1.
Liver
There are many applications of ultrasound elastography in liver.
The main application is liver fibrosis evaluation,
in which elastography techniques,
and in particular FibroScan,
are validated.
Elastography can also be used for:
- Cirrhosis complications prediction
- Hepatitis treatment monitoring
- Diagnosis of complications occurring after liver transplantation
- Steatosis evaluation
- Characterization of hepatic tumors.
A.
Liver fibrosis evaluation
For evaluation and quantification of liver fibrosis,
all elastography techniques have been widely studied.
Each technique,
their advantages and drawbacks,
and their diagnostic performance are detailed in tables 3-6 (Table 3, Table 4 , Table 5 , Table 6 ).
It must be kept in mind that fibrosis is not the only factor that can change the stiffness of the liver.
There are confounding factors such as hepatic congestion,
cholestasis,
and presence of acute hepatitis associated lesions,
that must be considered when interpreting the data.
B. Cirrhosis complications prediction
For predicting cirrhosis complications,
only the FibroScan and ARFI techniques were evaluated in liver and spleen.
Values are highly variable and these techniques can not be used routinely as a substitute for other methods validated in these indications.
However,
more stifness values increase,
more HCC or esophageal varices risks increase.
The areas under the ROC curve for predicting esophageal varices and HCC with FibroScan and ARFI methods are presented in Table 7.
C. Hepatitis treatment monitoring
Regarding viral hepatitis,
decreased elasticity values should be a marker of treatment response.
But these results must still be interpreted with caution.
D. Diagnosis of complications occurring after liver transplantation
Moreover,
elastography can help to highlight a liver graft injury,
as graft rejection or viral recurrence after transplantation,
showing an increase of stifness:
- graft damage by FibroScan 7.8 kPa (5.4 to 27.4) vs 5.3 kPa (3.1 to 7.4) (p <0.001)
- rejection SSI: 12.29 ± 8.13 kPa vs.
6.33 ± 2.10 kPa,
(p <0.001).
E. Steatosis evaluation
In the assessment of steatosis,
only the CAP ™ system (controlled attenuation parameter) coupled to the FibroScan Echosens system has been studied many times.
This technique measures the attenuation of ultrasonic waves in decibels per meter dB / m.
A value is provided only if a valid measurement of shear wave velocity is obtained.
This method has good performance for steatosis quantification,
regardless the etiology and is reproducible (ICC = 0.84) (Table 8).
The thresholds vary across studies.
It seems more efficient than simple B-mode ultrasound,
SteatoTest and FLI (Fatty Liver Index).
F. Characterization of hepatic tumors
In focal liver lesions characterization,
multiple studies are published:
- Some techniques with static elastography
- The majority with ARFI
- And one with the SSI technique.
There are significant variations within tumor types,
particularly for heterogeneous tumors and large tumors.
Several studies gave different and inconsistent results.
Two main studies assessed the interest of static elastography for FLL characterization.
The first one is a qualitative study and showed that HCC can be distinguished from metastases,
HCC appearing as soft or intermediate tumors while metastases appearing as hard tumors (Fig. 8).
The second study is a semi-quantitative study and found a strain ratio lower for benign lesions compared to malignant lesions:
Malignant lesions (2.82 ± 1.82) > benign lesions (1.45 ± 1.28) (p <0.001) (Fig. 9).
With the ARFI method,
the results are heterogeneous and involve either the value of the shear wave velocity V,
or the ratio between the velocity in the lesion and in the liver (this report seems to give better results).
The results of different studies are presented on Table 9.
Differentiating benign tumors - malignant tumors is not confirmed by all studies,
although the majority of studies found higher V in malignant tumors (Fig. 10, Fig. 11).
For lesion characterization,
the results are variable but trends are found:
- V FNH > adenoma
- V HCC > adenoma
- V metastases > adenoma.
With the SSI method,
only one study has been published and found that the technique could differentiate adenoma and FNH,
and HCC and cholangiocarcinoma.
These results should be confirmed.
2.
Breast
The patient is in supine position with the ipsilateral arm in an elevated position.
The pressure applied with the probe must be as low as possible.
All kind of elastography techniques with ultrasound guidance are applicable.
The main indication in breast is the characterization of a lesion detected in B mode.
It is based on the fact that malignant nodules are harder than benign lesions.
