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Paper text:
all tumours start out small , and treatment is more effective if the disease is detected early ; thus the current focus of imaging is to improve our ability to recognise small lesions . with colorectal liver metastases , careful mri or ct will detect 95% or more of lesions larger than about 15 mm , and it is now uncommon for our surgeons to discover lesions at the time of operation that were not already detected by earlier imaging .
the addition of intra - operative ultrasound ( ious ) has improved our ability to find small tumours , but in a current study in my own department , 20% of patients undergoing liver resection assisted by ious were found to have 
surgical findings at laparotomy , even with ious , no longer represent an adequate suitable standard of reference for measuring the accuracy of imaging .
one possible approach would be to look at the distribution of sizes amongst the lesions detected .
a recent study found that almost 60% of colorectal liver metastases were smaller than 20 mm at the time of detection , and 30% were smaller than 10 mm .
if we can detect more lesions in this size range we can be confident that our imaging techniques are improving .
once we have found a small lesion , we need to determine whether it is benign or malignant .
small ( < 15 mm ) lesions discovered incidentally in patients with no known primary malignancy are virtually always benign , but in patients with known malignant disease there are some metastases with ct appearances which overlap with those of benign lesions .
larger benign lesions are less problematic on ct , but at all sizes the mr appearances are more characteristic .
their contents are anechoic on us , show water attenuation on ct , low signal on t1 mri and high signal on t2 which is maintained on heavily t2-weighted sequences .
they are unchanged after gadolinium  an enhancing rim suggests necrotic metastasis , or in the appropriate clinical context , abscess .
characteristically , haemangiomas are hyper - echoic on sonography , show geographic shape with low attenuation on unenhanced ct and on t1-weighted mri .
their high signal on conventional t2-weighted mri is maintained with more heavy t2 weighting ( te = 180 m / s , or haste ) .
with contrast - enhanced ct or gadolinium - enhanced mri , the lesions show nodular enhancement at the periphery , followed by centripetal infilling over the next 510 min .
the discontinuity of the peripheral nodular enhancement is a specific feature , and peripheral wash - out does not occur .
in contrast , metastases with necrotic centres are usually round in shape and although they may show peripheral enhancement , the ring is more uniform .
whilst delayed enhancement of the necrotic centre of metastases is not uncommon , the brightly enhancing peripheral ring fades fairly rapidly .
some small haemangiomas show uniform and rapid enhancement starting in the arterial phase , similar to small hepatocellular lesions , but they usually show intensely bright signal on t2-weighted mri , in contrast to the iso - intense or mildly hyper - intense appearance of small benign hepatocellular lesions .
other atypical haemangiomas include small lesions with centrifugal enhancement from a single vascular nodule ( central dot sign ) and in other cases the lesion may be predominantly fibrous , so enhancement is patchy and delayed , with little of the typical peripheral nodular vascularity .
fatty infiltration is often heterogeneous , sometimes focal , and in other cases small focal areas of liver parenchyma may be spared from fatty change . on sonography , either focal fatty change or focal sparing can produce lesions which are difficult to distinguish from metastases .
one helpful feature with focal fatty change or sparing is that there is usually no mass effect 
focal fat may be recognisable on ct by its geographic distribution , normal contrast enhancement , and the lack of a mass effect , but a more reliable diagnosis can be made on mri . in patients with diffuse fatty change , metastases may be obscured on ct because their low attenuation is matched by the low attenuation of the surrounding liver .
occasionally , a ring of liver tissue immediately surrounding the lesion may be spared from fatty change , producing an irregular halo of denser tissue within an otherwise homogeneous liver .
this appearance can be seen with haemangiomas in a fatty liver , as well as with metastases . using inphase and opposed - phase t1 sequences
, mri offers a rapid and reliable approach for the recognition of focal fat , and reveals metastatic lesions in the diffusely fatty liver .
