IEEN TRINS.WTIOS.S ON SOSICY ASD ULTHISONICS, VOL. s1.-17. NO. 3, JULY 1970

Acoustical Imaging of Biological Tissue FREDRICK L. THURSTONE, MEMBEII. ~ E E E

Abstract-In any imaging system the subjective quality and in turn the usefulness of the system is dependent upon the information detection, processing, and display procedures that are employed. Numerous techniques have been investigated over a period of years for the purpose of imaging biological tissue structures using ultrasound as the investigating radiant energy. However, the clinical and research applicability of these techniques has not become widespread because of the limited diagnostic usefulness of the images. Several image factors that influence the diagnostic useful- ness of ultrasound images are discussed.

HE extension of radar and sonar concepts to pulse- echo ultrasound was a natural consequence of the concepts involved in target detection and localiza-

tion. The development of pulse-echo ultrasound techniques for the measurement and imaging of opaque structures has been pursued for both nondestructive materials testing and for medical diagnostic use. Although t,he diagnostic use of ultrasound as a nontraumatic method for measuring and imaging biological t'issue structures is indeed an intriguing and exciting possibility, it is probably fair to say that the clinical usefulness of these techniques has been severely limited and has not developed the char- acteristics envisioned by research workers in this field. There are a number of limiting factors involved in the development of a clinically useful system, and it will be the purpose of this paper to enumerate and define these limitations and perhaps to suggest means by which these factors may be reduced or overcome. It is the author's firm conviction that the primary limitations on the devel- opment of a clinically useful diagnostic imaging system are directly related to the problems associated with the detection of ultrasonic energy-that is, in a general sense, transducer limitations.

It is not diEcult to approach the question of producing visual images using ultrasound echo informat,ion from an abstract viewpoint and to conclude that excellent images should be obtainable. In turn, since the echo information comes from soft tissue interfaces, these images should have great diagnostic value. For example, ultrasound frequencies in the 1-10 MHz frequency range are commonly used and readiIy traverse several centimeters of tissue structures. With a relatively constant propagation velocity in soft tissue, these frequencies have wavelengths of 1.5-0.15 mm. Assuming relatively large detection apertures and either short pulses or coherent detection, image resolution should approximate the acoustical wavelength. Images of soft tissue structures wit,h millimeter or fractional

in part by USPIlS Grants GM-15892, HE-12715, HE-41, 131, and Manuscript received February 9, 1970. This work was supported

F. L. Thurstone is with the Division of Biomedical Engineering, HE-5716.

Duke University, Durham, N. C. 27706.

millimeter resolution would be very fine indeed, even if limited t'o a very shallow depth of focus. Using acoustical holographic techniques, similar resolution should be obtained over a very great depth of field. Unfortunately, limitations on the detection of acoustical energy through a large aperture prevent the development of a practical imaging technique having these anticipated characteristics.

It should be pointed out that a distinction is being made between the use of pulse-echo ultrasound for measure- ment and the development of a system that utilizes echo information for t,he generation of two-dimensional visible images. The use of echo mrlging for the measurement of linear dimensions has been highly refined and is widely used in medical diagnostics for such measurements as the position of the midline of the brain, the dimension of the fetal skull in pregnancy, and the measure of dimen- sions of ocular structures in ophthalmological diagnosis. With very few exceptions, the present use of imaging procedures is limited to clinical research application rather than routine diagnostic use.

There are a number of problems associated m-ith the development of an imaging system using acoustical energ). for the investigating form of radiant energy-. Some of these are characteristic of all imaging systems and some are unique t,o acoustical imaging. In any imaging system: certain factors such as image resolution, image registration. dynamic range of present'ation, image magnification, distortion, etc. must be optimized. Frequently this involvch considerable compromise. For practical purposes, factors such as these have not imposed severe limitations on system design, with the single exception of the dynamic range of image presentation. Most commonly this factor has been avoided entirely by the use of storage-type oscilloscopes for the image display. Such display techniques are essentially one-bit binary displays, i.e., entirely black or white images of extreme contrast. However, there is a very severe limit'ation on the development of an acoustical imaging syst'em, and that is the lack of a suitable two- dimensional detection system or acoustical "film." Without such a detection scheme, a11 imaging systems become basically transducer limited, i.e., it is not a question of better acoustical optics or geometry, but a question of the amount of data that can be detected and the available data acquisition rate. Numerous detection systems are available including thermal, mechanical, chemical, and electrical methods. However, none possesses the necessazy speed, sensitivity, resolution, and range that is required for high-quality imaging.

