The F-number significantly reduces image artifacts in all image formation methods using the delay-and-sum (DAS) algorithm, such as
The F-number, for a uniform linear transducer array, equals the quotient of the focal length and the width of the receive subaperture.
The usage of a fixed F-number results in a dynamic receive subaperture whose width increases with the focal length.
Established methods to compute the optimal F-number attribute the image artifacts to two different phenomena:
Both approaches, although yielding similar F-numbers (0.5 <= F <= 2), are mutually contradictory.
Wide array elements, for example, show
an increased directivity.
The "Noise" approach suggests the usage of narrow receive subapertures or, equivalently, large F-numbers for such elements to improve the signal-to-noise ratio.
The "Grating lobes" approach, in contrast, permits wide receive subapertures or, equivalently, small F-numbers for such elements because they attenuate the grating lobes.
Both approaches, moreover, yield F-numbers that increase with the frequency.
The DAS algorithm, however, requires a fixed F-number and typically uses the maximum F-number at the upper frequency bound. This F-number satifies the conditions for all lower frequencies but, owing to its suboptimal value, reduces the spatial resolution.
The proposed F-number not only eliminates image artifacts but also maintains the spatial resolution of the full aperture .
This F-number, in particular, prevents the first-order grating lobes from insonifying reflective image structures. The F-number, to this end, uses a closed-form expression, which derives from the far-field sensitivity of the focused receive subaperture, to impose a minimum angular distance on these grating lobes.
A Fourier-domain beamforming algorithm enables the usage of frequency-dependent F-numbers. The algorithm not only varies the width of the receive subaperture with the voxel position but also with the frequency. This additional frequency dependence, in contrast to a fixed F-number, includes additional frequency components that improve both the contrast and the spatial resolution.
|Method||Width of the receive subaperture||Spatial resolution||Grating lobe suppression|
|No F-number||always full||optimal||none|
|Proposed F-number||frequency- and position-dependent||almost optimal||optimal|
- Clone the repository or download the release to your local hard drive.
git clone https://github.com/mschiffn/f_number
- Add the repository to your MATLAB path using .
addpath( genpath( './f_number' ) )
The repository has the following structure:
. ├── +auxiliary # auxiliary functions (e.g., dimension and size check) ├── +f_numbers # classes for various types of F-numbers (e.g., constant, directivity-derived, proposed) ├── +windows # classes for various window functions (e.g., boxcar, Hann, Tukey) ├── das_pw.m # main function ├── LICENSE # license file └── README.md # this readme
The packages +f_numbers and +windows contain an exemplary class hierarchy to manage various types of F-numbers and window functions.
Use the function das_pw to form images.
In MATLAB type
to obtain an explanation of the input and output arguments.
The typical usage is:
[ image, F_number_values ] = das_pw( positions_x, positions_z, data_RF, f_s, e_theta, element_width, element_pitch, ( 1 - N_elements ) / 2, [ f_lb, f_ub ], c_ref, N_samples_shift, window, F_number);
The proposed F-number can be instantiated by
chi_lb = 45; % minimum angular distance of the first-order grating lobes F_ub = 3; % maximum permissible F-number F_number_rx = f_numbers.grating.angle_lb( chi_lb, F_ub );
width_over_pitch = 0.918; % element width-to-element pitch ratio (1) F_number_rx_1 = f_numbers.directivity.perrot( width_over_pitch ); F_number_rx_2 = f_numbers.directivity.szabo( width_over_pitch );
The standard fixed F-number is
F_number_rx_3 = f_numbers.constant( 3 );
M. F. Schiffner and G. Schmitz, "Frequency-Dependent F-Number Increases the Contrast and the Spatial Resolution in Fast Pulse-Echo Ultrasound Imaging," 2021 IEEE Int. Ultrasonics Symp. (IUS), in press.
G. Montaldo, M. Tanter, J. Bercoff, N. Benech, and M. Fink, “Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography," IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 56, no. 3, pp. 489–506, Mar. 2009.
B. Delannoy, R. Torguet, C. Bruneel, E. Bridoux, J. M. Rouvaen, and H. Lasota, “Acoustical image reconstruction in parallel-processing analog electronic systems,” J. Appl. Phys., vol. 50, no. 5, pp. 3153–3159, May 1979.
C. Bruneel, E. Bridoux, B. Delannoy, B. Nongaillard, J. M. Rouvaen, and R. Torguet, “Effect of spatial sampling on an acoustical image reconstruction,” J. Appl. Phys., vol. 49, no. 2, pp. 569–573, Feb. 1978.
mschiffn (2022). Frequency-Dependent F-Number for CPWC (https://github.com/mschiffn/f_number/releases/tag/0.1), GitHub. Retrieved .
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