Chapter 9 Real-time ultrasound instrumentation

1. Chapter 9Real-time ultrasound instrumentation

2. Image acquisition • A mechanical sector has a single crystal with one pulser and one transmit/receive switch • A system with 128 channels requires 128 pulsers, 128 T/R switches, and 128 crystals in the array and hence very complex structure • Beam formation in transmit mode include: beam steering/sequencing, focusing, time delay generation and apodizing • The processing in receiving signal is TGC, gain, beam formation, signal processing, scan conversion,, image processing, image storage and display

3. B-mode scanner block diagram

4. In beam former ADC of the amplified signal occurs at a sampling rate of 20 to 40 MHz with 8 to 12 bits. The dynamic range after TGC & gain amplification is 50 to 80 db. Dynamic range of the signal after TGC is 50 to 80dB and after 12 bits digitalization it reduce to 36dB Interpolation increase the effective sampling rate by a factor of 4 to allow nanosecond time delays for dynamic receive focusing. To remain in focus necessary time delay is applied to the received signal To reduce side lob apodization is performed Beam former

5. Image matrix • The echo signal is placed at the correct location in the image matrix by a position generator. • A position generator uses the time of travel for the ultrasound pulse & position sensors for beam direction. • The signal level and coordinate are placed in a temporary buffer before transfer to scan convertor • The format is changed from signal level along successive lines of sight to the matrix notation.

6. Operator control • Several operator adjustable controls are available to optimize image • The most important are scan range, transmit power, transmit focal zone, gain, TGC, log compression, edge enhancement, reject control, persistence, gray scale, preprocessing and post processing manipulation of the data

7. Transmission and Gain control • The depth control sets the maximum rang of the field of view. • The power control adjusts the intensity of ultrasound beam transmitted into the patient. • The transmit zone selection control adjusts the depth of focusing. • Gain and TGC change control the signal amplification • TGC should readjust when imaging depth changes

8. Signal processing • Received signals have a very wide range. They should accommodate in an eight bit word • Enabling to process an extremely wide range of signal levels, compression is necessary. • Weaker signals are enhanced by log compression. • Low signal levels occupy a greater portion of 0 to 255 compressed values. • The reject control reduces low-amplitude noise.

9. Image processing and persistence • Use different digital filtering to reduce noise and enhance signal • Digital filtering • Temporal averaging to reduce noise (persistence) • Edge enhancement • Contrast enhancement • Persistence: noise reduction through temporal signal averaging • Freeze frame • Freezing a frame of interest to see more carefully (the frame is held in the buffer and repeatedly displayed) • A single frame is hold in the output buffer • Cine loop • Acquired images allow the operator to review the dynamic relationships (up to 150 frame) • Many image can be stored in the memory and then played back with desired speed to study in more detail

10. Synthetic aperture • Lateral resolution is improved as the number of processing channels is increased. • Increasing lateral resolution in this way increase imaging time and reduce frame rate • To reduce imaging time, the number of channels in the beam former can be reduced to half the number of elements if a two pulse sequence is used.

11. Multiple beam former • Provide ability to acquire multiple scan line per transmitted pulse (up to 4) • Multiple scan lines per transmitted pulse increase: • Beam width. • Frame rate. • Parallel processing with multiple beam formers in transmit mode using multiple focal zones also can improve lateral resolution without loss in frame rate.

12. Coherent image formation • The narrow receive focusing & the more broad transmission beam don't share a common axis caused sampling along the scan line is curved. • Image composition with curved scan lines degrades spatial detail.

13. Coherent image formation • Coherent image formation correct scan line curvature to form synthetic straight scan lines using suing echo amplitudes for a pair of oppositely curved interrogation line averaged, depth-by-depth. • Coherent image processing includes both axial data along individual scan lines & lateral data from adjacent scan line.

