Kim Gammelmark

Recursive ultrasound imaging

Presents a new imaging method, applicable for both 2D and 3D imaging. It is based on Synthetic Transmit Aperture Focusing, but unlike previous approaches a new frame is created after every pulse emission. The elements from a linear transducer array emit pulses one after another. The same transducer element is used after N/sub xmt/ emissions. For each emission the signals from the individual elements are beam-formed in parallel for all directions in the image. A new frame is created by adding the new RF lines to the RF lines from the previous frame. The RF data recorded at the previous emission with the same element are subtracted. This yields a new image after each pulse emission and can give a frame rate of, for example, 5000 images/sec. The paper gives a derivation of the recursive imaging technique and compares simulations for fast B-mode imaging with measurements. A low value of N/sub xmt/ is necessary to decrease the motion artifacts and to make flow estimation possible. The simulations show that for N/sub xmt/=13 the level of grating lobes is less than -50 dB from the peak, which is sufficient for B-mode imaging and flow estimation. The measurements made with an off-line experimental system having 64 transmitting channels and 1 receiving channel, confirmed the simulation results. A linear array with a pitch of 208.5 /spl mu/m, central frequency f/sub otr/=7.5 MHz and bandwidth BW=70% was used. The signals from 64 elements were recorded, beam-formed and displayed as a sequence of B-mode frames, using the recursive algorithm. An excitation with a central frequency f/sub otr/=5 MHz (/spl lambda/=297 /spl mu/m in water) was used to obtain the point spread function of the system. The -6 dB width of the PSF is 1.056 mm at axial distance of 39 mm. For a sparse synthetic transmit array with N/sub xmt/=22 the expected grating lobes from the simulations are -53 dB down from the peak value at, positioned at /spl plusmn/28/spl deg/. The measured level was -51 dB at /spl plusmn/27/spl deg/ from the peak. Images obtained with the experimental system are compared to the simulation results for different sparse arrays. The application of the method for 3D real-time imaging and blood-velocity estimations is discussed.

Sequential beamforming for synthetic aperture imaging

Synthetic aperture sequential beamforming (SASB) is a novel technique which allows to implement synthetic aperture beamforming on a system with a restricted complexity, and without storing RF-data. The objective is to improve lateral resolution and obtain a more depth independent resolution compared to conventional ultrasound imaging. SASB is a two-stage procedure using two separate beamformers. The initial step is to construct and store a set of B-mode image lines using a single focal point in both transmit and receive. The focal points are considered virtual sources and virtual receivers making up a virtual array. The second stage applies the focused image lines from the first stage as input data, and take advantage of the virtual array in the delay and sum beamforming. The size of the virtual array is dynamically expanded and the image is dynamically focused in both transmit and receive and a range independent lateral resolution is obtained. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The lateral resolution increases with a decreasing F#. Grating lobes appear if F#≤2 for a linear array with λ-pitch. The performance of SASB with the virtual source at 20mm and F#=1.5 is compared with conventional dynamic receive focusing (DRF). The axial resolution is the same for the two methods. For the lateral resolution there is improvement in FWHM of at least a factor of 2 and the improvement at -40dB is at least a factor of 3. With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at -40dB is fluctuating with range. The theoretical potential improvement in SNR of SASB over DRF has been estimated. An improvement is attained at the entire range, and at a depth of 80mm the improvement is 8dB.

Synthetic Aperture Sequential Beamforming

A synthetic aperture focusing (SAF) technique denoted synthetic aperture sequential beamforming (SASB) suitable for 2D and 3D imaging is presented. The technique differ from prior art of SAF in the sense that SAF is performed on pre-beamformed data contrary to channel data. The objective is to improve and obtain a more range independent lateral resolution compared to conventional dynamic receive focusing (DRF) without compromising frame rate. SASB is a two-stage procedure using two separate beamformers. First a set of B-mode image lines using a single focal point in both transmit and receive is stored. The second stage applies the focused image lines from the first stage as input data. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The performance of SASB with a static image object is compared with DRF. For the lateral resolution the improvement in FWHM equals a factor of 2 and the improvement at -40 dB equals a factor of 3. With SASB the resolution is almost constant throughout the range. The resolution in the near field is slightly better for DRF. A decrease in performance at the transducer edges occur for both DRF and SASB, but is more profound for SASB.