with the widespread use of dual - phase ct , multiphase contrast - enhanced mr , and the recent introduction of vascular contrast agents in sonography , has come a realisation that fnh is not rare .
larger lesions typically show a rounded or lobulated shape with clear - cut margin , a mass effect displacing adjacent vessels , and a central scar sometimes with radiating spokes towards the periphery .
their echogenicity , attenuation and signal characteristics are little different to those of normal liver tissue .
the lesions show marked arterial phase contrast enhancement which usually fades rapidly to become iso - intense in the portal phase , although occasionally increased enhancement lasts for several minutes .
lesions smaller than about 2 cm are often homogeneous with no central scar , but show the same enhancement characteristics .
it is not rare for these small lesions to be multiple , and they may co - exist with other benign lesions . when found in patients with known malignancy , a confident diagnosis of fnh requires the demonstration of intact liver function within the lesion .
some fnh lesions will take up as much spio as normal liver , so the lesion appears black on the post - contrast t2-weighted images .
in other cases the lesion may take up some spio , but less than the surrounding liver , so that the lesion becomes more apparent after contrast . in these cases
it is important to use the same t2-weighted sequence before and after spio , and measure the change in signal intensity in both liver and lesion .
if the lesion shows substantial spio uptake ( more than 4050% reduction in signal ) , it may be regarded as benign .
metastases do not take up spio , but well differentiated hepatocellular cancers may take up a little .
another approach is to use one of the hepatocellular mr contrast agents ( mangafodipir , gadobenate , or gadoxetic acid ) .
all of these agents are taken up by the functioning hepatocytes present in fnh lesions ( and also in well differentiated hcc ) , but because there is no biliary excretory pathway , the lesions retain the contrast for much longer than the normal liver , so becoming brighter than liver on delayed t1-weighted images .
peripheral wedge - shaped areas of abnormal perfusion , usually seen only in the arterial dominant phases of enhanced ct and mri , are probably caused by segmental portal vein occlusion which in some cases is associated with parenchymal tumour deposits , but in other cases no cause can be found . such transient hepatic attenuation defects ( thads )
may also be associated with parenchymal oedema within the liver which appears hyper - intense on t2-weighted mri . around the periphery of segment 4 , particularly just to the right of the falciform ligament anteriorly , and also anterior to the bifurcation of the portal vein , small areas of liver parenchyma may receive nonportal venous supply from the parabiliary venous plexus and from anomalous drainage of gastric , duodenal and pancreatic veins .
this can produce small nodules offocal fatty change in these areas , or more commonly may result in small areas of altered enhancement after contrast .
biliary microhamartomas ( von meyenburg complexes ) show imaging characteristics which overlap between cysts and haemangiomas . these biliary malformations are often irregular or even angular in shape , and usually only a few millimetres in diameter .
incidental small benign lesions are relatively common in patients with malignant disease , and must be distinguished from metastatic disease in order to avoid erroneous management . focal fat , focal sparing and haemangiomas
may be diagnosed on sonography or ct if they are typical , but in other cases mri is usually decisive .
discriminating small cysts and haemangiomas from metastases is best done with mri using a heavily t2-weighted sequence followed by dynamic gadolinium - enhanced t1 .
tiny lesions which appear on ct to have an angular shape and low attenuation will usually be biliary microhamartomas .
the successful treatment of liver tumours is more likely if they are found early . 
 the use of high - resolution ct and mri with contrast enhancement allows more sub - centimetre liver lesions to be detected , but some small tumours remain occult even at surgery with intra - operative sonography . 
 an indication of the accuracy of imaging in detecting liver metastases may be given by the proportion of lesions found which are under 12 cm in size . 
 the characterisation of small lesions remains problematic on ct , with benign and malignant tumours showing overlapping imaging features . 
 however , with appropriate use of chemical shift , heavy t2 weighting , gadolinium enhancement , and liver - specific contrast agents , a carefully tailored mri examination will usually produce diagnostic appearances .