Majority of the people approaching ultrasound for the first time assume that this is one of the newer imaging techniques. Certainly ultrasound has only achieved wide clinical use since 1970. But medical use of ultrasound came about shortly after close of World War II. These were based on fundamental research into the principle of sound and its interaction with different material which took place centuries back.
The piezoelectric effect, upon which the generation and detection of ultrasound signal depends, was noted by Pierre and Jacques Curie in 1880.
The principle of pulse echo technique for detection of underwater bodies were already under investigation prior to the First World War and were given considerable impetus by the need to detect submarines during world war. The first major attempt at a practical application was made in the unsuccessful search for the sunken Titanic in the North Atlantic in 1912. Other early attempts at applying ultrasound met with same fate.
The physical principles on which our modern diagnostic techniques rely therefore probably owe their origin to Chilowsky and Laugavin who produced their early publication in 1916. Their work was on the basis for SONAR (Sound Navigation and Ranging), the first important successful application of ultrasound.
It was this pulse echo distance ranging technique which became the basis for modern diagnostic medical sonography.
The medical potential was not appreciated in early post First World War period. However, technological development occurred with small portable pulse echosystem that were developed for detecting flaws in manufactured structures.
Although it was these new flaw detectors which generated the greatest interest and several workers dedicated themselves to the detection and evaluation of biological effects.
Worod, Loomis and Johnson were probably the earliest researchers and they published their result on the bioeffect of this newly discovered form of energy in 1927 and 1929, respectively.
In view of destructive effect in both solid material and biological systems, both industrial and biological applications for these properties were soon investigated.
Lym and Putnam focused ultrasound in water bath and induced massive cerebral lesions in experimental animals by insonating them with focused 1 MHz ultrasound. Attempts were made to develop ultrasound as an alternative to radiotherapy, but the technology for beam manipulation and for interaction of the ultrasound generators with the patients was inadequate and early clinical results showed more destruction of normal tissue than with the currently available X-ray radiotherapeutic technique. In the late 30s and early 40s, Sakdov and Firestone took out patients on ultrasound device used for detecting metal flaws.2
Dussik brothers decided to use ultrasound as an alternative source of transmitted energy in systems. In their first apparatus sound was transmitted from a single crystal which was mechanically scanned across the area of interest and transmitted signal received by a second transducer on the far side of area under investigation. They are known to have been working with this apparatus in the mid and late 1930s and published some early clinical results in the late 1940s.
Their first publications showed transmission ultrasonograms of the human head in which ventricular system appeared to be represented. Several other groups also published transmission image of head in 1950 and 1951.
However, it was late discovered that transmission ultrasonogram of the head from which the brain had been removed and replaced with saline were indistinguishable from the ultrasound already published. Clearly the apparent image of the ventricular system was due merely to varying attenuation of the transmitted ultrasound by alteration in the thickness of the lateral skull wall.
In 1949, Ludwig and Struthess used pulse echo-ultrasound as medical imaging technique to detect gallstones and foreign bodies in soft tissues.
One problem of the all the early ultrasound system was the difficulties of conducting the transmitted sound into the patient and echo-out. Thus the early pulse echosystem required total immersion of the subject in a water tank in which the scanning apparatus was also submerged.
Using such a system Howery and Bliss built a scanner based on up turned gun tunnel from a war time bomber; then clinical results were of high quality.
In 1950, Howry and Bliss produced first cross- sectional ultrasound image. In 1957, Howry and Homes assembled the first compound scanner consisting of water-filled tank in which patient was immersed surrounded by track in which transducer was moved in a series of compound motions and published their cross-sectional image of improved quality. The echoes were displaced on phosphor screens and photographed.
Wild and Reid published their work on direct contact scanner showing images of muscles of forearm and later of breast tissue. They reported 90 percent accuracy in differentiating benign from malignant lesions.
Mid 1950s, Saw proliferation of wider clinical application of diagnostic ultrasound. Baun developed an ophthalmic scanner.
He subsequently repeated the sonographic visualization of intraocular and orbital tumors, foreign bodies and retinal detachments.
Luksell applied the principle of industrial flaw detector to human skull and detected cerebral midline echo. Luksell and Turner were fortunate in choosing an appropriate anatomical target for their system and they laid the foundation of mid line echoencephalography. This field of investigation was further developed in Europe by Gordon, Lek Shell in Sweden and Kazner and colleagues in Germany.
This technique was subsequently widely used in accident and emergency and in neurology department until the advent of CT scanner in the late 1970s.
