Focal nodular hyperplasia (FNH) in liver is the second common benign hepatic tumor. It usually shows hypervascular mass on imaging studies and it is not easy to differentiate with other hypervascular malignant tumor. For diagnosis of FNH, contrast-enhanced ultrasonography (US) has been used to detect 'spoke-wheel sign', which can be typically seen in FNH. However, temporal window of vascular phase using contrast-enhanced US (CEUS) is very short (about 10 sec) and coordination of patient's respiration during US exam is absolutely needed. Thus, the investigators will use Superb-Microvascular imaging (SMI, Toshiba, Japan) for detection of 'spoke-wheel sign' in patients with proven FNH, which enable to detect slow micro vascular flow without using CEUS.

First, to compare the detection rate of 'spoke-wheel sign' between CEUS using sonazoid (Perfluorobutane, GE healthcare) and SMI.

Second, to compare the accuracy of size measurement between gray-scale US and SMI (reference standard: CEUS using sonazoid.)

1. To evaluate the detection rate of 'spoke-wheel sign' on CEUS using sonazoid and SMI (primary outcome).

   Each US findings during exam for patients with FNH were recorded by radiologist. The results were interpreted by consensus using 4 scale bar.

   0. Absence of 'spoke-wheel sign'

   1. Mild suspicious of 'spoke-wheel sign'
   2. Highly suspicious of 'spoke-wheel sign'
   3. Definite "spoke-wheel sign'

2. To evaluated the accuracy of size measurement for FNH on gray-scale US and SMI (reference standard: CEUS using sonazoid).

   To calculate the measurement error between each method and CEUS using sonazoid on US images using measurement of maximum diameter of tumor.

3. Sample size calculation according to the primary outcome.

Expecting the detection rate of "spoke-wheel sign" on SMI, 20% (known detection rate on CEUS using sonazoid, 23.5%), a sample size of 62 patients was required using 95% confidence interval.

Inclusion Criteria:

1. Patients with biopsy or imaging proven focal nodular hyperplasia

Exclusion Criteria:

1. Pregnant women
2. Patient's age > 70
3. Egg allergy
4. Breast-feeding women

Undecided