14. Phase aberration • If the tissue velocity isn't uniform along the path, the ultrasound pulse arrives at a latter time then expected and cause phased aberration. • Phase aberration reduces the SNR & degrades the sensivity. • Correction for phase aberration is performed by adjusting the time delay applied independly to each element in the active aperture of multirow crystal array transducer so that the phase of the received signal from the two lines become identical. • Alternative method is to align the signal form each element to the center element

15. Broadband techniques • Broadband transducer provide: • Flexibility in operation. • As the frequency band width become wider; the possibility of matching optimal image acquisition parameters with the desired clinical information is improved

16. Broadband techniques include: Multi frequency imaging: The available broad bandwidth subdivide into two or more frequency ranges for transmission and reception for different depth Confocal imaging: Focal zone at each depth with different centre frequency can be used. The main problem is the reduction in frame rate Dynamic frequency filtering: Uses total transducer bandwidth for the transmitted pulse and then adjusts the receiver center for receiving signal at different depth Frequency compounding: If the received echo can be divided to low and high frequency bandwidth; then appropriate filtering can be applied and then combine the two spectrum to get frequency average image with less speckles Broadband techniques

17. Coded excitation • To improve SNR & penetration depth: • Increase applied power • Increase pulse duration • Increasing pulse duration reduce axial resolution. To over come the transmitted pulse can be coded. Coded excitation include: • Binary encoding. • Frequency modulation.

18. Binary encoding • A binary coding of the excitation pulse generate a specific transmission pattern pulse. • A reflected echo from a specific depth has different echo pattern from echo from other depth • The combined echoes from different depths can be analyzed mathematically

19. Frequency modulation • Frequency modulation of the transmitted pulse is achieved by varying the emitted frequency with time. • Axial resolution doesn't depend on the chirp pulse duration, but rather the fractional bandwidth.

20. Comparison of echo ranging and coded excitation • More energy per unit time is present in the coded waveform. • To improve SNR. • Echo ranging is more appropriate for superficial structures. • In pulse formation, side lobes can be reduced by proper weighting each frequency & window function.

21. Tissue harmonic imaging • This is a new imaging technique • It relies on the detection of the harmonic frequencies created by beam propagation through tissue • Although the detected signal is week by contrast is often better

22. Nonlinear propagation • The speed of the sound wave c(z) isn't constant at different depth but varies over the propagation path (z): • Because the speed of sound isn't constant during the wave cycle, the sinusoidal shape become distorted and change in frequency components of the sound wave. • is the nonlinearity coefficient

23. Nonlinear propagation • High-frequency harmonics are very rapidly attenuated in the medium. • Nonlinear propagation due to the high- frequency components exhibits more rapid pressure amplitude loss than predicted by the attenuation equation

24. Nonlinear propagation • Amplitude of the second harmonic: • To isolate the second harmonic component in tissue harmonic imaging are used two method: • Harmonic band filtering. • Pulse phase inversion.

25. Harmonic band filtering • The returning echo have a fundamental frequency component & a harmonic frequency component. • A filter is applied to remove the fundamental echo signal & only the tissue harmonic component is processed for image formation.

26. Pulse phase inversion • A two pulse sequences is transmitted along the same path in which the second one 180 degrees shifted • For linear propagation the two received signals have the same amplitude but an 180-degree phase difference. • In nonlinear propagation, harmonics are added in the same proportion to each waveform following.

27. Pulse phase inversion

28. Advantage of tissue harmonic imaging • Echo from clutter, grating & side lobes are suppressed in the tissue harmonic image. • Multiscattering produce low amplitude sound waves, which do not form harmonics • Reverberation artifacts are reduced. • Distortion & scattering are suppressed in the tissue harmonic image.

29. vmc

30. Advantage of tissue harmonic imaging • The reduction of acoustic noise enhances contrast resolution & border delineation. • Having narrower beam width improves lateral resolution. • THI are depicted with greater contrast & spatial detail.

