrasound pulse to travel from the transducer to the surface of the body, where the sound wave is re-

Fig. 2. Moire picture of an asymmetrical back due to scolio- sis. This is mainly a qualitative topographical representa- tion with level curves of the back surface, just like level curves of mountains and valleys on a map. It is possible to extract quantitative data from these pictures, but it involves considerable work as the resulting isocontour lines in the

image will not come out equidistantly spaced.

fleeted, and back, is measured. As the velocity of sound is known, it is a simple matter to calculate the distance from the transducer to the back of the pa- tient. The transducer is then mechanically moved along the measuring line to make a new measurement of the pulsed-echo time and a new calculation of the distance, and so on. This approach will work, but it is not an adequate solution for medical use. A complete scan of the back will take several minutes, and during this time, there will be a distinct risk for motional artifacts induced by breathing and other movements of the patient.

The slow scanning procedure can easily be im- proved upon if the mechanical scanning is exchanged for electronic scanning, e.g., by building a linear-array scanner with a number of transducers mounted in a row of about 50 cm, approximately the same length as the spinal column, As the curvature of the spine nor- mally is rather smooth, it will be sufficient to apply the transducers at an intertransducer distance of 45 mm, or about 12 transducers over 50 cm. To make

Transducer Distance no.

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524 Ultrasound in Medicine and Biology

one pulsed-echo measurement at a maximum target distance of 20 cm takes 1.2 X 1 O-’ s, which is equiva- lent to a maximal pulse repetition frequency of about 850 Hz. With 12 transducers in the array, this means a frame rate of about 70 frames/s. A recording of the curvature of a back, scanned with a one-linear array scanner, is shown in Fig. 3. Even if the quality of the scanned image is good, little diagnostic information which can be used to classify the asymmetry of the back, and hence the scoliosis, can be gained from it.

The advanced three linear array scanner (ATLAS) To extract more information on the asymmetry

and curvature of the back, two more linear arrays are added in parallel with the first one as seen in Fig. 4. The new scanning system, ATLAS, shown in Fig. 5, consists of three main parts: 0 patient console including ultrasound transducer

arrays, l microprocessor control unit, and 0 presentation equipment.

The patient console is the part of the scanner where the patient is in contact with the equipment. There is a mechanical fixation stud to enable the pa- tient to stand in the same standardized position every time a scanning is performed and without influence on the posture of the back. In a frame behind the patient are the three linear transducer arrays, with one

10 mmldiv

Fig. 3. A two-dimensional recording of the profile of the back along the spinal column, produced with an airborne

ultrasound linear-array scanner.

Volume 17, Number 5, 199 1

Fig. 4. Scanner head consisting of three separate linear-array transducers. The vertical position ofthe whole scanner head and the horizontal space between the outer transducer arrays can be adjusted to suit the individual patient. A low- intensity helium-neon laser, at the top of the scanner, is

used for aiming the middle array to the spinal column.

fixed array in the middle consisting of 12 ultrasound transducers in a line covering the length of the spinal column. Parallel to this are two laterally movable transducer arrays with 10 transducers in each, cover- ing the surface profiles symmetrical to the spine. On top of the scanner frame is a low-intensity, mounted helium-neon laser to produce a light plane for direc- tional purposes.

The microprocessor control unit is the heart of the scanner system, situated in a 19” main-frame at the lower part of the scanner frame, and designed around a 8085 microprocessor. It also has an analogue part with dynamically controlled amplifiers and a special detector for an optimal detection of the echoes. With individual connection lines to each of the 32 ultra- sound transducers, the control computer must ad- dress the right transducer at the right moment and produce transmission pulses of the right duration. It also has to calculate mean values of the measured re- sults and test if the results are in a permitted time interval (i.e., echo window). If the echo amplitude is

Real-time linear array scanner 0 L. MAURITZSON et al. 525

Fig. 5. Overview of the three-linear array airborne ultra- sound scanner system for studies of the curvature of the

human back.

fading out, or, just the opposite, getting too strong, the input amplifier is dynamically altered to fit the new condition. In the automatic mode, the control com- puter is testing if the detected echoes are stable for a few frames (to reduce possible artifacts). When all transducer channels comply with the prescribed crite- ria, the scanning is stopped.

The presentation equipment has a check and con- firm panel with 32 light-emitting diodes (one for each transducer) for quality control of the returning echoes before the result is presented on an oscilloscope used as a monitor for the real-time imaging of the 32 mea- suring points. Possible ringing from excitation of the transducer, old ghost echoes, etc., are not allowed to disturb the echo signal. So, if the patient is standing too close to the scanner, an audible alarm will warn both the operator and the patient. When the operator is satisfied with the collection of data from the scan- ner, these values can be frozen and transferred to a desk-top computer, HP-S, for further processing, storage and print-out of the result on a plotter for archiving.