Multi-element synthetic transmit aperture imaging using temporal encoding

A new method to increase the signal-to-noise-ratio (SNR) of synthetic transmit aperture (STA) imaging is investigated. The new approach is called temporally Encoded Multi-Element STA imaging (EMESTA). It utilizes multiple elements to emulate a single transmit element, and the conventional short excitation pulses are replaced by linear FM signals. Simulations using Field II and measurements are compared to linear array imaging. A theoretical analysis shows a possible improvement in SNR of 17 dB. Simulations are done using an 8.5 MHz linear array transducer with 128 elements. Spatial resolution results show better performance for EMESTA imaging after the linear array focus. Both methods have similar contrast performance. Measurements are performed using our experimental multi-channel ultrasound scanning system, RASMUS. The designed linear FM signal obtains temporal sidelobes below -55 dB, and SNR investigations show improvements of 4-12 dB. The depth performance is investigated using a multi-target phantom. Results show a 30 mm increase in penetration depth with improved spatial resolution. In conclusion, EMESTA imaging significantly increases the SNR of STA imaging, exceeding that of linear array imaging.

High resolution ultrasound beamforming using synthetic and adaptive imaging techniques

Ultrasound image quality can be improved by incorporating elevation information from high-order arrays and adjust the beamforming techniques to take advantage of the added information. We present the use of 1.75D ultrasound arrays for improving image quality with two approaches: 1) increasing the depth-of-field through synthetic transmit and receive imaging in elevation, and 2) implementing adaptive imaging techniques to remove tissue phase-aberrations to improve contrast and detail resolution. Phantom data is presented to show the increased visualization with synthetic elevation beamforming. Phantom and clinical results show the advantages of measuring and correcting images from two-dimensional phase aberration profiles using our 8 /spl times/ 128 1.75D array.

Synthetic elevation beamforming and image acquisition capabilities using an 8 /spl times/ 128 1.75D array

Ultrasound imaging can be improved with higher order arrays through elevation dynamic focusing in future, higher channel count systems. However, modifications to current system hardware could yield increased imaging depth-of-field with 1.75D arrays (arrays with individually addressable elements, several rows in elevation) through the use of synthetic elevation imaging. We describe synthetic elevation beamforming methods and its implementation with our 8 /spl times/ 128, 1.75D array (Tetrad Co., Englewood, CO). This array has been successfully interfaced with a Siemens Elegra scanner for summed RF and single channel RF data acquisition. Individual rows of the 8 /spl times/ 128 array can be controlled, allowing for different aperture configurations on transmit and receive beamforming. Advantages of using this array include finer elevation sampling, a larger array footprint for aberration measurements, and elevation focusing. We discuss system tradeoffs that occur in implementing synthetic receive and synthetic transmit/receive elevation focusing and show significant image quality improvements in simulation and phantom data results.

2-D tissue motion compensation of synthetic transmit aperture images

Synthetic transmit aperture (STA) imaging is susceptible to tissue motion because it uses summation of low-resolution images to create the displayed high-resolution image. A method for 2-D tissue motion correction in STA imaging is presented. It utilizes the correlation between highresolution images recorded using the same emission sequence. The velocity and direction of the motion are found by cross correlating short high-resolution lines beamformed along selected angles. The motion acquisition is interleaved with the regular B-mode emissions in STA imaging, and the motion compensation is performed by tracking each pixel in the reconstructed image using the estimated velocity and direction. The method is evaluated using simulations, and phantom and in vivo experiments. In phantoms, a tissue velocity of 15 cm/s at a 45° angle was estimated with relative bias and standard deviation of -6.9% and 5.4%; the direction was estimated with relative bias and standard deviation of -8.4% and 6.6%. The contrast resolution in the corrected image was -0.65% lower than the reference image. Abdominal in vivo experiments with induced transducer motion demonstrate that severe tissue motion can be compensated for, and that doing so yields a significant increase in image quality.

