Week 11 Additional Modes of Ultrasound Imaging

Tissue Harmonic Imaging (THI)

• If an ultrasound beam were a single frequency, it would be a perfect sine wave.

• Pulsed wave transducers produce multiple frequencies, so the beam is made up of multiple sine waves of different frequencies.

• Pulses become distorted as they travel through tissues due to non-linear propagation.

• Compressions travel faster than rarefactions.

• Harmonics refer to frequencies that are multiples of the transmit frequency (the fundamental frequency).

• The second harmonic (2f) is twice the fundamental frequency.

• Harmonic frequencies are generated at the center of the main beam because it's the highest intensity portion.

• Harmonics are produced due to distortion of the ultrasound wave.

• Only the highest intensity central portion of the beam can create harmonics.

• The harmonic beam is much narrower than the fundamental beam, leading to improved lateral resolution.

• As frequency increases, amplitude decreases.

• Only the 2nd harmonic is typically used, as higher-order harmonics are readily attenuated.

Advantages of Harmonic Imaging

1. Improved axial, lateral, and elevational resolution - reduces beam width and thickness

2. Reduced grating lobe artifacts - receiving echoes from centre of beam therefore eliminating weaker echoes from outside of the beam

3. Reduced reverberation artifact - because the harmonic beam is generated at a depth beyond where superficial reverberation occurs the image degradation that they cause is reduced or eliminated

4. Posterior shadowing artifact is increased - 2nd harmonic higher frequency therefore greater attenuation

Types of Harmonic Imaging

1. THI bandwidth filtering:

   • A bandpass filter is used to eliminate echoes from the fundamental frequency.

2. Pulse inversion THI:

   • Sends a normal pulse followed by an inverted pulse.

   • The two fundamental frequency pulses cancel each other out (destructive interference).

   • Only the 2nd harmonic frequency is received.

Disadvantages of Harmonic Imaging

• THI bandwidth filtering degrades axial resolution.

• Pulse inversion THI degrades temporal resolution slightly due to the use of two pulses for every scan line, and higher acoustic power is required.

• Depth penetration can be limited for both types due to the higher frequency.

Applications of Harmonic Imaging

• Differentiate solid and cystic lesions.

• Clean up artifactual echoes within fluid.

• Accentuate borders.

• Enhance shadowing.

• Improved detail and contrast resolution.

Compound Imaging (Sono CT)

• Uses phasing to send scan lines in multiple directions.

• Structures are interrogated more than once by the ultrasound beam with different incident angles.

• 3-9 images are acquired and combined to produce a compound, real-time image.

Advantages of Compound Imaging

1. Reduces speckle and noise by reinforcing true signals.

2. Reduces clutter or haze.

3. Reduces specular reflection artifacts.

4. Structures deep to highly attenuating objects are better seen.

5. Improved continuity of specular reflectors.

6. Reduced refractive (edge) shadowing, improving contrast resolution.

Disadvantages of Compound Imaging

• Decreased temporal resolution because multiple images are used to produce one frame.

• Improves contrast resolution but not spatial resolution.

• May create motion blur.

• Posterior acoustic shadowing and enhancement may be reduced.

Compound Imaging - Brand Names

• Advanced SciClear - Siemens

• CrossXbeam - GE

• APliPure - Toshiba (Canon)

• SonoCT - Philips

Panoramic Imaging

• Enables a wider field of view than the transducer face.

• Also called Extended Field of View (EFOV).

• Obtained by sliding the transducer along the scan plane.

• Earlier frames are retained and added to the new frames and “stitched together” electronically.

• Frames are “stitched together” electronically by spatially correlating where they overlap.

• The image processor uses image feature recognition software to combine the images.

Advantages of Panoramic Imaging

• Improved spatial relationships.

• Improved measurement accuracy.

• Best with linear, high-frequency transducers on stationary objects.

Limitations of Panoramic Imaging

• Tissue motion.

• Transducer rotation.

Applications of Panoramic Imaging

• When tissue motion is minimal.

• When transducer movement can be slow and steady.