How fast is the scanning system? The maximum time allowed between two ultrasound pulses is chosen to 3 ms (corresponding to a maximal measuring dis- tance of about 50 cm, to reduce the occurrence of ghost echoes), which means 32 X 3 ms = 96 ms per frame scan, or about 10 total scans/set. The echo arrives normally long before the time limit is ex- ceeded, but a certain amount of time is needed for control of the system, the computer and the data stor-

age, so the practical scanning speed turns out to be about 15 scans&c.

Detector Contrary to diagnostic ultrasound, airborne ul-

trasound is mainly used to measure the distance from the transducer to one solitary target. The target in this application is the human skin surface, which might generate strong reflections, but vary in magnitude due to its three-dimensional curvature. A straightforward approach to detect the arrival of the echoes uses the standard threshold-comparator detector. The first ar- riving echo that is larger than the threshold voltage of the comparator will trip it and the positive flank of the comparator output will make up the arrival time. However, as shown in Fig. 6, this measurement is very sensitive to amplitude variations in the echo signal. Relatively small amplitude variations in the echo sig- nal might displace the arrival time corresponding to a quarter of a wavelength or more.

How can the performance of this detector be im- proved? The only points in the echo curve, whose po-

A) I 1 1 B)

t2 TIME

Fig. 6. Operation of the standard threshold detector. (a) When the amplitude of a large returning ultrasound echo exceeds the preset threshold voltage, the comparator will trip (tr). Dependent on the amplitude of the echo, this time might differ corresponding to a quarter of a wavelength. (b) For a reduced echo amplitude (dotted line), the comparator might trip in the second period of the echo complex (tz), which results in a trigger error of at least one full wave-

length.

526 Ultrasound in Medicine and Biology Volume 17, Number 5, 1991

sitions are independent of the echo amplitude, are the zero-crossings, but it is not possible to set the thresh- old voltage to zero due to the presence of noise. In- stead, Fig. 7 shows a different, simple but very effi- cient dual-threshold comparator detector circuit. A negative threshold voltage is applied to the positive input, and the echo signal is applied to the inverting input of a comparator. As long as there is no echo at the signal input, the comparator output will be close to zero volts. When an ultrasound echo arrives and its amplitude goes below the threshold voltage, it will trip the comparator. As a result, the threshold voltage is changed to zero volts and the comparator will not switch back until the echo signal crosses the zero- voltage line. It is now the negative flank of the com- parator output signal which is used to constitute the echo arrival time. The magnitude of the initial thresh-

-----_ ----

h

DETECTED

Fig. 7. Operation of the dual-threshold comparator detec- tor. When no echo signal is applied to the inverting input of the comparator, its output will be close to zero. The resistive network will then create a negative threshold voltage, AV, connected to the noninverting input. When an echo crosses the threshold, the comparator will trip, and its output will go high. This, in turn, will shift the comparator threshold to zero voltage. The echo detection will, thereafter, occur at the next zero crossing of the echo voltage (t,), almost inde-

pendently of the echo amplitude.

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Fig. 8. Comparison of echo-detection accuracy between the standard and the dual-threshold detector, using the echo signal shown in Fig. 6. For large echo signals, the detection might differ more than 1.5 mm for the normal detector, and normally below 0.01 mm for the dual-threshold detector when the amplitude of the echo is reduced. If the amplitude of the arriving echo is further weakened, both detectors might trigger on the next part of the echo complex, which results in a trigger error of one whole wavelength, or about 7

mm.

old voltage is set by the resistor, R, and the accuracy of the zero-voltage threshold can be fine tuned by the trim potentiometer at the comparator output. A repro- ducible arrival time recording better than 0.1 mm is normally obtained with this detector used on an ultra- sound echo of varying amplitude. The excellent per- formance of this dual-threshold detector, when sub- jected to different echo amplitudes as compared to the standard threshold detector, is evident from Fig. 8. One word of caution: As this detector measures time of flight to the first zero-crossing in the arriving echo signal, this must be compensated for when used for absolute distance measurements. This is not a prob- lem with the present application, where the task is to scan the variation in distance from the transducer to the back of the patient.

RESULTS AND CONCLUSIONS

The main medical field of application for this airborne ultrasound linear-array scanner is preventive long-range screening for scoliosis in large populations of schoolchildren. The Moiri technique for qualita- tive assessment of the asymmetrical deformation of the back is not sufficient for this purpose. Instead, the method should accomplish automatic registration and storage of quantified measurement results for subsequent comparative computer analysis.

