TISSUE HARMONIC IMAGING (THI) SITI AISHAH BINTI SHUIB A141063

CONTENT Introduction of Tissue Harmonics Imaging What are Tissue Harmonics? Generation of Tissue Harmonics Artifact Reduction with Tissue Harmonics Image Formation in Tissue Harmonics Image Characteristics Clinical Effects of Tissue Harmonic Imaging

INTRODUCTION A new grayscale imaging technique Images derived from higher frequency that produced second harmonic sound when the ultrasound pulse passes through tissue within the body Several advantages: Improved contrast resolution Reduced noise and clutter Improved lateral resolution Reduced slice thickness Reduced artifacts Improved signal-to-noise ration

WHAT ARE TISSUE HARMONICS? Harmonics are multiples of the fundamental beam Transmitting a band of frequencies centered at 2MHz will result in production of harmonic frequency bands at 4 MHz, 6MHz This doubled frequency sound is called the second harmonic Only the lowest frequency harmonic is used to form images Higher order harmonics not used in image formation because of: Range of frequencies can detect is insufficiently large to capture these higher frequency signals It is progressively lower in amplitude More quickly lost as they pass through tissue

GENERATION OF TISSUE HARMONICS The peaks and troughs of the transmit pulse Tissue alternatively expand and contract, distorting its shape Tissue density increase causing the peak of the sound wave to travel slightly faster than the trough Result: non-linear propagation, the wave become progressively more asymmetrical The asymmetrical distortion result in harmonics

Continue… As ultrasound wave travels trough more tissues, more harmonics are generated The production of harmonics is proportional to the square of the fundamental intensity Harmonics are generated predominantly by the the main transmit beam The maximal production of harmonics is at the focal zone Beam formed from tissue harmonics signals have less side lobes, less noise, and improved contrast resolution.

ARTIFACT REDUCTION WITH TISSUE HARMONICS Exhibit a reduction in image noise and clutter with improvement in contrast resolution (improved signal-to-noise) Reduce artifact in several ways - Tissue harmonic signals produced in the tissue only pass through the body wall one time. Decrease scatter from the body wall - Harmonic imaging prefers reject echoes that come from weak parts of the beam (side lobe, reverberation) Reduced artifacts from weak echoes - The virtual elimination of reverberation artifacts from the body wall Less near field haze from body wall reverberation

IMAGE FORMATION IN TISSUE HARMONICS Techniques to detect harmonics and eliminate the unwanted fundamental echoes: Filtration Single Line Pulse Inversion Side-by-side Phase Cancellation Pulse Encoding Tissue harmonic imaging operates by transmitting a fundamental beam that has a lower frequency As it propagates through tissue inside the body, generates the higher frequency harmonic sound Echoes from the fundamental frequency are rejected.

Continue.. Single Line Pulse Inversion (PI) Filtration Temporal cancellation technique Adds the echoes from two opposite polarity pulse to cancel the fundamental echoes, leaving only the harmonic information Filtration Remove echoes from the fundamental frequency and allow the harmonic frequency to pass Side-by-side cancellation Spatial cancellation technique Sends both signals together at the same time with opposite phases Pulse Encoding Relatively complex sequences to give each a unique code Recognized by special decoder Useful in the near field

TISSUE HARMONIC Artifacts Decreases Image Dynamic Range Image Characteristics Decreases Image Dynamic Range - Amplitude of the harmonic signal is lower in magnitude than the fundamental signal because of dynamic range of harmonic imaging is lower than the fundamental beam. - It results in images with more contrast, improve visualization. Better Lateral Resolution – Reduced Slice Thickness - Slice thickness of harmonic frequencies is narrower than the fundamental pulse - The edges of any ultrasound beam produce fewer harmonics than central beam Artifacts - Reduces some detrimental artifacts while useful artifacts are made more visible - Square law in harmonic imaging, harmonic production is propotional to the square of the fundamental amplitude - Results in minimizing certain artifacts - Worse Images in ‘Glass Body’ Patient - The resolution in harmonic images may be worse than fundamental imaging due to filtration decrease bandwidth - Related to the better contrast resolution of the higher frequency fundamental image

CLINICAL EFFECTS OF TISSUE HARMONIC IMAGING Display pathology and normal structures Decrease the time required to scan some patient Lead to important clinical information Identify lesions that are invisible or nearly invisible Confidently diagnose renal cell carcinoma and kidney stone Reducing the need for follow-up CT exams Easier to obtain better quality images Improve the overall quality of ultrasound diagnosis

REFERENCE http://www.sonocredits.com/. Philip W. Ralls M.D. Tissue Harmonic Imaging-Updated. 2004

THANK YOU 