Breast Elastography in BI-RADS 3–4: Tsukuba Scale, Strain Ratio, and Biopsy Decision
Strain Elastography Methods
According to EFSUMB (Săftoiu et al., 2019), strain elastography (SE) images of the breast can be evaluated visually using the Tsukuba scale (also known as the Itoh or Ueno scale), semi-quantitatively with strain ratio (SR) or strain histograms (SH), and by the size ratio of the lesion on the elastogram to its size in B-mode (E/B ratio). An optimal elastogram includes glandular tissue, surrounding fat, and the lesion itself.
According to Diagnostic Imaging: Breast (Berg, Leung, 2019), strain reflects the longitudinal displacement of tissue under compression: soft tissues deform more than dense ones. The strain ratio compares the deformation of adjacent fat to the deformation of the lesion. Compression is achieved by the transducer, natural movement (heartbeat, breathing), or light manual pressure by the operator.
Five-Point Tsukuba Scale
According to EFSUMB (Cosgrove et al., 2013), the Tsukuba scale evaluates the lesion based on the volume of stiff tissue relative to the background:
| Score | Characteristic |
|---|---|
| 1 | The lesion is not stiffer than the surrounding tissues |
| 2 | Increasing proportion of stiff tissue in the lesion |
| 3 | Further increase in the proportion of stiff tissue |
| 4 | The lesion is stiff throughout |
| 5 | Stiffness extends beyond the borders of the lesion visible in B-mode |
Strain Ratio and E/B Ratio
According to Diagnostic Imaging: Breast (Berg, Leung, 2019), malignant lesions are characterized by an E/B ratio > 1. According to a meta-analysis, the E/B ratio is the most accurate strain indicator: sensitivity 96% and specificity 88%. In qualitative assessment, absolute elasticity is not calculated because the initial compression force varies and is not precisely measured.
Impact on Management in BI-RADS 3–4
Elastography has the potential to downgrade BI-RADS 4A and even 4B lesions to BI-RADS 3 with a malignancy risk <2% (Berg, Leung, 2019), allowing some BI-RADS 4A lesions to be managed with dynamic observation instead of biopsy. In the provided clinical example, an oval soft (blue), homogeneous lesion on SWE appeared benign; biopsy confirmed a fibroadenoma.
Shear Wave Elastography (SWE)
In SWE, shear waves are generated by the transducer's automatic "push pulse"; the longitudinal displacement of the lesion induces transverse shear waves, the speed of which is recorded by the system. In denser tissue, waves propagate faster; for the breast, the maximum scale is often set at 7.7 m/s (180 kPa) (Berg, Leung, 2019).
Frequently asked questions
What does a score of 5 on the Tsukuba scale mean?
According to EFSUMB (Cosgrove et al., 2013), a score of 5 means that stiffness extends beyond the borders of the lesion visible in B-mode.
Which strain indicator is the most accurate?
According to a meta-analysis (Berg, Leung, 2019), the most accurate is the E/B ratio: sensitivity 96%, specificity 88%. Malignant lesions are characterized by an E/B ratio > 1.
Can the BI-RADS category be downgraded using elastography?
Yes, elastography has the potential to downgrade BI-RADS 4A and even 4B to BI-RADS 3 with a malignancy risk <2% (Berg, Leung, 2019).
Why is absolute elasticity not calculated in qualitative SE?
Because the initial compression force varies and is not precisely measured (Berg, Leung, 2019).
What is the maximum SWE scale used for the breast?
The maximum scale is often set at 7.7 m/s (180 kPa) for breast applications (Berg, Leung, 2019).