The mechanical parts of the scanner are manu- factured in aluminum, therefore, they are easily mo-

Real-time linear array scanner 0 L. MAURITZSON et al. 521

bile, The scanning system can be erected on less than 2 m square floor space, i.e., in almost any classroom. The patient is positioned in front of the scanner-trans- ducer frame and aligned laterally using the helium- neon laser at the top of the frame. The laser emits a light plane which should always cross the line between the prominent spinal process of C7 and crena ani to standardize the patient set-up. A complete scan, in- cluding all three back profiles, can theoretically be carried out within a fraction of a second. However, in everyday use, the check and confirm system might indicate a need to adjust the direction of the trans- ducer frame, before echoes from all transducers are accepted. This might take a few seconds. Thereafter, the scanner microprocessor control unit freezes the acquired echo information, which then can be moni- tored on the oscilloscope screen. This display shows: one back profile representing the line between the prominent spinal process of C7 and crena ani, and two back profiles symmetrically lateral to the spine and corresponding to the posterior superior iliac spinae.

The information on the scanned back profiles is stored in the scanner as 32 measurement results (10 + 12 + lo), proportional to the distances between the respective transducer and the back. These are trans- ferred to the desk-top computer for data processing and long-time storage together with patient informa- tion on a magnetic tape. Each mini-data cartridge tape can store about 40 individual measurements, i.e., normally more than sufficient for preventive screen- ing of an average school class. Necessary patient prepa- ration and handling, such as alignment of the patient, input of name, date of birth and so forth, reduces the pace of the screening. A realistic screening throughput is 15 to 20 individual measurements per hour.

The diagnostic potential of the airborne ultra- sound scanner is presently under evaluation. In an ongoing study consisting of 50 scoliosis patients, the results from the ultrasound scanner are compared with results from alternative methods, like x-ray exam- ination. The outcome of the study will be published elsewhere.

The direct outcome from a patient scanning is a graphical presentation of the three back profiles, one along and two symmetric laterally to the spine. It was recognized early that the predictive value of the mea- surement could be further improved. Minor varia- tions in between the recorded back profiles turned out to be difficult to identify for the naked eye on patients with mild or early scoliosis. An additional calculation and presentation of the difference between the two lateral profile curves, displayed with increased sensi-

tivity, is shown in Fig. 9. If the patient just stands in an angle to the scanning frame, the difference curve is a straight line but displaced with a constant value. For a patient with scoliosis, this presentation is a way to emphasize the asymmetry of the back, which indi- cates how much the patient’s back is rotated around the spine.

Although the airborne ultrasound scanner shows great promise, some problems are still to be dealt with. The ultrasound beam is rather wide when it hits the target and the echo could, therefore, be generated from a not-too-well-specified area at the back of the patient. Thus, the spatial resolution should be im- proved upon, which seems to be feasible using air- borne ultrasound transducers of higher frequency and/or focused transducers (H. W. Persson, personal communication). The relatively low ultrasound fre- quency presently used (50 kHz), also makes the mea- surements slightly sensitive to acoustical disturbances from the surroundings. A spoon, stirring around in a cup of coffee, can easily produce acoustical noise up to many tens of kHz. This problem should be much

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Fig. 9. Print-out from the three-linear array scanner, show- ing two back profiles, scanned along two equidistant lines, offset symmetrically to the spine, and the calculated differ- ence between these two profiles (middle of the picture). Please, observe the different scale of the curves, which re- sults in a rapid, but very sensitive, detection of pathological

curvatures of the back.

528 Ultrasound in Medicine and Biology

reduced if ultrasound frequencies of a few hundred kHz or higher come into use. How do we select reli- able anatomical landmarks on the patient, which could be identified by the scanner? We are measuring the back along a straight line, while physicians would sometimes prefer to measure along the lateral S- shaped spine, present in patients with scoliosis. Some of these problems might be solved in a future enlarge- ment of the number of ultrasound transducers in the measurement system, aiming at the possibility of a true three-dimensional, scanned picture of the whole body surface.

Volume 17, Number 5, 1991

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Hill, C. R. Physical principles of medical ultrasonics. Chichester, UK Ellis Horwood Ltd; 1986:278-29 1.

Kinsler, L. E.; Frey, A. R. Fundamentals of acoustics. New York: Wiley; 1962:2 17-245.

Lindstrom, K.; Mauritzson, L.; Benoni, G.; Svedman, P.; Willner, S. Application of airborne ultrasound to biomedical measure- men&. Med. Biol. Eng. Comput. 20:393-400; 1982.

Olsson. H. F. Acoustical enaineerina. Princeton, NJ: Nostrand: 1957:205-2 11.

Willner, S. Moire topography-A method for school screening of scoliosis. Arch. Orthop. Trauma Surg. 95:181-185; 1979.