Molecular imaging may be defined as spatially localized and/or temporally resolved sensing of molecular and cellular processes in vivo. An imageable molecular event may be the result of the overexpression of a gene that produces a specific messenger RNA. The overexpressed protein could be an enzyme or could be incorporated into cell-surface transporters or receptors. Any step of this process is a potential target for molecular imaging. Current molecular imaging modalities include magnetic resonance imaging, nuclear imaging, ultrasound, and optical imaging. Nuclear medicine has been at the forefront of molecular imaging because of the relatively high sensitivity to detect nanomolar or picomolar quantities of the radiolabeled imaging probe. Imaging has had a central role in the diagnosis of hepatocellular carcinoma (HCC), which is considered the fifth most frequent malignancy worldwide. Nuclear imaging was one of the earlier modalities used for liver imaging. Traditional tracers included technetium 99m (99mTc) sulfur colloid, gallium 67, and 99mTc iminodiacetate acid analogues. Other less traditional probes include 99mTc diethylenetriamine pentaacetic acid-galactosyl-human serum albumin for evaluation of functional liver volume and 99mTc-labeled tetrofosmin and methoxisobutylisonitrile for detecting drug resistance. Fluorodeoxyglucose is the most widely used probe for positron emission tomography (PET) tumor imaging; however, carbon 11-labeled acetate appears to show improved sensitivity and specificity for HCC. Oxygen 15 PET imaging allows for the measurement of hepatic and tumor blood flow. Difficulties developing specific imaging methods for HCC are caused by the lack of obvious specific molecular targets, problems with drug delivery, and poor contrast-to-noise. No magic molecular imaging method exists today to accurately detect, characterize, and monitor HCC in vivo.
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