31. Spatial compounding • By acquiring multiple beam angles & averaging the data, improves the presentation of boundaries throughout the FOV. • A linear array with high number of elements (>192) is necessary. • Average of the acquired image is done real time • Steering angle is within 20 degrees • A wideband transducer is necessary to lower the center frequency for large angle steering in order to reduce grating lobes. • A weighted average of the scan lines is necessary to compensate for differences in amplitude. • Maximum amplitude from all scan lines may be used alternatively • The image is less noisy

32. Extended FOV • Real-time FOV is limited by transducer width or sector scanning angle. • Extended FOV overcomes this limitation by combining successive frames to form a panoramic image. Alternative method acquire real time images in a cine clip and the process the successive frame to form a panoramic image • Extended FOV use computer analysis of images to determine transducer location without sensors or articulating arm

33. Image registration • Multiple frame are obtained as a linear array moves across the patient. • Successive frames can be registered and then anatomical matching to find the transducer movement and the combining the frames is performed. • Computer matching of image feature in successive frame is accomplished.

34. Image registration • In the probe linear movement, image feature displacements are in the same direction & the same distance throughout the superimposed frame. • If the probe translates & rotates, then the image feature displacement change with depth.

35. Image registration

36. Image registration • The extended FOV, has both real-time & static components. • The real-time component shows the anatomy currently being imaged. • The static component displays the anatomy that has been scanned. • The primary factors that can cause artifacts are large-scale tissue motion & off plane rotation.

37. High frequency imaging • Increasing frequency results in higher resolution • With a 20MHz transducer 100um resolution is achievable • Nowadays 10MHz imaging is routine in different areas of medical practice • Problems with higher frequencies are less penetration and higher sampling rate

38. Three-dimensional imaging • In 3D imaging the spatial relationships of structures within a scanned volume are represented. • Adding successive frame results in a three dimensional image • 3D image can be a combination of parallel slice, wedge, cone or any arbitrary direction • Real time 3D image is called 4D image

39. Methods of volume sampling in 3D imaging are: Mechanical scanning Freehand scanning Use of 2D probe Mechanical scanning: linear motion: parallel 2D images acquired at defined intervals Tilt motion: parallel 2D images acquired at defined angle Rotation: parallel 2D images acquired around a defined axis Three-dimensional imaging

40. Three-dimensional imaging • Free hand • Free hand scanning without position sensing. • Free hand scanning with position sensing. • Rectangular probe.

41. Position-sensing techniques • Articulated arm. • Magnetic field sensor. • Acoustic ranging. • Image-based correlation.

42. Image reconstruction • The most common method to form 3D volume data set is voxel- based reconstruction. • Each pixel in the set of 2D images is placed at the proper location within the 3D volume. • If voxel wasn't sampled by a 2D image, then the value for that voxel is calculated by interpolation using the value from neighboring voxel.

43. Image display • The process of selecting & manipulating data for visualization is called rendering. • Three types of rendering from 3D images for display: • multiplanar formatting. • Surface rendering. • Volume rendering.

44. 4D imaging • Display of a surface-rendered plane or multiple planes of the sampled volume in real-time is called 4D ultrasound. • The fourth dimension is time • Methods are mechanically steered multi element and 2D array

45. Multielement arrays • In this method: Mechanically sweeps a 1.5D arrays back and forth across the beam port of transducer • The direction of the movement is perpendicular to the crystal rows • A frame rate of about 5volume per second is possible • Since fewer scan lines are acquired resolution deteriorate

46. 2D array • Approximately 4000 crystal elements comprise a 2D array • The FOV is smaller but the frame rate is higher • Maximum frame rate of 20 is obtainable • The future of 4D ultrasonic depends on the technological advances in the design of 2D arrays of transducers, computer speed and their costs

47. limitation • One major drawback of this 4D imaging method is the lack of orthogonal scan planes. • Reconstruction of scan data sets in to 3D display is more abstruse than CT & MRI . • The lack of a common unifying factor, speckle & small differences in signal levels makes some data manipulation techniques difficult to apply.

48. Elasticity imaging • It is a new technique • It is a quantitative map of tissue stiffness • Stiffness is a measure of how likely the tissue will maintain the shape when the force is applied • Deforming force per area is called stress • The relative displacement of tissue when subjected to stress is strain • The proportionality constant between strain and displacement is called elastic modulus

49. Elasticity image • Strain imaging depicts the relative tissue displacement between precompression and compression measured by ultrasound