Quality control of elastography measurements – influencing factors and pitfalls

Purpose: To evaluate the correctness of shear wave measurements of different vendors and scanheads.

Material and methods: For comparing different US machines and scan heads we used the 2nd generation elastography phantom of CIRS (039) with 4 different stiffness values from 3.5 kPa (1), 10 kPa (2), 25 kPa (3) and 45 kPa (4) simulating the range from normal liver to severe fibrosis. 3 US machines with up to three scan heads (4 MHz, 6 MHz and 9 MHz) were compared (Siemens S2000 and S3000 with 4C1, 6C1, and 9L4 scan head, Toshiba Applio 500 with 6C1 scan head).

In every phantom 5 measurements are done in 2, 4, 6 and 8 cm depth. Additionally the pressure of the scan head was modified.

Results: The best correlation was seen at 2 cm with the 4C1 scan head of the S2000 (3.63 kPa, 9.61 kPa, 22.69 kPa, 42.19 kPa) and with the Aplio 500 in single shot technique (3.2 kPa, 9.7 kPa, 20.4 kPa, 41.2 kPa). With the 9L4 comparable results are only achievable in phantom 1 and 2 with significant underestimation of the stiffer phantoms.

All machines and scan heads showed an increasing underestimation of the kPa values with depth.

Fig. 1: Depth dependence

Only the 4C1 scan head was able to measure correct values up to 8 cm depth, The 6C1 scanheads of Siermens and Toshiba are limited to 6 cm. Realistic measurements with the Aplio 500 are only possible with the One Shot technique.

Conclusion: Besides influence factors as compression there was a systematic underestimation in greater depth possibly due to loss of energy oft he push pulse, that induces the shear wave. Realistic measurements were possible up to 6 cm.