Ultrasonography

Evolution of Ultrasound

Its use has extended from obstetrics, as in the early days, to image almost every organ system of the body resolving structures down to couple of millimeters in size

Dr Sonali Maniar Chief of Conventional Radiology & Ultrasound Wockhardt Hospital, Mumbai

Ultrasound is the most widely used imaging modality. The technology has evolved considerably over time and so have its applications. The foundation of ultrasound can be traced back to 1880 when Pierre Curie introduced simple echo sounding methods, leading to discovery of Sound Navigating and Ranging or SONAR. SONAR was used to detect submarines during the world war. This inspired the early ultrasound investigations in various centres across the world.

The first published work on medical ultrasound was by Dr Karl Theodore Dussik in Austria

in 1942. He used ultrasound waves to detect brain tumors. His method consisted of an ultrasound emitter at one end and ultrasound receiver at the other, while the patient stayed between the two devices. By measuring the sound beams transmitted through the patient, he was able to detect any tumors in the brain.

Professor Ian Donald from Glasgow, Scotland along with his colleagues in 1950's did much to facilitate the development of the technology and applications leading to wider use of ultrasound in medical practice. He was an obstetrician with interest in machines and electronics. Along with Tom Brown he invented and constructed the prototype of the first Compound B Mode Contact Scanner. Professor Donald introduced several diagnostic techniques in obstetrics and gynaecology which are till today in use such as the measurement of foetal Biparietal Diameter.

Flashback

Early machines in the 1960s were primitive, where a single image took several seconds to build up. These images depicted only the structure but not the movement. The scanning equipment was very large and probes had a large scan head with heavy cables. The first real time scanner was developed in 1965 by Walter Klause and Richard Soldner. The ultrasound beam from a hand-held probe was swept quickly and repetitively through the body by mechanised or electronic means. Thus, large volumes of tissues could be scanned in a short time.

The application of ultrasound extended to scan abdominal organs to help in detection of gall stones and tumors. Breast evaluation with ultrasound also began to develop. Studies were also made to use ultrasound for musculoskeletal imaging. The first B Scan image of a human joint was published in 1972 by Daniel G McDonald and George R Leopold in the British Journal of Radiology.

In early 1980s, computer software was merged to ultrasound technology. Digital technology helped in making smaller, more portable and relatively low-cost machines. The convex and curvilinear abdominal transducers were introduced and probes were smaller, lighter and easy to operate. Spectral and Colour Doppler were introduced which led to the extension of ultrasound applications to evaluate blood vessels and also cardiac imaging. Tumour vascularity could also be studied with color doppler.

1990s saw further improvements in image quality. The new transducer materials and engineering techniques allowed the use of much wider frequency band widths and higher sensitivity. With improved contrast and spatial resolution and availability of multi-frequency probes, the applications of ultrasound increased. Small parts like thyroid, scrotum and eye could be scanned. With high frequency transducers, the skin and subcutaneous tissue can also be studied.

Endoluminal Devices

Endoluminal devices were introduced to provide images of the vessel wall structures. Endoscopic ultrasound developed with the use of high frequency ultrasound probes which are introduced into the upper or lower part of the gastrointestinal tract to visualised gastrointestinal wall and adjacent structures. This is very useful in diagnosis and staging of benign and malignant lesions of the gut wall and surrounding structures of the mediastinum, abdomen and pelvis. It is also useful to evaluate submucosal masses of the upper gastrointestinal tract and the rectosigmoid for locating pancreatic tumor and assessment of vascular disease. Guided interventions like FNA or drainage are also possible.

As the machines became smaller and portable their use in trauma units, emergency departments and critical care units has increased. Amongst the recent advances in ultrasound harmonic imaging, real time spatial compound imaging, adaptive image processing, power doppler imaging and contrast enhanced gray scale harmonic ultrasound improved the image quality significantly.

Harmonic imaging is a modality that produces artifact free images with high resolution. Real time spatial compounding sonography uses electronic beam steering of a transducer array and as many as nine scans of an object, which are acquired from different view angles, are merged in overlapping fashion and averaged to form a compound real time image. This reduces artifacts and noise, thereby enhancing image contrast.

Contrasts

The earliest use of ultrasound contrast agents was in 1968, but subsequently better agents were developed in the 1990s.These are gas filled microbubbles that are administered intravenously. Microbubbles have a high degree of echogenicity, which is the ability of an object to reflect the ultrasound waves. Contrast enhanced ultrasound can be used to image blood perfusion in organs and measure blood flow rate in the heart and other organs. It has also been useful to evaluate hepatic masses.

3D World

3D ultrasound was first developed by Olaf von Ramm and Stephen Smith at Duke University in 1987. But later 4D came into existence which adds the element of time to the process. It renders live images of the foetus, much to the delight of expecting mothers and radiologists. It is currently widely used in obstetrics to show live images of the foetal face and also helps in diagnosing anomalies like cleft-lip and spinal dysraphism. Improvements and advances in technology led to several newer applications.

In gynaecology, it helps to evaluate uterine cavity anomalies, ovarian volumes, volumes of fibroids and other masses. In the abdominal study, it helps to determine the volumes of masses, gallstones, congenital renal anomalies, bladder masses and diverticuli. It can also be used in breast imaging to evaluate the tumor margins and its relation to ductal structures. The spatio-temporal image correlation technology is particularly useful in imaging the foetal heart.

Other Innovations

Ultrasound elasticity imaging is another new innovation useful for imaging nearly every tissue. Studies have shown that in breast imaging this can enhance the specificity for cancer detection. Elastography can also be used to image lesions in the thyroid, prostate, pancreas and lymph nodes.

The introduction of intra-operative transducers has led to the use of ultrasound in diagnosing liver metastases or pancreatic masses during surgery. It also provides valuable information of the relationship of the tumour to the portal and hepatic veins, thus being helpful in planning the surgery. It is also used in liver transplant to map the hepatic veins in the donor and to evaluate the hepatic artery graft in the recipient.

Ultrasound guided interventions like guidance for drainage, FNA, biopsy and pigtail catheter insertions can be performed. Recently, ultrasound has also been used to assist radiofrequency ablation of tumors. The therapeutic effects of ultrasound have been used to treat an injured joint or muscle tissue. High-intensity focused ultrasound is now used to heat and destroy pathogenic tissue. It is being used to treat uterine fibroids and prostate cancers.

Today, ultrasound is a sophisticated computer integrated tool. Its use has extended from obstetrics, as in the early days, to image almost every organ system of the body resolving structures down to couple of millimeters in size. Additionally, it has the advantages of involving no ionising radiation, has no known side effects, is readily available, relatively cheap, non invasive and portable.

With the ongoing improvements in ultrasound technology and software development, the applications of ultrasound will keep expanding.

drsonalimaniar@hotmail.com