I n addition to the problems associated with any acoustical imaging technique, there are a number of problems associated with the application of acoustical

THURSTONE: .+COUSTICAI, IMAGING OF BIOLOGICAL TISSUE

imaging procedures in medical diagnosis. Lcor imaging relatively gross structures of the body, frequencies ill the range of 1 10 YIHz arc appropriate. This requires the use of a liquid-coupling medium between tlle source- detector and t'he body surface. This is normally accom- plished by either a thin film of couplant in "contact" scanning or by :L longer water path as in immersion scanning. In any diagnostic imaging procedure, problems arise conceruing the patient support procedure. This is particularly true with the acutely ill or aged patient. The stability of the patient support system becomes a factor as does the time required to obtain the necessary data. Restrictions on the t,ime available for data acquisition may take several forms depending on the situation and the particular body structure under study. This limit may be imposed by the pat,ient's ability to endure the procedure, by the time of the respiratory cycle, or even by the time of the cardiac cycle. ,4 somewhat more subtle but very real limitation on the diagnostic usefulness of ultrasound imaging procedures is the 1:~ck of standardiza- tion of technique, and in turn, the lack of a background of experience or c:xpertise in the interpretation of the images generated. Instrumentation presently available for diagnostic imaging is capable of producing a great variety of image characteristics at the control or whim of the operat'or. This fact has led many to t,lle cotlclusion that without a priori knowledge of the anatomical struc- tures to be imaged, including any pathology, images generated acoustically have little definit'ive diagnostic value.

Several factors have contributed to the lack of any uniformity of procedure or st'andardization of tecllnique in diagnost,ic imaging. The great variability of body structure among the patient population is certainly a major factor. The rctlatively small number of individuals active in clinical research using diagnostic ultrasound and their variety of specialized interests and equipments is also a major factor. The variability and relative in- stability of ultrasound transducers used for medical diagnosis is also a significant factor. Indeed, the variability and fundamental limitations imposed upon the simple linear piezoelectric crystals used for transmitting aud detecting ultrasound pulses havc been t,he primary limiting factors in the developnlent of a dingnosticall). useful imaging system.

Several transdl~cer fact,ors greatly affect the images obtairled using pulse-cello B-scan techniques, whether simple or compound in nature. Such factors include the lateral extent of the ultrasound beam that determines the available lateral resolution, and the mechanical damping that determines the minimum pulse duration, \vhich in turn controls the available range resolution. These factor$ can not be described in terms of simple parameters because of the dynamic range of target infor- mation as it is related to the complex near- and far-field- beam characteristics.

Advanced fabrication procedures have produced trans- .ducers with excellent mechanical damping. Transducers

155

can be obtained that are damped by a factor of 20 dB or more within two or three cycles. For imaging purposes these transducers provide a range resolution on the order of two or three wavelengths, which is considerably better than the available lateral resolution. It is this factor that has led to the development of numerous compound scanning techniques, which involve compromising the available lateral and range resolution in order to produce a more optimum image.

Fig. 1 is a diagrammatic illustratior~ of the basic transducer dilemma that must be considered. The tran- sition range from near-field region to far-field region for a simple circular piston transducer of radius a is given by the equation

D = a2,/ix

where D is the transition range and ix is the acoustical wavelength. As shown in Fig. l(a), the transition rango for a piston transducer 10 n-avelengths in radius is a t :L distance (Jf 100 wavelengths. 111 this case, the far-field beam divergence is not great; however, the near-held beamwidth is approximately equal to the 20-wavelength diameter of the transducer. If the size of the transducer is reduced in order to decrease the beamwidth in the ne:w field, then the far-field beamwidth is increased. This factor is illustrated in Fig. l(b) for a piston 5 wave- lengths in radius and a transition rang(: of 25 wavelengths. In general, a compromise must be made between reduced lateral resolution in the far field for small transducers and reduced resolution in the near-field region for large transducers. It is possible to focus the transducer either with a lens or a reflecting system or to produce a curved transducer as illustrated i n b'ig. l(c). This provides in+ proved lateral resolution in the focal region of the trans- ducer. As in any optical imaging system, improving the resolution by this method is at the expense of a t1egr:lded resolution a t distnnces greatly different from the focal distance. This reduced depth of focus with focused trans- ducers either degrades t,lle lateral resolution or decreases the data acquisition ratat: if range gating is employed us in a C-scan system.

The actual near- ancl far-field beam charact~rist~ics arc much more complex than indicated in Fig. 1. To illustrate this complexity, near- nrui f;w-ficld beam characteristics are illustrated for a circular piston transducer of 10 wavelengths in radius. This i3 typical o f the dimensioncom- monly used for diagnostic imaging procedures. Vig. 2 shows the far-field pressure distribution for such a trans- ducer in terms of the angular deviation from the axis of the piston. It should be noted t,llat the first side lobe of the transducer peaks a t an angle of approximately 3 degrees, and t'hat it has an amplit,ude that is only 17 dB below the central masimum.

