A validation study of the Spatially Modulated Ultrasound Radiation Force (SMURF) method for shear modulus estimation is presented. SMURF estimates of uniform gelatin and Zerdine™ phantoms covering a modulus range of 2 to 18 kPa are compared with results obtained by unconfined mechanical compression and sonoelastography. The results show agreement within the measurement uncertainties over the range indicated for all three methods. Repeatability and variation on the order of 5% of the phantom modulus are found for observations made at a single point within the phantom. Averaging of modulus estimates from several adjacent scan lines further decreases the variation. By using multiple radiation force peaks to induce a shear wave of known wavelength and measure the frequency of the wave, SMURF obtains modulus estimates from tracking data acquired along a single A-line. This is significant, as speckle can bias the measured phase of the shear wave. SMURF is shown to be insensitive to a constant phase error in the shear wave measurement. This results in greatly reduced correlated noise in the modulus estimates, in contrast with methods which track at multiple locations and do not cancel phase errors.

A val i da tion study of the Spa tially Mod u lated Ul tra sound Ra di a tion Force (SMURF) method for shear modulus es ti ma tion is pre sented. SMURF es ti mates of uni form gel a tin and Zerdine™ phan toms cov er ing a modulus range of 2 to 18 kPa are com pared with re sults ob tained by un con fined me chan i cal com pres sion and sonoelastography. The re sults show agree ment within the mea sure ment un cer tain ties over the range in di cated for all three meth ods. Re peat abil ity and vari a tion on the or der of 5% of the phantom modulus are found for ob ser va tions made at a sin gle point within the phan tom. Av er ag ing of modulus es ti mates from sev eral ad ja cent scan lines fur ther de creases the vari a tion. By us ing mul ti ple radi a tion force peaks to in duce a shear wave of known wave length and mea sure the fre quency of the wave, SMURF ob tains modulus es ti mates from track ing data ac quired along a sin gle A-line. This is sig nif icant, as speckle can bias the mea sured phase of the shear wave. SMURF is shown to be in sen si tive to a con stant phase er ror in the shear wave mea sure ment. This re sults in greatly re duced cor re lated noise in the modulus es ti mates, in con trast with meth ods which track at mul ti ple lo ca tions and do not can cel phase er rors.

Spatially modulated ultrasound radiation force (SMURF) [S. McAleavey et al., Ultrason. Imag. 29, 87–104, (2007)] is a novel method for ultrasonic estimation of the low-frequency shear modulus properties of elastic media. In this approach, radiation force with a known lateral magnitude variation is applied impulsively within a region of interest using short (∼30 μs) bursts of ultrasound. The spatial frequency k of the radiation force intensity variation is selectable and determined through beamforming. Application of the radiation force impulse gives rise to a low-frequency (500–2000 Hz) shear wave. The temporal frequency ω of this wave is measured using Doppler ultrasound methods. The modulus is estimated from the relationship G=ρ(ωk)2, where ρ is the material density. To validate this method, the moduli of five samples of Zerdine (CIRS, Incorporated), an ultrasonically tissue-equivalent elastic material, were measured using conventional unconfined cyclic compression and SMURF implemented on a Siemens Antares scanner. The samples were cylindrical with diam 54 mm and height 25 mm. The shear modulus of the samples ranged from 2.5–35 kPa. For all samples, the two methods agreed to within the estimated variation. An intrasample variation of 8% was observed for SMURF in the (assumed uniform) samples. [Work supported by NIH/NIBIB.]