DINFO Dipartimento di Ingegneria dell Informazione Department of Information Engineering Nuove tecnologie per ecografia ad ultrasuoni: da 2D a 4D Piero Tortoli Microelectronics Systems Design Lab 1
Introduction Historical overview Imaging systems Doppler systems Advanced methods/systems Research platforms 2
Echographic technique x TX t d RX t t
First medical applications J. Wild and John M. Reid: Application of Echo- Ranging Techniques to the Determination of Structure of Biological Tissues (Science, 1952)
First medical applications
Systems evolution
ULTRASOUND IMAGING SYSTEMS Array probes Beamforming B-Mode
Transmitter Receiver Processing
TX Beamformer TX waveform B-Mode imaging Ultrasound images are typically formed line-by-line, by using multiple transducers (arrays) RF Front-end Processing Demodulator RX Beamformer ADCs
B-Mode imaging For each transmitted pulse, the echo amplitudes are converted to gray levels to form a scan line. 100-200 pulses (lines) are needed to form one image. The Pulse Repetition Frequency (PRF) is limited by the max depth to be imaged
BEAMFORMING Multiple lines are scanned by using different groups of TX-RX elements How can the energy be focussed along a scan line with desired orientation? By exciting the individual elements of the active aperture with different signals: o to simulate a lens (beam-focusing) and/or o to steer the beam (beam-steering)
IMAGING ARRAY PROBES Linear Convex Sector
B-Mode IMAGE The envelope must be detected It is convenient doing this in baseband:
RF Envelope Detected Compressed
DOPPLER IMAGING Doppler effect Doppler receiver Doppler signal processing Doppler artefacts 15
Doppler effect Difference between Tx and Rx frequencies: f d f = 2 0 cos v c f 0 Tx frequency f 0 = 5 MHz c= 1500 m/s = 60 v = 30 cm/s f d 1 khz angle between directions of sound propagation and of target path c sound wave velocity
PW systems Transmitter Receiver Bursts of US energy are transmitted into the body at rate PRF Gate Processing Flow Display Audio output
PW RX: Gate (Sample/Hold) US transducer skin vessel TX burst Received echoes Range gate 1 st wall 2 nd wall Bursts are transmitted at PRF rate: for each TX burst, one sample of the Doppler signal is obtained (time sampling) The range gate selects the information backscattered only from the region of interest (sample volume) The Doppler signal is given by the sequence of samples collected in the slow-time domain
frequency Spectral analysis Spectral analysis of the Doppler signal allows distinct frequency (velocity) contributions to be discriminated time In Doppler spectrograms, subsequent spectra are grey-scale coded and displayed in adjacent vertical lines
Spectral analysis applications Detection of stenosis a. Healthy subject b. Patient with proximal stenosis (turbulence) (Courtesy of Johan Thijssen)
Duplex systems Ideal for stenosis assessment
Multigate spectral Doppler Slow time Fast ime FFT FFT FFT Real-time detection of velocity profiles in arteries and veins Depth Freq/ Vel
Flow imaging Mode (Color Doppler) Real-time 2D velocity maps are obtained By firing several pulses for each scan line By estimating the mean frequency (velocity) detected at each depth By color-coding consecutive pixels according to the detected mean frequencies By scanning a 2-D region Frame rate limitations Poor sensitivity Multigate processing applied to multiple scan lines
Doppler angle ambiguity The detected frequency depends on the Doppler angle f d = v c 2 f0 cos Frequency/color changes due to a change in the angle of insonation If the angle is not known, the frequency cannot be converted to velocity
Real-time Vector Doppler Directional beamforming along the symmetry line Left and right mean frequencies trigonometrically combined and converted to velocity
Real-time Vector Doppler
ADVANCED IMAGING SYSTEMS Parallel beamforming Volumetric imaging 27
Parallel beamforming US images are formed line by line: At 5 khz PRF, 20 ms are needed to form 100 lines Frame rates are typically limited to tens/s The only possibility of increasing the frame rate is creating multiple lines for each transmission event Multiline TX beamforming Parallel RX beamforming 28
Multiline TX beamforming It is possible transmitting multiple beams in the same TX event SINGLE LINE TRANSMISSION MULTI LINE TRANSMISSION By simultaneously transmitting N beams, N lines can be simultaneously formed An N-fold frame rate increase is achieved Frame rates up to several hundreds have been obtained 29
In vivo MLT imaging In collaboration with KU Leuven SINGLE LINE TRANSMISSION MULTI LINE TRANSMISSION Nominal frame rate: 39 fps 156 fps Playback Frame Rate: 10 fps 30
PARALLEL RECEIVE BEAMFORMING Real-time MLT operation is feasible provided the receiver can simultaneously beamform multiple lines (parallel RX beamforming) In this case, by transmitting wide beams, it is also possible to beamform multiple lines for each transmitted beam The approach can be extended to the transmission of a plane wave to cover the ROI in a single step (PLANE WAVE IMAGING). Since one image can be done with a single TX pulse, the term HIGH FRAME RATE (HFR) IMAGING is used.
Plane-wave imaging applications Super-resolution imaging Vector Doppler imaging 32
3D/4D IMAGING Probes Methods 33
3-D imaging: probes Freehand 1D-probe position tracking systems Step-motor driven linear arrays poor time resolution 2D Matrix arrays: electronically controlled to steer the beam in both the elevation and lateral directions much better time resolution but 34
MATRIX ARRAYS > 2k elements (up to over 8k) Need for micro-beamforming (ASIC design)
Matrix array probes Microbeamforming Significant reduction of the channel number Need for expensive integrated circuits (ASICs) 36
Sparse array probes Direct control of all active elements Reduced performance and sensitivity Need for carefully positioning the active elements 37
Spiral sparse array probes In collaboration with University of Delft/Rotterdam and RomaTre 30 20 10 y-axis [ ] 0-10 -20-30 -30-20 -10 0 10 20 30 x-axis [ ] Deterministic but aperiodic elements distribution 38
4-D imaging 4-D imaging requires producing at least 20 volumes per second One volume may include several thousands of voxels: Very high processing speed is needed 39
Research platforms Open scanners 40
TX Beamformer TX waveform Research platforms RF Front-end Processing Demodulator RX Beamformer ADCs Research needs: flexibility, expandable functionality, access to data open scanners /research platform
MAIN FEATURES OF OPEN SCANNERS Programmability Arbitrary waveform transmission Programmable beamforming Real time signal/image processing Data Storage Modes Pre-beamforming RF data Post-beamforming RF data Baseband (demodulated) data Capability of implementing a large class of (non standard) Imaging and Doppler Modes
HFR PLANE WAVE IMAGING PW 1 PW 3 PW 7 PW 11 Frame size [points] 96 lines 512 points Frame Rate Max [Hz] 1600 1267 543 345 345 fps 44
REAL-TIME VECTOR DOPPLER IMAGING 45
ULA-OP network ULA-OP map 46
US IMAGING: State-of-the art Electronics technology and open platforms have accelerated the experimental research, leading to: High quality B-Mode & vector Doppler imaging Ultra portable systems Matrix array probes (still expensive) and parallel processing devices (e.g. GPU) expected to: Extend 3D imaging to 4D (real-time)