The near-field beam characteristics of this samc 10 wavelength-radius transducer are illustrated in Figs. 3 and 4. Fig. 3 shows the variations in the near-field pattern along the axis of the transducer. The multiple nulls in the response along the axis are evident. Between the surface of the

156 IEEC TR.4NS.%CTIONS O N SONICS AND ULTRASONICS, JULY 1970

T 1 l l l I I I I l I

Resolution

(C) Fig. 1. Ultrasound transducer lateral-beam characteristics.

W O J

z 0 Q 5 W K 3 m u) W (L a W

t

2 a J W oi

I l l I l I ,

: \ t ) 2" 4 O 6 O 8" 10" 1Z0 74O

DEGREES OFF AXIS

Fig. 2. Far-field beam characteristic of a piston transducer

tramducer and a distance of 10 wavelengths removed from the surface, the pressure characteristic has nurnerow maxima and minima, and for clarity has not been plotted. The transverse character of the beam pattern is even more complex as shown in Fig. 4. This illustrates the transverse pattern of the pressure distribution at five distinct axial distances from the transducer: 20, 40, CO, 80, and 100 wavelengths. The 100-wavelength di-. stance again represents the transition from t'he Fresnel or near- field region to the Fraunhofer or far-field region.

It is clear that images obtained using transducers of the type just described n i l 1 have limited lateral resolution, and that the echo amplitudes will be very dependent upon target position. Biological tissue structures reflect ultrasound in a highly specular manner. In order to image curved and complex structures, it is necessary to use a compound-scanning technique and to include a large dynamic range of echo information in the image generated.

CN AXIS NEAR FIELJ PRESSURE OC Z,RCUL.AR -RL\NSDUCER,RADiUSlOX

D STANCE FROM 1KANSDUCER I N 'WAVELENGTHS

Fig. 3. Axial-beam characteristic of n piston transducer.

Utilizing a large dynamic range leads to the inclusion of art,ifacts produced by spurious responses in the side lobes of the transducer, etc. The dynamic range of echo information is one of the most sovere limitations on the development of a more practical imaging system. The limited dynamic range of image-presentation devices means that the range of echo information must be com- pressed in order to include all of the desired image in- formation without image saturation. The amount of compression that can be employed, however, is limited by the dyt~amic range of the transducer cl1:~racteristic. Although some cfforts have been directcd toward loga- rithmic compression of the ccho data and other procedures, most imaging techniques s:rturnte the imago at a selected level by the usc of storage-type oscilloscope displays.

In the development of an improved imaging system, i t \ d l be necessary to optimize the transducer cl~nrnc- teristics and the irlformation-processing and display techniques. AUthougI1 difficult,, the use of diffuse illumi- nation might signific:rntly improve the images obtained by reducing the dyrmnic range of the echo information. Some dcgrcBe o f signal compression cnn be employed to improve the imag(\s, but noise and spurious sigtml.5 will still limit the amount of useful compression. The image- prcsentation device should be chosen to provide the greatest possible dynamic range of prescntation consistent n-it11 image-rctbolution and datu-preserltation rntcs.

A s in any diagnostic procedure, the degree of confidence in the result is based upon the objectivity and specificity of the procedure and upon the experience of the diag- nostician. For a dingrtostic imaging system to have maximum clinical usefulness, it must be designed to provide images that have the greatest value or meaning to the observer. The psychometrics of visualization and the subjective quality of the images generated must also be considered, as well as the objective paramet'ers that have been discussed. This may be more readily accom- plished with certain diagnostic procedures that have very specific and identifiable alternatives. However, this area has been largely overlooked in past efforts using ultra- sound for image generation.

It has been the purpose of this paper to call attention to the problems and limitations inherent to the use of acoustical energy for the development of visible images of

157 THUESTONE: ACOUSTIC.4L IMAGING OF BIOLOGICAL TISSCE

WAUELENGTH OFF AXIS

Fig. 4. ?;ear-fieltlInteral-l)exm characteristic of a piston transducer

opaque biological materials. Such an enumeration of because there js every reason to be optimistic about such difficulties together with a review of the presently limited developments in the future. Improved information de- usefulness of these techniques tends to produce a pessi- tection, processing, and presentation techniques are certain mistic outlook toward future developments in this field. to be developed, and the diagnostic usefulness of improved It would be unfortunate to convey or instill such an imaging procedures may increase at a much faster rnt.e attitude because of the very great value that t,he develop- than the simple objective criteria of image evaluation ment of improved imaging techniques would have, and might indicate.