2.1.4.3　High-frequency ultrasound（HFUS）

HFUS involves the creation of images using the reflection of ultrasound waves off interfaces with different acoustic impedances.The probe is placed on the skin，and reflected sound waves are interpreted by a computer and generate images.Image brightness is determined by the amplitude of the echo reaching the transducer.High-intensity echoes（or large amplitudes）create hyperechoic images（white），whereas low-intensity echoes（or small amplitudes）produce hypoechoic images（gray）.Anechoic images are black images.In ultrasonography，the frequency is inversely proportional to the depth of penetration and directly proportional to image resolution.Thus，HFUS has a shorter wavelength，providing a better resolution but less penetration.In dermatology，the commonly used frequencies are 20-25 MHz，but can be as high as 100 MHz.With frequency is about 20 MHz，resolution reaches 50-200 μm with a depth of penetration of 6-7 mm.If frequency increases to 50 MHz，resolution reaches 39-120μm，but the depth of penetration is only 4 mm.It is mainly used to differentiate benign and malignant tumors.Before surgery，it can provide information about tumor type and size and surrounding vessels，helping identify the best area for incision.Doctors can discuss with the patients in front of the ultrasound screen about the range of skin lesion in real time.It can also help the clinician evaluate whether the tumor was completely removed or not.However，it is still a challenge to differentiate various tissues（e.g.，tumor，fat，and inflammatory infiltrates）under HFUS.(https://www.daowen.com)