Elastography increases the performance of Bi-Rads in which it will soon be integrated (only color map).
It should not be used alone but in addition to the BiRads score.
Its interest is mainly for lesions classed Bi-Rads 3 or 4a for which it may modify the management.
Elastography has no interest in benign lesions classed Bi-Rads 2 and in moderately or highly suspicious lesions classed Bi-Rads 4b or 5.
The ROI must cover the entire lesion and surrounding breast tissue to allow comparison.
False positives are mainly fibrous fibroadenomas and scarring (Table 10,
Table 11,
Table 12).
False negatives are mucinous cancer,
cancer with inflammatory stroma,
lesion <5 mm,
deep lesion,
lesion in a breast parenchyma of high density.
Few studies have evaluated the interest of elastography in the evaluation of axillary lymph nodes: metastatic lymph appearing more rigid than the reaction lymph,
in static elastography or shear wave elastography.
3.
Thyroid
The examination is performed with a superficial linear probe,
during the conventional examination.
The main indication of elastography in thyroid disease is the nodule characterization,
in addition to B-mode which it does not replace.
Thyroid cancers behave differently according to histology:
- Papillary cancer is hard;
- Follicular cancers do not increase stiffness.
The presence of calcification can cause false positive.
(Table 13,
Table 14,
Table 15)
A classification has been established using qualitative static elastography however.
The interest of quantitative elastography in diffuse disease is still in the evaluation phase and is not validated.
The value of elastography for cervical lymph nodes characterization is still being evaluated,
but it seems that metastatic lymph nodes have a higher stiffness.
4.
Kidney
The use of ultrasound elastography techniques is complex in the kidney.
Indeed,
the stifness of the renal tissue depends on:
- The presence of fibrosis;
- The vascularization;
- The anisotropy (and thus the direction of the beam emission U.S.
/ renal structures);
- The presence of hydronephrosis.
Few studies have reported normal values : 5.0 ± 2.9 kPa for the renal cortex,
23.6 ± 5.4 kPa for the pyelon (Arda K,
AJR 2011) (SSI method)
The two indications of elastography are:
- The evaluation of intra-renal fibrosis;
- The characterization of kidney tumors.
The applicable methods in the kidney are:
- ARFI and SWI
- Measurement performed after a B mode exam,
with the convex probe.
- Their reproducibility are 22-24% for the ARFI system; and 12-20% for the SWI system (SSI).
The non-applicable methods in the kidneys are:
- Static elastography systems because:
– The depth of the kidneys
– The inability to compare pathological and normal renal tissues in case of diffuse pathologies.
- FibroScan system (for native kidneys) due to the absence of ultrasound guidance.
A.
Kidney characterization
On native kidneys,
no clinical studies reporting the interest of the ultrasonic elastography in the evaluation of intra-renal fibrosis has been published.
An experimental study showed an increased stiffness with renal glomerulosclerosis evolution.
On kidney transplant,
the results presented in the literature are contradictory and are presented in Table 16.
B.
Renal tumour characterization
Two studies assessed the contribution of elastography.
The first study reported a series of 15 cases (2 pseudo-tumors,
2 hemorrhagic cysts,
8 renal cell carcinomas,
one chromophobe cell carcinoma,
2 tubulo-papillary carcinomas),
studied by ARFI technique,
with values between 1.61 and 3.97 m / s,
without distinction between the different types (Clevert,
Clin Hemorheol Microcirc 2009).
The second study compared the strain imaging and the ratios between the lesion and the adjacent parenchyma,
in 28 angiomyolipomas and 19 renal cell carcinomas,
and found a difference between the two groups (p <0.001) (Tan,
AJR 2013)(Fig. 12).
5.
Prostate
The examination is performed with an endo-rectal probe,.
withno special preparation.
The techniques of static elastography and shear wave elastography (SWE-SSI and ARFI) are applicable (Table 17).
The rationale of using elastography in prostate is based on the fact that prostate cancers are indurated lesions.
Three applications have been identified (Table 18,
Fig. 13, Fig. 14):
- Characterization of abnormal area identified on mode B or Doppler imaging
- Detection of hard areas
- Targeting biopsies to increase the rate of positive biopsies.
A hard hypoechoic lesion is suspected of malignancy,
but all cancers are not hard and all hard lesions are not cancers (calcifications,
fibrosis).