A multirate scan conversion method

Pixel based focusing for photoacoustic and ultrasound dual-modality imaging

It is desired that the same imaging functional modules such as beamformation, envelope detection, and digital scan conversion (DSC) are employed for the efficient development of a cross-sectional photoacoustic (PA) and ultrasound (US) dual-modality imaging system. The beamformation can be implemented using either delay-and-sum beamforming (DAS-BF) or adaptive beamforming methods, each with their own advantages and disadvantages for the dual-modality imaging. However, the DSC is always problematic because it causes blurring the fine details of an image, e.g., edges. This paper demonstrates that the pixel based focusing method is suitable for the dual-modality imaging; beamformation is directly conducted on each display pixel and thus DSC is not necessary. As a result, the artifacts by DSC are no longer a problem, so that the proposed method is capable of providing the maximum spatial resolution achievable by DAS-BF. The performance of the proposed method was evaluated through simulation and ex vivo experiments with a microcalcification-contained breast specimen, and the results were compared with those from DAS-BF and adaptive beamforming methods with DSC. The comparison demonstrated that the proposed method effectively overcomes the disadvantages of each beamforming method.

Compounded direct pixel beamforming for medical ultrasound imaging

In this paper, a new compounded direct pixel beamforming (CDPB) method is presented to remove blurring artifacts introduced by ultrasound scan conversion. In CDPB, receive focusing is directly performed on each display pixel in Cartesian coordinates using the raw RF data from adjacent transmit firings so that artifacts from the scan conversion can be removed. In addition, the energy variations resulting from the distance between the transmit scanline and display pixel are compensated by utilizing the gain factor obtained from the ultrasound beam pattern. The proposed CDPB method was evaluated using simulation and in vivo liver data acquired by a commercial ultrasound machine equipped with a research package. The experimental results showed that the proposed CDPB method improved the information entropy contrast (IEC) by 23.6% compared with the conventional scan conversion method and it reduced the blocking artifacts factor (BAF) by 16.4% over the direct pixel-based focusing method. These results indicate the proposed new direct pixel beamforming method could be used to enhance image quality in medical ultrasound imaging.

High-speed digital scan converter for high-frequency ultrasound sector scanners

This paper presents a high-speed digital scan converter (DSC) capable of providing more than 400 images per second, which is necessary to examine the activities of the mouse heart whose rate is 5-10 beats per second. To achieve the desired high-speed performance in cost-effective manner, the DSC developed adopts a linear interpolation algorithm in which two nearest samples to each object pixel of a monitor are selected and only angular interpolation is performed. Through computer simulation with the Field II program, its accuracy was investigated by comparing it to that of bilinear interpolation known as the best algorithm in terms of accuracy and processing speed. The simulation results show that the linear interpolation algorithm is capable of providing an acceptable image quality, which means that the difference of the root mean square error (RMSE) values of the linear and bilinear interpolation algorithms is below 1%, if the sample rate of the envelope samples is at least four times higher than the Nyquist rate for the baseband component of echo signals. The designed DSC was implemented with a single FPGA (Stratix EP1S60F1020C6, Altera Corporation, San Jose, CA) on a DSC board that is a part of a high-speed ultrasound imaging system developed. The temporal and spatial resolutions of the implemented DSC were evaluated by examining its maximum processing time with a time stamp indicating when an image is completely formed and wire phantom testing, respectively. The experimental results show that the implemented DSC is capable of providing images at the rate of 400 images per second with negligible processing error.

Local activity levels guided adaptive scan conversion algorithm

Digital scan converter in ultrasound machine prepares data in display buffer for displaying ultrasonic echo signals on monitor with interpolating missed pixel values. The popular bilinear interpolation and bicubic interpolation in scan conversion blurred image edges and detail regions. An adaptive scan conversion algorithm was proposed with applying warped distance interpolation to enhance image edges. And local activity level threshold distinguished smooth regions from detail regions to preserve sharpness in smooth regions. Simulation result indicated that result ultrasound images were superior to other common algorithms.

Real time 3D visualization of ultrasonic data using a standard PC

This paper describes a flexible, software-based scan converter capable of rendering 3D volumetric data in real time on a standard PC. The display system is used in the remotely accessible and software-configurable multichannel ultrasound sampling system (RASMUS system) developed at the Center for Fast Ultrasound Imaging. The display system is split into two modules: data transfer and display. These two modules are independent and communicate using shared memory and a predefined set of functions. It is, thus, possible to use the display program with a different data-transfer module which is tailored to another source of data (scanner, database, etc.). The data-transfer module of the RASMUS system is based on a digital signal processor from Analog Devices--ADSP 21060. The beamformer is connected to a PC via the link channels of the ADSP. A direct memory access channel transfers the data from the ADSP to a memory buffer. The display module, which is based on OpenGL, uses this memory buffer as a texture map that is passed to the graphics board. The scan conversion, image interpolation, and logarithmic compression are performed by the graphics board, thus reducing the load on the main processor to a minimum. The scan conversion is done by mapping the ultrasonic data to polygons. The format of the image is determined only by the coordinates of the polygons allowing for any kind of geometry to be displayed on the screen. Data from color flow mapping is added by alpha-blending. The 3D data are displayed either as cross-sectional planes, or as a fully rendered 3D volume displayed as a pyramid. All sides of the pyramid can be changed to reveal B-mode or C-mode scans, and the pyramid can be rotated in all directions in real time.

Beamforming with a reduced sampling rate

The beamforming process requires a high delay resolution to avoid the deteriorating effects of the delay quantization lobes on the image dynamic range and signal to noise ratio. Wideband transducers require delay resolutions in the order of 1/16 the signal period. If oversampling is used to achieve this timing resolution, a huge data volume has to be acquired and processed in real time. This is usually avoided by sampling just above the Nyquist rate and interpolating to achieve the required delay resolution. However this increases the hardware complexity. Baseband sampling has been alternatively proposed with sampling rates as low as the transducer frequency or even lower. This approach uses two A/D converters and processing chains for every channel, thus doubling the hardware requirements. Quadrature sampling can be used instead with a single A/D converter, but the sampling rate must be a multiple of four times the transducer frequency, decreasing the application flexibility. Furthermore, it produces relatively high errors in the detected envelope if wideband transducers are used. This work presents a new approach, the selective sampling technique (SST), which keeps the lowest sampling rate required by the imaging process or the signal bandwidth (whatever is larger) and, at the same time, provides a high delay resolution to keep the highest image dynamic range. The SST is based on a second order sampling process which, differently from the mentioned approaches, does not pose any constraints in the time interval between samples and produce lower errors in the detected envelope. The hardware requirements are low (a single A/D converter and processing chain for every transducer element), working at the lowest data rate compatible with the Nyquist criterion, thus reducing the data bandwidth. Furthermore, the sampling points can be also freely chosen, so that the SST simplify the usually required scan conversion process to a simple linear interpolation easily carried out by software in real-time.