In-phase and opposed-phase imaging: Applications of chemical shift and magnetic susceptibility in the chest and abdomen

For the radiologist performing body MRI, chemical shift imaging is a unique tool available to investigate the cellular composition of tissue. By leveraging the inherent difference in resonant frequency between water and fat protons that is due to their local chemical environment, chemical shift image acquisition can be timed to image water and fat protons when their collective signals are in phase or out of phase, to determine the relative amount of fat signal and water signal within an individual voxel. Loss of signal intensity between the in-phase and opposed-phase MR images indicates intravoxel lipid and can be used to make definitive benign diagnoses, characterize the tissue composition of solid organs, and serve as an adjunct imaging feature aiding in the diagnosis of a variety of benign and malignant processes in the chest and abdomen. Simultaneously, T2* decay can be used to advantage at chemical shift imaging to highlight a decrease in signal intensity or amplification of signal void (blooming artifact) on longer–echo time (TE) in-phase images, compared with the shorter-TE opposed-phase images. Susceptibility-related signal intensity loss owing to T2* decay allows the identification of iron deposition, hematomas, air, metal, and melanin to aid in diagnosis.