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Tissue Harmonic Imaging (THI) is a ultrasonographic technique that potentially
yields better-quality images than those obtained with conventional ultrasound
The production of harmonics derives from the non-linear distortion of the incident beam as it passes through the tissues.
The phenomenon occurs for ultrasound beams of a sufficiently high power. The harmonic signal is reduced on the surface and decreases in the deeper layers as a result of ultrasound beam attenuation. Current technology employs the second harmonic that has a frequency twice that of the incident beam.
In forming the image, a system of filters excludes the fundamental echoes with frequencies equal to the incident beam. Specific modalities can be used to improve separation between the two frequency bands—harmonic and fundamental—and enhance representation of the harmonic image. Our system uses a particular pulse configuration, whereas other systems use pulse-phase inversion techniques [1-5]. The potential advantages of harmonic imaging include increased axial resolution resulting from reduction in wavelength and better lateral resolution resulting from better ultrasound beam focussing.
Moreover, background noise is diminished as a result of reduced reading of backscattering echoes, there are fewer artefacts due to lateral lobes and less distortion of the ultrasound beam when it impacts the body wall. There are relatively few literature reports evaluating the potential benefits of harmonic imaging, and the few papers presented have mostly evaluated the increase in image quality obtained in the various abdominal regions [9-12].
Our study aimed at identifying the diagnostic advantages of harmonic imaging compared to conventional imaging in the evaluation of common bile duct lithiasis in patients who have undergone cholecystectomy. In the diagnosis of biliary obstruction, ultrasonography is an excellent method, able to identify the level of obstruction in 90% of cases [13-17]. For the identification of bile duct lithiasis varying percentages of diagnostic accuracy have been reported (32%, 71%) [18-19]. This has promoted the application of other techniques and methods for the study of this disease, such as endoscopic ultrasound (EUS), laparoscopic ultrasound (LUS), MR-cholangiography and endoscopic retrograde cholangiopancreatography (ERCP).
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