Howry and other with their compound scanner produced good quality image but requirement of total immersion of the patient in water was clinically unacceptable. Tom Brown in late 1950s produced a mechanical contact compound scanner. However image formation time was excessively long but image quality was good and rapidly gain clinical acceptance. Donald made a new apparatus on the basis of this produced direct contact manually operated scanner and applied in obstetrics and gynecology which gave a foundation of medical diagnostic ultrasound imaging as we know it today. Donald described two techniques still in use—coupling the transducer to the patient with mineral oil and 3using a full urinary bladder as a sonic window through which to visualize deep pelvic structures. He published a correlation of fetal biparietal diameter with gestational age.
Holm also developed a contact compound scanner which was in regular clinical use by the mid 1960s. Both Brown and Holmes scanner had ultrasound transducer mounted on an arm attached to a substantial rectilinear frame. Thereafter Wells produced probably the first of hinged arm scanner which rapidly became the main configuration for manually operated compound static scanners until their demise at the hand of automated real-time system in early 1980s.
The display system of very early experimental scanners all used conventional cathode ray oscilloscopes with open shutter photography to store the image. These system produced images with the equivalent of what we call a grey scale display. The development of scanner proceeded over the ensuing years. The bistable storage oscilloscope was introduced in order to improve the case of both scanning and photography. The price paid for this development was the display of low level echoes disappeared and resulting ultrasound images contained information only from organ boundaries and strong reflectors. Kossoff developed (1970s) a complex water bath scanner which produced excellent high resolution images with good dynamic range display on a non-storage oscilloscope. It was probably his work which stimulated the equipment manufacturers to reintroduce grey scale image display and this was greately facilitated by the development of television scan converter tube. This was a vacuum tube device somewhat similar to conventional cathode ray tube in which the image could be written onto a target in a random fashion during scanning and then read from the target as a conventional cathode ray tube. This device became readily available for inclusion in commercial scanners from about 1974 onwards. This revolutionized the ability of ultrasound to detect and display low echoes from soft tissue structures. Limitation of this scan converter was its average useful life which was very short and with electronic technology digital scan converter was developed in 1976. These systems were rapidly improved to give image resolution of greater than 512 × 512 pixels as the wide grey scan range.
The origin of real-time ultrasound imaging system lie in the late 1960s and early 1970s. Bom pioneered the development of linear array transducer as his first system having 20 elements giving 20 lines of information in the image. Sonar was simultaneously developing the phased array transducer. Both linear array and phased array scanner was practically demonstrated for its use in cardiac and abdominal injuries.
Prime clinical application of linear array real-time was in obstetrics while the scanner proved more satisfactory for cardiac and upper abdominal and pelvic imaging. The problem of small field of view with linear scanner was overcome in early 1980s by the introduction of curvilinear transducer. This design gave a very substantial improvement in image quality from these systems.
Siemens at the same time introduced mechanical real-time scanner called videoson. This system incorporated transducer mounted on a rotating wheel in front of parabolic mirror and produced good real-time images.
In early 1970s, a number of groups attempted to develop more compact real-time mechanical scanner. Their invention fell into two major groups, the rotating spinners and oscillating wobblers.
Mc Dicken (1974) published his initial work and subsequent development in this field has led to refinement incorporated in many rotating mechanical sector scanner. The wobblers have also undergone simultaneous further development and are currently the most popular mechanical real-time sector scanner.4
One of the main reason for continued popularity of mechanical sector scan systems is the symmetrical beam focusing which can be achieved with a single crystal element and suitable lenses. The main disadvantage of mechanical system has been that the focusing depth is fixed for any one crystal configuration.
In mid 1970s, annular array transducers were introduced which permitted beam focusing at any depth.
In 1980s, intracavitary mechanical rotating scanner were produced.
At the same time doppler ultrasound were being developed. The fundamental work in this field was done by Kallnus in 1954. The detection of fetal heart movement by doppler ultrasound was described by Callagan in 1964 and principle was rapidly developed for detection of blood flow within accessible superficial vessels throughout the body. Continuous wave doppler system was used. The major problem of this system was the inability to produce any form of image of the vessel or determine from what depth or from how many vessels the doppler signals were being received. In early 1970s, pulsed doppler system was employed which could permit measurement of depth within the patient from which echoes were arising. These systems suffered from the problem of extremely slow image production.
In mid 1970s to early 1980s, a duplex system was introduced in which a high resolution conventional real-time imaging scanner was linked to a pulsed doppler device. These systems were popular for evaluation of carotid circulation. The major limitation of this system lays in its inability of real-time imaging system to detect all plaque and thrombus reliably, especially fresh thrombus within a vessel.
This problem was overcome in late 1980s by Doppler color flow mapping. This technique became possible as a consequence of continuing development in rapid parallel computer signal processing.
Continuous rate of growth of improving image qualities and introducing new technological development continues unabated at present time which is making ultrasound a dynamic and essential part of imaging system in radiology department/hospital.