the most common and generally accepted ' typical ' us features of hepatic hemangiomas are their small size , uniform hyperechogenicity , well - defined margin , and posterior echo enhancement ( 1 - 3 ) ( fig .
1 ) . in addition , follow - up scanning only rarely shows a change in size , appearance , or detectability ( 4 ) .
they are usually composed of large blood - filled cavernous spaces , lined by a single layer of flat endothelial cells and separated by fibrous septa ; multiple interfaces between the walls of the sinuses and the blood within them account for the typical hyperechogenicity seen at us ( 4 ) .
the enhanced through - transmission observed in many cases reflects the low acoustic impedance of blood - filled spaces and tends to occur in hemangiomas larger than 25 mm ( 2 ) . because hemangiomas may undergo various changes including internal hemorrhage with necrosis , thrombosis , myxomatous change , fibrosis , and- rarely- calcification as they become larger ( 1 , 4 ) , ' atypical ' us features are frequently seen in larger ones . unlike a typical hemangioma , one that is atypical
has an internal echo pattern at least partially hypoechoic ; a hyperechoic mass with a hypoechoic central portion or decreased echogenicity can be seen throughout the entire lesion ( figs .
the most suggestive us feature of this type of hemangioma is an echogenic border , seen as a thick echogenic rind or thin rim around a tumor ( 4 ) ( figs . 2 and 6 ) .
although the prevalence of atypical hemangioma has not been precisely determined , it seems that approximately 20 - 40% of hemangiomas are of this kind ( 4 , 5 ) .
the echogenicity of hepatic parenchyma influences the us appearance of a hemangioma . because of the increased echogenicity of attenuating fatty liver parenchyma , diffuse fatty infiltration may lead to an atypical echo - poor appearance ( 6 ) ( fig . 8) . if a ' typical ' hemangioma is present in a liver in which fatty infiltration has occurred , its us appearance will be altered .
the lesion will initially appear less hyperechoic than isoechoic , and finally hypoechoic relative to infiltrated liver .
fatty infiltration of the liver may also cause obscuration of the echogenic border around the tumor ( 5 ) .
it has recently been shown that the speed of contrast enhancement occurring during incremental dynamic ct or multiphase dynamic mr imaging provides a basis for predicting the echo pattern at us , and vice versa . on dynamic ct or mr images ,
sonographically hypoechoic hemangiomas tend to show rapid enhancement , but if hyperechoic , they tend to enhance slowly ( 7 , 8) ( figs . 9 and 10 ) .
as techniques for evaluating the vascularity of various organs and diseases , color and power doppler us have recently shown promise . since power doppler us is superior to color doppler us in detecting slow flow , it has been used in atempts to diagnose hepatic hemangiomas specifically on the basis of their typical hemodynamic features .
though it was initially believed that power doppler is able to depict slow flow within hemangiomas ( 10 ) , it has been suggested that homogeneous noise from highly reflective interfaces can be misinterpreted as true flow ( 11 ) . to avoid these false signals ,
a recent study in which a doppler phantom / flow control system and a 2 - 4 mhz transducer were used found that for determining the vascularity of hyperechoic tissue , a pulse repetition frequency of 1,000 hz and a medium wall filter were adequate .
using these optimized parameters , the majority of hepatic hemangiomas produced either no power doppler signal or one that was minimal ( 12 ) ( fig .
the majority of hepatic hemangiomas show a typical enhancement pattern : peripheral nodular or globular enhancement during the bolus dynamic phase and centripetal fill - in and persistent enhancement during the delayed phase , and for this reason , dynamic contrast - enhanced ct or mr imaging has been widely used for diagnosis . since microbubble contrast agents for us have become available , the efficacy of these for the characterization of focal hepatic lesions has been the subject of numerous investigations ( 9 , 13 - 15 ) .
in hepatocellular carcinoma , the use of contrast agents greatly increases intratumoral doppler signals ( 13 , 14 ) , but in hemangiomas , the use of power doppler us with microbubble agents reveals either no internal vascularity or sparse peripheral flow ( 13 , 15 ) ( fig . 11 ) . due to its lack of sensitivity in detecting slow flow in hemangiomas ,
power doppler us can not therefore , be used for the specific diagnosis of hepatic hemangioma , even with the use of microbubble agents ( 15 ) .
pulse - inversion harmonic us is a newly introduced technique for displaying the amplitude of second harmonic signals resulting from non - linear echoes .
in contrast to power doppler us , pulse - inversion harmonic us with interval delay scanning can provide strong gray - scale enhancement by microbubble contrast agents and detect signals from microbubbles in very slow flow without doppler - related artifacts .
it has recently been shown that this technique can depict the typical enhancement patterns of three different types of liver lesions : hemangiomas , metastases , and hepatocellular carcinomas .
while most hemangiomas ( 95% ) show peripheral globular or rim - like enhancement with progressive centripetal fill - in , malignant tumors do not show this pattern ( 9 ) ( fig .
pulse - inversion harmonic us with interval delay scanning using a contrast agent is therefore believed to be potentially useful for the specific diagnosis of hepatic hemangiomas by demonstrating their characteristic enhancement features .
in the diagnosis of hepatic hemangiomas , the most important aspect is non - invasive differentiation from other tumors .
since us plays a key role in the initial evaluation of these hemangiomas , knowledge of the spectrum of those that are atypical is important and can help avoid most diagnostic errors . at the same time , however , in most cases of hemangioma with atypical sonographic features a specific diagnosis can be established by means of dynamic contrast - enhanced studies such as ct or mr imaging . because pulse - inversion harmonic us with interval delay scanning using a contrast agent is also believed to be potentially useful for the specific diagnosis of hepatic hemangiomas by demonstrating their characteristic enhancement features ,
simple and immediate characterization of a focal hepatic lesion newly detected by initial us examination is possible .
 because us plays a key role in the initial evaluation of hepatic hemangiomas , knowledge of the entire spectrum of us appearances of these tumors is important . 
 most hemangiomas have a distinctive us appearance , and even with those with atypical appearances on conventional gray - scale us , specific diagnoses can be made using pulse - inversion harmonic us with contrast agents . in this essay , we review the spectrum of us appearances of hepatic hemangiomas on conventional gray - scale , power doppler , and pulse - inversion harmonic us with contrast agents . 
Conventional Gray-Scale US
Power Doppler US
Power Doppler US with a Contrast Agent
Pulse-Inversion Harmonic US with a Contrast Agent
CONCLUSION