In-phase and out-of-phase sequences

In-phase (IP) and out-of-phase (OOP) sequences correspond to paired MRI gradient echo (GRE) sequences obtained with the same repetition time (TR) but with two different echo time (TE) values. 

Applications

The main application of the IP-OOP sequences is to identify pathological (microscopic) fat content of tissues in the abdomen by showing signal intensities drop on the OOP images compared to the IP images. Loss of signal intensity between the in-phase and out-of-phase MR images indicates fat. Examples where IP-OOP sequences are useful include ^1,2:

• fatty liver and focal fatty sparing/infiltration
• fat-rich adrenal lesions:
  • adrenal adenoma (helping differentiate it from carcinomas and metastases)
  • adrenal myelolipoma
• lipid-poor angiomyolipoma
• renal cell carcinoma (RCC)
• thymic hyperplasia
• osteoporotic versus neoplastic vertebral wedging: neoplastic lesions display persistence high signals on opposed-phase ^3,4
• vertebral metastases: metastatic lesions don't show loss of signal in out-of-phase sequence
• hemochromatosis of the liver ⁵
• intrahepatic pneumobilia ⁵
• metallic objects: by the effect of susceptibility artifact (e.g cholecystectomy clips)