Preliminary in-vivo evaluation of convex array synthetic aperture imaging

Synthetic transmit aperture (STA) imaging has previously been investigated and compared to traditional imaging techniques in simulations and phantom studies. However, a full in-vivo study evaluating its clinical potential has yet to be conducted. This paper presents a preliminary in-vivo study of STA imaging in comparison to conventional imaging. The purpose is to evaluate whether STA imaging is feasible in-vivo, and whether the image quality obtained is comparable to traditional scanned imaging in terms of penetration depth, spatial resolution, contrast resolution, and artifacts. Acquisition was done using our RASMUS research scanner and a 5.5 MHz convex array transducer. STA imaging applies spherical wave emulation using multi-element subapertures and a 20 µs linear FM signal as excitation pulse. For conventional imaging a 64 element aperture was used in transmit and receive with a 1.5 cycle sinusoid excitation pulse. Conventional and STA images were acquired interleaved yielding ensuring exact same anatomical location. Image sequences were recorded in real-time, and processing was done offline. Male volunteers were scanned abdominally, and resulting images were compared by medical doctors using randomized blinded presentation. Penetration and image quality were scored and evaluated statistically. Results show that in-vivo imaging using STA imaging is feasible with improved image quality compared to conventional imaging.

Equipment and methods for synthetic aperture anatomic and flow imaging

Conventional ultrasound imaging is done by sequentially probing in each image direction. The frame rate is, thus, limited by the speed of sound and the number of lines necessary to form an image. This is especially limiting in flow imaging, since multiple lines are used for flow estimation. Another problem is that each receiving transducer element must be connected to a receiver, which makes the expansion of the number of receive channels expensive. Synthetic aperture (SA) imaging is a radical change from the sequential image formation. Here ultrasound is emitted in all directions and the image is formed in all directions simultaneously over a number of acquisitions. SA images can therefore be perfectly focused in both transmit and receive for all depths, thus significantly improving image quality. A further advantage is that very fast imaging can be done, since only a few emissions are needed for forming an image, and a novel approach of recursive ultrasound imaging can be used to give several thousand images a second. A commercial SA imaging system has, however, not yet been introduced due to a number of problems. The fundamental problems are primarily that the signal-to-noise ratio and penetration depth are low and velocity imaging is thought not to be possible. This paper will address all the issues above and show that they can all be solved using various techniques. The SNR is increased significantly beyond that for normal systems by using coded imaging and grouping of elements to form larger defocused emitting apertures. It is also possible to have many more receive channels, since different elements can be sampled during different emissions. The paper also shows that velocity imaging can be performed by making a special grouping of the received signals without motion compensation by using recursive imaging. With this technique continuous imaging at all points in the image is possible, which can significantly improve velocity estimates, since the estimates can be formed from a large number of emissions (100-200). The research scanner RASMUS, capable of acquiring clinical SA images, has been constructed and will be described. A number of phantom and in-vivo images will be presented showing in-vivo SA B-mode and flow imaging.

Velocity estimation using recursive ultrasound imaging and spatially encoded signals

Previously we have presented a recursive beamforming algorithm for synthetic transmit aperture focusing. At every emission a beamformed low-resolution image is added to an existing high-resolution one, and the low-resolution image from the previous emission with the current active element is subtracted yielding a new frame at every pulse emission. In this paper the method is extended to blood velocity estimation, where a new color flow mapping (CFM) image is created after every pulse emission. The underlying assumption is that the velocity is constant between two pulse emissions and the current estimates can therefore be used for compensation of the motion artifacts in the data acquired in the next emission. Two different transmit strategies are investigated in this paper: (a) using a single defocused active aperture in transmit, and (b) emitting with all active transmit sub-apertures at the same time using orthogonal spatial encoding signals. The method was applied on data recorded by an experimental system. The estimates of the blood velocity for both methods had a bias less than 3% and a standard deviation around 2% making them a feasible approach for blood velocity estimations.