Dr N Jayalatha, Professor – Radiology & Imaging and Director, MNJ Institute of Oncology & Regional Cancer Centre
Ultrasounds are user friendly and readily available at bed side especially in high intensity focused ultrasound (HIFU) and harmonic imaging.
Few advances in ultrasonography (USG) include:
Digital beam forming. The beam former is the system of electronics that determines the shape of the beam. Earlier beam formers used either analog electronics or a combination of analog and digital electronics. In modern transducers the beam former is totally digital which enables the ultrasound beam to be focused with greater precision.
High-frequency imaging. High-frequency ultrasound imaging using frequencies above 20 MHz is being developed to enable the imaging of superficial structures at a very high resolution. Using conventional medical ultrasound at 5 MHz, it is possible to penetrate up to 20 cm and achieve a spatial resolution of 0.5–1.0 mm. However, with high-frequency ultrasound it is possible to image with a resolution of 50 microns (1/20 mm). The disadvantage is that because the attenuation is increased at high frequencies, penetration is reduced to a few millimeters.
Extended field of view imaging. This is an imaging process which combines static B-mode techniques with real-time imaging so that a large subject area can be viewed on a single static image. Extended field of view (FOV) images are obtained by sliding the probe over the area of interest and as the images are acquired they are stitched together electronically.
Compound imaging. This technique combines electronic beam steering with conventional linear array technology to produce real-time images acquired from different view angles.
Three-dimensional imaging. The production of a 3-D image requires a volume of tissue to be scanned. The data from this volume are then used to construct the types of image required. There are three approaches to scanning a volume of tissue: free-hand, mechanical, and electronic scanning.
Harmonic imaging. Harmonic imaging exploits non-linear propagation of ultrasound through the body tissues. The high-pressure portion of the wave travels faster than low pressure resulting in distortion of the shape of the wave. This change in waveform leads to the generation of harmonics (multiples of the fundamental or transmitted frequency) from a tissue. At present, second harmonic is being used to produce the image as the subsequent harmonics are of decreasing amplitude and hence insufficient to generate a proper image.
These harmonic waves that are generated within the tissue, increase with depth to the point of maximum intensity and then decrease with further depth due to attenuation. Hence the maximum intensity is achieved at an optimum depth below the surface.
Contrast agents. Contrast-enhanced ultrasound (CEUS) involves the administration of intravenous contrast agents containing microbubbles of perfluorocarbon or nitrogen gas. The bubbles greatly affect ultrasound backscatter and increase vascular contrast.
PI imaging. Pulse inversion (PI) technique plays an important role in ultrasonic nonlinear imaging. For tissue imaging, PI technique provides suppression of spectral leakage and, thus, produces better image contrast. For contrast imaging, contrast between the agents and surrounding tissues are also enhanced with this technique by distinguishing nonlinear microbubbles from the background in either Doppler domain or radiofrequency domain
Elastography. Elastography is a newer technique that exploits the fact that a pathological process alters the elastic properties of the involved tissue. This change in elasticity is detected and imaged using elastography.